@article{QiuHollaenderWirgesetal.2013,
  author    = {Qiu, Xunlin and Holl{\"a}nder, Lars and Wirges, Werner and Gerhard, Reimund and Basso, Heitor Cury},
  title     = {Direct hysteresis measurements on ferroelectret films by means of a modified Sawyer-Tower circuit},
  series = {Journal of applied physics},
  volume    = {113},
  journal   = {Journal of applied physics},
  number    = {22},
  publisher = {American Institute of Physics},
  address   = {Melville},
  issn      = {0021-8979},
  doi       = {10.1063/1.4809556},
  pages     = {8},
  year      = {2013},
  abstract  = {Ferro- and piezo-electrets are non-polar polymer foams or film systems with internally charged cavities. Since their invention more than two decades ago, ferroelectrets have become a welcome addition to the range of piezo-, pyro-, and ferro-electric materials available for device applications. A polarization-versus-electric-field hysteresis is an essential feature of a ferroelectric material and may also be used for determining some of its main properties. Here, a modified Sawyer-Tower circuit and a combination of unipolar and bipolar voltage waveforms are employed to record hysteresis curves on cellular-foam polypropylene ferroelectret films and on tubular-channel fluoroethylenepropylene copolymer ferroelectret film systems. Internal dielectric barrier discharges (DBDs) are required for depositing the internal charges in ferroelectrets. The true amount of charge transferred during the internal DBDs is obtained from voltage measurements on a standard capacitor connected in series with the sample, but with a much larger capacitance than the sample. Another standard capacitor with a much smaller capacitance-which is, however, still considerably larger than the sample capacitance-is also connected in series as a high-voltage divider protecting the electrometer against destructive breakdown. It is shown how the DBDs inside the polymer cavities lead to phenomenological hysteresis curves that cannot be distinguished from the hysteresis loops found on other ferroic materials. The physical mechanisms behind the hysteresis behavior are described and discussed.},
  language  = {en}
}
@article{BassoQiuWirgesetal.2013,
  author    = {Basso, Heitor Cury and Qiu, Xunlin and Wirges, Werner and Gerhard, Reimund},
  title     = {Temporal evolution of the re-breakdown voltage in small gaps from nanoseconds to milliseconds},
  series = {Applied physics letters},
  volume    = {102},
  journal   = {Applied physics letters},
  number    = {1},
  publisher = {American Institute of Physics},
  address   = {Melville},
  issn      = {0003-6951},
  doi       = {10.1063/1.4773518},
  pages     = {5},
  year      = {2013},
  abstract  = {A detailed understanding of electric breakdown in dielectrics is of scientific and technological interest. In gaseous dielectrics, a so-called re-breakdown is sometimes observed after extinction of the previous discharge. Although time-dependent re-breakdown voltage is essentially known, its behavior immediately after the previous discharge is not precisely understood. We present an electronic circuit for accurate measurements of the time-dependent re-breakdown voltage in small gaps from tens of nanoseconds to several milliseconds after the previous spark. Results from such experiments are compared with earlier findings, and relevant physical mechanisms such as heating of the gas, decay of the plasma, and ionization of excited atoms and molecules are discussed. It is confirmed that the thermal model is not valid at times below several microseconds.},
  language  = {en}
}