Biomedical Engineering Reference
In-Depth Information
Figures 11.9a and 11.9b shows SEM micrographs of carbon precursors. In
hardwoods, the pores typically follow a bimodal distribution, with small pores
with diameters ranging from 4
μ
m to 10
μ
m and large pores ranging from 30
μ
m to
200
m, depending on the precursor [Singh, 2000; Martínez, 2000; Singh, 2003;
Arellano, 2004]. The microstructure of the porous carbon performs are shown in
Figures 11.9a and 11.9b in two different directions, respective to tree growth.
There is a wide variation in the microstructure and density of the carbonaceous
performs, due to the structural differences within a wood sample and between
various types of wood. The variation of preform microstructure and properties
can be utilized to produce fi nal materials with controlled composition and phase
morphologies. The perform density and microstructure control the composition
and microstructure of fi nal materials.
Figure 11.10 shows the principal aspects of the microstructure in bioSiC, as
well as the main phases observed [Zollfrank, 2004; Zollfrank, 2005; Varela, 2005;
Arellano - L ó pez, 2007 ; Varela, 2007 ]. As seen in the XRD in Figure 11.10 a, both
Si and SiC are crystalline in bioSiC, while the carbon precursors are amorphous.
In the SEM micrographs in Figure 11.10b (1,2,3) crystalline Si is seen in brighter
contrast. Crystalline SiC is seen as darker grey, and presents a bimodal grain size
distribution consisting on both micron-sized grains (
μ
- SiC) and nanometersized
grains (n-SiC), as seen in TEM pictures (4 and 5). Finally, unreacted amorphous
C is seen as black. SEM/EDS micrograph (6) confi rms these observations.
Micron-size SiC grains are formed in the walls of sap channels (large chan-
nels with diameter over 5
μ
m). In small channels, where Si is usually depleted in
the reaction because micron-sized SiC grains closed the pores, nano-sized SiC
grains can be observed between the large SiC grains and the unreacted carbon.
These n-SiC grains are found forming rosettes in certain places, which is typical of
diffusion-controlled growth, while
μ
-SiC grains are faceted, which is typical of
solution-precipitation-controlled growth. Both unreacted Si and C areas can be
found, normally separated by a SiC layer that prevents them from reacting with
each other.
The previous observations make it possible to distinguish two reaction
methods, depending on the diameter of the channels in the carbon preform. In
both cases, molten Si penetrates through the carbon preform by capillarity and
reacts with the solid C spontaneously and exothermically. If the channel is large
and Si is abundant, Si-C groups are formed in Si solution until it saturates and
μ
μ
-SiC grains precipitate. In the small channels, the process is similar but the large
μ
-SiC grains that precipitate close the pores. Under these conditions, the Si must
diffuse through the previously formed SiC to react with the carbon perform. Thus,
nanometer-sized SiC grains are formed in a process controlled by diffusion.
Concerning the chemical composition of these ceramics, EDS analyses show
that this biomorphic material is a composite of a SiC skeleton with small amounts
of unreacted Si and C. Recent studies conclude that the residual silicon does not
present adverse physiological effects in the body; Si is safely excreted through the
urine and no accumulation was found distributed in the major organs [Lai, 2002].
Table 11.1 shows the XRF and XPS analyses of the three SiC types [González,
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