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Chapter IV
Department of Materials Science Page 96
CHAPTER – IV
STUDIES ON SILICA AND CARBON/SILICA BASED GLASSES USING
FURFURYL ALCOHOL
IV.1 Introduction
The silicon oxycarbide glasses have been prepared with carbon precursor from
furfuryl alcohol and silica precursor from silane. The Si-C bond in the precursor is
preserved after condensation and drying. The resulting gel contains Si atoms bonded
simultaneously to carbon and oxygen atoms, thus creating a silicon oxycarbide. In the
present chapter, furfuryl alcohol, carbon precursor has been co-polymerized with
partial hydrolyzed silica. The co-polymer was heat treated at different temperatures
and resulting products were characterized form their physico-chemical and surface
properties. The results obtained have been co-related with their processing parameter
in order to optimize processing parameter.
IV.2 Studies on Carbon/Silica based glasses using Furfuryl alcohol (FFA)
IV.2.1 Effect of TEOS/FFA molar ratio on physical structure of cured and
pyrolyzed samples
The organic-inorganic hybrids through sol-gel route were synthesized by co-
polymerization of hydrolyzed TEOS and furfuryl alcohol (FFA) in order to
incorporate carbon in the silica network. In all experiments, optimized parameters for
hydrolysis of TEOS to silica were used viz.
TEOS/ Water molar ratio = 4
TEOS/ Ethanol molar ratio = 2
pH = 2
Time of hydrolysis = 4 hours
Chapter IV
Department of Materials Science Page 97
During the sol–gel process, the cationic polymerization of furfuryl alcohol
(FA) yields polyfurfuryl alcohol (PFA). PFA is hydrophobic and tends to phase-
separate from silica. In our experiments, FA polymerizes considerably faster than the
silica in sol–gel process because of high acidity of the system and the temperature
employed. As a result, PFA-rich micro domains may be formed in the C–SiO2
composite after drying [1, 2].
The molar ratio of TEOS: furfuryl alcohol was varied from 0.75 to 4. The
samples were pyrolyzed to 1000oC in nitrogen atmosphere. Extent of
copolymerization of TEOS to FFA by measurements of zeta potential, particle size
and viscosity of sol.
IV.2.2 Measurements of Viscosity of TEOS/FFA sol
To study extent of completion of reaction viscosity of co-polymers were
measured as a function of time.
IV.2.2a Viscosity
Viscosity is a measure of resistance to flow of a fluid while flowing. In any
flow, layers move at different velocities and fluid's viscosity arises from the shear
stress between the layers that ultimately oppose any applied force. The viscosity of
the resin has a low value. It increases with increasing time of co-polymerization of
partially hydrolyzed TEOS and furfuryl alcohol. It depends on reaction rate of
copolymerization. In TEOS/FFA molar ratio equal to 3.25, the viscosity of sol was
very high after 50 hrs. But in other samples, the viscosity was less suggesting that the
co-polymerization reaction was slow reaction. After certain time intervals i.e. the
viscosity increased with reaction rate (time) resulting in the formation of solid
product.
Chapter IV
Department of Materials Science Page 98
Table IV.1 Viscosity of samples having different molar ratio of TEOS to FFA versus
different time intervals
Time
(hr)
Sample Name
SFFA0.75 SFFA2.15 SFFA2.75 SFFA3.25 SFFA3.8 SFFA4
02 4.21 4.22 4.36 4.48 4.91 4.45
04 4.33 4.36 4.82 5.04 5.08 5.51
20 5.23 5.25 5.86 5.53 6.3 6.31
22 5.41 5.44 5.88 5.95 6.31 7.55
26 6.08 6.10 6.19 6.64 6.86 8.76
50 6.68 6.70 6.70 9.11 7.55 10.7
Viscosity of samples with different molar ratio of TEOS:FFA as a function of
time are given in table IV.1. It is seen that viscosity rise in samples with different
molar ratio of TEOS to FFA was nearly same in samples and no particle separation
was seen upto the TEOS:FFA molar ratio of 1:3.25 on further increasing the molar
ratio to 3.8 and 4 the separation of solid particle was found to observed.
Chapter IV
Department of Materials Science Page 99
Fig.IV.1 Viscosity data of different composition of TEOS to FFA molar ratio
(A) TEOS : FFA :: 1:0.75 (D) TEOS : FFA :: 1:3.25
(B) TEOS : FFA :: 1:2.15 (E) TEOS : FFA :: 1:3.8
(C) TEOS : FFA :: 1:2.75 (F) TEOS : FFA :: 1:4
0 10 20 30 40 504.0
4.5
5.0
5.5
6.0
6.5
7.0
Vis
co
sit
y (
cP
)
Time (hr)
SFFA0.75
0 10 20 30 40 504.0
4.5
5.0
5.5
6.0
6.5
7.0
Vis
co
sit
y (
cP
)
Time (hr)
SFFA2.15
0 10 20 30 40 504.0
4.5
5.0
5.5
6.0
6.5
7.0
Vis
co
sit
y (
cP
)
Time (hr)
SFFA2.75
0 10 20 30 40 504
5
6
7
8
9
Vis
co
sit
y (
cP
)
Time (hr)
SFFA3.25
0 10 20 30 40 504.5
5.0
5.5
6.0
6.5
7.0
7.5
8.0
Vis
co
sit
y (
cP
)
Time (hr)
SFFA3.8
0 10 20 30 40 50
4
5
6
7
8
9
10
11
Vis
co
sit
y (
cP
)
Time (hr)
SFFA4
Chapter IV
Department of Materials Science Page 100
IV.2.3 Free carbon, %, free silica, %, black glass, % and silicon carbide, % in
the pyrolyzed samples
The copolymerized samples were heat treated at 1000oC in nitrogen
atmosphere. The resulting products were analyzed for yield %; free carbon %; free
silica %; and black glass %. The results are given in Table IV.2
The percentage of black glass formed was found to increase almost linearly
with an increase of molar ratio 0.75 to 3.25. On further increase of molar ratio to 3.8
and 4, the percentage of black glass was found to decrease from 31 to 25 and 18.
When TEOS to FFA molar ratio was 3.25, highest percentage of black glass i.e.
31.18% was achieved.
Table IV.2 Physical changes of different composition of TEOS to FFA molar ratio
No. Sample
Name Composition
% yield at
1000oC
% of
Free
carbon
% of
Free
silica
% of
black
glass
1. SFFA0.75 TEOS: FFA ::
1:0.75 58.66 27.42 44.17 28.41
2. SFFA2.15 TEOS: FFA ::
1:2.15 59.46 37.24 32.55 30.03
3. SFFA2.75 TEOS: FFA ::
1:2.75 59.91 40.7 28.45 30.85
4. SFFA3.25 TEOS: FFA ::
1:3.25 60.53 41.97 26.85 31.18
5. SFFA3.8 TEOS: FFA ::
1:3.80 57.64 47.08 28.16 24.76
6 SFFA4 TEOS: FFA :: 1:4 56.9 51.84 29.66 18.5
Chapter IV
Department of Materials Science Page 101
Fig. IV.2 shows different molar ratio of TEOS: FFA and (A) percentage yields
of products, (B) black glass, %; (C) free carbon, %; and (D) free silica, %. The graph
show clearly product yield increased up to the molar ratio (TEOS: FFA :: 1:3.25). On
further increasing the molar ratio to 3.8 and 4 the yield was found to decrease. This
may be attributed to a specific composition, at when carbon reacts with silica or
coated with silica. Therefore, strong bond formation takes place between carbon and
silicon to form silicon oxycarbide. On further increasing the molar ratio beyond
Fig. IV.2 (A) Graph of TEOS to FFA molar ratio versus % yield at 1000oC
(B) Graph of TEOS to FFA molar ratio versus % of black glass
(C) Graph of TEOS to FFA molar ratio versus % of free carbon
(D) Graph of TEOS to FFA molar ratio versus % of free silica
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.556.5
57.0
57.5
58.0
58.5
59.0
59.5
60.0
60.5
61.0
% y
ield
at
100
0oC
TEOS to FFA molar ratio
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.525
30
35
40
45
50
55
% o
f fr
ee c
arb
on
TEOS to FFA molar ratio
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
18
20
22
24
26
28
30
32
% o
f b
lack g
lass
TEOS to FFA molar ratio
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
26
28
30
32
34
36
38
40
42
44
46
% o
f fr
ee
sil
ica
TEOS to FFA molar ratio
A B
C D
Chapter IV
Department of Materials Science Page 102
1:3.25, formation of silicon oxycarbide was found to decrease. The amount of free
carbon content continuously increased with increasing molar ratio from 0.75 to 4.
This decrease in silicon oxycarbide glass formation may be attributed to reaction of
carbon with oxygen present in the organic molecule to form CO2. Due to removal of
carbon, less carbon being available for bond formation with hydrolyzed silica,
therefore the amount of silicon oxycarbide formed become less. The amount of silica
added was same throughout in all samples. Fig. IV.2(B) shows different molar ratio of
TEOS to FFA versus yield, % of black glass. The graph shows clearly that black glass
percentage formation was increased when the TEOS: FFA molar ratio reached to
1:3.25. On further increasing their molar ratio to 3.8 and 4 the black glass percentage
formation was found to decrease for similar reason. Fig. IV.2(C) shows different
molar ratio of TEOS to FFA free carbon %. The amount of free carbon % present in
the sample was found to increase as the molar increased from 0.75 to 4. The graph of
free silica obtained with different molar ratio of TEOS:FFA is shown in fig. IV.2(D).
The graph shows as the molar ratio of TEOS: FFA increased to 3.25, the amount of
free silica is found to decrease but beyond the molar ratio 1:3.25 an increase in the
free silica amount took place. On further increaseing the molar ratio to 3.8 and 4, the
yield was found to increase. This may be due to the fact that amount of silica obtained
from TEOS was same in all samples but the amount of carbon added which was
increased. More and more silica was used to react with carbon to form Si-C bond and
free silica, % was found to decrease. From above results it can be concluded that
molar ratio of TEOS: FFA as 1:3.25 is optimized molar ratio for developing silicon
oxycarbide.
Chapter IV
Department of Materials Science Page 103
IV.2.4 FTIR studies of cured and pyrolyzed TEOS/ FFA sample
FTIR spectra were recorded for all gelled samples as well as for samples heat
treated at different temperatures.
The FTIR spectra of the samples made with different molar ratio of
TEOS/FFA are shown in fig. IV.3. On co-polymerization of hydrolyzed TEOS with
furfuryl alcohol, there was a rapid decrease in intensity of peaks at 570 cm-1
and 950
cm-1
responsible for formation of Si-O-C or SiC network. The peak intensity was
small in case of TEOS/FFA molar ratio = 3.25 suggesting that condensation of Si-OH
and –OH groups of furfuryl alcohol, which is responsible for Si-O-C or Si-C network
in the structure. The band at 3400 cm-1
results from super position of vibration bands
of –OH group, which is less prominent in the gels with 0.75 FFA or higher FFA
molar ratio.
Chapter IV
Department of Materials Science Page 104
Fig IV.4 shows FTIR spectra of samples with different molar ratios of
TEOS/FFA, heat treated at 1000oC. The FTIR spectra of samples prepared from
copolymerization of hydrolyzed TEOS and FFA revealed that this polymeric material
though exhibit a peak at 450 cm-1
, characteristic of silica network, it does not show
band at 885, 1500 and 1560 cm-1
suggesting disappearance of furan ring. It shows
furan ring opening at curing temperature. The intensity of the band at 953 cm-1
assigned to Si-OH groups decreased appreciably suggesting co-condensation of Si-
OH groups of silica gel and –OH groups of furfuryl alcohol. The band at 1640 cm-1
Fig. IV.3 Infrared spectroscopy of cured samples
(A) TEOS : FFA :: 1:0.75 (D) TEOS : FFA :: 1:3.25
(B) TEOS : FFA :: 1: 2.15 (E) TEOS : FFA :: 1:3.8
(C) TEOS : FFA :: 1:2.75 (F) TEOS : FFA :: 1:4
Chapter IV
Department of Materials Science Page 105
attributed to deformation band of H-O-H which is prominent in gels of TEOS alone
but less prominent in gels co-polymerized with FFA. During pyrolysis, the band at
1200 cm-1
associated with Si-O also becomes more prominent.
The spectra of samples heated at 1000oC exhibited a band at 820 cm
-1 along
with band at 796 cm-1
suggesting that at least some of the silicon atoms are bonded to
carbon atoms. Therefore, it is inferred that in the pyrolyzed sample carbon can be
bonded to two silicon atoms. This agrees with the observations made by Gray M.
Renlund [3, 4] that silicon oxycarbide glasses consist of random network of silicon
oxygen tetrahedron and also contain silicon bonded to one or two carbon atoms which
in turn are tetrahedrally bonded to other silicon atoms.
So, the following structure of silicon oxycarbide could be suggested satisfying above
observations:
Chapter IV
Department of Materials Science Page 106
IV.2.5 Nitrogen adsorption-desorption isotherms
The adsorption–desorption isotherms of samples synthesized with different
composition of TEOS:FFA are given in fig. IV.5 ABCDEF. These isotherms belong
to type IV. The adsorption capacity rises sharply as the relative pressure rise range
from 0.6 to 0.9. In desorption process hysteresis loop is also present showing that
sample possess a mesoporous structure also. The isotherm for the samples has a
rounded knee indicating the difference between microporosity and mesoporosity. The
presence of mesopores is justified by the hysteresis effect and slope of the plateau
Fig. IV.4 Infrared spectroscopy of samples heated at 1000oC
(D) TEOS : FFA :: 1:0.75 (D) TEOS : FFA :: 1:3.25
(E) TEOS : FFA :: 1: 2.15 (E) TEOS : FFA :: 1:3.8
(F) TEOS : FFA :: 1:2.75 (F) TEOS : FFA :: 1:4
Chapter IV
Department of Materials Science Page 107
increased to yield type IV isotherms with a significant increase in the nitrogen uptake
through the entire pressure range.
Fig. IV.5 (A to F) also show that volume of nitrogen gas adsorbed increases
more gradually (on a relative basis) at a very high relative pressure (i.e., as p/po
approaches 1) for samples with increased extent of conversion. This is again
representative of the beginning of a shift from the nitrogen condensation onto an
essentially nonporous "bulk" surface (Type II behavior) to condensation within pores
(Type IV behavior) [6].
A very small, closed hysteresis loop was observed in Fig. IV.5 (E) and IV.5
(F) in the intermediate relative pressure range, i.e., from p/po values of ~0.3 at the low
end to ~0.9 at the high end. Hysteresis loops in the intermediate and upper relative
pressure ranges are characteristic of samples with mesoporosity [5]. The occurrence
of hysteresis in the isotherm in this pressure range indicates a difference in the
processes of liquid condensation into the pores (the adsorption branch) and liquid
evaporation from the pores the desorption branch. A variety of explanations have
been offered to explain this phenomenon [6]. The fact that the hysteresis loop closes
at pressures below p/po = 1 suggests that there is an upper size limit within the
mesopore size range. Also, development of hysteresis loop in these samples was
associated with the onset of some bimodality in the pore size distributions. The shape
of the hysteresis loop in gas adsorption/desorption isotherms is dependent upon
morphological structure of porosity in the sample.
Chapter IV
Department of Materials Science Page 108
0.0 0.2 0.4 0.6 0.8 1.0
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
Vo
lum
e a
bs
orb
ed
cm
3/g
m
Relative pressure P/Po
SFFA0.75
0.0 0.2 0.4 0.6 0.8 1.0
5
6
7
8
9
10
11
12
13
Vo
lum
e a
bs
orb
ed
cm
3/g
m
Relative pressure P/Po
SFFA2.15
0.0 0.2 0.4 0.6 0.8 1.06
7
8
9
10
11
12
13
14
15
Vo
lum
e a
bs
orb
ed
cm
3/g
m
Relative pressure P/Po
SFFA2.75
0.0 0.2 0.4 0.6 0.8 1.0
7
8
9
10
11
12
13
14
Vo
lum
e a
bs
orb
ed
cm
3/g
m
Relative Pressure P/Po
SFFA3.25
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
10
20
30
40
50
60
70
% o
f M
icro
po
re v
olu
me
TEOS to FFA molar ratio
0 10 20 30 40 50-10
0
10
20
30
40
50
60
Zeta
po
ten
tial
(mV
)
Time in hr
SSC1.25
0 10 20 30 40 50-10
0
10
20
30
40
50
60
Zeta
po
ten
tial
(mV
)
Time in hr
SSC1.25
A B
C D
0.0 0.2 0.4 0.6 0.8 1.0
4
5
6
7
8
9
10
Vo
lum
e a
bs
orb
ed
cm
3/g
m
Relative pressure P/Po
SFFA3.8
0.0 0.2 0.4 0.6 0.8 1.0
4
6
8
10
12
14
16
18
Vo
lum
e a
bso
rbed
cm
3/g
m
Relative pressure P/Po
SFFA4
E F
Fig. IV.5 Adsorption – Desorption isotherms of different composition of
TEOS to FFA molar ratio
(A) TEOS : FFA :: 1:0.75 (D) TEOS : FFA :: 1:3.25
(B) TEOS : FFA :: 1:2.15 (E) TEOS : FFA :: 1:3.8
(C) TEOS : FFA :: 1:2.75 (F) TEOS : FFA :: 1:4
Chapter IV
Department of Materials Science Page 109
IV.2.6 Pore size distribution curves
The pore size curves of samples prepared with different molar ratio of TEOS
to FFA are shown in fig.IV.6. The average pore diameter of sample SFFA3.25 is less
than 2 nm and pore size distribution is also very narrow, which makes this sample
more advantageous for the adsorption of small molecules and for gas phase
application. Formation of such a system of pores is due to the structure of the
relatively rigid i.e. three-dimensionally cross-linked polymer network.
0 500 1000 1500 20000.0000
0.0005
0.0010
0.0015
0.0020
0.0025
0.0030
0.0035
Po
re
vo
lum
e (
cm
3/g
m)
Average pore diameter (Angstrom)
SFFA0.75
0 100 200 300 400 500 6000.000
0.001
0.002
0.003
0.004
0.005
0.006P
ore
vo
lum
e (
cm
3/g
m)
Average pore diameter (Angstrom)
SFFA2.15
0 100 200 300 400 500 600
0.001
0.002
0.003
0.004
0.005
0.006
0.007
Po
re
vo
lum
e (
cm
3/g
m)
Average pore diameter (Angstrom)
SFFA2.75
0 100 200 300 400 500
0.000
0.001
0.002
0.003
0.004
0.005
0.006
Po
re v
olu
me (
cm
3/g
m)
Average pore diameter (Angstrom)
SFFA3.25
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
10
20
30
40
50
60
70
% o
f M
icro
po
re v
olu
me
TEOS to FFA molar ratio
0 10 20 30 40 50-10
0
10
20
30
40
50
60
Zeta
po
ten
tial
(mV
)
Time in hr
SSC1.25
0 10 20 30 40 50-10
0
10
20
30
40
50
60
Zeta
po
ten
tial
(mV
)
Time in hr
SSC1.25
A B
C D
Chapter IV
Department of Materials Science Page 110
IV.2.7 Surface characterization
The nitrogen adsorption/desorption isotherms studies were carried out by
using Micromeritics Gemini 2375 volumetric adsorption analyser. Table IV.3 shows
results of BET surface area, micropore area, total pore volume, micropore volume and
average pore diameter of samples prepared with different TEOS to FFA molar ratio.
A comparision of results show that sample SFFA3.25 was found to have highest
surface area and micropore area. The average pore diameter of sample SFFA3.25 was
very low i.e. 1.84nm as compared to sample with other molar ratios. Since it is less
than 2 nm. Therefore, SFFA3.25 is a microporous sample while all other samples of
different molar ratio were on an mesoporous. Table IV.4 shows % of microporosity
and % of mesoporosity of sample prepared with different molar ratio of TEOS to FFA
molar ratio. From the table, it can be concluded that highest % of micropore area &
micropore volume, % or lower mesopore area, % and mesopore volume were
observed in the sample SFFA3.25
0 50 100 150 200 250 300 350 400
0.000
0.001
0.002
0.003
0.004
0.005
0.006
0.007
0.008
Po
re v
olu
me
(c
m3/g
m)
Average pore diameter (Angstrom)
SFFA3.8
0 100 200 300 400 500 600
0.000
0.005
0.010
0.015
0.020
Po
re
vo
lum
e (
cm
3/g
m)
Average pore diameter (Angstrom)
SFFA4
E F
Fig. IV.6 Pore size distribution curves of different composition of TEOS to
FFA molar ratio
(A) TEOS : FFA :: 1:0.75 (D) TEOS : FFA :: 1:3.25
(B) TEOS : FFA :: 1:2.15 (E) TEOS : FFA :: 1:3.8
(C) TEOS : FFA :: 1:2.75 (F) TEOS : FFA :: 1:4
Chapter IV
Department of Materials Science Page 111
The specific surface area of samples was calculated, using BET equation. The
mesopore surface area was calculated by the t-plot method. The variation in surface
area of samples prepared by using different molar ratios on heat treatment at 1000oC.
The mesopore surface area was obtained by subtracting the micropore surface area
from the corresponding BET surface area of samples. Total pore volume of samples
was estimated from nitrogen adsorption data at a relative pressure of 0.99. Micropore
volume of the samples was obtained by the t-plot method. The mesopore volume was
calculated by subtracting the micropore volume from the total pore volume.
Table IV.3 Surface area analysis of different composition of TEOS to FFA molar ratio
Table IV.4 Microporosity and Mesoporosity of different composition of TEOS to
FFA molar ratio
Sample
Name
BET surface
area m2/gm
Micropore
area
m2/gm
Total
pore vol.
Cm3/gm
Micropore
vol. Cm3/gm
Average
pore
diameter
(nm)
SFFA0.75 13.69 10.49 0.00775 0.004244 2.26
SFFA2.15 36.41 24.31 0.018726 0.009557 2.06
SFFA2.75 42.40 33.59 0.021598 0.013235 2.03
SFFA3.25 45.94 36.42 0.021179 0.014425 1.84
SFFA3.8 27.16 14.25 0.014177 0.006057 2.08
SFFA4 33 8.59 0.02782 0.00373 3.34
Sample
Name
% of
Micropore
area
% of
Micropore vol.
% of
Mesopore
area
% of
Mesopore vol.
SFFA0.75 76.61 54.72 23.39 45.28
SFFA2.15 66.76 51.03 33.24 48.97
SFFA2.75 79.21 61.3 20.79 38.7
SFFA3.25 79.27 68.11 20.73 31.89
SFFA3.8 52.46 42.72 47.54 57.28
SFFA4 26.04 13.4 73.96 86.6
Chapter IV
Department of Materials Science Page 112
Graphical overview
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.510
15
20
25
30
35
40
45
50
Su
rfa
ce
are
a (
m2/g
m)
TEOS to FFA molar ratio
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.55
10
15
20
25
30
35
40
Mic
rop
ore
are
a (
m2/g
m)
TEOS to FFA molar ratio
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.520
30
40
50
60
70
80
% o
f M
icro
po
re a
rea
TEOS to FFA molar ratio
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
10
20
30
40
50
60
70%
of
Mic
rop
ore
vo
lum
e
TEOS to FFA molar ratio
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
10
20
30
40
50
60
70
% o
f M
icro
po
re v
olu
me
TEOS to FFA molar ratio
0 10 20 30 40 50-10
0
10
20
30
40
50
60
Zeta
po
ten
tial
(mV
)
Time in hr
SSC1.25
0 10 20 30 40 50-10
0
10
20
30
40
50
60
Zeta
po
ten
tial
(mV
)
Time in hr
SSC1.25
A B
C D
Chapter IV
Department of Materials Science Page 113
The variation in surface area of all samples was observed on heating the
sample at 1000oC. On heating the sample at 1000
oC, the decomposition of
copolymerized take place. There is removal of volatile solvents i.e. hydrocarbon gas,
carbon dioxide and other volatile impurities. At completion of reaction there was
formation of black glass along with free carbon and free silica formation in sample.
FFA was used as the carbon precursor. It is seen invariably that obtained carbon
materials from FFA has high ordered uniform mesoporous and microporous structures
depending on the two or three dimensional network structures. On heat treatment of
copolymerized product at 1000oC the bridged organic moieties between silicon atoms
in the mesopore walls are cleaved to Si-O-Si and Si-O-C, and the carbonized products
are produced with different carbon contents (relative to silica) at different pyrolysis
temperatures. So during heat treatment at 1000oC, pores are generated as a result of
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
1.8
2.0
2.2
2.4
2.6
2.8
3.0
3.2
3.4
Avera
ge p
ore
dia
mete
r (n
m)
TEOS to FFA molar ratioE
Fig. IV.7 Surface area analysis of different composition of TEOS to FFA molar ratio
(A) Graph of BET surface area versus different of TEOS to FFA molar ratio
(B) Graph of Micropore area versus of different of TEOS to FFA molar ratio
(C) Graph of % of Micropore area versus of different of TEOS to FFA molar ratio
(D) Graph of % of Micropore volume versus of different of TEOS to FFA molar
ratio
(E) Graph of Average pore diameter versus of different of TEOS to FFA molar
ratio
Chapter IV
Department of Materials Science Page 114
dehydration of FFA and subsequent formation of the mesoporous framework from
obtained carbon fragments.
IV.3 Heat treatment of copolymerized product of TEOS/FFA system
The pellets were prepared by using copolymerized product of different molar
ratio of TEOS:FFA by hot pressing. The powder samples were pressed in 25mm
circular die by applying pressure of 200kg/cm2 at 180
oC. The green pellets were
subjected to heat treatment at 1000oC in nitrogen and at 1550
oC in argon in the
furnace. During pyrolysis, decomposition of the product took place followed by
crosslinking involving chemical reactions. The decomposition process was associated
with weight changes due to chemical reaction and resulted in dimensional changes
also due to removal of volatiles. Therefore, the dimensional changes of sample during
pyrolysis of various samples were measured both before and after pyrolysis.
IV.3.1 Dimensional shrinkage
The dimensional shrinkage, % in thickness and diameter along with density
were measured before and after heat treatment. The results are given in Table IV.5.
Table IV.5 Changes in properties composites on heating of at 1000oC and 1550
oC
Sample
Name
Density
at 180oC
gm/cc
Density
at 1000oC
gm/cc
Density
at
1550oC
gm/cc
Shrinkage (%) Wt.
loss
(%)
Free
carbon
% Diameter thickness
SFFA0.75 1.34-1.37 -- -- -- -- -- 27.42
SFFA2.15 1.24-1.29 1.4-1.43 0.99-1.04 18-20 18-20 42-43 37.24
SFFA2.75 1.28-1.33 1.38-1.41 1.0 19-20 17-20 42-43 40.7
SFFA3.25 1.29-1.3 1.42-1.45 1.04-1.05 19-20 19-20 42-43 41.97
SFFA3.8 1.27-1.29 1.39-1.44 0.98-1.02 20-21 20-21 43-45 41.08
SFFA4 1.21-1.25 1.31-1.39 0.95-0.98 20-22 22-25 47-48 51.84
Chapter IV
Department of Materials Science Page 115
The results given in table IV.5 show that as the amount of FFA added to one
mole of silica is increased from 0.75 to 4, the density of cured samples decreased
from 1.34-1.25 gm/cc. On heat treatment to 1000oC, density increased due to Si-O-C
linkage formation. These results were compared with presence of free carbon, %
present in the samples after heat treatment. Samples with lower TEOS/FFA ratio,
show silica rich product with weight loss in temperature range of 300-1000oC during
pyrolysis and below 200oC due to evolution of water. On the other hand sample with
increasing amount of FFA resulted in more carbon content in the copolymerized
TEOS/FFA samples and carbon rich products were obtained with more weight loss in
the temperature range of 300-1000oC during pyrolysis. The former sample during
pyrolysis maintain structural integrity while latter sample exhibit cracking. Therefore,
it can be concluded that heavily microcracking occur with lower molar ratio of
TEOS:FFA sample during pyrolysis is due to evolution of water and alcohols.
The photographs of sample made with different molar ratio of TEOS/FFA and
heat treated at 1550oC are shown in fig. IV.10. the heat treatment at this temperature
results in densification of samples.
Fig.IV.8 Photographs of cured resin pellets with different molar ratio of TEOS/FFA
Chapter IV
Department of Materials Science Page 116
Fig.IV.10 Photographs of pellets made with different molar ratio of TEOS/FFA
samples heate treated at 1550oC
Fig.IV.9 Photographs of pellets made with different molar ratio of TEOS/FFA
samples heate treated at 1000oC
40 40-200 200-320 320 320-550 550 550-700 700 700-1000 1000 1000-40
-20
0
20
40
60
80
100
120
140
160
180
Tim
e (h
r)
Tempearture oC
Heating rate
Fig.IV.11 Heating and cooling rate for pyrolysis of pellets
Chapter IV
Department of Materials Science Page 117
IV.3.2 Carbothermal reduction
Heat treatment at 1550oC in argon atmosphere leads to chemical reaction
between carbon and silica to form silicon carbide. The reaction is known as
carbothermal reduction and it is a two step reaction. The first step is the reaction
between silica and carbon to form SiO and CO as shown by the following equation:
SiO2 (s) + C (s) → SiO (g) + CO (g)
The second step is believed to be the reaction between SiO vapor and solid
carbon to produce SiC and CO as shown by the following equation:
SiO (g) + 2C (s) → SiC (s) + CO (g)
Before the heat treatment reaction, the pyrolyzed material is composed of
randomly mixed nanoscale silica and carbon particle. Upon heating the pyrolyzed
material, the silica and carbon particle react with each other to generate SiO and CO,
and forms a network of porous carbon outside the original pyrolyzed particles. The
SiO from the first reaction migrates to neighbouring carbon regions and reacts at the
carbon surface to form SiC and CO. As a result outer layer of the porous carbon
outside particle is replaced by a porous SiC layer as shown in fig. IV.12. The rate of
reaction is very fast such that both reactions might actually happen simultaneously.
Most researchers believe that carbothermal reduction reaction is a multi-step process.
Chapter IV
Department of Materials Science Page 118
Fig. IV.12 Reaction Mechanism
Chapter IV
Department of Materials Science Page 119
IV.4 X-ray Difraction studies on TEOS/FFA system of pyrolyzed samples
The XRD of samples with different molar ratio of TEOS:FFA heat treated at
1550oC are given in fig. IV.12ABCD. It shows development of crystalline phases
from amorphous black glass. The major peaks of XRD graph for different samples
occurring at diffraction angles 2θ = 35.7o, 41.4
o 60.1
o, 71.9
o, 75.4
o, 90
o are attributed
to the (111), (200), (220), (311), (222), (400) planes of the cubic β-SiC phase. The
broad band at 2θ = 25o
is due to combined amorphous carbon and amorphous silica
structures. It is observed that as of silica: FFA as the molar ratio is increased, the band
becomes broader showing presence of more free carbon. From the graph concluded
that after heating at 1550oC, the silica-carbon product was converted into silicon
carbide and also free carbon and free silica was also present in the final product.
Fig.IV.12 XRD graph of A) SFFA2.15, B) SFFA2.75, C) SFFA3.25 and
D) SFFA3.8 at 1550oC
Chapter IV
Department of Materials Science Page 120
IV.5 References
1. S. Grund, A. Seifert, G. Baumann, W. Baumann, G. Marx,M. Kehr and S. Spange,
Microporous Mesoporous Mater., 95, 206, (2006).
2. S. Spange, H. Mu¨ ller, C. Ja¨ ger and C. Bellmann, Macromol. Symp., 177, 111,
(2002).
3. G.M. Renlund, S. Prochazka and R.H. Doremus, “Silicon oxycarbide Glasses: Part-
I. Preparation and Chemistry”, J.Mater. Res., vol.6, 2716-22, (1991).
4. G.M. Renlund, S. Prochazka and R.H. Doremus, “Silicon oxycarbide Glasses: Part-
II. Structure and Properties”, J.Mater. Res., 6, 2723-34, (1991).
5. Sing, K. S. W., "Characterization of Adsorbents," in Adsorption: Science and
echnology, edited by Rodrigues, A. E., LeVan, M. D. and Tondeur, D., Kluwer,
Academic Publishers, Netherlands, 3-14, (1989).
6. Gregg, S. J. and Sing, K. S. W., Adsorption, Surface Area and Porosity, Academic
Press, London, (1982).