Silicon oxynitride, precursors

Silicon nitride has also been prepared from SiCl4/NH4 [159], SiBr4/NH3 [160], and, more recently, Si2CVNH3 [161]. Silicon oxynitride (SiON) is readily prepared by use of any of the precursors used for silicon nitride with the addition of either N2O or NO as an oxygen source [162, 163, 164]. The composition and properties of the SiO N films may be varied linearly from Si02-like to Si3N4-like by the variation of the reactant flow rates. 

SiC fibers were produced using polycarbosilanes by Yajima et al. in 1975 [1,2]. Besides SiC fibers, Si-Ti-C-O fibers prepared from a polytitanocarbosi-lane have been obtained by adding a titanium tetrabutoxide to polycarbosilane or polysilane [3]. SiC fibers (Nicalon) and Si-Ti-C-O fibers (Tyranno) are manufactured on an industrial scale. Colorless silicon oxynitride fibers and silicon nitride fibers [4] have been obtained by the nitridation of polycarbosilanes in the author s laboratory. Polymers used for ceramic precursor and the resulting ceramic fibers are listed in Table 1. 

Silicon oxynitride exhibits better resistance to thermal downshock and oxidation than silicon nitride, although it decomposes under similar conditions with loss of N2 . Thus, it represents a practical target for precursor synthetic methods. Unfortunately, it is difficult to achieve the exact 2Si 1012N stoichiometry in a processable precursor and retain this ratio during pyrolysis. Hence, very little work has been directed toward developing Si20N2 precursors . Furthermore, the approaches used to date rely on processing with NH3 to achieve the correct stoichiometry. 

Poly(carbosilane) fibres have been employed recently as precursors to silicon nitride and silicon oxynitride fibres. The preparation of silicon nitride fibres involves a two-stage process. The precursor fibres are first crosslinked by electron irradiation then heated in ammonia at temperatures up to 1400 °C. Electron irradiation is used to effect the crosslinking reaction without incorporation of oxygen. The nitridation reaction takes place between 500 and 800 °C. At a pyrolysis temperature of 1400 °C, essentially all the carbon in the precursor is lost, giving a composition close to that of pure silicon nitride. At this temperature, however, the fibre was extensively crystallized and the tensile strength was very low. The tensile strength of the fibre was shown to reach a maximum level of about 1.4 GPa at a nitridation temperature of 1300°C. 

Si-C-N(O) fibers derived from HPZ precursor fibers are nanoporous and heterogeneous with a skin/core structure. The composition changes from SiOxCy in the external porous surface to SiNxC, in the core. The molecuiar formuia of this fiber is close to 4 mol. >4 SiOa, 81 mol.% SiNxCy (x = 1.02, y = 0.23) and 15 mol.% free C [22]. The presence of complex tetrahedral units is supported by the Si NMR spectrum which shows a broad signal covering the chemical shift region expected for silicon oxycarbide, siiicon oxynitride and silicon carbonitride units [21]. The occurrence of free carbon, expected from the nature of the precursor, is supported by the C Is XPS pattern [22]. 

Mutin PH. Control of the composition and structure of silicon oxycarbide and oxynitride glasses derived from polysUoxane precursors. J. Sol-Gel Sci. Technol. 1999 14 27-38 Nomura K., Takasuka Y., Kamiya K., Nasu H. Preparation ofNbN fibres by nitridation of sol-gel derived Nb20s fibres. J. Mater. Sci., Electronics 1994 5 53-58... 

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