DESIGN OF CONDUCTING POLYMER GAS SENSORS - MODELING AND EXPERIMENT

The use of conducting polymers as active materials in chemical sensors is growing rapidly; for example they have been used in the place of metal oxides to sense gases and vapours, such as NH3, NO2 and alcohols. Here we model a polymer gas sensor in terms of homogeneous diffusion coupled to simple adsorption within a bounded layer. From the model we present analytical expressions of the adsorbate profiles for the diffusion-rate limited, reaction-rate limited and intermediate cases in terms of fundamental dimensionless parameters. The model is then used to calculate the conductance of a typical chemiresistor which consists of a pair of co-planar electrodes below on electropolymerised thin polymer film and on an impermeable substrate. The analytical expression for the electric field is combined with the diffusion reaction equations by assuming an single carrier conduction model. Finally, the theoretical chemiresistor response is calculated in six limiting cases and compared with experimental data on pyrrole-based conducting polymers. In practice the gas-polymer interaction is likely to be much more complex and so we are extending our model to consider in more detail the conduction principles.