Novel Routes to High Purity Oxides

Adams, Jeffrey (1991) Novel Routes to High Purity Oxides. PhD thesis, University of Glasgow.

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Advances in the applications of ceramic material presently being introduced require products with chemical and physical properties tailored to the particular application. To attain the high standards necessary, a number of new methods of producing ceramics have been developed. One of these, organo-sol-gel processing, is the subject of the work described here. Silica is produced by the hydrolysis of tetraethylorthosilicate and the properties of the product are related to the reaction conditions leading to its production. Because the reaction is catalysed by both acid and base, the results are significantly different in each case, the two systems are dealt with separately. Under base catalysis the product morphology may be of interest as a support for metal catalysts so a third section deals with the incorporation of nickel compounds within the silica. The first section of the thesis deals with reactions prepared under neutral or acidic conditions. These conditions lead to the formation of clear rigid gels. The material was studied both during and after completion of the reaction by x-ray diffraction, infrared spectroscopy, thermogravimetric analysis, microanalysis, transmission electron microscopy and electron diffraction. The x-ray diffraction results were those of amorphous material and did not alter significantly on alteration of the reaction composition. Similarly the microanalysis and TGA results were not sensitive to alterations in the reaction conditions. The infrared spectra suffered from a similar problem giving the spectra of silica with little difference, but in a few cases peaks corresponding to carbonyl groups appear and may indicate the catalytic activity of the silica. TEM and electron diffraction were the most useful techniques. A number of sample preparation methods were used including dispersion in water or organic solvents, preparation of films, freeze drying, thin sectioning, removal of small areas by contact and staining with uranyl acetate. These all gave similar results where the product appeared to be small particles which form chains and 3D networks or material which forms plate-like areas of sample which appeared layered in nature. A number of these areas, especially the latter type, appeared crystalline when examined by electron diffraction but the patterns do not correspond to any known form of silica; this may indicate the formation of a crystalline hydrated silica. However it is not possible to determine the extent to which the preparation method influenced the observed morphology. The second section deals with the production of silica when the reaction is carried out under basic conditions. The reaction produced three distinct types of product; weak gels, colloidal dispersions and precipitates. The material was studied both during and after completion of the reaction by techniques including infrared spectroscopy, transmission and scanning electron microscopy. The product morphologies included small particles which aggregate to form 3D networks thus forming a gel, large particles of irregular shape which precipitate out of the reaction, and particles of spherical or near spherical shape with diameters up to 400nm. The size of these particles altered depending on the reaction conditions used to form them. Comparison of the size variation and particle growth rates with variations predicted from possible effects of the concentration changes on the rates of reaction and solvent properties indicate a possible nucleation and growth mechanism. The effect of including an ionic salt or polymer in the reaction support the growth mechanism proposed. Reactions employing tetrabutylorthosilicate in place of the tetraethylorthosilicate produced particles of diameter up to 1.2mum. The alteration with size, both within the tetrabutylorthosilicate reactions and in comparison with the tetraethylorthosilicate reactions, again supports the proposed mechanism. The observations also included SEM which indicated the solids formed by settling of spheres correspond to the structure of natural opal providing a dense solid with an iridescent sheen. The third section deals with the deposition of nickel complexes within the silica spheres as they grow. The original method was to deposit nickel bis(dimethylglyoximate) formed from the reaction of diacetyl, hydroxylamine and a nickel salt. However the presence of the ionic nickel salt altered the morphology of the silica and a nickel hydroxylamine chloride complex was precipitated, thus leading to the formation of particles including nickel hydroxylamine complex and silica.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Inorganic chemistry
Date of Award: 1991
Depositing User: Enlighten Team
Unique ID: glathesis:1991-78212
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 30 Jan 2020 15:36
Last Modified: 30 Jan 2020 15:36

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