Humidity sensors using manganese oxides

Unique humidity sensing properties of electrolytic manganese dioxide (EMD) and its calcination products (Mn2O3 and Mn3O4) were investigated. These oxides gave resistive type elements which reduced d.c. resistance on exposure to humidity at room temperature. The humidity sensing behavior was strongly dependent on the oxides, reflecting the differences in microstructure. The use of a mixture of these oxides (hybrid system) was found to be effective for optimizing humidity sensing performances not only in d.c. mode but also in a.c. mode. The elements attached with a pair of Pt and Cu electrodes, on the other hand: showed response to humidity in EMF mode in the high relative humidity (RH) range above 60%. This suggests that proton conduction in the oxides increases with increasing RH, becoming comparable to or larger than electronic conduction at high RH. The EMF response was discussed on an electrochemical cell model constructed on a mixed conducting diaphragm. (C) 1998 Elsevier Science S.A. All rights reserved.