Ferroelectric Polymers for Electrical Energy Storage

Open Access
Author:
Claude, Jason W.
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
August 22, 2008
Committee Members:
  • Qing Wang, Committee Chair
  • Ralph H Colby, Committee Member
  • Michael T Lanagan, Committee Member
  • Qiming Zhang, Committee Member
Keywords:
  • vinylidene fluoride
  • fluoropolymer
  • energy storage
  • electrical breakdown
  • capacitor
  • dielectric
Abstract:
The energy storage properties of vinylidene fluoride based fluoropolymers were explored. Energy density is a function of a materials permittivity and electrical breakdown strength. High values of each of these parameters are desirable for a high energy density and were explored in various fluoropolymer systems. Copolymers containing vinylidene fluoride (VDF), chlorofluoroethylene (CTFE), and trifluoroethylene (TrFE) were synthesized by a two-step approach beginning with the copolymerization of VDF and CTFE and the subsequent hydrogenation of the CTFE units to TrFE to create the terpolymer P(VDF-CTFE-TrFE). By changing the chemical composition of the fluoropolymers, the permittivity was varied from 12 to 50 due to changes in the crystal phase that converted the polymers from paraelectric to ferroelectric materials. The electrical breakdown mechanisms of a single copolymer composition of P(VDF-CTFE) was studied as a function of molecular weight and temperature. Energy density and breakdown strength increased as molecular weight increased and temperature decreased. An electromechanical breakdown mechanism was responsible for failure at 25C while a thermal breakdown mechanism operated at -35C which was below the glass transition of the material. In between at -15C, a combination of the two mechanisms was found to operate. Electromechanical breakdown was also found to operate in a copolymer system with a fixed amount of VDF and varying amounts of TrFE and CTFE. The molecular weights were identical for all the polymers. Maxwell stress is the primary contributor to the electromechanical stress in polymers with a high amount the CTFE. Electrostrictive stress due to a crystal phase change at high electric fields is a major contributor to the electromechanical stress in polymers containing a high amount of TrFE. Energy density and electrical breakdown strength increased with increasing amounts of TrFE. Nanometer sized silica particles were incorporated into a P(VDF-CTFE) matrix using an in-situ sol-gel process. Electrical breakdown strengths and energy densities were maximized in hybrids containing an optimized amount of silica due to the creation of new charge trap sites. These sites reduced space charge and electric loss delaying thermal breakdown in the hybrids. For optimized composites, high energy densities over 20 J/cm3 were achieved.