Varistor and humidity-sensitive properties of SnO2–Co3O4–Nb2O5–Cr2O3 ceramics with V2O5 addition

It is shown that tin dioxide based SnO2-Co3O4-Nb2O5-Cr2O3 ceramic material exhibits not only varistor properties discussed in the literature but also humidity-sensitive properties. At low electric fields conductivity of SnO2-Co3O4-Nb2O5-Cr2O3 ceramics is increased with relative humidity of air, however, highly nonlinear current-voltage characteristic is observed even at relative humidity 93%. Conduction mechanism in this material is grain-boundary controlled and decrease of the barrier height on voltage and on relative humidity is responsible, respectively, for varistor and sensor effects. Slow decrease of the low-field conductivity with time during several months after the sintering or high-temperature heat treatment is observed. Such ageing effect is accompanied by the decrease of the lowest value of the current density at highly nonlinear part of current-voltage characteristic. Addition of V2O5 to SnO2-Co3O4-Nb2O5-Cr2O3 causes strong decrease of the nonlinearity coefficient, increase of the low-field conductivity, decrease of the activation energy of electrical conduction and decrease of the humidity sensitivity. At quite small V2O5 addition (about 1mol.%) the transition to a percolation along the V2O5 phase is assumed. Secondary conductive V2O5 phase acts as a shunt to the grain boundaries.