Taylor, Alan (1992) The chemical synthesis and crystallisation sequence of mullite. PhD thesis, University of Warwick.

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Abstract

Mullite stoichiometry powders have been produced using a "water-free" sol-gel approach. The powders are fabricated by mixing aluminium isopropoxide and tetraethyl silicate (TEOS) in butan-l-ol, under conditions where the amount of water allowed into the system from external sources is minimised. The powders are obtained from the solution by rapid removal of the alcohol and other volatile organics. The sols that are produced by the unidentified condensation reactions are then deposited. A homogeneity coefficient has been defined, and is dependant on the crystalline phases that are produced on heating the material to completion of the 980°C exotherm. Two crystalline phases are seen on such a heat treatment, spinel and mullite. This semi-quantitative coefficient is defined as the ratio of the peak height of the x-ray reflections from the (440) plane of spinel and the (331) plane of mullite. Using this definition there is a reproducible and regular variation in the homogeneities of the powders yielded when compared with the length of time the solution was stirred before the sols were deposited. With the concentration used, 0.25M for the t'sopropoxide and 0.083M for the TEOS, the sols deposited early in the reaction were relatively inhomogeous. At approximately 19 hours after stirring of both reactants had begun, the homogeneity increased. That is, the amount of mullite that was yielded after completion of the exotherm increased. Continued stirring of the solution gave less homogeneous materials. This behaviour is novel in terms of sol- gel fabrication. The amount of water present in the system, from contamination of the reactants is insignificant compared to the amount of water demanded by the traditional water consuming reactions of sol-gel. Aluminium alkoxides have been found to react directly with alcohols and this reaction has been proposed as being central to the condensation reactions that have been inferred as occurring by the production of Al-O-Si bonds. A phenomenological explanation of the homogeneity has been proposed based on the aluminium alkoxide being the active species after reaction with the alcohol. Linear polymeric species are formed increasing the overall homogeneity of the system, a point is reached in the reaction where the polymers cross-link via aluminium atoms rather than grow in length, the homogeneity of the sols then decreases.
The crystallisation sequence of the materials has been explained in terms of the homogeneity In particular, the detailed structure of the materials has been related to the coordination of aluminium. At low levels of homogeneity, the material is grossly segregated in terms of the coordination polyhedra of aluminium and silicon. The alumina rich regions have aluminium preferentially in octahedral environments. These materials crystallise to predominantly spinel, an alumina polymorph that probably contains a small amount of silica. Increasing the homogeneity decreases the size of the alumina rich regions. These still exist however due to the stoichiometry of the system. As they decrease in size the interfacial surface area between these regions and the silica rich matrix increases. It is proposed that pentacoordinate aluminium is found in these interfacial areas. The pentacoordinate aluminium is metastable and transforms at 980°C to octahedral and tetrahedral coordinations. It is this transformation that initiates crystallisation of the material. The smaller the volume occupied by the inhomogeneities the more mullite is yielded since the aluminium and silicon are more intimately mixed. Heat treatments of the amorphous material below this temperature will yield spinel from the crystallisation of the unstable alumina rich regions, with some mullite being formed at the interface. Again the smaller the inhomogeneities the more mullite is produced by such a treatment

Item Type:	Thesis (PhD)
Subjects:	Q Science > QC Physics Q Science > QD Chemistry
Library of Congress Subject Headings (LCSH):	Mullite, Crystallization
Official Date:	December 1992
Dates:	Date Event December 1992 UNSPECIFIED
Institution:	University of Warwick
Theses Department:	Department of Physics
Thesis Type:	PhD
Publication Status:	Unpublished
Supervisor(s)/Advisor:	Holland, Diane
Sponsors:	Science and Engineering Research Council (Great Britain) ; Pilkington Glass Ltd.‏
Format of File:	pdf
Extent:	ix, 114 leaves : illustrations
Language:	eng

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