M.B. Hernández

Microstructure and electrical properties in SnO2 ceramics with sequential addition of Co, Sb and Ca

The effect of sequential additions of Co3O4 ,S b 2O5 and CaCO3 on the microstructure and electrical behaviour of SnO2-based ceramics was investigated. Although without conferring varistor properties, Co3O4 additions promote densification in the material. Successive addition of Sb2O5 provides the sample varistor properties but with low nonlinearity coefficient (α = 5.7) and high electric field (E1 = 2022 V cm −1 ) at current density 10 −3 Ac m −3 . Moreover, antimony oxide addition degrades the microstructure condition by inducing porosity. Subsequent addition of CaCO3 promotes densification and grain growth. However, it does not lead to the increase in the nonlinearity coefficient. It only lowers the electric field, thus making the material suitable for lower voltage applications. Observed significant increase in the relative dielectric permittivity up to a factor of about 60 and 200 in the cases of antimony or antimony and calcium addition to SnO2–Co3O4 (accompanied by the appearance of varistor effect) is due to the formation of barrier depletion layers at grain boundaries.

Current–voltage characteristics of SnO2–Co3O4–Cr2O3–Sb2O5 ceramics

The effect of mechanical treatment in a planetary mill on the microstructure and electrical properties of tin dioxide based varistor ceramics in the system SnO2–Co3O4–Cr2O3–Sb2O5 sintered in the range 1150–1450 °C was studied. The mechanical treatment leads to an increase in shrinkage, decrease in porosity, decrease in sample diameter, change in colour of the sintered samples from grey to black and enhancement of nonlinearity. For the sample sintered at 1350 °C the mechanical treatment enhances the nonlinearity coefficient from 11 to 31 and decreases the electric field E1 (at 10−3 A cm−2) from 3500 to 2800 V cm−1. The observed changes in physical properties are explained in terms of an additional size reduction of oxide particles and a better mixing of oxide powder followed by the formation of potential barriers at the grain boundaries throughout the whole sample. In spite of the low porosity, the low-field electrical conductivity of mechanically treated ceramics is significantly increased with the growth of relative humidity. A higher humidity sensitivity is found for mechanically treated ceramics with higher barrier height and higher nonlinearity coefficient.