Preceramic 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. 

Reaction (4) was the first reaction successfully used to synthesize preceramic polysilazanes. Verbeek et al. found that fusible polysilazane resins could be produced by pyrolysis of bis- or trisalkyl-aminosilazanes (or mixtures) (15,16) ... 

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. 

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... 

Objectives of Preceramic Polymer Design. The conversion of Tris and vinyl-Tris to preceramic polysilazane polymers was investigated, with the objective of developing processes with the following properties ... 

In the procedure described in this chapter, a preceramic polysilazane coating is synthesized on silica gel via CSC using Sic and NH3,but the chlorosilylations are performed in the liquid phase. In this way, a very small amount of the hazardous reagent is used in comparison with gas-phase reactions, and the total precursor synthesis can be performed at room temperature. 

ABSTRACT. The behavior of three preceramic polysilazane variations during pyrolysis is discussed. These polymers consist of the monomeric units -[H2SiNCH3]-, -[CH3SiHNH]-, and -[H2SiNCIl2CH3]-. Their differing pyrolytic performances are evaluated in relation to polymer structure, functional group, and pyrolytic environment... 

Understanding pyrolysis mechanisms as a function of substituting groups and configurations and the steric arrangements of preceramic polysilazanes is extremely important for the better design of future ceramic precursors. The assessment of the chemical pathway... 

Bis- and m-sanunosilanes, R2Si(NHMe)2 and RSi(NHMe)3, can be deami-nated to give preceramic polysilazanes (84) and cyclic oligomers, such as 83, at elevated temperatures (Scheme The efficiency of this commercial process... 

With the exception of reaction (3), the utility of all of the above reactions for the synthesis of oligo- and polysilazanes has been examined in varying detail. In the following paragraphs, we attempt to examine the pertinent studies in each area especially as it relates to the synthesis of preceramic polymers. 

Polymers of this type were found to have the highest molecular weight, Mjj 10K D, of any polysilazanes produced to date. Unfortunately, no one has examined the utility of these compounds as preceramic polymers. 

None of the above described ring opening polymerization methods has, as yet, proved useful for the formation of polysilazane preceramic polymers. However, Si-N bond cleavage and reformation, as it occurs in reaction (13), is probably responsible in part for the curing or thermoset step in transition metal catalyzed dehydrocoupling polymerization of hydridosilazanes (31), as described below. 

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. 

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. 

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. 

This chapter gives an introduction to the preceramic polymer route to ceramic materials and focuses on the reasons why this new approach was needed and on the chemical considerations important in its implementation, with examples from research on organosilicon polymers. Novel polysilazanes have been prepared by the dehydro-cyclodimerization reaction, a new method for polymerizing suitably substituted cyclooligosilazanes. The living polymer intermediate in this reaction has been used to convert Si-H-containing organosilicon polymers that are not suitable for pyrolytic conversion to ceramics into useful preceramic polymers. 

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. 

Characterization of Preceramic Polymers. The study of the pyrolysis products of preceramic polymers is not always straightforward. If crystalline species are produced (e.g., SiC and Si3N4 in the case of polysilazanes), then their identification by X-ray diffraction presents no problems. 

Thus the chemistry leading to the desired ceramic product is quite satisfactory most of the requirements mentioned earlier are met. Initial evaluation of the polysilazane shows that it has promise in three of the main potential applications of preceramic polymers in the preparation of ceramic fibers and ceramic coatings and as a binder for ceramic powders. 

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. 

Besides polysilazanes, PSCs are intensively investigated preceramic polymers for ternary Si-C-N materials. Their polymer backbone is composed of alternating Si-N=C=N nnits. Bis(silyl)carbodiimides, R3Si-N=C=N-SiR3 (R = alkyl) have been known since the early 1960s.Flowever, similar to low-molecular weight silazanes, they evaporate with heat treatment and are therefore not suitable as precursors for ceramics. PSCs were first obtained by Pump and Rochow ° in 1964 by metathesis reactions of dichlorosilanes and disilvercyanamide. 

Borazine and its derivatives are also possible educts to synthesize precursors for Si-B-N-C ceramics. Sneddon and co-workers prepared Si-B-N-C preceramic precursors via the thermal dehydrocoupling of polysilazanes and borazines [7]. A further synthesis route is the hydroboration of borazines. The work group of Sneddon found that definite transition metal reagents catalyze hydroboration reactions with olefins and alkynes to give 5-substituted borazines [8]. Recently, Jeon et al. reported the synthesis of polymer-derived Si-B-N-C ceramics even by uncatalyzed hydroboration reactions from borazines and dimethyldivinylsilane [9]. 

When compared to SiC, less work has been reported on the production of Si3N4 by the polymer pyrolysis route. Most efforts have focused on polymer precursors based on polysilazanes, a class of polymers having Si-N bonds in the main chain (58-61). The reactions to produce the Si-N bond in the chain backbone are based on the ammonolysis of methylchlorosilanes. A preceramic polymer can be prepared by the ammonolyis of methyldichlorosilane, followed by the polymerization of the silazane product catalyzed by potassium hydride (69) ... 

Polymeric organosilicon compounds consist of two kinds of compounds, one of which has a silicon atom in the side chain and the other having a silicon atom in the principal chain as shown in Table 8.6. In these compounds, polysilylmethylene, polysilylphenylene, polysilazane and polymetalosiloxane (e.g.. Si—O—Ti—O) are used as preceramic polymers of SiC, Si3N4 and Si—C—Ti—O [42]. 

Polycyclomethylsilazane (PCMS). Polysilazanes containing organic groups on the silicon backbone atoms are more typical as preceramic polymers, especially those substituted with methyl groups [7-12]. Polymers consisting of the monomeric units -[MeSiHNH]- are "isomeric" to the N-methylsilazanes discussed above. Comparison between the pyrolytic properties of the two is, Aerefore, of great interest. 

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