Texture development in ZnO-based varistors, effects of inversion boundaries on texture development and relationships between texture and electrical characteristics

Effects of morphological and crystallographic texture on the electrical properties of a ZnO-based varistor system were investigated regarding the texture axes. Highly textured ZnO-based varistors were fabricated via Templated Grain Growth (TGG) technique. Rod-like ZnO templates were produced by hydrothermal synthesis method. Behavior of the templates during heating was investigated by quenching the samples with following compositions to room temperature; pure ZnO, ZnO/Bi2O3, ZnO/Bi2O3/TiO2, ZnO/Bi2O3/SnO2 and a low voltage varistor system. Stereological analysis, electron back scattered diffractometer (EBSD) and x-ray diffractometer (XRD) were conducted to measure the texture distribution. The degree of orientation, r calculated from the (0001) EBSD pole figure was 0.34 and texture fraction, f (Lotgering factor) calculated from the XRD data was 0.98 for templated varistor system. A significant degree of texture was also observed in the template free varistor system probably as a result of initial particle alignment during tape casting. The r and f parameters calculated for this system were 0.47 and 0.52, respectively. The threshold voltages were found to be anisotropic for both systems in consistent with observed morphological texture. However, the nonlinear coefficients ? , did not exhibit a significant difference as a function of direction even in the highly textured samples. On the other hand, Schottky barrier heights calculated from the capacitance-voltage (C-V) measurements were also found to be anisotropic.