Polysilazane precursors

Trassl, S., Motz, G., Rossler, E., and Ziegler, G. Characterization of the free-carbon phase in precursor-derived Si-C-N ceramics. II. Comparison of different polysilazane precursors. 2tAtlk, S SMlLSSL- 2002 85 1268-1274. 

In a recent article we tried to evaluate the relationship between the stmcture and content of polysilazanes, precursors to silicon nitride and their pyrolytic characteristics and to interpret the observations in terms of chemical pathways [1]. Other related studies on the effects of organic groups [2] and pyrolysis under ammonia [3] have recently been published. These articles support and refine some of the concepts summarized in recent reviews [4,5]. 

The release of ammonia indicated the existence of transamination reactions corresponding to further crosslinking of the polysilazane precursor. The major mass loss (17.6%) occurred between 350 and 800 °C. This mass loss corresponds to the escape of hydrocarbons and traces of silicon species in the temperature range from 350 to 650 "C. The masses of the... 

This chapter discusses the development of thermosetting preceramic polymers, emphasizing peroxide-curable polysilazanes. These polymers are excellent precursors for both silicon nitride and silicon carbide. Vinyl-substituted polysilazanes may be readily thermoset with peroxide initiators. In addition, a new class of polysilazanes which contain peroxide substituents directly boxmd to the polymer has been developed The utility of peroxide-cured polysilazane precursors for the formation of silicon nitride articles has been demonstrated. 

West (p. 6), Miller (p. 43), Zeigler (10), and Sawan (p. 112) outline the synthesis of a wide variety of soluble, processable polydiorganosilanes, a class of polymers which not long ago was thought to be intractable. Matyjaszewski (p. 78) has found significant improvements in the synthetic method for polydiorganosilane synthesis as well as new synthetic routes to unusual substituted polydiorganosilanes. Seyferth (p. 21, 143) reports synthetic routes to a number of new polycarbosilanes and polysilazanes which may be used as precursors to ceramic materials. 

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. 

Surprisingly, pyrolysis of these polysilazane polymers gave mostly SiC rather than the expected Si3N. It was later found that the use of HSiCl-5 in place of MeSiCl-i provides useful precursors to Si-iN/. (42) J J ... 

We have also explored the use of the dehydrocoupling reaction for the synthesis of preceramlc polysilazanes starting from the precursor MeNH-[H2SlNMe]x-H. As described above, MeNH-[H2SlNMe]X H Is produced by aminolysis of H2SiCl2 with MeNH2 under conditions where x 18-20 (Mn - 1150). 

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. 

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 ... 

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... 

Precursors that fall in this category are generically called polysilazanes or polysilsesquiazanes. Synthetic routes to these materials have been reviewed63,24 thus, we provide an overview of typical methods. The most common route to polysilazanes is via ammonolysis/aminolysis of chlorosilanes25-27. 

Polysilazanes can also be synthesized by catalytic dehydrocoupling (equation 4)28,29. The reaction works best with monosubstituted silanes (MeSiH3 is flammable) however, as with the ammonolysis reactions, molecular weights are normally less than 2000 Da. Hence, as-formed oligomers are not useful precursors. However, combinations of ammonolysis followed by catalytic dehydrocouphng provide access to useful precursors as discussed in the SiCN section. 

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. 

Although Si—N—B—O materials containing trace amounts of nitrogen (2-3 wt%) have been prepared solely via a processing approach (ammonolysis of a borosilicate gel), complete conversion to Si—N—B was not achieved86. However, there are several literature examples of routes to Si—N—B materials that couple synthesis with a specific processing approach. These include early work on Si—N—B precursors by Seyferth and Plenio, who reacted the now ubiquitous — [MeHSiNH] — polysilazane with H3B SMe287 as shown in equation 26. 

One other Si—N—B—X precursor type, where X = Ti, has been made by several groups85,90,91. The approach used has been to react Ti(NMe2)4 with a polysilazane as illustrated in equation 32. Note that the initial reaction with Ti(NMe2)4 can be exothermic and even violent if Si—H bonds are present. However, processable polymers are... 

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. 

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). 

Pyrolysis of Preceramic Polysilazanes Prepared by the Dehy-drocyclodimerization Reaction. Whatever the structure of the polysilazanes obtained by KH treatment of the cyclo-(CH3SiHNH) oligomers, these polymers are excellent ceramic precursors. Examination of the polymers from various preparations by TGA (thermogravimetric analysis) showed the weight loss on pyrolysis to be between only 15 and 20%. Pyrolysis appears... 

Highly branched and even cross-linked polysilazane polymers are also desirable, particularly for the formation of ceramic powders. The transamination of Tris and vinyl-Tris with ammonia is a convenient process for the preparation of these precursors. The reactions were effectively catalyzed by strong acids such as trifluoroacetic acid ... 

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. 

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