Introduction
The amazing physical and mechanical properties of carbon nanotubes (CNT) make them one
of the smartest reinforcements for 21st century futuristic composites. Their special
geometrical features like high aspect ratio and diameter in the nanoscale along with their low
density ensure the high quality of CNT reinforced structural materials due to the increase in
strength as a result of the presence of only atomic scale defects attributed to their nano size.
The extensively small size of CNT reinforcements provides very high surface area for matrix-
reinforcement interaction producing striking effects which are still under research. Though
CNTs are being used in polymeric, metallic and ceramic matrix composites, they are not as
popular in amorphous glasses and partially crystalline glass ceramics silicate based
composites. The enhancement of fracture toughness along with other mechanical, functional
and technological properties of these composites with brittle matrices are of great interest to
the scientists to realise the potential applications of these novel composites.
Carbon Nanotubes
Carbon Nanotubes, long, thin cylinders of carbon, were discovered in 1991 by Sumio Iijima.
They can be thought of as a sheet of graphite (a hexagonal lattice of carbon) rolled into a
cylinder. CNTs are sp2 hybridized like graphite. However, graphene layers in CNTs are rolled
up to form seamless hollow tubes; an individual atomic carbon layer forms a single walled
carbon nanotube, while multiple layers rolled up concentrically constitute a multi-walled
carbon nanotube. A structure containing pentagons or half fullerenes closes the tube ends.
The interlayer spacing of multi-walled CNTs is around 3.35 Ǻ. The diameter of the innermost
tube can be as small as 0.4 nm and up to a large fraction of the outer diameter, which possibly
can reach more than 100 nm. These intriguing structures have sparked much excitement in
recent years and a large amount of research has been dedicated to their understanding.
Production of Carbon NanotubesCNTs are produced mainly by the following methods:
Electric Arc Discharge (EAD)
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Fig.1. Multi walled Carbon Nanotube
Fig.2. Single walled Carbon Nanotubes
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This technique was used to characterize CNTs for the first time by Iijima in 1991. Typically,
a current of 50-100 A is passed through a pair of graphite electrodes of diameter 6-12 mm at
a voltage of 20-40V. The electrodes are separated by a distance of 1-4 mm in inert
atmosphere provided using helium. Carbon vaporises away from the anode and arranges itself
into multi-walled CNTs, which deposit on the cathode as a soft and dark black fibrous
material. Transition metal catalysts are used to produce single walled CNTs which deposit on
the walls of the reaction chamber. This method gives single walled CNTs of diameter 1-5 nm
and length of up to 1 μm. The CNTs produced thereof are highly graphitized with very less
defects and good electrical, thermal and mechanical properties.
Laser Ablation or Evaporation (LAE)
This method is quite similar to the previous method. It uses a heating mechanism which
involves a high power laser to evaporate graphite. The flow of an inert gas such as argon or
helium drives the vaporised carbon atoms away from the high temperature zone on a cold
copper collector where CNTs deposit. The processing parameters such as the type of the inert
gas, intensity of the laser and the furnace temperature influence the morphology and
properties of the CNTs produced. This technique produces good quality CNTs with yields as
high as 70 per cent. It also offers good diameter control.
Chemical Vapour Deposition (CVD)
This method was used first in 1998 for the synthesis of CNTs. In this technique, a source of
carbon (usually a hydrocarbon or CO) is heated inside a quartz tube at an intermediate
temperature range (500-1100°C) in the presence of a catalyst under an inert atmosphere of
argon or helium gas. Hydrocarbon molecules decompose into hydrogen and carbon in the
presence of catalyst. This is followed by the rearrangement of carbon atoms into hexagonal
networks on the metal catalyst to grow CNTs. The hydrocarbon source can be a solid
(camphor, naphthalene), a liquid (benzene, alcohol and hexane) or a gas (acetylene, methane
and ethylene). The technique has a high production rate at relatively low cost. It is possible to
closely monitor the growth of the CNTs by adjusting process parameters like temperature,
catalyst, hydrocarbon source and the flow rate of gases.
The quality and yield of CNTs depend largely upon the synthesis path and various process
parameters. Highly crystalline CNTs can be created by using high temperature EAD and LAE
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techniques. However, these techniques are not cost effective as the yield is poor due to the
presence of undesirable carbonaceous by-products such as fullerenes and graphitic
nanoparticles. On the other hand, CVD technique produces good quantities of cheap CNTs.
But, these CNTs are structurally defective and often coated with amorphous carbon.
Key Properties of Carbon Nanotubes
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By measurement of the amplitude of their thermal vibrations in a transmission electron microscope, the moduli value of multi walled CNTs and single walled CNTs were found to be 1.8 TPa and 1.25 TPa respectively. Axial Stiffness of highly crystalline CNTs = 1.1 TPaCNTs produced by CVD have lower stiffness due to the presence of a large number of structural defectsModulus of ElasticityTheoretical Tensile Strength = 75-135 GPa Tensile loadings of single walled CNTs lead to the failure by brittle fracture mode.Multi walled CNTs fail in tension via a sword-in-sheath mechanism; bending failure of multi walled CNTs is due to stepwise or brittle fractures.Presence of defects is responsible for the low strength values observed experimentally. The defects provide ideal sites for failure.Tensile StrengthMetallic single walled CNTs = 105-106 S/mSemiconductor single walled CNTs = 10 S/mIn-plane conductivity of graphite = 2.5×106 S/m (for comparison)Multi walled CNTs = 20-2×107 S/mThe presence of defects affect the measured electrical conductivity of CNTs significantly.Electrical ConductivityTheoretically predicted value for individual single walled CNTs = 6600W/mKThe experimental thermal conductivities of individual multi walled CNTs are very high, e.g. 3000 W/mKThermal Conductivity
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Fig 3. Crack Bridging Mechanism
Fig.4. Multi walled CNTs
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Applications of Carbon Nanotubes Supercapacitors
Transparent electrodes for organic light emitting diodes
Lithium ion batteries
Nanowires
Field-effect transistors
Molecular switches, sensors and filters
Electrochemical electrodes and catalyst supports
Immobilization of biomacromolecules
Organic reaction catalysis
Nanoscale reinforcing elements in composites
Glass and glass-ceramic matrix composites
Glasses and glass-ceramics are becoming one of the favourite matrix materials for aerospace,
automotive, electronics, biomedical and other specialized field applications because of the
flexibility they offer in terms of composition, properties and processing. It is possible to
manufacture glasses with a wide range of thermal expansion coefficients (TECs) which
match the TECs of the reinforcement. High composite densities can be achieved by
promoting viscous flow during sintering by controlling the viscosity of glasses.
Manufacturing Process: CNT Reinforced Glass/glass-ceramic matrix
compositesManufacturing Requirements:
a) The availability of high quality CNTs with intrinsically good mechanical properties.
b) A uniform dispersion of CNTs in the matrix
c) The development of a suitable interfacial bonding.
d) The prevention of oxidation of CNTs during high temperature sintering, and
e) The consolidation of composites to near theoretical densities
The Stages in Manufacturing:
1. Composite powder preparation using a suitable dispersion process:
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The techniques to achieve this include conventional powder mixing, sol-gel
techniques, colloidal mixing processes and in-situ CNT synthesis in the matrix
material. The lack of driving force in conventional powder mixing processes leads to
a poor dispersion of CNTs which results in the formation of agglomerates in the
sintered composites. On the other hand, sol-gel and colloidal mixing techniques
produce homogeneous dispersions of CNTs due to the absence of CNT agglomerates.
Conventional Powder Mixing: In this process, as-synthesized CNTs are mixed with
glass or glass-ceramic powders and a composite slurry or suspension is prepared
which is ultrasonicated and/or ball-milled to disperse the CNTs. The composite
slurry/suspension is then dried, ground and sieved before the final compaction and
sintering to obtain composite bodies. The lack of dispersion is not surprising in these
systems, given that the matrix particles are typically large compared to the desired
CNT-CNT separation, and the fact that the CNTs tend to be forced into mutual
contact around the perimeter of the matrix particles. There is no effective mechanism
to disagglomerate these CNTs or to distribute them within the original matrix particles
during consolidation.
Sol-gel Processing: In this process, functionalized or surfactant-stabilized CNTs are
mixed in a usually aqueous solution (sol) of molecular precursor of the matrix which
is subsequently gelled (gel) and dried to obtain the inorganic composite. The obtained
composite body is either used as the final composite after heat treatment or crushed,
ground, sieved and then sintered to obtain composite samples of the desired shape. In
one of the earliest studies on sol-gel processing, tetraethoxysilane was used as the
precursor in acidified water to produce a silica glass matrix.
Colloidal Mixing: In this process, the surface chemistry of CNTs and the glass
powder suspensions is adjusted to encourage the coating of glass particles on CNTs or
vice versa, depending upon their size. The manipulation of the surface chemistry of
the two composite constituents results in the development of opposite surface charges
on them. As a result, similarly charged particles repel each other and attract
oppositely charged particles during the mixing of the two suspensions, a process
called heterocoagulation. The surface charges on CNTs and glass powders can be
easily developed using organic surfactants and dispersants. Colloidal mixing
processes providing high quality CNT dispersions in glass matrices are being
developed. Aqueous CNT dispersions are obtained by treating CNTs with a mixture
of sulphuric and nitric acids which not only purify them from catalyst particles and
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amorphous carbon produced during their synthesis but also shorten their lengths and
decorate them with acidic functional groups (i.e. carboxylic acid and other oxygen
containing groups). These surface functionalities stabilize CNTs electrostatically in
aqueous suspensions and develop a negative surface charge. The development of
negative surface charge on CNTs requires positive surface charge on the glass
particles to encourage heterocoagulation which is produced using cationic surfactants,
and finally suitable composite powder suspensions are obtained by colloidal mixing.
The composite powder suspensions are dried, ground and sieved to obtain powders for
subsequent sintering into solid compacts. A calcinations process is usually performed
on dried composite powders before sintering to remove organic surfactants and CNT
oxidation debris at temperatures less than 400°C.
In-situ Synthesis: This process involves the direct growth of CNTs on matrix powders
by CVD.
2. Composite Densification by an Appropriate Sintering Technique:
The densification techniques used to consolidate CNT-glass/glass-ceramic matrix
composites are:
Hot-Press Sintering (HPS)
Spark Plasma Sintering (SPS)
Pressureless Sintering (PLS)
Laser Treatment
High Pressure Techniques
In SPS, composite powders are internally heated by passing pulsed DC current
through a graphite die, while in HPS and PLS, composite powders are consolidated
by an external heat source. The rapid heating in SPS results in lower sintering
temperatures and shorter durations compared with HPS and PLS. Vacuum or
protective atmospheres of nitrogen and argon are used during sintering to avoid the
oxidation of CNTs. In addition, alignment of CNTs has been observed in densified
composites sintered by SPS and HPS, as these techniques involve uniaxial pressure.
In contrast, hot isostatic pressing (HIP) and PLS provide randomly oriented CNTs in
the final composites, provided composite powders are pre-compacted by cold
isostatic pressing (CIP). In short, SPS is a time efficient and effective route to good
densities, but PLS is cheaper and more flexible in terms of composite size and shape,
but at the cost of a comparatively lower density.
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Mechanical PropertiesHardness
The properties of CNT-glass/glass-ceramic matrix composites are still under research. The
actual effect of CNTs on hardness is still unclear. The following tendencies have been
observed during experiments:
1. Dense matrices and good interfacial bonding tends to increase the hardness. There is
no proof of a strong interfacial bonding. The simple mechanical bonds exist.
2. Inhomogeneous dispersion of CNTs in matrices and poor densities of sintered
composites cause decrement in the hardness value.
3. If there is a combination of the above two, a compromise is generally observed in the
hardness value.
Sharp increases at very low filler contents are most likely related to changes in matrix
morphology or crystallinity.
Elastic Modulus
Considering the high stiffness of CNTs, which should be greater than that of glasses/glass-
ceramics, an increase in elastic modulus of the composites is expected. For three-dimensional
randomly aligned short fibre composites, a modified rule of mixtures called Krenchel’s rule
incorporating an orientation efficiency factor (η0) and length efficiency parameter (ηL) is
often used:
EC = ηO ηL EF VF + EM (1-VF)
In which EC, EF and EM are the elastic moduli of the composite, fibres and matrix respectively
and VF is the fibre volume fraction. The value of ηo for three dimensional random orientation
of fibres is 0.2 and the value of ηL depends upon the fibre length, diameter, inter-fibre spacing
and shear modulus of the matrix.
Fracture Strength
Significant increase in the fracture strength of CNT-glass/glass-ceramic matrix composites
have been observed at low to moderate CNT loadings using three and four point flexural and
compression strength tests. The main parameters in this case are microstructure and the
degree of agglomeration.
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Fracture Toughness
This property is very important because glass based systems are brittle. The aim has been to
introduce additional toughening mechanisms through the incorporation of CNTs. To compare
the toughness values of the composites and unreinforced glass/glass-ceramic matrices, a
simple method known as Vickers Indentation Fracture Toughness Technique (VIF) has been
employed in most of the studies. It has been observed that the composites developed have
higher fracture toughness. High CNT loadings lead to a drop again in the fracture toughness
value due to inhomogeneous CNT dispersion. Improvements in inorganic matrix CNT
composite toughness are usually attributed to conventional fibre mechanisms, such as crack
deflection, CNT bridging and CNT pull-out as characteristic features are often observed by
fractography. However, scaling considerations highlight the lower absolute performance
expected for nanofibres compared to microfibers, if only these conventional mechanisms
operate. Thus, the possibility of new toughening mechanisms such as shear banding of
hollow nanostructures or pull-out of flexible single walled CNTs over convoluted contour
lengths.
Functional PropertiesElectrical Conductivity
The electrical conductivity of glasses and glass ceramics increases by a huge margin on
incorporation of CNTs. At very low CNT contents, electrical conductivity is not increased
significantly, but the formation of a conducting percolating network of CNTs rapidly
increases electrical conductivity by several orders of magnitude following the scaling law
according to percolation theory:
σ C = σCNT (Ф-ФC)t
where σ C and σ CNT are the conductivities of composites and CNTs respectively, Ф is the
volume fraction of CNTs in composites, ФC is the critical volume fraction (percolation
threshold) and the exponent t is the dimensionality of the system.
Thermal Conductivity
The thermal diffusivity (α) and specific heat capacity (CP) of CNT-glass/glass-ceramic matrix
composites are generally measured by laser flash technique and differential scanning
calorimetry, respectively. Thermal conductivity (KC) is then calculated by computing the two
measured values along the sintered density (ρ) of composites using the following equation:
KC = αCP ρ
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CNTs exhibit high individual thermal conductivities greater than graphite and diamond. But,
the increment in thermal conductivity due to CNT reinforcement in glass/glass ceramic
matrix is not very high due to the high interfacial density associated with nanoscale particles.
The characteristic percolation behaviour observed for electrical conductivity is not mirrored
in the thermal response because of fewer orders of magnitude of variation between the
thermal conductivities of fibre and matrix than in the electrical case. Many reason may limit
the enhancement, including the presence of structural defects in CNTs, non-uniform
dispersion of CNTs producing agglomerates, insufficient densification of composites leading
to porosity, preferential alignment of CNTs producing anisotropic composite properties and
the presence of new phases at the CNT-glass interfaces. The thermal resistance due to large
interfacial surface area between CNTs and glass matrix is the most likely reason for the
reduced increment of thermal conductivity of composites.
Applications The CNT-glass/glass ceramic matrix composites seem to be appealing because of low
density, improved fracture toughness, enhanced electrical and thermal conductivity,
resistance to oxidation at moderate to high temperatures and higher hardness and stiffness
than polymers and some metals. CNT-glass/glass-ceramic composites, due to their higher
thermal conductivities than the pure matrices, may be used for space structural applications
and as heat sink materials to dissipate heat from electronic components. Applications
requiring a sudden temperature change may also be considered for CNT composites due to
their resistance to thermal shock and cycling, for example in the handling of low-melting
metals and glasses. CNT composites can also be used for applications demanding
intermediate temperatures up to 500°C and normal air environments. Glass matrix
composites containing CNTs can be developed as thermal barrier coatings. The low friction
resistance of CNT-glass matrix composites can be exploited for applications demanding anti-
friction materials such as in pump manufacturing and components for the automotive
industry. Components such as bearings, seals, brake and gear systems can be manufactured
using CNT-glass/glass ceramic composites. Finally, the process of developing a porous glass
after burning incorporated CNTs may also be used for certain applications where controlled
(nano)porosity is required, such as in filters, catalyst supports or sorbents.
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Conclusions and ScopeThere is much about carbon nanotubes that is still unknown. More research needs to be done regarding the environmental and health impacts of producing large quantities of them. There is also much work to be done towards cheaper mass-production and incorporation with other materials before many of the current applications being researched can be commercialized. There is no doubt however that carbon nanotube will play a significant role in a wide range of commercial applications in the very near future. Not only will they help create some very cool tech gadgets, they may also help solve the world's energy problems.
Although the effect of randomly dispersed CNTs on the hardness and stiffness of glass/glass-ceramic matrices is still not clearly understood, moderate improvements in fracture strength and toughness and thermal conductivity have been conclusively observed; the significant increase in electrical conductivity is particularly promising. Incorporating highly aligned CNTs of high aspect ratio in one or more well-specified directions remains to be achieved. Although challenging, such a configuration of CNTs in composites will increase their functional and structural properties tremendously, especially thermal conductivity.
The interfaces in nanocomposites occupy a large area. It might be possible in future to develop in-situ interfaces formed by the chemical reaction between CNTs and the surrounding glass phase resulting in better composite properties. CNTs produced by different techniques offer varying characteristics in their purity, crystallinity, straightness, diameter and aspect ratio. A useful increase in functional properties, moderate increase in mechanical characteristics and improved technological properties suggest the potential use of CNT-glass/glass-ceramic matrix composites in numerous industrial applications. The possibility of tailoring the functional properties and the simplicity of production (in comparison with continuous fibre reinforced composites) justifies continued R&D efforts in this field. Combinations of CNT fillers with conventional fibre reinforcements may prove particularly beneficial in the near term, as has begun to be demonstrated in polymer matrix systems.
References
Ceramic Nanocomposites, Woodhead Publishing Series In Composites Science And Engineering, Edited by Rajat Banerjee and Indranil Manna
http://www.answers.com/topic/timeline-of-carbon-nanotubes?cat=technology http://www.pa.msu.edu/cmp/csc/nttimeline.html
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