HUMIDITY SENSORS BASED ON SnO2 -Co3 O4 -Nb2 O5 -Cr2 O3 SEMICONDUCTOR VARISTOR CERAMICS

Abstract

The varistor effect in SnO2 -Co3 O4 -Nb2 O5 -Cr2 O3 ceramics with different additions is accompanied by a strong humidity-sensitivity effect in the low-field electrical conductivity. The low-field electrical conductivity and dielectric permittivity of ceramics are increased when air relative humidity rises from 10 to 93%. Such effects are observed both in the samples prepared by solid-phase sintering without or with CaO addition and in the samples prepared by liquid-phase sintering with CuO or V2 O5 additions. The lowest humidity-sensitivity effect was found in the liquid-phase sintering samples due to the more solid structures and the secondary phases which are located between the SnO2 grains. For the studied samples the values of humidity sensitivity coefficient 1.8·103 -3.2·105 at low electric fields and the nonlinearity coefficient 30-57 at high electric fields (E1 = 5.3-7 kV·cm-1) were calculated. The properties of oxide ceramics are explained by electrical conductivity which is controlled by the grain-boundary potential barriers. The barrier height decreases with the increase both of air relative humidity at low electric fields and of voltage at high electric fields. Strong humidity sensitivity is a key factor for sensor application of SnO2 - based ceramics.