An Initial Physics-Based Model for the Impedance Spectrum of a Hydrocarbon Sensor with a Zeolite/Cr₂O₃ Interface

Planar interdigital capacitors (IDC) covered with a thin Cr₂O₃ layer and a thick film of platinum-doped Na-ZSM-5 zeolite, are known to act as very selective impedometric hydrocarbon gas sensors. In this contribution, the impedance spectra are calculated taking into account the ionic conductivity of the zeolite and the p-type semiconductor properties of Cr₂O₃ as well as the blocking electrode characteristics of the zeolite/Cr₂O₃ interface. A simplified differential equation for the charge velocity in the zeolite is derived and solved for a driving sinusoidal voltage. Thus, the time-dependent current through the zeolite and the Cr₂O₃ is obtained. By Fourier transforming the current, the IDC impedance spectrum is calculated. Since the ambient hydrocarbon concentration influences the charge carrier density in the Cr₂O₃ film, the calculated sensor current responds to the concentration change, accordingly leading to a good agreement between measured and calculated impedance spectra.