Barium titanate polar ceramics

The bending piezoelectricity in drawn and polarized polymer films was studied in detail by Kawai (1) (1970). Kitayama and Nakayama (1971) reported a very high piezoelectricity in composite films of polymer (PVDF, nylon 11, PVC) and powdered ceramics (barium titanate, PZT) after poling. In the case of PVDF and nylon, the piezoelectric constant increase by a factor of 102 when the ceramics make up 50% of the volume. The pyroelectricity and optical nonlinearity of polarized PVDF films have been studied by Bergmann, McFee, and Crane (1971). 

The effect of k on d is most clearly demonstrated in the experiment by Fukada and Date (1970) on the polyester resin film, filled with powdered barium titanate and polarized under a d.c. field. The strong piezoelectricity, as shown in Fig. 29, is ascribed to the polarization charge of the ceramic filler and heterogeneous strain due to the composite structure. The real part d exhibits a maximum at 90° C and d" has a peak and a succeeding dip at this temperature where the primary relaxation of polyester resin occurs. The behavior of d and d" is quite similar to that of k and k" in Fig. 16, respectively, in which decreasing X = an corresponds to increasing temperature. 

Since the polar axes in barium titanate and PZT (see Fig. 2.40(b) and Fig. 2.44) are longer than the perpendicular axes, ceramics expand in the polar direction during poling. The application of a high compressive stress in the polar direction to a poled ceramic causes depoling since the 90° domains switch direction as a result of the ferroelastic effect and the polar directions of the crystallites become randomized. 

L. M. Eng, H. J. Giintherodt, G. A. Schneider, U. Kopke, and J. Munoz Saldana, Nanoscale reconstruction of surface crystaUography from three dimensional polarization distribution in ferro electric barium titanate ceramics, Appl Phys. Lett. 74,233 235 (1999). 

Usually, the value of the spontaneous polarization P, depends on the temperature. Figure 22-4 shows the temperature dependence of the spontaneous polarization in barium titanate (BaTiOs) crystal. As temperature changes, a variation of the charge density can be observed on those surfaces of the sample which are perpendicular to the unique polar axis in a crystal without twins, or in a poled polycrystalline solid (ceramic) with oriented grains. The phenomenon, where the spontaneous polarization changes with temperature, is called pyroelectric effect . All ferroelectric materials exhibit pyroelectric effect. 

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