Journal of the Korean Ceramic Society

Vol. 47, No. 4, pp. 308~311, 2010.

− 308 −

ReviewDOI:10.4191/KCERS.2010.47.4.308

†Corresponding author : Jung-Soo Ha

E-mail : jsha@andong.ac.kr

Tel : +82-54-820-5637 Fax : +82-54-820-6211

Properties of Porous SiC Ceramics Prepared by Wood Template Method

Jung-Soo Ha†, Byong Gu Lim*, Geum-Hyun Doh**, In-Aeh Kang**, and Chang-Sam Kim***

School of Advanced Materials Engineering, Andong National University, Andong 760-749, Korea*Head Office & Factory, CoorsTek Asia Inc., Gumi 730-400, Korea

**Department of Forest Products Technology, Korea Forest Research Institute, Seoul 130-012, Korea***Battery Research Center, KIST, Seoul 130-650, Korea

(Received July 6, 2010; Revised July 15, 2010; Accepted July 16, 2010)

ABSTRACT

Porous SiC samples were prepared with three types of wood (poplar, pine, big cone pine) by simply embedding the wood char-

coal in a powder mixture of Si and SiO2 at 1600 and 1700oC. The basic engineering properties such as density, porosity, pore size

and distribution, and strength were characterized. The samples showed full conversion to mostly β-SiC with good retention of the

cellular structure of the original wood. More rigid SiC struts were developed for 1700oC. They showed similar bulk density (0.5~

0.6 g/cm3) and porosity (81~84%) irrespective of the type of wood. The poplar sample showed three pore sizes (1, 8, 60 µm) with a

main size of 60 µm. The pine sample showed a single pore size (20 µm). The big cone pine sample showed two pore sizes (10,

80 µm) with a main size of 10 µm. The bend strength was 2.5 MPa for poplar, 5.7 MPa for pine, 2.8 MPa for big cone pine, indi-

cating higher strength with pine.

Key words : Porous SiC, Wood template, Embedding, Properties

1. Introduction

orous ceramics have been found to be essential for a

variety of technical applications such as thermal insu-

lators, filters, catalyst supports, gas sensors, and bone graft

substitutes.1) The pore size and shape, porosity, and

mechanical properties should be tailored to meet the

requirements for each application. Many methods have

been developed to produce porous ceramics, such as replica-

tion of polymer foams, foaming of a ceramic slurry, incorpo-

ration of a fugitive phase, sintering of hollow spheres, etc.2,3)

Wood has been used as a template to produce porous

ceramics that replicate the strongly anisotropic cellular

structure of wood with 50~80 vol% porosity.4-10) Hardwood

has a heterogeneous pore size, while softwood has a

homogeneous pore size.11) A wide range of ceramic materials

(e.g., SiC, SiC/Si, TiC, C/C, and TiO2) can be derived from

wood.5,7) Among these, porous SiC is particularly attractive

due to its high mechanical and thermal resistance as well as

its bioinert and corrosion resistant properties, enabling

versatile applications in energy and environmental

technology as well as in medicine and biotechnology.12-14)

For the fabrication of SiC, dried wood is first carbonized

and then siliconized by infiltration reactions with various

sources of Si such as Si melt,4) Si vapor,6,7) SiO gas,8) or tetra-

ethylorthosilicate (TEOS).5,10) The processes involving a gas-

phase reaction are advantageous for avoiding residual Si

and high conversion to SiC. Previously, porous SiC was pre-

pared by the reaction of wood charcoal with gaseous SiO

that was generated remotely from an equimolar mixture of

Si and SiO2.8) Recently, we successfully produced porous cel-

lular SiC by simply embedding poplar charcoal into a pow-

der mixture of Si and SiO2 at 1600oC.15) In the present work,

porous SiC samples were prepared with three types of wood

(poplar, pine, big cone pine) by the embedding process at

1600 and 1700oC. The basic engineering properties such as

density, porosity, pore size and distribution, and strength

were investigated and the results are presented.

2. Experimental Procedure

The wood samples were dried without any warping and

cracks at 85oC for 24 h for poplar and at 60oC for 4 days for

pine and big cone pine. Rectangular specimens (45×10×10 mm) were cut from the dried wood and carbonized at 800oC

for 4 h in flowing N2 in a graphite furnace with heating rates

of 1oC/min up to 500oC and 5oC/min up to 800oC. The

carbonization caused a weight loss of 73~75% and shrinkage

of 17~22% axially, 28~36% radially, and 22~36% tangentially.

The resulting wood charcoal (i.e., carbon template) was

embedded in an equimolar mixture of Si and SiO2 powders

in a BN-coated graphite crucible and then heated at 1600 or

1700oC for 1 h in Ar in a graphite furnace. The amount of

the embedding powder was greater (>100 wt% excess) than

the stoichiometric amount necessary for the two successive

reactions: Si(s,l) + SiO2(s) = 2SiO(g), and SiO(g) + 2C(s) =

SiC(s) + CO(g). The weight changes of the samples after the

P

July 2010 Properties of Porous SiC Ceramics Prepared by Wood Template Method 309

reaction were measured to calculate the degree of SiC

conversion (R) from the following equation:8)

Here, Wc is the initial weight of the carbon template, ∆W

the weight change after the reaction, MSiC

the molecular

weight of SiC, and Mc the molecular weight of C.

For the carbon templates and SiC samples, the phase and

microstructure were examined using an X-ray diffractome-

ter (XRD; RINT 2000, Rigaku Co., Japan) and a scanning

electron microscope (SEM; JSM 6300, JEOL, Japan). The

density and porosity were measured by the Archimedes

method. A mercury porosimeter (Pascal 140+440, Thermo

Finnigan, Italy) was employed to investigate the pore size

along with its distribution. The strength of the SiC samples

was measured by a three-point flexural test with a span of

20 mm and a cross head speed of 5 mm/min. The tensile

surfaces were ground with a 400 grit diamond wheel to

ensure a uniform surface finish.

3. Results and Discussion

Fig. 1 shows the XRD results of the carbon templates pre-

pared from poplar, pine, and big cone pine. It can be seen that

the three types of wood were pyrolyzed successfully into

amorphous carbon. The SEM results in Fig. 2 show that the

carbon templates retained the porous cellular structures of

their original wood in each case. The poplar consisted of large

(50~100 µm) and small (10~20 µm) pores, while the pine and

big cone pine had a pore structure of single size (20~30 µm). It

appears that their surfaces were very smooth due to the

amorphous nature as shown more clearly in Fig. 2(b), which

shows an image of the poplar taken at a higher magnification.

Calculation of the SiC conversion degree indicated that

three types of carbon templates were all converted to SiC in

100% by the embedding process. Previously,15) we attempted

to prepare porous SiC using a Si melt infiltration method in

which a carbon template from poplar was embedded in Si

powder at 1600oC for 2 h. In this case, only 52% conversion

to SiC was obtained. XRD analyses confirmed that the

present samples were comprised of β-SiC and a slight

amount of α-SiC, as shown in Fig. 3. The SEM results of the

SiC samples prepared at 1600 and 1700oC are given in Figs.

4 and 5, respectively. It can be seen that they all retained

the pore structure of the carbon templates. Additionally,

more rigid and thicker SiC struts were obtained with the

samples prepared at 1700oC.

Table 1 shows the results of measurements of bulk den-

sity, porosity, pore size, and strength of the carbon tem-

R %( ) W∆Wc

---------MSiC 2Mc–

2Mc

----------------------------⎝ ⎠⎛ ⎞

1–

100× W∆Wc

---------1

0.67----------- 100×⋅= =

Fig. 1. XRD results of carbon templates.

Fig. 2. SEM micrographs of carbon templates: (a, b) poplar,(c, d) pine, (e, f) big cone pine.

Fig. 3. XRD results of SiC samples prepared at 1600oC.

310 Journal of the Korean Ceramic Society - Jung-Soo Ha et al. Vol. 47, No. 4

plates and SiC samples. The bulk densities of the carbon

templates were approximately 0.3 g/cm3 for the three types

of wood and increased to nearly 0.5~0.6 g/cm3 after SiC con-

version. The porosities were 79~84% for the carbon tem-

plates and 80~84% for the SiC samples, indicating nearly no

change. The pore sizes of the carbon templates were 1, 15,

and 70 (the main pore size) µm for poplar; 30 µm for pine;

and 20 (the main pore size) and 80 µm for big cone pine.

Upon conversion to SiC, they decreased by 10 µm so that

the pore sizes of the SiC samples were 1, 8, and 60 (the main

pore size) µm for poplar; 20 µm for pine; and 10 (the main

pore size) and 80 µm for big cone pine. Fig. 6 shows the pore

size distribution curves obtained from the porosimeter. The

carbon templates and the SiC samples are very similar in

the distribution, indicating that the carbon templates con-

verted to SiC while mostly maintaining their pore structure.

The strength of the SiC samples were 2.5 MPa for poplar,

5.7 MPa for pine, and 2.8 MPa for big cone pine. The higher

strength with the pine sample is attributed to its small and

homogeneous pore size. In the case of carbon templates,

there are no strength data because they were too weak for

the strength test to be performed.

4. Conclusions

Porous cellular SiC ceramics were produced from poplar,

pine, and big cone pine by the embedding process with good

retention of the pore structure of the original wood. More

rigid SiC struts were developed at 1700oC. They showed

similar bulk density (0.5~0.6 g/cm3) and porosity (81~84%)

irrespective of the type of wood. The poplar sample showed

three pore sizes (1, 8, and 60 µm), with the main size of

60 µm. The pine sample showed a single pore size (20 µm).

The big cone pine sample showed two pore sizes (10 and

Fig. 4. SEM micrographs of SiC samples prepared at 1600oC:(a, b) poplar, (c, d) pine, (e, f) big cone pine.

Fig. 5. SEM micrographs of SiC samples prepared at 1700oC:(a, b) poplar, (c, d) pine, (e, f) big cone pine.

Table 1. Properties of Carbon Templates and SiC Samples Prepared at 1600oC

PropertiesCarbon templates SiC samples

Poplar Pine Big cone pine Poplar Pine Big cone pine

Bulk density (g/cm3) 0.28 0.33 0.28 0.49 0.58 0.58

Porosity (%) 83 79 84 84 80 81

Pore size (µm) 1, 15, 70† 30 20†, 80 1, 8, 60† 20 10†, 80

Flexural strength (MPa) - - - 2.5 5.7 2.8†Main pore sizeNo strength data for carbon templates because they were too weak for the test.

July 2010 Properties of Porous SiC Ceramics Prepared by Wood Template Method 311

80 µm), with the main size of 10 µm. The flexural strength

was 2.5 MPa for poplar, 5.7 MPa for pine, and 2.8 MPa for

big cone pine, showing higher strength with pine due to its

small and homogeneous pore size.

Acknowledgment

One of the authors (JSH) was supported by Andong

National University through the research professor pro-

gram in 2008.

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Fig. 6. Pore size distributions for carbon templates and SiCsamples prepared at 1600oC: (a) poplar, (b) pine, (c)big cone pine.