Zinc(II) oxide-zinc(II) molybdate composite humidity sensor

Humidity sensing properties of composites made from ZnMoO4 (ZM) and ZnO (ZO) were investigated. Sintered polycrystalline disks of ZnMoO4 (ZMZO-10) and ZnO (ZMZO-01) and composites of ZM and ZO in the mole ratios 8:2, 6:4, 4:6 and 2:8 designated as ZMZO-82, 64, 46 and 28, respectively, were subjected to dc resistance measurements as a function of relative humidity in the range of 5–98% RH. The dc conductance measurements were carried out on the samples from which the activation energies were determined. Thermoelectric power measurements in the temperature range 250–400 °C revealed the materials to be n-type. The stability of the composites was evaluated by emf measurements by constructing solid oxide electrolyte galvanic cells with a view to (a) correlate the metastable coexistence of ZnO with ZnMoO4 without forming ZnO+Zn3MoO6 and (b) to detect moisture in hydrogen streams. The metastable coexistence of ZnO with ZnMoO4 in the composites was confirmed by SEM studies. The dc resistance of these hybrid oxides decreased on exposure to humidity at room temperature and the use of a mixture of oxides was found to be effective for optimizing humidity sensing performance in dc mode. The composite ZMZO-28 has the highest humidity sensitivity, greater than 2×103, with a response time of approximately 3 min, attributed mainly to the optimization of pore size and its even distribution in a biphasic matrix which is a metastable mixture of the two terminal phases, viz. ZnMoO4 and ZnO.