Effect of high Al2O3 content on the microstructure and electrical properties of Co- and Ta-doped SnO2 varistors

Varistor behavior shows significant differences when the addition levels of different dopants like In2O3, Cr2O3, and Al2O3 are changed, thus stimulating current investigations on the SnO2–Co3O4–Ta2O5 ceramic system. In this contribution, the influence of high additions of Al2O3 on the microstructure, structure and the electrical properties of SnO2–Co3O4–Ta2O5 varistors ([98.95-X]%SnO2–1% Co3O4–X% Al2O3–0.05% Ta2O5, where X = 0, 0.05, 0.1, 1 or 2 mol%) is investigated. Characterization techniques such as thermal analysis, scanning electron microscopy and X-ray diffraction with Rietveld refinement were used for specimen analysis. The endothermic peaks in the ceramic system containing Al2O3 additions between 0.05 and 1% are ascribed to the formation of the Co2SnO4 and CoAl2O4 spinel type phases. Doping the ceramic system with 1 and 2 mol% Al2O3 leads to the formation of 1.163 and 3.449%, respectively, of the spinel phase Al2CoO4, which acts as a grain growth inhibitor because grain size decreases in about 16% for both addition levels. The apparent grains homogeneity and narrowest monomodal grain size distribution for the specimens with 2 mol% Al2O3 confirm the inhibitory role. With the lowest level of Al2O3 (0.05 mol%) the nonlinearity coefficient reaches a maximum, after which it decreases and fades at the highest alumina level. A remarkable decrease of about 50% in the leakage current from the reference specimen´s value to that of the one with 0.05 mol% Al2O3 concurrently with an increase in about 40% in the nonlinearity coefficient favors the potential use of alumina in the SnO2–Co3O4–Ta2O5 ceramic system.