Silicon/carbon high-temperature pyrolysis

The earliest work on silicon-carbide-related fibers was by Verbeek and Winter (4). Using the principles developed earlier by Fritz and co-workers (5), Verbeek and Winter (4) reported that the high-temperature pyrolysis of tetramethylsilane or methylchlorosilanes gives branched polycarbosilane (PCS) polymers containing a structure with alternating silicon and carbon atoms (equation 1). 

There are many methods to manufacture a CMC. Only a small selection is discussed in this section. Let us first have a look at the production process of a SiC matrix reinforced with SiC fibres. First a model is made of fibres, the so-called preform, then CVI (Chemical Vapour Infiltration) is applied to produce a coating on the fibres in order to ensure a better attachment to the matrix. The next step is resin infiltration. After pyrolysis (heating to a high temperature without oxygen) a network matrix of the porous carbon arises. Silicon which has first been melted in an oven is then introduced into this network it reacts with the carbon to form the following matrix ... 

The pyrolysis products from 9.13 can give essentially pure silicon carbide in 89% yield. Intermediates that contain from 5 to 25% allyl functionalized units give ceramics with progressively larger amounts of carbon beyond the 1 1 Si C ratio. This process has been developed into a manufacturing sequence for the production of reinforced composites for aircraft brakes and high temperature coatings. 

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. 

Silazanes function as binders for high density SiC monoliths, because they decompose to give silicon carbide and carbon below the sintering temperature of silicm carbide. Hence, other binder materials became obvious. We believed that we could successfully employ a polymer that would be a Si, C, O precursor upon pyrolysis to 1200 C and remove CO at even higher temperatures to give a SiC/C ceramic (equation 2). 

Ng, S.H., Wang, J., Wexler, D., Chew, S.Y., Liu, H.K., 2007. Amorphous carbon-coated silicon nanocomposites a low-temperature synthesis via spray pyrolysis and their application as high-capacity anodes for lithium-ion batteries. Journal of Physical Chemistry C 111 (29), 1131-11138. 

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