Ferroelectric Properties and Polarization Switching Kinetic of Poly (vinylidene fluoride-trifluoroethylene) Copolymer

Abstract

The discovery of the piezoelectric properties of poly(vinylidene fluoride) (PVDF) by Kawai [Kawai, 1969], and the study of its pyroelectric and nonlinear optical properties [Bergman et al., 1971; Glass, 1971] led to the discovery of its ferroelectric properties in the early 1970s. Since that time, considerable development and progress have been made on both materials and devices based on PVDF. This work helped establish the field of ferroelectric polymer science and engineering [Nalwa, 1995a]. There are many novel ferroelectric polymers, such as poly(vinylidene fluoride) (PVDF) copolymers, poly(vinylidene cyanide) copolymers, odd-numbered nylons, polyureas, ferroelectric liquid crystal polymers and polymer composites of organic and inorganic piezoelectric ceramics [Nalwa, 1991 and Kepler & Anderson, 1992 as cited in Nalwa, 1995b; Nalwa, 1995a]. Among them, PVDF, and its copolymers are the most developed and promising ferroelectric polymers because of their high spontaneous polarization and chemical stability. Ferroelectricity is caused by the dipoles in crystalline or polycrystalline materials that spontaneously polarize and align with an external electric field. The polarization of the dipoles can be switched to the opposite direction with the reversal of the electric field. Similar to inorganic ferroelectric materials such as PbZr0.5Ti0.5O3 (PZT) and SrBi2Ta2O9 (SBT), organic ferroelectric materials exhibit ferroelectric characteristics such as Curie temperature (the transition temperature from ferroelectrics to paraelectrics), coercive field (the minimum electric field to reverse the spontaneous polarization) and remanent polarization (the restored polarization after removing the electric field). However, the low temperature and low fabrication cost of organic ferroelectric materials enable them to be used in a large number of applications, such as flexible electronics. In this chapter, the discussion is focused on poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)], one of the most promising PVDF ferroelectric copolymers. The main objective of this chapter is to describe the ferroelectric properties of P(VDF-TrFE) copolymer and review the current research status of ferroelectric devices based on this material. The chapter is divided in six sections. The first section introduces the topic of organic ferroelectrics. The second section describes the material properties of the ferroelectric phase of P(VDF-TrFE) including phase structures, surface morphology, crystallinity and molecule chain orientation. Next, the electrical properties such as polarization, switching current, etc.