SELECTIVITY AND COMPOSITION DEPENDENCE OF RESPONSE OF GAS-SENSITIVE RESISTORS .1. PROPANE CARBON-MONOXIDE SELECTIVITY OF BA6FEXNB10-XO30 (1-LESS-THAN-OR-EQUAL-TO-X-LESS-THAN-OR-EQUAL-TO-2)

A series of tetragonal tungsten bronze structure compounds of formula Ba6FexNb10-xO30 (1 < x < 2) have been made and characterised by X-ray powder diffraction and electron microprobe analysis. The compounds were ostensibly single-phase materials, but since their electrical conductivity did not vary with composition, their structure was interpreted as being Ba6FeNb9O30 with intergrowths of, possibly, BaFe2O4. The gas-sensing behaviour to propane and carbon monoxide was investigated. The compounds were n-type semiconductors, exhibiting a resistance decrease in the presence of ppm levels of the gases in air. Pellets were found to show some selectivity to propane over carbon monoxide which increased with temperature in the range 400-485 degrees C. The selectivity was shown to be dependent on the porosity of the pellets. There was no systematic variation of either sensitivity or selectivity with stoichiometry, x. Temperature-programmed mass spectrometric experiments showed that the onset of carbon monoxide oxidation occurred at 310 degrees C over these compounds, while propane combustion started at 420 degrees C. The propane-carbon monoxide selectivity was interpreted as being due to differences in the combustion kinetics of the two gases which resulted in different gas concentration gradients within the pellet. This was confirmed by using a novel electrode geometry, which enabled the resistance to be measured simultaneously at both the centre and the edge of a pellet.