Selectivity and composition dependence of response of wolframite-based gas sensitive resistors (MWO4)x([Sn-Ti]O2)1-x, (0<x<1, M = Mn, Fe, Co, Ni, Cu, Zn)

The response to carbon monoxide, methane, ammonia and water of gas;sensitive 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 studied. The conductance and the activation energy for conductance vary systematically with composition, reflecting the energy and number density of 3d-transition metal states which depend upon the nature of the transition metal ion and its concentration. Tungsten tended to be strongly surface segregated. The surface trap-limited conductivity model can explain, at least partially, the switch from resistance decreasing to resistance increasing response 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 consistent with that expected with the other reactive gases. The sign of resistance 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 appear to be surface binding sites for water vapour which are specifically associated with particular transition metal ions.