Metal stannates and their role as potential gas-sensing elements

Abstract

A selective gas sensor, sensitive to the presence of carbon monoxide in preference to the lower hydrocarbons, can be fabricated from a mixture of bismuth oxide and tin dioxide when sintered at 800 °C. At temperatures above ca. 650 °C a solid-state reaction takes place in which bismuth stannate (Bi₂Sn₂O₇) is formed and in the above sensor all of the Bi₂O₃is converted to the stannate. This material is one of a group of mixed oxide stannates which possess a pyrochlore structure and have the general formulae M₂Sn₂O₇. Several of these materials can be produced by heating an intimate mixture of lanthanum metal oxides (M₂O₃ where M = La, Nd, Sm, Gd, Yb, Dy, Tm and Ho) and tin dioxide at temperatures of 1500 °C. Sensors were produced containing these materials in an attempt to reproduce the behaviour of the original device and further understand the chemical, physical and topographical features responsible for conferring selectivity. However, none of the new sensors produce results consistent with those observed for the tin–bismuth system. It has subsequently been shown that when SnO₂ is subjected to heat treatment at 1500 °C, it can exhibit both resistance increases and decreases upon exposure to the same gas, depending on the operating conditions. In order to reduce these interfering effects the sintering temperature was lowered to 1350 °C and sensors fabricated from pure tin dioxide fired at this temperature respond in a conventional manner to all reducing gases tested. A series of sensors produced from some of the SnO₂/M₂O₃ materials listed above exhibit a general trend of increasing carbon monoxide and hydrogen sensitivity with decreasing M³⁺ ionic radius.