Humidity sensing characteristics of sol–gel derived Sr(II)-added ZnAl2O4 composites

Strontium(II)-added ZnAl2O4 composites prepared by a sol–gel method were characterised by X-ray diffraction, scanning electron microscopy (SEM) and nitrogen adsorption/desorption isotherms at 77 K and studied for humidity sensing characteristics. The composites were prepared with the addition of Sr(II) of different molar ratios (0, 0.2, 0.4, 0.6, 0.8 and 1.0) to ZnAl2O4 and were labeled as ZnSA1, ZnSA2, ZnSA3, ZnSA4, ZnSA5 and ZnSA6, 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 of 5–98% and the results revealed that the humidity response (Sf = R5%/R98%; R5% and R98% were the dc resistances at 5 and 98% RH) increased with an increase in strontium molar ratio. Among the different composites prepared, ZnSA5 possessed the highest humidity response of Sf = 38,744, while the pure composite ZnSA1 possessed a low humidity response of Sf = 10,000. The response and recovery times of the most sensitive sample ZnSA5 were 120 and 50 s.