Microstructure development in (Co,Ta)-doped SnO2-based ceramics with promising varistor and dielectric properties

Abstract

Following the 3 Sn(IV)Sn(IV)×⇋Co(II)Sn(IV)″+2 Ta(V)Sn(IV)⋅ charge compensation mechanism we optimized densification and electrical properties of Ta2O5-doped SnO2–CoO ceramics. We show that incorporation of acceptor dopant Co2+ in SnO2 is promoted after the addition of donor dopant like Ta5+, whereas any surplus of Co would form secondary Co2SnO4 phase. A balanced addition of both dopants is needed to promote densification, and any surplus of donor dopants that remain present at the grain boundaries retard the grain growth and deteriorate electrical properties. Varistor and dielectric properties are then strongly influenced by donor doping. Optimum varistor properties (α = 40, UT = 272 V/mm, IL = 1.2 μA) were measured for the sample with 1 mol% Ta2O5 and the best dielectric properties (ε = 6525; tan(δ) = 0.057@1kHz) were measured for the sample with 0.10 mol% Ta2O5 with the largest SnO2 grain sizes.