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In this paper, a new nonlinear diffusion-adsorption model for the steady-state and transient response of thick-film tin oxide gas sensors in the presence of multicomponent gas mixtures is discussed in detail. The model, which takes into account gas interaction during the adsorption process, accurately accounts for the sensor behavior for a wide range of gas concentrations. The model predictions were validated by measuring binary mixtures of ethanol, toluene, and ethylbenzene with commercially available thick-film tin oxide gas sensors. New parameters that may be useful for identifying components in a gas mixture were obtained by adjusting experimental and theoretical transients.

Most of the chemical sensors described in the literature are based on inorganic semiconducting oxides. In some cases, the main oxide is modified by doping with small amounts of additives such as other oxides and/or metals. The sensing principle is based on the change in the conductance undergone by the oxide film when gases are adsorbed and react on its surface. A survey of typical sensor materials for detecting different gases in characteristic temperature ranges can be found elsewhere. 1,2 In recent years, some gas-sensitivity studies with tungsten trioxide (WO 3 ) based semiconductors have been reported. Pure or doped tungsten oxide is a promising material for the detection of nitrogen oxides (NO and NO 2 ) 3-6 and sulfur dioxide, 7 two substances which are considered to be responsible for ambient degradation together with carbon oxides and hydrocarbons. The gas-sensing properties of WO 3 are highly dependent on the deposition method. Tungsten oxide is generally deposited by reactive rf sputtering. 8-10 However, other techniques such as thermal evaporation, 11,12 sol-gel methods, 13 screen printing, 14 or photochemical production 6 have been reported to grow thin or thick active