Flammable gas sensors based on sol-gel materials

The sol-gel route to the fabrication of metal oxide thin film gas sensors offers promising advantages over traditional manufacturing technologies. In particular, the low temperature fabrication process and reliable doping procedure allows the production of high surface area, high porosity materials. Metal alkoxide and metal chloride precursors have been used to prepare high surface area SnO₂ materials; Pt and Pd catalysts were added to the surface by evaporation of the appropriate metal salt. X-ray Powder Diffraction (XRPD) studies have shown these materials to be nanocrystalline with an average particle size of 2-3 nm. The use of polyacrylic acid as an organic template has been explored to improve the porosity of the materials. In this paper we present the preliminary results of a study into the detection of mixtures of low ppm concentrations of flammable gases and vapours. Initial studies of the response of unmodified SnO₂ to simple binary mixtures of vapours and gases have shown that the response is not easily interpreted in terms of the individual gas responses and there is some evidence of interference effects. The response of Pt or Pd modified SnO₂ materials to gas mixtures does however show additive effects. This can be attributed to the complete oxidation of the gases and hence the removal of interfering reaction intermediates. However, reductions in response are observed due to the increased competition for active catalytic sites on the surface. The materials are capable of selectively detecting carbon monoxide in the presence of high concentrations of methane at very low temperatures (less than 100°C) making them suitable for use as inexpensive domestic CO sensors