-
Chinese Materials Research Society
Progress in Natural Science: Materials International
Progress in Natural Science: Materials International
2013;23(2):157–163
1002-0071 & 2013 Chhttp://dx.doi.org/10.10
nCorresponding authE-mail address: g
Peer review under r
www.elsevier.com/locate/pnsmiwww.sciencedirect.com
ORIGINAL RESEARCH
Microstructure and flexural properties of carbon/carboncomposite
with in-situ grown carbon nanotube assecondary reinforcement
Hai Zhang, Lingjun Guon, Qiang Song, Qiangang Fu, Hejun Li,
Kezhi Li
State Key Laboratory of Solidification Processing, Northwestern
Polytechnical University, Xi’an 710072, China
Received 15 October 2012; accepted 20 January 2013Available
online 6 April 2013
KEYWORDS
Carbon nanotube;C/C composite;Chemical
vaporinfiltration;Microstructure;Flexural property
inese Materials Res16/j.pnsc.2013.03.00
or. Tel./fax: þ86 [email protected]
esponsibility of Chin
Abstract Carbon nanotubes (CNTs) were in-situ grown in carbon
felts using ferric chloride as catalystand natural gas as carbon
precursor via thermal gradient chemical vapor infiltration
(TGCVI).Subsequently, the carbon felts were densified to obtain CNT
reinforced carbon/carbon (C/C) compositesin the same furnace.
Effects of CNTs on the microstructure and flexural property of C/C
composites wereinvestigated by polarized light microscopy, Raman
spectroscopy, scanning electron microscopy anduniversal mechanical
testing machine. The results of PLM observation and Raman analysis
showed thatCNTs have two-sided effects on the microstructure of
pyrocarbon: the pyrocarbons in the region withoutCNTs show medium
texture; while, in the region full of CNTs, the microstructure was
low-textured oreven isotropic though the TGCVD conditions would
lead to the deposition of pure low texturepyrocarbons. Analysis
based on stress–strain curves demonstrated that the flexural
strength increasedfirst and then decreased with the CNT content
increasing. When the CNT content was 5.23 wt%, theflexural strength
was maximum and had a nearly 35% improvement compared with pure C/C
composite.Besides, after adding CNTs, the flexural modulus of the
composites decreased and the ductility increasedobviously,
indicating CNTs can toughen C/C composites.
& 2013 Chinese Materials Research Society. Production and
hosting by Elsevier B.V. All rights reserved.
earch Society. Production and hostin1
9 8849 4197.u.cn (L. Guo).
ese Materials Research Society.
1. Introduction
Carbon/carbon (C/C) composites have been widely used inaviation
and aerospace industries due to their good thermal andmechanical
properties such as high thermal conductivity, lowdensity, high
specific strength, good chemical and mechanicalablation resistance
[1]. The performance of C/C composites mainlydepends on the type of
pyrocarbon matrix, spatial structure of
g by Elsevier B.V. All rights reserved.
www.elsevier.com/locate/pnsmidx.doi.org/10.1016/j.pnsc.2013.03.001dx.doi.org/10.1016/j.pnsc.2013.03.001dx.doi.org/10.1016/j.pnsc.2013.03.001mailto:[email protected]/10.1016/j.pnsc.2013.03.001
-
Felt Φ70mm Specimens for observation
of pristine CNT
Center of
edge of specimen
H. Zhang et al.158
primary reinforcement and interfacial strength between
carbonfiber and pyrocarbon matrix. Too weak interfacial strength
andbrittle pyrocarbon can lead to bad performance [2].
Carbon nanotubes (CNTs) have attracted many
researchers’attention for their excellent mechanical properties and
uniquerolled graphitic layers since it was discovered by Sumio
Iijima.Abundant theoretical and experimental studies demonstrated
thattheir Young's modulus can be up to 1 TPa and tensile strength
canapproach to 60 GPa [3,4], which makes CNTs one of the
stiffestmaterials available. With such promising properties, CNTs
arenatural candidates for reinforcement of advanced structural
com-posites. For example, CNTs have been used for addictives in
metal[5], polymer [6], cement [7] and ceramic [8] matrix
composites.Hung et al. [9] conducted CNTs grown on carbon fibers
andemployed the CNT-attached fibers to make composites with
multi-scaled reinforcement for modulate composites’ intrinsical
proper-ties. Their results showed that increasing of interfacial
area andpulling out of nanotubes can improve the tensile strength
of as-prepared composites. Li [10] et al. prepared CNT reinforced
C/Ccomposites by film boiling CVI method with ferrocene and
tolueneas catalyst and precursor, respectively. The test revealed
that CNTsembedded in the matrix of C/C composites can be the
bridgebetween fiber and carbon matrix and offer both intra-laminar
andinter-laminar reinforcement, thus improving the
delaminationresistance and the flexural performance of C/C
composites.
Up to now, the main motivation of doped nano-scaled CNTs isto
alleviate the existing limitations associated with the
matrixdominated material and to improve the performances of
thecomposites. In general, there still are a few reports on
thedevelopment of CNTs as reinforcement for brittle C/C
composites.In our study, C/C composites with multi-scaled
reinforcements(i.e. micro-scaled carbon fiber and nano-scaled
carbon nanotube)were fabricated by densifying two-dimensional
carbon fiber feltsdoped with CNTs in-situ grown in them via thermal
gradientchemical vapor infiltration (TGCVI). The microstructure
andflexural mechanical property of these composites were
alsoinvestigated further.
specimen
Fig. 1 Schematic diagram of sampling for observation of
pristineCNT (a), device for three points flexural test (b).
2. Experimental
2.1. Preparation of CNT-C/C composites
Two-dimensional (2D) carbon fiber felts (carbon fiber: T300;
size:Φ 70 mm� 10 mm; 0.4 g/cm3) were used as the starting
materials.They were immersed into FeCl3 � 6H2O aqueous solution
withdifferent concentrations (0.5 wt%, 1 wt%, 1.5 wt%, 2 wt% and 4
wt%) at room temperature for 10 h and dried in air at 80 1C.
Afterwards,they were introduced into a TGCVI reactor for CNT
growth.
In our work, the CNT growing temperature was set to be1000 1C
and growing time was 5 h. Before reaching the desiredtemperature,
hydrogen and Ar (0.1 m3/h and 0.08 m3/h, respec-tively) were fed
into the TGCVI reactor for protection andreduction of catalyst.
Upon reaching 1000 1C, the hydrogen andAr were turned off and
Natural gas was fed into the furnace with aflow rate of 0.6 m3/h as
the carbon source of CNT growth. After5 h growth, the felts were
taken out from reactor and weighedusing an analytical balance. The
mass gain was ascribed to theformation of CNT, content of which are
1.01 wt%, 5.23 wt%,6.64 wt%, 8.41 wt%, 12.93 wt%, respectively. And
then, theywere densified by TGCVI to obtain CNT reinforced C/C
(CNT-C/C) composites in the same furnace at 1000 1C with natural
gas
as carbon source under ambient pressure. These composites
weremarked by S1, S2, S3, S4 and S5, respectively. For
comparison,pure C/C composite marked by S0 was also prepared under
thesame conditions. Finally, all the samples were treated at 2100
1Cfor 2 h under Ar atmosphere.
2.2. Characterizations of CNTs and the composites
The morphology of CNTs and the fracture surfaces of all
thecomposites were observed using Quanta-600FEG field
emissionscanning electron microscopy (SEM) and Tecnai VEGA3
microscopy.The sampling for SEM observation was shown in Fig. 1(a).
In order toinvestigate the microstructure of CNTs, the carbon felts
with in situgrown CNTs was cut down and put into alcohol solution
for ultrasonicvibration. A copper grid was immersed into alcohol to
capture thesuspended CNTs, and then, the copper grid was dried for
TEMobservation (TEM), which was carried out on a Tecnai F30G2
filedemission transmission electron microscopy. Specimens, with
dimen-sions approximately 30 mm� 10 mm� 10 mm, were cut from
thedensified composites. These specimens were pre-polished using
SiCabrasive paper (600, 1000, 1500, 2000 grit successively). The
finalpolishing was completed with 0.5 μm grain size diamond
polishingpaste. Finishing the preparation of the specimens,
microstructure ofpyrocarbon was studied with polarized light
microscopy (PLM, LeicaDMLP optical microscope) and Raman
spectroscopy (Renishaw inVIA, with 514.5 nm excitation) at polished
cross-section.
-
Microstructure and flexural properties of carbon/carbon
composite with in-situ grown carbon nanotube as secondary
reinforcement 159
Three points flexural test was carried out with a
universaltesting machine (INSTRON 8872) to evaluate the effects of
CNTson the mechanical properties of C/C composites at room
tempera-ture. Testing specimens were machined into rectangular bars
of55 mm� 10 mm� 4 mm from the densified composites along theaxial
direction of carbon fiber bundles. Before tests, the bulkdensities
of the samples were measured. The tests were carriedwith the
constant loading rate of 0.5 mm/min and a span of40 mm. At least
six specimens were tested for each composite.Fig. 1(b) is the
device of flexural test in our experiment. The stressand strain
values were recorded automatically. A ductility factor(FD) was used
to determine the ductility of the composites. FD canbe calculated
from the following equation [11].
FD ¼ 1−ðEsecant=EoriginÞ ¼ 1−ðεlin=εtÞ ð1Þwhere Esecant is
secant modulus (the slope of the line from theorigin to the stress
at failure in the stress–strain curve), Eorigin iselastic modulus
(the slope of the linear part of the stress–straincurve), εlin is
the strain in the linear part of the stress–strain curveat failure
and the εt means the strain at failure.
3. Results and discussion
3.1. Morphology and microstructure of CNTs
SEM observations of pristine CNTs in the center and edge
ofspecimen are presented in Fig. 2(a and b), respectively. It can
be
500 nm
Fig. 2 (a) SEM micrograph of as obtained CNTs in the edge of
specim(c) TEM micrograph of CNTs in the carbon felts. (d) HRTEM
micrograp
seen that the straight CNTs with the diameter of 100–150 nm
inFig. 2(a) are distributed disorderly in the space between
carbonfibers. The surface of carbon fiber is smooth (Fig. 2(a)) and
noCNT is grown on it. The length of most CNTs is over 5 μm andeven
up to 15 μm (Fig. 2(a)). It can be seen that the amounts ofCNTs in
the center of the specimen are almost similar comparedwith that on
the edge. However, there is still a little differencebetween the
center and edge in CNT's morphology. The inset inFig. 2(b) reveals
that besides some coarsen and straight CNTs,there are lots of
thinner and shorter CNTs intertwining with thebigger sized CNTs.
TEM images (Fig. 2(c)) show the innerdiameter and the wall
thickness of CNTs are 30 nm and 40 nm,respectively. According to
Fig. 2(d), the CNTs are multi-walledand the crystal lattice fringes
are somewhat bent, implying thepartial crystallization of CNTs.
3.2. Pyrocarbon microstructure of the composites
Microstructures of pyrocarbon matrix of C/C composites withCNTs
and pure C/C were investigated by PLM and Ramanspectroscopy.
According to PLM observation of the polishedspecimens in Fig. 3, it
is evident that pyrocarbons in pure C/Ccomposite (S0) are
low-textured due to the low optical activitiesand the unclear
extinction cross around carbon fibers (Fig. 3(a)).However, CNT
doped composites (Fig. 3(b and c)) have differentoptical activities
in the reflection of polarized light compared withpure C/C
composite (Fig. 3(a)). The thickness of pyrocarbon is
500 nm
5 nm
en S2. (b) SEM micrograph of CNTs in the center of specimen S2.h
of carbon nanotube wall.
-
H. Zhang et al.160
about 10 mm and the boundary of pyrocarbons grown
arounddifferent fibers is extremely distinct. As for CNT-C/C
composites,the pyrocarbons have higher optical activities,
indicating a highertexture than that of pure C/C composite. While,
in the region fullof CNTs (labeled by white rectangular frame in
Fig. 3(c)), low-textured pyrocarbon or isotropic pyrocarbon is
induced to form.The texture diversity of pyrocarbon is more
apparent in Fig. 3(cand d). It can be seen that more close to the
fiber, denser carbonnanotubes are and lower the optical activities
of pyrocarbon is. Theformation of low-textured pyrocarbon should be
attributed tothe CNTs agglomeration and the depositing randomness
ofpyrocarbon around them [12]. As shown in Fig. 2(b), thenanometer
scaled interval among CNTs and twisted CNTs willprovide large
surface area in a small space, i.e. lager surface area/volume
ratio, which can accelerate the deposition of pyrocarbonon the CNTs
as active nucleation sites [13]. The nano-scaled poreswould
constrict the size of pyrocarbon grain around CNTs. Thepyrocarbon
grain originated from adjacent CNT would contacteach other soon in
densification process. This deposition ofpyrocarbon is largely
dependent on the randomly oriented CNTsas the substrate of
deposition. As a result, the pyrocarbon growingfrom CNTs would
align in different angles controlled by thepristine disordered
three dimensional CNTs network. Therefore,the randomness of
pyrocarbon in orientation, especially in theCNTs agglomeration
region, would exhibit characterization of ISOlayer or lower texture
in polarized light. Nevertheless, pyrocarbon(shown by white arrows
in Fig. 3(c)) deposited around theindividual CNT can grow larger
(the thickness is about severalmicrometers) and shows some
characteristics of medium-texturedpyrocarbon. The π–π conjugation
effect of CNT curved graphiticlayers, which attracts similar
structured polyaromatic molecules to
20 �m
20 �m
Carbon fiber
Individual CNT Agglomeration
zone of CNT
Fig. 3 PLM images of composites. (a) Microstructure of co
arrange parallel each other to form regular pyrocarbon perhaps
haswider influence on microstructure, which is considered to
inducethe formation of medium-textured pyrocarbon. For all the
C/Ccomposites doped by CNTs, the microstructure of pyrocarbon isnot
homogeneous at all. Namely, medium-textured and ISOpyrocarbons
coexist in the matrix. In order to further study themicrostructure
of CNTs doped composites. Raman characteriza-tion of pyrocarbon in
S0, S3 and the pristine CNTs was conducted.As shown in Fig. 4(b),
the characteristic peak (D band) at1350 cm−1 and shoulder peak at
1620 cm−1 (D' band) correspondto presence of disorder and
distortion in carbon atomic layers.The second prominent band is G
band related to high-frequencyE2g first order mode and the band at
2700 cm
−1 is the overtone ofD band, called 2D band. The D and G band
position of CNTsare several cm−1 lower than composites. This
downshift is due tothe less carbon atomic layers of CNTs. The ratio
of IG to ID(IG means the intensity of G band) depends on the
perfect crystalplanar domain size and the degree of order of
pyrocarbon [14].The IG/ID value for pyrocarbon in pure C/C is the
lowest,corresponding to the lowest crystal ordered degree [15].The
second lowest IG/ID value corresponding to pyrocarbonoriginated
from fiber (labeled with point 1). Meanwhile, the IG/ID value of
pyrocarbon grown from individual CNT (labeledwith point 2) is a
little larger compared with that at point 1, whichis normally due
to the improvement of crystal ordered degree andfewer defects in
microstructure. According to the highest ratiovalue presented in
the Raman spectrum of pristine CNTs, there-fore, it is considered
that the CNTs have the highest texture. As forpyrocarbon of CNTs
agglomeration region (labeled with point 3),the second larger IG/ID
ratio value may virtually be attributedto the presence of CNTs
within the laser spot. In our work, these
CNT
fiber
20 �m
20 �m
Medium
texture ISO pyrocarbon fiber
mposite S0. (b and c) Composite S2. (d) Composite S3.
-
600 900 1200 1500 1800 2100 2400 2700 300-200
0200400600800
100012001400160018002000220024002600280030003200
2D band
G band
pure C/C
pristine CNTs
point 2
point 3
inte
nsity
/a.u
.
Raman shift /cm-1
point 1
D band
D' band
Point 1
Point 2
Point 3
20 �m
Fig. 4 (a) Microstructures of composite S3 under normal light
(left) and polarized light (right). Point 1, 2 and 3 show the
positions of Ramantesting (b) Raman spectra of pure C/C, composite
S3 at different points and pristine CNTs.
Table 1 Three point flexural strength and modulus for all
composites.
Composites S0 S1 S2 S3 S4 S5
Density (g cm−3) 1.75 1.62 1.57 1.60 1.57 1.59Flexural strength
(MPa) 8673 9772 116715 10878 6576 50.978Flexural modulus (GPa)
16.7772.1 12.2472.2 12.9773.5 13.4373.8 8.871.2 10.2870.8FD o0.005
0.23370.059 0.48870.073 0.45370.098 0.41170.055 0.42470.92
0.000 0.005 0.010 0.015 0.020 0.025 0.030
0
20
40
60
80
100
120
140
stre
ss (M
Pa)
strain
S0
S3
S1
S4
S5
S2
Fig. 5 Stress–strain curves for composites S0, S1, S2, S3, S4
and S5.
Microstructure and flexural properties of carbon/carbon
composite with in-situ grown carbon nanotube as secondary
reinforcement 161
findings indicate that besides the CVI deposition
parameters,CNTs and its distribution can also affect the
microstructureof pyrocarbon. Concretely, CNTs can induce the
formation ofhigher- or lower-textured pyrocarbon, which is similar
to other’swork [16].
3.3. Flexural properties and fracture behaviors of
C/Ccomposites
The results of three points bending test are listed in Table 1.
Itpresents the average flexural strength and modulus for the
sixcomposites. The average flexural strengths of composites S0,
S1,S2, S3, S4, and S5 are 80 MPa, 97 MPa, 116 MPa, 108 MPa,65 MPa
and 50.9 MPa, respectively. The flexural strengthincreases first
and then decreases with the CNT content increasing.When the CNT
content is 5.23 wt%, the flexural strength is
maximum and has a nearly 35% improvement compared with pureC/C
(S0). However, when the CNT content increases to 8.41 wt%or more,
the flexural strength decreases from the maximum value.On the hand,
this may be the result of the degradation of carbonfiber tensile
strength caused by the dissolution of more iron catalystinto fibers
and more formation of low texture pyrocarbon incomposites with
higher concentration catalyst [17]. On the otherhand, another
reason for the reducing of flexural strength might bethe
aggregation of the CNTs, which leads to the sealing of nano-scaled
pores and poor infiltration of the precursor and deposition
ofpyrocarbon. The defects in the aggregation region of CNTs,
forinstance, pores exists in matrix (Fig. 6g) and make the
continuouspyrocarbon matrix separated, causing lower the
contribution ofmatrix to the flexural strength. Flexural modulus
for all hybridcomposites S1–S5 are less than that of composite S0
completely,which is different from variation of flexural strength.
This variationof flexural modulus is similar with that in
literature [18]. Thisreducing of flexural modulus can be attributed
to the ISOpyrocarbon formed in matrix under the influence of
agglomeratedCNT. As seen in Fig. 6(g and h), the coarsened CNTs and
theconcave pits are obvious, and most coarsened CNTs in
theagglomeration zone are more liable to detach from the
matrixrather than the expected fulling out of CNTs or cracking. As
asequence, the strength and modulus of CNT cannot be fullyutilized.
However, with CNT content increasing, the CNTsaggregation becomes
more serious in composites S4 and S5,which deteriorate the ultimate
properties due to the defects in theregion as discussed above.
The stress–strain curves for composites are shown in Fig. 5.
Asseen from the curves, it is noticeable that the composites S1,
S2,S3 have higher fracture strengths and larger strains than
compositeS0. For CNT-doped composites (S1, S2, S3, S4, S5), after
themaximum stress, the curves drop slowly and have a larger
ductilityfactor FD, exhibiting a pseudo plastic fracture mode,
whereas the
-
Debonding
of fiber
Pulling out
of fiber Debonding
of CNTs
Bridge
Pulling out of CNT
Holes left by pulled out CNTs
Agglomeration zone of
CNTs with low texture Pulling out of
CNT
Pores
Fracture of coarsened
CNTs
Pyrocarbon
Pulling out of CNT
Fracture of
coarsened CNTs
Concave pits left by the coarsened
CNTs separation from matrix
Fig. 6 (a) SEM images of the fracture surfaces of composite S0.
(b) Magnified images of fracture surfaces of composite S0.
((c)–(f)) SEM imagesof the fracture surface of composite S2: (c)
Debonding and pulling out of fibers occurs in the surface. (d)
Bridge and debonding of CNTs.(e) Fracture surface of pyrocarbon
deposited in the agglomeration zone of CNTs. (f) Fracture surface
of pyrocarbon deposited around individualCNT. ((g) and (h))
Fracture morphology of composite S4.
H. Zhang et al.162
-
Microstructure and flexural properties of carbon/carbon
composite with in-situ grown carbon nanotube as secondary
reinforcement 163
pure C/C composite S0 shows a brittle fracture behavior due
tosharp decline of stress and small fracture strain
indicatingintroduction of CNTs is contributed to the improvement of
flexuralductility. In the platform of curves, it is thought that
the partialfibers have cracked and the rest fibers and CNTs still
bear the load,thus, they absorb the energy during being pulled out
resulting information of platform. The curves for hybrid composites
S1–S5display obvious platform, meaning better ductility.
To investigate the reinforcing mechanism of CNTs, the
fracturesurfaces of composite S0, S2 and S4 are observed by SEM
(Fig. 6).For pure C/C S0, the fracture surface is flat (Fig. 6(a))
and only afew fibers are pulled out from carbon matrix (Fig. 6(b))
whichconfirms the strong fiber/matrix interface bonding and
brittlefracture mode. As seen from Fig. 6(c), the extraction
anddebonding of carbon fibers shown by white arrows can be seenin
composite S2, besides, CNT pull-out from matrix is also found,both
of which can toughen C/C composites. It is evident that thedoping
of CNTs into matrix can tremendously increase theinterface area
between reinforcement and matrix. Therefore, whenthe stress is
loaded on the specimens, fracture is more difficult tooccur for
CNT-doped composites by virtue of additional surfaceenergy,
pull-out work and friction work because of the newlyformed surface
area [19]. The CNTs inserted in pyrocarbon shownin Fig. 6(d) link
adjacent pyrocarbon layers together as a bridge,which can avoid the
formation of concentric crack around carbonfiber to a certain
extent. Hence, it is beneficial to the loadtransmitting and
sufficient utilization of carbon fiber. And whenthe CNTs were
pulled out, an overview of many holes left inmatrix is clearly
shown in Fig. 6(e). The multilayer carbon matrix(Fig. 3(c)) has
been formed under the influence of interlaced CNTsshown in Fig. 6(e
and f). During the fracture, multi-layered carbonmatrix can lead
cracks to spread along multiple paths and results inan uneven
fracture surface. Therefore, the crackle would havemuch greater
interaction with the pyrocarbon matrix duringpropagating, which
further improves mechanical interlockingbetween reinforcements and
matrix and toughness [20,21]. Fromthe discussed, it will be
significant on improvement of themechanical properties of CNTs
doped composite to control theproper content and distribution of
pristine CNTs in felts.
4. Conclusions
C/C composites doped by CNTs (as secondary reinforcements)were
prepared by TGCVI. According the observations of PLM andthe Raman
analysis, CNTs in-situ grown in carbon felts had two-side effects
on the microstructure of pyrocarbon. ISO pyrocarbonwas obtained in
the aggregation region of CNTs; while, medium-textured pyrocarbon
was induced in the sparse region of CNTs andaround individual CNT.
As contrast, the pyrocarbon of pure C/Cwas low-textured. Mechanical
tests and fracture surface analysisshow that the suitable CNTs
content can improve the flexuralstrength and ductility, while, the
modulus of prepared multi-scaledC/C composites with CNTs generally
decreased compared withpure C/C composite.
Acknowledgments
This work has been supported by the programme of
introducingtalents of discipline to universities (B08040), key
grant project ofChinese ministry of education (313047) and national
naturalscience foundations of China (51275417 and 51221001).
References
[1] T. Windhorst, G. Blount, Carbon–carbon composite: a summary
ofrecent developments and application, Materials and Design 18
(1)(1997) 11–15.
[2] A. Oberlin, Pyrocarbons, Carbon 40 (1) (2002) 7–18.[3] J.P.
Lu, Elastic properties of carbon nanotubes and nanoropes,
Physical Review Letters 79 (7) (1997) 1297–1300.[4] M.
Sammalkorpi, A. Krasheninnikov, A. Kuronen, Mechanical
properties of carbon nanotubes with vacancies and related
defects,Physical Review B: Condensed Matter 70 (24) (2004)
245416–245418.
[5] J. Stein, B. Lenczowsk, N. Frety, Mechanical reinforcement
of a high-performance aluminium alloy AA5083 with homogeneously
dispersedmulti-walled carbon nanotubes, Carbon 50 (6) (2012)
2264–2272.
[6] S.S. Wicks, R.G. de Villoria, B.L. Wardle, Interlaminar and
intrala-minar reinforcement of composite laminates with aligned
carbonnanotubes, Composites Science and Technology 70 (1) (2010)
20–28.
[7] B.M. Tyson, R. Al-Rub, A. Yazdanbakhsh, et al., Carbon
nanotubesand carbon nanofibers for enhancing the mechanical
properties ofnanocomposite cemetitious materials, Journal of
Materials in CivilEngineering 23 (7) (2011) 1028–1035.
[8] A. Peigney, C. Laurent, E. Flahaut, A. Rousset, Carbon
nanotubes innovel ceramic matrix nanocomposites, Ceramics
International 26 (6)(2000) 677–683.
[9] K.H. Hung, W.S. Kuo, Processing and tensile characterization
ofcomposites composed of carbon nanotube-grown carbon
fibers,Composites Part A: Applied Science and Manufacturing 40
(8)(2009) 1299–1304.
[10] X.T. Li, K.Z. Li, H.J. Li, Microstructures and mechanical
propertiesof carbon/carbon composites reinforced with carbon
nanofibers/nanotubes produced in situ, Carbon 45 (8) (2007)
1662–1668.
[11] B. Reznik, M. Guellali, D. Gerthsen, Microstructure and
mechanicalproperties of carbon–carbon composites with multilayered
pyrocarbonmatrix, Materials Letters 52 (1-2) (2002) 14–19.
[12] K.Z. Li, Q. Song, Influence of carbon nanotube content on
micro-structures and mechanical properties of carbon/carbon
composite,Chinese Journal of Inorganic Chemistry 27 (5) (2011)
1001–1008.
[13] J. Antes, Z. Hu, W. Zhang, Chemistry and kinetics of
chemical vapordeposition of pyrocarbon VII: Confirmation of the
influence of thesubstrate surface area/reactor volume ratio, Carbon
37 (1999) 2031–2039.
[14] W.Z. Li, H. Zhang, C.Y. Wang, Raman characterization of
alignedcarbon nanotubes produced by thermal decomposition of
hydrocarbonvapor, Applied Physics Letters 70 (20) (1997)
2684–2686.
[15] E.L. Honorato, P.J. Meadows, Fluidized bed chemical vapor
deposi-tion of pyrolytic carbon-I. Effect of deposition conditions
onmicrostructure, Carbon 47 (2009) 396–410.
[16] Q.M. Gong, Z. Li, Fabrication and structure: a study of
alignedcarbon nanotube/carbon nanocomposites, Solid State
Communica-tions 131 (6) (2004) 399–404.
[17] Q. Hui, A. Bismarck, E.S. Greenhalgh, Carbon nanotube
graftedcarbon fibers: a study of wetting and fibers fragmentation,
CompositesPart A: Applied Science and Manufacturing 41 (9) (2010)
1107–1114.
[18] K.Z. Li, H.L. Deng, Microstructure and mechanical
properties ofcarbon/carbon composites doped with LaCl3, Materials
Science andEngineering: A 529 (25) (2011) 177–183.
[19] X.W. Wu, R.Y. Luo, Deposition mechanism and microstructure
ofpyrocarbon prepared by chemical vapor infiltration with kerosene
asprecursor, Carbon 47 (6) (2009) 1429–1435.
[20] Q. Song, K.Z. Li, H.L. Li, Grafting straight carbon
nanotubes radiallyonto carbon fibers and their effect on the
mechanical properties ofcarbon/carbon composites, Carbon 50 (10)
(2012) 3949–3952.
[21] H.L. Li, H.J. Li, J.H. Lu, Effects of a pre-deposited
pyrocarbon layeron the microstructure and mechanical properties of
carbon/carboncomposites, Materials Science and Engineering: A 556
(2012) 295–301.
Microstructure and flexural properties of carbon/carbon
composite with in-situ grown carbon nanotube as
secondary...IntroductionExperimentalPreparation of CNT-C/C
compositesCharacterizations of CNTs and the composites
Results and discussionMorphology and microstructure of
CNTsPyrocarbon microstructure of the compositesFlexural properties
and fracture behaviors of C/C composites
ConclusionsAcknowledgmentsReferences