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

Abstract

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.

Exploitation of Unique Properties of Zeolites in the Development of Gas Sensors

“…The other group of molecules that has been extensively studied are hydrocarbons [ 58 – 60 ]. Impedance spectra of a device made by coating Cr 2 O 3 on top of an ion-conducting Pt-doped ZSM-5 exhibited several semicircles, with a hydrocarbon dependent semicircle in the medium frequency range [ 58 ] (though later studies from the same group showed that it is the low frequency part that exhibits the increase in the presence of the hydrocarbons) [ 59 , 61 ].…”

“…A lower cost fabrication technology for the Cr 2 O 3 /zeolite sensor has also been proposed. A recent paper examined the mechanism at the Cr 2 O 3 /zeolite interface more carefully, and concluded that the hydrocarbon alters the charge carrier density in the Cr 2 O 3 layer, which leads to the observed impedance changes [ 60 ]. In order to observe the response to hydrocarbon, the design has to include electrode-Cr 2 O 3 – zeolite – Cr 2 O 3 – electrode, or electrode – zeolite – Cr 2 O 3 – zeolite-electrode.…”

Insights in the Ionic Conduction inside Nanoporous Metal-Organic Frameworks by Using an Appropriate Equivalent Circuit

“…Appropriate equivalent circuits where each component has a clear physical meaning have also been introduced for various nanoporous systems. Examples are MOF films with (low) electronic conductivity on substrates with interdigitated gold electrodes (IDEs) [ 35 ], zeolite-Cr 2 O 3 films on IDEs used as gas sensors [ 36 , 37 ], zeolites embedded in polymer membranes [ 38 ], zeolite films as coatings on alloys [ 39 ], or for the film growth of a zeolite membrane [ 40 ]. To the best of our knowledge, a critical discussion of an equivalent circuit for the ionic conduction in nanoporous non-conducting materials, like (most) MOFs between two inert electrodes like gold is missing.…”

Characterization of zeolite-trench-embedded microcantilevers with CMOS strain gauge for integrated gas sensor applications

“…[15][16][17][18] There are also gas sensors based on the nature of ion exchange for reading the change in conductivity. [19][20][21][22][23] Optical sensors use zeolite for gas detection as well. [24][25][26] The lowering of the sensitivity limit has been continued by the development of mass sensors such as QCM by MEMS=NEMS cantilevers.…”