Investigation of the influence of solid ionic conductor admixtures on the performance of gas sensors based on tin oxide

Abstract

Tin oxide is a well-known and widely used gas sensitive material for detection of toxic and hazardous gas components in air. Different admixtures such as catalysts, aliovalent ions and solid ionic conductors were added to improve the sensitivity, selectivity and stability of these sensors. For material preparation a sol-gel route is usually used and catalysts are added before sintering to improve the sensitivity and to favour the sensors to specific gas components. In this work we investigated the influence of added solid ionic conductors on the gas sensitivity of tin oxide based ceramic admixtures dependent on the relative parts of the oxidic sodium ionic conductor, the ionic conductivity and the grain size in relation to the tin oxide grains. Almost ideal model substances for these studies are solid electrolytes with the NASICON (Na1+xZr2SixP3−xO12) structure. They can be prepared in the sol-gel route and in a solid state reaction as well resulting in different powder grain sizes. The sodium ionic conductivity of the solid electrolytes in the composition range 0≤×≤3 varies by several orders of magnitude. Both materials, the NASICON and the tin oxide, were characterized with respect to morphology and structure by BET and XRD measurements. Powders of tin oxide were homogeneously mixed with different NASICON powders, pressed as discs, sintered, and contacted with gold electrodes for measurements of the conductivity in air at various temperatures (200°C≤T≤400 °C). For preparation of sensor layers the powder mixtures were transferred to a paste, screen printed on an alumina substrate with interdigital gold electrodes and sintered for gas sensitivity measurements. A dramatic sensitivity enhancement of the composite to alcohol and significant sensitivity reduction to other gases like CO, H2, NH3, CH4, C3H8 was found.