Silicon nitride polymer precursors

Turning now to other types of ceramic fibre, the most important material made by pyrolysis of organic polymer precursors is silicon carbide fibre. This is commonly made from a poly(diorgano)silane precursor, as described in detail by Riedel (1996) and more concisely by Chawla (1998). Silicon nitride fibres are also made by this sort of approach. Much of this work originates in Japan, where Yajima (1976) was a notable pioneer. 

Soluble polysilane polymers can also be used as precursors to silicon carbide. The first such application, using (PhMeSi)n-(Me2Si)m copolymers ("Polysilastyrene"), was to strengthen silicon nitride ceramics. The Si3N4 ceramic body was soaked in polysilane and refired, leading to the formation of silicon carbide whiskers in the pore spaces and a consequent increase in strength. (U)... 

Organometallic polymer precursors offer the potential to manufacture shaped forms of advanced ceramic materials using low temperature processing. Polysilazanes, compounds containing Si-N bonds in the polymer backbone, can be used as precursors to silicon nitride containing ceramic materials. This chapter provides an overview of the general synthetic approaches to polysilazanes with particular emphasis on the synthesis of preceramic polysilazanes. 

The spectrum of silicon based polymers is enriched by high tech ceramics like silicon nitride and carbide, respectively. These materials are produced by pyrolysis of appropriate polymeric precursors such as polysilanes, polycarbosilanes and polysilazanes (preceramics). These synthetic ceramics display a certain analogy to silicates, having SiC, SiN, or Si(C,N) as structural subunits instead ofSiO. 

Current interest in saturated Si-N rings stems primarily from the industrial uses of silicon nitride (and related materials), which is a hard, chemically resistant insulator, and as precursors to silicon-nitrogen polymers (poly-silazanes). The formation and precursor chemistry of these polymers are discussed in Section 10.3.3. 

A less explored area of transition metal catalysis involves bond formation between Group 14 elements and nitrogen. In direct analogy to previously discussed areas of research, silicon-nitrogen bonds can be formed by dehydrocoupling, hydrosilylation, and dehydrogenative silylation. The compounds produced are valuable for use in organic synthesis or as polymer precursors to silicon nitride ceramics. 

The coupling of a trialkylsilane and an amine with loss of H2, catalyzed by palladium on carbon, was first reported by Sommer and Citron in 1967.178 More recent work by Laine and Blum has involved the application of catalytic dehydrocoupling of compounds containing Si-H and N-H bonds to form aligo- and polysilazanes. These polymers, with silicon-nitrogen bonds in the backbone, are useful precursors to silicon nitride. In the presence of Ru3(CO)i2, silicon-nitrogen bonds are cleaved and reformed... 

Preparation of Preceramic Polysilazanes. At MIT (Massachusetts Institute of Technology), our initial research on silicon-based preceramic polymers was aimed at developing a precursor for silicon nitride. To this end, we studied the ammonolysis of dichlorosilane, H2SiCl2 (ii). This reaction had already been carried out on a millimolar scale in the gas phase and in benzene solution by Stock and Somieski in 1921 (12). We found that this reaction gave a much better yield of soluble ammonolysis product when it was carried out in more polar solvents such as dichloromethane or diethyl ether (ii). 

Kanner King New Polymer Precursors for Silicon Nitride... 

The drawing of these precursor polysilazane polymers to form fibers and their subsequent pyrolysis to silicon nitride fibers is a complex process that will be reported separately. 

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. 

The ammonolysis of dimethyidichlorosilane offers another route to PSZ precursor fibers and ultimately silicon nitride fibers. In a first step, Me2SiCl2 and MeHSiCl2 are mixed in nearly a 1 1 molar ratio in benzene. The mixture is treated with NHs, NH4CI precipitates, and the solvent is removed. The ammonolysis product, a viscous polymer, is converted in a second step at 400°C into melt spinnable polymer, and is melt spun. The resulting PSZ precursor fiber has a (Si-N) backbone. Carbon is present in pendent methyl groups, and the empirical formula of the fiber is SiCi.7No.9oH57 [23]. 

Silicon nitride has been obtained by the pyrolysis (in a stream of NHj) of the perhydropolysilazane prepared by the ammonialysis of the H SiClj-pyridine adduct [23, 24]. The gas stream employed during the pyrolysis of the preceramic polymer plays a crucial role [25]. Pyrolysis of [B, H,j diamine] polymers in an NHj stream gives BN [26]. TiN is similarly obtained by the pyrolysis of an amine precursor [27]. TiN has been prepared from titanazane [28]. Pyrolysis of Nicalon in NHj is reported to give SijN [24]. Besides single-phase ceramics, multiphase ceramics (e.g. composites of SiC and TiC, BiN and SijN ) have been prepared from precursors [29, 30]. Group 13 metal nitrides (GaN, AIN, hiN) have been prepared by the decomposition of urea complexes [31]. This method has been extended for the synthesis of BN, TiN and NbN [32]. 

Since the development of the first useful organosilicon compound, a silicon carbide precursor, by Yajima [11], there has been extensive research on the design of polymers which transform to silicon—containing ceramics upon pyrolysis. In recent years, progress has been made in the synthesis of polymeric precursors of silicon nitride. Seyferth and coworkers [12] developed a methylsilazane compound, basically an ammonolysis product of methyldichlorosilane, and reported the... 

Polysilazanes have been shown to be excellent polymeric precursors to amorphous silicon carbonitride (SiCN), silicon nitride, silicon carbide (SiC) and their composites. The actual chemical and phase compositions of the ceramic products depend on the polymer composition and pyrolysis conditions, such as temperature, time and atmosphere. Polymeric silazanes consist of amorphous networks, which transform to amorphous SiCN ceramics by pyrolysis under inert atmosphere at around 1000 C. These ceramic products remain amorphous up to 1400 °C in an inert atmosphere [a.322]. However, at higher temperatures the non-stoichiometric SiCN matrix decomposes, with nitrogen loss, giving the thermodynamically stable phases, namely Si3N4 and SiC. Polysilanes, polycarbosilanes and polysilazanes are commonly used for the preparation of high-performance ceramics such as silicon carbide, silicon nitride and silicon carbonitride. 

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