Sintering and humidity-sensitive behaviour of the ZnCr2O4-K2CrO4 ceramic system

The solid state reaction at elevated temperatures (calcination temperatures 800-1300 C) and the sintering behaviour of zinc chromite formed from ZnO and Cr2O3 were investigated. The effects of adding varying amounts of potassium compounds (10, 20, 30% K2CrO4) on the crystal structure, surface and fracture morphologies and humidity sensing properties were then studied. Effects of small CuO additions on the humidity sensitivity were also assessed. The fired ceramic body, which proved to be mainly constructed from ZnCr2O4 spinel grains, was porous. The humidity characteristics of the sensor showed that the resistance decreased as a logarithmic function with an increase in humidity. The resistance values obtained were about 6 x 10 exp(9) ohm and 3 x 10 exp(4) ohm at 25 and 93% relative humidity, respectively. Based on ac impedance measurements, an equivalent circuit associated with a network of resistors together with series capacitors is suggested. It is assumed that such an equivalent circuit model of the sensor under the moist conditions represents the sensing mechanism as a diffusion process. 19 refs.