:: AZoNanotechnology Article
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CNTs Electrical Conductivity
CNTs Strength and Elasticity
CNTs Thermal Conductivity and Expansion
CNTs Field Emission
CNTs High Aspect Ratio
Applications of Carbon Nanotubes
CNTs Thermal Conductivity
CNTs Field Emission Applications
CNTs Conductive Plastics
CNTs Energy Storage
CNTs Conductive Adhesives and Connectors
CNTs Molecular Electronics
CNTs Thermal Materials
CNTs Structural Composites
CNTs Fibers and Fabrics
CNT Catalyst Supports
CNTs Biomedical Applications
CNTs Air and Water Filtration
CNTs Ceramic Applications
Other CNT Applications
CNTs Electrical
Conductivity
There has been considerable practical interest in the
conductivity of CNTs. CNTs with particular combinations
of N and M (structural parameters indicating how much the nanotube is
twisted) can be highly conducting, and hence can be said to be
metallic. Their conductivity has been shown to be a function of their
chirality (degree of twist), as well as their diameter. CNTs can be either metallic or
semi-conducting in their electrical behavior.
Conductivity in MWNTs is quite complex. Some types
of “armchair”-structured CNTs appear to conduct better than
other metallic CNTs. Furthermore, interwall
reactions within MWNTs have been found to
redistribute the current over individual tubes non-uniformly. However,
there is no change in current across different parts of metallic
single-walled CNTs. However, the behavior of
ropes of semi-conducting SWNTs is different, in that the
transport current changes abruptly at various positions on the CNTs.
The conductivity and resistivity of ropes of SWNTs has been measured by placing
electrodes at different parts of the CNTs. The resistivity of the SWNT ropes was in the order of
10–4 ohm-cm at 27°C. This means that SWNT ropes are the most conductive
carbon fibers known. The current density that was possible to achieve
was 107 A/cm2, however in theory the SWNT ropes should be able to
sustain much higher stable current densities, as high as 1013 A/cm2.
It has been reported that individual SWNTs may contain defects.
Fortuitously, these defects allow the SWNTs to act as transistors.
Likewise, joining CNTs together may form
transistor-like devices. A nanotube with a natural junction (where a
straight metallic section is joined to a chiral semiconducting section)
behaves as a rectifying diode – that is, a half-transistor in
a single molecule. It has also recently been reported that SWNTs can route electrical signals
at high speeds (up to 10 GHz) when used as interconnects on
semi-conducting devices.
CNTs Strength and
Elasticity
The carbon atoms of a single (graphene) sheet of graphite
form a planar honeycomb lattice, in which each atom is connected via a
strong chemical bond to three neighboring atoms. Because of these
strong bonds, the basal-plane elastic modulus of graphite is one of the
largest of any known material. For this reason, CNTs are expected to be the
ultimate high-strength fibers. SWNTs are stiffer than steel, and
are very resistant to damage from physical forces. Pressing on the tip
of a nanotube will cause it to bend, but without damage to the tip.
When the force is removed, the tip returns to its original state. This
property makes CNTs very useful as probe tips for
very high-resolution scanning probe microscopy.
Quantifying these effects has been rather difficult, and an
exact numerical value has not been agreed upon. Using an atomic force
microscope (AFM), the unanchored ends of a freestanding nanotube can be
pushed out of their equilibrium position and the force required to push
the nanotube can be measured. The current Young’s modulus
value of SWNTs is about 1 TeraPascal, but
this value has been disputed, and a value as high as 1.8 Tpa has been
reported. Other values significantly higher than that have also been
reported. The differences probably arise through different experimental
measurement techniques. Others have shown theoretically that the
Young’s modulus depends on the size and chirality of the SWNTs, ranging from 1.22 Tpa to
1.26 Tpa. They have calculated a value of 1.09 Tpa for a generic
nanotube. However, when working with different MWNTs, others have noted that the
modulus measurements of MWNTs using AFM techniques do not
strongly depend on the diameter. Instead, they argue that the modulus
of the MWNTs correlates to the amount of
disorder in the nanotube walls. Not surprisingly, when MWNTs break, the outermost layers
break first.
CNTs Thermal Conductivity
and Expansion
New research from the University of Pennsylvania indicates
that CNTs may be the best
heat-conducting material man has ever known. Ultra-small SWNTs have even been shown to
exhibit superconductivity below 20°K. Research suggests that
these exotic strands, already heralded for their unparalleled strength
and unique ability to adopt the electrical properties of either
semiconductors or perfect metals, may someday also find applications as
miniature heat conduits in a host of devices and materials. The strong
in-plane graphitic C-C bonds make them exceptionally strong and stiff
against axial strains. The almost zero in-plane thermal expansion but
large inter-plane expansion of SWNTs implies strong in-plane
coupling and high flexibility against nonaxial strains. Many
applications of CNTs, such as in nanoscale
molecular electronics, sensing and actuating devices, or as reinforcing
additive fibers in functional composite materials, have been proposed.
Reports of several recent experiments on the preparation and
mechanical characterization of CNT-polymer composites have also
appeared. These measurements suggest modest enhancements in strength
characteristics of CNT-embedded matrixes as compared
to bare polymer matrixes. Preliminary experiments and simulation
studies on the thermal properties of CNTs show very high thermal
conductivity. It is expected, therefore, that nanotube reinforcements
in polymeric materials may also significantly improve the thermal and
thermo-mechanical properties of the composites.
CNTs Field Emission
Field emission results from the tunneling of electrons from a
metal tip into vacuum, under application of a strong electric field.
The small diameter and high aspect ratio of CNTs is very favorable for field
emission. Even for moderate voltages, a strong electric field develops
at the free end of supported CNTs because of their sharpness.
This was observed by de Heer and co-workers at EPFL in 1995. He also
immediately realized that these field emitters must be superior to
conventional electron sources and might find their way into all kind of
applications, most importantly flat-panel displays. It is remarkable
that after only five years Samsung actually realized a very bright
color display, which will be shortly commercialized using this
technology.
Studying the field emission properties of MWNTs, Bonard and co-workers at
EPFL observed that together with electrons light is emitted, as well.
This luminescence is induced by the electron field emission, since it
is not detected without applied potential. This light emission occurs
in the visible part of the spectrum, and can sometimes be seen with the
naked eye.
CNTs High Aspect Ratio
CNTs represent a very small, high
aspect ratio conductive additive for plastics of all types. Their high
aspect ratio means that a lower loading (concentration) of CNTs is needed compared to other
conductive additives to achieve the same electrical conductivity. This
low loading preserves more of the polymer resins’ toughness,
especially at low temperatures, as well as maintaining other key
performance properties of the matrix resin. CNTs have proven to be an
excellent additive to impart electrical conductivity in plastics. Their
high aspect ratio (about 1000:1) imparts electrical conductivity at
lower loadings, compared to conventional additive materials such as
carbon black, chopped carbon fiber, or stainless steel fiber.
Applications of Carbon
Nanotubes
The special nature of carbon combines with the molecular
perfection of single-wall CNTs to endow them
with exceptional material properties, such as very high electrical and
thermal conductivity, strength, stiffness, and toughness. No other
element in the periodic table bonds to itself in an extended network
with the strength of the carbon-carbon bond. The delocalized
pi-electron donated by each atom is free to move about the entire
structure, rather than remain with its donor atom, giving rise to the
first known molecule with metallic-type electrical conductivity.
Furthermore, the high-frequency carbon-carbon bond vibrations provide
an intrinsic thermal conductivity higher than even diamond.
In most materials, however, the actual observed material
properties - strength, electrical conductivity, etc. - are degraded
very substantially by the occurrence of defects in their structure. For
example, high-strength steel typically fails at only about 1% of its
theoretical breaking strength. CNTs, however, achieve values very
close to their theoretical limits because of their molecular perfection
of structure. This aspect is part of the unique story of CNTs. CNTs are an example of true
nanotechnology: they are only about a nanometer in diameter, but are
molecules that can be manipulated chemically and physically in very
useful ways. They open an incredible range of applications in materials
science, electronics, chemical processing, energy management, and many
other fields.
CNTs Thermal Conductivity
CNTs have extraordinary electrical
conductivity, heat conductivity, and mechanical properties. They are
probably the best electron field-emitter possible. They are polymers of
pure carbon and can be reacted and manipulated using the well-known and
tremendously rich chemistry of carbon. This provides opportunity to
modify their structure, and to optimize their solubility and
dispersion. Very significantly, CNTs are molecularly perfect,
which means that they are normally free of property-degrading flaws in
the nanotube structure. Their material properties can therefore
approach closely the very high levels intrinsic to them. These
extraordinary characteristics give CNTs potential in numerous
applications.
CNTs Field Emission
Applications
CNTs are the best known field
emitters of any material. This is understandable, given their high
electrical conductivity, and the incredible sharpness of their tip
(because the smaller the tip’s radius of curvature, the more
concentrated will be an electric field, leading to increased field
emission; this is the same reason lightning rods are sharp). The
sharpness of the tip also means that they emit at especially low
voltage, an important fact for building low-power electrical devices
that utilize this feature. CNTs can carry an astonishingly
high current density, possibly as high as 1013 A/cm2. Furthermore, the
current is extremely stable. An immediate application of this behavior
receiving considerable interest is in field-emission flat-panel
displays. Instead of a single electron gun, as in a traditional cathode
ray tube display, in CNT-based displays there is a
separate electron gun (or even many of them) for each individual pixel
in the display. Their high current density, low turn-on and operating
voltages, and steady, long-lived behavior make CNTs very attractive field
emitters in this application. Other applications utilizing the
field-emission characteristics of CNTs include general types of
low-voltage cold-cathode lighting sources, lightning arrestors, and
electron microscope sources.
CNTs Conductive Plastics
Much of the history of plastics over the last half-century
has involved their use as a replacement for metals. For structural
applications, plastics have made tremendous headway, but not where
electrical conductivity is required, because plastics are very good
electrical insulators. This deficiency is overcome by loading plastics
up with conductive fillers, such as carbon black and larger graphite
fibers (the ones used to make golf clubs and tennis rackets). The
loading required to provide the necessary conductivity using
conventional fillers is typically high, however, resulting in heavy
parts, and more importantly, plastic parts whose structural properties
are highly degraded. It is well-established that the higher the aspect
ratio of filler particles, the lower the loading required needed to
achieve a given level of conductivity. CNTs are ideal in this sense,
since they have the highest aspect ratio of any carbon fiber. In
addition, their natural tendency to form ropes provides inherently very
long conductive pathways even at ultra-low loadings.
Applications that exploit this behavior of CNTs include EMI/RFI shielding
composites; coatings for enclosures, gaskets, and other uses;
electrostatic dissipation (ESD); and antistatic materials and (even
transparent!) conductive coatings; and radar-absorbing materials for
low-observable (“stealth”) applications.
CNTs Energy Storage
CNTs have the intrinsic
characteristics desired in material used as electrodes in batteries and
capacitors, two technologies of rapidly increasing importance. CNTs have a tremendously high
surface area (~1000 m2/g!!), good electrical conductivity, and very
importantly, their linear geometry makes their surface highly
accessible to the electrolyte.
Research has shown that CNTs have the highest reversible
capacity of any carbon material for use in lithium-ion batteries. In
addition, CNTs are outstanding materials for
supercapacitor electrodes and are now being marketed for this
application.
CNTs also have applications in a
variety of fuel cell components. They have a number of properties,
including high surface area and thermal conductivity, which make them
useful as electrode catalyst supports in PEM fuel cells. They may also
be used in gas diffusion layers, as well as current collectors, because
of their high electrical conductivity. CNTs' high strength and
toughness-to-weight characteristics may also prove valuable as part of
composite components in fuel cells that are deployed in transport
applications, where durability is extremely important.
CNTs Conductive
Adhesives and Connectors
The same properties that make CNTs attractive as conductive
fillers for use in electromagnetic shielding, ESD materials, etc., make
them attractive for electronics packaging and interconnection
applications, such as adhesives, potting compounds, and coaxial cables
and other types of connectors.
CNTs Molecular Electronics
The idea of building electronic circuits out of the essential
building blocks of materials - molecules - has seen a revival the past
five years, and is a key component of nanotechnology. In any electronic
circuit, but particularly as dimensions shrink to the nanoscale, the
interconnections between switches and other active devices become
increasingly important. Their geometry, electrical conductivity, and
ability to be precisely derived, make CNTs the ideal candidates for the
connections in molecular electronics. In addition, they have been
demonstrated as switches themselves.
CNTs Thermal Materials
The record-setting anisotropic thermal conductivity of CNTs is enabling many applications
where heat needs to move from one place to another. Such an application
is found in electronics, particularly advanced computing, where
uncooled chips now routinely reach over 100°C.
The technology for creating aligned structures and ribbons of
CNTs is a step toward realizing
incredibly efficient heat conduits. In addition, composites with CNTs have been shown to
dramatically increase their bulk thermal conductivity, even at very
small loadings.
CNTs Structural Composites
The superior properties of CNTs are not limited to electrical
and thermal conductivities, but also include mechanical properties,
such as stiffness, toughness, and strength. These properties lead to a
wealth of applications exploiting them, including advanced composites
requiring high values of one or more of these properties.
CNTs Fibers and Fabrics
Fibers spun of pure CNTs have recently been
demonstrated and are undergoing rapid development, along with CNT composite fibers. Such super
strong fibers will have many applications including body and vehicle
armor, transmission line cables, woven fabrics and textiles. CNTs are also being used to make
textiles stain resistant.
CNT Catalyst Supports
CNTs intrinsically have an
enormously high surface area; in fact, for SWNTs every atom is not just on a
one surface - each atom is on two surfaces, the inside and outside of
the nanotube! Combined with the ability to attach essentially any
chemical species to their sidewalls (functionalization) provides an
opportunity for unique catalyst supports. Their electrical conductivity
may also be exploited in the search for new catalysts and catalytic
behavior.
CNTs Biomedical
Applications
The exploration of CNTs in biomedical applications is
just underway, but has significant potential. Since a large part of the
human body consists of carbon, it is generally though of as a very
biocompatible material. Cells have been shown to grow on CNTs, so they appear to have no
toxic effect. The cells also do not adhere to the CNTs, potentially giving rise to
applications such as coatings for prosthetics, as well as anti-fouling
coatings for ships.
The ability to functionalize (chemically modify) the
sidewalls of CNTs also leads to biomedical
applications such as vascular stents, and neuron growth and
regeneration. It has also been shown that a single strand of DNA can be
bonded to a nanotube, which can then be successfully inserted into a
cell.
CNTs Air and Water
Filtration
Many researchers and corporations have already developed CNT based air and water filtration
devices. It has been reported that these filters can not only block the
smallest particles but also kill most bacteria. This is another area
where CNTs have already been
commercialized and products are on the market now.
CNTs Ceramic Applications
A ceramic material reinforced with carbon nanotubes has been
made by materials scientists at UC Davis. The new material is far
tougher than conventional ceramics, conducts electricity and can both
conduct heat and act as a thermal barrier, depending on the orientation
of the nanotubes.
Ceramic materials are very hard and resistant to heat and
chemical attack, making them useful for applications such as coating
turbine blades, but they are also very brittle. The researchers mixed
powdered alumina (aluminum oxide) with 5 to 10 percent carbon nanotubes
and a further 5 percent finely milled niobium. The researchers treated
the mixture with an electrical pulse in a process called spark-plasma
sintering. This process consolidates ceramic powders more quickly and
at lower temperatures than conventional processes.
The new material has up to five times the fracture toughness
-- resistance to cracking under stress -- of conventional alumina. The
material shows electrical conductivity seven times that of previous
ceramics made with nanotubes. It also has interesting thermal
properties, conducting heat in one direction, along the alignment of
the nanotubes, but reflecting heat at right angles to the nanotubes,
making it an attractive material for thermal barrier coatings
Other CNT Applications
There is a wealth of other potential applications for CNTs, such as solar collection;
nanoporous filters; catalyst supports; and coatings of all sorts. There
are almost certainly many unanticipated applications for this
remarkable material that will come to light in the years ahead, and
which may prove to be the most important and valuable ones of all. Many
researchers are looking into conductive and or water proof paper made
with CNTs. CNTs have also been shown to
absorb Infrared light and may have applications in the I/R Optics
Industry.
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