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 1 mol.%) the transition to a percolation along the V2O5 phase is assumed. Secondary conductive V2O5 phase acts as a shunt to the grain boundaries.