ELECTRICAL AND HUMIDITY SENSING PROPERTIES OF COMPOSITES PREPARED FROM MnO_2 AND MnCr_2O_4

摘要

MnCr_2O_4 was prepared by the high temperature solid-state method by the reaction of Cr_2O_3 and MnO_2 heated at 1173 K. Experimental results on the composites made from MnCr_2O_4 (MCO) and MnO_2 (MO) for electrical and humidity-sensing properties are described. The compounds and composites of MnCr_2O_4 and MnO_2 in the mole ratios (1:0, 0:1, 1:1, 1:2, 2:1, 3:2 and 1:4) were sintered in the form of the disc and subjected to DC conductance measurements over the temperature range 373-673 K from which the activation energies were determined. The activation energy value lies in the range of 0.18-0.38 eV for the composites and 0.57 and 0.45 eV for the terminal phases MCMO-10 and 01 respectively which falls in the wide gap of the semiconductor range. The powder XRD revealed that the composite is a complex material containing oxides of manganese and chromium. The composites were subjected to DC resistance measurements as a function of relative humidity in the range of 5-98 percent, achieved by different water vapor buffers thermostated at room temperature. The sensitivity factor. S_f (R_(5 percent)/R_(98 percent)) measured at 298 K revealed that MCMO-32 (three moles of MnCr_2O_4 and two moles of MnO_2) has the highest humidity sensitivity of 7.35 X 10~3 for which the response and recovery time have been assessed. The scanning electron microscope (SEM) showed the porous structure of the composite.