Applications of Carbon Nanotubes

There are numerous carbon nanotubes properties and applications which take full advantage of CNTs unique properties of aspect ratio, mechanical strength, electrical and thermal conductivity. In this article, a list of properties and applications of carbon nanotubes has been presented.

Carbon Nanotubes Properties and Applications

Carbon Nanotubes

Carbon Nanotubes Properties

  • CNTs have high thermal conductivity
  • CNTs have high electrical conductivity
  • CNTs aspect ratio
  • CNTs are very elastic ~18% elongation to failure
  • CNTs have very high tensile strength
  • CNTs are highly flexible — can be bent considerably without damage
  • CNTs have a low thermal expansion coefficient
  • CNTs are good electron field emitters

Carbon Nanotubes Applications

  • CNTs field emission
  • CNTs thermal conductivity
  • CNTs energy storage
  • CNTs conductive properties
  • CNTs conductive adhesive
  • CNTs thermal materials
  • Molecular electronics based on CNTs
  • CNTs structural applications
  • CNTs fibers and fabrics
  • CNTs biomedical applications
  • CNTs Air & Water Filtration
  • CNTs catalyst supports
  • Other CNT applications

CNTs Electrical Conductivity

There has been significant practical interest in the conductivity of CNTs. CNTs with particular combinations of M and N (structural parameters indicating how much the nanotube is twisted) can be highly conducting, and hence can be considered as metallic. Their conductivity has been proved to be a function of their diameter as well as their chirality (degree of twist). CNTs can be either semi-conducting or metallic in their electrical behavior.

CNTs Electrical Conductivity

Conductivity in multi-walled nanotubes (MWNTs) is somewhat intricate. The conductivity of some types of “armchair”-structured CNTs appear to be superior to other metallic CNTs. Moreover, interwall reactions within MWNTs have been found to non-uniformly redistribute the current over individual tubes. However, the current does not change across different parts of metallic single-walled CNTs. However, the behavior of ropes of semi-conducting SWNTs is not similar, as the transport current changes immediately at different positions on the CNTs.

By placing electrodes at different parts of the CNTs, the resistivity and conductivity of ropes of SWNTs have been measured. The resistivity of the SWNT ropes was in the order of 10–4 ohm-cm at 27 °C. This shows that SWNT ropes are the most conductive carbon fibers known. SWNT ropes were able to achieve a current density of 107 A/cm2; however, theoretically, they should be able to sustain much higher stable current densities, as high as 1013 A/cm2.

It has been reported that individual SWNTs may have defects. Unexpectedly, these defects enable the SWNTs to act as transistors. In the same way, combining CNTs together might result in transistor-like devices. A nanotube with a natural junction (where a straight metallic section is joined to a chiral semiconducting section) acts as a rectifying diode, or a half-transistor in a single molecule. In addition, it has recently been reported that SWNTs can direct 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 graphene (a single sheet of graphite) form a planar honeycomb lattice, in which each atom is connected to three neighboring atoms by a strong chemical bond. These strong bonds make the basal-plane elastic modulus of graphite one of the largest among any known material. Therefore, CNTs are expected to be the ultimate high-strength fibers. SWNTs are stiffer compared to steel and are extremely resistant to damage from physical forces. When the tip of a nanotube is pressed, it bends without causing any damage to the tip, and on the removal of the force, the tip returns to its original state. Due to this property, CNTs are very useful as probe tips for very high-resolution scanning probe microscopy.

It has been quite difficult to quantify these effects, and an exact numerical value has not been agreed upon. An atomic force microscope (AFM) can be used to push the unanchored ends of a freestanding nanotube 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 around 1 TPa; however, this value has been uncertain, and a value as high as 1.8 TPa has been reported. Additionally, other values considerably higher than that have been reported. Different experimental measurement techniques might be the reason for the differences. Others have proven theoretically that the Young’s modulus depends on the chirality and size of the SWNTs, ranging from 1.22 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 noticed that the modulus measurements of MWNTs using AFM techniques do not have a strong dependence on the diameter. Instead, they argue that the modulus of the MWNTs and the amount of disorder in the nanotube walls are correlated. As expected, when MWNTs break, the outermost layers break first.

CNTs Thermal Conductivity and Expansion

New research from the University of Pennsylvania signifies that CNTs may be the best heat-conducting material ever known to mankind. Ultra-small SWNTs have been shown to exhibit superconductivity even below 20 K. Research suggests that these exotic strands, already heralded for their incomparable strength and unique ability to adopt the electrical properties of either perfect metals or semiconductors, may soon also find applications as miniature heat conduits in a host of materials and devices. Due to the strong in-plane graphitic C-C bonds, they are made remarkably stiff and strong against axial strains. The almost zero in-plane thermal expansion but large inter-plane expansion of SWNTs implies high flexibility and strong in-plane coupling against nonaxial strains. Many applications of CNTs, such as in sensing and actuating devices, nanoscale molecular electronics, or as reinforcing additive fibers in functional composite materials, have been proposed.

Reports of many recent experiments on the preparation and mechanical characterization of CNT-polymer composites have also been presented. These measurements imply modest improvements in strength characteristics of CNT-embedded matrixes in comparison with bare polymer matrixes. Preliminary experiments and simulation studies on the thermal properties of CNTs show very high thermal conductivity. Therefore, nanotube reinforcements in polymeric materials are expected to considerably improve the thermal and thermo-mechanical properties of the composites.

CNTs Field Emission

Field emission is associated with the tunneling of electrons from a metal tip into vacuum, under application of a strong electric field. The high aspect ratio and small diameter of CNTs are very suitable for field emission. A strong electric field is developed at the free end of supported CNTs even for moderate voltages due to their sharpness. De Heer and co-workers observed this at EPFL in 1995. He also immediately realized that these field emitters must be superior to traditional electron sources and might find their way into all kind of applications, most significantly flat-panel displays. It is notable that Samsung actually accomplished a very bright color display only after five years, which will be soon commercialized using this technology.

CNTs Field Emission

During their research on field emission properties of MWNTs, Bonard and co-workers at EPFL observed that light is also emitted along with electrons. This luminescence is induced by the electron field emission because it is not detected when potential is not applied. This light is emitted in the visible part of the spectrum and can sometimes be seen with the naked eye.

CNTs Field Emission

CNTs High Aspect Ratio

CNTs represent a very small, high-aspect-ratio conductive additive for all kinds of plastics. Their high aspect ratio means that a lower loading (concentration) of CNTs is required to realize the same electrical conductivity when compared to other conductive additives. This low loading not only preserves more of the polymer resins’ toughness, especially at low temperatures but also maintains other main performance properties of the matrix resin. CNTs have been established to be an outstanding additive to impart electrical conductivity in plastics. Thanks to their high aspect ratio (about 1000:1), electrical conductivity can be imparted at lower loadings, compared to traditional additive materials such as chopped carbon fiber, stainless steel fiber, or carbon black.

Applications of Carbon Nanotubes

The unique nature of carbon combines with the molecular perfection of single-wall CNTs to endow them with extraordinary material properties, such as very high thermal and electrical conductivity, stiffness, strength, and toughness. It is the only element in the periodic table which 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 whole structure, instead of remaining with its donor atom, resulting in the first known molecule with metallic-type electrical conductivity. Moreover, an intrinsic thermal conductivity higher than even diamond is offered by the high-frequency carbon-carbon bond vibrations.

Carbon Nanotube

In most materials, however, due to the occurrence of defects in their structure, the actual observed material properties such as strength, electrical conductivity, and so on are degraded very significantly. For example, high-strength steel typically fails at only around 1% of its theoretical breaking strength. However, CNTs achieve values very near to their theoretical limits owing to their molecular perfection of structure. This aspect is part of the unique story of CNTs. CNTs are examples of true nanotechnology: they are only about a nanometer in diameter, but are molecules that can be manipulated physically and chemically in very useful ways. They find an incredible range of applications in electronics, materials science, energy management, chemical processing, and many other fields.

CNTs Thermal Conductivity

CNTs have outstanding heat conductivity, electrical conductivity, and mechanical properties. They are probably the best electron field-emitter possible. They are polymers of pure carbon and can be made to and manipulated using the recognized and extremely rich chemistry of carbon. This offers the opportunity to alter their structure and to optimize their dispersion and solubility. Most notably, CNTs are molecularly perfect, in the sense that they are generally free of property-degrading flaws in the nanotube structure. Their material properties can thus reach close to the very high levels intrinsic to them. Due to these extraordinary characteristics, CNTs can be prospectively used in a number of applications.

CNTs Field Emission Applications

CNTs are the best known field emitters of any material. This is understandable, with regard to their high electrical conductivity, and the unbelievable sharpness of their tip (as the tip’s radius of curvature becomes smaller, the electric field will be more concentrated, resulting in increased field emission; this is the same reason lightning rods are sharp). In addition, the sharpness of the tip also indicates that they emit at specifically low voltage, a key fact for building low-power electrical devices that employ this feature. CNTs can carry an amazingly high current density, probably as high as 1013 A/cm2. Additionally, the current is extremely stable. Field-emission flat-panel displays are an immediate application of this behavior, receiving considerable interest. Unlike conventional cathode ray tube display where a single electron gun is used, CNT-based displays use a separate electron gun (or even many of them) for each individual pixel in the display. Their low turn-on and operating voltages, high current density, and steady, long-lived behavior make CNTs very attractive field emitters in this application. General types of low-voltage cold-cathode lighting sources, electron microscope sources, and lightning arrestors are other applications utilizing the field-emission characteristics of CNTs. [B.Q. Wei, et al, Appl. Phys. Lett. 79 1172 (2001)].

CNTs Conductive Plastics

Over the past five decades, much of the history of plastics has involved their use as a substitute for metals. For structural applications, plastics have progressed tremendously, but not where electrical conductivity is needed, since plastics are very good electrical insulators. This deficiency can be ruled out 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). In order to offer the necessary conductivity using conventional fillers, the loading required is typically high, however, leading to heavy parts and, more prominently, plastic parts whose structural properties are highly degraded. It is well known that as the aspect ratio of filler particles becomes high, the loading required to achieve a given level of conductivity becomes low. For this reason, CNTs are perfect because they have the highest aspect ratio of any carbon fiber. Furthermore, their natural tendency to form ropes offers inherently very long conductive pathways even at ultra-low loadings.

This behavior of CNTs is utilized in applications such as electrostatic dissipation (ESD); EMI/RFI shielding composites; coatings for gaskets, enclosures, and other uses; radar-absorbing materials for low-observable (“stealth”) applications; and antistatic materials and (even transparent!) conductive coatings.

CNTs Energy Storage

The intrinsic properties of CNTs make them the preferred material for use as electrodes in capacitors and batteries — two technologies of fast-growing significance. CNTs possess good electrical conductivity, an extremely high surface area (~1000 m2/g), and most importantly, their linear geometry makes their surface very accessible to the electrolyte.

Research has demonstrated that CNTs have the highest reversible capacity of any carbon material for use in lithium-ion batteries [B. Gao, Chem. Phys. Lett. 327, 69 (2000)]. Moreover, CNTs are excellent materials for supercapacitor electrodes [R.Z. Ma, et al., Science in China Series E-Technological Sciences 43 178 (2000)] and are currently being marketed for this application.

In addition, CNTs hold applications in various fuel cell components. They have several properties, such as high thermal conductivity and surface area, making them valuable as electrode catalyst supports in PEM fuel cells. Owing to their high electrical conductivity, they may also be used in gas diffusion layers, besides current collectors. The high strength and toughness-to-weight characteristics of CNTs may also prove useful as part of composite components in fuel cells that are used in transport applications, where durability is paramount.

CNTs Conductive Adhesives and Connectors

The exact properties that make CNTs desirable as conductive fillers for use in ESD materials, electromagnetic shielding, and so on make them suitable for interconnection applications and electronics packaging, including coaxial cables, potting compounds, and adhesives and other types of connectors.

CNTs Molecular Electronics

The idea of building electronic circuits out of the critical building blocks of materials — molecules — has seen growth in the past five years, and is a vital part of nanotechnology. In any electronic circuit, but specifically when dimensions reduce in size to the nanoscale, the interconnections between switches and other active devices become more and more essential. Their ability to be precisely derived, electrical conductivity, and geometry make CNTs the most suitable candidates for the connections in molecular electronics. Furthermore, they have been shown as switches themselves.

CNTs Thermal Materials

The record-setting anisotropic thermal conductivity of CNTs is opening doors to several applications that involve heat transfer. Such an application is found in electronics, specifically advanced computing, where uncooled chips currently regularly exceed 100 °C.

The technology for creating aligned structures and ribbons of CNTs [D.Walters, et al., Chem. Phys. Lett. 338, 14 (2001)] is a step toward achieving extremely efficient heat conduits. Furthermore, composites with CNTs have been demonstrated to significantly increase their bulk thermal conductivity, even at incredibly small loadings.

CNTs Structural Composites

The superior properties of CNTs are not just restricted to thermal and electrical conductivities but also include mechanical properties, such as strength, toughness, and stiffness. These properties pave the way for use in a range of applications exploiting them, including advanced composites that need high values of one or more of these properties.

CNTs Fibers and Fabrics

Recently, fibers spun from pure CNTs have been demonstrated [R.H. Baughman, Science 290, 1310 (2000)] and are experiencing rapid development, together with CNT composite fibers. Such super strong fibers will have several applications such as woven fabrics and textiles, transmission line cables, and body and vehicle armor. CNTs are also being employed in order to make textiles stain resistant.

CNT Catalyst Supports

CNTs intrinsically possess an enormously high surface area; actually, for SWNTs, every atom is not just on one surface — but two surfaces, the interior and exterior of the nanotube. Along with the ability to attach basically any chemical species to their sidewalls (functionalization) offers a prospect for unique catalyst supports. Their electrical conductivity may also be used propitiously in the quest for new catalysts and catalytic behavior.

CNTs Biomedical Applications

Although the exploration of CNTs in biomedical applications is just in progress, it has great potential. Since a great part of the human body is made up of carbon, it is usually considered a very biocompatible material. The growth of cells on CNTs has been demonstrated; therefore, they apparently have no toxic effect. The cells also do not adhere to the CNTs, opening doors for applications such as anti-fouling coatings for ships and coatings for prosthetics.

The ability to functionalize (chemically modify) the sidewalls of CNTs also gives rise to biomedical applications including neuron growth and regeneration, and vascular stents. It has also been demonstrated that a single strand of DNA can be bonded to a nanotube, which can subsequently be effectively inserted into a cell.

CNTs Air and Water Filtration

Several corporations and researchers have already developed CNT-based water and air filtration devices. It has been reported that these filters, apart from blocking the tiniest particles, can also destroy most bacteria. This is one more area where CNTs have already been commercialized and products are available now.

CNTs Ceramic Applications

Materials scientists at UC Davis have produced a ceramic material reinforced with carbon nanotubes. The new material is significantly tougher than traditional ceramics, conducts electricity, and can both conduct heat and function as a thermal barrier, with respect to the nanotube orientation.

Since ceramic materials are very hard and resistant to heat and chemical attack, they are valuable for applications such as coating turbine blades; however, they are also very brittle. The researchers mixed powdered alumina (aluminum oxide) with 5%–10% carbon nanotubes, in addition to 5% finely milled niobium. The mixture was treated with an electrical pulse in a process called spark-plasma sintering by the researchers. This process collates ceramic powders more rapidly and at lower temperatures than traditional processes.

The fracture toughness (resistance to cracking under stress) of the new material is up to five times of that of traditional alumina. The material exhibits electrical conductivity seven times of that of earlier ceramics made with nanotubes. It also has fascinating thermal properties, conducting heat in one direction, along the alignment of the nanotubes and, on the other hand, reflecting heat at right angles to the nanotubes, making it a preferred material for thermal barrier coatings.

Other Carbon Nanotubes Applications

There are several other potential applications for CNTs, including solar collection, nanoporous filters, catalyst supports, and all kinds of coatings. There are almost certainly several surprising applications for this excellent material that will be revealed in the future, and which may prove to be the most significant and valuable ones of all. A number of researchers have been studying the conductive and/or waterproof paper produced using CNTs. CNTs have also been demonstrated to absorb infrared light and may hold applications in the I/R optics industry.

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