TY - JOUR A1 - Sun, Zhuanlan A1 - Zhang, Xiaoqing A1 - Xia, Zhongfu A1 - Qiu, Xunlin A1 - Wirges, Werner A1 - Gerhard, Reimund A1 - Zeng, Changchun A1 - Zhang, Chuck A1 - Wang, Ben T1 - Polarization and piezoelectricity in polymer films with artificial void structure JF - Applied physics : A, Materials science & processing N2 - Laminated polymer-film systems with well-defined void structures were prepared from fluoroethylenepropylene (FEP) and polytetrafluoroethylene (PTFE) layers. First the PTFE films were patterned and then fusion-bonded with the FEP films. The laminates were subjected to either corona or contact charging in order to obtain the desired piezoelectricity. The build-up of the "macro-dipoles" in the laminated films was studied by recording the electric hysteresis loops. The resulting electro-mechanical properties were investigated by means of dielectric resonance spectroscopy (DRS) and direct measurements of the stress-strain relationship. Moreover, the thermal stability of the piezoelectric d (33) coefficient was investigated at elevated temperatures and via thermally stimulated discharge (TSD) current measurements in short circuit. For 150 mu m thick laminated films, consisting of one 25 mu m thick PTFE layer, two 12.5 mu m thick FEP layers, and a void of 100 mu m height, the critical voltage necessary for the build-up of the "macro-dipoles" in the inner voids was approximately 1400 V, which agrees with the value calculated from the Paschen Law. A quasi-static piezoelectric d (33) coefficient up to 300 pC/N was observed after corona charging. The mechanical properties of the film systems are highly anisotropic. At room temperature, the Young's moduli of the laminated film system are around 0.37 MPa in the thickness direction and 274 MPa in the lateral direction, respectively. Using these values, the theoretical shape anisotropy ratio of the void was calculated, which agrees well with experimental observation. Compared with films that do not exhibit structural regularity, the laminates showed improved thermal stability of the d (33) coefficients. The thermal stability of d (33) can be further improved by pre-aging. E.g., the reduction of the d (33) value in the sample pre-aged at 150A degrees C for 5 h was less than 5% after annealing for 30 h at a temperature of 90A degrees C. Y1 - 2011 U6 - https://doi.org/10.1007/s00339-011-6481-2 SN - 0947-8396 VL - 105 IS - 1 SP - 197 EP - 205 PB - Springer CY - New York ER - TY - JOUR A1 - Rychkov, Dmitry A1 - Gerhard, Reimund T1 - Stabilization of positive charge on polytetrafluoroethylene electret films treated with titanium-tetrachloride vapor JF - Applied physics letters N2 - The surface of polytetrafluoroethylene films was treated with titanium-tetrachloride vapor. The treatment was carried out in a flow reactor by means of molecular-layer deposition, a method from the arsenal of chemical nanotechnology. X-ray photoelectron spectroscopy reveals that such a treatment results in considerable changes in the chemical composition at and near the surface of the fluoropolymer film. Both, defluorination and oxidation of the surface were observed. At the same time, samples treated with titanium tetrachloride show a significant enhancement in the thermal stability of the positive homocharge. The thermally stimulated surface-potential-decay curves were observed to shift to higher temperatures by more than 100 degrees C. Y1 - 2011 U6 - https://doi.org/10.1063/1.3565166 SN - 0003-6951 VL - 98 IS - 12 PB - American Institute of Physics CY - Melville 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 - Qiu, Xunlin A1 - Gerhard, Reimund A1 - Mellinger, Axel T1 - Turning polymer foams or polymer-film systems into ferroelectrets dielectric barrier discharges in voids JF - IEEE transactions on dielectrics and electrical insulation N2 - Polymer foams and void-containing polymer-film systems with internally charged voids combine large piezoelectricity with mechanical flexibility and elastic compliance. This new class of soft materials (often called ferro-or piezoelectrets) has attracted considerable attention from science and industry. It has been found that the voids can be internally charged by means of dielectric barrier discharges (DBDs) under high electric fields. The charged voids can be considered as man-made macroscopic dipoles. Depending on the ferroelectret structure and the pressure of the internal gas, the voids may be highly compressible. Consequently, very large dipole-moment changes can be induced by mechanical or electrical stresses, leading to large piezoelectricity. DBD charging of the voids is a critical process for rendering polymer foams piezoelectric. Thus a comprehensive exploration of DBD charging is essential for the understanding and the optimization of piezoelectricity in ferroelectrets. Recent studies show that DBDs in the voids are triggered when the internal electric field reaches a threshold value according to Townsend's model of Paschen breakdown. During the DBDs, charges of opposite polarity are generated and trapped at the top and bottom internal surfaces of the gas-filled voids, respectively. The deposited charges induce an electric field opposite to the externally applied one and thus extinguish the DBDs. Back discharges may eventually be triggered when the external voltage is reduced or turned off. In order to optimize the efficiency of DBD charging, the geometry (in particular the height) of the voids, the type of gas and its pressure inside the voids are essential factors to be considered and to be optimized. In addition, the influence of the plasma treatment on the internal void surfaces during the DBDs should be taken into consideration. KW - Ferroelectret KW - piezoelectret KW - dielectric barrier discharge (DBD) KW - effective polarization KW - man-made dipole Y1 - 2011 U6 - https://doi.org/10.1109/TDEI.2011.5704490 SN - 1070-9878 VL - 18 IS - 1 SP - 34 EP - 42 PB - Inst. of Electr. and Electronics Engineers CY - Piscataway 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 - 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 - 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 - Kalbitz, Rene A1 - Frübing, Peter A1 - Gerhard, Reimund A1 - Taylor, D. M. T1 - Stability of polarization in organic ferroelectric metal-insulator-semiconductor structures JF - Applied physics letters N2 - Dielectric measurements have been carried out on all-organic metal-insulator-semiconductor structures with the ferroelectric polymer poly(vinylidenefluoride-trifluoroethylene) as the gate insulator. It is shown that the polarization states remain stable after poling with accumulation and depletion voltage. However, negative charge trapped at the semiconductor-insulator interface during the depletion cycle masks the negative shift in flatband voltage expected during the sweep to accumulation voltages. Y1 - 2011 U6 - https://doi.org/10.1063/1.3543632 SN - 0003-6951 VL - 98 IS - 3 PB - American Institute of Physics CY - Melville ER - TY - GEN A1 - Gerhard, Reimund T1 - Honoring Professor Gerhard M. Sessler on his 80th Birthday (15th of February 2011) T2 - IEEE transactions on dielectrics and electrical insulation Y1 - 2011 U6 - https://doi.org/10.1109/TDEI.2011.5704485 SN - 1070-9878 VL - 18 IS - 1 SP - 1 EP - 2 PB - Inst. of Electr. and Electronics Engineers CY - Piscataway ER - TY - JOUR A1 - Fang, Peng A1 - Wang, Feipeng A1 - Wirges, Werner A1 - Gerhard, Reimund A1 - Basso, Heitor Cury T1 - Three-layer piezoelectrets from fluorinated ethylene-propylene (FEP) copolymer films JF - Applied physics : A, Materials science & processing N2 - A process for preparing three-layer piezoelectrets from fluorinated ethylene-propylene (FEP) copolymer films is introduced. Samples are made from commercial FEP films by means of laser cutting, laser bonding, electrode evaporation, and high-field poling. The observed dielectric-resonance spectra demonstrate the piezoelectricity of the FEP sandwiches. Piezoelectric d (33) coefficients up to a few hundred pC/N are achieved. Charging at elevated temperatures can increase the thermal stability of the piezoelectrets. Isothermal experiments for approximately 15 min demonstrate that samples charged at 140A degrees C keep their piezoelectric activity up to at least 120A degrees C and retain 70% of their initial d (33) even at 130A degrees C. Acoustical measurements show a relatively flat frequency response in the range between 300 Hz and 20 kHz. Y1 - 2011 U6 - https://doi.org/10.1007/s00339-010-6008-2 SN - 0947-8396 SN - 1432-0630 VL - 103 IS - 2 SP - 455 EP - 461 PB - Springer CY - New York ER -