Ferroelectric poling direction

Yamada et al. [9,10] demonstrated that the copolymers were ferroelectric over a wide range of molar composition and that, at room temperature, they could be poled with an electric field much more readily than the PVF2 homopolymer. The main points highlighting the ferroelectric character of these materials can be summarized as follows (a) At a certain temperature, that depends on the copolymer composition, they present a solid-solid crystal phase transition. The crystalline lattice spacings change steeply near the transition point, (b) The relationship between the electric susceptibility e and temperature fits well the Curie-Weiss equation, (c) The remanent polarization of the poled samples reduces to zero at the transition temperature (Curie temperature, Tc). (d) The volume fraction of ferroelectric crystals is directly proportional to the remanent polarization, (e) The critical behavior for the dielectric relaxation is observed at Tc. 

Because a ceramic is composed of a large number of randomly oriented crystallites it would normally be expected to be isotropic in its properties. The possibility of altering the direction of the polarization in the crystallites of a ferroelectric ceramic (a process called poling ) makes it capable of piezoelectric, pyroelectric and electro-optic behaviour. The poling process - the application of a static electric field under appropriate conditions of temperature and time -aligns the polar axis as near to the field direction as the local environment and the crystal structure allow. 

In the majority of ferroelectric tungsten bronzes the polar axes are parallel to the A-site tunnels. PbNb206 is an exception below the Curie point it undergoes an orthorhombic distortion in the plane perpendicular to the A-site tunnels as indicated in Fig. 6.16. It possesses four possible polar directions and can be poled successfully in ceramic form. 

Pyro- and Piezoelectric Properties The electric field application on a ferroelectric nanoceramic/polymer composite creates a macroscopic polarization in the sample, responsible for the piezo- and pyroelectricity of the composite. It is possible to induce ferroelectric behavior in an inert matrix [Huang et al., 2004] or to improve the piezo-and pyroelectricity of polymers. Lam and Chan [2005] studied the influence of lead magnesium niobate-lead titanate (PMN-PT) particles on the ferroelectric properties of a PVDF-TrFE matrix. The piezoelectric and pyroelectric coefficients were measured in the electrical field direction. The Curie point of PVDF-TrFE and PMN-PT is around 105 and 120°C, respectively. Different polarization procedures are possible. As the signs of piezoelectric coefficients of ceramic and copolymer are opposite, the poling conditions modify the piezoelectric properties of the sample. In all cases, the increase in the longitudinal piezoelectric strain coefficient, 33, with ceramic phase poled) at < / = 0.4, the piezoelectric coefficient increases up to 15 pC/N. The decrease in da for parallel polarization is due primarily to the increase in piezoelectric activity of the ceramic phase with the volume fraction of PMN-PT. The maximum piezoelectric coefficient was obtained for antiparallel polarization, and at < / = 0.4 of PMN-PT, it reached 30pC/N. 

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