| Detailed theoretical models are presented, and used to facilitate the design of industrial metal oxide gas sensors. By modeling both the sensing mechanisms and the conduction process, these methods optimize the sensitivity and selectivity.; A phenomenological approach to the operation of metal oxide gas sensors, the Integrated Reaction Conduction (IRC) model, is introduced. The IRC model consolidates the surface reaction kinetics and electrical conduction through the granular sensor microstructure. A simple relation is established between the sensing mechanism reaction energies and the sensing behavior of n-type oxide sensors. The adsorption and oxidation reaction energies for anatase-phase based TiO2 sensors are extracted from comparisons with experimental data. By predicting the effects of dopants, temperature, and sintering on the sensor response, the IRC model foretells improvement of the sensitivity and other gas sensor properties.; The depletion width prior to reducing gas exposure in an n-type metal oxide sensor is calculated by combining the known defect chemistry and the various phenomena determining electron depletion near the oxide surface. These include the charged surface of adsorbed oxygen ions, dopant segregation, and ionic defect accumulation. The sensor response is found to enhance if the depletion width is comparable to the grain size. Affecting the initial depletion width via dopants or ionic defect equilibration provides an alternate route towards sensor response optimization. The Polychromatic Percolation Model (PPM) is introduced, and facilitates the design of a selective n/p composite sensor by predicting the sensor response as a function of the n-type/p-type composition. Conduction through an n-type/p-type composite containing randomly distributed and oriented grains is characterized as a percolation phenomenon, where the overall resistance of the material combines contributions from n-type and p-type pathways. The composite microstructure governs the relative fraction of each pathway type and its respective percolation threshold, while the effective resistance through each pathway is calculated using experimentally measured resistances of single-phase n-type and p-type materials.; PPM model calculations match the experimentally measured response of an anatase/rutile TiO2 composite gas sensor, and the conductivity of a ZnO/CuO n/p composite. Using the predictions of the PPM model, a selective n/p composite sensor may be designed. |
