Materials Transactions, Vol. 49, No. 9 (2008) pp. 1929 to 1933
#2008 The Japan Institute of Metals
Low-Temperature Synthesis and Mechanical Properties
of SiC Porous Granules from Activated Charcoal with a Na Flux
Haruhiko Morito, Hisanori Yamane, Takahiro Yamada, Shu Yin and Tsugio Sato
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan
SiC porous granules were synthesized from activated granular charcoal and Si powder at 973 K by using a Na flux. The SiC granules
maintained the shape of the charcoal with a dimension of about 5 mm in diameter and 7–10 mm in length. X-ray diffraction showed the structure
of the formed SiC to be cubic -type. Agglomerates of a few dozen nm of SiC grains and an electron diffraction ring pattern of -SiC were
observed with a transmission electron microscope. A micropore size distribution of < 4 nm and mesopores in the range of 20–40 nm were shown
by a nitrogen adsorption technique. The SiC granules had a specific surface area of 3:4 1:0 m2 /g and a pore volume of 0:006 0:002 cm3 /g.
The fracture stress of the SiC porous granules was evaluated to be 47 MPa by a compressive test at room temperature. The Vickers hardness of
the granule surface was 13:1 1:0 GPa. [doi:10.2320/matertrans.MAW200859]
(Received January 21, 2008; Accepted March 12, 2008; Published August 25, 2008)
Keywords: silicon carbide, activated charcoal, low temperature synthesis, sodium flux, compressive strength, Vickers hardness
1.
Introduction
Silicon carbide (SiC) ceramics are attractive materials due
to their potential properties in high strength, high stiffness,
and high thermal stability. In particular, porous SiC ceramics
can be used for a variety of applications including high
temperature exhaust gas filter, molten metal filter, catalyst
support, and lightweight components of machines.1–3) Furthermore, they have also been studied as light weight implant
materials for bone substitutions with taking advantage of
corrosion resistant.2–4)
Several processing methods of porous SiC ceramics have
been developed to their associated requirements for porosity,
pore size, and degree of interconectivity.5,6) One of the
representative methods for the synthesis of cellular ceramics
utilizes natural templates of wood which has an intricate
anatomy. The carbonized wood can be converted into a
silicon carbide cellular ceramic by various methods, such as
infiltration and reaction with liquid Si,4,7,8) Si containing
vapors,9) and infiltration and carbothermal reduction of a sol
containing colloidal silica.10–14) Biomorphic SiC has also
been produced directly from raw wood impregnated with
sodium silicate.15) However, high-temperature conditions
above 1573–1973 K are necessary for these processes and
the crystallization of SiC.
Recently, we found that -type cubic SiC could be
prepared from a Si and fullerene powder mixture or a Si
and amorphous carbon (carbon black) powder mixture by the
Na flux method at 900–1000 K.16,17) Furthermore, woods
from balsa and Japanese cypress were transformed into
carbonaceous preforms by pyrolysis and subsequently converted into cellular SiC ceramics by heating with Na and
Si at 973 K.18) Cellular structures similar to those of the
preforms were observed for the resulting porous SiC
ceramics. However, these SiC pellets and cellular SiC could
not have the adequate strength and stiffness because of their
low density. Therefore, mechanical properties of the SiC
porous ceramics prepared by the Na flux method were not
investigated in the previous studies.
In the present study, to obtain adequate strength and
stiffness SiC porous ceramics, we synthesized porous SiC
granules at 973 K by using activated granular charcoal as
high dense carbon templates and characterized their mechanical properties.
2.
Experimental
The following materials were placed in a boron nitride
crucible (Showa Denko, 99.95%, inside diameter 28 mm,
depth 25 mm): granules of activated charcoal (Wako, S
grade, about 5 mm in diameter and 7–10 mm in length),
Si powder (High Purity Chemicals, 99.999% purity, 75 mm
pass) and Na (Nippon Soda, 99.95% purity) at a Si : C : Na
molar ratio of 1:2 : 1:0 : 0:5. The crucible was set into a
container as illustrated in Fig. 1. This procedure was carried
out in an Ar gas-filled glove box (O2 and H2 O < 1 ppm).
The container was connected to an Ar gas-feed line and Ar
gas (Nippon Sanso, > 99:9999%) was introduced up to
about 0.3 MPa. The specimens were heated at 973 K for
24 h with an electric furnace and then cooled in the
furnace. The BN crucible was taken out of the container
in the glove box.
sheath heater BN crucible
Na metal
Si powder
activated
granular charcoal
Ni crucible
Ni cap
thermocouple
Fig. 1 Schematic illustration of the experimental apparatus used for the
synthesis of SiC.
1930
H. Morito, H. Yamane, T. Yamada, S. Yin and T. Sato
(a)
(b)
(c)
10 mm
Photographs of activated charcoal granules (a), as-prepared samples (b) and samples after washing (c).
3.
Results and Discussion
Figure 2 shows photographs of the samples in the BN
crucibles; (a) activated charcoal granules, (b) as-prepared
samples, and (c) samples after removing Na. A thin metallic
layer covered on the granules shown in Fig. 2(b) was
2mm
Photograph of a porous SiC plate prepared by filing a SiC granule.
Fig. 4
311
220
200
20°°
30°
40°
222
Fig. 3
111
The morphology of the samples was observed with an
optical microscope in the glove box. The Na and a Na-Si
intermetallic compound were removed by reaction with
2-propanol and ethanol in air. The samples were then
washed with distilled water and dried at 473 K in air. The
X-ray diffraction (XRD) patterns of the samples were
measured on an X-ray diffractometer (Rigaku RINT2200)
using CuK radiation produced at 40 kV and 40 mA with a
graphite monochromator. A scanning electron microscope
with a field emission cathode (SEM, Philips ESEM XL30)
at an acceleration voltage of 15 kV was used for the
observation of the fracture surface. The samples were
cracked with a nipper. Elemental analysis of the fractured
samples was performed with an energy dispersive X-ray
analyzer (EDX, EDAX) attached to the SEM. Specimens
for transmission electron microscopy (TEM) were prepared
by dispersing crushed samples on holey carbon films. TEM
images and selected-area electron diffraction (SAD) patterns were taken with a 200 kV electron microscope (JEOL
JEM-2000EX). The samples were also characterized with a
specific surface area analyzer (Quantachrome NOVA4200e) by N2 adsorption. Each sample was heated at
393 K for 2 h in vacuum to remove adsorbed water and gas
on the sample surface before measurement. Barrett-JoynerHalenda (BJH) and Brunauer-Emmett-Teller (BET) analyses were used to determine the mesopore size distribution
and specific surface area. The granular samples were filed
into a circular cylinder with the flat surfaces, 4:5 6:5 mm for a compressive test. The tests were performed at
room temperature using a compressive testing machine
(Shimadzu AG-20kNG). The constant cross-head speed
corresponded to the stroke rates of 0.1 mm/min. Vickers
hardness (HV) was determined for the mechanically
polished surface of the granular samples with a MicroVickers hardness tester (Akashi MVK-HO). Indentation
was performed with a Vickers indenter under 4.9 N. The
load was maintained for 15 seconds. The indenter print was
observed by means of a SEM.
Intensity, I / a. u.
Fig. 2
50°
2θ
60°
70°
80°
X-ray diffraction pattern from the surface of the porous SiC plate.
identified as Na and NaSi by X-ray diffraction under Ar
atmosphere. Na and NaSi were washed away with alcohol
first and then with water. The obtained samples maintained
the granular shape of activated charcoal (Fig. 2(c)).
The mass of the obtained sample was 3.17 g, which is close
to the expected mass of 3.10 g by the formation of SiC with
0.93 g of the charcoal granules. Some amounts of Na might
be trapped in the closed pores formed in the sample. As
shown in Fig. 3, the granular sample was ground into a
rectangular shape with a dimension of 3:34 2:96 5:76 mm3 by using a sanding machine. The density of this
sample was 2.2 Mg/m3 , which was 69% of the density of
SiC (3.2 Mg/m3 ).
The XRD pattern measured for the rectangular bulk
sample is shown in Fig. 4. The diffraction angles and relative
Low-Temperature Synthesis of SiC Porous Granules with a Na Flux
(a)
1931
(c)
200µ
µm
(b)
200µm
(d)
10µm
10µm
Fig. 5 SEM images of the fracture surfaces of the activated charcoal (a), (b) and the SiC porous granule (c), (d).
Si
C
I / a. u.
Intensity, I / a. u.
intensities of the diffraction peaks were consistent with a
previous report for -SiC (cubic type, lattice parameter
a ¼ 0:436 nm).19) Various crystal structures reported for SiC
depend on the synthesis temperature and -SiC is stable at
low temperature.20) The crystallite size of 12 nm calculated
with the XRD peak widths by the Scherrer’s equation was
larger than the sizes reported for the powders (9 nm) and the
porous bulk ceramics (4–7 nm) of -SiC prepared in the
previous studies by using a Na flux at 1000 K.16,17) These
results suggest that the high density of the carbon template
helps the grain growth in the porous ceramics.
Figure 5 shows the SEM images of the fracture surfaces of
the activated charcoal granule (a) and (b) and the obtained
SiC porous granule (c) and (d). Although the granular shape
was kept before and after heating (Fig. 2), the morphology of
the fractured surface was changed (Fig. 5(a) and (c)). Many
open spaces between the submicrometer-sized grains of the
activated charcoal (Fig. 5(b)) were buried with round cohering SiC grains of 1–3 mm as seen in Fig. 5(d).
Figure 6 shows an EDX spectrum measured for the
fracture surface of the SiC granule. The peaks of C and Si
were observed around 0.25 and 1.75 keV, respectively.
The composition determined by the EDX analysis was C
42–45 at%, Si 55–58 at%, O < 0.5 at % and Na 1 at %, which
was close to the ideal composition of SiC.
The TEM image of the fragments of the SiC granule is
shown in Fig. 7. It was observed that small particles with a
size of a few dozen nm aggregated and formed secondary
particles or agglomerates of a few hundred nm. The SAD
pattern from the agglomerates consisted of rings with some
spots, which corresponded to the diffractions of -SiC (111),
(220) and (311) planes.
Na
O
0
0
5
1
E / keV
10
2
15
Energy, E / keV
Fig. 6
EDX spectrum of the fracture surface of the SiC porous granule.
Figure 8 shows the adsorption-desorption isotherm curves
for the nitrogen sorption at 77 K in the activated charcoal and
the SiC porous granule. The volume adsorbed in the activated
charcoal was large at low relative pressure and saturated at
P=P0 ¼ 0:2. The curves belonged to type I isotherm plots in
the IUPAC classification,21) indicating that it possessed a
micropores structure. On the other hand, the adsorption and
desorption isotherm curves for the SiC granule exhibited an
abrupt change when the relative pressure was greater than
0.9, which belonged in the type II and showed the presence
of macropores.21) The hysteresis loop in the curves suggested
that the micropores were also included in the sample.21)
H. Morito, H. Yamane, T. Yamada, S. Yin and T. Sato
30
311
200nm
Fig. 7 TEM image and SAD pattern of fragments of the SiC porous
granules.
Cumulative pore volume, Vp / cm3 g -1 × 10 3
220
111
20
200
10
100
0
8
(a)
0
8
(b)
6
6
4
4
2
2
0
400
300
(a)
1
10
100
dV/d(log D) / cm3 g-1 × 10 3
1932
0
Pore diameter, D / nm
300
Volume adsorbed, Va / cm3 g-1
Fig. 9 Pore-size distribution diagrams of the activated charcoal (a) and the
SiC porous granule (b).
200
100
0
5
(b)
4
3
2
1
0
0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure, P/P 0
Fig. 8 Nitrogen adsorption and desorption isotherms at 77 K of the
activated charcoal (a) and the SiC porous granules (b).
The pore size distributions of the activated charcoal and
the SiC porous granules are shown in Fig. 9. Many pores with
a diameter of 4 nm were detected in the activated charcoal.
The calculated BET specific surface areas of the activated
charcoal were 870 50 m2 /g, and a pore volume of 0:028 0:010 cm3 /g was estimated. The SiC had micropores of
< 4 nm and mesopores with a diameter of around 30 nm. A
specific surface area and a pore volume of the SiC granules
were 3:4 1:0 m2 /g and 0:006 0:002 cm3 /g, respectively.
No phase diagram has been reported for the Na-Si binary
system. However, as discussed in the previous reports,17,18)
some amount of Si is considered to be dissolved in a Na melt.
In the present study, the Na melt containing Si probably
infiltrated into the pores and cavities of the activated charcoal
by capillary force or owing to the wettability of the melt to
the activated charcoal, and the formation of SiC buried the
micropores of the activated charcoal, which reduced porosity
in the bulk. On the other hand, the previous SiC porous
ceramics from the pelletized powder mixture of Si and
carbon had a low bulk density (29–34%) because of the
volume shrinkage by the SiC formation.17) Consequently, the
present SiC porous granules were more dense than the SiC
porous ceramics prepared in the previous study.17)
The stress-strain curves measured at room temperature for
the activated charcoal and the SiC porous granule are shown
in Fig. 10. The stress was applied along the cylinder axis
and the strain " along its axis was illustrated in the insert
figure of Fig. 10. The symbol of ‘‘’’ marks the point of the
fracture stress, at which crack growth began. From the
compressive tests, the fracture stresses of the SiC porous
granule and the activated charcoal granule were evaluated to
be 47 MPa and 7 MPa, respectively. On the other hand, the
fracture strain of the SiC granule was smaller than that of the
charcoal granule due to the difference of their elasticities.
While the compressive strength of well-sintered dense SiC
ceramics is over 3000 MPa,22) that of the present -SiC
porous granules is small due to its porosity. The compressive strength of the sample decreased with increasing its
porosity.23)
The hardness of the SiC porous granule was examined
with a micro-Vickers hardness tester. The Vickers indenter
was knocked in a polished surface of the SiC granule as
shown in Fig. 11. The Vickers hardness was then determined
by the ratio F=A, where F was the force applied to the
diamond and A was the surface area of the resulting
Low-Temperature Synthesis of SiC Porous Granules with a Na Flux
4.
Compressive stress, σ / MPa
50
1933
Conclusions
×
σ
40
SiC
ε
30
σ
20
activated charcoal
10
×
0
0
1
2
3
4
5
6
Strain, ε / %
Fig. 10 Stress-strain curves measured at room temperature for the
activated charcoal and the SiC porous granule.
The porous granules of -SiC ceramics were fabricated by
heating activated charcoal granules with Si and Na at 973 K.
The agglomerates of SiC grains with a size of a few dozen nm
were observed by TEM. A micropore size distribution of
< 4 nm and mesopores in the range of 20–40 nm were shown
by a nitrogen adsorption technique. The SiC granules had a
specific surface area of 3:4 1:0 m2 /g. The SiC porous
granules had a strength of 47 MPa and hardness of 13:1 1:0 GPa. The present results demonstrated that the SiC
ceramics synthesized with the Na flux have enough strength
for self-standing porous materials.
The authors wish to thank Dr. M. Ohtsuka in Tohoku
University for his help on the micro-Vickers hardness
measurement. This study was supported in part by a Science
Research Expenses Subsidy from the Ministry of Education,
Culture, Sports, Science and Technology, Grant-in-Aid for
Exploratory Research, 18655083, 2007.
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Fig. 11 Micro-Vickers indentation of the SiC surface.
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