Síntese e caracterização de compósitos ferroelétricos à Base de Poli (fluoreto de vinilideno)/(Pb,La)(Zr,Ti)O3 e Poli (fluoreto de vinilideno)/BaTiO3

Abstract

The poly(vinylidene fluoride) (PVDF) is a ferroelectric polymer that has been intensively studied since the discovery of their piezoelectric properties in the late 60s. Despite this polymer exhibit higher piezo and pyroelectric coefficients among all polymers, the low values of their coefficients over the other ferroelectric materials, even single-crystals or poly-crystals, limit their applications. Ferroelectric ceramics of barium titanate (BT) and lanthanum modified lead zirconate titanate (PLZT) have higher dielectric constant and spontaneous polarization, which ensures a wide application in capacitors and memory devices. However, low mechanical strength of these materials limits their applications. Thus, it appears that conventional materials such as ceramics and polymers in many cases do not meet the requirements for specific applications. In this context, development of composite materials has become an alternative to this deal. With the purpose of combine the properties piezo, pyro and ferroelectric of PLZT and BT ceramics and the mechanical and electrical properties of the PVDF polymer, this work proposes the synthesis of ferroelectric composites in the form of films, with potential applications in sensors, actuators and transducers, keeping the properties of these two classes of materials. The PVDF films were prepared by dissolving the polymer in dimethylformamide and subsequent heat treatment, at different temperatures, to solvent removal and crystallization of the polymer. The composites were prepared by dispersing the ceramic in a solution containing the dissolved polymer. In the PVDF films it was found that increasing the crystallization temperature results in films with less porosity and greater crystallinity, but, however, with smaller fraction of β phase (ferroelectric phase and this work desired for that material). While low crystallization temperatures resulted in porous films with a low degree of crystallinity, but with high values of β phase fraction. For composites, it was found the presence of two modes of accommodation of the ceramic particles in the polymer matrix. One case the ceramic powders are incorporated in the polymeric matrix, and the other case, the powder accommodation occurs preferentially at the pores of the polymer matrix. The crystalline properties of the polymeric matrix and electric properties of the composite have large dependence with the accommodation of the ceramics particles in the polymer. Therefore, a new form for visualization of the characteristics of these materials must be taken into account, and the properties, in turn, now are described not only in terms of the ratio of ceramic/polymer, but also by mode of accommodation of these phases.