Flat interfaces in zinc oxide-based varistor ceramics
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Four types of structurally different, flat interfaces have been observed in ZnO-based varistor ceramics containing metal-oxides additives (Bi, Mn, and Ti) by bright-field and high-resolution imaging in a transmission electron microscope. Orientation relationships have been characterized by selected-area diffraction. The faceting of ZnO grains when in contact with /}—Bi2O3 is discussed in relation to the anisotropic growth of the ZnO grains which leads to pronounced faceting parallel to (0001) planes.
I. INTRODUCTION Since the development of the zinc oxide-based varistor with highly nonohmic current-voltage characteristics and high-energy absorption capabilities,1 the electrophysical behavior of nonohmic ZnO varistors has been studied and related to the microstructure, the conduction and degradation mechanisms, and the dielectric properties of the material. An extensive research effort has been aimed at characterizing ZnO grain boundary regions in order to explain the origin of the nonlinear currentvoltage characteristics of these materials.2"4 A typical ZnO varistor material contains a small amount of several metal oxides (e.g., Bi 2 O 3 , CoO, MnO, Sb 2 O 3 , and C^Os). Co and Mn are contained within the ZnO grains, while the other impurities are present in the intergranular phases. Several polymorphic forms of Bi2O3, the spinel Zn7Sb2Oi2, and the pyrochlore Zn^BisS^O^'-''" 7 have been identified as intergranular phases. The electrical barrier models for the nonohmic current-voltage characteristics have been proposed in relation to the microstructural location of the Bi-rich intergranular phase.8"12 Recently, it was reported that the intergranular phase exhibits temperature-dependent grain boundary wetting, with an average equilibrium dihedral angle of 0° at 1140 °C and over 55° at 610 °C in the varistor system ZnO-Bi 2 O 3 -Co 2 O3. 13 - 15 The application of ZnO varistors in low and high electric fields has been developed by the control of the ZnO grain size, because the breakdown voltage depends on the number of grain boundaries between the electrodes of the ZnO varistor device.16"21 The addition of Sb2O3 or &2O3 results in the formation of a spinel phase which is frequently located at the grain boundaries and/or occasionally within the ZnO grains. These "'Present address: Advanced Technology Department, Electronics and Telecommunications Research Institute, P.O. Box 8, Daeduk Science Town, Daejeon, 302-350, Korea. b 'Present address: Department of Chemical Engineering and Materials Science, Amundson Hall, University of Minnesota, Minneapolis, Minnesota. Address correspondence to C. Barry Carter. 474
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J. Mater. Res., Vol. 7, No. 2, Feb 1992
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additions are known to act as a grain-growth moderator by anchoring grain boundaries during sintering.22 In contrast, titanium dioxide has been reported as a graingrowth enhancer of the ZnO-based varistor ceramic.17 In the varistor system ZnO-Bi 2 O 3 -MnO-TiO 2 , ZnO grains have a tendency to grow preferentially alon
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