Precursors to silicon carbide

The first type of polycarbosilane synthesized by using ADMET methodology was a poly[carbo(dimethyl)silane].14c Linear poly(carbosilanes) are an important class of silicon-containing polymers due to their thermal, electronic, and optical properties.41 They are also ceramic precursors to silicon carbide after pyrolysis. ADMET opens up a new route to synthesize poly(carbosilanes), one that avoids many of the limitations found in earlier synthetic methods.41... 

The history and development of polysilane chemistry is described. The polysilanes (polysilylenes) are linear polymers based on chains of silicon atoms, which show unique properties resulting from easy delocalization of sigma electrons in the silicon-silicon bonds. Polysilanes may be useful as precursors to silicon carbide ceramics, as photoresists in microelectronics, as photoinitiators for radical reactions, and as photoconductors. 

Possible ways in which polysilanes may be useful include, 1. As precursors to silicon carbide ceramics 2. As photoinitiators in radical reactions 3. As photoconductive materials, and 4. As photoresists in microelectronics. The last of these uses will be treated in the chapter by Miller,(31) and so will not be covered here. 

Soluble polysilane polymers can also be used as precursors to silicon carbide. The first such application, using (PhMeSi)n-(Me2Si)m copolymers ("Polysilastyrene"), was to strengthen silicon nitride ceramics. The Si3N4 ceramic body was soaked in polysilane and refired, leading to the formation of silicon carbide whiskers in the pore spaces and a consequent increase in strength. (U)... 

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

Polysilanes can be regarded as one-dimensional analogues to elemental silicon, on which nearly all of modern electronics is based. They have enormous potential for technological uses [1-3]. Nonlinear optical and semiconductive properties, such as high hole mobility, photoconductivity, and electrical conductivity, have been investigated in some detail. However, their most important commercial use, at present, is as precursors to silicon carbide ceramics, an application which takes no advantage of their electronic properties. 

Polysilanes. Following the first reports of soluble and processable polysilanes in the late 1970s, these macromolecules have attracted substantial interest from both fundamental and applied perspectives." The backbone of silicon atoms gives rise to unique electronic and optical properties as a result of the delocalisation of a-electrons. Several polysilanes have also been found to function as useful thermal precursors to silicon carbide fibres and these materials have also attracted attention with respect to microlithographic applications and as polymerisation initiators." ... 

The growth of interest in poly(silylene)s is indicated in Figure 5.1, which shows the number of literature publications on polysilanes (and oligosilanes) plotted versus time. Note that few publications appeared before 1980, but during the past 20 years scientific activity in this field has increased dramatically. The interest in polysilanes stems partly from their novel constitution and behavior and partly from their potential utility. The possibility that soluble polysilanes might be precursors to silicon carbide was evident when they were first synthesized, but other technological uses were not so obvious and arose only in the course of research. 

For some uses it is important to form bonds that link different polysilane chains, to transform soluble, meltable polysilanes into insoluble resins. This process is vital if the polysilanes are to be used as precursors to silicon carbide ceramics, since, if cross-linking is not carried out, most of the polymer is volatilized before thermolysis to silicon carbide can take place. Several methods have therefore been developed to bring about cross-linking of polysilanes.109 110... 

There are several reports of the condensation of bis-silanes of the type H3Si-X-SiH3 (primary silanes) and H2RSi-X-SiR H2 (secondary silanes) principally with the aim of producing polymers that would be precursors to silicon carbide ceramic material. The polymers would, in principle,... 

Although it was one of the first, the Yajima polycarbosilane is not the only polymeric precursor to silicon carbide that has been developed. Another useful system that merits mention is the polysilane developed by Schilling and his co-workers at Union Carbide Corporation (iO). As already mentioned, a useful polymeric precursor for silicon nitride has been developed more recently by workers at Dow Corning Corporation (8). 

The recent upsurge of interest in polysilanes stems from their uses in technology, both actual and potential. They are now being sold and used as precursors to silicon carbide... 

Figure 3.53 (Bottom) The NMR spectrum of polymethylvinylsilane and elemental silicon, the precursors to silicon carbide, SiC. (Top) Spectrum of the pyrolyzed product, fi-SiC. (From Apple, used with permission.)...

MAJOR APPLICATIONS Precursor to silicon carbide ceramics via intermediate pyrolysis to polycarbosilane. ... 

West R, David LD, Djurovich PI, Yu H, Sinclair R, Polysilastyrene, Phenylmethylsilane-dimethylsilane Copolymers as Precursors to Silicon Carbide, Am Ceramic Soc Bull, 62(8), 899-903, 1983. 

Polysilanes have many applications [1-5]. These are based on the ability of some members of their family to serve as precursors to silicon carbide. Other applications of polysilanes stem from their interesting electronic and photophysical properties. 

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. 

R. West, L. D. David, P. I. Djuiovich and H. Yu (1983), Polysilastyrene phenyl methylsilane-dimethylsilane copolymers as precursors to silicon carbide, Cer Bull. 62, 899. 

Polysilastyrene Phenylmethylsilanc-Dimcthylsilanc Copolymers as Precursors to Silicon Carbide, (1983) Am. Ceram.Soc. Bull. 62, 899. 

In the non-oxide area, research on precursors to silicon carbide, silicon nitride, and related ceramic materials continues to receive impetus from the pioneering work of Seichi Yajima in the TO s. ... 

Poly(carbosilane)s are organosilicon polymers with a backbone structure composed essentially of Si-C bonds. They represent the most widely studied class of precursors to silicon carbide fibre, and are the starting polymers in the manufacture of the Nicalon silicon carbide fibre produced by Nippon Carbon Co. 

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