Rice husk ash as a new humidity sensing material and its aging behavior

Rice husk ash (RHA), a by-product of a biomass combustion plant, was investigated as a new humidity sensing material. The raw material was first calcined at 500 ◦C for 3 h and then characterized by X-ray diffraction (XRD), N2 adsorption, laser granulometry and field emission-scanning electron microscopy (FE-SEM). Thick-film sensors were fabricated by screen-printing technique onto α-alumina substrates with Pt interdigitated electrodes and fired at 600 ◦C. The sensors were characterized by X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FT-IR) and Raman spectroscopy as well as through FE-SEM observations. Furthermore, they were tested towards humidity at room temperature and interferences with some gases such as NH3 (50 ppm), CO2 (50 ppm), CH4 (50 ppm), O3 (0.5 ppm) and N2O (15 ppm) were studied too. Unfortunately, though very promising, these sensors exhibited a rather fast aging due to a decrease of the sensing performances after 6 months from the fabrication. FT-IR and Raman spectroscopy, as well as XPS, were then used to understand the degradation phenomena involved in this process.