Porosity control of humidity-sensitive ceramics and theoretical model of humidity-sensitive characteristics

Abstract

In this paper, a new method for porosity control of porous humidity-sensitive ceramics is presented. Graphite powder is chosen as the pore former. It is convenient to control the pore size and porosity of porous humidity-sensitive ceramics made from CoFe spinel and MgCr spinel by changing the content and grain size of the pore former. The electrical behaviours can be kept stable for a long time because of the completeness of the solid phase reaction. Experiments indicate that when the weight percentage of graphite is 10–20 wt.%, the humidity sensitivity of these two porous ceramics increases remarkably and the linearity of the resistance and humidity characteristics is clearly improved. It has been shown by XRD, SEM and AES analyses that graphite powder has only a slight influence on the matrix material, but its pore-forming effects are quite favourable. From the electronic—ionic comprehensive conduction model, and use of Grotthuss' transmission mechanism and the phase law of two mixed phases, a new equation of resistance and humidity has been derived theoretically, thus successfully explaining the experimental data. Moreover, the characteristics of resistance and humidity, and those of capacitance and humidity of various types of materials can be determined.