Solid state ceramic gas sensors based on interfacing ionic conductors with semiconducting oxides

Solid-state ceramic NO_x sensors based on interfacing an ionic conductor (NASICON) with semiconducting oxides (rare earth perovskite-type oxides) were investigated. NASICON powders were pressed into thimbles 12 mm long with 3 mm inner diameter and 4 mm outer diameter, then sintered at 1270°C in air. A Pt wire was attached to the outer surface of the tubes using a platinum paste. A uniform Au/Pd (60 wt.%) coating, permeable to oxygen but not to NO_x, was sputtered for 40 min on the sensor external surface to allow the exposure of both electrodes to the gas atmosphere without using reference air. Windowless energy-dispersive spectroscopy (EDS) was used to evaluate the chemical composition of the Au-Pd layer before and after the performance of sensing tests. Sodalite powder as an auxiliary phase was tightly packed into the NASICON thimbles with a Pt lead for the electrical contact. To get an in-situ NO conversion to NO₂, a Pt-loaded alumina powder was used as a catalyst and incorporated with the sensor on the top of the auxiliary phase. Nano-sized and chemically-pure rare earth perovskite-type oxide (LaFeO₃, SmFeO₃, NdFeO₃ and LaCoO₃) powders, prepared by the thermal decomposition of the corresponding hexacyanocomplexes, were also used in the electrochemical cells. Each of the tested oxides was packed into the thimbles replacing the sodalite and the Pt-loaded alumina catalyst. Tests were performed also using only the perovskitic oxides. The microstructure of the materials tested was evaluated using scanning electron microscopy (SEM). The NO₂ sensing properties of the prototype sensors were investigated at controlled temperature (in the range 300-600°C) by measuring the electromotive force (EMF) at different NO₂ concentrations (in the range 2-2000 ppm in air). Some measurements were performed at various NO concentrations diluted with Ar. The results obtained showed a promising NO₂ sensing performance when ferrites were used. SmFeO₃ has a lower catalytic effect on NO oxidation than the Pt-loaded alumina catalyst, and has a similar effect to sodalite when used as auxiliary phase. The perovskite-type oxides are more preferable as auxiliary phase than sodalite because they improve the stability of the electrochemical sensor performances.