Abstract
Humidity sensing properties of composites made from ZnMoO 4 (ZM) and ZnO (ZO) were investigated. Sintered polycrystalline disks of ZnMoO 4 (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 ZnMoO 4 without forming ZnO+Zn 3MoO 6 and (b) to detect moisture in hydrogen streams. The metastable coexistence of ZnO with ZnMoO 4 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 2×10 3, 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. ZnMoO 4 and ZnO.
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