METAL STANNATES AND THEIR ROLE AS POTENTIAL GAS-SENSING ELEMENTS

A selective gas sensor, sensitive to the presence of carbon monoxide i n preference to the lower hydrocarbons, can be fabricated from a mixtu re of bismuth oxide and tin dioxide when sintered at 800 degrees C. At temperatures above ca. 650 degrees C a solid-state reaction takes pla ce in which bismuth stannate (Bi2Sn2O7) is formed and in the above sen sor all of the Bi2O3 is converted to the stannate. This material is on e of a group of mixed oxide stannates which possess a pyrochlore struc ture and have the general formulae M(2)Sn(2)O(7). Several of these mat erials can be produced by heating an intimate mixture of lanthanum met al oxides (M(2)O(3) where M = La, Nd, Sm, Gd, Yb, Dy, Tm and Ho) and t in dioxide at temperatures of 1500 degrees C. Sensors were produced co ntaining these materials in an attempt to reproduce the behaviour of t he original device and further understand the chemical, physical and t opographical features responsible for conferring selectivity. However, none of the new sensors produce results consistent with those observe d for the tin-bismuth system. It has subsequently been shown that when SnO2 is subjected to heat treatment at 1500 degrees 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 inte rfering effects the sintering temperature was lowered to 1350 degrees C and sensors fabricated from pure tin dioxide fired at this temperatu re respond in a conventional manner to all reducing gases tested. A se ries of sensors produced from some of the SnO2/M(2)O(3) materials list ed above exhibit a general trend of increasing carbon monoxide and hyd rogen sensitivity with decreasing M(3+) ionic radius.