Adewoyin, Aiyeyemi Anike; (1998) Gas sensitive resistor materials for carbon dioxide measurements in air. Doctoral thesis (Ph.D.), University College London (United Kingdom).

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Abstract

The work described in this thesis explores the use of barium stannate, BaSnO3 as a gas-sensitive resistor material, sensitive at elevated temperature to the presence of small concentrations of carbon dioxide in air. The behaviour of the substituted compounds (Ba,A')(Sn,B')O3, where A' = Ca and B' = Sb, have also been explored. Separation of effects on the measured resistance of processes occurring at the inter-grain boundaries from effects due to changes in the bulk resistance of the crystallites was made by impedance spectroscopy. Some exploration of the nature of the surface species present, and comparison with those present on other oxides, was made using Fourier transform infra-red spectroscopy (FTIR). For BaSnO3, effects on the resistivity were distinguished for concentrations of carbon dioxide in air ranging from 100 ppm to 1.0 x 105 ppm, in dry conditions. At concentrations exceeding 1.0 x 105 ppm the response law of BaSnO3 tended to saturate. These effects were clearly distinguishable from the effects of changing oxygen partial pressure. In the presence of moisture (50% r.h.) the lower detection limit was reduced to 600 ppm and the saturation effect observed in dry conditions was not evident; the response of BaSnO3 continued to increase as the carbon dioxide concentration increased. The origin of the response was at the grain boundaries of the crystallites. Both Sb- and Ca-substitution greatly decreased the response. For both BaSnO3 and Ba(Sn,Sb)O3, the sign of the response was temperature dependent. The transition temperature between the different regimes changed upon Sb-substitution, the interference due to water vapour was also changed by Sb-substitution. Surface carbonates were detected by FTIR following gas exposure at ambient temperature. A mechanism postulating the formation of a surface carbonate satisfactorily rationalised the results. Attempts to implement diffuse reflectance FTIR at elevated temperature were unsuccessful. Literature reports of the successful realisation of carbon dioxide sensor employing a BaSnO3-CuO composite addressed by impedance spectroscopy, were followed up. The possibility of enhancement of the response near the percolation limit of the conductance of the composite was investigated. Composites of BaSnO3-CuO, SnO2-A12O3 and BaSnO3-A12O3 were studied. For the BaSnO3-CuO composites the main effect of CuO addition was simply to decrease to resistivity into an easily measurable range. No enhancement of response near the percolation limit in semiconductor-insulator composites was found.

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