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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