Effect of rate controlled sintering on microstructure and electrical properties of ZnO doped with bismuth and antimony o
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Effect of rate controlled sintering on microstructure and electrical properties of ZnO doped with bismuth and antimony oxides Gaurav Agarwal and Robert F. Speyer School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332 (Received 21 August 1996; accepted 24 February 1997)
Various rate controlled sintering (RCS) schedules were used on isostatically pressed particulate compacts of ZnO with Bi2 O3 and Sb2 O3 additives. For low additive content, smaller average grain sizes with more rapid RCS schedules were attributable to thermal schedules which minimized the time at elevated temperatures where grain growth could occur. b –Bi2 O3 , Zn7 Sb2 O12 , and Zn2 Sb3 Bi3 O14 phases formed during/after sintering. Elevated heat-treatment temperatures favored the formation of Zn7 Sb2 O12 and additional b –Bi2 O3 , while Zn2 Sb3 Bi3 O14 was dominant in sintered samples where the RCS schedule did not result in temperatures in excess of 1100 ±C. Zn2 Sb3 Bi3 O14 precipitated during sintering, functioning as grain boundary pinning sites which impeded ZnO grain growth. Bismuth and antimony oxide-based liquid facilitated sintering at lower temperatures, which in turn resulted in decreased average grain size. Rapid RCS schedules for samples with low dopant content resulted in lower sintering temperatures, since time was not allowed for Zn2 Sb3 Bi3 O14 precipitation to deplete the liquid phase. For higher dopant contents, liquid phase was adequately plentiful, wherein longer RCS schedules resulted in lower sintering temperatures. Increasing concentration of second phase generally fostered decreased grain size and attenuated the effect of thermal schedule on the microstructure. Electrical resistance and breakdown voltage increased consistent with decreasing ZnO average grain size.
I. INTRODUCTION
Zinc oxide varistors are polycrystalline ceramic oxides whose electrical characteristics behave similarly to back-to-back zener diodes, but with comparatively much greater energy-handling capabilities.1 They are used predominantly in the area of circuit overvoltage protection. The electrical conductivity of zinc oxide is known to vary with atmosphere and schedule used during thermal processing. At elevated temperatures, thermal dilation of the wurtzite-structured lattice promotes an exponential increase in the density of zinc atoms adopting interstitial sites in the lattice. Pairs of neutral oxygens then leave the lattice in the form of O2 gas, in order to maintain space charge neutrality. The interstitial zinc atoms have a low ionization energy,2 contributing electrons to the lattice as a whole (e.g., conduction band electrons), making the material conductive (n-type semiconductor). Thus ZnO, which is heat-treated in air and then quenched, is consequently conductive (relative to slowly cooled ZnO) at room temperature. Additions of multivalent oxides to ZnO powders result in a sintered body with highly nonlinear I-V characteristics. Additives, such as cobalt,
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