Proceedings of the 12th Conference of the European Ceramic Society – ECerS XII
Stockholm, Sweden - 2011
Impact of the processing conditions on the specific surface area of carbides
powders derived from a colloidal route
Deschanels X.1, El Ghazzal M.1, Delchet C.1, Herault D.1, Hodaj F.2 and Zemb T1
– UMR CNRS 5266, Grenoble INP
UJF, BP 75
1ICSM
2SIMaP
CEA Marcoule - UMR 5257
BP 17171
30207 Bagnols-sur-Cèze (France)
38402 St Martin d'Hères Cedex (France)
xavier.deschanels@cea.fr
problem is to reduce the size of starting products
from micrometric to nanometric scale3. Recently,
several authors4-8 have used this route to obtain
carbide.
In this work, a process similar to that described
by Martin6-7 has been used to synthesize carbides. It
consists in the carbothermal reduction of the parent
metallic oxide (colloidal or micrometric) by carbon
obtained after the pyrolysis of a carbohydrate. This
technique provides the ability to intimately mix the
reagents and to encapsulate the oxide in a carbon
matrix. The characteristics of the final products
(surface area, crystallite size) were studied
according to the size and the nature of the
precursors. The main objective of this work was to
study the relationship between the initial surface
contact between carbohydrate and silica/zirconia,
and the final surface area of the porous material
obtained.
Abstract
Carbides compounds (SiC, ZrC) powders were
synthesized by carbothermal reduction of
corresponding metal oxides (colloidal or
micrometric size) by carbon resulting from the
pyrolysis of carbohydrate at temperatures ranging
between 1300 and 1550°C under flowing argon.
The specific surface area and the crystallite size of
the final products were investigated as a function of
the size of the starting oxides and the excess of
carbohydrate used for the carboreduction. The final
products elaborated from colloidal precursors have
a mesoporous structure localised into the porous
carbon matrix surrounding the carbides particles.
SEM and TEM observations showed that the
porosity of the samples is due to the release of
gaseous species produced during the carbothermal
process. The highest surface area measured is close
to 300m2/g for a sample ZrC+C. The use of
nanometric precursors could be an interesting way
to control the pore size in these supported
mesoporous carbon materials.
Keywords:
Carbides,
Carbothermal reduction, Colloid
Experimental
The carbothermal reduction of metal oxides
(ZrO2, SiO2…) leading to the synthesis of carbides
can be described as depicted in equation (1) :
MO2 + 3C → MC + 2 CO
(1)
It has been suggested that under reducing
conditions, the mechanism of the silica
carbothermal reduction is more complex and leads
to the formation of the gaseous SiO species9. Thus,
the carbothermal reduction of silica can be
described as follow:
SiO2 + C → SiO(g) + CO(g)
(2)
SiO + 2C → SiC(s) + CO(g)
(3)
Experimentally, the condensation of silica,
indicating the presence of SiO gas, was observed on
the crucibles after the pyrolysis of the samples.
Sucrose was the carbohydrate used as carbon source
for the carbothermal reduction of fumed silica (P1),
micrometric silica (P2), colloidal zirconia (P3),
micrometric zirconia (P4). Fumed silica is
amorphous silica; it has a chain-like particle
morphology composed of submicron-sized spheres
(7 nanometers), which are highly branched
(nominal size 0.1-0.2 microns). Its surface area is
close to 360m2/g. The colloidal zirconia was
purchased from Nyacol Company (particle size
10nm). These colloidal particles are stabilized by
Mesoporous,
Introduction
Carbides constitute a class of materials that
exhibit the characteristics of ceramic compounds
such as high hardness, high melting point, high
strength, good wear and good corrosion stability1.
These properties allow them to be a good candidate
for many technological applications in the field of
advanced ceramic industry (cutting tools
application, wear resistance….). Metallic carbide
compounds are developed in the field of catalysis
because of the availability of metal carbide with
high surface areas2. Several approaches have been
used to synthesize carbides. Direct methods consist
of the reaction of the oxide or the metal oxide with
solid carbon at high temperature (~ 2000°C). The
main drawback of these routes is the use of high
temperature (>2000°C) which leads to coarsegrained powders with particle size higher than 1
micron. These methods lead to compounds with
low surface area. One possibility to overcome this
1
Proceedings of the 12th Conference of the European Ceramic Society – ECerS XII
Stockholm, Sweden - 2011
Table 1 Phases detected by XRD, C and O analyses of
the powders elaborated at 1550°C-4h Ar (M=Si for P1
and P2; M=Zr for P3 and P4).
acetate ions. Micrometric silica and zirconia
powders were purchased from Aldrich Company.
The grains size of these powders is in the order of
1-10 micrometers.
The synthesis of the precursors was done by
mixing the oxide particles together with a quantity
of sucrose solution corresponding to the target final
stoichiometry. The suspension obtained guarantees
a homogeneous dispersive mixing in a simple way.
The amount of sucrose is adjusted in a proportion
ranging between 1 and 8 times the quantity needed
by the reaction (1) to obtain a complete conversion
of the parent compounds. The ratio C/M quantifies
the deviation from stoichiometry, i.e. C/M=3 means
the reagents are mixed together in the proportion of
the reaction (1). In a second stage, the suspension is
freeze dried to obtain a powder. In a last step, the
powders were pre-heated at 800°C during 3 hours
in flowing argon to decompose the sucrose into
carbon, and then the carbides formation was carried
out at higher temperature.
Thermal analyses were carried out with a
Netzsch STA 409 thermo gravimetric analyzer. The
specific surface areas were obtained by BET on a
Micromeritics ASAP after 4h vacuum (10-2 mbar)
at 350 ºC. Elemental analyses (C, O) were
performed respectively by LECO CS230 and
TCH600. The oxygen content in the powder was
determined by combustion analysis under helium
and IR detection of the CO2-vibration. Combustion
method was also used to determine the carbon
content. The carbides powder was heated in an
oxygen flow. Added Fe powder was used to assist
the combustion. The formed CO2 was used again to
detect the carbon content. Transmission Electron
Microscopy (TEM) observations were carried out at
100 kV (JEOL 1200 EXII). Samples for TEM
measurements were prepared by milling of the
samples in a agate mortar, followed the dispersion
of such powder in a ethanol under ultrasonification
and deposition on copper grids. Powder X-ray
diffraction patterns were measured on a Bruker D8
advance diffractometer using CuK radiation in
Bragg-Brentano geometry. SEM analyses were
obtained with a FEI QUANTA FEG 200 ESEM
environmental scanning electron microscope.
C/M
3
6
9
12
15
18
24
4.5
9
12
4.8
6
9
12
21
24
6
12
24
P1
P2
P3
P4
SBET
(m2/g)
18
16
194
194
173
136
69
15
53
29
39
128
251
305
243
248
3.4
20
6
C
(wt.%)
28.3
28.2
50.2
54.9
73.6
78.1
80.1
28.8
51.7
58.0
12.0
19.1
28.6
38.1
52.8
57.1
11.6
32.7
37.8
O
(wt.%)
2.1
1.3
1.5
1.1
1.1
0.7
0.4
3.1
5.0
n.d.
4.3
5.1
5.3
4.8
4.0
3.5
4.8
6.1
2.9
XRD
SiC
SiC
SiC
SiC+C
SiC+C
SiC+C
SiC+C
SiC
SiC+SiO2
SiC+C
ZrC+m-ZrO2
ZrC
ZrC
ZrC
ZrC
ZrC
ZrC
ZrC
ZrC
Consequently a slightly higher temperature
(1550°C-4h) was chosen for samples heat-treated in
the furnace. The conversion of these precursors has
required a flow of argon higher over 30 l/h to
evacuate the gaseous species produced during the
heat-treatment. Below this flow the reaction yield is
very low and it remains primarily the oxide in the
product pyrolyzed. In these conditions, X-Rays
diffraction patterns show the formation of carbides
(Table 1). No change in the temperature conversion
of the carbides has been observed according to the
size of the oxides (colloidal, micrometric).
Fig. 1 TGA analysis of P1 and P3 under argon flow.
Results and Discussion
x
TGA analysis under flowing argon revealed a
first weight loss below 800°C attributed to the
evacuation of the water adsorbed on the precursors
and to the decomposition of sucrose and acetate
present in the colloidal zirconia. A second weight
loss was observed above 1300°C, it can be
attributed to the carbothermal reduction. The mass
of the final products is stabilized at 1500°C,
indicating that the reaction is complete (Fig. 1).
x Cubic SiC
a Amorphous carbon
holder
C/Si=12
C/Si=6
C/Si=15
C/Si=9
C/Si=18
u.a.
x
x
a
x
10
20
30
40
50
60
70
80
2q(°)
Fig. 2 X-rays diffraction pattern of heat-treated
precursors P1 versus the ratio C/Si (1550°C-4h-Ar).
2
Proceedings of the 12th Conference of the European Ceramic Society – ECerS XII
Stockholm, Sweden - 2011
Complied with the reaction (1), a ratio C/Si
higher than 3 is necessary to obtain silicon carbide,
for the zirconia it has been necessary to increase the
ratio C/Zr up to 6 in order to completely
transformed ZrO2 in ZrC. When the ratio C/Si
increases above 9, amorphous carbon has been
detected on the X-Rays patterns (Fig. 2).
Elementary analyses confirm the presence of
residual carbon in such samples, jointly with a
decrease of the amount of oxygen (Table 1).
The specific surface area (SBET) of the materials
P1, P3 and P2, P4 elaborated respectively from the
nanometric and the micrometric oxides are plotted
versus the volume fraction of residual carbon on
Fig. 3. The volume fraction of carbon is inferred
from the results of the elementary analysis of
carbon, by considering that the final material
consists of only 2 phases, i.e. carbides (SiC or ZrC)
and carbon. The evolution of the surface area is
correlated with the size of the initial oxides. The
specific surface area SBET of P1 and P3-type
materials increases up to 200 and 300 m2/g
respectively and then decreases for higher carbon
content to tend to the value measured on pyrolyzed
sucrose (2m2/g). These materials exhibits,
especially for samples with the highest specific area,
a mesoporous structure which is located in the
matrix of residual carbon as it can be seen on the
SEM observations (Fig.4). No significant change
has been observed on the grain size of the carbides
particles according to the volume fraction of
residual carbon. Such particles present a grain size
of 100nm and 50nm respectively for P1 and P3type powders (Fig. 5). The calculated outer surface
(S) which is developed by a SiC powders with a
particle size of R=50 nm is close to 20 m2/g
(S=3/(R), SiC=3.2g/cm3). This value is one order
of magnitude lower than the maximum surface area
developed by SiC P1-type material (195m2/g). This
point supports the hypothesis of the localization of
the porosity in the matrix of residual carbon. The
N2 adsorption-desorption isotherm of the P1-type
sample for a ratio C/Si=12 (Fig. 6) undergo abrupt
changes when the relative pressure is in the medium
range from 0.7 to 0.9, and this is salient feature of
mesoporous materials.
350
d)
c)
Fig. 5 TEM observations of powders pyrolyzed at
1550°C C/M=24, a)P1-type (SiC+C), b) P3-type (ZrC+C).
The shape and the hysteresis loop of P1 and P3type powders (Fig.6) suggest that these samples
possess a mesoporous structure. In contrast, the N2
adsorption-desorption isotherms of powders made
from micrometric oxide (P2, P4) do not show
hysteresis loops which indicate they are not
mesoporous.
This evolution of the surface area (SBET) versus
the volume fraction of residual carbon (%vol. Cres.)
can be explained if one considers that the formation
of the mesoporosity is a consequence of the release
of CO gas produced by the carbothermal reduction
of the oxides at high temperature. For low volume
fraction of carbon, SiC grains are bonded together
by a thin layer of amorphous carbon and the gases
produced during the carboreduction can percolate
by the interstices between these grains. Therefore
the specific surface area is close to the external
surface of the carbides particles, i.e. 20m2/g for P1type powder as noted above, and roughly the same
value for P3-type powder if one considers a particle
size of 50nm and a density of 5.85 g/cm3 for ZrC
compound.
P1
P2
P3
P4
Sucrose
300
250
300
200
P1
P2
P3
P4
250
150
Vads. (cm3/g)
Specific area (m2/g)
a)
b)
Fig. 4 SEM observations of powders pyrolyzed at
1550°C C/M=24, a)P1-type (SiC+C), b) P3-type (ZrC+C).
100
50
0
0
20
40
60
80
200
150
100
50
100
%vol Cres.
0
0
Fig. 3 SBET of precursors P1, P2, P3, P4 pyrolysed at
1550°C versus volume fraction of residual carbon.
Specific surface area of pyrolyzed sucrose is 2 m2/g.
0,2
0,4
0,6
P/P0
3
0,8
1
Proceedings of the 12th Conference of the European Ceramic Society – ECerS XII
Stockholm, Sweden - 2011
carbides are <20 and about 250-300 m2/g
respectively.
The increase of the volume fraction of the
residual carbon up to of 50 vol.% induces an
increase of the surface area of both carbides
elaborated with the nanometric precursors. For
higher amount of carbon the surface area decreases
to tend to the value of pyrolyzed sucrose (2m2/g).
Mesoporosity was observed on these samples. The
porosity is mainly located in the residual carbon
phase resulting from the pyrolysis of the sucrose. A
mechanism based on the release of CO gas during
the carboreduction of the oxides has been proposed
to explain the formation of the mesoporosity in the
nanosized carbides.
Fig. 6 Nitrogen adsorption-desorption isotherms of
precursors P1, P2, P3, P4 after pyrolysis at 1550°C-4h in
flowing Ar (C/M=12 for all the samples).
Increase in the volume fraction of residual carbon
Carbide
Carbon + Porosity
References
Fig. 7 Schematic formation of porosity due to the release
of CO gas during the carboreduction process.
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one pot new supported porous carbon materials. No
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carbothermal reaction when varying the precursor
grain size (micrometric, nanometric) or the quantity
of sucrose surrounding the oxide grains. The initial
contact surface between oxide and sucrose is in the
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4