Humidity Sensing Elements Based on Si-Bi-O Surface Layers Prepared via a Sol-Gel Method

This paper presents humidity sensing elements based on Si-Bi-O surface layers prepared via a sol-gel method and sintered at 400 °C and 800 °C. Tetraethoxysilane has been used as silica precursor and Bismuth acetate as dopant source. Morphological and map-data analysis of these layers has been carried out by means of scanning electron microscopy and dispersive spectroscopy. X-ray diffraction analysis has been carried out to study the structural composition of the material of the sensing layers. The electrical properties of the sensing elements have been studied with a precise impedance analyzer. The influence of frequency within the range of 20 Hz to 1 MHz on the characteristics of samples has also been studied. The developed sensing elements have very high sensitivity to humidity, small hysteresis, and short response and recovery time. The change in the resistance of the samples, sintered at 400 °C, reaches about six orders of magnitude within the range from 17% to 93% relative humidity at a frequency of 20 Hz and at 25 °C.