Some thoughts on carbon-carbon processing

Chemical vapor deposition:

In this process, a hydrocarbon gas is thermally degraded onto a hot carbon surface depositing pyrolytic carbon and releasing H2. Liebermann has reported the formation of acetylene compounds or aromatic species during the decomposition of CH4. The mechanisms and morphologies of these depositions have been described by Bokros. The rate of deposition is controlled by the substrate surface area and concentration of the gas, which can be diluted with an inert gas to control the concentration of the cracking gas. As the gas penetrates the pores of the substrate, its concentration decreases, thus establishing a form of concentration gradient which tends to further induce gas to enter the pores, resulting in a deposition gradient within the pore.

Liquid infiltration:

A liquid impregnation process essentially provides the means to avoid the closure of narrow pores. The liquid impregnation process is normally performed under both vacuum and applied pressure to achieve more efficient pore filling. Molten pitch is a common material selected for liquid impregnation and it is possible to select a fraction of pitch with the most appropriate working viscosity for processing at a given temperature. The yield increases with increasing C/H ratio. A pitch can be categorized by measuring its Tg and the minimum viscosity occurs about 100-150C above the Tg. Normally, as the impregnation cycles proceed, the fraction of pitch can be changed to provide more effective filler for the gaps. Eventually, the surface tension becomes a limiting factor. It is at this stage that the benefits of using a combination of liquid impregnation followed by CVD /CVI become apparent, with the latter impregnants penetrating the small pores more readily.

A higher pressure favours an increase in the carbon yield, but as the pressure is increased, the deposit becomes more coarse and isotropic, probably due to the evolved gases becoming compressed and being unable to escape. When pitch is carbonized, it evolves gases, a process known as bloating, which can force pitch out of the carbon matrix. Bloating can be overcome by oxidation of the pitch prior to carbonization through treatment with O2 for up to 100h at about 220C, rendering the mesophase fraction infusible and hence permitting carbonization to proceed without disrupting the microstructure. The bulk density of the carbon-carbon composite shows an initial drop in density and then increases with the number of cycles of carbonization impregnation, the increase diminishing as the pores become blocked. Graphitization at 2200-3000C will enable further densification to occur, reaching density levels of 1.84 g/cm3. The flexural strength increases with the number of process cycles. The use of mesophase pitch will limit the number of process cycles.


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