LETTER TO THE EDITOR
Journal of Non-Crystalline Solids 416 (2015) 1–3
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Journal of Non-Crystalline Solids
journal homepage: www.elsevier.com/ locate/ jnoncrysol
Letter to the Editor
Synthesis and characterization of carbon fiber/silica
aerogel nanocomposites
Ślosarczyk Agnieszka a,⁎, Strauchmann Wojciech a, Ziółkowski Piotr a, Jakubowska Paulina b
a
b
Poznan University of Technology, Institute of Structural Engineering, Poland
Poznan University of Technology, Institute of Chemical Technology, Poland
a r t i c l e
i n f o
Article history:
Received 5 January 2015
Received in revised form 13 February 2015
Accepted 15 February 2015
Available online xxxx
Keywords:
Silica aerogel;
Sol–gel synthesis;
Carbon fibers;
Chemical modification
a b s t r a c t
Nanoporous silica aerogel–carbon fiber composites were prepared via a sol–gel process by surface modification at
ambient pressure. Oxidizing modification of the carbon microfibers improved the adhesion between hydrophilic
silica gel and the carbon material surface. Suggested solution contributed to the blocking of hydroxide bonds of silica
gel via reaction with the oxidized carbon material surface, which lowered the contraction of gel volume during its
drying in atmospheric pressure and led to the more stable structure.
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
Receiving good structural parameters of aerogel does not improve
the main drawback of silica aerogels, which is their brittleness. One of
the ways to improve the aerogel brittleness is the use of fibers. Nevertheless, there are very few articles on the synthesis of nanocomposites
as silica aerogel-fibers. First research on this subject was conducted in
1990's [1]. Paramenter and Milstein modified silica aerogel with silica
fibers, alumina fibers and aluminaborosilicate fibers. Those fibers are
characterized by a relatively high mass density, which significantly
changes the density of the created nanocomposites. Research proved
that applied fibers considerably limited the contraction of silica aerogel,
but they lowered its mechanical parameters, such as compressive
strength and elastic modulus. Much better parameters were achieved
by Meador and co-workers while modifying silica aerogel with much
lighter carbon nanofibers and di-isocyanate [2]. In the case of this solution an improvement of compressive strength by 5% was achieved.
However, Zhang and co-workers were the first to apply polypropylene
fibers as silica aerogel component [3]. They used polypropylene fibers
12 mm long obtaining good structural properties of silica aerogel without significant changes in its density. A drawback of hitherto solutions
for this type of modification was the lack of fiber surface modification
and lack of chemical bonding between fibers and silica gel.
The main aim of presented studies was to form the silica aerogel–
carbon fiber nanocomposites with more durable structure. The synthesis of aerogel and its composites with carbon fibers was performed
⁎ Corresponding author at: Piotrowo 5 str., 60–965 Poznan, Poland.
E-mail address: agnieszka.slosarczyk@put.poznan.pl (Ś. Agnieszka).
http://dx.doi.org/10.1016/j.jnoncrysol.2015.02.013
0022-3093/© 2015 Elsevier B.V. All rights reserved.
in two-step sol–gel process using the drying in atmospheric pressure.
The chemical modification of carbon fibers in nitric acid was carried
out to enhance the adhesion between carbon fibers and silica aerogel
skeleton. In addition, the chemical modification of aerogel nanocomposites in TMCS/n-hexane mixture was performed. The structure and
temperature stability of silica aerogel–carbon fiber composites were
tested through the following methods: BET analysis, thermogravimetric
measurements TGA and Fourier Transform Infrared Spectroscopy FTIR.
2. Materials and methods
As the precursor of silica aerogel synthesis the tetramethylsilane
TMOS (Sigma-Aldrich) in methanol solution (CHEMPUR) was used.
The catalyst of reaction was the aqueous solution of ammonium with
the concentration of 29–30% (Sigma-Aldrich). The chemical modification of silica aerogel was performed using the trimethylchlorosilane
TMCS (Sigma-Aldrich) and n-hexane (CHEMPUR). As component of
silica aerogel–carbon fiber composites, low-modulus carbon fibers from
coal-tar pitch with density of 1.64 g/cm3 were used (OsakaGas Corp.).
Carbon fibers have the diameter of 13 μm and length of 700 μm, and are
characterized by shape factor of L/d = 50. They have a low graphitization
level and small specific surface area 0.96 m2/g (according the BET method) with very small micropore content (0.0009 cm3/g).
The physical and chemical characterization of synthesized nanomaterials was performed by the using of BET and thermo-gravimetric
analysis. Surface area and pore volume of aerogel composites were estimated based on adsorption isotherms in low-temperature nitrogen sorption in the temperature of 77 K using the equation of BET isotherm and
analyzer ASAP 2010 (Micrometrics). Thermo-gravimetric analysis of
LETTER TO THE EDITOR
Ś. Agnieszka et al. / Journal of Non-Crystalline Solids 416 (2015) 1–3
2
Table 1
Physical properties of aerogel–carbon fiber nanocomposites.
Surface area BET, m2/g
Density, g/cm3
Average pore diameter, nm
Shrinkage of composites, %
Aerogel without
fibers
Aerogel with fibers in the
amount of 0.01 g
Aerogel with fibers in the
amount of 0.03 g
Aerogel with fibers in the
amount of 0.05 g
Aerogel with fibers in the
amount of 0.07 g
572.1
0.302
12.1
62
583.5
0.337
11.9
48.7
586.8
0.358
11.6
41.3
529.0
0.400
11.9
35.2
559.2
0.406
12.3
29.9
aerogels was performed in nitrogen atmosphere by means of NETSCH apparatus, type TG 209 F3. During measurements the following parameters
were used: flow rate of inert gas 30 ml/min, speed of sample heating
10 °C/min, and temperature range 30–1000 °C. To identify functional
group on the silica aerogel–carbon fiber composite surface the infrared
spectroscopy was applied. Infrared absorption spectra ATR-FTIR for tested
aerogels and its composites were determined by means of FT-IR NICOLET
5700 apparatus (Thermo Electron Corporation). All measurements were
performed in the wave number range from 600 to 4000 cm−1.
3. Experimental
Silica gel was made in two-step method using the sol–gel synthesis.
Two solutions were mixed together — solution A (3.0 ml of TMOS mixed
with methanol to obtain 5 ml of liquid) and solution B (0.72 ml NH4OH
at concentration of 0.5 mol/l mixed with methanol to obtain 5 ml of
liquid). The connection of these two solutions initiates a hydrolysis
reaction, as a result of which sol is created, that is further transformed
into gel in the process of condensation (gelation). In the subsequent
step gel was aging, first in the mixture of water and methanol for two
days, and next only in methanol for 7 days.
The synthesis of silica aerogel composites was performed in the
same way. Before introducing the carbon fibers to TMOS solution they
were treated chemically in hot nitric acid. Carbon fibers were placed
in a three-neck flask, poured by concentrated nitric acid and then heated gradually to 120 °C using the reflux condenser. The oxidative treatment was performed by 3 h. After that, carbon fibers were washed out
with water and dried at 110 °C in air for 5 h. The estimation of oxidation
degree of carbon fiber surface by means of electrochemical as well as
FTIR analysis was presented in earlier publications [4,5].
The last stage of silica aerogel and its composite with carbon fiber
synthesis was the elimination of dissolvent from the nanostructure of
aerogel by drying in ambient pressure. Firstly, the water and methanol
located in the pores were changed into a water-free solvent, and then
the reaction with TMCS/n-hexane mixture at 50 °C was performed.
4. Results and discussion
The structural properties of pure silica aerogel and carbon fiber–
silica aerogel nanocomposites are shown in Table 1. The average
diameters of the pores were calculated on the basis of nitrogen adsorption isotherms with BET method, basing on the 4 V/Å formula,
where V stands for total pore volume determined in a single point of
adsorption isotherm with p/p0 = 0,99. It was shown that the presence
of carbon fibers in silica aerogel composite does not disturb the
structural properties of aerogel. Aerogels with carbon fibers have
similar average structural parameters as those without fibers:
surface areas (529–587 m2/g) and pore diameters (11.6–12.3 nm).
In the case of pure silica aerogel surface area equals 572.1 m2 /g
and pore diameter was calculated as 12.1 nm. Taking into consideration
the effective diameter, the pores are classified according to IUPAC as follows: ultra-micropores (below 0.8 nm), micropores (between 0.8 and
2 nm), mesopores (from 2 nm to 50 nm), and macropores (above
50 nm). Based on the IUPAC classification and structural parameters
obtained in BET analysis, it is obvious that the synthesized silica aerogel
and silica aerogel–carbon fiber nanocomposites maintain a porous
structure and belong to mesopore materials. In addition a confirmation
of mesopore structure is the shape of adsorption and desorption isotherms shown in Fig. 1. According to the classification of porous materials
made by de Boer in 1958 [6], from the shape of the desorption hysteresis
loop it can be found that pore structures are both cylindrical capillary
pores open at both ends and cylindrical pores closed at one end with a
narrow neck at the other, like an “ink-bottle”, which are assigned to the
materials with the mesopore structure.
In comparison to pure silica aerogel, the aerogels with carbon fibers
exhibited higher density. For unmodified aerogel density equaled
0.302 g/cm3, and for aerogels with different amounts of carbon fibers
the values of density were varied from 0.337 to 0.406 g/cm3. The higher
1200
100
1
Silica aerogel without CF
Silica aerogel with CF
98
2
800
96
600
94
TG, %
3
Volume Adsorbed (cm /g)
1000
400
3
92
4
1 - without CF
2 - with 0.01g CF
3 - with 0.03g CF
4 - with 0.05g CF
5 - with 0.07g CF
90
200
88
0
0,0
0,2
0,4
0,6
0,8
1,0
Relative Pressure (p/p 0)
Fig. 1. N2 adsorption and desorption isotherms for pure silica aerogel and its composites
with carbon fibers.
5
86
0
100
200
300
400
500
600
o
temperature, C
Fig. 2. TGA for pure silica aerogel and its composites with carbon fibers.
700
LETTER TO THE EDITOR
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3
104
100
96
100
96
88
84
92
reflection, %
reflection, %
92
88
84
5
3
80
4
2
76
3
72
68
80
1
1200
1100
1000
900
800
wavenumber, cm
72
68
4000
4
2
1 - without CF
2 - with 0.01g CF
3 - with 0.03g CF
4 - with 0.05g CF
5 - with 0.07g CF
3500
1
700
-1
76
5
1
3000
2500
2000
wavenumber, cm
1500
1000
-1
Fig. 3. FTIR for pure silica aerogel and its composites with carbon fibers.
values of density noted for aerogel composites are the result of additive
of carbon fibers which have much higher density than pure silica
aerogel. Moreover, the presence of fibers lowers the thermal resistance
of the aerogel. Fig. 2 presents thermogravimetric curves gained for the
pure aerogel and for the nanocomposite of siliceous aerogel–carbon
fibers. In the whole range of measured temperatures the pure aerogel
showed higher thermal stability than the nanocomposites with carbon fibers. Except for the aerogel with the highest amount of fibers (0.07 g) the
remaining composites showed a very small decline in weight (up to 5%)
in the temperature up to 450 °C, which was just slightly lower than that
of pure aerogel. In the temperature of 700 °C the decline averaged less
than 12%. The lowest thermal stability in the whole research range
was shown by the composite with the highest amount of fibers. This is
probably related to the loosening of the aerogel nanostructure due to
the admixture of more and more carbon fibers. Micro-sized carbon fibers
hinder the growth of homogeneous silica aerogel structure and lead to the
decrease of aerogel thermal stability. On the other hand, the composites
with carbon fibers showed lower contraction of samples during drying
in ambient pressure. In the case of pure aerogel the volume decline
equalled almost 60%, whereas for the composite with the highest fiber
amount (0.07 g) the contraction equalled less than 30%. Lowering the
aerogel contraction to this level during the drying is possible thanks to
the application of carbon fibers after surface modification. On the surface
of the carbon fibers carboxyl and hydroxyl groups are built, as a result of
carbon fiber oxidation in the nitric acid. Those groups can react with
hydroxyl groups present on the surface of the siliceous aerogel during
its synthesis. Fig. 3 presents the FTIR analysis of the nanocomposites.
The peaks appearing in the spectrum for wavenumbers from 1050 to
1100 cm−1 and 845 cm−1 are attributed to the Si–O–Si bond and Si–
CH3 bond of a siliceous structure, respectively [7,8]. In the nanocomposites with carbon fibers a decline in the peak intensity was observed.
This testifies that as a result of adding more and more modified carbon
fibers, the number of free hydroxyl and hydrocarbon groups on the
siliceous aerogel surface decreases. This is a consequence of an ion bond
between hydroxyl groups present on the aerogel surface and the carboxyl
and hydroxyl groups built up on the carbon fiber surface during their
modification. Moreover, as a result of carbon fiber surface oxidation,
their character is changed from hydrophobic to hydrophilic, which enables their dispersion in the aqueous solution of methanol.
5. Conclusion
In the aspect of the aforementioned results of the research, the
modification of the silica aerogel structure with carbon fibers seems to
be an interesting solution. Hitherto research proved that the application
of carbon fibers as components of silica aerogels gave positive results in
limiting the aerogel contraction during drying. Chemical treatment of
the carbon materials resulted in building on their surface oxygen functional groups and enabled a chemical reaction and creation of ionic
bonds between those groups and hydroxide groups on the surface of
silica gel, which strengthen the silica aerogel frame. Moreover, the
blocking of hydroxide bonds of silica gel via reaction with oxidized carbon fiber surface lowered the contraction of gel volume during its drying in atmospheric pressure, prevented the adsorption of water and
finally led to the more stable aerogel nanostructure.
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