Effect of rate controlled sintering on microstructure and electrical properties of ZnO doped with bismuth and antimony oxides

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Abstract
Various rate controlled sintering (RCS) schedules were used on isostatically pressed particulate compacts of ZnO with Bi2O3 and Sb2O3 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. β–Bi2O3, Zn7Sb2O12, and Zn2Sb3Bi3O14 phases formed during/after sintering. Elevated heat-treatment temperatures favored the formation of Zn7Sb2O12 and additional β–Bi2O3, while Zn2Sb3Bi3O14 was dominant in sintered samples where the RCS schedule did not result in temperatures in excess of 1100 °C. Zn2Sb3Bi3O14 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 Zn2Sb3Bi3O14 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.

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