Polymeric precursors silicon carbide

Another process for silicon carbide fibers, developed by Verbeek and Winter of Bayer AG [45], also is based on polymeric precursors which contain [SiCH2] units, although linear polysilmethylenes are not involved. The pyrolysis of tetramethylsilane at 700°C, with provision for recycling of unconverted (CHg Si and lower boiling products, gave a polycarbosilane resin, yellow to red-brown in color, which was soluble in aromatic and in chlorinated hydrocarbons. Such resins could be melt-spun but required a cure-step to render them infusible before they were pyrolyzed to ceramic... 

Soluble polydiorganosilane homo and copolymers have recently shown great potential in such areas as precursors for the preparation of silicon carbide fibers (1), as photoinitiators in alkene polymerization (2), as photoconductors (3), and as positive or negative self-developing photoresists for photolithographic applications (4). A number of copolydiorganosilane copolymers have been reported recently (5) in which the copolymer contained equal amounts of both monomers in the feed. 

The Yajima polycarbosilane, while it was one of the first, is not the only polymeric precursor to silicon carbide which has been developed. 

Another useful system which merits mention is the polycarbosilane which resulted from research carried out by C.L. Schilling and his coworkers in the Union Carbide Laboratories in Tarrytown, New York [6]. More recently, a useful polymeric precursor for silicon nitride has been developed by workers at Dow Corning Corporation [7]. 

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. 

As observed by D. Johnson and J. Stiegler, "Polymer-precursor routes lor fabricating ceramics offer one potential means or producing reliable, cost-effective ceramics. Pyrolysis of polymeric metalloorganic compounds can be used to produce a wide variety of ceramic materials." Silicon carbide and silicon oxycarbide fibers have been produced and sol gel methods have been used In prepare line oxide ceramic powders, such as spherical alumina, as well as porous and fully dense monolithic forms. 

Silicon carbides are generally synthesized by the pyrolysis of precursors, prepared by liquid phase methods. One possible way for precursor synthesis is the addition of carbon black or sucrose, to a gelling silica.8 In this method, the carbon is introduced from an external source. A more intimate contact between the carbon and silicon in the precursor is assured with the use of organometallic polymer precursors. The use of silane polymers for silicon carbide production was initiated by Yajima.9,10 Polymers having a -[Si-C]- backbone are crosslinked and pyrolysed to yield SiC." In the initial work, dimethyldichlorosilane was used as a starting monomer, which was subjected to a sodium catalyzed polymerization (reaction (C)). 

Another silicon carbide multifilament fiber, made via a polymeric precursor by Dow Corning Corp., US A, is called Sylramia According to the manufacturer, this textile grade silicon carbide fiber has a nanocrystalline, stoichiometric... 

Polysilanes have been the first class of precursors used to prepare silicon carbide ceramics. In all cases, on pyrolysis, polysilanes are converted into polycarbosilanes by a Kumada rearrangement prior to the formation of an amorphous silicon carbide network. Several synthetic routes including dehydro-polymerization, ring-opening polymerization of strained cyclosilanes, polymerization of masked disilenes, or base catalyzed disproportionation reactions of disilanes have been described to prepare linear or branched polysilanes but despite its drawbacks the Wurtz-coupling route remains the method applied most, especially to prepare linear polysilanes. 

Summary Novel poly(silylenemethylene)s have been prepared by ring-opening polymerization of 1,3-disilacyclobutanes followed by a protodesilylation reaction with triflic acid. Reactions of the triflate derivatives with organomagnesium compounds, LiAlH4, amines or alcohols gave functional substituted and branched poly(silylene-methylene)s, which may serve as suitable precursors for silicon carbide and Si/C/N-based materials. 

Nonoxides can be formed from polymeric precursors that are already mixed in the right proportion on an atomic scale. The atoms in silicon carbide are bonded with covalent bonds and are not mobile. Similar to most other carbides and nitrides SiC is difficult to sinter and so has slow solid state reactions even at high temperatures. A good precursor for solid state formation of SiC is the polymer polycarbosilane, which is made at 450°C from polysilanes formed from dimethyl-dichlorosilane and sodium ... 

MAJOR APPLICATIONS Precursor for silicon carbide ceramic. Initiator for free-radical polymerization. ... 

Organometallic polymers such as poly(ferrocenylsilane)s have been shown to be precursors for new types of magnetic ceramics [9, 42]. Similarly poly(silyleneethylene)s [41] and some polysilanes are polymeric precursors for silicon carbide ceramics [19]. 

USGS. The delocalization of a electrons in polysilanes gives rise to unique electronic and optical properties. Also, several polysilanes have been foimd to function as useful thermal precursors to silicon carbide fibers and these materials have attracted attention with respect to microlithographic applications and as polymerization initiators (1,27,28). The use of these materials as hole transport layers in electroluminescent devices has also been explored (42). Indeed, the photoconductivity of poly(methylphenylsilane) doped with Geo has been studied and has been found to be comparable with the best materials available (43). 

From the present study, it is clear that methylsilane can be easily polymerized to poly(methylsilane), using either DMT or DMZ as catalyst. By proper choice of conditions, a polymer completely soluble in common organic solvents can be obtained, but prolonged reaction eventually leads to insoluble gel. As will be described elsewhere, this cross-linking reaction is of some advantage when the polymers are used as precursors for synthesis of silicon carbide. 

H.-J. Wu and L. V Interrante (1989), Preparation of a Polymeric Precursor to Silicon Carbide via Ring-Opening Polymerization Syntheses Poly[(methylchlorosilylene)-methylene] and Poly(sila-propylene), Chem. Mater. 1, 564. 

Since the development of the first useful organosilicon compound, a silicon carbide precursor, by Yajima [11], there has been extensive research on the design of polymers which transform to silicon—containing ceramics upon pyrolysis. In recent years, progress has been made in the synthesis of polymeric precursors of silicon nitride. Seyferth and coworkers [12] developed a methylsilazane compound, basically an ammonolysis product of methyldichlorosilane, and reported the... 

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. 

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