Biomedical Engineering Reference
In-Depth Information
around 170 °C. During the second step of weight loss, the decomposition of hemi-
cellulose takes place at around 190-280 °C and volatile products are released
(CO 2 , CO, and other organics vapors). In the temperature range of 280-500 °C,
major weight loss takes place due to the decomposition of cellulose and lignin.
The decomposition of lignin increases above 300 °C and it results in a rapid
increase in carbon content of the material. Typically, the majority of the decom-
position is completed by 500 °C, and minimal weight loss occurs afterwards. The
major weight loss events are shown in DSC curves, which exhibit the endother-
mic and exothermic nature of the reactions. Although the TGA curves for differ-
ent wood species are quite similar in their nature, the DSC curves may show slight
differences due to differences in the chemical contents.
The pyrolyzed preforms were infi ltrated with silicon under vacuum. Gener-
ally, the infi ltration time and temperature depends on the melting point of the
infi ltrants and dimensions and properties of the preforms. For silicon, infi ltration
at 1450 °C for 30 minutes is adequate. The subsequent SiC formation reaction is
spontaneous and exothermic [Singh, 2000 ; Mart í nez, 2000 ; Singh, 2003 ; Arellano -
L ó pez, 2004 ; Greil, 1998 ].
The theoretical density of graphite is 2.16 g.cm − 3 [Phule, 2002]. From the
density of the carbon templates and the SiC density of Si-free fi nal products, it was
also possible to estimate, with linear correlation of r 2 = 0.97, that [Varela, 2002 ]:
ρ
(
28
.
04
.
)
ρ
(11.1)
SiC
C
and then:
ρ
175
.
ρ
(11.2)
SiC
Wood
Reproducible microstructures and good properties can be obtained with carbon
template pore sizes
m. Final SiC materials are Si/SiC composites, with some
unreacted C (Figure 11.5). To minimize the amount of unreacted C, which is del-
eterious for mechanical properties, an excess amount of Si over the stoichiometric
amount is always necessary (20-100%), with the excess amount increasing as the
typical pore size of the template increases. The reaction to form SiC results in a
volume expansion which decreases the pore size and closes off some of the smaller
pores compared to the C preforms, but the overall structure of the precursor
wood is retained (Figure 11.6). The melt infi ltration method allows good control
of the shape and density of the fi nal product by choosing the appropriate wood
[Varela, 2002 ; Singh, 2002 ].
Nitric acid (HNO 3 ) and hydrofl uoric acid (HF) can be used to remove the
excess unreacted silicon, typically fi lling the larger pores. After etching away the
excess silicon, a porous silicon carbide skeleton remains (Figure 11.7). An impor-
tant advantage of this process is the possibility to tune the density of this material
in a wide range (1.1 - 2.5 g.cm − 3 ). Its density is very low in comparison with the
literature values of the theoretical density of SiC (3.21 g.cm − 3 ) and both medical-
grade Ti (4,51 g.cm − 3 ) and Ti - alloy (4,42 g.cm − 3 ) [Mangonon, 1999 ].
>
5
μ
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