@article{QiuGerhardMellinger2011,
  author    = {Qiu, Xunlin and Gerhard, Reimund and Mellinger, Axel},
  title     = {Turning polymer foams or polymer-film systems into ferroelectrets dielectric barrier discharges in voids},
  series = {IEEE transactions on dielectrics and electrical insulation},
  volume    = {18},
  journal   = {IEEE transactions on dielectrics and electrical insulation},
  number    = {1},
  publisher = {Inst. of Electr. and Electronics Engineers},
  address   = {Piscataway},
  issn      = {1070-9878},
  doi       = {10.1109/TDEI.2011.5704490},
  pages     = {34 -- 42},
  year      = {2011},
  abstract  = {Polymer foams and void-containing polymer-film systems with internally charged voids combine large piezoelectricity with mechanical flexibility and elastic compliance. This new class of soft materials (often called ferro-or piezoelectrets) has attracted considerable attention from science and industry. It has been found that the voids can be internally charged by means of dielectric barrier discharges (DBDs) under high electric fields. The charged voids can be considered as man-made macroscopic dipoles. Depending on the ferroelectret structure and the pressure of the internal gas, the voids may be highly compressible. Consequently, very large dipole-moment changes can be induced by mechanical or electrical stresses, leading to large piezoelectricity. DBD charging of the voids is a critical process for rendering polymer foams piezoelectric. Thus a comprehensive exploration of DBD charging is essential for the understanding and the optimization of piezoelectricity in ferroelectrets. Recent studies show that DBDs in the voids are triggered when the internal electric field reaches a threshold value according to Townsend's model of Paschen breakdown. During the DBDs, charges of opposite polarity are generated and trapped at the top and bottom internal surfaces of the gas-filled voids, respectively. The deposited charges induce an electric field opposite to the externally applied one and thus extinguish the DBDs. Back discharges may eventually be triggered when the external voltage is reduced or turned off. In order to optimize the efficiency of DBD charging, the geometry (in particular the height) of the voids, the type of gas and its pressure inside the voids are essential factors to be considered and to be optimized. In addition, the influence of the plasma treatment on the internal void surfaces during the DBDs should be taken into consideration.},
  language  = {en}
}
@article{FangMaLietal.2017,
  author    = {Fang, Peng and Ma, Xingchen and Li, Xiangxin and Qiu, Xunlin and Gerhard, Reimund and Zhang, Xiaoqing and Li, Guanglin},
  title     = {Fabrication, Structure Characterization, and Performance Testing of Piezoelectret-Film Sensors for Recording Body Motion},
  series = {IEEE Sensors Journal},
  volume    = {18},
  journal   = {IEEE Sensors Journal},
  number    = {1},
  publisher = {Inst. of Electr. and Electronics Engineers},
  address   = {Piscataway},
  issn      = {1530-437X},
  doi       = {10.1109/JSEN.2017.2766663},
  pages     = {401 -- 412},
  year      = {2017},
  abstract  = {During muscle contractions, radial-force distributions are generated on muscle surfaces due to muscle-volume changes, from which the corresponding body motions can be recorded by means of so-called force myography (FMG). Piezo- or ferroelectrets are flexible piezoelectric materials with attractive materials and sensing properties. In addition to several other applications, they are suitable for detecting force variations by means of wearable devices. In this paper, we prepared piezoelectrets from cellular polypropylene films by optimizing the fabrication procedures, and developed an FMG-recording system based on piezoelectret sensors. Different hand and wrist movements were successfully detected on able-bodied subjects with the FMG system. The FMG patterns were evaluated and identified by means of linear discriminant analysis and artificial neural network algorithms, and average motion-classification accuracies of 96.1\% and 94.8\%, respectively, were obtained. This paper demonstrates the feasibility of using piezoelectret-film sensors for FMG and may thus lead to alternative methods for detecting body motion and to related applications, e.g., in biomedical engineering or structural-health monitoring.},
  language  = {en}
}
@article{FangHollaenderWirgesetal.2012,
  author    = {Fang, Peng and Hollaender, Lars and Wirges, Werner and Gerhard, Reimund},
  title     = {Piezoelectric d(33) coefficients in foamed and layered polymer piezoelectrets from dynamic mechano-electrical experiments, electro-mechanical resonance spectroscopy and acoustic-transducer measurements},
  series = {Measurement science and technology},
  volume    = {23},
  journal   = {Measurement science and technology},
  number    = {3},
  publisher = {IOP Publ. Ltd.},
  address   = {Bristol},
  issn      = {0957-0233},
  doi       = {10.1088/0957-0233/23/3/035604},
  pages     = {9},
  year      = {2012},
  abstract  = {Piezoelectrets are novel transducer materials which can be widely applied in sensors and actuators. Here, three techniques for determining piezoelectric d(33) coefficients of piezoelectrets are reviewed and compared. Two types of piezoelectrets, polyethylene-naphthalate (PEN) polymer-foam piezoelectrets and fluorinated ethylene-propylene (FEP) copolymer-layer piezoelectrets, have been prepared and measured by means of dynamic, resonance, and acoustical methods. The dynamic measurements show that the d(33) coefficient of PEN-foam samples clearly decreases with increasing stress, but 80\% of the initial d(33) can be retained after 1800 cycles of a continuous dynamic measurement in a mechanical fatigue test. The resonance measurements demonstrate that both PEN-foam and FEP-layer samples exhibit clear electro-mechanical resonances. PEN-foam samples show elastic moduli in the range from 1 to 12 MPa and d(33) values up to 500 pC N-1, while FEP-layer samples show homogeneous elastic moduli of about 0.3 MPa and d(33) values of about 280 pC N-1. The acoustical measurements reveal that both PEN-foam and FEP-layer samples exhibit stable frequency responses in the range from 5.7 to 20 kHz. In addition, d(33) coefficients obtained with different experimental methods are in good agreement with each other, which confirms the reliability of all three techniques.},
  language  = {en}
}