Resistivity measurements of carbon–polymer composites in chemical sensors: impact of carbon concentration and geometry

Chemiresistor sensors comprised of conductive polymer composites have shown great potential in identifying gaseous analytes. The performance of these sensors depends on a number of parameters, including the geometry and concentration of the conductive component dispersed in the polymer. In this study, 64 chemiresistors representing eight different carbon concentrations (8–60 vol.% carbon) were constructed by depositing thin films of a carbon black–polyisobutylene composite onto concentric spiral platinum electrodes on a silicon chip. 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, and the resistance of each sensor was measured in dry air at room temperature. Finite element modeling was used to correlate the thickness and measured electrical resistance with the intrinsic resistivity of the polymer–carbon composite. 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. Further 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 herein 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.