Biomedical Engineering Reference
In-Depth Information
high surface area (170-200 m
2
/g) may be widely applied in catalysis, catalyst support,
and photovoltaics.
12.3.2
SiC Nanorods
Thermal treatment of carbon/silica composites in Ar at high temperature produces SiC
materials showing as below (Equation 12.1).
SiO
2
+
3C
→
SiC
+
2CO
(12.1)
In this reaction, excess carbon is required to achieve high-quality SiC; residual carbon
can easily be removed by subsequent oxidation. Our early experiment for the preparation
of biomorphic SiC materials through mineralization of silica into wood cellular network
followed by carbothermal reduction at 1400
◦
C showed that cubic SiC materials formed
through the quantitative conversion of mineralized silica into SiC (28). After further
treatment at 700
◦
C for 2 h in air, pure greenish yellow SiC was obtained. Since the
amount of silica in the composite materials is smaller than that of carbon on a mole
basis, the formation of SiC is directly related to the quantity of SiO
2
mineralized within
the wood. SEM images revealed that entire cellular structures including cells, pits, and
lumens remained after the thermal treatment, and SiC whiskers (
10%) were randomly
deposited on the biomorphic SiC surface, which consisted of nanosized crystalline SiC
powders (
∼
90%).
The same carbothermal reduction process has been applied to the preparation of SiC
nanorods from wood pulp (29) and CNXL (30). XRD patterns (not shown here) of SiC
rod samples synthesized from both unbleached and bleached pulp, and CNXL revealed
mainly the cubic crystal polymorph (
β
-SiC, unbleached:
∼
2
.
51 A;
2
θ
=
35
.
74,
d
111
=
2
.
52 A) after thermal treatment at 1400
◦
C. The
d
values
of four diffraction peaks for SiC rods synthesized from different celluloses are deter-
mined to be 2.51, 2.18, 1.54, and 1.31 A. SiC rods synthesized from unbleached pulp
(31, 32) show that unbleached pulp retains its original morphology after thermal treat-
ment at 1400
◦
C, with a small amount of SiC nanorods randomly dispersed on the sample
surface (Figure 12.4a and b). The size (5-25
µ
m in diameter and up to a few cm long) of
wood fibers remained nearly the same after high temperature reaction. SiC rods are not
uniform and are entangled with one another. SiC materials synthesized from bleached
pulp were completely converted to straight and uniform SiC nanorods under the same
processing conditions (Figure 12.4c and d). Energy dispersive X-ray spectroscopy (EDS)
analysis demonstrated that the atomic ratio of Si/C was nearly 1:1 in the inner part of
the nanorods. In particular, they exhibit a camelback-type structure (inset). The uniform
SiC nanorods are 80 nm in diameter and 50
µ
m long. SiC nanorods synthesized from
the unbleached pulp exhibited nonuniform patterns, along with a variable thickness of
amorphous SiO
2
layer. Products synthesized from the bleached pulp exhibited uniform
camelback structures with a 10-15 nm SiO
2
coating. They possessed a high density of
stacking faults, which are perpendicular to the nanowire axes. CNXL materials produce
high quality SiC nanorods (Figure 12.4e and f) when the silica mineralized material is
carbothermally reduced. SiC rod products are very uniform (70 nm in diameter and
>
100
µ
m in length) and do not have camelback structures as seen in the mineralized
samples. This homogeneity results from uniform crystalline structure of CNXL.
bleached: 2
θ
=
35
.
62,
d
111
=
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