Abstract
Ceramics are useful materials for humidity sensing device because the electric impedance of porous ceramics is strongly affected by the humidity in atmosphere. The sensitivity could be controlled by the pore structure, surface area and/or chemical modification of the pore wall surface. For the sample where the surface occupancy area of acidic protons was larger than ca. 0.1nm2, the impedance value at 90% r.h. was controlled by the surface area and the morphologies of pores. In the range below 0.1nm2, the impedance especially for >40% r.h. could be lowered without any increase in the surface area and was reciprocally proportional to the concentration of acidic protons. The introduction of alkali ions to the pore wall surface instead of protons is a preferable method to improve the humidity sensitivity and durability. It is difficult to lower the impedance for <30% r.h. by the introduction of acidic protons and/or alkali cations. To lower the impedance for the lower humidity region, the use of super ionic conductor or ferroelectrics instead of usual insulating metal-oxide was proposed.
