International Journal of Applied Ceramic Technology, 3(6): pp. 485-490.Biomorphous β-SiC ceramics were produced at 1400°C from pine wood impregnated with
silica. This one-step carbothermal reduction process decreases the cost of manufacturing of
SiC ceramics compared to siliconization of carbonized wood in silicon vapor. The
synthesized sample exhibits 14 m2/g surface area and has a hybrid pore structure with large 5-
20 μm tubular macropores and small (< 50 nm) slit shaped mesopores. SiC whiskers of 20-
400 nm in diameter and 5-20 μm in length formed within the tubular pores. These whiskers
are expected to improve the filtration by removing dust particles that could otherwise
penetrate through large pores. After ultrasonic milling, the powdered sample showed an
average particle size of ~30 nm. The SiC nanopowder produced in this process may be used
for manufacturing SiC ceramics for structural, tribological, and other applications

Gogotsi, Yury G.

Pereselentseva, L.

Vyshnyakova, K.

Yushin, Gleb

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Formation of porous SiC ceramics by pyrolysis of wood impregnated with silica 
 
K. Vyshnyakova1, G. Yushin2, L. Pereselentseva1, Y. Gogotsi2 * 
 
1 - Frantsevich Institute for Problems of Materials Science of NAS of Ukraine,  
3 Krzhyzhanovsky Str., Kiev, 03142, Ukraine 
 
 
2 - Drexel University, Department of Materials Science and Engineering and A. J. Drexel 
Nanotechnology Institute, Philadelphia, Pennsylvania 19104, USA 
Fax:1 215 895 6760; Tel: 1 215 895 6446   
 
* Contact author 
Additional contact information: 
K. Vyshnyakova e-mail: katia_vi@yahoo.it 
G. Yushin e-mail: yushin@cbis.ece.drexel.edu 
L. Pereselentseva e-mail: lnp212@i.com.ua  
Y. Gogotsi e-mail: gogotsi@drexel.edu  
 
 
ABSTRACT 
 
 
 
Biomorphous β-SiC ceramics were produced at 1400°C from pine wood impregnated with 
silica. This one-step carbothermal reduction process decreases the cost of manufacturing of 
SiC ceramics compared to siliconization of carbonized wood in silicon vapor. The 
synthesized sample exhibits 14 m2/g surface area and has a hybrid pore structure with large 5-
20 µm tubular macropores and small (< 50 nm) slit shaped mesopores. SiC whiskers of 20-
400 nm in diameter and 5-20 µm in length formed within the tubular pores. These whiskers 
are expected to improve the filtration by removing dust particles that could otherwise 
penetrate through large pores. After ultrasonic milling, the powdered sample showed an 
average particle size of ~30 nm. The SiC nanopowder produced in this process may be used 
for manufacturing SiC ceramics for structural, tribological, and other applications.       
 
 
 
 
 
 
 
 
Introduction 
Synthesis of ceramics from natural fibrous and vegetable precursors attracted much 
interest in recent years [1-4]. The research was mainly driven by economical reasons since 
natural materials serve as a renewable and relatively inexpensive source of carbon. In 
addition, biomorphic ceramics templated by cellular biological objects exhibit complex 
hierarchical microstructures, which are attractive for applications requiring a low specific 
weight and high stiffness [5, 6]. Synthesis, microstructure and selected properties of 
biomorphic SiC, Si3N4 and oxide ceramics from different types of wood (pine, eucalyptus, 
fir, aspen, etc.), bast and cereal fibers (rice, jute etc.) have been reported [7-9]. SiC and Si3N4 
ceramics with cellular structure are usually produced by methods based on a two-step 
process: (1) a controlled pyrolysis of natural precursors, and (2) high temperature reactive 
infiltration of the porous carbon preform by molten Si, Si vapor, or SiO in a controlled 
environment (vacuum, Ar, or N2) [10-15]. The excellent strength and specific stiffness of the 
biomorphic ceramics open a wide range of applications for these attractive materials [16, 17].  
In this paper we demonstrate a simple synthesis route, in which biomorphic SiC is 
produced directly from raw wood impregnated with silica (sodium silicate) using waste 
products of pine wood processing. This method is more economical as it allows eliminating 
the pyrolysis step and does not require the use of elemental silicon. In addition, the described 
procedure reduces the temperature and duration of the SiC synthesis and generates SiC 
nanofibers in the pores.  
Experimental 
Figure 1 shows the process flow chart for the manufacturing of SiC ceramics. We 
used pine wood chips mostly consisting of about 5x1x1 mm particles. The chips were dried to 
a constant weight, and then held in a sodium silicate aqueous solution at 20-50 ºC for 3-5 
hours. The saturated chips were dried, treated with a hydrochloric acid solution, rinsed with 
water, and dried again at the temperature of 100±5 ºC to a constant weight. The chips 
impregnated with ~40 wt. % silica were then heated in a graphite reactor under an Ar flow to 
1400 ºC, held at this temperature for 60 min, cooled down to room temperature and taken out 
for further analyses. Part of the sample was milled using an UZDM-A (Russia) ultrasonic 
miller at 22 kHz for 5 min, in 1-wt.% sodium hexametaphosphate aqueous solution.  
 X-ray diffraction (XRD) of the synthesized products was performed using a DRON-
1.5 diffractometer (Burevestnik, Russia) using Cu Kα radiation (45kV, 20A, λ=1.54056 Å) 
with a Ni filter, step size of 0.25° (2θ) and hold time of 1min. Scanning electron microscopy 
(SEM) and energy dispersive X-ray spectroscopy (EDS) was performed using a FEI XL-30 
environmental field-emission scanning electron microscope (FEI, The Netherlands) equipped 
with an EDAX EDS detector. SEM samples were studied without coating and were attached 
to a metal sample holder using a conductive carbon tape. Accelerating voltage of 5 and 20 kV 
and a working distance of ~6 and ~11 mm were employed for SEM and EDS analyses, 
respectively. The semiquantitative analysis of the EDS spectra was done using the Genesis 
software supplied by EDAX. Gas sorption measurements were performed using an automated 
micropore gas analyzer Autosorb-1 (Quantachrome Instruments, USA). The Ar sorption 
isotherms collected at liquid nitrogen temperature (-196 oC) were analyzed using the 
Brunauer, Emmet, Teller (BET) equation to reveal the specific surface area (SSA) of the 
produced ceramics. Quantachrome’s data reduction software Autosorb v.1.51 was employed 
for these calculations. Particle size analysis of the grinded sample was done using a Zetasizer 
1000 (Malvern Instruments, Ltd., UK).  
Results and discussion 
Powder X-ray diffraction 
XRD analysis of the synthesized and milled material allowed us to determine the 
minimal temperature of the SiC synthesis as ~1400 ºC. Below this temperature (1350 ºC) the 
main phases formed are α-cristobalite (diffraction peaks at 2θ ~31.1º, corresponding to (102) 
planes, and 35.8º - (200)) and graphite ((002) peak at 2θ ~26.6º). Figure 2 shows the XRD 
pattern of the sample produced at 1400 ºC during the 60 min annealing. The position of the 
observed peaks indicated the formation of cubic β-SiC. The SiC synthesis proceeding through 
the carbothermal reduction reaction [18] is almost completed, and no substantial amount of 
crystalline SiO2 phase can be detected. A weak peak at ~26.6º, visible in the diffraction 
pattern (Fig. 2), indicates the presence of graphitic carbon in the sample and can be explained 
by the excess of carbon in the pyrolysis products of the wood/SiO2 composite. The remaining 
carbon can be removed from the sample by oxidation at ~ 600° C in air (data not shown).  
Scanning electron microscopy and energy-dispersive X-ray spectroscopy 
Figure 3 shows SEM micrographs of the SiC sample synthesized at 1400 ºC. Large 
tubular pores of about 5-20 μm in diameter, observed in most of the samples, are believed to 
originate from the biological vessels existing in wood. The cross-section shape of these 
macro-channels varies from round/elliptical (Fig. 3a) to nearly rectangular (Fig.3c). The wall 
thickness is generally in the 1-5 μm range. These walls demonstrate a porous nano-crystalline 
structure with the grain size of 20-200 nm (Fig.3b).  
The EDS analysis showed up to 17 at. % of oxygen and a slight excess of carbon in 
the SiC samples (Fig. 3f), confirming the presence of graphite detected by XRD (Fig. 2). 
Oxygen is believed to be in the form of amorphous silicon oxycarbide, since no crystalline 
SiO2 was detected by XRD (Fig. 2). The composition of the material is somewhat similar to 
SiC produced from polymeric precursors, in which nano-sized grains of SiC are surrounded 
by silicon oxycarbide and carbon (Tyranno fibers). The yield of SiC is expected to increase at 
higher temperatures or longer annealing time, but this may increase the grain size of SiC and 
the overall cost of the process.   
Roughly half of the pores in the sample studied by SEM showed the presence of SiC 
whiskers grown on the walls inside the macro-channels (Figs. 3 c,d,e). The diameter and the 
length of the whiskers ranged from ~20 to ~400 nm and ~5 to ~20 µm, respectively. Larger 
SiC fibers with the diameter of ~1µm were also occasionally observed (Fig. 3e). Formation 
of SiC whiskers during the synthesis of biomorphic SiC ceramics from mineralized wood was 
reported in several recent publications [19, 20]. The growth of SiC whiskers is believed to 
proceed via the vapor-liquid-solid (VLS) mechanism where the chemical reaction between 
gaseous SiO and CO provides transport of carbon and silicon needed for the whisker growth 
[18]. Impurities coming from wood or sodium silicate might have acted as nucleation sites. 
Silica deposited on the carbon surface is known to promote the growth of SiC whiskers [21]. 
Particle size analysis  
Preliminary particle size measurements performed on the ultrasonically milled sample 
showed the formation of ultra-fine particles (Figs. 4 a,b) with an average particle size of ~30 
nm (Figs. 4 b). The relatively small grain size in the synthesized SiC ceramics could be due 
to the relatively low synthesis temperature which impedes re-crystallization and grain growth. 
In addition silicon acid gel precipitated onto the wood sample surface is known to consist of 
SiO2 (cristobalite) nanocrystals [22] which may determine the size of SiC crystallites formed 
during the carbothermal synthesis process. A network of small pores existing in wood and 
transferred to the produced SiC ceramics make milling and separation into small grains 
relatively easy. Pine wood is a light-weight wood with relatively low carbon content, 
producing a loose and open pore structure. 
Argon sorption  
Figure 5 shows Ar sorption on the surface of the synthesized sample. The shape of the 
adsorption isotherm (Type II according to the Brunauer classification) is typical for 
macroporous (>50 nm) solids with unrestricted multilayer adsorption [23]. However, the 
presence of adsorption-desorption hysteresis and the shape of the desorption isotherm 
indicate the formation of slit-shaped mesopores (2-50 nm) [23], presumably within the walls 
of the macropores (Fig. 3b). The BET SSA was found to be ~14 m2/g, which is typical for 
biomorphic SiC ceramics.    
Thus, the material produced in our one-step process is similar to ceramics made by 
silicon infiltration of carbonized wood. It has a high surface area and well-developed pore 
structure, including macropores and mesopores. Since the produced material looks both 
robust and porous enough for making high-temperature filters, we were satisfied with the 
achieved yield and did not pursue the synthesis at higher temperatures. A detailed 
characterization of mechanical properties and gas permeation should be conducted to 
evaluate potential applications, but it is outside the scope of this study, which mainly reports 
on the composition and structure of the synthesis products. 
Summary  
Porous β-SiC ceramics have been synthesized at 1400 °C from pine wood infiltrated 
with silica gel. SEM and gas sorption techniques revealed the presence of both long 
cylindrically-shaped macropores and slit-shaped mesopores, with the accessible specific 
surface area of ~14 m2/g. SEM has also detected formation of SiC whiskers with a diameter 
of 20-400 nm and a length of 5-20 µm inside the macropores. The simplicity and relatively 
low cost of the suggested process may be attractive for industrial production of high-
temperature filters, catalytic converters for exhaust gas purification, and heat exchangers, as 
well as fillers for composites with metallic, polymer, or ceramic matrices. Ultrasonic milling 
of the produced ceramics produced SiC nanopowders with an average particles size of ~30 
nm.  
Acknowledgement 
SEM, EDS, and gas sorption studies were performed at the Centralized Materials 
Characterization Facility of the A.J. Drexel Nanotechnology Institute. 
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Adsorption and Desorption Isotherms,” Colloids and Surfaces, 187–188 11–21 (2001). 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
  
 
 
Fig. 1. Process flow used for the fabrication of porous SiC ceramics from wood. 
 
 
20 30 40 50 60 70 80
0.2
0.4
0.6
0.8
1.0
In
te
ns
ity
, a
.u
.
2 Θ, degree
Cubic SiC (JCPDS 29-1129)(111)
(220)
(311)
(200)
Graphite
 
Fig. 2. XRD pattern of the sample produced at 1400ºC. Cubic β-SiC crystal phase has been 
identified. The peak at ~26° corresponds to (002) planes of graphite. All other peaks originate 
from β-SiC. 
 
 
 
Fig. 3. SEM micrographs (a-e) and EDS spectrum (f) of the SiC samples produced from pine 
wood at 1400°C. The macro-channels of 5-20 µm observed in the produced ceramics mimics 
the biological vessels in wood. The 1-5 µm walls show a porous nanocrystalline structure. 
SiC whiskers and fibers of 0.05-1 µm in diameter are clearly seen in most of the channels.  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
element   at. % 
Si 39 
C 44 
O 17 
(a) (b)
(c) (d)
(e) (f)
1 µm
20 µm 5 µm
1 µm 
100 µm
20 µm 
 
 
 
 
10 100 1000
P
ar
tic
le
 v
ol
um
e,
 a
.u
.
Particle size, nm
   
10 100 1000
N
um
be
r o
f p
ar
tic
le
s,
 a
.u
.
Particle size, nm
 
 
Fig. 4. Particle size distribution by volume (a) and number (b) of the ultrasonically milled 
SiC samples produced from wood at 1400°C. 
 
 
0.0 0.2 0.4 0.6 0.8 1.0
0
5
10
15
20
adsorption 
V
ol
um
e 
of
 A
r a
ds
or
be
d,
 c
c/
g
Relative pressure, P/P0
desorption 
BET-SSA = 14 m2/g
 
 
Fig. 5. Argon sorption isotherm recorded at 77 K.   
 
 
 
 
 
 
 
 
 
 
(a) (b)


Formation of porous SiC ceramics by pyrolysis of wood impregnated with silica

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Formation of porous SiC ceramics by pyrolysis of wood impregnated with silica

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