Silicon polysilazanes

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. 

Reactions (l)-(5) illustrate known methods for forming silicon-nitrogen bonds of potential use in the formation of oligo- and polysilazanes. The most common method of forming silazanes is via ammo-nolysis or aminolysis as shown in reaction (1) (4,5) ... 

In the area of preceramic polysilazanes, sufficient progress has been made to produce precursors for silicon nitride fibers, coatings and as binders for silicon nitride powder. However, particular problems still remain to be solved particularly with regard to reducing impurity levels and improving densification during pyrolysis. 

Introductory Remarks. In contrast with the popularity and usefulness of the polysiloxane chains, which constitute the structural backbone of silicones, the knowledge of polymers based on the silazane unit is still limited. In the sixties there was still some hope of the possibility of producing long chain polysilazane molecules and a number of laboratories were active in seeking convenient methods for their synthesis (e.g. see review by Aylett (12)). 

The introduction of small amounts of boron into precursors that produce silicon nitride have been known to improve the ceramic yields of silicon nitride and Si—B—C—N ceramics as first reported in 1986.110 Several reports have appeared in the past couple of years alone that utilize borazine precursors such as 2,4-diethylb-orazine and other cyclic boron precursors, such as pinacolborane, 1,3-dimethyl-1, 3-diaza-2-boracyclopentane, for their reactions with silanes, polysilazanes, and polysilylcarbodiimides for the high-yield production of Si—B—N—C ceramics.111... 

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. 

Polysilazanes that can serve as precursors for silicon carbonitride have been prepared using a Ru3(CO)i2-catalyzed Si-H/N-H dehydrogenative coupling process by workers at SRI [21]. Thus the ammonolysis product of CH3SiHCl2, whose composition approximates [CH3Si(H)NH]n, could be crosslinked by heating at 40 °C with a catalytic quantity of Ru3(CO)i2. Other polysilazanes were prepared by this procedure ... 

Summary A brief review of the preparation of silicon containing preceramic polymers to prepare silicon carbide and silicon carbonitride fibers is given. Methylchlorodisilanes are converted to polysilanes and polysilazanes which yield ceramic fibers after meltspinning, curing, and pyrolysis. 

The polysilazanes were also melt spun, cured, and pyrolyzed to give silicon carbonitride fibers (Eq. 7). The carbon content of these fibers depends on the molecular composition of the polysilazane and the pyrolysis gas. When ammonia is used as reactive gas pure silicon nitride fibers will be obtained (Eq. 8) [14]. 

Figure 4. shows the route from the high boiling residue of the direct synthesis to silicon carbo-nitride fibers. Methylchlorodisilanes and trichlorosilanes as additives are mixed in a specific ratio and react with methylamine and a small amount of ammonia to form an aminodisilane/oligosilazane. The subsequent polycondensation reaction of this mixture by heating to 250 °C yields a soluble and melt spinnable polysilazane. In comparision with the polysilane the properties of the polysilazane depend on the ratios of the disilanes/silanes and methylamine/ammonia and also on the reaction conditions. 

Polysilazane fibers are rendered infusible by humidity or in the absence of oxygen by ammonia. The final step of producing ceramic fibers is the pyrolysis. The cured fibers are heated at 1200 -1300°C in argon, nitrogen, or in vacuo, and SiC- or SiC/SijN fibers with a diameter of around 15 /xm are obtained. Heating up silicon-polymers, whether polysilanes or polysilazanes, results in the evolution of CH4 and H2. 

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... 

Access to phase pure silicon nitride materials via processable precursors is limited to just three approaches. The first, shown in reaction 6, provides one of the first oligomers exploited as a preceramic polymer24,253. This simple polysilazane, containing only Si, N and H, is known to be relatively unstable and will crosslink on its own to give intractable gels. Furthermore, it does not offer the 3Si I4N stoichiometry required for Si3N4. Nonetheless, it is useful as a binder and for fiber-reinforced ceramic matrix composites (CMCs)31. 

The most important class of silicon-containing polymers that have not yet been covered are the polysilazanes, shown in 7.1. These polymers, or precursors to them, are generally prepared by the reaction of organic-substituted chlorosilanes with ammonia or amines as is shown in reaction (l).32... 

Summary Polysilacarbosilanes and polysilasilazanes prepared according to a copolymer strategy offer an easy, coherent approach to polycarbosilanes and silazanes, precursors of SiC and SiCN-based materials with variable C/Si and C/Si/N ratios. In contrast with the polysilazane route which leads, upon pyrolysis, to carbon-containing silicon nitride, the synthesized polycarbosilazanes are finally converted into nitrogen-containing silicon carbide. 

A further method for the synthesis of the title compounds with only hydrogen as byproduct is the base-catalyzed dehydrogenative coupling (index D) of ammonia and tris(hydridosilylethyl)boranes, B[C2H4Si(R)H2]3 (R = H, CH3). Initially, the strong base, e.g. n-butyl lithium, deprotonates ammonia. The highly nucleophilic amide replaces a silicon-bonded hydride to form a silylamine and lithium hydride, which then deprotonates ammonia, resuming the catalytic cycle. Under the conditions used, silylamines are not stable and by elimination of ammonia, polysilazane frameworks form. In addition, compounds B[C2l-L Si(R)H2]3 can be obtained from vinylsilanes, H2C=CHSi(R)H2 (R - H, CH3), and borane dimethylsulfide. 

In the pyrolysis of a preceramic polymer, the maximum temperature used is important. If the maximum temperature is too low, residual functionality (C-H, N-H, and Si-H bonds in the case of polysilazanes) will still be present. On the other hand, too high a pyrolysis temperature can be harmful because of solid-state reactions that can take place. For instance, if the polysilazane-derived silicon carbonitride contains a large amount of free carbon, a high-temperature reaction between carbon and silicon nitride (equation 1) (7) is a possibility. 

However, in many cases the pyrolysis products are amorphous. For instance, the polysilazane-derived silicon carbonitride mentioned above crystallizes only when it is heated above 1450 C, and its characterization is difficult. Elemental analysis poses problems, in part because the pyrolysis product is very porous. As a result of its high surface area, the material adsorbs moisture and volatiles very readily, and improper prior handling and preparation for analysis can result in misleading results. 

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). 

Seyferth and co-workers (12) used the ammonolysis of dichlorosilane to prepare carbon-free polysilazanes that could be converted into silicon nitride (equation 8),... 

A novel process for polysilazane preparation has been developed via transaminationy which does not require the use of chlorosilane intermediates. Silicon nitride, especially as fibers or coatings,... 

Polysilazanes prepared by these processes are readily converted to silicon nitride or silicon-nitride-containing ceramic compositions upon pyrolysis at temperatures up to 1600 °C. High-purity a-Si3N4 (a-phase crystalline form of silicon nitride) has been efficiently prepared by these processes,... 

Polysilazane Routes to Silicon Nitride-Silicon Carbide... 

Like the preparative procedures for polysilazane intermediates for silicon nitride, the reactions leading to silicon nitride-silicon carbide involve chlorosilanes and generate substantial quantities of amine hydrochlorides, which must be removed by extensive purification, because their presence in the later stages of the process can be deleterious (28). 

The reaction is surprisingly clean under these vigorous hydrosilation conditions, with no evidence for the formation of l,2-bis[tris(dimethyl-amino)]ethane (32). Thus, very satisfactory processes have been developed for both Tris and vinyl-Tris that do not involve chlorosilane intermediates. Both monomers are suitable intermediates for the preparation of polysilazane preceramic polymers that could be converted thermally to silicon nitride and mixtures of silicon nitride and silicon carbide. 

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