Effect of Sr addition on the humidity sensing properties of CoAl2O4 composites

The effect of Sr addition on the CoAl2O4 composites prepared by sol–gel technique was investigated for humidity sensing properties. X-ray diffraction, scanning electron microscopy (SEM), FT-IR spectroscopy and nitrogen adsorption–desorption isotherm at 77 K was employed respectively to identify the structural phases, surface morphology, vibrational stretching frequencies and BET surface area of the composites. The composites were prepared with the addition of strontium of different molar ratios (0, 0.2, 0.4, 0.6, 0.8 and 1.0) to CoAl2O4 and were labeled as CoSA1, CoSA2, CoSA3, CoSA4, CoSA5 and CoSA6, respectively. The samples sintered at 900 °C for 5 h, were subjected to dc resistance measurements as a function of relative humidity (RH) in the range 5–98% and the results revealed that the sensitivity factor increased with an increase in strontium molar ratio. Among the different strontium-added composites, CoSA5 possessed the highest humidity sensitivity of 56,000 while the pure composite CoSA1 possessed a low sensitivity of 14,000. The response and recovery characteristics of the maximum sensitivity sample CoSA5 were 120 and 70 s.