@article{GerhardKacprzyk2022,
  author    = {Gerhard, Reimund and Kacprzyk, Ryszard},
  title     = {Paul B{\"o}ning - early electret researcher in Shanghai and Wroclaw (1922-1945)},
  series = {IEEE transactions on dielectrics and electrical insulation},
  volume    = {29},
  journal   = {IEEE transactions on dielectrics and electrical insulation},
  number    = {3},
  publisher = {Institute of Electrical and Electronics Engineers},
  address   = {New York, NY},
  issn      = {1070-9878},
  doi       = {10.1109/TDEI.2022.3168372},
  pages     = {853 -- 858},
  year      = {2022},
  abstract  = {The scientific career and the research activities of Paul Boening, especially during his tenures at Tongji University in Shanghai (Woosung Campus, 1922-1936) and the Technical University of Wroclaw (TH Breslau, 1936-1945), are briefly reviewed. In particular, Boening's pioneering investigations in the area of electrets and space charge in dielectrics are emphasized. We attempt to shed some light on the significant achievements of a virtually unknown contributor to the early history of electrets and of space-charge research and high-voltage engineering, during the 1920s and 1930s. It should be noted that dielectrics research was a truly international endeavor already at that time.},
  language  = {en}
}
@article{RamanVenkatesanSmykallaPlossetal.2022,
  author    = {Raman Venkatesan, Thulasinath and Smykalla, David and Ploss, Bernd and W{\"u}bbenhorst, Michael and Gerhard, Reimund},
  title     = {Tuning the relaxor-ferroelectric properties of Poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) Terpolymer films by means of thermally induced micro- and nanostructures},
  series = {Macromolecules : a publication of the American Chemical Society},
  volume    = {55},
  journal   = {Macromolecules : a publication of the American Chemical Society},
  number    = {13},
  publisher = {American Chemical Society},
  address   = {Washington},
  issn      = {0024-9297},
  doi       = {10.1021/acs.macromol.2c00302},
  pages     = {5621 -- 5635},
  year      = {2022},
  abstract  = {The effects of thermal processing on the micro- and nanostructural features and thus also on the relaxor-ferroelectric properties of a P(VDF-TrFE-CFE) terpolymer were investigated in detail by means of dielectric experiments, such as dielectric relaxation spectroscopy (DRS), dielectric hysteresis loops, and thermally stimulated depolarization currents (TSDCs). The results were correlated with those obtained from differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), and Fourier-transform infrared spectroscopy (FTIR). The results from DRS and DSC show that annealing reduces the Curie transition temperature of the terpolymer, whereas the results from WAXD scans and FTIR spectra help to understand the shift in the Curie transition temperatures as a result of reducing the ferroelectric phase fraction, which by default exists even in terpolymers with relatively high CFE contents. In addition, the TSDC traces reveal that annealing has a similar effect on the midtemperature transition by altering the fraction of constrained amorphous phase at the interphase between the crystalline and the amorphous regions. Changes in the transition temperatures are in turn related to the behavior of the hysteresis curves on differently heat-treated samples. During heating, evolution of the hysteresis curves from ferroelectric to relaxor-ferroelectric, first exhibiting single hysteresis loops and then double hysteresis loops near the Curie transition of the sample, is observed. When comparing the dielectric-hysteresis loops obtained at various temperatures, we find that annealed terpolymer films show higher electric-displacement values and lower coercive fields than the nonannealed sample, irrespective of the measurement temperature, and also exhibit ideal relaxor- ferroelectric behavior at ambient temperatures, which makes them excellent candidates for applications at or near room temperature. By tailoring the annealing conditions, it has been shown that the application temperature could be increased by fine tuning the induced micro- and nanostructures.},
  language  = {en}
}
@article{RamanVenkatesanWuebbenhorstGerhard2022,
  author    = {Raman Venkatesan, Thulasinath and W{\"u}bbenhorst, Michael and Gerhard, Reimund},
  title     = {Structure-property relationships in three-phase relaxor-ferroelectric terpolymers},
  series = {Ferroelectrics},
  volume    = {586},
  journal   = {Ferroelectrics},
  number    = {1},
  publisher = {Routledge, Taylor \& Francis Group},
  address   = {Abingdon},
  issn      = {0015-0193},
  doi       = {10.1080/00150193.2021.2014260},
  pages     = {60 -- 81},
  year      = {2022},
  abstract  = {Poly(vinylidenefluoride-trifluoroethylene)-based (P(VDF-TrFE)-based) terpolymers represent a new class of electroactive polymer materials that are relaxor-ferroelectric (RF) polymers and that offer unique and attractive property combinations in comparison with conventional ferroelectric polymers. The RF state is achieved by introducing a fluorine-containing termonomer as a "defect" into the ferroelectric P(VDF-TrFE) copolymer, which reduces the interaction between the VDF/TrFE dipoles. The resulting terpolymer exhibits a low Curie transition temperature and small remanent and coercive fields yielding a slim hysteresis loop that is typical for RF materials. Though the macroscopic behavior is similar to RF ceramics, the mechanisms of relaxor ferroelectricity in semi-crystalline polymers are different and not fully understood yet. Structure-property relationships play an important role in RF terpolymers, as they govern the final RF properties. Hence, a review of important characteristics, previous studies and relevant developments of P(VDF-TrFE)-based terfluoropolymers with either chlorofluoroethylene (CFE) or chlorotrifluoroethylene (CTFE) as the termonomer is deemed useful. The role of the termonomer and of its composition, as well as the effects of the processing conditions on the semi-crystalline structure which in turn affects the final RF properties are discussed in detail. In addition, the presence of noteworthy transition(s) in the mid-temperature range and the influence of preparation conditions on those transitions are reviewed. A better understanding of the fundamental aspects affecting the semi-crystalline structures will help to elucidate the nature of RF activity in VDF-based terpolymers and also help to further improve their applications-relevant electroactive properties.},
  language  = {en}
}
@article{HeWangHeetal.2022,
  author    = {He, Yushuang and Wang, Feipeng and He, Li and Wang, Qiang and Li, Jian and Qian, Yihua and Gerhard, Reimund and Plath, Ronald},
  title     = {An insight Into the role of Nano-Alumina on DC Flashover-Resistance and surface charge variation of Epoxy Nanocomposites},
  series = {IEEE transactions on dielectrics and electrical insulation},
  volume    = {29},
  journal   = {IEEE transactions on dielectrics and electrical insulation},
  number    = {3},
  publisher = {Inst. of Electr. and Electronics Engineers},
  address   = {Piscataway},
  issn      = {1070-9878},
  doi       = {10.1109/TDEI.2022.3173510},
  pages     = {1022 -- 1029},
  year      = {2022},
  abstract  = {The addition of nano-Al2O3 has been shown to enhance the breakdown voltage of epoxy resin, but its flashover results appeared with disputation. This work concentrates on the surface charge variation and dc flashover performance of epoxy resin with nano-Al2O3 doping. The dispersion of nano-Al2O3 in epoxy is characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The dc flashover voltages of samples under either positive or negative polarity are measured with a finger-electrode system, and the surface charge variations before and after flashovers were identified from the surface potential mapping. The results evidence that nano-Al2O3 would lead to a 16.9\% voltage drop for the negative flashovers and a 6.8\% drop for positive cases. It is found that one-time flashover clears most of the accumulated surface charges, regardless of positive or negative. As a result, the ground electrode is neighbored by an equipotential zone enclosed with low-density heterocharges. The equipotential zone tends to be broadened after 20 flashovers. The nano-Al2O3 is noticed as beneficial to downsize the equipotential zone due to its capability on charge migration, which is reasonable to maintain flashover voltage at a high level after multiple flashovers. Hence, nano-Al2O3 plays a significant role in improving epoxy with high resistance to multiple flashovers.},
  language  = {en}
}
@article{WangZhangYanetal.2022,
  author    = {Wang, Feipeng and Zhang, Zheng and Yan, Yuyang and Shen, Zijia and Wang, Qiang and Gerhard, Reimund},
  title     = {Surface reconstruction on electro-spun PVA/PVP nanofibers by water evaporation},
  series = {Nanomaterials},
  volume    = {12},
  journal   = {Nanomaterials},
  number    = {5},
  publisher = {MDPI},
  address   = {Basel},
  issn      = {2079-4991},
  doi       = {10.3390/nano12050797},
  pages     = {7},
  year      = {2022},
  abstract  = {Tailoring the secondary surface morphology of electro-spun nanofibers has been highly desired, as such delicate structures equip nanofibers with distinct functions. Here, we report a simple strategy to directly reconstruct the surface of polyvinyl alcohol/polyvinylpyrrolidone (PVA/PVP) nanofibers by water evaporation. The roughness and diameter of the nanofibers depend on the temperature during vacuum drying. Surface changes of the nanofibers from smooth to rough were observed at 55 degrees C, with a significant drop in nanofiber diameter. We attribute the formation of the secondary surface morphology to the intermolecular forces in the water vapor, including capillary and the compression forces, on the basis of the results from the Fourier-transform infrared (FTIR) and X-ray photoelectron (XPS) spectroscopy. The strategy is universally effective for various electro-spun polymer nanofibers, thus opening up avenues toward more detailed and sophisticated structure design and implementation for nanofibers.},
  language  = {en}
}
@article{RamanVenkatesanSmykallaPlossetal.2021,
  author    = {Raman Venkatesan, Thulasinath and Smykalla, David and Ploss, Bernd and W{\"u}bbenhorst, Michael and Gerhard, Reimund},
  title     = {Non-linear dielectric spectroscopy for detecting and evaluating structure-property relations in a P(VDF-TrFE-CFE) relaxor-ferroelectric terpolymer},
  series = {Applied physics : A, Materials science \& processing},
  volume    = {127},
  journal   = {Applied physics : A, Materials science \& processing},
  number    = {10},
  publisher = {Springer},
  address   = {Berlin ; Heidelberg ; New York},
  issn      = {0947-8396},
  doi       = {10.1007/s00339-021-04876-0},
  pages     = {10},
  year      = {2021},
  abstract  = {Non-linear dielectric spectroscopy (NLDS) is employed as an effective tool to study relaxation processes and phase transitions of a poly(vinylidenefluoride-trifluoroethylene-chlorofluoroethylene) (P(VDF-TrFE-CFE)) relaxor-ferroelectric (R-F) terpolymer in detail. Measurements of the non-linear dielectric permittivity epsilon 2 ' reveal peaks at 30 and 80 degrees C that cannot be identified in conventional dielectric spectroscopy. By combining the results from NLDS experiments with those from other techniques such as thermally stimulated depolarization and dielectric-hysteresis studies, it is possible to explain the processes behind the additional peaks. The former peak, which is associated with the mid-temperature transition, is found in all other vinylidene fluoride-based polymers and may help to understand the non-zero epsilon 2 ' values that are detected on the paraelectric phase of the terpolymer. The latter peak can also be observed during cooling of P(VDF-TrFE) copolymer samples at 100 degrees C and is due to conduction and space-charge polarization as a result of the accumulation of real charges at the electrode-sample interface.},
  language  = {en}
}
@article{Gerhard2021,
  author    = {Gerhard, Reimund},
  title     = {50 years of International Symposia on Electrets from 1967 to 2017},
  series = {IEEE electrical insulation magazine / Institute of Electrical and Electronics Engineers},
  volume    = {37},
  journal   = {IEEE electrical insulation magazine / Institute of Electrical and Electronics Engineers},
  number    = {2},
  publisher = {IEEE},
  address   = {New York, NY},
  issn      = {0883-7554},
  doi       = {10.1109/MEI.2021.9352710},
  pages     = {50 -- 55},
  year      = {2021},
  abstract  = {The prehistory of electrets is not known yet, but it is quite likely that the electrostatic charging behavior of amber (Greek: τ{\`o} ηλεκτρoν, i.e., "electron") already was familiar to people in ancient cultures (China, Egypt, Greece, etc.), before the Greek philosopher and scientist Thales of Miletus (6th century BCE)-or rather his disciples and followers-reported it in writing (cf. Figure 1). More than two millennia later, William Gilbert (1544-1603), the physician of Queen Elizabeth I, coined the term "electric" in his book De Magnete, Magneticisque Corporibus, et de Magno Magnete Tellure (1600) for dielectric materials that attract like amber and that included sulfur and glass [1]. The second half of the 18th century saw the invention of the electrophorus or electrophore [2], a capacitive electret device, in 1762 by Johan Carl Wilcke (1732-1796).},
  language  = {en}
}
@article{WangRychkovGerhard2021,
  author    = {Wang, Jingwen and Rychkov, Dmitry and Gerhard, Reimund},
  title     = {Space-charge electret properties of polypropylene films with transcrystalline or spherulitic structures},
  series = {Journal of applied physics : AIP's archival journal for significant new results in applied physics / publ. by the American Institute of Physics},
  volume    = {129},
  journal   = {Journal of applied physics : AIP's archival journal for significant new results in applied physics / publ. by the American Institute of Physics},
  number    = {6},
  publisher = {American Institute of Physics},
  address   = {Melville},
  issn      = {0021-8979},
  doi       = {10.1063/5.0039867},
  pages     = {7},
  year      = {2021},
  abstract  = {Spherulite-related space-charge electret properties of polypropylene (PP) have been widely discussed in the past decades. In the present paper, a less-common crystalline structure in PP-transcrystalline PP-is studied regarding its electret behavior in comparison with the typical spherulitic morphology. Polarized light microscopy and differential scanning calorimetry were employed to characterize the crystallite types and crystallinities of transcrystalline and spherulitic PP. Their electret functionality is investigated by means of thermally stimulated discharge experiments, where the cross-over phenomenon is observed on transcrystalline PP films, whereas surface-potential saturation and undercharging on the surface occur on the spherulitic samples. Besides, an asymmetrical behavior of positive and negative surface-charge stabilities is found on PP with spherulites, the negatively charged spherulitic surfaces show a better charge stability. It is shown that PP electrets are very sensitive to changes in the microscopic crystalline structures and their interfaces as well as in the molecular conformations controlled through adjustments of the respective processing steps. In addition, surface and bulk nanocomposites of PP or low-density polyethylene with inorganic particles are included in the comparison. In view of recent developments in the areas of PP-based electret-fiber filters and cellular-foam ferroelectrets, the observed changes in the charge-storage properties may have particular relevance, as the required film, fiber, or foam processing might significantly modify crystalline morphologies and nano-scale interfaces in PP electrets. Limitations in the charge-storage capabilities of interface structures may also be of interest in the context of high-voltage electrical-insulation materials where reduced space-charge accumulation and slightly increased charge transport can be advantageous.},
  language  = {en}
}
@article{WangDanielsConnellyetal.2021,
  author    = {Wang, Ningzhen and Daniels, Robert and Connelly, Liam and Sotzing, Michael and Wu, Chao and Gerhard, Reimund and Sotzing, Gregory A. and Cao, Yang},
  title     = {All-organic flexible ferroelectret nanogenerator with fabric-based electrodes for self-powered body area networks},
  series = {Small : nano micro},
  volume    = {17},
  journal   = {Small : nano micro},
  number    = {33},
  publisher = {Wiley-VCH},
  address   = {Weinheim},
  issn      = {1613-6810},
  doi       = {10.1002/smll.202103161},
  pages     = {11},
  year      = {2021},
  abstract  = {Due to their electrically polarized air-filled internal pores, optimized ferroelectrets exhibit a remarkable piezoelectric response, making them suitable for energy harvesting. Expanded polytetrafluoroethylene (ePTFE) ferroelectret films are laminated with two fluorinated-ethylene-propylene (FEP) copolymer films and internally polarized by corona discharge. Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)-coated spandex fabric is employed for the electrodes to assemble an all-organic ferroelectret nanogenerator (FENG). The outer electret-plus-electrode double layers form active device layers with deformable electric dipoles that strongly contribute to the overall piezoelectric response in the proposed concept of wearable nanogenerators. Thus, the FENG with spandex electrodes generates a short-circuit current which is twice as high as that with aluminum electrodes. The stacking sequence spandex/FEP/ePTFE/FEP/ePTFE/FEP/spandex with an average pore size of 3 mu m in the ePTFE films yields the best overall performance, which is also demonstrated by the displacement-versus-electric-field loop results. The all-organic FENGs are stable up to 90 degrees C and still perform well 9 months after being polarized. An optimized FENG makes three light emitting diodes (LEDs) blink twice with the energy generated during a single footstep. The new all-organic FENG can thus continuously power wearable electronic devices and is easily integrated, for example, with clothing, other textiles, or shoe insoles.},
  language  = {en}
}
@article{StubningDenesGerhard2021,
  author    = {Stubning, Tobias and Denes, Istvan and Gerhard, Reimund},
  title     = {Tuning electro-mechanical properties of EAP-based haptic actuators by adjusting layer thickness and number of stacked layers},
  series = {Engineering research express},
  volume    = {3},
  journal   = {Engineering research express},
  number    = {1},
  publisher = {Institute of Physics},
  address   = {London},
  issn      = {2631-8695},
  doi       = {10.1088/2631-8695/abd286},
  pages     = {13},
  year      = {2021},
  abstract  = {In our fast-changing world, human-machine interfaces (HMIs) are of ever-increasing importance. Among the most ubiquitous examples are touchscreens that most people are familiar with from their smartphones. The quality of such an HMI can be improved by adding haptic feedback-an imitation of using mechanical buttons-to the touchscreen. Thin-film actuators on the basis of electro-mechanically active polymers (EAPs), with the electroactive material sandwiched between two compliant electrodes, offer a promising technology for haptic surfaces. In thin-film technology, the thickness and the number of stacked layers of the electroactive dielectric are key parameters for tuning a system. Therefore, we have experimentally investigated the influence of the thickness of a single EAP layer on the electrical and the electro-mechanical performance of the transducer. In order to achieve high electro-mechanical actuator outputs, we have employed relaxor-ferroelectric ter-fluoropolymers that can be screen-printed. By means of a model-based approach, we have also directly compared single- and multi-layer actuators, thus providing guidelines for optimized transducer configurations with respect to the system requirements of haptic applications for which the operation frequency is of particular importance.},
  language  = {en}
}
@article{RamanVenkatesanGulyakovaGerhard2020,
  author    = {Raman Venkatesan, Thulasinath and Gulyakova, Anna A. and Gerhard, Reimund},
  title     = {Influence of film stretching on crystalline phases and dielectric properties of a 70/30 mol\% poly(vinylidenefluoride-tetrafluoroethylene) copolymer},
  series = {Journal of advanced dielectrics},
  volume    = {10},
  journal   = {Journal of advanced dielectrics},
  number    = {5},
  publisher = {World Scientific},
  address   = {Singapore},
  issn      = {2010-135X},
  doi       = {10.1142/S2010135X2050023X},
  pages     = {10},
  year      = {2020},
  abstract  = {Polyvinylidene fluoride (PVDF)-based copolymers with tetrafluoroethylene (P(VDF-TFE)), trifluoroethylene (P(VDF-TrFE)) or hexafluoropropylene (P(VDF-HFP)) are of strong interest due to the underlying fundamental mechanisms and the potential ferro-, pyro- and piezo-electrical applications. Their flexibility and their adaptability to various shapes are advantageous in comparison to inorganic ferroelectrics. Here, we study the influence of stretching temperature on the crystalline phases and the dielectric properties in P(VDF-TFE) films by means of Dielectric Relaxation Spectroscopy (DRS), Fourier-Transform InfraRed spectroscopy (FTIR), Wide-Angle X-ray Diffraction (WAXD), Differential Scanning Calorimetry (DSC) and Dynamic Mechanical Analysis (DMA). Especially, the effect of stretching and the influence of the temperature of stretching on the mid-temperature (T-mid) transition are studied in detail. The results show that stretching has a similar effect as that on PVDF, and we observe an increase in the fraction of ferroelectric beta-phase with a simultaneous increment in both melting point (T-m) and crystallinity (chi(c)) of the copolymer. While an increase in the stretching temperature does not have a profound impact on the amount of ferroelectric phase, the stability of the ferroelectric phase seems to improve - as seen in the reduction of the Full Width at Half Maximum (FWHM) of the WAXD peaks in both parallel and perpendicular directions to the molecular chain axis. The observation is also supported by the reduction of dissipation losses with an increase in stretching temperature - as seen in DRS measurements. Finally, both stretching itself and the temperature of stretching affect the various molecular processes taking place in the temperature range of the T-mid transition.},
  language  = {en}
}
@article{SpelzhausenIonianGerhardetal.2020,
  author    = {Spelzhausen, Simon and Ionian, Mario-Rafael and Gerhard, Reimund and Plath, Ronald},
  title     = {Time-resolved measurement of space-charge evolution in dielectric films or slabs by means of repeatable laser-induced pressure pulses},
  series = {Review of scientific instruments : a monthly journal devoted to scientific instruments, apparatus, and techniques},
  volume    = {91},
  journal   = {Review of scientific instruments : a monthly journal devoted to scientific instruments, apparatus, and techniques},
  number    = {5},
  publisher = {American Institute of Physics},
  address   = {Melville},
  issn      = {0034-6748},
  doi       = {10.1063/1.5142443},
  pages     = {7},
  year      = {2020},
  abstract  = {A new variant of the Laser-Induced Pressure-Pulse (LIPP) method for repeatable, time-resolved space-charge profile measurements is proposed and demonstrated. Automated deposition of a fresh laser-target film before each illumination leads to good repeatability of the LIPP and thus allows for the detection of time-resolved changes in the space-charge distribution over many hours. We describe and discuss the experimental setup and its features, compare the repeatability of the LIPP measurements on the same sample without and with re-preparation of the test cell, and present the time-resolved evolution of the space-charge profile in a two-layer arrangement of a silicone-grease and a silicone-elastomer film as an example. Finally, the temperature dependence of the space-charge evolution during polarization under high voltage and during depolarization in short circuit is shown. Possible uses and future developments of the new LIPP approach are also discussed.},
  language  = {en}
}
@article{RamanVenkatesanGerhard2020,
  author    = {Raman Venkatesan, Thulasinath and Gerhard, Reimund},
  title     = {Origin of the mid-temperature transition in vinylidenefluoride-based ferro-, pyro- and piezoelectric homo-, co- and ter-polymers},
  series = {Materials Research Express},
  volume    = {7},
  journal   = {Materials Research Express},
  publisher = {IOP Publ.},
  address   = {Bristol},
  issn      = {2053-1591},
  doi       = {10.1088/2053-1591/ab842c},
  pages     = {8},
  year      = {2020},
  abstract  = {The existence of an intermediate transition between the glass and the Curie/melting temperatures in Poly(vinylidene fluoride) (PVDF) and some of its co- and ter-polymers has been reported by several authors. In spite (or because?) of various different explanations in the literature, the origins of the transition are still not clear. Here, we try to understand the extra transition in more detail and study it with thermal and dielectric methods on PVDF, on its co-polymers with trifluoroethylene (P(VDF-TrFE)) and tetrafluoroethylene (P(VDF-TFE)), and on its ter-polymer with trifluoroethylene and chlorofluoroethylene (P(VDF-TrFE-CFE). Based on interpretations from the literature and our experimental studies, we propose the new hypothesis that the intermediate transition should have several interrelated origins. Especially since the relevant range is not far above room temperature, better understanding and control of their properties may also have practical implications for the use of the respective polymer materials in devices.},
  language  = {en}
}
@article{QiuBenjaminRamanVenkatesanetal.2020,
  author    = {Qiu, Xunlin and Benjamin, Aravindan Joseph and Raman Venkatesan, Thulasinath and Schmidt, Georg C. and Soler, Ricardo Alonso Quintana and Panicker, Pramul Muraleedhara and Gerhard, Reimund and H{\"u}bler, Arved Carl},
  title     = {Dielectric and electroacoustic assessment of screen-printed piezoelectric polymer layers as flexible transducers},
  series = {IEEE transactions on dielectrics and electrical insulation},
  volume    = {27},
  journal   = {IEEE transactions on dielectrics and electrical insulation},
  number    = {5},
  publisher = {Institute of Electrical and Electronics Engineers},
  address   = {New York, NY},
  issn      = {1070-9878},
  doi       = {10.1109/TDEI.2020.008864},
  pages     = {1683 -- 1690},
  year      = {2020},
  abstract  = {Here, piezoelectric transducers consisting of a P(VDF-TrFE) layer with either silver or PEDOT:PSS screen-printed electrodes are studied. The influence of electrodes on the dielectric and electroacoustic properties are studied in dielectric-spectroscopy and ferroelectric-hysteresis measurements. Only when both the bottom and the top electrodes are made of silver, the typical dielectric relaxation of the P(VDF-TrFE) layer is clearly observed. When one or two of the electrodes are of PEDOT:PSS, a Debye-like relaxation is present. Compared with silver electrodes, PEDOT:PSS electrodes allow for moderate self-healing. Consequently, samples with bottom and top PEDOT:PSS electrodes can be poled to saturation, while samples with silver electrodes can hardly be poled to saturation due to destructive electric breakdown. Acoustic transducer measurements show that silver electrodes facilitate higher and broader frequency operation, while PEDOT:PSS electrodes bring slightly lower total harmonic distortion. Overall, the acoustic performance shows no significant deviations between differently electroded samples so that silver electrodes do not offer any advantages for the transducers studied here due to their much higher tendency for destructive electric breakdown.},
  language  = {en}
}
@article{WangRychkovNguyenetal.2020,
  author    = {Wang, Jingwen and Rychkov, Dmitry and Nguyen, Quyet Doan and Gerhard, Reimund},
  title     = {The influence of orthophosphoric-acid surface modification on charge-storage enhancement in polypropylene electrets},
  series = {Journal of applied physics},
  volume    = {128},
  journal   = {Journal of applied physics},
  number    = {3},
  publisher = {American Institute of Physics},
  address   = {Melville},
  issn      = {0021-8979},
  doi       = {10.1063/5.0013805},
  pages     = {6},
  year      = {2020},
  abstract  = {Bipolar electrets from polypropylene (PP) are essential, e.g., in electret air filters and in cellular-foam ferroelectrets. Therefore, the mechanism of surface-charge stability enhancement on PP electrets via orthophosphoric-acid surface treatment is investigated in detail. It is shown that the significant charge-stability enhancement can be mainly attributed to deeper surface traps originating from deposited chemicals and topographic features on the modified surfaces. Thermally stimulated discharge of chemically treated and non-treated PP films with different surface-charge densities is used to test the limits of the newly formed deep traps in terms of the capacity for hosting surface charges. When the initial surface-charge density is very high, more charges are forced into shallower original traps on the surface or in the bulk of the treated PP samples, reducing the effect of the deeper surface traps brought by the surface modification. The well-known crossover phenomenon (of the surface-charge decay curves) has been observed between modified PP electrets charged to +/- 2kV and to +/- 3kV. Acoustically probed charge distributions in the thickness direction of PP electrets at different stages of thermal discharging indicate that the deep surface trapping sites may have preference for negative charges, resulting in the observed asymmetric charge stability of the modified PP films.},
  language  = {en}
}
@article{WangRychkovNguyenetal.2020,
  author    = {Wang, Jingwen and Rychkov, Dmitry and Nguyen, Quyet Doan and Gerhard, Reimund},
  title     = {Unexpected bipolar space-charge polarization across transcrystalline interfaces in polypropylene electret films},
  series = {Journal of applied physics},
  volume    = {128},
  journal   = {Journal of applied physics},
  number    = {13},
  publisher = {American Institute of Physics, AIP},
  address   = {Melville, NY},
  issn      = {0021-8979},
  doi       = {10.1063/5.0022071},
  pages     = {7},
  year      = {2020},
  abstract  = {A double-layer transcrystalline polypropylene (PP) film with a flat central interface layer between its two transcrystalline layers is obtained by recrystallization from the melt between two polytetrafluoroethylene (PTFE) surfaces on both sides of the PP film. Its electret properties are studied and compared with those of a single-layer transcrystalline PP film re-crystallized in contact with only one PTFE surface. Within experimental uncertainty, the two types of transcrystalline films exhibit the same thermal properties and crystallinities. After thermal poling, however, two hetero-charge layers of opposite polarity are found on the internal interfaces of the double-layer transcrystalline films and may together be considered as micrometer-sized dipoles. The unexpected phenomenon does not occur in single-layer transcrystalline samples without a central interface layer, suggesting that the interfaces between the transcrystalline layers and the micrometer-thick central interface layer may be the origin of deeper traps rather than the crystalline structures in the transcrystallites or the spherulites. The origin of the interfacial charges was also studied by means of an injection-blocking charging method, which revealed that intrinsic charge carriers introduced during recrystallization are most likely responsible for the interfacial charges. It is fascinating that a material as familiar as PP can exhibit such intriguing properties with a special bipolar space-charge polarization across the central interface layer after quasi-epitaxial surface moulding into a double-layer transcrystalline form. In addition to applications in electret (micro-)devices for electro-mechanical transduction, the highly ordered structures may also be employed as a new paradigm for studying charge storage and transport in polymer electrets and in dielectrics for DC electrical insulation.},
  language  = {en}
}
@article{AssagraAltafimdoCarmoetal.2020,
  author    = {Assagra, Yuri A.O. and Altafim, Ruy Alberto Pisani and do Carmo, Joao P. and Altafim, Ruy A.C. and Rychkov, Dmitry and Wirges, Werner and Gerhard, Reimund},
  title     = {A new route to piezo-polymer transducers: 3D printing of polypropylene ferroelectrets},
  series = {IEEE transactions on dielectrics and electrical insulation},
  volume    = {27},
  journal   = {IEEE transactions on dielectrics and electrical insulation},
  number    = {5},
  publisher = {Inst. of Electr. and Electronics Engineers},
  address   = {Piscataway},
  issn      = {1070-9878},
  doi       = {10.1109/TDEI.2020.008461},
  pages     = {1668 -- 1674},
  year      = {2020},
  abstract  = {Here, a promising approach for producing piezo-polymer transducers in a one-step process is presented. Using 3D-printing technology and polypropylene (PP) filaments, we are able to print a two-layered film structure with regular cavities of precisely controlled size and shape. It is found that the 3D-printed samples exhibit piezoelectric coefficients up to 200 pC/N, similar to those of other PP ferroelectrets, and their temporal and thermal behavior is in good agreement with those known of PP ferroelectrets. The piezoelectric response strongly decreases for applied pressures above 20 kPa, as the pressure in the air-filled cavities strongly influences the overall elastic modulus of ferroelectrets.},
  language  = {en}
}
@misc{RamanVenkatesanGerhard2020,
  author    = {Raman Venkatesan, Thulasinath and Gerhard, Reimund},
  title     = {Origin of the mid-temperature transition in vinylidenefluoride-based ferro-, pyro- and piezoelectric homo-, co- and ter-polymers},
  series = {Postprints der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe},
  journal   = {Postprints der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe},
  number    = {977},
  issn      = {1866-8372},
  doi       = {10.25932/publishup-47467},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-474672},
  pages     = {9},
  year      = {2020},
  abstract  = {The existence of an intermediate transition between the glass and the Curie/melting temperatures in Poly(vinylidene fluoride) (PVDF) and some of its co- and ter-polymers has been reported by several authors. In spite (or because?) of various different explanations in the literature, the origins of the transition are still not clear. Here, we try to understand the extra transition in more detail and study it with thermal and dielectric methods on PVDF, on its co-polymers with trifluoroethylene (P(VDF-TrFE)) and tetrafluoroethylene (P(VDF-TFE)), and on its ter-polymer with trifluoroethylene and chlorofluoroethylene (P(VDF-TrFE-CFE). Based on interpretations from the literature and our experimental studies, we propose the new hypothesis that the intermediate transition should have several interrelated origins. Especially since the relevant range is not far above room temperature, better understanding and control of their properties may also have practical implications for the use of the respective polymer materials in devices.},
  language  = {en}
}
@misc{NguyenWangRychkovetal.2019,
  author    = {Nguyen, Quyet Doan and Wang, Jingwen and Rychkov, Dmitry and Gerhard, Reimund},
  title     = {Depth Profile and Transport of Positive and Negative Charge in Surface (2-D) and Bulk (3-D) Nanocomposite Films},
  series = {2nd International Conference on Electrical Materials and Power Equipment (ICEMPE 2019)},
  journal   = {2nd International Conference on Electrical Materials and Power Equipment (ICEMPE 2019)},
  publisher = {IEEE},
  address   = {New York},
  isbn      = {978-1-5386-8434-4},
  doi       = {10.1109/ICEMPE.2019.8727256},
  pages     = {298 -- 300},
  year      = {2019},
  abstract  = {In the present study, the charge distribution and the charge transport across the thickness of 2- and 3-dimensional polymer nanodielectrics was investigated. Chemically surface-treated polypropylene (PP) films and low-density polyethylene nanocomposite films with 3 wt \% of magnesium oxide (LDPE/MgO) served as examples of 2-D and 3-D nanodielectrics, respectively. Surface charges were deposited onto the non-metallized surfaces of the one-side metallized polymer films and found to broaden and to thus enter the bulk of the films upon thermal stimulation at suitable elevated temperatures. The resulting space-charge profiles in the thickness direction were probed by means of Piezoelectrically-generated Pressure Steps (PPSs). It was observed that the chemical surface treatment of PP which led to the formation of nano-structures or the use of bulk nanoparticles from LDPE/MgO nanocomposites enhance charge trapping on or in the respective polymer films and also reduce charge transport inside the respective samples.},
  language  = {en}
}
@article{RamanVenkatesanGulyakovaFruebingetal.2019,
  author    = {Raman Venkatesan, Thulasinath and Gulyakova, Anna A. and Fr{\"u}bing, Peter and Gerhard, Reimund},
  title     = {Electrical polarization phenomena, dielectric relaxations and structural transitions in a relaxor-ferroelectric terpolymer investigated with electrical probing techniques},
  series = {Materials research express},
  volume    = {6},
  journal   = {Materials research express},
  number    = {12},
  publisher = {IOP Publ. Ltd.},
  address   = {Bristol},
  issn      = {2053-1591},
  doi       = {10.1088/2053-1591/ab5352},
  pages     = {7},
  year      = {2019},
  abstract  = {Dielectric Relaxation Spectroscopy (DRS) and Thermally Stimulated Depolarization Current (TSDC) measurements were employed to study dielectric-relaxation processes, structural transitions and electric-polarization phenomena in poly(vinylidenefluoride-trifluoroethylene-chlorofluoroethylene) (P(VDF-TrFE-CFE)) terpolymer films. Results from DRS confirm the existence of two separate dispersion regions related to a para-to-ferroelectric phase transition and to the glass transition. The dipolar TSDC peak correlates with the loss peak of the ? relaxation that represents the glass transition. The electric polarization calculated from the dipolar TSDC peak (glass transition) shows a non-linear electric-field dependence and saturates at high electric poling fields. As the observed behaviour is essentially the same as that of the electric polarization obtained from direct polarization-versus-electric-field hysteresis measurements, TSDC experiments are also suitable for studying the polarization in relaxor-ferroelectric polymers. A saturation polarization of 44 mC m(?2) was found for an electric field of 190 MV m(?1).},
  language  = {en}
}
@misc{GerhardKaltenbrunner2019,
  author    = {Gerhard, Reimund and Kaltenbrunner, Martin},
  title     = {In Memoriam Siegfried Bauer},
  series = {IEEE electrical insulation magazine},
  volume    = {35},
  journal   = {IEEE electrical insulation magazine},
  number    = {2},
  publisher = {IEEE},
  address   = {Piscataway},
  issn      = {0883-7554},
  doi       = {10.1109/MEI.2019.8636175},
  pages     = {76 -- 78},
  year      = {2019},
  abstract  = {Siegfried Bauer, an internationally renowned, very creative applied physicist, who also was a prolific materials scientist and engineer, died on December 30, 2018, in Linz, Austria, after a one-year battle with cancer. He was full professor of soft-matter physics at the Johannes Kepler University Linz, Austria, and a scientific leader and innovator across the fields but mainly in the areas of electro-active materials (including electrets) and stretchable and imperceptible electronics.},
  language  = {en}
}
@misc{Gerhard2019,
  author    = {Gerhard, Reimund},
  title     = {Dielectric materials for electro-active (electret) and/or electro-passive (insulation) applications},
  series = {2nd International Conference on Electrical Materials and Power Equipment (ICEMPE 2019)},
  journal   = {2nd International Conference on Electrical Materials and Power Equipment (ICEMPE 2019)},
  publisher = {IEEE},
  address   = {New York},
  isbn      = {978-1-5386-8434-4},
  doi       = {10.1109/ICEMPE.2019.8727276},
  pages     = {91 -- 96},
  year      = {2019},
  abstract  = {Dielectric materials for electret applications usually have to contain a quasi-permanent space charge or dipole polarization that is stable over large temperature ranges and time periods. For electrical-insulation applications, on the other hand, a quasi-permanent space charge or dipole polarization is usually considered detrimental. In recent years, however, with the advent of high-voltage direct-current (HVDC) transmission and high-voltage capacitors for energy storage, new possibilities are being explored in the area of high-voltage dielectrics. Stable charge trapping (as e.g. found in nano-dielectrics) or large dipole polarizations (as e.g. found in relaxor ferroelectrics and high-permittivity dielectrics) are no longer considered to be necessarily detrimental in electrical-insulation materials. On the other hand, recent developments in electro-electrets (dielectric elastomers), i.e. very soft dielectrics with large actuation strains and high breakdown fields, and in ferroelectrets, i.e. polymers with electrically charged cavities, have resulted in new electret materials that may also be useful for HVDC insulation systems. Furthermore, 2-dimensional (nano-particles on surfaces or interfaces) and 3-dimensional (nano-particles in the bulk) nano-dielectrics have been found to provide very good charge-trapping properties that may not only be used for more stable electrets and ferroelectrets, but also for better HVDC electrical-insulation materials with the possibility to optimize charge-transport and field-gradient behavior. In view of these and other recent developments, a first attempt will be made to review a small selection of electro-active (i.e. electret) and electro-passive (i.e. insulation) dielectrics in direct comparison. Such a comparative approach may lead to synergies in materials concepts and research methods that will benefit both areas. Furthermore, electrets may be very useful for sensing and monitoring applications in electrical-insulation systems, while high-voltage technology is essential for more efficient charging and poling of electret materials.},
  language  = {en}
}
@misc{LouposDamigosTsertouetal.2019,
  author    = {Loupos, Konstantinos and Damigos, Yannis and Tsertou, Athanasisa and Amditis, Angelos and Lenas, Sotiris-Angelos and Chatziandreoglou, Chistos and Malliou, Christina and Tsaoussidis, Vassilis and Gerhard, Reimund and Rychkov, Dmitry and Wirges, Werner and Frankenstein, Bernd and Camarinopoulos, Stephanos and Kalidromitis, Vassilis and Sanna, C. and Maier, Stephanos and Gordt, A. and Panetsos, P.},
  title     = {Innovative soft-material sensor, wireless network and assessment software for bridge life-cycle assessment},
  series = {Life-cycle analysis and assessmanet in civil engineering : towards an integrated vision},
  journal   = {Life-cycle analysis and assessmanet in civil engineering : towards an integrated vision},
  publisher = {CRC Press, Taylor \& Francis Group},
  address   = {Boca Raton},
  isbn      = {978-1-315-22891-4},
  pages     = {2085 -- 2092},
  year      = {2019},
  abstract  = {Nowadays, structural health monitoring of critical infrastructures is considered as of primal importance especially for managing transport infrastructure however most current SHM methodologies are based on point-sensors that show various limitations relating to their spatial positioning capabilities, cost of development and measurement range. This publication describes the progress in the SENSKIN EC co-funded research project that is developing a dielectric-elastomer sensor, formed from a large highly extensible capacitance sensing membrane and is supported by an advanced micro-electronic circuitry, for monitoring transport infrastructure bridges. The sensor under development provides spatial measurements of strain in excess of 10\%, while the sensing system is being designed to be easy to install, require low power in operation concepts, require simple signal processing, and have the ability to self-monitor and report. An appropriate wireless sensor network is also being designed and developed supported by local gateways for the required data collection and exploitation. SENSKIN also develops a Decision-Support-System (DSS) for proactive condition-based structural interventions under normal operating conditions and reactive emergency intervention following an extreme event. The latter is supported by a life-cycle-costing (LCC) and life-cycle-assessment (LCA) module responsible for the total internal and external costs for the identified bridge rehabilitation, analysis of options, yielding figures for the assessment of the economic implications of the bridge rehabilitation work and the environmental impacts of the bridge rehabilitation options and of the associated secondary effects respectively. The overall monitoring system will be evaluated and benchmarked on actual bridges of Egnatia Highway (Greece) and Bosporus Bridge (Turkey).},
  language  = {en}
}
@article{LiChenQiuetal.2019,
  author    = {Li, Changsheng and Chen, Gangjin and Qiu, Xunlin and Gao, Meng and Gerhard, Reimund},
  title     = {Modified polytetrafluoroethylene},
  series = {Applied physics express : APEX},
  volume    = {13},
  journal   = {Applied physics express : APEX},
  number    = {1},
  publisher = {IOP Publ. Ltd.},
  address   = {Bristol},
  issn      = {1882-0778},
  doi       = {10.7567/1882-0786/ab5b23},
  pages     = {5},
  year      = {2019},
  abstract  = {Three poly(tetrafluoroethylene-hexafluoropropylene-vinylidenefluoride) (TFE-HFP-VDF or THV) terpolymers (Dyneon (R)) with different monomer ratios are investigated to demonstrate the concept of "modified" PTFE for space-charge electrets. HFP and VDF monomers distort the highly ordered PTFE molecules, which effectively enhances processability and adversely affects space-charge storage. Particularly, VDF component renders the material polar and probably also more conductive, partially undermining the space-charge-storage capabilities of PTFE. Nevertheless, the terpolymer THV815 with a TFE/HFP/VDF wt\% ratio of 76.1/10.9/13 combines easy processability and relatively good space-charge stability. Our results shed light on novel concepts for space-charge electret materials with enhanced processing properties and reasonable charge-storage capabilities.},
  language  = {en}
}
@article{RamanVenkatesanGulyakovaFruebingetal.2018,
  author    = {Raman Venkatesan, Thulasinath and Gulyakova, Anna A. and Fr{\"u}bing, Peter and Gerhard, Reimund},
  title     = {Relaxation processes and structural transitions in poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) relaxor-ferroelectric terpolymers as seen in dielectric spectroscopy},
  series = {IEEE transactions on dielectrics and electrical insulation},
  volume    = {25},
  journal   = {IEEE transactions on dielectrics and electrical insulation},
  number    = {6},
  publisher = {Institut of Electrical and Electronics Engineers},
  address   = {Piscataway},
  issn      = {1070-9878},
  doi       = {10.1109/TDEI.2018.007440},
  pages     = {2229 -- 2235},
  year      = {2018},
  abstract  = {Dielectric relaxation processes and structural transitions in Poly(vinylidenefluoride-trifluoroethylene-chlorofluoroethylene) (P(VDF-TrFE-CFE)) terpolymer films with two different monomer compositions were investigated in comparison with Poly(vinylidenefluoride-trifluoroethylene) (P(VDF-TrFE)) copolymer films as reference material. Differential Scanning Calorimetry was employed to assess annealing effects on phase transitions and crystalline structure, while relaxation processes were investigated by means of Dielectric Relaxation Spectroscopy, the results of which indicate the existence of two separate dispersion regions, denoted as processes A and B, respectively. Process A appears at a certain temperature independent of frequency, but is strongly influenced by the crystallisation temperature and the CFE content, while peak B shows typical features of a relaxation process and is less influenced by crystallisation temperature and CFE content. Furthermore, peak B is related to the glass transition which is more pronounced in the terpolymer than in P(VDF-TrFE). A closer analysis indicates that the addition of CFE and thermal annealing gradually shift the ferro-to-paraelectric transition in P(VDF-TrFE) to lower temperatures, while the phase transition is transformed more and more into a relaxation.},
  language  = {en}
}
@article{QiuGrothWirgesetal.2018,
  author    = {Qiu, Xunlin and Groth, Frederick and Wirges, Werner and Gerhard, Reimund},
  title     = {Cellular polypropylene foam films as DC voltage insulation and as piezoelectrets},
  series = {IEEE transactions on dielectrics and electrical insulation},
  volume    = {25},
  journal   = {IEEE transactions on dielectrics and electrical insulation},
  number    = {3},
  publisher = {Institut of Electr. and Electronics Engineers},
  address   = {Piscataway},
  issn      = {1070-9878},
  doi       = {10.1109/TDEI.2018.007192},
  pages     = {829 -- 834},
  year      = {2018},
  abstract  = {Polymer foams are in industrial use for several decades. More recently, non-polar polymer foams were found to be piezoelectric (so-called piezoelectrets) after internal electrical charging of the cavities. So far, few studies have been carried out on the electrical-insulation properties of polymer foams. Here, we compare the piezoelectric and the DC-voltage electrical-insulation properties of cellular polypropylene (PP) foams. Their cavity microstructure can be adjusted via inflation in high-pressure nitrogen gas in combination with a subsequent thermal treatment. While inflation is effective for improving the piezoelectricity, it is detrimental for the electrical-insulation properties. The original cellular PP foam shows a breakdown strength of approximately 230 MV/m, within the same range as that of solid PP. The breakdown strength decreases with increasing degree of inflation, and the dependence on the foam thickness follows an inverse power law with an exponent of 1.2. Nevertheless, up to a thickness of 140 mu m (3.5 times the original thickness), the breakdown strength of cellular-foam PP films is at least 7 times that of an air gap with the same thickness. In addition, the influence of high temperatures and high humidities on the piezoelectricity and the breakdown strength of cellular PP was studied. It was found that the piezoelectric d(33) coefficient decays rapidly already at 70 degrees C, while the breakdown strength slightly increases during storage at 70 or 90 degrees C. Under a relative humidity of 95\%, the breakdown strength increases with storage time, while the piezoelectric d(33) coefficient slightly decreases.},
  language  = {en}
}
@article{RychkovGerhardKuznetsovetal.2018,
  author    = {Rychkov, Dmitry and Gerhard, Reimund and Kuznetsov, Alexey and Rychkov, Andrey},
  title     = {Influence of charge density on the trap energy spectrum in fluoroethylenepropylene copolymer films with chemically modified surfaces},
  series = {IEEE transactions on dielectrics and electrical insulation},
  volume    = {25},
  journal   = {IEEE transactions on dielectrics and electrical insulation},
  number    = {3},
  publisher = {Inst. of Electr. and Electronics Engineers},
  address   = {Piscataway},
  issn      = {1070-9878},
  doi       = {10.1109/TDEI.2018.007437},
  pages     = {840 -- 844},
  year      = {2018},
  abstract  = {Tetrafluoroethylene-hexafluoropropylene copolymer (FEP) films were treated with titanium-tetrachloride vapor by means of molecular-layer deposition. The treatment leads to considerable improvements of the electret-charge stability on positively charged films. A slight improvement is also observed for negatively charged films. In line with our previous findings, we attribute the improvement in electret properties to the formation of deeper traps on the chemically modified polymer surfaces. Here, we investigate the influence of the charge density on electret stability of FEP films with modified surfaces. Trap-energy spectra obtained from thermally-stimulated-discharge measurements indicate that the charge stability on modified FEP films depends on how the surface traps are populated and on the availability of additional deeper traps.},
  language  = {en}
}
@misc{CheilakouTsopelasAnastasopoulosetal.2018,
  author    = {Cheilakou, E. and Tsopelas, N. and Anastasopoulos, A. and Kourousis, D. and Rychkov, Dmitry and Gerhard, Reimund and Frankenstein, B. and Amditis, A. and Damigos, Y. and Bouklas, C.},
  title     = {Strain monitoring system for steel and concrete structures},
  series = {Procedia Structural Integrity},
  volume    = {10},
  journal   = {Procedia Structural Integrity},
  publisher = {Elsevier},
  address   = {Amsterdam},
  issn      = {2452-3216},
  doi       = {10.1016/j.prostr.2018.09.005},
  pages     = {25 -- 32},
  year      = {2018},
  abstract  = {The present work is part of a collaborative H2020 European funded research project called SENSKIN, that aims to improve Structural Health Monitoring (SHM) for transport infrastructure through the development of an innovative monitoring and management system for bridges based on a novel, inexpensive, skin-like sensor. The integrated SENSKIN technology will be implemented in the case of steel and concrete bridges, and tested, field-evaluated and benchmarked on actual bridge environment against a conventional health monitoring solution developed by Mistras Group Hellas. The main objective of the present work is to implement the autonomous, fully functional strain monitoring system based on commercially available off-the-shelf components, that will be used to accomplish direct comparison between the performance of the innovative SENSKIN sensors and the conventional strain sensors commonly used for structural monitoring of bridges. For this purpose, the mini Structural Monitoring System (mini SMS) of Physical Acoustics Corporation, a comprehensive data acquisition unit designed specifically for long-term unattended operation in outdoor environments, was selected. For the completion of the conventional system, appropriate foil-type strain sensors were selected, driven by special conditioners manufactured by Mistras Group. A comprehensive description of the strain monitoring system and its peripheral components is provided in this paper. For the evaluation of the integrated system's performance and the effect of various parameters on the long-term behavior of sensors, several test steel pieces instrumented with different strain sensors configurations were prepared and tested in both laboratory and field ambient conditions. Furthermore, loading tests were performed aiming to validate the response of the system in monitoring the strains developed in steel beam elements subject to bending regimes. Representative results obtained from the above experimental tests have been included in this paper as well.},
  language  = {en}
}
@misc{RamanVenkatesanFruebingGerhard2018,
  author    = {Raman Venkatesan, Thulasinath and Fr{\"u}bing, Peter and Gerhard, Reimund},
  title     = {Influence of Composition and Preparation on Crystalline Phases and Morphology in Poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) Relaxor-Ferroelectric Terpolymer},
  series = {2018 IEEE 2nd International Conference on Dielectrics (ICD)},
  journal   = {2018 IEEE 2nd International Conference on Dielectrics (ICD)},
  publisher = {IEEE},
  address   = {New York},
  isbn      = {978-1-5386-6389-9},
  doi       = {10.1109/ICD.2018.8514758},
  pages     = {4},
  year      = {2018},
  abstract  = {The influence of chemical composition and crystallisation conditions on the ferroelectric and paraelectric phases and the resulting morphology in Poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) (P(VDF-TrFE-CFE)) terpolymer films with 55.4/37.2/7.3 mol\% or with 62.2/29.4/8.4 mol\% of VDF/TrFE/CFE was studied. Poly(vinylidene fluoride trifluoroethylene) (P(VDF-TrFE)) with 75/25 mol\% VDF/TrFE was employed as reference material. Fourier-Transform Infrared Spectroscopy (FTIR) was used to determine the fractions of the relevant terpolymer phases, and X-Ray Diffraction (XRD) was employed to assess the crystalline morphology. The FTIR results show an increase of the fraction of paraelectric phases after annealing. On the other hand, XRD results indicate a more stable paraelectric phase in the terpolymer with higher CFE content.},
  language  = {en}
}
@misc{WangRychkovGerhard2018,
  author    = {Wang, Jingwen and Rychkov, Dmitry and Gerhard, Reimund},
  title     = {Influence of Charge Density on Charge Decay in Chemically Modified Polypropylene Films},
  series = {2018 IEEE 2nd International Conference on Dielectrics (ICD)},
  journal   = {2018 IEEE 2nd International Conference on Dielectrics (ICD)},
  publisher = {IEEE},
  address   = {New York},
  isbn      = {978-1-5386-6389-9},
  doi       = {10.1109/ICD.2018.8514718},
  pages     = {4},
  year      = {2018},
  abstract  = {Previous work has shown that surface modification with orthophosphoric acid can significantly enhance the charge stability on polypropylene (PP) surface by generating deeper traps. In the present study, thermally stimulated potential-decay measurements revealed that the chemical treatment may also significantly increase the number of available trapping sites on the surface. Thus, as a consequence, the so-called "cross-over" phenomenon, which is observed on as-received and thermally treated PP electrets, may be overcome in a certain range of initial charge densities. Furthermore, the discharge behavior of chemically modified samples indicates that charges can be injected from the treated surface into the bulk, and/or charges of opposite polarity can be pulled from the rear electrode into the bulk at elevated temperatures and at the high electric fields that are caused by the deposited charges. In the bulk, a lack of deep traps causes rapid charge decay already in the temperature range around 95 degrees C.},
  language  = {en}
}
@misc{NguyenGerhard2018,
  author    = {Nguyen, Quyet Doan and Gerhard, Reimund},
  title     = {LDPE/MgO Nanocomposite Dielectrics for Electrical-Insulation and Ferroelectret-Transducer Applications},
  series = {2018 IEEE 2nd International Conference on Dielectrics (ICD)},
  journal   = {2018 IEEE 2nd International Conference on Dielectrics (ICD)},
  publisher = {IEEE},
  address   = {New York},
  isbn      = {978-1-5386-6389-9},
  pages     = {4},
  year      = {2018},
  abstract  = {Published results on LDPE/MgO nanocomposites (3wt\%) show that they promise to be good electrical-insulation materials. In this work, the nanocomposites are examined as a potential (ferro-)electret material as well. Isothermal surface-potential decay measurements show that charged LDPE/MgO films still exhibit significant surface potentials after heating for 4 hours at 80 degrees C, which suggests good capabilities of LDPE/MgO nanocomposites to hold electric charges of both polarities. Open-tubular-channel ferroelectrets prepared from LDPE/MgO nanocomposite films show significant piezoelectricity with d(33) coefficients of about 20 pC/N after charging and are stable up to temperatures of at least 80 degrees C. Thus LDPE/MgO nanocomposites may become available as a new ferroelectret material. To increase their d(33) coefficients, it is desirable to optimize the charging conditions and the ferroelectret structure.},
  language  = {en}
}
@article{GidionGerhard2018,
  author    = {Gidion, Gunnar and Gerhard, Reimund},
  title     = {The Bow on a String},
  series = {Acta Acustica united with Acustica},
  volume    = {104},
  journal   = {Acta Acustica united with Acustica},
  number    = {2},
  publisher = {Hirzel Verlag},
  address   = {Stuttgart},
  issn      = {1610-1928},
  doi       = {10.3813/AAA.919174},
  pages     = {315 -- 322},
  year      = {2018},
  abstract  = {The interaction between a bowed string instrument and its player is conveyed by the bow, the vibrational properties of which can be measured either separately on the bow or during the bowing procedure. Here, two piezoelectric film sensors, made of a ferroelectret material, are installed on a violin bow, one sensor at the tip and one at the frog. With these sensors, a violin is played under normal conditions, and the signals are analysed. The features in the resulting spectrograms are identified as string harmonics and longitudinal bow-hair resonances. The bow-hair sections on both sides of the bow-string contact exhibit separate resonances which are observed as absorption dips in the spectra. Owing to the sensor positions at the bow-hair terminations, it can be inferred that the two bow-hair sections act as mutual vibration absorbers. From a regression of the observed resonances, the longitudinal bow-hair velocity can be obtained. With additional film sensors under the violin bridge, body vibrations were also detected providing further details of the coupling mechanisms.},
  language  = {en}
}
@article{GulyakovaGorokhovatskyfruebingetal.2017,
  author    = {Gulyakova, Anna A. and Gorokhovatsky, Yuri A. and fr{\"u}bing, Peter and Gerhard, Reimund},
  title     = {Relaxation Processes Determining the Electret Stability of High-Impact Polystyrene/Titanium-Dioxide Composite Films},
  series = {IEEE transactions on dielectrics and electrical insulation},
  volume    = {24},
  journal   = {IEEE transactions on dielectrics and electrical insulation},
  publisher = {Inst. of Electr. and Electronics Engineers},
  address   = {Piscataway},
  issn      = {1070-9878},
  doi       = {10.1109/TDEI.2017.006587},
  pages     = {2541 -- 2548},
  year      = {2017},
  abstract  = {The influence of relaxation processes on the thermal electret stability of high-impact polystyrene (HIPS) free-standing films filled with titanium dioxide (TiO2) of the rutile modification are investigated by means of a combination of dielectric methods (dielectric relaxation spectroscopy (DRS), thermally stimulated depolarization current (TSDC) and thermally stimulated surface-potential decay (TSSPD)), supplemented by differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). Films with 2, 4, 6 and 8 vol.\% TiO2 are compared to each other and to non-filled samples. Filling HIPS with up to 8 vol.\% of TiO2 enhances the elastic modulus below the glass transition and increases the thermal electret stability above the glass transition without significantly increasing the DC conductivity. The improvement of the electret stability is caused by the build-up of an interface polarization which decays only gradually if the glass transition is exceeded. Two kinds of Arrhenius processes are considered in order to explain the decay of the composite-polymer electrets: (1) charge release from chemical traps located at the phenyl rings of the polymer chain with an activation energy of E-a = 1.1 eV after passing the glass transition at about 100 degrees C and (2) charge release from traps formed by the TiO2 particles with E-a = 2.4 eV at temperatures above 130 degrees C. Finally, the activation energies are discussed with respect to their significance.},
  language  = {en}
}
@article{WangRychkovGerhard2017,
  author    = {Wang, Jingwen and Rychkov, Dmitry and Gerhard, Reimund},
  title     = {Chemical modification with orthophosphoric acid enhances surface-charge stability on polypropylene electrets},
  series = {Applied physics letters},
  volume    = {110},
  journal   = {Applied physics letters},
  publisher = {American Institute of Physics},
  address   = {Melville},
  issn      = {0003-6951},
  doi       = {10.1063/1.4983348},
  pages     = {5},
  year      = {2017},
  abstract  = {The low surface-charge stability of polypropylene (PP) frequently limits its application as an electret material. In this paper, we demonstrate how the treatment of PP-film surfaces with orthophosphoric acid (H3PO4) enhances their charge stability. To discriminate between the effects of chemical modification and thermal treatment, as-received and annealed PP films are used as reference samples. The electret properties of treated and non-treated PP films are characterized with thermally stimulated discharge (TSD) and isothermal surface-potential decay (ISPD) experiments, from which considerable improvement in thermal and temporal charge stability is observed for samples modified with H3PO4. The half-value temperature (T-1/2) observed on TSD curves of chemically treated PP increases to 131 and 145 degrees C for positive and negative charges, respectively. The enhancement might be attributed to the phosphoric compounds detected on the H3PO4-modified surfaces via attenuated-total-reflection infrared spectroscopy. Deeper surface traps formed at the "foreign" phosphorus-containing structures are able to capture the charges over longer time periods and at higher temperatures, thus leading to significant improvements in the temporal and thermal surface-charge stabilities of PP electrets. Published by AIP Publishing.},
  language  = {en}
}
@article{FangMaLietal.2017,
  author    = {Fang, Peng and Ma, Xingchen and Li, Xiangxin and Qiu, Xunlin and Gerhard, Reimund and Zhang, Xiaoqing and Li, Guanglin},
  title     = {Fabrication, Structure Characterization, and Performance Testing of Piezoelectret-Film Sensors for Recording Body Motion},
  series = {IEEE Sensors Journal},
  volume    = {18},
  journal   = {IEEE Sensors Journal},
  number    = {1},
  publisher = {Inst. of Electr. and Electronics Engineers},
  address   = {Piscataway},
  issn      = {1530-437X},
  doi       = {10.1109/JSEN.2017.2766663},
  pages     = {401 -- 412},
  year      = {2017},
  abstract  = {During muscle contractions, radial-force distributions are generated on muscle surfaces due to muscle-volume changes, from which the corresponding body motions can be recorded by means of so-called force myography (FMG). Piezo- or ferroelectrets are flexible piezoelectric materials with attractive materials and sensing properties. In addition to several other applications, they are suitable for detecting force variations by means of wearable devices. In this paper, we prepared piezoelectrets from cellular polypropylene films by optimizing the fabrication procedures, and developed an FMG-recording system based on piezoelectret sensors. Different hand and wrist movements were successfully detected on able-bodied subjects with the FMG system. The FMG patterns were evaluated and identified by means of linear discriminant analysis and artificial neural network algorithms, and average motion-classification accuracies of 96.1\% and 94.8\%, respectively, were obtained. This paper demonstrates the feasibility of using piezoelectret-film sensors for FMG and may thus lead to alternative methods for detecting body motion and to related applications, e.g., in biomedical engineering or structural-health monitoring.},
  language  = {en}
}
@misc{LouposDamigosAmditisetal.2017,
  author    = {Loupos, Konstantinos and Damigos, Yannis and Amditis, Angelos and Gerhard, Reimund and Rychkov, Dmitry and Wirges, Werner and Schulze, Manuel and Lenas, Sotiris-Angelos and Chatziandreoglou, Christos and Malliou, Christina and Tsaoussidis, Vassilis and Brady, Ken and Frankenstein, Bernd},
  title     = {Structural health monitoring system for bridges based on skin-like sensor},
  series = {IOP conference series : Materials science and engineering},
  volume    = {236},
  journal   = {IOP conference series : Materials science and engineering},
  publisher = {IOP Publ. Ltd.},
  address   = {Bristol},
  issn      = {1757-8981},
  doi       = {10.1088/1757-899X/236/1/012100},
  pages     = {10},
  year      = {2017},
  abstract  = {Structural health monitoring activities are of primal importance for managing transport infrastructure, however most SHM methodologies are based on point-based sensors that have limitations in terms of their spatial positioning requirements, cost of development and measurement range. This paper describes the progress on the SENSKIN EC project whose objective is to develop a dielectric-elastomer and micro-electronics-based sensor, formed from a large highly extensible capacitance sensing membrane supported by advanced microelectronic circuitry, for monitoring transport infrastructure bridges. Such a sensor could provide spatial measurements of strain in excess of 10\%. The actual sensor along with the data acquisition module, the communication module and power electronics are all integrated into a compact unit, the SENSKIN device, which is energy-efficient, requires simple signal processing and it is easy to install over various surface types. In terms of communication, SENSKIN devices interact with each other to form the SENSKIN system; a fully distributed and autonomous wireless sensor network that is able to self-monitor. SENSKIN system utilizes Delay-/Disruption-Tolerant Networking technologies to ensure that the strain measurements will be received by the base station even under extreme conditions where normal communications are disrupted. This paper describes the architecture of the SENSKIN system and the development and testing of the first SENSKIN prototype sensor, the data acquisition system, and the communication system.},
  language  = {en}
}
@article{QiuWirgesGerhard2016,
  author    = {Qiu, Xunlin and Wirges, Werner and Gerhard, Reimund},
  title     = {Thermal poling of ferroelectrets: How does the gas temperature influence dielectric barrier discharges in cavities?},
  series = {Applied physics letters},
  volume    = {108},
  journal   = {Applied physics letters},
  publisher = {American Institute of Physics},
  address   = {Melville},
  issn      = {0003-6951},
  doi       = {10.1063/1.4954263},
  pages     = {1687 -- 1697},
  year      = {2016},
  abstract  = {The influence of the temperature in the gas-filled cavities on the charging process of ferroelectret film systems has been studied in hysteresis measurements. The threshold voltage and the effective polarization of the ferroelectrets were determined as functions of the charging temperature TP. With increasing TP, the threshold voltage for triggering dielectric barrier discharges in ferroelectrets decreases. Thus, increasing the temperature facilitates the charging of ferroelectrets. However, a lower threshold voltage reduces the attainable remanent polarization because back discharges occur at lower charge levels, as soon as the charging voltage is turned off. The results are discussed in view of Paschen's law for electrical breakdown, taking into account the respective gas temperature and a simplified model for ferroelectrets. Our results indicate that the thermal poling scheme widely used for conventional ferroelectrics is also useful for electrically charging ferroelectrets. Ferroelectrets (sometimes also called piezoelectrets) are relatively new members of the family of piezo-, pyro-, and ferroelectric materials.1-5 As their name indicates, ferroelectrets are space-charge electrets that show ferroic behavior. They are non-uniform electret materials or materials systems with electrically charged internal cavities. As space-charge electrets, ferroelectrets usually do not contain any molecular dipoles. However, the cavities inside the material can be turned into macroscopic dipoles through a series of micro-plasma discharges at high electric fields, so-called dielectric barrier discharges (DBDs).6-8 The gas inside the cavities is ionized when the internal electric field exceeds the threshold for electrical breakdown, generating charges of both polarities.9 The positive and negative charges travel in opposite directions, and are eventually trapped at the internal top and bottom surfaces of the cavities, respectively. After charging, the cavities may be regarded as macroscopic dipoles that can be switched by reversing the applied voltage. An electric-polarization-vs.-electric-field (P(E)) hysteresis is considered as an essential criterion for ferroelectricity. P(E)-hysteresis curves are usually characterized by the spontaneous polarization, the coercive field, and the remanent polarization. Recently, we have demonstrated P(E)-hysteresis loops on two different types of ferroelectrets, namely, cellular polypropylene ferroelectrets and tubular-channel fluoroethylene-polypropylene copolymer ferroelectrets.10,11 The P(E)-hysteresis loops not only prove the ferroic behavior of ferroelectrets, but also allow us to determine such parameters as the coercive field and the remanent polarization. It is widely accepted that Paschen breakdown is the underlying mechanism for the inception of DBDs in ferroelectrets.12-14 On this basis, the charging behavior and the resulting piezoelectricity of ferroelectrets in different gases at various pressures have been studied.15-17 Paschen's law describes the conditions for electrical breakdown in a gas at a constant temperature (usually room temperature), and it needs to be modified for gas breakdown at other temperatures. The temperature stability of the piezoelectricity in ferroelectrets after charging at elevated temperatures was investigated by several researchers.18-21 Recently, a preliminary report about the effects of the charging temperature on the hysteresis loops in ferroelectrets has been presented.22 In this letter, the influence of the gas temperature on the charging of ferroelectret systems is investigated in more detail by means of quasi-ferroelectric hysteresis-loop measurements. Teflon™ fluoroethylenepropylene (FEP) copolymer samples with tubular channels were prepared via thermal lamination as described previously.23 To this end, two FEP films with a thickness of 50 \&\#956;m each were laminated at 300 ° C around a 100 \&\#956;m thick polytetrafluoroethylene (PTFE) template (total area 35 mm × 45 mm) that contains parallel rectangular openings (area 1.5 mm × 40 mm each). After lamination, the template was removed, which results in an FEP film system with open tubular channels. The samples were metallized on both surfaces with aluminum electrodes of 20 mm diameter. P(E)-hysteresis loops were obtained with a modified Sawyer-Tower (ST) circuit.10,11 A high-voltage (HV) capacitor C1 (3 nF) and a large standard capacitor Cm (1 \&\#956;F) were connected in series with the sample. A bipolar sinusoidal voltage with a frequency of 10 mHz was applied from an HV power supply (FUG HCB 7-6500) controlled by an arbitrary-waveform generator (HP 33120a). The voltage Vout on Cm is measured by means of an electrometer (HP 3458a), and the charge flowing through the circuit is determined as Q(t)=CmVout(t) . The experiments were carried out at isothermal conditions in a Novocontrol® Quatro cryosystem. With the modified ST circuit, Q-V loops have been measured on a tubular-channel FEP ferroelectret system at different temperatures. The sample capacitance of about 34.5 pF is determined by a linear fit of the initial part of the Q-V curve recorded at 20 °C , where the voltage has been raised up from zero on a fresh sample. The hysteresis loops are obtained from the Q-V curves by subtracting the contribution that results from charging of the sample capacitance.10 Figure 1 shows the hysteresis loops of the sample at \&\#8722;100, 0, and +100 ° C, respectively. According to previous theoretical and experimental studies,24,25 the length of each of the horizontal sides of the parallelogram-like hysteresis loops is given by 2Vth where Vth is the threshold voltage. As the charging temperature decreases, the hysteresis loop becomes wider and less high, i.e., the threshold voltage increases, while the polarization at maximum voltage decreases.},
  language  = {en}
}
@article{MazurekYuGerhardetal.2016,
  author    = {Mazurek, P. and Yu, L. and Gerhard, Reimund and Wirges, Werner and Skov, A. L.},
  title     = {Glycerol as high-permittivity liquid filler in dielectric silicone elastomers},
  series = {Journal of applied polymer science},
  volume    = {133},
  journal   = {Journal of applied polymer science},
  publisher = {Wiley-Blackwell},
  address   = {Hoboken},
  issn      = {0021-8995},
  doi       = {10.1002/app.44153},
  pages     = {8},
  year      = {2016},
  abstract  = {A recently reported novel class of elastomers was tested with respect to its dielectric properties. The new elastomer material is based on a commercially available poly(dimethylsiloxane) composition, which has been modified by embedding glycerol droplets into its matrix. The approach has two major advantages that make the material useful in a dielectric actuator. First, the glycerol droplets efficiently enhance the dielectric constant, which can reach astonishingly high values in the composite. Second, the liquid filler also acts as a softener that effectively decreases the elastic modulus of the composite. In combination with very low cost and easy preparation, the two property enhancements lead to an extremely attractive dielectric elastomer material. Experimental permittivity data are compared to various theoretical models that predict relative permittivity changes as a function of filler loading, and the applicability of the models is discussed. (c) 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 44153.},
  language  = {en}
}
@article{HollaenderKossackKolloscheetal.2016,
  author    = {Holl{\"a}nder, Lars and Kossack, Wilhelm and Kollosche, Matthias and Wirges, Werner and Kremer, Friedrich and Gerhard, Reimund},
  title     = {Influence of the remanent polarisation on the liquid crystal alignment in composite films of ferroelectric poly(vinylidene fluoride-trifluoroethylene) and a cyanobiphenyl-based liquid crystal},
  series = {Liquid crystals : an international journal of science and technology},
  volume    = {43},
  journal   = {Liquid crystals : an international journal of science and technology},
  publisher = {Editions Rodopi BV},
  address   = {Abingdon},
  issn      = {0267-8292},
  doi       = {10.1080/02678292.2016.1185174},
  pages     = {1514 -- 1521},
  year      = {2016},
  abstract  = {Polymer-dispersed liquid crystals (PDLCs) of ferroelectric poly(vinylidene fluoride-trifluoroethylene) and nematic 4-cyano-4\&\#697;-n-hexylbiphenyl (6CB) or 4-cyano-4\&\#697;-n-pentylbiphenyl (5CB) were prepared to study the effect of the remanent polarisation of the polymer on the liquid crystal alignment. We measured the macroscopic alignment of the liquid crystal molecules in the thickness direction by means of Infrared Transition-Moment Orientational Analysis. Electrical poling at 100 V/µm caused an increased order parameter up to 0.15. After subsequent annealing above the nematic-to-isotropic phase-transition temperature, the order parameter was reduced to 0.02. Nevertheless, the order parameter was still higher than for non-poled film indicating a slight orientation in thickness direction. Both values are lower than those expected from model calculations. In agreement with dielectric measurements, we attribute this result to the shielding effect of mobile charge carriers within the liquid crystal inclusions.},
  language  = {en}
}
@article{SborikasQiuWirgesetal.2014,
  author    = {Sborikas, Martynas and Qiu, Xunlin and Wirges, Werner and Gerhard, Reimund and Jenninger, Werner and Lovera, Deliani},
  title     = {Screen printing for producing ferroelectret systems with polymer-electret films and well-defined cavities},
  series = {Applied physics : A, Materials science \& processing},
  volume    = {114},
  journal   = {Applied physics : A, Materials science \& processing},
  number    = {2},
  publisher = {Springer},
  address   = {New York},
  issn      = {0947-8396},
  doi       = {10.1007/s00339-013-7998-3},
  pages     = {515 -- 520},
  year      = {2014},
  abstract  = {We report a process for preparing polymer ferroelectrets by means of screen printing-a technology that is widely used for the two-dimensional patterning of printed layers. In order to produce polymer-film systems with cavities that are suitable for bipolar electric charging, a screen-printing paste is deposited through a screen with a pre-designed pattern onto the surface of a polymer electret film. Another such polymer film is placed on top of the printed pattern, and well-defined cavities are formed in-between. During heating and curing, the polymer films are tightly bonded to the patterned paste layer so that a stable three-layer system is obtained. In the present work, polycarbonate (PC) films have been employed as electret layers. Screen printing, curing and charging led to PC ferroelectret systems with a piezoelectric d (33) coefficient of about 28 pC/N that is stable up to 100 C-a similar to. Due to the rather soft patterned layer, d (33) strongly decreases already for static pressures of tens of kPa. The results demonstrate the suitability of screen printing for the preparation of ferroelectret systems.},
  language  = {en}
}
@article{QiuWirgesGerhard2014,
  author    = {Qiu, Xunlin and Wirges, Werner and Gerhard, Reimund},
  title     = {Polarization and Hysteresis in Tubular-Channel Fluoroethylenepropylene-Copolymer Ferroelectrets},
  series = {Ferroelectrics},
  volume    = {472},
  journal   = {Ferroelectrics},
  number    = {1},
  publisher = {Routledge, Taylor \& Francis Group},
  address   = {Abingdon},
  issn      = {0015-0193},
  doi       = {10.1080/00150193.2014.964603},
  pages     = {100 -- 109},
  year      = {2014},
  abstract  = {Polarization-vs.-applied-voltage hysteresis curves are recorded on tubular-channel fluoroethylene-propylene (FEP) copolymer ferroelectrets by means of a modified Sawyer-Tower circuit. Dielectric barrier discharges (DBDs) inside the cavities are triggered when the applied voltage is sufficiently high. During the DBDs, the cavities become man-made macroscopic dipoles which build up an effective polarization in the ferroelectret. Therefore, a phenomenological hysteresis curve is observed. From the hysteresis loop, the remanent polarization and the coercive field can be determined. Furthermore, the polarization can be related to the respective piezoelectric coefficient of the ferroelectret. The proposed method is easy to implement and is useful for characterization, further development and optimization of ferro- or piezoelectrets.},
  language  = {en}
}
@unpublished{HilczerGerhardScott2014,
  author    = {Hilczer, B{\"o}rn and Gerhard, Reimund and Scott, James F.},
  title     = {Special Issue of Ferroelectrics in Honor of S. B. Lang},
  series = {Ferroelectrics},
  volume    = {472},
  journal   = {Ferroelectrics},
  number    = {1},
  publisher = {Routledge, Taylor \& Francis Group},
  address   = {Abingdon},
  issn      = {0015-0193},
  doi       = {10.1080/00150193.2014.964099},
  pages     = {VII -- VIII},
  year      = {2014},
  language  = {en}
}
@unpublished{Gerhard2014,
  author    = {Gerhard, Reimund},
  title     = {Sidney Lang - his collaboration with the University of Potsdam},
  series = {Ferroelectrics},
  volume    = {472},
  journal   = {Ferroelectrics},
  number    = {1},
  publisher = {Routledge, Taylor \& Francis Group},
  address   = {Abingdon},
  issn      = {0015-0193},
  doi       = {10.1080/00150193.2014.967090},
  pages     = {5 -- 5},
  year      = {2014},
  language  = {en}
}
@article{BassoMonteiroMazulquimetal.2013,
  author    = {Basso, Heitor Cury and Monteiro, Jose Roberto B. de A. and Mazulquim, Daniel Baladelli and de Paula, Geyverson Teixeira and Goncalves Neto, Luiz and Gerhard, Reimund},
  title     = {Alternating current-generated plasma discharges for the controlled direct current charging of ferroelectrets},
  series = {Journal of applied physics},
  volume    = {114},
  journal   = {Journal of applied physics},
  number    = {10},
  publisher = {American Institute of Physics},
  address   = {Melville},
  issn      = {0021-8979},
  doi       = {10.1063/1.4821113},
  pages     = {6},
  year      = {2013},
  abstract  = {The standard charging process for polymer ferroelectrets, e. g., from polypropylene foams or layered film systems involves the application of high DC fields either to metal electrodes or via a corona discharge. In this often-used process, the DC field triggers the internal breakdown and limits the final charge densities inside the ferroelectret cavities and, thus, the final polarization. Here, an AC + DC charging procedure is proposed and demonstrated in which a high-voltage high-frequency (HV-HF) wave train is applied together with a DC poling voltage. Thus, the internal dielectric-barrier discharges in the ferroelectret cavities are induced by the HV-HF wave train, while the final charge and polarization level is controlled separately through the applied DC voltage. In the new process, the frequency and the amplitude of the HV-HF wave train must be kept within critical boundaries that are closely related to the characteristics of the respective ferroelectrets. The charging method has been tested and investigated on a fluoropolymer-film system with a single well-defined cylindrical cavity. It is found that the internal electrical polarization of the cavity can be easily controlled and increases linearly with the applied DC voltage up to the breakdown voltage of the cavity. In the standard charging method, however, the DC voltage would have to be chosen above the respective breakdown voltage. With the new method, control of the HV-HF wave-train duration prevents a plasma-induced deterioration of the polymer surfaces inside the cavities. It is observed that the frequency of the HV-HF wave train during ferroelectret charging and the temperature applied during poling of ferroelectrics serve an analogous purpose. The analogy and the similarities between the proposed ferroelectret charging method and the poling of ferroelectric materials or dipole electrets at elevated temperatures with subsequent cooling under field are discussed.},
  language  = {en}
}
@article{QiuHollaenderWirgesetal.2013,
  author    = {Qiu, Xunlin and Holl{\"a}nder, Lars and Wirges, Werner and Gerhard, Reimund and Basso, Heitor Cury},
  title     = {Direct hysteresis measurements on ferroelectret films by means of a modified Sawyer-Tower circuit},
  series = {Journal of applied physics},
  volume    = {113},
  journal   = {Journal of applied physics},
  number    = {22},
  publisher = {American Institute of Physics},
  address   = {Melville},
  issn      = {0021-8979},
  doi       = {10.1063/1.4809556},
  pages     = {8},
  year      = {2013},
  abstract  = {Ferro- and piezo-electrets are non-polar polymer foams or film systems with internally charged cavities. Since their invention more than two decades ago, ferroelectrets have become a welcome addition to the range of piezo-, pyro-, and ferro-electric materials available for device applications. A polarization-versus-electric-field hysteresis is an essential feature of a ferroelectric material and may also be used for determining some of its main properties. Here, a modified Sawyer-Tower circuit and a combination of unipolar and bipolar voltage waveforms are employed to record hysteresis curves on cellular-foam polypropylene ferroelectret films and on tubular-channel fluoroethylenepropylene copolymer ferroelectret film systems. Internal dielectric barrier discharges (DBDs) are required for depositing the internal charges in ferroelectrets. The true amount of charge transferred during the internal DBDs is obtained from voltage measurements on a standard capacitor connected in series with the sample, but with a much larger capacitance than the sample. Another standard capacitor with a much smaller capacitance-which is, however, still considerably larger than the sample capacitance-is also connected in series as a high-voltage divider protecting the electrometer against destructive breakdown. It is shown how the DBDs inside the polymer cavities lead to phenomenological hysteresis curves that cannot be distinguished from the hysteresis loops found on other ferroic materials. The physical mechanisms behind the hysteresis behavior are described and discussed.},
  language  = {en}
}
@article{RychkovRychkovEfimovetal.2013,
  author    = {Rychkov, Dmitry and Rychkov, A. and Efimov, N. and Malygin, A. and Gerhard, Reimund},
  title     = {Higher stabilities of positive and negative charge on tetrafluoroethylene-hexafluoropropylene copolymer (FEP) electrets treated with titanium-tetrachloride vapor},
  series = {Applied physics : A, Materials science \& processing},
  volume    = {112},
  journal   = {Applied physics : A, Materials science \& processing},
  number    = {2},
  publisher = {Springer},
  address   = {New York},
  issn      = {0947-8396},
  doi       = {10.1007/s00339-013-7821-1},
  pages     = {283 -- 287},
  year      = {2013},
  abstract  = {Tetrafluoroethylene-hexafluoropropylene copolymer (FEP) films were treated with titanium-tetrachloride vapor in a molecular-layer deposition process. As a result of the surface treatment, significant improvements of the thermal and temporal charge stability were observed. Charge-decay measurements revealed enhancements of the half-value temperatures and the relaxation times of positively charged FEP electrets by at least 120 A degrees C and two orders of magnitude, respectively. Beyond previous publications on fluoropolymer electrets with surface modification, we here report enhanced charge stabilities of the FEP films charged in negative as well as in positive corona discharges. Even though the improvement for negatively charged FEP films is moderate (half-value temperature about 20 A degrees C higher), our experiments show that the asymmetry in positive and negative charge stability that is typical for FEP electrets can be overcome by means of chemical surface treatments. The results are discussed in the context of the formation of modified surface layers with enhanced charge-trapping properties.},
  language  = {en}
}
@article{BassoQiuWirgesetal.2013,
  author    = {Basso, Heitor Cury and Qiu, Xunlin and Wirges, Werner and Gerhard, Reimund},
  title     = {Temporal evolution of the re-breakdown voltage in small gaps from nanoseconds to milliseconds},
  series = {Applied physics letters},
  volume    = {102},
  journal   = {Applied physics letters},
  number    = {1},
  publisher = {American Institute of Physics},
  address   = {Melville},
  issn      = {0003-6951},
  doi       = {10.1063/1.4773518},
  pages     = {5},
  year      = {2013},
  abstract  = {A detailed understanding of electric breakdown in dielectrics is of scientific and technological interest. In gaseous dielectrics, a so-called re-breakdown is sometimes observed after extinction of the previous discharge. Although time-dependent re-breakdown voltage is essentially known, its behavior immediately after the previous discharge is not precisely understood. We present an electronic circuit for accurate measurements of the time-dependent re-breakdown voltage in small gaps from tens of nanoseconds to several milliseconds after the previous spark. Results from such experiments are compared with earlier findings, and relevant physical mechanisms such as heating of the gas, decay of the plasma, and ionization of excited atoms and molecules are discussed. It is confirmed that the thermal model is not valid at times below several microseconds.},
  language  = {en}
}
@article{WangLackXieetal.2012,
  author    = {Wang, Feipeng and Lack, Alexander and Xie, Zailai and Fr{\"u}bing, Peter and Taubert, Andreas and Gerhard, Reimund},
  title     = {Ionic-liquid-induced ferroelectric polarization in poly(vinylidene fluoride) thin films},
  series = {Applied physics letters},
  volume    = {100},
  journal   = {Applied physics letters},
  number    = {6},
  publisher = {American Institute of Physics},
  address   = {Melville},
  issn      = {0003-6951},
  doi       = {10.1063/1.3683526},
  pages     = {4},
  year      = {2012},
  abstract  = {Thin films of ferroelectric beta-phase poly(vinylidene fluoride) (PVDF) were spin-coated from a solution that contained small amounts of the ionic liquid (IL) 1-ethyl-3-methylimidazolium nitrate. A remanent polarization of 60 mC/m(2) and a quasi-static pyroelectric coefficient of 19 mu C/m(2)K at 30 degrees C were observed in the films. It is suggested that the IL promotes the formation of the beta phase through dipolar interactions between PVDF chain-molecules and the IL. The dipolar interactions are identified as Coulomb attraction between hydrogen atoms in PVDF chains and anions in IL. The strong crystallinity increase is probably caused by the same dipolar interaction as well.},
  language  = {en}
}
@article{KalbitzGerhardTaylor2012,
  author    = {Kalbitz, Rene and Gerhard, Reimund and Taylor, D. M.},
  title     = {Fixed negative interface charges compromise organic ferroelectric field-effect transistors},
  series = {Organic electronics : physics, materials and applications},
  volume    = {13},
  journal   = {Organic electronics : physics, materials and applications},
  number    = {5},
  publisher = {Elsevier},
  address   = {Amsterdam},
  issn      = {1566-1199},
  doi       = {10.1016/j.orgel.2012.01.034},
  pages     = {875 -- 884},
  year      = {2012},
  abstract  = {Capacitance-voltage (C-V) and current-voltage measurements have been undertaken on metal-ferroelectric-semiconductor capacitors and ferroelectric field-effect transistors (FeFETs) using the ferroelectric polymer poly(vinylidenefluoride-trifluoroethylene) as the gate insulator and poly(3-hexylthiophene) as the active semiconductor. C-V measurements, voltage-dependence of gate currents and FeFET transfer characteristics all confirm that ferroelectric polarization is stable and only reverses when positive/negative coercive fields are exceeded for the first time. The apparent instability observed following the application of depletion voltages arises from the development of a negative interfacial charge which more than compensates the ferroelectric-induced shift, resulting in a permanent shift in threshold voltage to positive values. Application of successive bipolar voltage sweeps to a diode-connected FeFET show that significant remanent polarization is only induced in an unpoled device when the coercive field is exceeded during the first application of accumulation voltages. This initial polarization and its growth during subsequent bipolar voltage sweeps is accompanied by the accumulation of the fixed interfacial negative charges which cause the positive turn on voltages seen in C-V and transfer characteristics. The origin of the negative charge is ascribed either to layers of irreversible ferroelectric domains at the insulator surface or to the drift to the insulator-semiconductor interface of F-ions produced electrolytically during the application of accumulation voltages.},
  language  = {en}
}
@article{WertheimerStGeorgesRobillardLerougeetal.2012,
  author    = {Wertheimer, Michael R. and St-Georges-Robillard, Amelie and Lerouge, Sophie and Mwale, Fackson and Elkin, Bentsian and Oehr, Christian and Wirges, Werner and Gerhard, Reimund},
  title     = {Amine-rich organic thin films for cell culture - possible electrostatic effects in cell-surface interactions},
  series = {Japanese journal of applied physics},
  volume    = {51},
  journal   = {Japanese journal of applied physics},
  number    = {11},
  publisher = {Japan Soc. of Applied Physics},
  address   = {Tokyo},
  issn      = {0021-4922},
  doi       = {10.1143/JJAP.51.11PJ04},
  pages     = {5},
  year      = {2012},
  abstract  = {In recent communications from these laboratories, we observed that amine-rich thin organic layers are very efficient surfaces for the adhesion of mammalian cells. We prepare such deposits by plasma polymerization at low pressure, atmospheric pressure, or by vacuum-ultraviolet photo-polymerization. More recently, we have also investigated a commercially available material, Parylene diX AM. In this article we first briefly introduce literature relating to electrostatic interactions between cells, proteins, and charged surfaces. We then present certain selected cell-response results that pertain to applications in orthopedic and cardiovascular medicine: we discuss the influence of surface properties on the observed behaviors of two particular cell lines, human U937 monocytes, and Chinese hamster ovary cells. Particular emphasis is placed on possible electrostatic attractive forces due to positively charged R-NH3+ groups and negatively charged proteins and cells, respectively. Experiments carried out with electrets, polymers with high positive or negative surface potentials are added for comparison.},
  language  = {en}
}
@article{AltafimRychkovWirgesetal.2012,
  author    = {Altafim, Ruy Alberto Pisani and Rychkov, Dmitry and Wirges, Werner and Gerhard, Reimund and Basso, Heitor Cury and Altafim, Ruy Alberto Pisani and Melzer, Martin},
  title     = {Laminated tubular-channel ferroelectret systems from low-density Polyethylene Films and from Fluoroethylene-propylene Copolymer Films - A comparison},
  series = {IEEE transactions on dielectrics and electrical insulation},
  volume    = {19},
  journal   = {IEEE transactions on dielectrics and electrical insulation},
  number    = {4},
  publisher = {Inst. of Electr. and Electronics Engineers},
  address   = {Piscataway},
  issn      = {1070-9878},
  pages     = {1116 -- 1123},
  year      = {2012},
  abstract  = {A template-based lamination technique for the manufacture of ferroelectrets from uniform electret films was recently reported. In the present work, this technique is used to prepare similar ferroelectret structures from low-density polyethylene (LDPE) films and from fluoro-ethylene-propylene (FEP) copolymer films. A comparative analysis of the pressure-, temperature-, and frequency-dependent piezoelectric properties has been performed on the two ferroelectret systems. It is observed that the FEP ferroelectrets exhibit better piezoelectric responses and are thermally more stable. The difference between the piezoelectric d(33) coefficients of the two ferroelectret systems is partially explained here by their different elastic moduli. The anti-resonance peaks of both structures have been investigated by means of dielectric resonance spectroscopy and electroacoustic sound-pressure measurements. A difference of more than 10 kHz is observed between the anti-resonance frequencies of the two ferroelectret systems.},
  language  = {en}
}
@article{AltafimAltafimQiuetal.2012,
  author    = {Altafim, Ruy Alberto Pisani and Altafim, Ruy Alberto Pisani and Qiu, Xunlin and Raabe, Sebastian and Wirges, Werner and Basso, Heitor Cury and Gerhard, Reimund},
  title     = {Fluoropolymer piezoelectrets with tubular channels resonance behavior controlled by channel geometry},
  series = {Applied physics : A, Materials science \& processing},
  volume    = {107},
  journal   = {Applied physics : A, Materials science \& processing},
  number    = {4},
  publisher = {Springer},
  address   = {New York},
  issn      = {0947-8396},
  doi       = {10.1007/s00339-012-6848-z},
  pages     = {965 -- 970},
  year      = {2012},
  abstract  = {Ferro- or piezoelectrets are dielectric materials with two elastically very different macroscopic phases and electrically charged interfaces between them. One of the newer piezoelectret variants is a system of two fluoroethylenepropylene (FEP) films that are first laminated around a polytetrafluoroethylene (PTFE) template. Then, by removing the PTFE template, a two-layer FEP structure with open tubular channels is obtained. After electrical charging, the channels form easily deformable macroscopic electric dipoles whose changes under mechanical or electrical stress lead to significant direct or inverse piezoelectricity, respectively. Here, different PTFE templates are employed to generate channel geometries that vary in height or width. It is shown that the control of the channel geometry allows a direct adjustment of the resonance frequencies in the tubular-channel piezoelectrets. By combining several different channel widths in a single ferroelectret, it is possible to obtain multiple resonance peaks that may lead to a rather flat frequency-response region of the transducer material. A phenomenological relation between the resonance frequency and the geometrical parameters of a tubular channel is also presented. This relation may help to design piezoelectrets with a specific frequency response.},
  language  = {en}
}
@article{RychkovAltafimQiuetal.2012,
  author    = {Rychkov, Dmitry and Altafim, Ruy Alberto Pisani and Qiu, Xunlin and Gerhard, Reimund},
  title     = {Treatment with orthophosphoric acid enhances the thermal stability of the piezoelectricity in low-density polyethylene ferroelectrets},
  series = {Journal of applied physics},
  volume    = {111},
  journal   = {Journal of applied physics},
  number    = {12},
  publisher = {American Institute of Physics},
  address   = {Melville},
  issn      = {0021-8979},
  doi       = {10.1063/1.4729866},
  pages     = {5},
  year      = {2012},
  abstract  = {Ferroelectrets have been fabricated from low-density polyethylene (LDPE) films by means of a template-based lamination. The temperature dependence of the piezoelectric d(33) coefficient has been investigated. It was found that low-density polyethylene ferroelectrets have rather low thermal stability with the piezoelectric coefficient decaying almost to zero already at 100 degrees C. This behavior is attributed to the poor electret properties of the polyethylene films used for the fabrication of the ferroelectrets. In order to improve the charge trapping and the thermal stability of electret charge and piezoelectricity, LDPE ferroelectrets were treated with orthophosphoric acid. The treatment resulted in considerable improvements of the charge stability in LDPE films and in ferroelectret systems made from them. For example, the charge and piezoelectric-coefficient decay curves shifted to higher temperatures by 60 K and 40 K, respectively. It is shown that the decay of the piezoelectric coefficient in LDPE ferroelectrets is governed by the relaxation of less stable positive charges. The treatment also leads to noticeable changes in the chemical composition of the LDPE surface. Infrared spectroscopy reveals absorption bands attributed to phosphorus-containing structures, while scanning electron microscopy shows new island-like structures, 50-200 nm in diameter, on the modified surface.},
  language  = {en}
}
@article{RychkovGerhardIvanovetal.2012,
  author    = {Rychkov, Dmitry and Gerhard, Reimund and Ivanov, Vadim and Rychkov, Andrey},
  title     = {Enhanced electret charge stability on Polyethylene Films treated with Titanium-Tetrachloride vapor},
  series = {IEEE transactions on dielectrics and electrical insulation},
  volume    = {19},
  journal   = {IEEE transactions on dielectrics and electrical insulation},
  number    = {4},
  publisher = {Inst. of Electr. and Electronics Engineers},
  address   = {Piscataway},
  issn      = {1070-9878},
  pages     = {1305 -- 1311},
  year      = {2012},
  abstract  = {Low-density polyethylene (LDPE) films have been treated with titanium-tetrachloride vapor by means of the molecular-layer-deposition method. It is shown that such a treatment leads to a considerable improvement of the electret properties for both positively and negatively charged films. The temperature stability of the electret homo-charge has been increased by approximately 60 degrees C. At the same time, the temporal stability of charge is also considerably improved. Modified low-density polyethylene films show no "cross-over phenomenon" when charged to higher voltages. Thus, it is now possible to produce electrets from polyethylene films with high initial charge densities, but without a strongly reduced charge stability. The influence of a chemical treatment with titanium-tetrachloride vapor on charge injection from aluminum electrodes into polyethylene films was also investigated. It is found that the interface between an aluminum electrode and a modified LDPE surface layer has different injection properties for positive and negative charges. Electrons can be injected across the modified interface, whereas injection of holes is either very limited or non-existent.},
  language  = {en}
}
@article{FangHollaenderWirgesetal.2012,
  author    = {Fang, Peng and Hollaender, Lars and Wirges, Werner and Gerhard, Reimund},
  title     = {Piezoelectric d(33) coefficients in foamed and layered polymer piezoelectrets from dynamic mechano-electrical experiments, electro-mechanical resonance spectroscopy and acoustic-transducer measurements},
  series = {Measurement science and technology},
  volume    = {23},
  journal   = {Measurement science and technology},
  number    = {3},
  publisher = {IOP Publ. Ltd.},
  address   = {Bristol},
  issn      = {0957-0233},
  doi       = {10.1088/0957-0233/23/3/035604},
  pages     = {9},
  year      = {2012},
  abstract  = {Piezoelectrets are novel transducer materials which can be widely applied in sensors and actuators. Here, three techniques for determining piezoelectric d(33) coefficients of piezoelectrets are reviewed and compared. Two types of piezoelectrets, polyethylene-naphthalate (PEN) polymer-foam piezoelectrets and fluorinated ethylene-propylene (FEP) copolymer-layer piezoelectrets, have been prepared and measured by means of dynamic, resonance, and acoustical methods. The dynamic measurements show that the d(33) coefficient of PEN-foam samples clearly decreases with increasing stress, but 80\% of the initial d(33) can be retained after 1800 cycles of a continuous dynamic measurement in a mechanical fatigue test. The resonance measurements demonstrate that both PEN-foam and FEP-layer samples exhibit clear electro-mechanical resonances. PEN-foam samples show elastic moduli in the range from 1 to 12 MPa and d(33) values up to 500 pC N-1, while FEP-layer samples show homogeneous elastic moduli of about 0.3 MPa and d(33) values of about 280 pC N-1. The acoustical measurements reveal that both PEN-foam and FEP-layer samples exhibit stable frequency responses in the range from 5.7 to 20 kHz. In addition, d(33) coefficients obtained with different experimental methods are in good agreement with each other, which confirms the reliability of all three techniques.},
  language  = {en}
}
@article{FangWangWirgesetal.2011,
  author    = {Fang, Peng and Wang, Feipeng and Wirges, Werner and Gerhard, Reimund and Basso, Heitor Cury},
  title     = {Three-layer piezoelectrets from fluorinated ethylene-propylene (FEP) copolymer films},
  series = {Applied physics : A, Materials science \& processing},
  volume    = {103},
  journal   = {Applied physics : A, Materials science \& processing},
  number    = {2},
  publisher = {Springer},
  address   = {New York},
  issn      = {0947-8396},
  doi       = {10.1007/s00339-010-6008-2},
  pages     = {455 -- 461},
  year      = {2011},
  abstract  = {A process for preparing three-layer piezoelectrets from fluorinated ethylene-propylene (FEP) copolymer films is introduced. Samples are made from commercial FEP films by means of laser cutting, laser bonding, electrode evaporation, and high-field poling. The observed dielectric-resonance spectra demonstrate the piezoelectricity of the FEP sandwiches. Piezoelectric d (33) coefficients up to a few hundred pC/N are achieved. Charging at elevated temperatures can increase the thermal stability of the piezoelectrets. Isothermal experiments for approximately 15 min demonstrate that samples charged at 140A degrees C keep their piezoelectric activity up to at least 120A degrees C and retain 70\% of their initial d (33) even at 130A degrees C. Acoustical measurements show a relatively flat frequency response in the range between 300 Hz and 20 kHz.},
  language  = {en}
}
@article{SunZhangXiaetal.2011,
  author    = {Sun, Zhuanlan and Zhang, Xiaoqing and Xia, Zhongfu and Qiu, Xunlin and Wirges, Werner and Gerhard, Reimund and Zeng, Changchun and Zhang, Chuck and Wang, Ben},
  title     = {Polarization and piezoelectricity in polymer films with artificial void structure},
  series = {Applied physics : A, Materials science \& processing},
  volume    = {105},
  journal   = {Applied physics : A, Materials science \& processing},
  number    = {1},
  publisher = {Springer},
  address   = {New York},
  issn      = {0947-8396},
  doi       = {10.1007/s00339-011-6481-2},
  pages     = {197 -- 205},
  year      = {2011},
  abstract  = {Laminated polymer-film systems with well-defined void structures were prepared from fluoroethylenepropylene (FEP) and polytetrafluoroethylene (PTFE) layers. First the PTFE films were patterned and then fusion-bonded with the FEP films. The laminates were subjected to either corona or contact charging in order to obtain the desired piezoelectricity. The build-up of the "macro-dipoles" in the laminated films was studied by recording the electric hysteresis loops. The resulting electro-mechanical properties were investigated by means of dielectric resonance spectroscopy (DRS) and direct measurements of the stress-strain relationship. Moreover, the thermal stability of the piezoelectric d (33) coefficient was investigated at elevated temperatures and via thermally stimulated discharge (TSD) current measurements in short circuit. For 150 mu m thick laminated films, consisting of one 25 mu m thick PTFE layer, two 12.5 mu m thick FEP layers, and a void of 100 mu m height, the critical voltage necessary for the build-up of the "macro-dipoles" in the inner voids was approximately 1400 V, which agrees with the value calculated from the Paschen Law. A quasi-static piezoelectric d (33) coefficient up to 300 pC/N was observed after corona charging. The mechanical properties of the film systems are highly anisotropic. At room temperature, the Young's moduli of the laminated film system are around 0.37 MPa in the thickness direction and 274 MPa in the lateral direction, respectively. Using these values, the theoretical shape anisotropy ratio of the void was calculated, which agrees well with experimental observation. Compared with films that do not exhibit structural regularity, the laminates showed improved thermal stability of the d (33) coefficients. The thermal stability of d (33) can be further improved by pre-aging. E.g., the reduction of the d (33) value in the sample pre-aged at 150A degrees C for 5 h was less than 5\% after annealing for 30 h at a temperature of 90A degrees C.},
  language  = {en}
}
@article{KappelAbelGerhard2011,
  author    = {Kappel, Marcel and Abel, Markus and Gerhard, Reimund},
  title     = {Characterization and calibration of piezoelectric polymers in situ measurements of body vibrations},
  series = {Review of scientific instruments : a monthly journal devoted to scientific instruments, apparatus, and techniques},
  volume    = {82},
  journal   = {Review of scientific instruments : a monthly journal devoted to scientific instruments, apparatus, and techniques},
  number    = {7},
  publisher = {American Institute of Physics},
  address   = {Melville},
  issn      = {0034-6748},
  doi       = {10.1063/1.3607435},
  pages     = {6},
  year      = {2011},
  abstract  = {Piezoelectric polymers are known for their flexibility in applications, mainly due to their bending ability, robustness, and variable sensor geometry. It is an optimal material for minimal-invasive investigations in vibrational systems, e.g., for wood, where acoustical impedance matches particularly well. Many applications may be imagined, e. g., monitoring of buildings, vehicles, machinery, alarm systems, such that our investigations may have a large impact on technology. Longitudinal piezoelectricity converts mechanical vibrations normal to the polymer-film plane into an electrical signal, and the respective piezoelectric coefficient needs to be carefully determined in dependence on the relevant material parameters. In order to evaluate efficiency and durability for piezopolymers, we use polyvinylidene fluoride and measure the piezoelectric coefficient with respect to static pressure, amplitude of the dynamically applied force, and long-term stability. A known problem is the slow relaxation of the material towards equilibrium, if the external pressure changes; here, we demonstrate how to counter this problem with careful calibration. Since our focus is on acoustical measurements, we determine accurately the frequency response curve - for acoustics probably the most important characteristic. Eventually, we show that our piezopolymer transducers can be used as a calibrated acoustical sensors for body vibration measurements on a wooden musical instrument, where it is important to perform minimal-invasive measurements. A comparison with the simultaneously recorded airborne sound yields important insight of the mechanism of sound radiation in comparison with the sound propagating in the material. This is especially important for transient signals, where not only the long-living eigenmodes contribute to the sound radiation. Our analyses support that piezopolymer sensors can be employed as a general tool for the determination of the internal dynamics of vibrating systems.},
  language  = {en}
}
@article{KussmaulRisseKofodetal.2011,
  author    = {Kussmaul, Bjoern and Risse, Sebastian and Kofod, Guggi and Wache, Remi and Wegener, Michael and McCarthy, Denis N. and Kr{\"u}ger, Hartmut and Gerhard, Reimund},
  title     = {Enhancement of dielectric permittivity and electromechanical response in silicone elastomers molecular grafting of organic dipoles to the macromolecular Network},
  series = {Advanced functional materials},
  volume    = {21},
  journal   = {Advanced functional materials},
  number    = {23},
  publisher = {Wiley-VCH},
  address   = {Weinheim},
  issn      = {1616-301X},
  doi       = {10.1002/adfm.201100884},
  pages     = {4589 -- 4594},
  year      = {2011},
  abstract  = {A novel method is established for permittivity enhancement of a silicone matrix for dielectric elastomer actuators (DEAs) by molecular level modifications of the elastomer matrix. A push-pull dipole is synthesized to be compatible with the silicone crosslinking chemistry, allowing for direct grafting to the crosslinker molecules in a one-step film formation process. This method prevents agglomeration and yields elastomer films that are homogeneous down to the molecular level. The dipole-to-silicone network grafting reaction is studied by FTIR. The chemical, thermal, mechanical and electrical properties of films with dipole contents ranging from 0 wt\% to 13.4 wt\% were thoroughly characterized. The grafting of dipoles modifies the relative permittivity and the stiffness, resulting in the actuation strain at a given electrical field being improved by a factor of six.},
  language  = {en}
}
@article{QiuWirgesGerhard2011,
  author    = {Qiu, Xunlin and Wirges, Werner and Gerhard, Reimund},
  title     = {Beneficial and detrimental fatigue effects of dielectric barrier discharges on the piezoelectricity of polypropylene ferroelectrets},
  series = {Journal of applied physics},
  volume    = {110},
  journal   = {Journal of applied physics},
  number    = {2},
  publisher = {American Institute of Physics},
  address   = {Melville},
  issn      = {0021-8979},
  doi       = {10.1063/1.3610507},
  pages     = {8},
  year      = {2011},
  abstract  = {Cellular polypropylene (PP) ferroelectrets combine a large piezoelectricity with mechanical flexibility and elastic compliance. Their charging process represents a series of dielectric barrier discharges (DBDs) that generate a cold plasma with numerous active species and thus modify the inner polymer surfaces of the foam cells. Both the threshold for the onset of DBDs and the piezoelectricity of ferroelectrets are sensitive to repeated DBDs in the voids. It is found that the threshold voltage is approximately halved and the charging efficiency is clearly improved after only 10(3) DBD cycles. However, plasma modification of the inner surfaces from repeated DBDs deteriorates the chargeability of the voids, leading to a significant reduction of the piezoelectricity in ferroelectrets. After a significant waiting period, the chargeability of previously fatigued voids shows a partial recovery. The plasma modification is, however, detrimental to the stability of the deposited charges and thus also of the macroscopic dipoles and of the piezoelectricity. Fatigue from only 10(3) DBD cycles already results in significantly less stable piezoelectricity in cellular PP ferroelectrets. The fatigue rate as a function of the number of voltage cycles follows a stretched exponential. Fatigue from repeated DBDs can be avoided if most of the gas molecules inside the voids are removed via a suitable evacuation process.},
  language  = {en}
}
@article{KofodStoyanovGerhard2011,
  author    = {Kofod, Guggi and Stoyanov, Hristiyan and Gerhard, Reimund},
  title     = {Multilayer coaxial fiber dielectric elastomers for actuation and sensing},
  series = {Applied physics : A, Materials science \& processing},
  volume    = {102},
  journal   = {Applied physics : A, Materials science \& processing},
  number    = {3},
  publisher = {Springer},
  address   = {New York},
  issn      = {0947-8396},
  doi       = {10.1007/s00339-010-6066-5},
  pages     = {577 -- 581},
  year      = {2011},
  abstract  = {A simple dip-coating technique was employed to manufacture coaxial actuators with multiple layers of alternating dielectric and conducting layers. A thin rubber string was coated with an electrode-insulator-electrode structure, giving rise to a thin, fiber-like actuator with coaxial geometry. The process was repeated to achieve a compact multilayer actuator with up to three coaxial dielectric layers. Mechanical and electromechanical characterization of the actuators is presented, showing actuation strains up to 8\% and proper voltage-thickness scaling behavior. Also presented is a capacitance vs. extension plot, demonstrating that these structures can be used for compact and accurate capacitive strain sensing.},
  language  = {en}
}
@article{RychkovGerhard2011,
  author    = {Rychkov, Dmitry and Gerhard, Reimund},
  title     = {Stabilization of positive charge on polytetrafluoroethylene electret films treated with titanium-tetrachloride vapor},
  series = {Applied physics letters},
  volume    = {98},
  journal   = {Applied physics letters},
  number    = {12},
  publisher = {American Institute of Physics},
  address   = {Melville},
  issn      = {0003-6951},
  doi       = {10.1063/1.3565166},
  pages     = {3},
  year      = {2011},
  abstract  = {The surface of polytetrafluoroethylene films was treated with titanium-tetrachloride vapor. The treatment was carried out in a flow reactor by means of molecular-layer deposition, a method from the arsenal of chemical nanotechnology. X-ray photoelectron spectroscopy reveals that such a treatment results in considerable changes in the chemical composition at and near the surface of the fluoropolymer film. Both, defluorination and oxidation of the surface were observed. At the same time, samples treated with titanium tetrachloride show a significant enhancement in the thermal stability of the positive homocharge. The thermally stimulated surface-potential-decay curves were observed to shift to higher temperatures by more than 100 degrees C.},
  language  = {en}
}
@article{QiuGerhardMellinger2011,
  author    = {Qiu, Xunlin and Gerhard, Reimund and Mellinger, Axel},
  title     = {Turning polymer foams or polymer-film systems into ferroelectrets dielectric barrier discharges in voids},
  series = {IEEE transactions on dielectrics and electrical insulation},
  volume    = {18},
  journal   = {IEEE transactions on dielectrics and electrical insulation},
  number    = {1},
  publisher = {Inst. of Electr. and Electronics Engineers},
  address   = {Piscataway},
  issn      = {1070-9878},
  doi       = {10.1109/TDEI.2011.5704490},
  pages     = {34 -- 42},
  year      = {2011},
  abstract  = {Polymer foams and void-containing polymer-film systems with internally charged voids combine large piezoelectricity with mechanical flexibility and elastic compliance. This new class of soft materials (often called ferro-or piezoelectrets) has attracted considerable attention from science and industry. It has been found that the voids can be internally charged by means of dielectric barrier discharges (DBDs) under high electric fields. The charged voids can be considered as man-made macroscopic dipoles. Depending on the ferroelectret structure and the pressure of the internal gas, the voids may be highly compressible. Consequently, very large dipole-moment changes can be induced by mechanical or electrical stresses, leading to large piezoelectricity. DBD charging of the voids is a critical process for rendering polymer foams piezoelectric. Thus a comprehensive exploration of DBD charging is essential for the understanding and the optimization of piezoelectricity in ferroelectrets. Recent studies show that DBDs in the voids are triggered when the internal electric field reaches a threshold value according to Townsend's model of Paschen breakdown. During the DBDs, charges of opposite polarity are generated and trapped at the top and bottom internal surfaces of the gas-filled voids, respectively. The deposited charges induce an electric field opposite to the externally applied one and thus extinguish the DBDs. Back discharges may eventually be triggered when the external voltage is reduced or turned off. In order to optimize the efficiency of DBD charging, the geometry (in particular the height) of the voids, the type of gas and its pressure inside the voids are essential factors to be considered and to be optimized. In addition, the influence of the plasma treatment on the internal void surfaces during the DBDs should be taken into consideration.},
  language  = {en}
}
@article{KalbitzFruebingGerhardetal.2011,
  author    = {Kalbitz, Rene and Fr{\"u}bing, Peter and Gerhard, Reimund and Taylor, D. M.},
  title     = {Stability of polarization in organic ferroelectric metal-insulator-semiconductor structures},
  series = {Applied physics letters},
  volume    = {98},
  journal   = {Applied physics letters},
  number    = {3},
  publisher = {American Institute of Physics},
  address   = {Melville},
  issn      = {0003-6951},
  doi       = {10.1063/1.3543632},
  pages     = {3},
  year      = {2011},
  abstract  = {Dielectric measurements have been carried out on all-organic metal-insulator-semiconductor structures with the ferroelectric polymer poly(vinylidenefluoride-trifluoroethylene) as the gate insulator. It is shown that the polarization states remain stable after poling with accumulation and depletion voltage. However, negative charge trapped at the semiconductor-insulator interface during the depletion cycle masks the negative shift in flatband voltage expected during the sweep to accumulation voltages.},
  language  = {en}
}
@misc{Gerhard2011,
  author    = {Gerhard, Reimund},
  title     = {Honoring Professor Gerhard M. Sessler on his 80th Birthday (15th of February 2011)},
  series = {IEEE transactions on dielectrics and electrical insulation},
  volume    = {18},
  journal   = {IEEE transactions on dielectrics and electrical insulation},
  number    = {1},
  publisher = {Inst. of Electr. and Electronics Engineers},
  address   = {Piscataway},
  issn      = {1070-9878},
  doi       = {10.1109/TDEI.2011.5704485},
  pages     = {1 -- 2},
  year      = {2011},
  language  = {en}
}
@article{WangFruebingWirgesetal.2010,
  author    = {Wang, Feipeng and Fr{\"u}bing, Peter and Wirges, Werner and Gerhard, Reimund and Wegener, Michael},
  title     = {Enhanced Polarization in Melt-quenched and Stretched Poly(vinylidene Fluoride-Hexafluoropropylene) Films},
  issn      = {1070-9878},
  doi       = {10.1109/TDEI.2010.5539679},
  year      = {2010},
  abstract  = {beta-phase poly(vinylidene fluoride-hexafluoropropylene) (P(VDF-HFP)) copolymer films were prepared by uniaxially stretching solution-cast or melt-quenched samples. Different preparation routes lead to different amounts of the crystalline alpha and beta phases in the films, as detected by means of Fourier-transform infrared spectroscopy and X-ray diffractometry. The beta phase is significantly enhanced in melt-quenched and stretched films in comparison to solution-cast and stretched films. This is particularly true for copolymer samples with higher HFP content. The beta- phase enhancement is also observed in ferroelectric-hysteresis experiments where a rather high polarization of 58 mC/ m(2) was found on melt-quenched and stretched samples after poling at electric fields of 140 MV/m. After poling at 160 MV/m, one of these samples exhibited a piezoelectric d(33) coefficient as high as 21 pC/N. An electric-field-induced partial transition from the alpha to the beta phase was also observed on the melt-quenched and stretched samples. This effect leads to a further increase in the applications-relevant dipole polarization. Uniaxially stretched ferroelectric- polymer films are highly anisotropic. Dielectric resonance spectroscopy reveals a strong increase of the transverse piezoelectric d(32) coefficient and a strong decrease of the transverse elastic modulus c(32) upon heating from 20 to 50 degrees C.},
  language  = {en}
}
@article{ChinagliaGregorioStefanelloetal.2010,
  author    = {Chinaglia, Dante Luis and Gregorio, Rinaldo and Stefanello, Josiani Cristina and Altafim, Ruy Alberto Pisani and Wirges, Werner and Wang, Feipeng and Gerhard, Reimund},
  title     = {Influence of the solvent evaporation rate on the crystalline phases of solution-cast poly(vinylidene fluoride) films},
  issn      = {0021-8995},
  doi       = {10.1002/App.31488},
  year      = {2010},
  abstract  = {The influence of the solvent-evaporation rate on the formation of of. and P crystalline phases in solution-cast poly(vinylidene fluoride) (PVDF) films was systematically investigated. Films were crystallized from PVDF/N,N- dimethylformamide solutions with concentrations of 2.5, 5.0, 10, and 20 wt \% at different temperatures. During crystallization, the solvent evaporation rate was monitored in situ by means of a semianalytic balance. With this system, it was possible to determine the evaporation rate for different concentrations and temperatures of the solution under specific ambient conditions (pressure, temperature, and humidity). Fourier-Transform InfraRed spectroscopy with Attenuated Total Reflectance revealed the P-phase content in the PVDF films and its dependence on previous evaporation rates. Based on the relation between the evaporation rate and the PVDF phase composition, a consistent explanation for the different amounts of P phase observed at the upper and lower sample surfaces is achieved. Furthermore, the role of the sample thickness has also been studied. The experimental results show that not only the temperature but also the evaporation rate have to be controlled to obtain the desired crystalline phases in solution-cast PVDF films.},
  language  = {en}
}
@article{FloresSuarezGanesanWirgesetal.2010,
  author    = {Flores Su{\´a}rez, Rosaura and Ganesan, Lakshmi Meena and Wirges, Werner and Gerhard, Reimund and Mellinger, Axel},
  title     = {Imaging liquid crystals dispersed in a ferroelectric polymer matrix by means of thermal-pulse tomography},
  issn      = {1070-9878},
  doi       = {10.1109/TDEI.2010.5539683},
  year      = {2010},
  abstract  = {A new arrangement of the optical elements in a Thermal-Pulse-Tomography (TPT) setup allows to scan micrometer structures in composite and heterogeneous samples such as polymer-dispersed liquid crystals (PDLCs). The non-destructive TPT technique allows the determination of three-dimensional profiles of polarization and space charge in dielectrics. The samples under study were 12 mu m thick films of a copolymer of vinylidene fluoride with trifluoroethylene P(VDF- TrFE) (65/35) with embedded liquid-crystal droplets. The poling process was performed in direct contact well above the coercive field of the copolymer. The 3D map obtained from scanning with a 10 mu m wide spot shows elliptically shaped areas with liquid-crystal droplets. Considering the droplets as oblate spheroids, their major axis lies in the x-y plane, while their minor axis in the z direction measures 0.5 mu m or more. This result is in good agreement with scanning electron micrographs. It is believed that the major axis is overestimated due to imaging of liquid-crystal clusters.},
  language  = {en}
}
@article{FangQiuWirgesetal.2010,
  author    = {Fang, Peng and Qiu, Xunlin and Wirges, Werner and Gerhard, Reimund and Zirkel, Larissa},
  title     = {Polyethylene-naphthalate (PEN) ferroelectrets : cellular structure, piezoelectricity and thermal stability},
  issn      = {1070-9878},
  doi       = {10.1109/TDEI.2010.5539678},
  year      = {2010},
  abstract  = {Cellular polyethylene-naphthalate (PEN) ferroelectrets are useful as soft and flexible electromechanical transducer materials. Improved cellular PEN foams are prepared by means of a "voiding + inflation + stretching" process and investigated with respect to their structure and their applications-relevant properties. It is found that most of the cellular voids have heights below 8 mu m. The polymer walls do not allow sufficient gas exchange between the voids and the ambient atmosphere, when the cellular films are exposed to atmospheric pressures between a millibar and a few bars. As expected for ferroelectrets, a threshold voltage for charging is observed: A reasonable piezoelectric coefficient d(33) is only found when the charging voltage is higher than 4 kV. Furthermore, d(33) increases with charging voltage and reaches saturation at approximately 8 kV. Annealing after charging or charging at elevated temperatures may enhance the thermal stability of the PEN ferroelectrets. The d(33) of properly annealed samples is stable up to the respective annealing temperatures, but the annealing process reduces the piezoelectric activity of charged ferroelectret films to some extent. Samples charged at suitable elevated temperatures show much better thermal stability than those charged at room temperature, but the charging temperature should be limited to values below the material's glass-transition temperature T-g. Furthermore, the relevant elastic modulus c(33) of PEN ferroelectrets may decrease upon thermal treatment.},
  language  = {en}
}
@article{AltafimQiuWirgesetal.2009,
  author    = {Altafim, Ruy Alberto Pisani and Qiu, Xunlin and Wirges, Werner and Gerhard, Reimund and Altafim, Ruy Alberto Pisani and Basso, Heitor Cury and Jenninger, Werner and Wagner, Joachim},
  title     = {Template-based fluoroethylenepropylene piezoelectrets with tubular channels for transducer applications},
  issn      = {0021-8979},
  doi       = {10.1063/1.3159039},
  year      = {2009},
  abstract  = {We describe the concept, the fabrication, and the most relevant properties of a piezoelectric-polymer system: Two fluoroethylenepropylene (FEP) films with good electret properties are laminated around a specifically designed and prepared polytetrafluoroethylene (PTFE) template at 300 degrees C. After removing the PTFE template, a two-layer FEP film with open tubular channels is obtained. For electric charging, the two-layer FEP system is subjected to a high electric field. The resulting dielectric barrier discharges inside the tubular channels yield a ferroelectret with high piezoelectricity. d(33) coefficients of up to 160 pC/N have already been achieved on the ferroelectret films. After charging at suitable elevated temperatures, the piezoelectricity is stable at temperatures of at least 130 degrees C. Advantages of the transducer films include ease of fabrication at laboratory or industrial scales, a wide range of possible geometrical and processing parameters, straightforward control of the uniformity of the polymer system, flexibility, and versatility of the soft ferroelectrets, and a large potential for device applications e.g., in the areas of biomedicine, communications, production engineering, sensor systems, environmental monitoring, etc.},
  language  = {en}
}
@article{PhamPetreBerquezetal.2009,
  author    = {Pham, Cong Duc and Petre, Anca and Berquez, Laurent and Flores Su{\´a}rez, Rosaura and Mellinger, Axel and Wirges, Werner and Gerhard, Reimund},
  title     = {3D high-resolution mapping of polarization profiles in thin poly(vinylidenefluoride-trifluoroethylene) (PVDF- TrFE) films using two thermal techniques},
  issn      = {1070-9878},
  doi       = {10.1109/TDEI.2009.5128505},
  year      = {2009},
  abstract  = {In this paper, two non-destructive thermal methods are used in order to determine, with a high degree of accuracy, three-dimensional polarization distributions in thin films (12 mu m) of poly(vinylidenefluoride- trifluoroethylene) (PVDF-TrFE). The techniques are the frequency-domain Focused Laser Intensity Modulation Method (FLIMM) and time-domain Thermal-Pulse Tomography (TPT). Samples were first metalized with grid-shaped electrode and poled. 3D polarization mapping yielded profiles which reproduce the electrode-grid shape. The polarization is not uniform across the sample thickness. Significant polarization values are found only at depths beyond 0.5 mu m from the sample surface. Both methods provide similar results, TPT method being faster, whereas the FLIMM technique has a better lateral resolution.},
  language  = {en}
}
@article{RosenhahnFinlayPettitetal.2009,
  author    = {Rosenhahn, Axel and Finlay, John A. and Pettit, Michala E. and Ward, Andy and Wirges, Werner and Gerhard, Reimund and Callow, Maureen E. and Grunze, Michael and Callow, James A.},
  title     = {Zeta potential of motile spores of the green alga Ulva linza and the influence of electrostatic interactions on spore settlement and adhesion strength},
  issn      = {1559-4106},
  doi       = {10.1116/1.3110182},
  year      = {2009},
  abstract  = {The zeta potential of the motile spores of the green alga (seaweed) Ulva linza was quantified by video microscopy in combination with optical tweezers and determined to be -19.3{\~n}1.1 mV. The electrostatic component involved in the settlement and adhesion of spores was studied using electret surfaces consisting of PTFE and bearing different net charges. As the surface chemistry remains the same for differently charged surfaces, the experimental results isolate the influence of surface charge and thus electrostatic interactions. Ulva spores were demonstrated to have a reduced tendency to settle on negatively charged surfaces and when they did settle the adhesion strength of settled spores was lower than with neutral or positively charged surfaces. These observations can be ascribed to electrostatic interactions.},
  language  = {en}
}
@article{GanesanFruebingMellingeretal.2009,
  author    = {Ganesan, Lakshmi Meena and Fr{\"u}bing, Peter and Mellinger, Axel and Gerhard, Reimund},
  title     = {Dielectric relaxation behaviour of nematic liquid crystals dispersed in poly(vinylidene fluoride- trifluoroethylene)},
  issn      = {0022-3727},
  doi       = {10.1088/0022-3727/42/9/092006},
  year      = {2009},
  abstract  = {Polymer-dispersed liquid crystals (PDLCs) are prepared from poly(vinylidene fluoride-trifluoroethylene) (P(VDF- TrFE)) and a nematic liquid-crystal (LC). The anchoring effect was studied using dielectric relaxation spectroscopy. Two dispersion regions are observed in the dielectric spectra of the pure P(VDF-TrFE) film. They are related to the glass transition and to a space-charge relaxation. In PDLC films containing 10 wt\% of LC, an additional, bias field-dependent relaxation peak is found that can be attributed to the motion of LC molecules. Due to the hindered movement of the LC molecules, this relaxation process is considerably slowed down, compared with the related process in the pure LC.},
  language  = {en}
}
@article{QiuMellingerWirgesetal.2007,
  author    = {Qiu, Xunlin and Mellinger, Axel and Wirges, Werner and Gerhard, Reimund},
  title     = {Spectroscopic study of dielectric barrier discharges in cellular polypropylene ferroelectrets},
  doi       = {10.1063/1.2786597},
  year      = {2007},
  abstract  = {The transient light emission from the dielectric barrier discharges (DBDs) in cellular polypropylene ferroelectrets subjected to high electric poling fields was spectroscopically measured. The spectrum shows strong emission from the second positive system of molecular nitrogen, N-2(C (3)Pi(u))-> N-2(B (3)Pi(g)), and the first negative system of N-2(+), N-2(+)(B (2)Sigma(+)(u))-> N-2(+)(X (2)Sigma(+)(g)), consistent with a DBD in air. When a dc voltage is applied stepwise to the ferroelectret film, light emission starts above a threshold, coinciding with the threshold voltage in obtaining piezoelectricity. From selected vibronic band strength ratios, the electric field in the discharge was determined and found to agree with Townsend breakdown.},
  language  = {en}
}
@article{BassoAltafimAltafimetal.2007,
  author    = {Basso, Heitor Cury and Altafim, Ruy Alberto Pisani and Altafim, Ruy Alberto Pisani and Mellinger, Axel and Fang, Peng and Wirges, Werner and Gerhard, Reimund},
  title     = {Three-layer ferroelectrets from perforated Teflon-PTFE films fused between two homogeneous Teflon-FEP films},
  isbn      = {978-1-4244-1482-6},
  year      = {2007},
  language  = {en}
}
@article{WegenerWirgesPaajanenetal.2007,
  author    = {Wegener, Michael and Wirges, Werner and Paajanen, Mika and Gerhard, Reimund},
  title     = {Charging behaviour and thermal stability of porous and non-porous polytetrafluoroethylene (PTFE) electrets},
  isbn      = {978-1-4244-1482-6},
  year      = {2007},
  language  = {en}
}
@article{QiuMellingerWirgesetal.2007,
  author    = {Qiu, Xunlin and Mellinger, Axel and Wirges, Werner and Gerhard, Reimund},
  title     = {Dielectric barrier discharges during the generation of ferroelectrets : optical spectroscopy for process monitoring},
  isbn      = {978-1-4244-1482-6},
  year      = {2007},
  language  = {en}
}
@article{QiuMellingerWegeneretal.2007,
  author    = {Qiu, Xunlin and Mellinger, Axel and Wegener, Michael and Wirges, Werner and Gerhard, Reimund},
  title     = {Barrier discharges in cellular polypropylene ferroelectrets : how do they influence the electromechanical properties?},
  year      = {2007},
  language  = {en}
}
@article{MellingerFloresSuarezSinghetal.2007,
  author    = {Mellinger, Axel and Flores Su{\´a}rez, Rosaura and Singh, Rajeev and Wegener, Michael and Wirges, Werner and Gerhard, Reimund},
  title     = {Zerst{\"o}rungsfreie Tomographie von Raumladungs- und Polarisationsverteilungen mittles W{\"a}rmepulsen},
  issn      = {0171-8096},
  doi       = {10.1524/teme.2007.74.9.437},
  year      = {2007},
  abstract  = {Non-destructive, three-dimensional imaging of space-charge and polarization distributions in electret materials has been implemented by means of laser-induced thermal pulses. In pyroelectric films of poled poly(vinylidene fluoride), images of up to 45 x 45 pixels with a depth resolution of less than 0.5 mu m and a lateral resolution of 40 mu m were recorded, the latter being limited by fast thermal diffusion in the absorbing metallic front electrode. Initial applications include the analysis of polarization distributions in corona-poled piezoelectric sensor cables and the detection of patterned space-charge distributions in polytetrafluoroethylene films.},
  language  = {de}
}
@misc{Gerhard2007,
  author    = {Gerhard, Reimund},
  title     = {Katzir, S., The beginnings of piezoelectricity: a study in mundane physics; Dordrecht, Springer, 2006},
  year      = {2007},
  language  = {en}
}
@article{MontanariFabianiCianietal.2007,
  author    = {Montanari, Gian Carlo and Fabiani, Davide and Ciani, Franco and Motori, Antonio and Paajanen, Mika and Gerhard, Reimund and Wegener, Michael},
  title     = {Charging properties and time-temperature stability of innovative polymeric cellular ferroelectrets},
  issn      = {1070-9878},
  doi       = {10.1109/TDEI.2007.302892},
  year      = {2007},
  abstract  = {After appropriate mechanical and electrical treatments, some cellular polymers become able to retain space charge for a long time, i.e. they acquire electret behavior. The electrical treatment consists of charging under high levels of DC electric field. The mechanical treatment, based on the application of stretching forces to cellular polymer slabs that were before expanded under pressurized gas, affects the cavity size and shape, and therefore also the effectiveness of the charging process itself. An investigation of charging mechanisms, as well as of mechanical treatment, is therefore fundamental for optimizing the ferro- and piezo-electret properties. The aim of this paper is to discuss the effect of the physical dimension of the cavities on the charging behavior of cellular ferroelectrets and to focus on the time-temperature stability for two families of polymeric cellular ferroelectrets based on polypropylene (PP) and on a cyclo-olefin copolymer (COC). Emphasis will be given to the stretching process and in particular to the expansion rate applied during the manufacturing process (which affects the radial dimension and the height of the cavities, respectively). Space-charge and partial-discharge measurements as a function of time and temperature are the main tools to infer the influence of the cavity size on charging and stability characteristics.},
  language  = {en}
}
@article{LancaWirgesNeaguetal.2007,
  author    = {Lanca, M. C. and Wirges, Werner and Neagu, Eugen R. and Gerhard, Reimund and Marat-Mendes, Jos{\´e} Narciso},
  title     = {Influence of humidity on the electrical charging properties of cork agglomerates},
  issn      = {0022-3093},
  doi       = {10.1016/j.jnoncrysol.2007.03.037},
  year      = {2007},
  abstract  = {Cork is a natural cellular and electrically insulating material which may have the capacity to store electric charges on or in its cell walls. Since natural cork has many voids, it is difficult to obtain uniform samples with the required dimensions. Therefore, a more uniform material, namely commercial cork agglomerate, usually used for floor and wall coverings, is employed in the present study. Since we know from our previous work that the electrical properties of cork are drastically affected by absorbed and adsorbed water, samples were protected by means of different polymer coatings (applied by spin-coating or soaking). Corona charging and isothermal charging and discharging currents were used to study the electrical trapping and detrapping capabilities of the samples. A comparison of the results leads to the conclusion that the most promising method for storing electric charges in this cellular material consists of drying and coating or soaking with a hydrophobic, electrically insulating polymer such as polytetraflouroethylene (Teflon (R)). (c) 2007 Elsevier B.V. All rights reserved.},
  language  = {en}
}
@article{FangWegenerWirgesetal.2007,
  author    = {Fang, Peng and Wegener, Michael and Wirges, Werner and Gerhard, Reimund and Zirkel, Larissa},
  title     = {Cellular polyethylene-naphthalate ferroelectrets : foaming in supercritical carbon dioxide, structural and electrical preparation, and resulting piezoelectricity},
  issn      = {0003-6951},
  doi       = {10.1063/1.2738365},
  year      = {2007},
  abstract  = {Polymer foams with electrically charged cellular voids, the so-called ferroelectrets, are soft piezoelectric transducer materials. Several polymers such as polyethylene terephthalate or cyclo-olefin copolymers are under investigation with respect to their suitability as ferroelectrets. Here, the authors report an additional ferroelectret polymer, cellular polyethylene-naphthalate (PEN), which was prepared from commercial uniform polymer films by means of foaming in supercritical carbon dioxide, inflation, biaxial stretching, electrical charging, and metallization. Piezoelectric d(33) coefficients of up to 140 pC/N demonstrate the suitability of such cellular PEN films for transducer applications. Their piezoelectricity is partially stable at elevated temperatures as high as 100 degrees C.},
  language  = {en}
}
@article{WirgesWegenerVoroninaetal.2007,
  author    = {Wirges, Werner and Wegener, Michael and Voronina, Olena and Zirkel, Larissa and Gerhard, Reimund},
  title     = {Optimized preparation of elastically soft, highly piezoelectric, cellular ferroelectrets from nonvoided poly(ethylene terephthalate) films},
  doi       = {10.1002/adfm.200600162},
  year      = {2007},
  abstract  = {Electrically charged cellular polymer films can exhibit very high piezoelectric activity and are therefore more and more often employed in advanced electromechanical and electro-acoustical transducers. In this paper, we report an optimized sequence of steps for preparing such ferroelectrets from commercial nonvoided ploy(ethylene terephthalate) (PETP) films by means of foaming with CO2 biaxial mechanical stretching, controlled void inflation, and bipolar electric charging. The nonvoid PETP films foamed with supercritical CO2 at a suitably high pressure and subsequently annealed for stabilization. The cellular foam structure was further optimized by means of well controlled biaxial stretching in a commercial stretcher and sometimes subsequent inflamation in a pressure chamber. Bipolar electric charging of the internal voids was achieved through the application of high electric fields in an SF0 atmosphere. The new optimized PETP ferroelectric exhibit quite large piezoelectric coefficients up to almost 500 pCN(-1), for which unusually low elastic stiffness of only around 0.3 MPa are essential. The PETP foam ferroelectrics posses unclamped thickenss-extension resonance frequences between approximately 120 and 250 kHz, and are thus highly suitable for several established as well as novel ultrasonic-transductant applications.},
  language  = {en}
}
@article{AltafimBassoAltafimetal.2006,
  author    = {Altafim, Ruy Alberto Pisani and Basso, Heitor Cury and Altafim, Ruy Alberto Pisani and Lima, L. and de Aquino, C. V. and Goncalves Neto, Luiz and Gerhard, Reimund},
  title     = {Piezoelectrets from thermo-formed bubble structures of fluoropolymer-electret films},
  issn      = {1070-9878},
  doi       = {10.1109/TDEI.2006.247822},
  year      = {2006},
  abstract  = {The acetone extracts of the root bark and stem bark of Erythrina sacleuxii showed antiplasmodial activities against the chloroquine-sensitive (D6) and chloroquine-resistant (W2) strains of Plasmodium falciparum. Chromatographic separation of the acetone extract of the root bark afforded a new isoflavone, 7-hydroxy-4 -methoxy-3'- prenylisoflavone (trivial name 5-deoxy-3' - prenylbiochanin A) along with known isoflavonoids as the antiplasmodial principles. Flavonoids and isoflavonoids isolated from the stem bark of E. sucleuxii were also tested and showed antiplasmodial activities. The structures were determined on the basis of spectroscopic evidence.},
  language  = {en}
}
@article{MellingerWegenerWirgesetal.2006,
  author    = {Mellinger, Axel and Wegener, Michael and Wirges, Werner and Mallepally, Rajendar Reddy and Gerhard, Reimund},
  title     = {Thermal and temporal stability of ferroelectret films made from cellular polypropylene/air composites},
  year      = {2006},
  abstract  = {Ferroelectrets are thin films of polymer foams, exhibiting piezoelectric properties after electrical charging. Ferroelectret foams usually consist of a cellular polymer structure filled with air. Polymer-air composites are elastically soft due to their high air content as well as due to the size and shape of the polymer walls. Their elastically soft composite structure is one essential key for the working principle of ferroelectrets, besides the permanent trapping of electric charges inside the polymer voids. The elastic properties allow large deformations of the electrically charged voids. However, the composite structure can also possibly limit the stability and consequently the range of applications because of, e. g., penetration of gas and liquids accompanied by discharge phenomena or because of a mechanical pre-load which may be required during the application. Here, we discuss various stability aspects related to the piezoelectric properties of polypropylene ferroelectrets. Near and below room temperature, the piezoelectric effect and the stability of the trapped charges are practically independent from humidity during long-time storage in a humid atmosphere or water, or from operating conditions, such as continuous mechanical excitation. Thermal treatment of cellular polypropylene above -10 degrees C leads to a softening of the voided structure which is apparent from the decreasing values of the elastic modulus. This decrease results in an increase of the piezoelectric activity. Heating above 60 degrees C, however, leads to a decrease in piezoelectricity},
  language  = {en}
}
@article{MellingerFloresSuarezSinghetal.2006,
  author    = {Mellinger, Axel and Flores Su{\´a}rez, Rosaura and Singh, Rajeev and Wegener, Michael and Wirges, Werner and Lang, Sidney B. and Gerhard, Reimund},
  title     = {High-resolution space-charge and polarization tomography with thermal pulses},
  isbn      = {3-8007-2939-3},
  year      = {2006},
  abstract  = {Die Arbeit wurde am 13.03.2006 mit dem "BEST PAPER AWARD" des deutschen IEEE Instrumentation and Measurement (I\&M) Chapter ausgezeichnet.},
  language  = {en}
}
@article{AbelBergweilerGerhard2006,
  author    = {Abel, Markus and Bergweiler, Steffen and Gerhard, Reimund},
  title     = {Synchronization of organ pipes : experimental observations and modeling},
  issn      = {0001-4966},
  doi       = {10.1121/1.217044},
  year      = {2006},
  abstract  = {We report measurements on the synchronization properties of organ pipes. First, we investigate influence of an external acoustical signal from a loudspeaker on the sound of an organ pipe. Second, the mutual influence of two pipes with different pitch is analyzed. In analogy to the externally driven, or mutually coupled self-sustained oscillators, one observes a frequency locking, which can be explained by synchronization theory. Further, we measure the dependence of the frequency of the signals emitted by two mutually detuned pipes with varying distance between the pipes. The spectrum shows a broad '' hump '' structure, not found for coupled oscillators. This indicates a complex coupling of the two organ pipes leading to nonlinear beat phenomena.},
  language  = {en}
}
@article{WegenerKuenstlerGerhard2006,
  author    = {Wegener, Michael and K{\"u}nstler, Wolfgang and Gerhard, Reimund},
  title     = {Poling behavior and optical absorption of partially dehydrofluorinated and uniaxially stretched polyvinylidene fluoride},
  doi       = {10.1080/00150190600694761},
  year      = {2006},
  abstract  = {Polyvinylidene fluoride was dissolved together with solid sodium hydroxide as catalyst in a dimethylsulfoxide/ acetone mixture and moderately dehydrofluorinated. The dehydrofluorination leads to a partial degradation of the fluorohydrocarbons, and in particular to main-chain scission and to formation of carbon double or triple bonds. This enhances the absorption at UV-vis frequencies. The degradation process also generates a large amount of excess charges in the polymer, which influence the electrical polarization behavior of the dehydrofluorinated polymer. Uniaxial stretching of moderately dehydrofluorinated polyvinylidene fluoride leads to films in a polar phase. Dipole polarization in the degraded and stretched films is demonstrated by means of switching experiments},
  language  = {en}
}
@article{WegenerBergweilerZscherpeletal.2006,
  author    = {Wegener, Michael and Bergweiler, Steffen and Zscherpel, Detlef and Wirges, Werner and Gerhard, Reimund},
  title     = {Detection of elliptical oscillations and monopole breathing of organ-pipe bodies with piezoelectric polymer sensors},
  year      = {2006},
  abstract  = {In this paper, a measuring technique is presented for the detection of radial oscillations of tube walls excited by changes in internal air pressure. On organ pipes, the oscillations were investigated by means of piezoelectric polymer films slightly tensioned around the pipe bodies. Employing sensors with patterned electrodes, the well-known elliptical oscillation of the cross section as well as an additional monopole breathing of the organ-pipe body were detected. For the monopole breathing, a close relationship between the pressure distribution of the air-column resonances inside the pipe and the circumference variations along the pipe was observed},
  language  = {en}
}
@article{FruebingKremmerGerhardetal.2006,
  author    = {Fr{\"u}bing, Peter and Kremmer, Alexander and Gerhard, Reimund and Spannoudaki, Anna and Pissis, Polycarpos},
  title     = {Relaxation processes at the glass transition in polyamide 11 : From rigidity to viscoelasticity},
  issn      = {0021-9606},
  doi       = {10.1063/1.2360266},
  year      = {2006},
  language  = {en}
}
@article{WegenerTuncerGerhardetal.2006,
  author    = {Wegener, Michael and Tuncer, Enis and Gerhard, Reimund and Bauer, Siegfried},
  title     = {Elastic properties and electromechanical coupling factor of inflated polypropylene ferroelectrets},
  isbn      = {1-4244-0547-5},
  year      = {2006},
  language  = {en}
}
@article{MellingerWegenerWirgesetal.2006,
  author    = {Mellinger, Axel and Wegener, Michael and Wirges, Werner and Mallepally, Rajendar Reddy and Gerhard, Reimund},
  title     = {Thermal and temporal stability of ferroelectret films made from cellular polypropylene/air composites},
  year      = {2006},
  language  = {en}
}
@article{GerhardFariaOliveiraJretal.2006,
  author    = {Gerhard, Reimund and Faria, Roberto M. and Oliveira Jr., O. N. and Giacometti, Jose A.},
  title     = {Useful effects of space charge and dipole polarization: Recent developments in polymer electrets and organic semiconductors (Editorial)},
  issn      = {1070-9878},
  doi       = {10.1109/TDEI.2006.247817},
  year      = {2006},
  language  = {en}
}
@article{Gerhard2006,
  author    = {Gerhard, Reimund},
  title     = {Electret science has a long history and a bright interdisciplinary future},
  year      = {2006},
  language  = {en}
}
@article{SaarimaekiPaajanenSavijaervietal.2006,
  author    = {Saarim{\"a}ki, Eetta and Paajanen, Mika and Savij{\"a}rvi, Ann-Mari and Minkkinen, Hannu and Wegener, Michael and Voronina, Olena and Schulze, Robert and Wirges, Werner and Gerhard, Reimund},
  title     = {Novel heat durable electromechanical film : processing for electromechanical and electret applications},
  year      = {2006},
  abstract  = {New ferroelectrets were developed on the basis of foams from cyclo-olefin polymers and copolymers. The results obtained on the cyclo-olefin polymer foam demonstrate a significant improvement of the service temperature for ferroelectret transducer materials. Suitable compounding and preparation led to cyclo-olefin ferroelectrets with an electromechanical activity of around 15 pC/N, which is thermally stable at least up to 110 degrees C. The properties in sensor and actuator applications are strongly dependent on the processing parameters related to film-making, sensor and actuator preparation, gas content and electric charging. The processing window for the film stretching was very narrow compared to the earlier developed polypropylene ferroelectrets. The film porosity, softness and thus the electromechanical activity are adjusted by gas-diffusion expansion. The activity of the electromechanically operating sensors and actuators was increased by stacking several layers of cellular cyclo-olefin film. For applications such as flat loudspeakers, the foamed films are tuned by tensioning them on a support frame. Correct tensioning was essential also for reducing the distortion levels.},
  language  = {en}
}
@article{FruebingKremmerGerhardetal.2006,
  author    = {Fr{\"u}bing, Peter and Kremmer, Alexander and Gerhard, Reimund and Spanoudaki, Anna and Pissis, Polycarpos},
  title     = {Relaxation processes at the glass transition in polyamide 11: From rigidity to viscoelasticity},
  series = {The journal of chemical physics : bridges a gap between journals of physics and journals of chemistr},
  volume    = {125},
  journal   = {The journal of chemical physics : bridges a gap between journals of physics and journals of chemistr},
  number    = {12},
  publisher = {American Institute of Physics},
  address   = {Melville},
  issn      = {0021-9606},
  doi       = {10.1063/1.2360266},
  pages     = {8},
  year      = {2006},
  abstract  = {Relaxation processes associated with the glass transition in nonferroelectric and ferroelectric polyamide (PA) 11 are investigated by means of differential scanning calorimetry, dynamic mechanical analysis, and dielectric relaxation spectroscopy (DRS) in order to obtain information about the molecular mobility within the amorphous phase. In particular, the effects of melt quenching, cold drawing, and annealing just below the melting region are studied with respect to potential possibilities and limitations for improving the piezoelectric and pyroelectric properties of PA 11. A relaxation map is obtained from DRS that shows especially the crossover region where the cooperative alpha relaxation and the local beta relaxation merge into a single high-temperature process. No fundamental difference between quenched, cold-drawn, and annealed films is found, though in the cold-drawn (ferroelectric) film the alpha relaxation is suppressed and slowed down, but it is at least partly recovered by subsequent annealing. It is concluded that there exists an amorphous phase in all structures, even in the cold-drawn film. The amorphous phase can be more rigid or more viscoelastic depending on preparation. Cold drawing not only leads to crystallization in a ferroelectric form but also to higher rigidity of the remaining amorphous phase. Annealing just below the melting region after cold drawing causes a stronger phase separation between the crystalline phase and a more viscoelastic amorphous phase.},
  language  = {en}
}
@article{FloresSuarezMellingerWegeneretal.2006,
  author    = {Flores Su{\´a}rez, Rosaura and Mellinger, Axel and Wegener, Michael and Wirges, Werner and Gerhard, Reimund and Singh, Rajeev},
  title     = {Thermal-pulse tomography of polarization distributions in a cylindrical geometry},
  series = {IEEE transactions on dielectrics and electrical insulation},
  volume    = {13},
  journal   = {IEEE transactions on dielectrics and electrical insulation},
  number    = {5},
  publisher = {IEEE},
  address   = {Piscataway},
  issn      = {1070-9878},
  doi       = {10.1109/TDEI.2006.258210},
  pages     = {1030 -- 1035},
  year      = {2006},
  abstract  = {Fast, three-dimensional polarization mapping in piezoelectric sensor cables was performed by means of the novel thermal-pulse tomography (TPT) technique with a lateral resolution of 200 mum. The active piezoelectric cable material (a copolymer of polyvinylidene fluoride with trifluoroethylene) was electrically poled with a point-to-cable corona discharge. A focused laser was employed to heat the opaque outer electrode, and the short-circuit current generated by the thermal pulse was used to obtain 3D polarization maps via the scale transformation method. The article describes the TPT technique as a fast non-destructive option for studying cylindrical geometries.},
  language  = {en}
}
@article{BassoAquinoAltafimetal.2006,
  author    = {Basso, Heitor Cury and Aquino, C. V. and Altafim, Ruy Alberto Pisani and Altafim, Ruy Alberto Pisani and Gerhard, Reimund},
  title     = {Piezoelectricity of a single bubble formed by two oppositely charged Teflon-FEP films},
  isbn      = {1-4244-0547-5},
  year      = {2006},
  language  = {en}
}
@article{TuncerWegenerGerhard2005,
  author    = {Tuncer, Enis and Wegener, Michael and Gerhard, Reimund},
  title     = {Modeling electro-mechanical properties of layered electrets : application of the finite-element method},
  issn      = {0304-3886},
  year      = {2005},
  abstract  = {We present calculations on the deformation of two- and three-layer electret systems. The electrical field is coupled with the stress-strain equations by means of the Maxwell stress tensor. In the simulations, two-phase systems are considered, and intrinsic relative dielectric permittivity and Young's modulus of the phases are altered. The numerically calculated electro-mechanical activity is compared to an analytical expression. Simulations are performed on two- and three-layer systems. Various parameters in the model are systematically varied and their influence on the resulting piezoelectricity is estimated. In three-layer systems with bipolar charge, the piezoelectric coefficients exhibit a strong dependence on the elastic moduli of the phases. However, with mono-polar charge, there is no significant piezoelectric effect. A two-dimensional simulation illustrated that higher piezoelectric coefficients can be obtained for non-uniform surface charges and low Poisson's ratio of phases. Irregular structures considered exhibit low piezoelectric activity compared to two-layer structures. (C) 2004 Elsevier B.V. All rights reserved},
  language  = {en}
}