Experimental studies on physical deterioration and electrical fatigue behavior in ferroelectric polymers
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Ferroelectric materials are widely used in various electronic applications based upon their excellent electrical bi-stabilities and dielectric performance in response to the applied electric field. They have been utilized to make nonvolatile electronic memories by exploiting the hysteretic behavior and high energy density capacitors in regard to the high capability of electrical energy storage. One critical issue is that the ferroelectrics are required to endure a large number of electrical cycles. A large body of scientific efforts has been devoted to high fatigue failure resistance of ferroelectric-based electronic devices. Fatigue failure of ferroelectric materials still needs to be solved. It is the objective of this work to explore the intrinsic origin of fatigue failure mechanisms. In this study, it was found that electric-field-induced stress relaxation in α-phase poly(vinylidene fluoride) (PVDF) films can be well described by using the Kohlraush function groups, also known as the stretched exponential relaxation function. The electric strength of the dielectric is strongly dependent on its elastic properties due to the electromechanical coupling effect. Our fitting result of the stretched exponent is in accordance with a Weibull cumulative distribution function. This indicates that the elastic properties of insulating polymers are crucial to the capability of electrical energy storage. In ferroelectric materials, the electromechanical coupling may be indicative of the microscopic origin of polarization fatigue. Further experiments were focused on the polarization fatigue in semi-crystalline poly(vinylidene fluoride trifluoroethylene) [P(VDF-TrFE)] copolymers films, whose ferroelectric response is superior to PVDF homopolymer films. Fatigue resistance of normal virgin P(VDF-TrFE) films was compared to that of P(VDF-TrFE) films modulated by using magnetic field. It was shown that normal P(VDF-TrFE) films exhibit a higher fatigue resistance. The artificially introduced lattice reorientation in magnetic-field-modulated P(VDF-TrFE) films would be closely related to the fatigue resistance. Under an ac electric field, the correspondingly microstructures may also influence the electrically induced lattice defects. Polarization fatigue data in P(VDF-TrFE) films was also analyzed by a dynamic Coffin-Manson law, wherein the corresponding coefficients and the exponent of the function can be estimated via different Weibull distribution function. The smallest scale found to be significant in electrical fatigue is the irreversible atomic movements. Studies on electrical failure behavior were also performed in P(VDF-TrFE) copolymer films. Experiment results consistently show that the measured electric polarization near the breakdown limit with respect to the failure life cycles obeys the Coffin-Manson law that is the most widely used to describe the mechanical fatigue failure behavior. The corresponding Coffin-Manson exponents remain constant. Our experimental evidence indicates that accumulation of the disordered structure at the atomic level is closely related to the physical origin of the fatigue in dielectric materials. It is the intrinsic atomic movement that constitutes the major finding in this work.