@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{QiuWirgesGerhard2011,
  author    = {Qiu, Xunlin and Wirges, Werner and Gerhard, Reimund},
  title     = {Beneficial and detrimental fatigue effects of dielectric barrier discharges on the piezoelectricity of polypropylene ferroelectrets},
  series = {Journal of applied physics},
  volume    = {110},
  journal   = {Journal of applied physics},
  number    = {2},
  publisher = {American Institute of Physics},
  address   = {Melville},
  issn      = {0021-8979},
  doi       = {10.1063/1.3610507},
  pages     = {8},
  year      = {2011},
  abstract  = {Cellular polypropylene (PP) ferroelectrets combine a large piezoelectricity with mechanical flexibility and elastic compliance. Their charging process represents a series of dielectric barrier discharges (DBDs) that generate a cold plasma with numerous active species and thus modify the inner polymer surfaces of the foam cells. Both the threshold for the onset of DBDs and the piezoelectricity of ferroelectrets are sensitive to repeated DBDs in the voids. It is found that the threshold voltage is approximately halved and the charging efficiency is clearly improved after only 10(3) DBD cycles. However, plasma modification of the inner surfaces from repeated DBDs deteriorates the chargeability of the voids, leading to a significant reduction of the piezoelectricity in ferroelectrets. After a significant waiting period, the chargeability of previously fatigued voids shows a partial recovery. The plasma modification is, however, detrimental to the stability of the deposited charges and thus also of the macroscopic dipoles and of the piezoelectricity. Fatigue from only 10(3) DBD cycles already results in significantly less stable piezoelectricity in cellular PP ferroelectrets. The fatigue rate as a function of the number of voltage cycles follows a stretched exponential. Fatigue from repeated DBDs can be avoided if most of the gas molecules inside the voids are removed via a suitable evacuation process.},
  language  = {en}
}