Solid state electrical conductivity and humidity sensing studies on metal molybdate–molybdenum trioxide composites (M = Ni2+, Cu2+ and Pb2+)

Metal molybdates, prepared by the solution route, were sintered at 750 °C in the form of cylindrical discs. The humidity sensing characteristics of composites MMoO4 (MM; M=Ni2+, Cu2+ and Pb2+) and MoO3 materials are described. Sintered polycrystalline discs of MMoO4 (MMMO-10) and MoO3 (MMMO-01) composites in the mole ratios of 80:20, 60:40, 40:60 and 20:80 designated as MMMO-82, MMMO-64, MMMO-46 and MMMO-28, respectively, are doped with 2 mol% of Li+. The composites were subjected to dc conductance measurements over the temperature range 100–300 °C in air atmosphere from which activation energies were determined. The current was found to increase linearly with the applied field and exponentially against temperature indicated the ohmic contact and semiconductive nature of the electrode samples, respectively. The activation energy for dc conductance was found to be in the range of 0.20–1.82 eV for different composites and terminal phases. The scanning electron microscopy studies of NMMO-28 composite indicated pores structure of the sensor materials. The surface adsorption studies (BET) of NMMO-28 composite showed that the radius of the pores was found to be distributed from 10 to 65 Å with pore specific volume of 0.27 cm3 g−1. As the micropore structures are preferred for the humidity sensing properties, the composites were subjected to dc resistance measurements as a function of relative humidity in the range of 5–98% relative humidity, achieved by different water vapour buffers thermostated at room temperature. The sensitivity factor, Sf (R5%/R98%) measured at ambient temperature revealed that NMMO-28, CMMO-28 and PMMO-28 composites have the highest humidity sensitivity factors of 16,543±2800, 6945±80 and 4814±300, respectively, when compared to undoped composites. The response and recovery characteristics for these humidity sensing composites were studied.