Solid state ceramic gas sensors based on interfacing ionic conductors withsemiconducting oxides

Solid-state ceramic NOx sensors based on interfacing an ionic conductor (NA SICON) 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 3 mm outer diameter, then sintered at 1270 degreesC i n air. A Pt wire was attached to the outer surface of the tubes using a pla tinum paste. A uniform Au/Pd (60 wt.%) coating, permeable to oxygen but not to NOx, was sputtered for 40 min on the sensor external surface to allow t he exposure of both electrodes to the gas atmosphere without using referenc e air. Windowless energy-dispersive spectroscopy (EDS) was used to evaluate the chemical composition of the Au-Pd layer before and after the performan ce of sensing tests. Sodalite powder as an auxiliary phase was tightly pack ed 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 ph ase. Nano-sized and chemically-pure rare earth perovskite-type oxide (LaFeO 3, SmFeO3, NdFeO3 and LaCoO3) powders, prepared by the thermal decompositio n of the corresponding hexacyanocomplexes, were also used in the electroche mical cells. Each of the tested oxides was packed into the thimbles replaci ng the sodalite and the Pt-loaded alumina catalyst. Tests were performed al so using only the perovskitic oxides. The microstructure of the materials t ested was evaluated using scanning electron microscopy (SEM). The NO2 sensi ng properties of the prototype sensors were investigated at controlled temp erature tin the range 300-600 degreesC) by measuring the electromotive forc e (EMF) at different NO2 concentrations tin the range 2-2000 ppm in air). S ome measurements were performed at various NO concentrations diluted with A r. The results obtained showed a promising NO2 sensing performance when fer rites were used. SmFeO3 has a lower catalytic effect on NO oxidation than t he PI-loaded alu mina catalyst, and has a similar effect to sodalite when u sed as auxiliary phase. The perovskite-type oxides are more preferable as a uxiliary phase than sodalite because they improve the stability of the elec trochemical sensor performances. (C) 2000 Elsevier Science Ltd. All rights reserved.