Inception and development of micro-descharges in the voids of ferroelectrets during charging

• Applicant: Dr. Xunlin Qiu
• Subject Area: Experimental Condensed Matter Physics
• Term: from 2007 to 2009
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 39440867

• Final Report Year: 2009

Final Report Abstract

Capacitance changes of cellular polypropylene (PP) films in high vacuum and at high pressures indicate that air can escape from or penetrate into the voids, respectively, as a result of gas diffusion. In order to understand the generation, deposition and storage of space charge in airfilled voids, samples were subjected to high electric fields, while the light emission and the piezoelectric activity were monitored in situ. Timeresolved images obtained with an EMCCD photon-counting camera show discharge processes in individual voids. A spectroscopic study of the microplasma discharges reveals electronically excited N2 molecules and N2+ molecular ions. The vibronic band strengths indicate an internal electric field of approx. 25 MV/m during charging, consistent with Townsend's model of Paschen breakdown. There is a strong correlation between the emitted light and the piezoelectric d33 coefficient. When the external electric field is reduced back to zero after completion of the charging process, the field built up by the deposited charges triggers back discharges, thus destroying part of the piezoelectric activity. A polarization-voltage (P-V) hysteresis loop is obtained by acoustical measurements in combination with dielectric resonance spectroscopy. Suppressing the back discharges could provide a pathway to a significantly enhanced piezoelectric activity. The influence of the internal gas pressure during charging on the piezoelectricity of ferroelectrets has also been studied. Optimal piezoelectricity is achieved by charging at a pressure of 170 kPa. This is explained with the void-height distribution in combination with Townsend’s model of the Paschen breakdown. The repeated dielectric barrier discharges in air result in significant fatigue of the polarization and the piezoelectricity due to plasma-surface interactions at the internal void surfaces. The fatigue rate strongly depends on the amplitude and the number of the voltage cycles, whereas the frequency and the waveform do not have a strong influence, which indicates the influence of molecular dynamics and physico-chemical processes at the internal polymer surfaces.

Publications

• "Dielectric barrier discharges during the generation of ferroelectrets: Optical spectroscopy for process monitoring", IEEE, 2007 Annual Report Conference on Electrical Insulation and Dielectric Phenomena, 1-4244–1482-2/07, 828-831 (2007)
  Xunlin Qiu, Axel Mellinger, Werner Wirges, and Reimund Gerhard
  
• "Spectroscopic study of dielectric barrier discharges in cellular polypropylene ferroelectrets", Applied Physics Letters 91, 132905, 3pp. (2007)
  Xunlin Qiu, Axel Mellinger, Werner Wirges, and Reimund Gerhard
  
• Barrier discharges in cellular polypropylene ferroelectrets: How do they influence the electromechanical properties? Journal of Applied Physics 101, 104112, 7pp. (2007)
  Xunlin Qiu, Axel Mellinger, Michael Wegener, Werner Wirges, and Reimund Gerhard
  
• "Effective polarization fatigue from repeated dielectric barrier discharges in cellular polypropylene ferroelectrets", Applied Physics Letters 93, 152902, 3pp. (2008)
  Xunlin Qiu and Reimund Gerhard
  
• "Influence of gas pressure in the voids during charging on the piezoelectricity of ferroelectrets", Applied Physics Letters 92, 052901, 3pp. (2008)
  Xunlin Qiu, Axel Mellinger, and Reimund Gerhard
  
• “In-situ acoustical investigation of the polarization build-up in cellular polypropylene ferroelectrets”, IEEE Transactions on Dielectrics and Electrical Insulation
  Xunlin Qiu, Reimund Gerhard, and Axel Mellinger