Metal stannates and their role as potential gas-sensing elements

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 about 650 C a solid-state reaction takes place in which bismuth stannate (Bi2Sn2O7) is formed and in the above sensor all of the Bi2O3 is 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. 12 refs.