CHAPTER IV STUDIES ON SILICA AND CARBON/SILICA BASED...

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


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.

http://en.wikipedia.org/wiki/Drag_(physics)

http://en.wikipedia.org/wiki/Velocity

Chapter IV


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


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


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


1:2.15 59.46 37.24 32.55 30.03


1:2.75 59.91 40.7 28.45 30.85


1:3.25 60.53 41.97 26.85 31.18


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


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


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


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


A B

C D

Chapter IV


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


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


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


(B) TEOS : FFA :: 1: 2.15 (E) TEOS : FFA :: 1:3.8


Chapter IV


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


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


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


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


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


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


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


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


SFFA4

E F

Fig. IV.5 Adsorption – Desorption isotherms of different composition of





Chapter IV


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)


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)


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)


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


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


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)


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)


SFFA4

E F

Fig. IV.6 Pore size distribution curves of different composition of TEOS to

FFA molar ratio




Chapter IV


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


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)


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)


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


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


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


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


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


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


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


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


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


Fig. IV.12 Reaction Mechanism

Chapter IV


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


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).

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CHAPTER IV STUDIES ON SILICA AND CARBON/SILICA BASED...

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