Charge compensation and electrical characteristics of Ta2O5–doped SnO2–CoO ceramics

Abstract

We investigated the effect of pentavalent donor dopant Ta2O5 on microstructure development, electric and dielectric characteristics of SnO2–CoO based ceramics. Already low additions of Ta2O5 (0.05 mol%) effectively reduce the porosity, improve densification and dielectric permittivity and trigger a 3–fold increase in SnO2 growth rate. Rietveld analysis shows that the amount of Co2SnO4 spinel phase drops with the addition of Ta2O5 due to incorporation of Co2+ and Ta5+ into SnO2 structure. With higher additions, however, Ta2O5 segregates to the grain boundaries and hinders SnO2 grain growth, which in turn improves electrical properties. TEM/EDS analysis shows that above 0.5 mol% of Ta2O5 the Co:Ta ratio in SnO2 grains is constant 1:2, which means that a twice lower amount of Ta5+ is incorporated in the SnO2 structure compared to the Nb2O5-doped SnO2–CoO system. Accordingly, the following charge compensation mechanism is proposed: 3 Sn(IV)S˟n (IV) ⇋ Co(II)Sn ̎(IV) + 2 Ta(V)˙Sn (IV).