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Abstract

The purpose of this project was to determine the suitability of employing the technique of chemical vapour deposition for the production of refractory oxide supports, for use in catalytic gas detectors. These are usually prepared by successive thermal decompositions of an appropriate salt solution. Chemical vapour deposition is an attractive alternative as the oxide bead is deposited in a single continuous process, thus minimizing contamination problems.

Attempts were made first to coat platinum coils with alumina using aluminium isopropoxide as the starting reagent. Various coil temperatures, flow rates and deposition times were used, but dense, coherent, thick deposits could not be produced. Extensive amounts of impurities were incorporated into the product and dendritic-type growth was observed when the preparation of thick deposits was attempted.

Attention was then turned to the production of titania from titanium tetrachloride. Dense, blue/grey polycrystalline rutile titania was produced between the temperatures of 480 and 800°C using three oxygen concentrations. A Fortran IV computor programme was constructed in order to predict the maximum growth rates obtainable under mass transport control. Good agreement was obtained between experimental and theoretical maximum growth rates and a temperature of ~g00°C, indicating that the boundary layer model used in the calculations was applicable and that mass transport was rate controlling at this temperature.

At lower temperatures the experimental growth rates and calculated percentage conversions showed that rate-control involved both mass transport and surface kinetics. On
the basis of evidence, the reaction was assumed to be first order, and the growth rate by surface kinetics alone was calculated. A value for the "true" energy of activation was thus obtained. The surface mechanism appears to conform to Rideal-Eley kinetics with oxygen dissociatively adsorbed on titania and reacting with gaseous titanium tetrachloride. The rate-controlling step for reaction in the gas phase is considered to be dissociation of the first chlorine from titanium tetrachloride; results obtained in this work are compared with other data in the literature in the light of this assumption.

In summary, the technique of chemical vapour deposition appears to be a highly satisfactory method for the preparation of gas detectors using titania as support. For preparation of alumina supports, the method is not effective.

Publication Type:	Thesis (Doctoral)
Subjects:	Q Science > Q Science (General) Q Science > QD Chemistry
Departments:	School of Science & Technology > School of Science & Technology Doctoral Theses Doctoral Theses

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