Barium polarization directions

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. 

Figure 1.7 Various properties of barium titanate as a function of temperature. Anisotropic properties are shown with respect to the lattice direction, (a) Lattice constants, (b) spontaneous polarization Ps and (c) relative permittivity er.

Figure 9.2 Polarization-isotropic photo-induced light scattering in strontium-barium-niobate doped with 0.66 mol% Cerium (SBN61 Ce). Strongly amplified scattered light occurs in a wide solid angle opposite to the polar c-axis. The label denotes the apex angle measured with respect to the directly transmitted laser beam in air. Ap = 632.8 nm, Ep c. The insets show a photograph of the sbn crystal (left) and the schematic setup of the experiment (right), respectively.

Whereas all of the methods proposed for large-scale fractionation of starch that have been discussed depend directly on the ability of amylose to form itLsoluble complexes with polar organic compounds. Cantor and Wimmer s process is based on a totally different principle. If a molecularly disperse solution of starch contains a sufficient amount of calcium chloride and caustic alkali is added, a rapid and quantitative precipitation of the starch occurs, because of the formation of complexes (of calcium hydroxide with the starch polysaccharides) which are insoluble in an aqueous, saturated solution of calcium hydroxide. The same phenomenon is observed with the hydroxides of barium and strontium. 

Barium titanate is cubic with a perovskite structure. However, at room temperature (actually below the Curie temperature of 120°C) it is tetragonal with a spontaneous electric polarization in the direction of the c-axis (only the higher temperatures form is shown in Figure 7.2). In this ferroelectric condition a crystal of BaTiOs has a domain structure. 

Barium titanate is ferroelectric and, by implication, also pyroelectric and piezoelectric. The characteristic of a ferroelectric is that it is polarized in the absence of an applied electric field and the direction of polarization can be reversed. Figure 31.10 shows a rectangular hysteresis loop for a singledomain single crystal of BaTiOs. This loop was obtained at room temperature using a 50 Hz supply. 

Barium titanate (BaTi03> is one of the most thoroughly studied and most widely used piezoelectric materials. Just below the Curie temperature (120°C), the vector of the spontaneous polarization points in the [0011 direction... 

Domain. In a ferroelectric or ferromagnetic crystal, e.g. barium titanate, a domain is a small area within which the polarization is uniform. If the crystal is exposed to a high electric or magnetic field, those domains in which the polarization is in a favourable direction will grow at the expense of other domains. A domain structure gives rise to hysteresis (q.v.). 

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