Gellman, Laura Jane; (1995) The effects of microstructure and surface chemistry on the response of gas-sensitive resistors. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This work describes the gas-sensing properties of niobate oxide semiconductors. Three groups of compounds were investigated; (a) tetragonal tungsten bronze-type series Ba6FexNb10-xO30 (this was found to be basically Ba6FeNb9O30 with an impurity phase and (speculatively) intergrowths of a spinel phase), (b) rutile type solid solution series CrxFe1-xNbO4 and (c) mixed phase FexNb2-xO4 (found to be a mixture of different phases of FeNbO4 and the unreacted oxides). These materials function as gas-sensitive resistors in that they exhibit considerable changes in their electrical resistance upon exposure to reducing gases present in low concentrations in air. Variation of stoichiometry and systematic substitution to form solid solutions were investigated with respect to their influence on the gas response to propane and carbon monoxide. These materials were synthesised using standard solid state ceramic methods and used in pellet form. They were characterized by X-ray powder diffraction (XRD), scanning-electron microcopy (SEM), Mossbauer spectroscopy, energy dispersive analysis of X-rays (EDAX) and electron probe microanalysis (EPMA) techniques. Two major factors influencing the gas response were identified. Firstly, the response was strongly dependent on the microstructure of the material as a result of the effects of reaction and gas diffusion within the porous body. The consequence of such a reaction was a concentration gradient within the pellet and hence a variation in electrical conductivity. These effects were particularly pronounced for Ba6FexNb10-xO30. Secondly, in the series CrxFe1-xNbO4, the responses were strongly inhibited by a surface species which could be desorbed at higher temperature. This species was found to be hydroxyl groups bound to the oxide surface. In addition, it was found that both the chromium iron niobates and the iron niobates were poisoned by prolonged continuous exposure to carbon monoxide at high temperature. It was proposed that this was due to reaction of carbon monoxide with lattice oxygen to form subsurface oxygen vacancies.

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