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.
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Document Type: Research Article
Publication date: December 1, 2008
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