@article{SunZhangXiaetal.2011,
  author    = {Sun, Zhuanlan and Zhang, Xiaoqing and Xia, Zhongfu and Qiu, Xunlin and Wirges, Werner and Gerhard, Reimund and Zeng, Changchun and Zhang, Chuck and Wang, Ben},
  title     = {Polarization and piezoelectricity in polymer films with artificial void structure},
  series = {Applied physics : A, Materials science \& processing},
  volume    = {105},
  journal   = {Applied physics : A, Materials science \& processing},
  number    = {1},
  publisher = {Springer},
  address   = {New York},
  issn      = {0947-8396},
  doi       = {10.1007/s00339-011-6481-2},
  pages     = {197 -- 205},
  year      = {2011},
  abstract  = {Laminated polymer-film systems with well-defined void structures were prepared from fluoroethylenepropylene (FEP) and polytetrafluoroethylene (PTFE) layers. First the PTFE films were patterned and then fusion-bonded with the FEP films. The laminates were subjected to either corona or contact charging in order to obtain the desired piezoelectricity. The build-up of the "macro-dipoles" in the laminated films was studied by recording the electric hysteresis loops. The resulting electro-mechanical properties were investigated by means of dielectric resonance spectroscopy (DRS) and direct measurements of the stress-strain relationship. Moreover, the thermal stability of the piezoelectric d (33) coefficient was investigated at elevated temperatures and via thermally stimulated discharge (TSD) current measurements in short circuit. For 150 mu m thick laminated films, consisting of one 25 mu m thick PTFE layer, two 12.5 mu m thick FEP layers, and a void of 100 mu m height, the critical voltage necessary for the build-up of the "macro-dipoles" in the inner voids was approximately 1400 V, which agrees with the value calculated from the Paschen Law. A quasi-static piezoelectric d (33) coefficient up to 300 pC/N was observed after corona charging. The mechanical properties of the film systems are highly anisotropic. At room temperature, the Young's moduli of the laminated film system are around 0.37 MPa in the thickness direction and 274 MPa in the lateral direction, respectively. Using these values, the theoretical shape anisotropy ratio of the void was calculated, which agrees well with experimental observation. Compared with films that do not exhibit structural regularity, the laminates showed improved thermal stability of the d (33) coefficients. The thermal stability of d (33) can be further improved by pre-aging. E.g., the reduction of the d (33) value in the sample pre-aged at 150A degrees C for 5 h was less than 5\% after annealing for 30 h at a temperature of 90A degrees C.},
  language  = {en}
}
@article{GerhardXiaKuenstleretal.1999,
  author    = {Gerhard, Reimund and Xia, Zhongfu and K{\"u}nstler, Wolfgang and Pucher, Andreas},
  title     = {Preliminary study of multi-layer space-charge electrets with piezoelectric properties from porous and non- porous Teflon films},
  year      = {1999},
  language  = {en}
}
@article{XiaGerhardKuenstleretal.1999,
  author    = {Xia, Zhongfu and Gerhard, Reimund and K{\"u}nstler, Wolfgang and Wedel, Armin and Danz, Rudi},
  title     = {High surface-charge stability of porous polytetrafluoroethylene electret films at room and elvated temperatures},
  year      = {1999},
  language  = {en}
}
@article{KuenstlerXiaWeinholdetal.2000,
  author    = {K{\"u}nstler, Wolfgang and Xia, Zhongfu and Weinhold, Till and Pucher, Andreas and Gerhard, Reimund},
  title     = {Piezoelectricity of porous polytetrafluoroethylene single- and multiple-film electrets containing high charge densities of both polarities},
  year      = {2000},
  language  = {en}
}
@article{GerhardKuenstlerGoerneetal.2000,
  author    = {Gerhard, Reimund and K{\"u}nstler, Wolfgang and G{\"o}rne, Thomas and Pucher, Andreas and Weinhold, Till and Seiß, Martin and Xia, Zhongfu and Wedel, Armin and Danz, Rudi},
  title     = {Porous polytetrafluoroethylene space-charge electrets for piezoelectrical applications},
  year      = {2000},
  language  = {en}
}
@article{QiuWegenerWirgesetal.2005,
  author    = {Qiu, X. L. and Wegener, Michael and Wirges, Werner and Zhang, X. Q. and Hillenbrand, J. and Xia, Zhongfu and Gerhard, Reimund and Sessler, G. M.},
  title     = {Penetration of sulfur hexafluoride into cellular polypropylene films and its effect on the electric charging and electromechanical response of ferroelectrets},
  issn      = {0022-3727},
  year      = {2005},
  abstract  = {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},
  language  = {en}
}