@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}
}
@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}
}