Selectivity and composition dependence of response of wolframite-based gassensitive resistors (MWO4)(x)([Sn-Ti]O-2)(1-x), (0 < x < 1, M = Mn, Fe, Co, Ni, Cu, Zn)

The response to carbon monoxide, methane, ammonia and water of gas;sensitiv e resistors fabricated from wolframite-based solid solution compounds (MWO4 )(x)([Sn-Ti] O-2)(1-x) (0< x < 1; M = Mn, Fe, Co, Ni, Cu, Zn) has been stud ied. The conductance and the activation energy for conductance vary systema tically with composition, reflecting the energy and number density of 3d-tr ansition metal states which depend upon the nature of the transition metal ion and its concentration. Tungsten tended to be strongly surface segregate d. The surface trap-limited conductivity model can explain, at least partia lly, the switch from resistance decreasing to resistance increasing respons e to reducing gases if assumptions are made about the local state occupancy and the energy of these states with respect to the energy associated with O adsorbed on the surface. The sign;of response to water was not always con sistent with that expected with the other reactive gases. The sign of resis tance change associated with adsorption of water can be explained in terms of the surface OH trap energy relative to the O-ads(2-) trap state. There a ppear to be surface binding sites for water vapour which are specifically a ssociated with particular transition metal ions.