TY - JOUR A1 - Pham, Cong Duc A1 - Petre, Anca A1 - Berquez, Laurent A1 - Flores Suárez, Rosaura A1 - Mellinger, Axel A1 - Wirges, Werner A1 - Gerhard, Reimund T1 - 3D high-resolution mapping of polarization profiles in thin poly(vinylidenefluoride-trifluoroethylene) (PVDF- TrFE) films using two thermal techniques N2 - 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. Y1 - 2009 UR - http://ieeexplore.ieee.org/servlet/opac?punumber=94 U6 - https://doi.org/10.1109/TDEI.2009.5128505 SN - 1070-9878 ER - TY - JOUR A1 - Gerhard, Reimund T1 - 50 years of International Symposia on Electrets from 1967 to 2017 BT - a global history JF - IEEE electrical insulation magazine / Institute of Electrical and Electronics Engineers N2 - The prehistory of electrets is not known yet, but it is quite likely that the electrostatic charging behavior of amber (Greek: τò 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). Y1 - 2021 U6 - https://doi.org/10.1109/MEI.2021.9352710 SN - 0883-7554 SN - 1558-4402 VL - 37 IS - 2 SP - 50 EP - 55 PB - IEEE CY - New York, NY ER - TY - JOUR A1 - Assagra, Yuri A.O. A1 - Altafim, Ruy Alberto Pisani A1 - do Carmo, Joao P. A1 - Altafim, Ruy A.C. A1 - Rychkov, Dmitry A1 - Wirges, Werner A1 - Gerhard, Reimund T1 - A new route to piezo-polymer transducers: 3D printing of polypropylene ferroelectrets JF - IEEE transactions on dielectrics and electrical insulation N2 - 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. KW - 3D printing KW - polymer ferroelectrets KW - sensors and actuators KW - piezoelectrets KW - electret polymers KW - soft electro-active materials KW - functional materials KW - soft matter Y1 - 2020 U6 - https://doi.org/10.1109/TDEI.2020.008461 SN - 1070-9878 SN - 1558-4135 VL - 27 IS - 5 SP - 1668 EP - 1674 PB - Inst. of Electr. and Electronics Engineers CY - Piscataway ER - TY - JOUR A1 - Gerhard, Reimund A1 - Sessler, G. M. T1 - A tribute to Professor Bernhard Gross Y1 - 1996 ER - TY - JOUR A1 - Brinker, Walter A1 - Wirges, Werner A1 - Molzow, Wolf-Dietrich A1 - Gerhard, Reimund A1 - Melcher, R. A1 - Budde, W. A1 - Fiedler, H. T1 - Active silicon CMOS addressing matrices for light-valve projection displays Y1 - 1995 ER - TY - JOUR A1 - Wang, Ningzhen A1 - Daniels, Robert A1 - Connelly, Liam A1 - Sotzing, Michael A1 - Wu, Chao A1 - Gerhard, Reimund A1 - Sotzing, Gregory A. A1 - Cao, Yang T1 - All-organic flexible ferroelectret nanogenerator with fabric-based electrodes for self-powered body area networks JF - Small : nano micro N2 - 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. KW - all-organic ferroelectret nanogenerator (FENG) KW - all-organic KW - piezoelectric nanogenerator (PENG) KW - expanded polytetrafluoroethylene KW - ferroelectret KW - micro-energy harvesting KW - (PEDOT KW - PSS)-coated porous KW - fabric electrodes KW - wearable electronics Y1 - 2021 U6 - https://doi.org/10.1002/smll.202103161 SN - 1613-6810 SN - 1613-6829 VL - 17 IS - 33 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Basso, Heitor Cury A1 - Monteiro, Jose Roberto B. de A. A1 - Mazulquim, Daniel Baladelli A1 - de Paula, Geyverson Teixeira A1 - Goncalves Neto, Luiz A1 - Gerhard, Reimund T1 - Alternating current-generated plasma discharges for the controlled direct current charging of ferroelectrets JF - Journal of applied physics N2 - 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. Y1 - 2013 U6 - https://doi.org/10.1063/1.4821113 SN - 0021-8979 SN - 1089-7550 VL - 114 IS - 10 PB - American Institute of Physics CY - Melville ER - TY - JOUR A1 - Wertheimer, Michael R. A1 - St-Georges-Robillard, Amelie A1 - Lerouge, Sophie A1 - Mwale, Fackson A1 - Elkin, Bentsian A1 - Oehr, Christian A1 - Wirges, Werner A1 - Gerhard, Reimund T1 - Amine-rich organic thin films for cell culture - possible electrostatic effects in cell-surface interactions JF - Japanese journal of applied physics N2 - 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. Y1 - 2012 U6 - https://doi.org/10.1143/JJAP.51.11PJ04 SN - 0021-4922 VL - 51 IS - 11 PB - Japan Soc. of Applied Physics CY - Tokyo ER - TY - JOUR A1 - Gerhard, Reimund T1 - Amplitudengesteuerte Beugungsgitter für die Lichtventil-Großbildprojektion Y1 - 1995 ER - TY - JOUR A1 - He, Yushuang A1 - Wang, Feipeng A1 - He, Li A1 - Wang, Qiang A1 - Li, Jian A1 - Qian, Yihua A1 - Gerhard, Reimund A1 - Plath, Ronald T1 - An insight Into the role of Nano-Alumina on DC Flashover-Resistance and surface charge variation of Epoxy Nanocomposites JF - IEEE transactions on dielectrics and electrical insulation N2 - 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. KW - surface morphology KW - Epoxy resins KW - Electric potential KW - Surface treatment KW - Doping KW - Epoxy resin KW - multiple KW - flashover KW - nanocomposite KW - surface charge Y1 - 2022 U6 - https://doi.org/10.1109/TDEI.2022.3173510 SN - 1070-9878 SN - 1558-4135 VL - 29 IS - 3 SP - 1022 EP - 1029 PB - Inst. of Electr. and Electronics Engineers CY - Piscataway ER - TY - JOUR A1 - Wegener, Michael A1 - Bauer, Siegfried A1 - Gerhard, Reimund T1 - Apparent dielectric hysteresis without significant dipolar movement? Y1 - 1999 ER - TY - JOUR A1 - Qiu, Xunlin A1 - Mellinger, Axel A1 - Wegener, Michael A1 - Wirges, Werner A1 - Gerhard, Reimund T1 - Barrier discharges in cellular polypropylene ferroelectrets : how do they influence the electromechanical properties? Y1 - 2007 ER - TY - JOUR A1 - Qiu, Xunlin A1 - Wirges, Werner A1 - Gerhard, Reimund T1 - Beneficial and detrimental fatigue effects of dielectric barrier discharges on the piezoelectricity of polypropylene ferroelectrets JF - Journal of applied physics N2 - 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. Y1 - 2011 U6 - https://doi.org/10.1063/1.3610507 SN - 0021-8979 SN - 1089-7550 VL - 110 IS - 2 PB - American Institute of Physics CY - Melville ER - TY - JOUR A1 - Bergweiler, Steffen A1 - Bergner, André A1 - Görne, Thomas A1 - Wegener, Michael A1 - Gerhard, Reimund T1 - Breathing modes and sound radiation of metallic organ pipes Y1 - 2003 ER - TY - JOUR A1 - Wegener, Michael A1 - Gerhard, Reimund A1 - Wirges, Werner A1 - Bergner, André A1 - Bergweiler, Steffen T1 - Breathing modes of organ-pipe bodies : experimental detection with ring-shaped piezoelectric-polymer sensors Y1 - 2003 ER - TY - JOUR A1 - Fang, Peng A1 - Wegener, Michael A1 - Wirges, Werner A1 - Gerhard, Reimund A1 - Zirkel, Larissa T1 - Cellular polyethylene-naphthalate ferroelectrets : foaming in supercritical carbon dioxide, structural and electrical preparation, and resulting piezoelectricity N2 - 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. Y1 - 2007 UR - http://apl.aip.org/ U6 - https://doi.org/10.1063/1.2738365 SN - 0003-6951 ER - TY - JOUR A1 - Qiu, Xunlin A1 - Groth, Frederick A1 - Wirges, Werner A1 - Gerhard, Reimund T1 - Cellular polypropylene foam films as DC voltage insulation and as piezoelectrets BT - a comparison JF - IEEE transactions on dielectrics and electrical insulation N2 - 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. KW - Cellular polypropylene (PP) KW - polymer-foam films KW - ferro- and piezoelectrets KW - charge storage KW - electrical breakdown KW - dielectric barrier discharges Y1 - 2018 U6 - https://doi.org/10.1109/TDEI.2018.007192 SN - 1070-9878 SN - 1558-4135 VL - 25 IS - 3 SP - 829 EP - 834 PB - Institut of Electr. and Electronics Engineers CY - Piscataway ER - TY - JOUR A1 - Kappel, Marcel A1 - Abel, Markus A1 - Gerhard, Reimund T1 - Characterization and calibration of piezoelectric polymers in situ measurements of body vibrations JF - Review of scientific instruments : a monthly journal devoted to scientific instruments, apparatus, and techniques N2 - 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. KW - acoustic transducers KW - calibration KW - durability KW - electric sensing devices KW - piezoelectricity KW - polymers Y1 - 2011 U6 - https://doi.org/10.1063/1.3607435 SN - 0034-6748 VL - 82 IS - 7 PB - American Institute of Physics CY - Melville ER - TY - JOUR A1 - Künstler, Wolfgang A1 - Gerhard, Reimund T1 - Charge profiles in corona- or electrode-charged fluoropolymers Y1 - 1998 ER - TY - JOUR A1 - Wegener, Michael A1 - Wirges, Werner A1 - Richter, Kristin A1 - Künstler, Wolfgang A1 - Gerhard, Reimund T1 - Charge stability and piezoelectric properties of porous fluoro-polymer space-charge electrets in layer systems Y1 - 2001 ER - TY - JOUR A1 - Wegener, Michael A1 - Wirges, Werner A1 - Paajanen, Mika A1 - Gerhard, Reimund T1 - Charging behaviour and thermal stability of porous and non-porous polytetrafluoroethylene (PTFE) electrets Y1 - 2007 SN - 978-1-4244-1482-6 ER - TY - JOUR A1 - Paajanen, Mika A1 - Wegener, Michael A1 - Gerhard, Reimund T1 - Charging of cellular space-charge electret films in various gas atmospheres Y1 - 2001 SN - 0-7803-7053-8 ER - TY - JOUR A1 - Montanari, Gian Carlo A1 - Fabiani, Davide A1 - Ciani, Franco A1 - Motori, Antonio A1 - Paajanen, Mika A1 - Gerhard, Reimund A1 - Wegener, Michael T1 - Charging properties and time-temperature stability of innovative polymeric cellular ferroelectrets N2 - 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. Y1 - 2007 UR - http://ieeexplore.ieee.org/servlet/opac?punumber=94 U6 - https://doi.org/10.1109/TDEI.2007.302892 SN - 1070-9878 ER - TY - JOUR A1 - Wang, Jingwen A1 - Rychkov, Dmitry A1 - Gerhard, Reimund T1 - Chemical modification with orthophosphoric acid enhances surface-charge stability on polypropylene electrets JF - Applied physics letters N2 - 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. Y1 - 2017 U6 - https://doi.org/10.1063/1.4983348 SN - 0003-6951 SN - 1077-3118 VL - 110 PB - American Institute of Physics CY - Melville ER - TY - JOUR A1 - Elling, B. A1 - Pinnow, M. A1 - Danz, Rudi A1 - Wegener, Michael A1 - Wirges, werner A1 - Gerhard, Reimund T1 - Coating of porous polytetrafluoroethylene films with other polymers for electret applications Y1 - 2001 SN - 0-7803-7053-8 ER - TY - JOUR A1 - Frübing, Peter A1 - Blischke, D. A1 - Salah Khalil, M. A1 - Gerhard, Reimund T1 - Complete relaxation map of polyethylene : filler-induced chemical modifications as dielectric probes Y1 - 2001 ER - TY - JOUR A1 - Wegener, Michael A1 - Wirges, Werner A1 - Gerhard, Reimund A1 - Dansachmüller, M. A1 - Schwödiauer, R. A1 - Bauer-Gogonea, Simona A1 - Bauer, Siegfried A1 - Paajanen, Mika A1 - Minkkinen, Hannu A1 - Raukola, J. T1 - Controlled inflation of voids in cellular polymer ferroelectrets : optimizing electromechanical transducer properties N2 - When exposed to sufficiently high electric fields, polymer-foam electret materials with closed cells exhibit ferroelectric-like behavior and may therefore be called ferroelectrets. In cellular ferroelectrets, the influence of the cell size and shape distributions on the application-relevant properties is not yet understood. Therefore, controlled inflation experiments were carried out on cellular polypropylene films, and the resulting elastical and electromechanical parameters were determined. The elastic modulus in the thickness direction shows a minimum with a corresponding maximum in the electromechanical transducer coefficient. The resonance frequency shifts as a function of the elastic modulus and the relative density of the inflated cellular films. Therefore, the transducer properties of cellular ferroelectrets can be optimized by means of controlled inflation. (C) 2004 American Institute of Physics Y1 - 2004 ER - TY - JOUR A1 - Wegener, Michael A1 - Paajanen, Mika A1 - Wirges, Werner A1 - Gerhard, Reimund T1 - Corona-induced partial discharges, internal charge separation and electromechanical transducer properties in cellular polymer films Y1 - 2002 SN - 0-7803-7560-2 ER - TY - JOUR A1 - Wegener, Michael A1 - Bergweiler, Steffen A1 - Zscherpel, Detlef A1 - Wirges, Werner A1 - Gerhard, Reimund T1 - Detection of elliptical oscillations and monopole breathing of organ-pipe bodies with piezoelectric polymer sensors N2 - 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 Y1 - 2006 ER - TY - JOUR A1 - Qiu, Xunlin A1 - Benjamin, Aravindan Joseph A1 - Raman Venkatesan, Thulasinath A1 - Schmidt, Georg C. A1 - Soler, Ricardo Alonso Quintana A1 - Panicker, Pramul Muraleedhara A1 - Gerhard, Reimund A1 - Hübler, Arved Carl T1 - Dielectric and electroacoustic assessment of screen-printed piezoelectric polymer layers as flexible transducers BT - influence of the electrode material JF - IEEE transactions on dielectrics and electrical insulation N2 - 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. KW - poly(vinylidenefluoride-trifluoroethylene) P(VDF-TrFE) KW - dielectric KW - spectroscopy KW - ferroelectricity and piezoelectricity in polymers KW - screen KW - printing KW - printed electroacoustic thin-film transducers Y1 - 2020 U6 - https://doi.org/10.1109/TDEI.2020.008864 SN - 1070-9878 SN - 1558-4135 SN - 0018-9367 VL - 27 IS - 5 SP - 1683 EP - 1690 PB - Institute of Electrical and Electronics Engineers CY - New York, NY ER - TY - JOUR A1 - Qiu, Xunlin A1 - Mellinger, Axel A1 - Wirges, Werner A1 - Gerhard, Reimund T1 - Dielectric barrier discharges during the generation of ferroelectrets : optical spectroscopy for process monitoring Y1 - 2007 SN - 978-1-4244-1482-6 ER - TY - JOUR A1 - Bauer-Gogonea, Simona A1 - Bauer, Siegfried A1 - Wirges, Werner A1 - Gerhard, Reimund T1 - Dielectric investigation of photo-induced chromophore degradation in nonlinear optical side-chain polymer electrets N2 - Organic materials with non-centrosymmetric chromophores are known to be susceptible to a number of photochemical processes, including reversible isomerization reactions as well as irreversible photo-oxidation or photo- reduction reactions. Reversible isomerization is the basis for a variety of applications, such as photo-induced poling, optical data storage and optical grating formation. The irreversible processes that involve the destruction of the chromophores have been found useful for the fabrication of optical waveguides, but they also limit the life times of polymeric photonic devices. In this paper, it is demonstrated that dielectric measurements allow for an in-depth investigation of non-reversible chromophore degradation processes in a typical side-chain polymer. The time- and temperature-dependent dielectric function of the polymer at 1 kHz enables us to follow the chromophore-degradation kinetics and to monitor the bleaching depth as a function of time at room and elevated temperatures Y1 - 2004 ER - TY - JOUR A1 - Bauer-Gogonea, Simona A1 - Cheng, Z. Y. A1 - Wirges, Werner A1 - Bauer, Siegfried A1 - Gerhard, Reimund A1 - Das-Gupta, D. K. T1 - Dielectric investigation of thermally-induced chromophore degradation in nonlinear optical polymer electrets Y1 - 1998 ER - TY - JOUR A1 - Kremmer, Alexander A1 - Ganser, J. A1 - Pissis, Polycarpos A1 - Gerhard, Reimund T1 - Dielectric relaxation and resonance spectra of ferroelectric polyamide 11 Y1 - 2003 SN - 0-7803-7560-2 ER - TY - JOUR A1 - Ganesan, Lakshmi Meena A1 - Frübing, Peter A1 - Mellinger, Axel A1 - Gerhard, Reimund T1 - Dielectric relaxation behaviour of nematic liquid crystals dispersed in poly(vinylidene fluoride- trifluoroethylene) N2 - 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. Y1 - 2009 UR - http://iopscience.iop.org/0022-3727/ U6 - https://doi.org/10.1088/0022-3727/42/9/092006 SN - 0022-3727 ER - TY - JOUR A1 - Frübing, Peter A1 - Kremmer, Alexander A1 - Neumann, Werner A1 - Gerhard, Reimund A1 - Guy, I. L. T1 - Dielectric relaxation in piezo-, pyro- and ferroelectric polyamide 11 N2 - Ferroelectric polyamide 11 films were prepared by melt-quenching, cold-drawing and electrical poling. Their ferroelectricity was studied by means of dielectric-hysteresis measurements. A remnant polarisation of up to 35 mC/m(2) and a coercive field of 75 MV/m were obtained. The piezoelectric d(33) coefficient and the pyroelectric coefficient of the films are reduced by annealing just below the melting region, but remain at about 3 pC/N and 8 muC/(m(2)K), respectively, during further heat treatment. Differential scanning calorimetry (DSC), dielectric relaxation spectroscopy (DRS) and thermally stimulated depolarisation (TSD) were applied for investigating the conformational changes induced by melt-quenching, cold-drawing and annealing. The results indicate that the cold-drawn film mainly consists of a rigid amorphous phase which exhibits considerably lower conductivity, no glass transition and consequently no dielectric a relaxation. Instead, an a, relaxation is found, which is related to chain motions in regions of the rigid amorphous phase where the amide-group dipoles are not perfectly ordered. Annealing removes imperfectly ordered structures, but does not affect the ferroelectric polarisation. Therefore, it may be concluded that essentially the a, relaxation causes the thermally non-stable part of the piezo- and pyroelectricity in polyamide 11 Y1 - 2004 ER - TY - JOUR A1 - Frübing, Peter A1 - Blischke, D. A1 - Salah Khalil, M. A1 - Gerhard, Reimund T1 - Dielectric relaxations in titanium-dioxide-filled polyethylene: Filler-induced chemical modifications of the polymer provide the dipolar response Y1 - 2001 SN - 0-7803-7053-8 ER - TY - JOUR A1 - Mellinger, Axel A1 - Frübing, Peter A1 - Prescher, Dietrich A1 - Gerhard, Reimund T1 - Dielectric spectra of a partially fluorinated chromophore / amorphous Teflon AF guest-host system Y1 - 1999 ER - TY - JOUR A1 - Yilmaz, Sükrü A1 - Wirges, Werner A1 - Bauer-Gogonea, Simona A1 - Bauer, Siegfried A1 - Gerhard, Reimund A1 - Michelotti, F. A1 - Toussaere, E. A1 - Levenson, R. A1 - Liang, J. A1 - Zyss, Joseph T1 - Dielectric, pyroelectric and electro-optic monitoring of the cross-linking process and photo-induced poling of Red Acid Magly Y1 - 1997 ER - TY - JOUR A1 - Frübing, Peter A1 - Wegener, Michael A1 - Gerhard, Reimund A1 - Buchsteiner, Alexandra A1 - Neumann, Werner A1 - Brehmer, Ludwig T1 - Different molecular mechanisms for the dielectric hysteresis and pyroelectricity in a poly(vinyl alcohol) with azobenzene-alkoxy side chains N2 - By means of pyroelectrical measurements and dielectric spectroscopy as well as structural information from differential scanning calorimetry, it is shown that, in a poly(vinyl alcohol) with azobenzene-alkoxy side chains, pyroelctricity and dielectric hysteresis which are usually related to each other have different origins. The pyroelectric effect is explained with reversible dipole-density changes upon thermal expansion, whereas the dielectric hysteresis is proposed to result from a charge-carrier polarisation. Y1 - 1999 SN - 0277-786X ER - TY - JOUR A1 - Qiu, Xunlin A1 - Holländer, Lars A1 - Wirges, Werner A1 - Gerhard, Reimund A1 - Basso, Heitor Cury T1 - Direct hysteresis measurements on ferroelectret films by means of a modified Sawyer-Tower circuit JF - Journal of applied physics N2 - 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. Y1 - 2013 U6 - https://doi.org/10.1063/1.4809556 SN - 0021-8979 VL - 113 IS - 22 PB - American Institute of Physics CY - Melville ER - TY - JOUR A1 - Tuncer, Enis A1 - Wegener, Michael A1 - Gerhard, Reimund T1 - Distribution of relaxation times in alpha-phase polyvinylidene fluoride N2 - In this paper, a recently developed numerical method to analyze dielectric-spectroscopy data is applied to alpha-phase polyvinylidene fluoride (PVDF). The numerical procedure is non-parametric and does not contain any of the extensively used empirical formulas mentioned in the literature. The method basically recovers the unknown distribution of relaxation times of the generalized dielectric function representation by simultaneous application of the Monte Carlo integration method and of the constrained least-squares optimization. The relaxation map constructed after the numerical analysis is compared to a-phase PVDF data presented in the literature and results of the parametric analysis with a well- known empirical formula. (c) 2005 Elsevier B.V. All rights reserved Y1 - 2005 SN - 0022-3093 ER - TY - JOUR A1 - Wegener, Michael A1 - Tuncer, Enis A1 - Gerhard, Reimund A1 - Bauer, Siegfried T1 - Elastic properties and electromechanical coupling factor of inflated polypropylene ferroelectrets Y1 - 2006 SN - 1-4244-0547-5 ER - TY - JOUR A1 - Gerhard, Reimund T1 - Electret science has a long history and a bright interdisciplinary future Y1 - 2006 ER - TY - JOUR A1 - Gerhard, Reimund T1 - Electrets Y1 - 1999 ER - TY - JOUR A1 - Wegener, Michael A1 - Gerhard, Reimund T1 - Electric poling and electromechanical characterization of 0.1-mm-thick sensor films and 0.2-mm-thick cable layers from piezoelectric poly(vinylidene fluoride-trifluoroethylene) Y1 - 2003 SN - 0885-3010 ER - TY - JOUR A1 - Gerhard, Reimund T1 - Electric-field poling of polymers Y1 - 1997 ER - TY - JOUR A1 - Bauer-Gogonea, Simona A1 - Wirges, Werner A1 - Bauer, Siegfried A1 - Gerhard, Reimund A1 - Liang, J. A1 - Zyss, Joseph T1 - Electrical determination of the degree of cross-linking in a poled nonlinear optical polymer Y1 - 1996 ER - TY - JOUR A1 - Raman Venkatesan, Thulasinath A1 - Gulyakova, Anna A. A1 - Frübing, Peter A1 - Gerhard, Reimund T1 - Electrical polarization phenomena, dielectric relaxations and structural transitions in a relaxor-ferroelectric terpolymer investigated with electrical probing techniques JF - Materials research express N2 - 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). KW - P(VDF-TrFE-CFE) terpolymer KW - relaxor-ferroelectric polymers KW - dielectric relaxation spectroscopy KW - thermally stimulated depolarization current KW - electrical polarization hysteresis Y1 - 2019 U6 - https://doi.org/10.1088/2053-1591/ab5352 SN - 2053-1591 VL - 6 IS - 12 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Gerhard, Reimund A1 - Bauer, Siegfried A1 - Bauer-Gogonea, Simona A1 - Brinker, Walter A1 - Dinger, Claudia A1 - Molzow, Wolf-Dietrich A1 - Wirges, Werner A1 - Yilmaz, Sükrü T1 - Electro-optical investigation of the dipole orientation in poled polymers Y1 - 1995 ER - TY - JOUR A1 - Gerhard, Reimund A1 - Wegener, Michael A1 - Wirges, Werner A1 - Giacometti, J. A. A1 - Altafim, Ruy Alberto Pisani A1 - Santos, Lucas F. A1 - Faria, Roberto M. A1 - Paajanen, Mika T1 - Electrode polling of cellular polypropylene films with short high-voltage pulses Y1 - 2002 SN - 0-7803-7502-5 ER - TY - JOUR A1 - Dinger, Claudia A1 - Yilmaz, Sükrü A1 - Brinker, Walter A1 - Wirges, Werner A1 - Bauer, Siegfried A1 - Gerhard, Reimund T1 - Ellipsometry and Michelson interferometry for fixed- and variable- frequency electro-optical measurements on poled polymers Y1 - 1996 ER - TY - JOUR A1 - Rychkov, Dmitry A1 - Gerhard, Reimund A1 - Ivanov, Vadim A1 - Rychkov, Andrey T1 - Enhanced electret charge stability on Polyethylene Films treated with Titanium-Tetrachloride vapor JF - IEEE transactions on dielectrics and electrical insulation N2 - 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. KW - Electrets KW - chemical modification KW - surface treatment KW - space-charge stability KW - charge-carrier transport KW - polyethylene KW - low-density polyethylene KW - injection Y1 - 2012 SN - 1070-9878 VL - 19 IS - 4 SP - 1305 EP - 1311 PB - Inst. of Electr. and Electronics Engineers CY - Piscataway ER - TY - JOUR A1 - Wang, Feipeng A1 - Frübing, Peter A1 - Wirges, Werner A1 - Gerhard, Reimund A1 - Wegener, Michael T1 - Enhanced Polarization in Melt-quenched and Stretched Poly(vinylidene Fluoride-Hexafluoropropylene) Films N2 - 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. Y1 - 2010 UR - http://ieeexplore.ieee.org/servlet/opac?punumber=94 U6 - https://doi.org/10.1109/TDEI.2010.5539679 SN - 1070-9878 ER - TY - JOUR A1 - Kussmaul, Bjoern A1 - Risse, Sebastian A1 - Kofod, Guggi A1 - Wache, Remi A1 - Wegener, Michael A1 - McCarthy, Denis N. A1 - Krüger, Hartmut A1 - Gerhard, Reimund T1 - Enhancement of dielectric permittivity and electromechanical response in silicone elastomers molecular grafting of organic dipoles to the macromolecular Network JF - Advanced functional materials N2 - 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. KW - dipole grafting KW - silicone based dielectric elastomer actuators KW - permittivity enhancement Y1 - 2011 U6 - https://doi.org/10.1002/adfm.201100884 SN - 1616-301X VL - 21 IS - 23 SP - 4589 EP - 4594 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Fang, Peng A1 - Ma, Xingchen A1 - Li, Xiangxin A1 - Qiu, Xunlin A1 - Gerhard, Reimund A1 - Zhang, Xiaoqing A1 - Li, Guanglin T1 - Fabrication, Structure Characterization, and Performance Testing of Piezoelectret-Film Sensors for Recording Body Motion JF - IEEE Sensors Journal N2 - 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. KW - Forcemyography KW - motion registration KW - piezoelectret KW - film sensor KW - wearable Y1 - 2017 U6 - https://doi.org/10.1109/JSEN.2017.2766663 SN - 1530-437X SN - 1558-1748 VL - 18 IS - 1 SP - 401 EP - 412 PB - Inst. of Electr. and Electronics Engineers CY - Piscataway ER - TY - JOUR A1 - Mellinger, Axel A1 - Singh, Rajeev A1 - Gerhard, Reimund T1 - Fast thermal-pulse measurements of space-charge distributions in electret polymers N2 - Space-charge depth profiles in various electret polymers have been measured in both the time and the frequency domain using thermal pulses and waves, respectively. A comparison of the two techniques on corona-charged polytetrafluoroethylene showed that the thermal-pulse method yielded similar results as the thermal-wave technique, but approximately 20-50 times faster. The article discusses sensitivity limitations as well as possible applications, including the real-time monitoring of space-charge decay under UV irradiation. (C) 2005 American Institute of Physics Y1 - 2005 SN - 0034-6748 ER - TY - JOUR A1 - Gerhard, Reimund T1 - Ferroelectrets : electrically charged polymer foams as quasi-piezoelectric transducer materials Y1 - 2004 SN - 3-18-091829-2 ER - TY - JOUR A1 - Bauer, Siegfried A1 - Gerhard, Reimund A1 - Sessler, G. M. T1 - Ferroelectrets : soft electroactive foams for transducers Y1 - 2004 ER - TY - JOUR A1 - Wegener, Michael A1 - Künstler, Wolfgang A1 - Richter, Kristin A1 - Gerhard, Reimund T1 - Ferroelectric polarization in stretched piezo- and pyroelectric poly(vinylidene fluoride-hexafluoropropylene) copolymer films Y1 - 2002 ER - TY - JOUR A1 - Kalbitz, Rene A1 - Gerhard, Reimund A1 - Taylor, D. M. T1 - Fixed negative interface charges compromise organic ferroelectric field-effect transistors JF - Organic electronics : physics, materials and applications N2 - 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. KW - Organic TFT KW - Memory device KW - Ferroelectric polymer KW - Organic FeFET KW - Ferroelectric polarization KW - Trapped charges Y1 - 2012 U6 - https://doi.org/10.1016/j.orgel.2012.01.034 SN - 1566-1199 VL - 13 IS - 5 SP - 875 EP - 884 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Altafim, Ruy Alberto Pisani A1 - Altafim, Ruy Alberto Pisani A1 - Qiu, Xunlin A1 - Raabe, Sebastian A1 - Wirges, Werner A1 - Basso, Heitor Cury A1 - Gerhard, Reimund T1 - Fluoropolymer piezoelectrets with tubular channels resonance behavior controlled by channel geometry JF - Applied physics : A, Materials science & processing N2 - 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. Y1 - 2012 U6 - https://doi.org/10.1007/s00339-012-6848-z SN - 0947-8396 VL - 107 IS - 4 SP - 965 EP - 970 PB - Springer CY - New York ER - TY - JOUR A1 - Roeder, H. A1 - Ehrke, R. A1 - Gerhard, Reimund A1 - Ipp, E. A1 - Menzel, I. T1 - Full-colour diffraction-based optical system for light-valve projection displays Y1 - 1995 ER - TY - JOUR A1 - Mazurek, P. A1 - Yu, L. A1 - Gerhard, Reimund A1 - Wirges, Werner A1 - Skov, A. L. T1 - Glycerol as high-permittivity liquid filler in dielectric silicone elastomers JF - Journal of applied polymer science N2 - 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. KW - crosslinking KW - dielectric properties KW - elastomers KW - sensors and actuators Y1 - 2016 U6 - https://doi.org/10.1002/app.44153 SN - 0021-8995 SN - 1097-4628 VL - 133 PB - Wiley-Blackwell CY - Hoboken ER - TY - JOUR A1 - Gerhard, Reimund A1 - Künstler, Wolfgang A1 - Eberle, G. A1 - Eisenmenger, E. A1 - Yang, G. M. T1 - High space densities in the bulk of fluoropolymer elctrets detected with piezoelectrically generated pressure steps Y1 - 1998 ER - TY - JOUR A1 - Xia, Zhongfu A1 - Gerhard, Reimund A1 - Künstler, Wolfgang A1 - Wedel, Armin A1 - Danz, Rudi T1 - High surface-charge stability of porous polytetrafluoroethylene electret films at room and elvated temperatures Y1 - 1999 ER - TY - JOUR A1 - Mellinger, Axel A1 - Flores Suárez, Rosaura A1 - Singh, Rajeev A1 - Wegener, Michael A1 - Wirges, Werner A1 - Lang, Sidney B. A1 - Gerhard, Reimund T1 - High-resolution space-charge and polarization tomography with thermal pulses N2 - Die Arbeit wurde am 13.03.2006 mit dem "BEST PAPER AWARD" des deutschen IEEE Instrumentation and Measurement (I&M) Chapter ausgezeichnet. Y1 - 2006 SN - 3-8007-2939-3 ER - TY - JOUR A1 - Rychkov, Dmitry A1 - Rychkov, A. A1 - Efimov, N. A1 - Malygin, A. A1 - Gerhard, Reimund T1 - Higher stabilities of positive and negative charge on tetrafluoroethylene-hexafluoropropylene copolymer (FEP) electrets treated with titanium-tetrachloride vapor JF - Applied physics : A, Materials science & processing N2 - 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. Y1 - 2013 U6 - https://doi.org/10.1007/s00339-013-7821-1 SN - 0947-8396 VL - 112 IS - 2 SP - 283 EP - 287 PB - Springer CY - New York ER - TY - JOUR A1 - Flores Suárez, Rosaura A1 - Ganesan, Lakshmi Meena A1 - Wirges, Werner A1 - Gerhard, Reimund A1 - Mellinger, Axel T1 - Imaging liquid crystals dispersed in a ferroelectric polymer matrix by means of thermal-pulse tomography N2 - 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. Y1 - 2010 UR - http://ieeexplore.ieee.org/servlet/opac?punumber=94 U6 - https://doi.org/10.1109/TDEI.2010.5539683 SN - 1070-9878 ER - TY - JOUR A1 - Rychkov, Dmitry A1 - Gerhard, Reimund A1 - Kuznetsov, Alexey A1 - Rychkov, Andrey T1 - Influence of charge density on the trap energy spectrum in fluoroethylenepropylene copolymer films with chemically modified surfaces JF - IEEE transactions on dielectrics and electrical insulation N2 - 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. KW - Electrets KW - chemical modification KW - surface treatment KW - space-charge stability KW - charge-carrier transport KW - polytetrafluoroethylene (PTFE) KW - fluoroethylenepropylene (FEP) copolymer Y1 - 2018 U6 - https://doi.org/10.1109/TDEI.2018.007437 SN - 1070-9878 SN - 1558-4135 VL - 25 IS - 3 SP - 840 EP - 844 PB - Inst. of Electr. and Electronics Engineers CY - Piscataway ER - TY - JOUR A1 - Raman Venkatesan, Thulasinath A1 - Gulyakova, Anna A. A1 - Gerhard, Reimund T1 - Influence of film stretching on crystalline phases and dielectric properties of a 70/30 mol% poly(vinylidenefluoride-tetrafluoroethylene) copolymer JF - Journal of advanced dielectrics N2 - 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. KW - P(VDF-TFE) copolymer KW - dielectric properties KW - crystalline phases KW - film KW - stretching KW - mid-temperature transition Y1 - 2020 U6 - https://doi.org/10.1142/S2010135X2050023X SN - 2010-135X SN - 2010-1368 VL - 10 IS - 5 PB - World Scientific CY - Singapore ER - TY - JOUR A1 - Lanca, M. C. A1 - Wirges, Werner A1 - Neagu, Eugen R. A1 - Gerhard, Reimund A1 - Marat-Mendes, José Narciso T1 - Influence of humidity on the electrical charging properties of cork agglomerates N2 - 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. Y1 - 2007 UR - http://www.sciencedirect.com/science/journal/00223093 U6 - https://doi.org/10.1016/j.jnoncrysol.2007.03.037 SN - 0022-3093 ER - TY - JOUR A1 - Holländer, Lars A1 - Kossack, Wilhelm A1 - Kollosche, Matthias A1 - Wirges, Werner A1 - Kremer, Friedrich A1 - Gerhard, Reimund T1 - 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 JF - Liquid crystals : an international journal of science and technology N2 - Polymer-dispersed liquid crystals (PDLCs) of ferroelectric poly(vinylidene fluoride-trifluoroethylene) and nematic 4-cyano-4ʹ-n-hexylbiphenyl (6CB) or 4-cyano-4ʹ-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. KW - Polymer-dispersed liquid crystal KW - ferroelectric polymer KW - remanent polarisation KW - liquid crystal alignment Y1 - 2016 U6 - https://doi.org/10.1080/02678292.2016.1185174 SN - 0267-8292 SN - 1366-5855 VL - 43 SP - 1514 EP - 1521 PB - Editions Rodopi BV CY - Abingdon ER - TY - JOUR A1 - Chinaglia, Dante Luis A1 - Gregorio, Rinaldo A1 - Stefanello, Josiani Cristina A1 - Altafim, Ruy Alberto Pisani A1 - Wirges, Werner A1 - Wang, Feipeng A1 - Gerhard, Reimund T1 - Influence of the solvent evaporation rate on the crystalline phases of solution-cast poly(vinylidene fluoride) films N2 - 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. Y1 - 2010 UR - http://www3.interscience.wiley.com/cgi-bin/jhome/30035 U6 - https://doi.org/10.1002/App.31488 SN - 0021-8995 ER - TY - JOUR A1 - Gerhard, Reimund A1 - Wegener, Michael A1 - Künstler, Wolfgang A1 - Wirges, Werner A1 - Görne, Thomas A1 - Urayama, D. A1 - Neher, Dieter T1 - Inverse piezoelectricity of porous PTFE films with bipolar space charge Y1 - 2000 ER - TY - JOUR A1 - Wang, Feipeng A1 - Lack, Alexander A1 - Xie, Zailai A1 - Frübing, Peter A1 - Taubert, Andreas A1 - Gerhard, Reimund T1 - Ionic-liquid-induced ferroelectric polarization in poly(vinylidene fluoride) thin films JF - Applied physics letters N2 - 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. KW - dielectric polarisation KW - ferroelectric thin films KW - polymer films KW - pyroelectricity KW - spin coating Y1 - 2012 U6 - https://doi.org/10.1063/1.3683526 SN - 0003-6951 VL - 100 IS - 6 PB - American Institute of Physics CY - Melville ER - TY - JOUR A1 - Altafim, Ruy Alberto Pisani A1 - Rychkov, Dmitry A1 - Wirges, Werner A1 - Gerhard, Reimund A1 - Basso, Heitor Cury A1 - Altafim, Ruy Alberto Pisani A1 - Melzer, Martin T1 - Laminated tubular-channel ferroelectret systems from low-density Polyethylene Films and from Fluoroethylene-propylene Copolymer Films - A comparison JF - IEEE transactions on dielectrics and electrical insulation N2 - 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. KW - Ferroelectrets KW - piezoelectrets KW - piezoelectricity KW - electro-mechanical transducers KW - static-pressure dependence KW - temperature dependence KW - frequency response Y1 - 2012 SN - 1070-9878 VL - 19 IS - 4 SP - 1116 EP - 1123 PB - Inst. of Electr. and Electronics Engineers CY - Piscataway ER - TY - JOUR A1 - Gerhard, Reimund T1 - Less can be more : holes in polymers lead to a new paradigm of piezoelectric materials for electet transducers Y1 - 2002 SN - 0018-9367, ER - TY - JOUR A1 - Gerhard, Reimund A1 - Mahler, G. T1 - Light-valve projection displays - an introduction Y1 - 1995 ER - TY - JOUR A1 - Wirges, Werner A1 - Przyrembel, G. A1 - Brinker, Walter A1 - Gerhard, Reimund A1 - Klemberg-Sapieha, J. A1 - Martinu, L. A1 - Poitras, D. A1 - Wertheimer, M. R. T1 - Metallised viscoelastic control layers for light-valve projection displays Y1 - 1995 ER - TY - JOUR A1 - Tuncer, Enis A1 - Wegener, Michael A1 - Gerhard, Reimund T1 - Modeling electro-mechanical properties of layered electrets : application of the finite-element method N2 - 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 Y1 - 2005 SN - 0304-3886 ER - TY - JOUR A1 - Li, Changsheng A1 - Chen, Gangjin A1 - Qiu, Xunlin A1 - Gao, Meng A1 - Gerhard, Reimund T1 - Modified polytetrafluoroethylene BT - towards easy-to-process space-charge electret materials JF - Applied physics express : APEX N2 - 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. Y1 - 2019 U6 - https://doi.org/10.7567/1882-0786/ab5b23 SN - 1882-0778 SN - 1882-0786 VL - 13 IS - 1 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Bauer-Gogonea, Simona A1 - Bauer, Siegfried A1 - Gerhard, Reimund T1 - Monomorphs, bimorphs and multimorphs from polar polymer electrets Y1 - 1999 ER - TY - JOUR A1 - Kofod, Guggi A1 - Stoyanov, Hristiyan A1 - Gerhard, Reimund T1 - Multilayer coaxial fiber dielectric elastomers for actuation and sensing JF - Applied physics : A, Materials science & processing N2 - 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. Y1 - 2011 U6 - https://doi.org/10.1007/s00339-010-6066-5 SN - 0947-8396 SN - 1432-0630 VL - 102 IS - 3 SP - 577 EP - 581 PB - Springer CY - New York ER - TY - JOUR A1 - Raman Venkatesan, Thulasinath A1 - Smykalla, David A1 - Ploss, Bernd A1 - Wübbenhorst, Michael A1 - Gerhard, Reimund T1 - Non-linear dielectric spectroscopy for detecting and evaluating structure-property relations in a P(VDF-TrFE-CFE) relaxor-ferroelectric terpolymer JF - Applied physics : A, Materials science & processing N2 - 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. KW - Non-linear dielectric spectroscopy KW - P(VDF-TrFE-CFE) KW - Relaxor-ferroelectric polymer KW - Dielectric hysteresis KW - Curie-transition KW - Mid-temperature transition Y1 - 2021 U6 - https://doi.org/10.1007/s00339-021-04876-0 SN - 0947-8396 SN - 1432-0630 VL - 127 IS - 10 PB - Springer CY - Berlin ; Heidelberg ; New York ER - TY - JOUR A1 - Bauer-Gogonea, Simona A1 - Gerhard, Reimund T1 - Nonlinear optical polymer electrets Y1 - 1999 ER - TY - JOUR A1 - Gerhard, Reimund T1 - Nonlinear optical polymer electrets Y1 - 1996 ER - TY - JOUR A1 - Bauer-Gogonea, Simona A1 - Gerhard, Reimund T1 - Nonlinear optical polymer electrets : current practice Y1 - 1996 ER - TY - JOUR A1 - Saarimäki, Eetta A1 - Paajanen, Mika A1 - Savijärvi, Ann-Mari A1 - Minkkinen, Hannu A1 - Wegener, Michael A1 - Voronina, Olena A1 - Schulze, Robert A1 - Wirges, Werner A1 - Gerhard, Reimund T1 - Novel heat durable electromechanical film : processing for electromechanical and electret applications N2 - 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. Y1 - 2006 ER - TY - JOUR A1 - Gerhard, Reimund A1 - Lang, Sidney B. T1 - Obituaries - Dilip Kumar Das-Gupta Y1 - 2002 ER - TY - JOUR A1 - Wirges, Werner A1 - Wegener, Michael A1 - Voronina, Olena A1 - Zirkel, Larissa A1 - Gerhard, Reimund T1 - Optimized preparation of elastically soft, highly piezoelectric, cellular ferroelectrets from nonvoided poly(ethylene terephthalate) films N2 - 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. Y1 - 2007 UR - http://onlinelibrary.wiley.com/doi/10.1002/adfm.200600162/pdf U6 - https://doi.org/10.1002/adfm.200600162 ER - TY - JOUR A1 - Tuncer, Enis A1 - Wegener, Michael A1 - Frubing, Peter A1 - Gerhard, Reimund T1 - Origin of temperature dependent conductivity of alpha-polyvinylidene fluoride N2 - The conductivity of alpha-polyvinylidene fluoride is obtained from dielectric measurements performed in the frequency domain at several temperatures. At temperatures above the glass-transition, the conductivity can be interpreted as an ionic conductivity, which confirms earlier results reported in the literature. Our investigation shows that the observed ionic conductivity is closely related to the amorphous phase of the polymer. (C) 2005 American Institute of Physics Y1 - 2005 SN - 0021-9606 ER - TY - JOUR A1 - Raman Venkatesan, Thulasinath A1 - Gerhard, Reimund T1 - Origin of the mid-temperature transition in vinylidenefluoride-based ferro-, pyro- and piezoelectric homo-, co- and ter-polymers JF - Materials Research Express N2 - 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. KW - mid-temperature transition KW - vinylidenefluoride (VDF)-based polymers KW - Differential Scanning Calorimetry (DSC) KW - Dielectric Relaxation Spectroscopy (DRS) KW - ferroelectric polymers KW - pyroelectric polymers KW - piezoelectric polymers Y1 - 2020 U6 - https://doi.org/10.1088/2053-1591/ab842c SN - 2053-1591 VL - 7 PB - IOP Publ. CY - Bristol ER - TY - JOUR A1 - Wegener, Michael A1 - Hesse, J. A1 - Wegener, T. A1 - Gerhard, Reimund T1 - Patterned ferro-, pyro- and piezoelectricity in poly(vinylindene fluoride) by means of a laser-induced irreversible beta to alpha phase transformation Y1 - 2002 ER - TY - JOUR A1 - Wegener, Michael A1 - Hesse, J. A1 - Wegener, T. A1 - Gerhard, Reimund T1 - Patterned ferro-, pyro- and piezoelectricity in poly(vinylindene fluoride) by means of a laser-induced irreversible beta to alpha phase transformation [Erratum] Y1 - 2002 ER - TY - JOUR A1 - Gerhard, Reimund A1 - Kacprzyk, Ryszard T1 - Paul Böning - early electret researcher in Shanghai and Wroclaw (1922-1945) JF - IEEE transactions on dielectrics and electrical insulation N2 - 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. KW - dielectrics KW - electrets KW - electrostatic KW - experiments KW - (high-)voltage measurements KW - space charge Y1 - 2022 U6 - https://doi.org/10.1109/TDEI.2022.3168372 SN - 1070-9878 SN - 1558-4135 VL - 29 IS - 3 SP - 853 EP - 858 PB - Institute of Electrical and Electronics Engineers CY - New York, NY ER - TY - JOUR A1 - Qiu, X. L. A1 - Wegener, Michael A1 - Wirges, Werner A1 - Zhang, X. Q. A1 - Hillenbrand, J. A1 - Xia, Zhongfu A1 - Gerhard, Reimund A1 - Sessler, G. M. T1 - Penetration of sulfur hexafluoride into cellular polypropylene films and its effect on the electric charging and electromechanical response of ferroelectrets N2 - Cellular polypropylene (PP) films were treated with sulfur hexafluoride (SF6) gas in order to study the SF6 penetration behaviour and optimize the electric charging conditions. There were differences in the penetration of SF6 for different cellular PP materials, depending on the microscopic properties, which manifest themselves in the voided structure as well as in the mechanical stiffnesses of the cellular films. The penetration of SF6 after long-term pressure treatment is confirmed in strongly inflated cellular PP films with a low mechanical stiffness of about 1 MPa. No SF6 penetration occurs for slightly inflated cellular PP films with smaller void sizes and higher mechanical stiffnesses of around 5.8 MPa. The observed thickness variations, the higher charging fields during corona charging because of SF6 penetration and the SF6 environment, as well as the resulting electromechanical properties are discussed Y1 - 2005 SN - 0022-3727 ER - TY - JOUR A1 - Brinker, Walter A1 - Yilmaz, Sükrü A1 - Wirges, Werner A1 - Bauer, Siegfried A1 - Gerhard, Reimund T1 - Phase-shift interference microscope for the investigation of dipoleorientation distributions Y1 - 1995 ER - TY - JOUR A1 - Gonzalez, Francisco Camacho A1 - Mellinger, Axel A1 - Gerhard, Reimund A1 - Santos, Lucas F. A1 - Faria, Roberto M. T1 - Photo-stimulated discharge of highly insulating polymers (PTFE and PETP) Y1 - 2002 SN - 0-7803-7502-5 ER - TY - JOUR A1 - Mellinger, Axel A1 - Camacho González, Francisco A1 - Gerhard, Reimund T1 - Photostimulated discharge in electret polymers : an alternative approach for investigating deep traps N2 - The stability of space charge in electrets such as polytetrafluoroethylene (PTFE), polyethylene terephthalate (PETP) and polypropylene (PP) under ultraviolet irradiation has been investigated using photostimulated discharge spectroscopy. While only weak discharge currents were observed in PTFE coated with semitransparent gold electrodes, up to 15 pA/cm(2) were found in PETP around the UV absorption edge near 310 nm. Space charge profiles obtained with the piezoelectrically generated pressure step method indicate that near-surface charges were almost completely removed. In PP foam, recent findings of a UV-reduced d(33) coefficient were confirmed for exposure times of up to 3.5 h, and a discharge peak at 200 urn could be assigned to the charges stored on the surfaces of the voids. The unique morphology and the (quasi-) piezoelectric properties of cellular PP make it a role model for the future investigation of charge storage in electrets Y1 - 2004 ER -