Integrated impedance based hydro‐carbon gas sensors with Na‐zeolite/Cr2O3 thin‐film interfaces: From physical modeling to devices

The impedance at temperatures of some hundred degree Celsius of sodium ion conducting zeolites applied on planar interdigital gold electrodes covered with a thin Cr2O3 film changes very sensitively and selectively when exposed to hydrocarbons. In contrast to comparable ammonia sensors, it was found that the sensing effect occurs at the electrodes, namely at the zeolite/Cr2O3 interface. To explain the sensor effect, impedance spectra are calculated with a model that considers the zeolite conductivity, the semiconducting properties of Cr2O3, and the zeolite/Cr2O3 interface characteristics. A differential equation to describe the time‐dependent current through zeolite and Cr2O3 is derived. The impedance spectra are then extracted from the complex amplitude of the first harmonic I1 in the Fourier series associated with this periodic current function. The hydrocarbon concentration influences the charge carrier density in the Cr2O3 film, thus leading to the observed impedance changes. The simulated impedance spectra reproduce the important features of the measured spectra quite well.To avoid photolithographic and thin‐film processes for manufacturing a prototype sensor, the technology was transferred to the established industrial thick‐film hybrid technology. Electrodes and zeolites were screen‐printed and the Cr2O3 film was electroplated. Prototype sensors show a good and long‐term stable sensitivity toward hydrocarbons. Interfering gases like NO, CO, or H2 do not affect the sensor signal very strongly, but an unexpected pronounced response toward ammonia was observed.