Preceramic polymer route

Boron-containing nonoxide amorphous or crystalline advanced ceramics, including boron nitride (BN), boron carbide (B4C), boron carbonitride (B/C/N), and boron silicon carbonitride Si/B/C/N, can be prepared via the preceramic polymers route called the polymer-derived ceramics (PDCs) route, using convenient thermal and chemical processes. Because the preparation of BN has been the most in demand and widespread boron-based material during the past two decades, this chapter provides an overview of the conversion of boron- and nitrogen-containing polymers into advanced BN materials. 

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. 

R. M. Laine and F. Babonneau, Preceramic polymer routes to silicon carbide, Chem. Mater., 5,260-279 (1993). 

In conclusion, the lesson learned from the research carried out to date on the subject of polycarbosilanes is that the general rule that linear, noncrosslinked polymers are not suitable preceramic polymers applies here as well. Crosslinked network-type polymers are needed. Such structures can be generated in more than one way, but in the case of the polycarbosilanes they have, to date, been obtained mainly by thermolytic routes thermal treatment (with or without other chemical additives) in the case of the Yajima polycarbosilanes and the thermolysis of tetramethylsilane in the case of the Bayer process-derived polycarbosilane. 

The conventional industrial method for the synthesis of a-silicon carbide is to heat silica (sand) with coke in an electric furnace at 2,000-2,500 °C. However, because of the high melting point of the product, it is difficult to fabricate by sintering or melt techniques. Thus, the discovery of a lower temperature fabrication and synthesis route to silicon carbide by Yajima and coworkers in 197526,27 proved to be an important technological breakthrough. This is a preceramic polymer pyrolysis route that has been developed commercially for the production of ceramic fibers. 

The traditional synthesis route involves the direct reaction of silicon with nitrogen at temperatures above 1,300 °C, or by heating silica with carbon (coke) in a stream of nitrogen and hydrogen at 1,500 °C.41 However, as in the case of silicon carbide, the high processing and fabrication temperatures focused attention on the need for alternative access routes based on preceramic polymers. 

A preceramic, carrier polymer route to boron carbide has been reported via the pyrolysis of a polynorbomene that bears decaborane side groups.69 An important feature of this development is the ability to produce nanofibers of boron carbide in the following way. A solution of the poly(norbomenyldecaborane) in THF is subjected to the process of... 

Various preceramic oligomer and polymer routes to aluminum nitride have been investigated.70 For example, the reaction of LiAlH4 or A1H3 with ammonia initially yields A1(NH2)3, which loses ammonia and hydrogen during pyrolysis and leaves AIN contaminated by carbon from the initial reaction solvent.71... 

Atomic Structure. The control of atomic structure is fundamental to any system, and an incomplete understanding of atomic structure can limit advancement. For example, our understanding of preceramic polymers, up through the formation of networks, is improving but the full exploitation of this chemistry is still limited by the lack of detailed knowledge of the structure of the resulting ceramic at the atomic level. Even with more familiar silicone polymer systems, synthetic barriers are encountered as polymers other than poly(dimethylsiloxane) are used. Stereochemical control is inadequate in the polymerization of unsymmetrical cyclic siloxanes to yield novel linear materials. Reliable synthetic routes to model ladder systems are insufficient. 

Boron nitride can be prepared by many routes such as CVD (NH3 + organoborane), 2 pyrolysis of preceramic polymers derived from borazine, 2 and solid-state metathesis... 

In the past few years, similar reactions using numerous dichlorosilane derivatives were investigated to yield preceramic polymers. Some routes use several different types of chlorosilanes as starting materials (e.g., Refs. 33-35) Analogous to the preparation of polysilanes, polycarbosilanes can also be synthesized using a similar route. Alkyl chloroalkyl chlorosilanes are used as starting materials for this purpose [2]. 

The following discussion covers the chemical synthesis of ceramics derived from organometallic polymers BN, AIN, TiN, TiC, and TiBa. It should be emphasized here that some of the syntheses involve starting materials, monomers, and intermediates, as well as polymers, that are oxidatively unstable and/or susceptible to hydrolysis. These syntheses therefore generally require inert atmospheres and the extensive use of vacuum (Schlenk type) line or dry-box techniques. This makes it obvious that collaborations between synthetic chemists and materials and ceramic scientists and engineers is important. Here we outline a selected number of synthetic routes to preceramic polymers. 

Alkylamines bearing longer hydrocarbon chains show no improvement but cause higher costs. In particular, including these chains in Ae preceramic polymer is not a feasible route to increase the carbon content of the fiinal ceramic, since they facilitate undesired segregation of graphite [86, 87]. 

Up to now, polymer pyrolysis has been investigated especially to develop ceramic fibers [46,47] and ceramic matrix for ceramic matrix composites [48-50]. More recently studies have been undertaken to exploit this method to develop ceramic thin films [51-53], foams [54], joints [55], and bulk materials [56]. Moreover, noncon-ventional heating systems such as laser [57], microwave heating [53], or even athermal conversion processes such as ion bombardment are just now starting to be applied to the polymer route and the preliminary results are very promising [58-60]. In this chapter we focus on the polymer processing of bulk ceramics obtained by pyrolysis of partially cross-linked preceramic bodies and of thin ceramic films (obtained either by traditional pyrolysis or by the innovative ion irradiation process). 

The purpose of this chapter is to provide an overview of the chemistry, processing and application of boron-containing preceramic polymers in the BN system. The nonoxide precursor route, also called the Polymer Derived Ceramics (PDCs) route, represents a chemical approach based on the use of air- and/or moisture-sensitive (molecular or polymeric) precursors by means of standard Schlenk techniques and vacuum/argon lines. This precursor route allows the chemistry (e.g., elemental composition, compositional homogeneity and atomic architecture) of molecular precursors to be controlled and tailored in order to provide the ensuing preceramic polymers... 

Polymers included in this category are generally called polyborazylenes. Synthetic route to these polymers have been reviewed [8] thus, we provide an overview of typical synthesis procedures starting from borazine derivatives and leading to BN preceramic polymers. 

Polymer pyrolysis refers to the pyrolytic decomposition of metal-organic polymeric compounds to produce ceramics. The polymers used in this way are commonly referred to as preceramic polymers in that they form the precursors to ceramics. Unlike conventional organic polymers (e.g., polyethylene), which contain a chain of carbon atoms, the chain backbone in preceramic polymers contains elements other than carbon (e.g., Si, B, and N ) or in addition to carbon. The pyrolysis of the polymer produces a ceramic containing some of the elements present in the chain. Polymer pyrolysis is an extension of the well-known route for the production of carbon materials (e.g., fibers from pitch or polyacrylonitrile) by the pyrolysis of carbon-based polymers (54). It is also related to the solution sol-gel process described in the previous section where a metal-organic polymeric gel is synthesized and converted to an oxide. 

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) ... 

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