Thick-film humidity sensor based on porous MnWO4 material

A new compact, robust, yet fast and highly sensitive ceramic humidity sensor based on the semiconducting metal oxide MnWO4 has been developed using thick-film technology. The sensor element possesses a novel 'sandwich' configuration with a 40 mu m porous MnWO4 ceramic layer sandwiched by two 10 mu m polarity-reversed interdigitated metal films. Instead of traditional glass frits, LiCl powders were used as adhesion promoters. The sintered ceramic layer exhibits a porous structure. The degree of the porosity is controlled by the amount of LiCl added and by the firing conditions for the ceramic. The surfaces of ceramic grains behave like electrolytes and easily adsorb water vapour through the pores. The novel electrode arrangement combines the advantages of humidity sensors in the form of a parallel capacitor with those in the form of an interdigital capacitor. The influence of temperature on the sensor characteristics has been compensated for by integrating a thick-film NTC resistor. The results of studies on the material processing, the fabrication and the characterization of this novel thick-film humidity sensor are described.