Carbon nanotubes (CNTs) are cylinders of one or more layers of graphene (lattice). Diameters of single-walled carbon nanotubes (SWNTs) and multi-walled carbon nanotubes (MWNTs) are typically 0.8 to 2 nm and 5 to 20 nm, respectively, although MWNT diameters can exceed 100 nm. CNT lengths range from less than 100 nm to 0.5 m.
Individual CNT walls can be metallic or semiconducting depending on the orientation of the lattice with respect to the tube axis, which is called chirality. MWNT's cross-sectional area offers an elastic modulus approaching 1 TPa and a tensile strength of 100 GPa, over 10-fold higher than any industrial fiber. MWNTs are typically metallic and can carry currents of up to 109 A cm−2. SWNTs can display thermal conductivity of 3500 W m−1 K−1, exceeding that of diamond.
As of 2013[ [update]], carbon nanotube production exceeded several thousand tons per year, used for applications in energy storage, device modelling, automotive parts, boat hulls, sporting goods, water filters, thin-film electronics, coatings, actuators and electromagnetic shields. CNT-related publications more than tripled in the prior decade, while rates of patent issuance also increased. Most output was of unorganized architecture. Organized CNT architectures such as "forests", yarns and regular sheets were produced in much smaller volumes. CNTs have even been proposed as the tether for a purported space elevator.
Recently, several studies have highlighted the prospect of using carbon nanotubes as building blocks to fabricate three-dimensional macroscopic ( ;1 mm in all three dimensions) all-carbon devices. Lalwani et al. have reported a novel radical initiated thermal crosslinking method to fabricated macroscopic, free-standing, porous, all-carbon scaffolds using single- and multi-walled carbon nanotubes as building blocks. These scaffolds possess macro-, micro-, and nano- structured pores and the porosity can be tailored for specific applications. These 3D all-carbon scaffolds/architectures may be used for the fabrication of the next generation of energy storage, supercapacitors, field emission transistors, high-performance catalysis, photovoltaics, and biomedical devices and implants.
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Biological and biomedical research.
Researchers from Rice University and State University of New York – Stony Brook have shown that the addition of low weight % of carbon nanotubes can lead to significant improvements in the mechanical properties of biodegradable polymeric nanocomposites for applications in tissue engineering including bone, cartilage, muscle and nerve tissue. Dispersion of low weight % of graphene (~0.02 wt.%) results in significant increases in compressive and flexural mechanical properties of polymeric nanocomposites. Researchers at Rice University, Stony Brook University, Radboud University Nijmegen Medical Centre and University of California, Riverside have shown that carbon nanotubes and their polymer nanocomposites are suitable scaffold materials for bone tissue engineering and bone formation.
Compared to the commonly used polyurethane in prosthetic feet, CNT natural rubber composites with just 1% carbon nanotubes show a 25% boost in tensile strength. Additionally, this small CNT addition improves dimensional stability by 20% and heat resistance by 15% over natural rubber.
CNTs exhibit dimensional and chemical compatibility with biomolecules, such as DNA and proteins. CNTs enable fluorescent and photoacoustic imaging, as well as localized heating using near-infrared radiation.
SWNT biosensors exhibit large changes in electrical impedance and optical properties, which is typically modulated by adsorption of a target on the CNT surface. Low detection limits and high selectivity require engineering the CNT surface and field effects, capacitance, Raman spectral shifts and photoluminescence for sensor design. Products under development include printed test strips for estrogen and progesterone detection, microarrays for DNA and protein detection and sensors for NO2 and cardiac troponin. Similar CNT sensors support food industry, military and environmental applications.
CNTs can be internalized by cells, first by binding their tips to cell membrane receptors. This enables transfection of molecular cargo attached to the CNT walls or encapsulated by CNTs. For example, the cancer drug doxorubicin was loaded at up to 60 wt % on CNTs compared with a maximum of 8 to 10 wt % on liposomes. Cargo release can be triggered by near-infrared radiation. However, limiting the retention of CNTs within the body is critical to prevent undesirable accumulation.
CNT toxicity remains a concern, although CNT biocompatibility may be engineerable. The degree of lung inflammation caused by injection of well-dispersed SWNTs was insignificant compared with asbestos and with particulate matter in air. Medical acceptance of CNTs requires understanding of immune response and appropriate exposure standards for inhalation, injection, ingestion and skin contact. CNT forests immobilized in a polymer did not show elevated inflammatory response in rats relative to controls. CNTs are under consideration as low-impedance neural interface electrodes and for coating of catheters to reduce thrombosis.
CNT enabled x-ray sources for medical imaging are also in development. Relying on the unique properties of the CNTs, researchers have developed field emission cathodes that allow precise x-ray control and close placement of multiple sources. CNT enabled x-ray sources have been demonstrated for pre-clinical, small animal imaging applications, and are currently in clinical trials.
In November 2012 researchers at the American National Institute of Standards and Technology (NIST) proved that single-wall carbon nanotubes may help protect DNA molecules from damage by oxidation.
A highly effective method of delivering carbon nanotubes into cells is Cell squeezing, a high-throughput vector-free microfluidic platform for intracellular delivery developed at the Massachusetts Institute of Technology in the labs of Robert S. Langer.
Carbon nanotubes have furthermore been grown inside microfluidic channels for chemical analysis, based on electrochromatography. Here, the high surface-area-to-volume ratio and high hydrophobicity of CNTs are used in order to greatly decrease the analysis time of small neutral molecules that typically require large bulky equipment for analysis.
Composite materials.
Because of the carbon nanotube's superior mechanical properties, many structures have been proposed ranging from everyday items like clothes and sports gear to combat jackets and space elevators. However, the space elevator will require further efforts in refining carbon nanotube technology, as the practical tensile strength of carbon nanotubes must be greatly improved.
Applications of CNTs/polymer composites in the automotive sector highlight technological advancements across various systems, particularly in body components, electrical systems, and engine parts. Incorporating CNTs with fiberglass reinforcement in epoxy composites can boost strength by 60% and impact energy resistance by 30%. These improvements could significantly reduce fuel consumption by 16% and greenhouse gas emissions by 26%. Additionally, CNT fillers, effective even at low concentrations (0.2 wt.%), enhance dimensional and thermal stability while reducing overall weight. A 25% reduction in vehicle weight could save up to 250 milion barrels of crude oil annually.
For perspective, outstanding breakthroughs have already been made. Pioneering work led by Ray H. Baughman at the NanoTech Institute has shown that single and multi-walled nanotubes can produce materials with toughness unmatched in the man-made and natural worlds.
Carbon nanotubes are also a promising material as building blocks in hierarchical composite materials given their exceptional mechanical properties (~1 TPa in modulus, and ~100 GPa in strength). Initial attempts to incorporate CNTs into hierarchical structures (such as yarns, fibres or films) has led to mechanical properties that were significantly lower than these potential limits. The hierarchical integration of multi-walled carbon nanotubes and metal/metal oxides within a single nanostructure can leverage the potentiality of carbon nanotubes composite for water splitting and electrocatalysis. Windle "et al." have used an "in situ" chemical vapor deposition (CVD) spinning method to produce continuous CNT yarns from CVD-grown CNT aerogels. CNT yarns can also be manufactured by drawing out CNT bundles from a CNT forest and subsequently twisting to form the fibre (draw-twist method, see picture on right). The Windle group have fabricated CNT yarns with strengths as high as ~9 GPa at small gage lengths of ~1 mm, however, strengths of only about ~1 GPa were reported at the longer gage length of 20 mm. The reason why fibre strengths have been low compared to the strength of individual CNTs is due to a failure to effectively transfer load to the constituent (discontinuous) CNTs within the fibre. One potential route for alleviating this problem is via irradiation (or deposition) induced covalent inter-bundle and inter-CNT cross-linking to effectively 'join up' the CNTs, with higher dosage levels leading to the possibility of amorphous carbon/carbon nanotube composite fibres. Espinosa "et al." developed high performance DWNT-polymer composite yarns by twisting and stretching ribbons of randomly oriented bundles of DWNTs thinly coated with polymeric organic compounds. These DWNT-polymer yarns exhibited an unusually high energy to failure of ~100 J·g−1 (comparable to one of the toughest natural materials – spider silk), and strength as high as ~1.4 GPa. Effort is ongoing to produce CNT composites that incorporate tougher matrix materials, such as Kevlar, to further improve on the mechanical properties toward those of individual CNTs.
Because of the high mechanical strength of carbon nanotubes, research is being made into weaving them into clothes to create stab-proof and bulletproof clothing. The nanotubes would effectively stop the bullet from penetrating the body, although the bullet's kinetic energy would likely cause broken bones and internal bleeding.
Carbon nanotubes can also enable shorter processing times and higher energy efficiencies during composite curing with the use of carbon nanotube structured heaters. Autoclaving is the ‘gold standard’ for composite curing however, it comes at a high price and introduces part size limitations. Researchers estimate that curing a small section of the Boeing 787 carbon fiber/epoxy fuselage requires 350 GJ of energy and produces 80 tons of carbon dioxide. This is about the same amount of energy that nine households would consume in one year. In addition, eliminating part size limitations eliminates the need to join small composite components to create large scale structures. This saves manufacturing time and results in higher strength structures.
Carbon nanotube structured heaters show promise in replacing autoclaves and conventional ovens for composite curing because of their ability to reach high temperatures at fast ramping rates with high electrical efficiency and mechanical flexibility. These nanostructured heaters can take the form of a film and be applied directly to the composite. This results in conductive heat transfer as opposed to convective heat transfer used by autoclaves and conventional ovens. Lee et al. reported that only 50% of the thermal energy introduced in an autoclave is transferred to the composite being cured regardless of part size, while about 90% of the thermal energy is transferred in a nanostructured film heater depending on the process.
Lee et al. were able to successfully cure aerospace-grade composites using a CNT heater made by “domino-pushing” a CNT forest onto a Teflon film. This film was then laid on top of an 8-ply OOA prepreg layup. Thermal insulation was incorporated around the assembly. The entire setup was subsequently vacuum bagged and heated using a 30 V DC power supply. Degree-of-cure and mechanical tests were conducted to compare conventionally cured composites against their OOA set-up. Results showed that there was no difference in the quality of the composite created. However, the amount of energy required to cure the composite OOA was reduced by two orders of magnitude from 13.7 MJ to 118.8 kJ.
Before carbon nanotubes can be used to cure Boeing 787 fuselages however, further development needs to occur. The largest challenge associated with creating reliable carbon nanotube structured heaters is being able to create a uniform carbon nanotube dispersion in a polymer matrix to ensure heat is applied evenly. CNTs high surface area results in strong Van Der Waals forces between individual CNTs, causing them to agglomerate together and yielding non-uniform heating properties. In addition, the polymer matrix chosen needs to be carefully chosen such that it can withstand the high temperatures generated and the repetitive thermal cycling required to cure multiple composite components.
Mixtures.
MWNTs were first used as electrically conductive fillers in metals, at concentrations as high as 83.78 percent by weight (wt%). MWNT-polymer composites reach conductivities as high as 10,000 S m−1 at 10 wt % loading. In the automotive industry, CNT plastics are used in electrostatic-assisted painting of mirror housings, as well as fuel lines and filters that dissipate electrostatic charge. Other products include electromagnetic interference (EMI)–shielding packages and silicon wafer carriers.
For load-bearing applications, CNT powders are mixed with polymers or precursor resins to increase stiffness, strength and toughness. These enhancements depend on CNT diameter, aspect ratio, alignment, dispersion and interfacial interaction. Premixed resins and master batches employ CNT loadings from 0.1 to 20 wt%. Nanoscale stick-slip among CNTs and CNT-polymer contacts can increase material damping, enhancing sporting goods, including tennis racquets, baseball bats and bicycle frames.
CNT resins enhance fiber composites, including wind turbine blades and hulls for maritime security boats that are made by enhancing carbon fiber