Production of doped ZnO powders for varistor applications using sol-gel techniques

Abstract

The electrical behaviour of ZnO varistors is controlled by the characteristics of the ceramic microstructure which depends strongly on the properties of the initial powder. This paper describes a study of the effect of different chemical methods to synthesise doped ZnO powders. The ceramic powders were synthesised by the following routes: a) classical mixing of oxides (for comparison purposes), b) aqueous solution of inorganic salts and c) hydrolysis and polycondensation of metalorganic compounds in an organic solvent. The physicochemical characteristics of these powders were evaluated using a scanning electron microscope and thermoanalytical instrumentation. Standard spray drying technology was used to pelletise the powders to obtain an agglomerated powder suitable for uniaxial pressing. The discs were sintered in an electric furnace under air atmosphere using several temperature programmes. The ceramic microstructure was characterised using SEM and X-ray diffraction. The effects of powder processing route on sintering and microstructural development are discussed. Powders prepared by the metalorganic route exhibited somewhat lower sintering temperatures than conventional powders. However, the rate of sintering was slower for metalorganic and aqueous solution powders. These observations were related to powder morphology. The ZnO−Bi2O3−CoO system exhibited the best varistor characteristics as it was expected, whereas the binary systems supported much lower voltages at low currents than ternary systems.