@article{MellingerSinghWegeneretal.2005,
  author    = {Mellinger, Axel and Singh, Rajeev and Wegener, Michael and Wirges, Werner and Gerhard, Reimund and Lang, Sidney B.},
  title     = {Three-dimensional mapping of polarization profiles with thermal pulses},
  issn      = {0003-6951},
  year      = {2005},
  abstract  = {High-resolution, large-area three-dimensional mapping of polarization profiles in electret polymers was carried out by means of a fast thermal pulse technique with a focused laser beam. A lateral resolution of 38 mu m and a near- surface depth resolution of less than 0.5 mu m was achieved. At larger depths, fast thermal diffusion in the metal electrode rather than the laser spot size becomes the limiting factor for the lateral resolution. (c) 2005 American Institute of Physics},
  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}
}
@article{TuncerWegenerFrubingetal.2005,
  author    = {Tuncer, Enis and Wegener, Michael and Frubing, Peter and Gerhard, Reimund},
  title     = {Origin of temperature dependent conductivity of alpha-polyvinylidene fluoride},
  issn      = {0021-9606},
  year      = {2005},
  abstract  = {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},
  language  = {en}
}
@article{TuncerWegenerGerhard2005,
  author    = {Tuncer, Enis and Wegener, Michael and Gerhard, Reimund},
  title     = {Modeling electro-mechanical properties of layered electrets : application of the finite-element method},
  issn      = {0304-3886},
  year      = {2005},
  abstract  = {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},
  language  = {en}
}
@article{TuncerWegenerGerhard2005,
  author    = {Tuncer, Enis and Wegener, Michael and Gerhard, Reimund},
  title     = {Distribution of relaxation times in alpha-phase polyvinylidene fluoride},
  issn      = {0022-3093},
  year      = {2005},
  abstract  = {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},
  language  = {en}
}
@article{WegenerBergweilerWirgesetal.2005,
  author    = {Wegener, Michael and Bergweiler, Steffen and Wirges, Werner and Pucher, Andreas and Tuncer, Enis and Gerhard, Reimund},
  title     = {Piezoelectric two-layer stacks of cellular polypropylene ferroelectrets : transducer response at audio and ultrasound frequencies},
  issn      = {0885-3010},
  year      = {2005},
  abstract  = {Piezoelectric cellular polypropylene films, so-called ferroelectrets, are assembled in a stack with two active transducer layers. The stack is characterized with respect to its linear and quadratic response in a frequency range from 1 kHz to 80 kHz. A relatively smooth frequency response in the sound-pressure level is found for the individual layers as well as for both layers driven in phase. The piezoelectric response of the two-layer stack is twice the response of an individual layer over a rather broad frequency range. Furthermore, the influence of the preparation conditions on the resonance frequency and the effect of the quadratic distortion on the radiated sound are investigated both for the individual transducer films in the stack and for the stack system as a whole},
  language  = {en}
}
@article{WegenerWirgesGerhard2005,
  author    = {Wegener, Michael and Wirges, Werner and Gerhard, Reimund},
  title     = {Piezoelectric polyethylene terephthalate (PETP) foams : specifically designed and prepared ferroelectret films},
  year      = {2005},
  language  = {en}
}