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Queen Mary University of LondonQueen Mary University of London
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School of Engineering and Materials Science
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PhD Thesis: Novel precursors for SiC-AIN ceramics: synthesis, structural characterisation and conversion

Author: LI, Xudong

Year: 2004

Supervisor(s): Mohan Edirisinghe

A novel Si-Al-C-0 precursor was developed via the reactions between polysilane (PS) and aluminum acetylacetonate (AlAce). The PS used in this research was synthesized by the Wurtz sodium metal dechlorination of a monomer combination of dichloromethylphenylsilane, dichloromethylvinylsilane and trichlorophenylsilane. Varying ratios (by weight) of PS and AlAce, e.g. 0.25, 0.50, 1.00, 2.00 and 4.00, were applied during synthesis to produce a family of the Si-Al-C-O precursors which pyrolyze in nitrogen at 1700°C to SiC-AIN ceramics with composition dependent on the initial AlAce/PS ratio.

Fourier transform infrared and solid state 29Si, 27A1 and 13C nuclear magnetic resonance (NMR) spectroscopies revealed that during synthesis chemical reactions occurred, including the Si-H, Si-Si bonds and the phenyl group of PS and the ligands of AlAce, which resulted in the changes in the ligands of AlAce and formation of Al-O, Si-0 and Si-O-Al linkages. The structure of the precursors is controlled by the initial AlAce/PS ratio, which finally leads to different complete phase assemblages of the pyrolysis products as indicated by X-ray diffraction and NMR analyses. The products of the precursors with a higher AlAce/PS ratio, such as 2.00 and 4.00, are SiC-AIN solid solutions, whereas the products from a low AlAce/PS ratio are a mixture of β-, α-SiC and A1N. 27Al NMR results further indicated there exists an intermediate Al local coordination environment between AlOn and AlN4 sites.

Structural evaluation of polysilanes with increasing temperature was also studied in terms of SiC crystallization and free carbon organization. The related studies provide useful information for elucidation of the precursor-to-ceramic conversion which is associated with carbothermal reduction and nitridation and SiC crystallization from the amorphous SiC4 moieties.