Modeling Chemiresistor Sensors 1: Conductivity Model

Chemiresistor sensors made from carbon black – polyisobutylene composite, perform based on the change of the resistivity of the composite when they swell in the analytes. Two models are necessary to describe the mechanism of carbon-polymer chemiresistors theoretically: the conductivity model and the thermodynamic model. In this work, the conductivity model was studied. Sixty-four chemiresistors representing 8 different carbon concentrations (8 to 60 vol% carbon) were constructed by depositing thin films of a carbon black/polyisobutylene composite onto concentric spiral platinum electrodes. The impact of carbon concentration and geometry on the measured resistance and derived resistivity of the polymer composite was determined. The thickness and surface topography of each sensor was measured with a mechanical profilometer. The derived resistivity data fit the general effective media (GEM) model adequately, and the fitted parameters predicted values for percolation threshold and carbon resistivity that were consistent with published literature. Finite element modeling showed that resistivity was a strong function of composite composition and thickness, but was relatively insensitive to the surface roughness of the composite on the sensor. The correlations developed can be used in reverse to calculate the thickness of the composite polymer film deposited on the solid substrate from a measurement of resistance in dry air.