Semiconducting oxide gas-sensitive resistors

The overall aim of this thesis is to describe the gas sensing behaviour of a wide range of metal oxide semiconductors which exhibit tremendous changes in their electrical resistance at high temperatures (typically > 300°C) upon exposure to traces (ppm) of reactive gases present in the air. The effects of surface segregation in antimony-doped tin dioxide (Sn1-ySbyo2) on both the electrical response to water vapour and the catalytic combustion of methane in the presence of water vapour were demonstrated. Effects of microstructure, and especially particle size, on the behaviour (sensitivity and selectivity) of these compounds to carbon monoxide and methane were also demonstrated. A change in behaviour correlating with the Debye length was shown. Theoretical calculation methods were used to model surface segregation and surface defects. Antimony segregates as Sb3+ and the complex [Snsn-Vooo] is a stable surface species. A model for gas response and surface reaction involving this complex is proposed. The properties of solid solution series prepared by systematic cation substitution as a way of understanding the gas response mechanism linked to the surface chemistry has been examined in (CrNbO4)x(Sn1-ySbyO2)1-x, Tix(Sn1-ySby)1-xO2, and (MW04)x([Sn-Ti]O2)1-x [with M; Mn, Fe, Co, Ni, Cu, Zn]. Effects of stoichiometry, microstructure, combustion gradient and surface segregation on gas (water, carbon monoxide, methane, propane and ammonia) sensitivity and selectivity have been observed and discussed