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 wer e ostensibly single-phase materials, but since their electrical conduc tivity 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 decreas e in the presence of ppm levels of the gases in air. Pellets were foun d to show some selectivity to propane over carbon monoxide which incre ased 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 stoich iometry, x. Temperature-programmed mass spectrometric experiments show ed that the onset of carbon monoxide oxidation occurred at 310 degrees C over these compounds, while propane combustion started at 420 degre es 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. T his was confirmed by using a novel electrode geometry, which enabled t he resistance to be measured simultaneously at both the centre and the edge of a pellet.