Alteration of pore size distribution by sol-gel impregnation for dynamic range and sensitivity adjustment in Kelvin condensation-based humidity sensors

It has been theoretically established that the dynamic range of sensitivity in porous dielectric ceramic-based resistive or capacitive humidity sensors is mainly determined by the open pore size distribution in their sensing pallets. Here, we directly apply the concept on ceramic resistive humidity sensors and utilize the results for the experimental verification of the theoretical model. The open pore size distribution in a number of identical titanium dioxide pallets, fabricated by the sintering of the slip-cast powder at 1073 K, is modified in two different directions: (a) impregnation with titanium tetraisopropoxide, followed by a heat treatment at 773 K increases the proportion of the finer pores and (b) re-sintering the original pallets at 1223 K closes the finer pores and substantially shifts the distribution towards larger pore dimensions. Sensitivity measurement results are consistent with the theoretically established concept: increasing the population of larger pores heightens the sensitivity to high relative humidity (RH) levels and shifts the dynamic range of the pallets from the original 20-85% to 50-95% RH. On the contrary, increasing the proportion of the finer pores in the microstructure of the ceramic pallet enhances the sensitivity at the lower RH range, shifting the dynamic range of the sensor to the 2-25% RH range.