Electrical barriers formation at the grain boundaries of Co-doped SnO2 varistor ceramics

The nonlinear electrical properties of CoO-doped SnO2 ceramics were characterized. A second phase is observed at the surface of the microstructure of the specimens. Energy-dispersive X-ray analysis reveals that the second phase is composed of tin, cobalt and oxygen with an approximate atomic ratio estimated for Co and Sn: 2:1. The phase was identified by X-ray diffraction and unambiguously establish the identity of the phase: Co2SnO4. Electrical characterization of the binary CoO–SnO2 was carried out on a very large range of current from 10−9 up to 100 A cm−2 showing that the doping of CoO increases drastically the resistivity of the ceramic from 104 to 1010 Ω cm. This value appears to be a threshold for which the nonlinear effect appears.