Conduction and gas-surface reaction modeling in metal oxide gas sensors

A phenomenological approach to the operation of metal oxide gas sensors, the Integrated Reaction Conduction (IRC) model, is proposed which integrates the gas-surface reactions with the electrical conduction process in a weakly sintered, porous metal oxide. An effective medium approximation is employed to relate the mesoscopic microstructure and the carrier depletion at the granular surface to the macroscopic electrical conduction. For a given ambient gas concentration and temperature, the electron concentration in the depletion layer is calculated from the gas-surface reaction kinetics. The adsorption and oxidation reaction energies of the gas sensing reactions are extracted for a TiO2-x CO sensor by comparing experimental data with three-dimensional plots of IRC model resistance as a function of the ambient [CO(g)] and temperature. The IRC model predicts novel properties of the gas sensor, including the sensitivity and the response range, which depend on the doping of the sensor material, the temperature, the grain size, and the geometry of the necks between grains.