@article{FangQiuWirgesetal.2010,
  author    = {Fang, Peng and Qiu, Xunlin and Wirges, Werner and Gerhard, Reimund and Zirkel, Larissa},
  title     = {Polyethylene-naphthalate (PEN) ferroelectrets : cellular structure, piezoelectricity and thermal stability},
  issn      = {1070-9878},
  doi       = {10.1109/TDEI.2010.5539678},
  year      = {2010},
  abstract  = {Cellular polyethylene-naphthalate (PEN) ferroelectrets are useful as soft and flexible electromechanical transducer materials. Improved cellular PEN foams are prepared by means of a "voiding + inflation + stretching" process and investigated with respect to their structure and their applications-relevant properties. It is found that most of the cellular voids have heights below 8 mu m. The polymer walls do not allow sufficient gas exchange between the voids and the ambient atmosphere, when the cellular films are exposed to atmospheric pressures between a millibar and a few bars. As expected for ferroelectrets, a threshold voltage for charging is observed: A reasonable piezoelectric coefficient d(33) is only found when the charging voltage is higher than 4 kV. Furthermore, d(33) increases with charging voltage and reaches saturation at approximately 8 kV. Annealing after charging or charging at elevated temperatures may enhance the thermal stability of the PEN ferroelectrets. The d(33) of properly annealed samples is stable up to the respective annealing temperatures, but the annealing process reduces the piezoelectric activity of charged ferroelectret films to some extent. Samples charged at suitable elevated temperatures show much better thermal stability than those charged at room temperature, but the charging temperature should be limited to values below the material's glass-transition temperature T-g. Furthermore, the relevant elastic modulus c(33) of PEN ferroelectrets may decrease upon thermal treatment.},
  language  = {en}
}
@article{WirgesWegenerVoroninaetal.2007,
  author    = {Wirges, Werner and Wegener, Michael and Voronina, Olena and Zirkel, Larissa and Gerhard, Reimund},
  title     = {Optimized preparation of elastically soft, highly piezoelectric, cellular ferroelectrets from nonvoided poly(ethylene terephthalate) films},
  doi       = {10.1002/adfm.200600162},
  year      = {2007},
  abstract  = {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.},
  language  = {en}
}
@article{FangWegenerWirgesetal.2007,
  author    = {Fang, Peng and Wegener, Michael and Wirges, Werner and Gerhard, Reimund and Zirkel, Larissa},
  title     = {Cellular polyethylene-naphthalate ferroelectrets : foaming in supercritical carbon dioxide, structural and electrical preparation, and resulting piezoelectricity},
  issn      = {0003-6951},
  doi       = {10.1063/1.2738365},
  year      = {2007},
  abstract  = {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.},
  language  = {en}
}