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

Abstract The nonlinear electrical properties of CoO-doped SnO 2 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: Co 2 SnO 4 . Electrical characterization of the binary CoO–SnO 2 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 10 4 to 10 10  Ω cm. This value appears to be a threshold for which the nonlinear effect appears.