Integrated impedance based hydro-carbon gas sensors with Na-zeolite/Cr_2O_3 thin-film interfaces: From physical modeling to devices

摘要

The impedance at temperatures of some hundred degree Celsius of sodium ion conducting zeolites applied on planar interdigital gold electrodes covered with a thin Cr_2O_3 Sim 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/ Cr_2O_3 interface. To explain the sensor effect, impedance spectra are calculated with a model that considers the zeolite conductivity, die semiconducting properties of Cr_2O_3, and die zeolite/Cr_2O_3 interface characteristics. A differential equation to describe die time-dependent current through zeolite and Cr_2O_3 is derived. The impedance spectra are then extracted from the complex amplitude of the first harmonic I_ in the Fourier series associated with this periodic current function. The hydrocarbon concentration influences the charge carrier density in the Cr_2O_3 film, thus leading to the observed impedance changes. The simulated impedance spectra reproduce the important features of the measured spectra quite well.To avoid photolidiographic and thin-film processes for manufacturing a prototype sensor, the technology was transferred to die established industrial thick-film hybrid technology, Electrodes and zeolites were screen-printed and the Cr_2O_3 film was electroplated. Prototype sensors show a good and long-term stable sensitivity toward hydrocarbons. Interfering gases like NO, CO, or H_2 do not affect die sensor signal very strongly, but an unexpected pronounced response toward ammonia was observed.