Silicon nitride, 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. 

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. 

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]. 

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. 

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. 

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. 

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. 

Volatility can be a problem during the thermal conversion of polysilazanes to silicon nitride. The efiBciency of the process not only depends on losses due to stripping of excessive side groups but also decreases because of losses due to polymer volatility. Therefore, the polysilazanes must have suflSciently high molecular weights to minimize such losses. 

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. 

Tris transaminates readily with ammonia or primary amines when catalyzed by carbon dioxide or strong organic acids. The polysilazane products range from discrete solid disilazanes, to liquid distillable oligomers, and to highly cross-linked infusible polymers. Some of these polysilazanes can be pyro-lyzed to amorphous silicon nitride or mixtures of silicon nitride and silicon carbide below 1550 or to crystalline ceramics above that temperature. 

Another special type of silicone containing polymers is polysilazanes. These materials are used in for the preparation of high performance ceramics, silicon nitride, etc. Polysilazane can be prepared from substituted methylchlorodisilanes and gaseous ammonia in the following reaction [4] ... 

D. Synthesis of Silicon Nitride Fibers from Polysilazanes... 

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]. 

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. 

Polysilazane Thermosets as Precursors for Silicon Carbide and Silicon Nitride... 

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. 

Solid state NMR spectroscopy showed complete consumption of the vinyl moieties in the thermoset polymer 2. Pyrolysis of the cured polysilazane under NH3 from ambient to 1000 C, followed by heating under Ar to 1600°C gave a mixture of a- and P-Si3N4 19). In contrast, pyrolysis under Ar fh)m ambient to 1600°C gave P-SiC. Indeed, this thermoset polysilazane is an excellent precursor for silicon carbide as well as silicon nitride 19). 

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