Investigation of SnO 2/Na +-ionic conductor composites for new gas sensitive layers

Abstract

The influence of the sodium-ions in SnO 2/NASICON (Na super ionic conductor) admixtures on the selectivity and sensitivity of metal oxide gas sensors was studied. NASICON (Na 1+ x Zr 2Si x P 3− x O 12, 0≤ x≤3) was chosen as a model substance in which the content of sodium ions can be varied in the range 0≤ x≤3. For that purpose SnO 2 powder was prepared and mixed in a volume ratio of 8:2 with NASICON powder, screen printed and sintered to obtain a gas sensitive thick film composite. A drastic improvement of the sensitivity to ROH, RCHO and RCOOH ending molecules and a reduction of the sensitivity to molecules like CO, H 2, NH 3, propane, propene, 2-propanol and propanone was observed in comparison to pure SnO 2. This effect is clearly pronounced with increasing concentration of the sodium ions in the NASICON structure. For comparison, admixtures with the anionic conductor yttrium stabilized zirconia (YSZ) and with the typical isolator Al 2O 3 showed equal or also higher sensitivities to the most tested substances compared to pure SnO 2 but no pronounced selectivity. A preliminary model is presented which explains the unusual gas sensitivity of SnO 2/NASICON composites to some individual gas components by specific molecular interactions at the NASICON-surface and by sensitivity amplification due to e −–Na + interactions across the SnO 2/NASICON interface.