Comparative study on micromorphology and humidity sensitive properties of thick film and disc humidity sensors based on semiconducting SnWO4–SnO2 composites

Sintered materials of SnWO4 (hubnerite structure) and SnO2, mixed in the molar ratios 80:20, 60:40, 40:60 and 20:80 and doped with 2 mol% of Li+, were fabricated through sol–gel technique, and the humidity sensing properties of the SnWO4–SnO2 (SWSO) composites were measured employing the sensor configurations of a polycrystalline cylindrical disc and a thick film. The thick film sensor showed a smaller humidity sensitivity factor than that of the polycrystalline disc. However, the former exhibited a fast response to humidity change and also had a very low temperature coefficient in the temperature range of 20–60 °C. The composites were characterized by solid state electrical measurement, SEM and BET studies to analyze activation energy for conduction, surface morphology and pore structure of the sensor materials, respectively. The discs and thick films were subjected to dc resistance measurements as a function of relative humidity in the range of 5–98% at 300 K. It was found that the Li+-doped SWSO-46 disc (SnWO4:SnO2 = 40:60) had the highest sensitivity factor of 22,338 (±3060), compared to that of the undoped composite (463 ± 17). Studies on thick film sensors based on SnO and WO3 indicated that the W6+ sites in the hubnerite material contributed strongly to the humidity sensing mechanism.