Potential applications of carbon nanotubes

Part of a series of articles on Nanomaterials Fullerenes Carbon nanotubes Buckminsterfullerene Fullerene chemistry Applications In popular culture Timeline Carbon allotropes Nanoparticles Quantum dots Nanostructures Colloidal gold Silver nanoparticles Iron nanoparticles Platinum nanoparticles See also Nanotechnology

Carbon nanotubes, a type of fullerene, have potential in fields such as nanotechnology, electronics, optics, materials science, and architecture. Over the years new applications have taken advantage of their unique electrical properties, extraordinary strength, and efficiency in heat conduction.

Contents 1 Structural 2 Electromagnetic 3 Electroacoustic 4 Chemical 5 Mechanical 6 Electrical circuits 6.1 Interconnects 6.2 Transistors 6.3 Electronic design and design automation 7 Medicine 8 References 9 External links

Structural

Carbon nanotubes have valuable qualities as structural materials. Potential uses include:

• Textiles—CNT can make waterproof and/or tear-resistant fabrics
• Body armor—MIT is working on combat jackets that use CNT fibers to stop bullets and to monitor the condition of the wearer.^[1] Cambridge University developed the fibres and licensed a company to make them.^[2]
• Concrete—CNT in concrete increase its tensile strength, and halt crack propagation.^[3]
• Polyethylene—Adding CNT to polyethylene can increase the polymer's elastic modulus by 30%.
• Sports equipment—Stronger and lighter tennis rackets, bicycle parts, golf balls, golf clubs, and baseball bats.
• Space elevator—CNT are under investigation as possible components of the tether up which a space elevator can climb. This requires tensile strengths of more than about 70 GPa.
• synthetic muscles: Due to their high contraction/extension ratio given an electric current, CNTs are ideal for synthetic muscle.^[4]
• High tensile strength fibers—Fibers produced with polyvinyl alcohol required 600 J/g to break.^[5] In comparison, the bullet-resistant fiber Kevlar fails at 27–33 J/g.
• Bridges—CNT may be able to replace steel in suspension and other bridges.
• Flywheels—The high strength/weight ratio enables very high rotational speeds.
• Carbon nanotube springs—Single-walled carbon nanotubes aligned in parallel can be elastically stretched for an energy density 10 times greater than that of current lithium-ion batteries, with the additional advantages of long cycling durability, temperature insensitivity, no spontaneous dischange, and arbitrary discharge rate.
• Fire protection—Thin layers of buckypaper can significantly improve fire resistance due to the efficient reflection of heat by the dense, compact layer of CNT or carbon fibers.^[6]

Electromagnetic

CNT can be fabricated as electrical conductors, insulators, and semiconductors. Applications include:

• Artificial muscles—CNT's have sufficient contractility to make them candidates to replace muscle tissue.^[7]
• Buckypaper—Thin nanotube sheets are 250 times stronger than steel and 10 times lighter and could be used as a heat sink for chipboards, a backlight for LCD screens or as a faraday cage to protect electrical devices/aeroplanes.
• Chemical nanowires—CNTs can be used to produce nanowires of other elements/molecules, such as gold or zinc oxide. These nanowires in turn can be used to cast nanotubes of other chemicals, such as gallium nitride. These can have very different properties from CNTs—for example, gallium nitride nanotubes are hydrophilic, while CNTs are hydrophobic, giving them possible uses in organic chemistry.
• Conductive films— Canatu ^[8] of Helsinki, Finland, Eikos Inc of Franklin, Massachusetts and Unidym Inc.^[9] of Silicon Valley are developing transparent, electrically conductive CNT films and NanoBuds to replace indium tin oxide (ITO) in LCDs, touch screens, and photovoltaic devices. Nanotube films show promise for use in displays for computers, cell phones, Personal digital assistants, and automated teller machines.
• Electric motor brushes—Conductive CNTs are used in brushes for commercial electric motors. They replace traditional carbon black. The nanotubes improve electrical and thermal conductivity because they stretch through the plastic matrix of the brush. This permits the carbon filler to be reduced from 30% down to 3.6%, so that more matrix is present in the brush. Nanotube composite motor brushes are better-lubricated (from the matrix), cooler-running (both from better lubrication and superior thermal conductivity), less brittle (more matrix, and fiber reinforcement), stronger and more accurately moldable (more matrix). Since brushes are a critical failure point in electric motors, and also don't need much material, they became economical before almost any other application.
• Light bulb filament: alternative to tungsten filaments in incandescent lamps.
• Magnets—Multi-walled nanotubes (MWNT coated with magnetite can generate strong magnetic fields.)
• Optical ignition—A layer of 29% iron enriched single-walled nanotubes (SWNT) is placed on top of a layer of explosive material such as PETN, and can be ignited with a regular camera flash.^[10]
• Solar cells—GE's CNT diode exploits a photovoltaic effect. Nanotubes can replace ITO in some solar cells to act as a transparent conductive film in solar cells to allow light to pass to the active layers and generate photocurrent.
• Superconductor—Nanotubes have been shown to be superconducting at low temperatures.^[11]
• Ultracapacitors—MIT is researching the use of nanotubes bound to the charge plates of capacitors in order to dramatically increase the surface area and therefore energy storage ability.^[12]
• Displays—CNTs can be used as extremely fine electron guns, which could be used as miniature cathode ray tubes in thin high-brightness, low-energy, low-weight displays. This type of display would consist of a group of many tiny CRTs, each providing the electrons to hit the phosphor of one pixel, instead of having one giant CRT whose electrons are aimed using electric and magnetic fields. These displays are known as field emission displays (FEDs).
• Transistor—CNT transistors have been developed at Delft, IBM, and NEC.
• Electromagnetic antenna—CNTs can act as antennas for radios and other electromagnetic devices.^[13]

Electroacoustic

• Loudspeaker—In November, 2008 Tsinghua-Foxconn Nanotechnology Research Centre in Beijing announced that it had created loudspeakers from sheets of parallel CNT, generating sound similar to how lightning produces thunder. Near-term commercial uses include replacing piezoelectric speakers in greeting cards.^[14]

Chemical

• Desalination— water molecules can be separated from salt by forcing them through networks of carbon nanotubes, which require far lower pressures than conventional reverse osmosis methods ^[15]
• Air pollution filter—CNT membranes can filter carbon dioxide from power plant emissions.
• Biotech container—CNT can be filled with biological molecules, aiding biotechnology.
• Hydrogen storage—CNT have the potential to store between 4.2 and 65% hydrogen by weight. If they can be mass produced economically, 13.2 litres (2.9 imp gal; 3.5 US gal) of CNT could contain the same amount of energy as a 50 litres (11 imp gal; 13 US gal) gasoline tank. See Hydrogen Economy.

Mechanical

• Oscillator—Oscillators based on CNT have achieved higher speeds than other technologies (> 50 GHz).
• Nanotube membrane—Liquid flows up to five orders of magnitude faster than predicted by classical fluid dynamics.
• Slick surface—Some CNT-based fabrics have shown lower friction than Teflon.
• Waterproof—Some CNT-fabrics are waterproof.
• Carbon nanotube actuators—
• Infrared detector—The reflectivity of the buckypaper produced with "super-growth" chemical vapor deposition method is 0.03 or less, potentially enabling performance gains for pyroelectric infrared detector.^[16][17]
• Radiometric standard—As a standard of the black.
• Thermal radiation—For thermal emission in space such as space satellites.
• stealth—Absorbance is high in wide ranges from FUV to FIR.

Electrical circuits

A nanotube formed by joining two nanotubes of different diameters end to end can act as a diode, suggesting the possibility of constructing computer circuits entirely of nanotubes. Because of their good thermal transmission properties, CNT can potentially dissipate heat from computer chips. The longest electricity conducting circuit is a fraction of an inch long.^[18]

Fabrication difficulties are major hurdles for CNT. Standard IC fabrication processes use chemical vapor deposition to add layers to a wafer. CNT can so far not be mass produced using such techniques.

Researchers can manipulate nanotubes one-by-one with the tip of an atomic force microscope in a time-consuming process. Using standard fabrication techniques would still require designers to position one end of the nanotube. During the deposition process, an electric field can potentially direct the growth of the nanotubes, which tend to grow along the field lines from negative to positive polarity. Another technique for self-assembly uses chemical or biological techniques to move CNT in solution to determinate places on a substrate.

Even if nanotubes can be precisely positioned, engineers have been unable to control the types (conducting, semiconducting, SWNT, MWNT) of nanotubes that appear.

Interconnects

Metallic carbon nanotubes have aroused research interest for their applicability as very-large-scale integration (VLSI) interconnects because of their high thermal stability, high thermal conductivity and large current carrying capacity.^{[19][20][21][22][23][24]} An isolated CNT can carry current densities in excess of 1000 MA/sq-cm without damage even at an elevated temperature of 250 °C (482 °F), eliminating electromigration reliability concerns that plague Cu interconnects. Recent modeling work comparing the two has shown that CNT bundle interconnects can potentially offer advantages over copper.^[25] Recent experiments demonstrated resistances as low as 20 Ohms using different architectures,^[26] detailed conductance measurements over a wide temperature range were shown to agree with theory for a strongly disordered quasi-one-dimensional conductor.

Hybrid interconnects that employ CNT vias in tandem with copper interconnects offers advantages from a reliability/thermal-management perspective.

Transistors

Semiconducting CNTs have been used to fabricate field effect transistors (CNTFETs), which show promise due to their superior electrical characteristics over silicon based MOSFETs. Since the electron mean free path in SWCNTs can exceed 1 micrometer, long channel CNTFETs exhibit near-ballistic transport characteristics, resulting in high speed devices. CNT devices are projected to operate in the frequency range of hundreds of Gigaherz. Recent work detailing the advantages and disadvantages of various forms of CNTFETs have also shown that tunneling CNTFET offers better characteristics compared to other CNTFET structures. This device has been found to be superior in terms of subthreshold slope - a very important property for low power applications.^{[27][28][29][30][31][32]}

Nanotubes are usually grown on nanoparticles of magnetic metal (Fe, Co) that facilitates production of electronic (spintronic) devices. In particular control of current through a field-effect transistor by magnetic field has been demonstrated in such a single-tube nanostructure.^[33]

Electronic design and design automation

Although CNT devices and interconnects separately have been shown to be promising in their own respects, there have been few efforts to combine them in a realistic circuit. Most CNTFET structures employ the silicon substrate as a back gate. Applying different back gate voltages might become a concern when designing large circuits out of these elements. Several top-gated structures have also been demonstrated, which can alleviate this concern. Recently, a fully integrated logic circuit built on a single nanotube was reported. This circuit employs a back-gate. Several process-related challenges need to be addressed before CNT-based devices and interconnects can enter mainstream VLSI manufacturing. Remaining problems include purification, separation, control over length, chirality and desired alignment, low thermal budget and high contact resistance. Innovative ideas have been proposed to build practical transistors out of nano-networks. Since lack of control on chirality produces a mix of metallic as well as semi-conducting CNTs from any fabrication process and it is difficult to control the growth direction of the CNTs, easily-produced random arrays of SWCNTs have been proposed to build thin film transistors. This idea can be further exploited to build practical CNT based transistors and circuits without the need for precise growth and assembly.

Medicine

Research at University of California, Riverside has shown that carbon nanotubes are suitable scaffold materials for osteoblast proliferation and bone formation.^[34]

References

Specific references:

External links

• Lecture by Ray Baughman (YouTube)
• Applications of Carbon Nanotubes