ABSTRACT
It is well-known that many porous metal oxides can be used as humidity-sensing materials (Seiyama 1983, Shimizu 1982). The adsorption of water vapor can enhance the surface electrical conductivity and dielectric constant of the metal oxides. There are some major requirements for a good humidity sensor: sensitivity, reversibility, fast response time, long life time, high-humidity selectivity and chemical and thermal stability. In general, the ceramic humidity sensors are more chemically and thermally stable than the polymeric humidity sensors, but not all metal oxides can be used as humidity-sensing materials. The conductancehumidity sensitivity of the ceramic sensors is determined by the material itself, the preparation method, and the sintering conditions. The response time of the ceramic humidity sensor depends on the material properties, porous structure of the sample, and the electrode contact material.
