Biomedical Engineering Reference
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observed by Pampuch et al. [Pampuch, 1986; Pampuch, 1987] by Differential
Thermal Analysis of the infi ltration and reaction process of molten Si in a preform
made of carbon fi bres, and by Sangsuwan et al. [Sangsuwan, 1999]. Therefore, if
the molten Si temperature is 1550 °C, then the process can reach 2000 °C inside
the C preform. For this temperature, and considering a wall thickness of one
μ
m,
all the carbon of the wall is dissolved in about a minute.
A model for the formation of bioSiC based in the C-Si solubility [Kleykamp,
1993] has been proposed [Varela, 2007], as shown in Figure 11.11. If Si is major-
ity, then a liquid solution of Si/SiC is formed and micrometric SiC is precipitated
upon cooling or saturation of the solution. Wherever C is majority, there is
no Si/C liquid interface and the SiC is formed by a diffusion process that yields
nanosized SiC grains. Which scenario occurs depends mainly on the channel
diameter.
In Figure 11.11a, the schematics of the SiC formation inside the large chan-
nels is shown, where liquid Si is majority. Liquid Si penetrates the carbon preform
by capillary forces and because in the preforms the porosity is connected, the
molten Si reaches all pores and channels. This process takes place in a few seconds,
as has been previously shown. Silicon then reacts with the carbon preform forming
Si-C groups dissolved in the liquid by corrosion of the carbon walls that surround
the channels. As the walls are 1-2
m thick, the amount of available C is small
compared to the Si volume and C is quickly depleted. This process take about a
minute as has been previously shown.
The solution process is aided by the heat generated during the reaction which
in turn raises the liquid Si temperature and enhances the C solubility. The SiC/Si
solution supersaturates for small C concentrations, producing the precipitation of
SiC near the C walls, yielding SiC grains in the micron range. This is supported by
the faceted morphology of the large SiC grains. The process ends when all the
carbon is depleted and all SiC has precipitated.
If the channel diameter is comparable to the C wall thickness, then the pores
can become closed by the precipitation of SiC. In that case, the SiC barrier pre-
vents the complete reaction of the C preform in that zone, and so some unreacted
C remains in the sample. The process in this scenario is schematized in Figure
11.11b. As happens when the channels are large, the Si infi ltration occurs by
means of capillary pressure. The liquid Si then reacts with the C in the perform,
forming Si-C groups dissolved in the melt. Because there is more carbon than
silicon, the solution is supersaturated earlier than in the previous case, and so
micron-sized grains precipitate inside the channel and end up choking it. Up
to now, the process is similar to the previous scenario and happens in the Si-rich
part of the Si-C phase diagram. The Si is less and less available as the reaction
takes place and so the system is displaced to the C rich part of the phase diagram.
In this zone there is no solubility of Si into C so it is impossible for a solution-
precipitation mechanism to occur. The only possibility available to the system is
the formation of SiC by diffusion through the already formed SiC layer. At the
SiC/C interface the grains have a size in the nanometre range because the kinetic
of the reaction is extremely slow. For practical effects and considering the time
μ
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