DWNT synthesis on the gram-scale was first proposed in  by the CCVD technique, from the selective reduction of oxides solid solutions in methane and hydrogen. Fullerites are the solid-state manifestation of fullerenes and related compounds and materials.
Being highly incompressible nanotube forms, polymerized single-walled nanotubes P-SWNT are a class of fullerites and are comparable to diamond in terms of hardness. However, due to the way that nanotubes intertwine, P-SWNTs don't have the corresponding crystal lattice that makes it possible to cut diamonds neatly. This same structure results in a less brittle material, as any impact that the structure sustains is spread out throughout the material.
A nanotorus is a theoretically described carbon nanotube bent into a torus doughnut shape. Nanotori have many unique properties, such as magnetic moments times larger than previously expected for certain specific radii. Carbon nanobuds are a newly created material combining two previously discovered allotropes of carbon: carbon nanotubes and fullerenes. In this new material fullerene-like "buds" are covalently bonded to the outer sidewalls of the underlying carbon nanotube.
This hybrid material has useful properties of both fullerenes and carbon nanotubes.
In particular, they have been found to be exceptionally good field emitters. Carbon nanotubes can have a length-to-diameter ratio as large as 28,, The bonding of atoms in a nanotube is described by applied quantum chemistry, specifically, orbital hybridization. The chemical bonding of nanotubes is composed entirely of sp 2 bonds , similar to those of graphite. This bonding structure, which is stronger than the sp 3 bonds found in diamonds , provides the molecules with their unique strength. Nanotubes naturally align themselves into "ropes" held together by Van der Waals forces.
Carbon nanotubes are the strongest and stiffest materials yet discovered on Earth, in terms of tensile strength and elastic modulus respectively. 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.
This, for illustration, translates into the ability to endure weight of kg on a cable with cross-section of 1 mm 2. Since carbon nanotubes have a low density for a solid of 1. Under excessive tensile strain, the tubes undergo plastic deformation, which means the deformation is permanent. This deformation begins at strains of approximately 5 percent and can increase the maximum strain the tube undergo before fracture by releasing strain energy. CNTs are not nearly as strong under compression.
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Because of their hollow structure and high aspect ratio, they tend to undergo buckling when placed under compressive, torsional or bending stress. The above discussion referred to axial properties of the nanotube, whereas simple geometrical considerations suggest that carbon nanotubes should be much softer in the radial direction than along the tube axis.
Indeed, TEM observation of radial elasticity suggested that even the van der Waals forces can deform two adjacent nanotubes . Nanoindentation experiments, performed by several groups on multiwalled carbon nanotubes,   indicated Young's modulus of the order of several GPa confirming that CNTs are indeed rather soft in the radial direction. Multi-walled nanotubes, multiple concentric nanotubes precisely nested within one another, exhibit a striking telescoping property whereby an inner nanotube core may slide, almost without friction, within its outer nanotube shell thus creating an atomically perfect linear or rotational bearing.
This is one of the first true examples of molecular nanotechnology, the precise positioning of atoms to create useful machines. Already this property has been utilized to create the world's smallest rotational motor . Future applications such as a gigahertz mechanical oscillator are also envisaged. Because of the symmetry and unique electronic structure of graphene, the structure of a nanotube strongly affects its electrical properties. All nanotubes are expected to be very good thermal conductors along the tube, exhibiting a property known as "ballistic conduction," but good insulators laterally to the tube axis.
It is predicted that carbon nanotubes will be able to transmit up to watts per meter per Kelvin at room temperature; compare this to copper, a metal well-known for its good thermal conductivity , which only transmits watts per meter per Kelvin. The temperature stability of carbon nanotubes is estimated to be up to degrees Celsius in vacuum and about degrees Celsius in air.
As with any material, the existence of defects affects the material properties. Defects can occur in the form of atomic vacancies. Another form of defect that may occur in carbon nanotubes is known as the Stone Wales defect, which creates a pentagon and heptagon pair by rearrangement of the bonds. Because of the very small structure of CNTs, the tensile strength of the tube is dependent on the weakest segment of it in a similar manner to a chain, where a defect in a single link diminishes the strength of the entire chain. The tube's electrical properties are also affected by the presence of defects.
A common result is the lowered conductivity through the defective region of the tube. Some defect formation in armchair-type tubes which can conduct electricity can cause the region surrounding that defect to become semiconducting. Furthermore single monoatomic vacancies induce magnetic properties . The tube's thermal properties are heavily affected by defects. Such defects lead to phonon scattering, which in turn increases the relaxation rate of the phonons.
This reduces the mean free path, and reduces the thermal conductivity of nanotube structures. Phonon transport simulations indicate that substitutional defects such as nitrogen or boron will primarily lead to scattering of high frequency optical phonons. However, larger scale defects such as Stone Wales defects cause phonon scattering over a wide range of frequencies, leading to a greater reduction in thermal conductivity . Due to their nanoscale dimensions, electron transport in carbon nanotubes will take place through quantum effects and will only propagate along the axis of the tube.
Techniques have been developed to produce nanotubes in sizeable quantities, including arc discharge, laser ablation, high pressure carbon monoxide HiPCO , and chemical vapor deposition CVD. Most of these processes take place in vacuum or with process gases. Large quantities of nanotubes can be synthesized by these methods; advances in catalysis and continuous growth processes are making CNTs more commercially viable.
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Nanotubes were observed in in the carbon soot of graphite electrodes during an arc discharge, by using a current of amps, that was intended to produce fullerenes. During this process, the carbon contained in the negative electrode sublimates because of the high temperatures caused by the discharge. Because nanotubes were initially discovered using this technique, it has been the most widely used method of nanotube synthesis. The yield for this method is up to 30 percent by weight and it produces both single- and multi-walled nanotubes with lengths of up to 50 micrometers.
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In the laser ablation process, a pulsed laser vaporizes a graphite target in a high temperature reactor while an inert gas is bled into the chamber. The nanotubes develop on the cooler surfaces of the reactor, as the vaporized carbon condenses. A water-cooled surface may be included in the system to collect the nanotubes.
It was invented by Richard Smalley and co-workers at Rice University, who at the time of the discovery of carbon nanotubes, were blasting metals with the laser to produce various metal molecules. When they heard of the discovery they substituted the metals with graphite to create multi-walled carbon nanotubes. This method has a yield of around 70 percent and produces primarily single-walled carbon nanotubes with a controllable diameter determined by the reaction temperature. However, it is more expensive than either arc discharge or chemical vapor deposition.
The catalytic vapor phase deposition of carbon was first reported in ,  but it was not until  that carbon nanotubes could be formed by this process. In , researchers at the University of Cincinnati UC developed a process to grow 18 mm long aligned carbon nanotube arrays on a FirstNano ET carbon nanotube growth system. During CVD, a substrate is prepared with a layer of metal catalyst particles, most commonly nickel, cobalt  , iron , or a combination . The metal nanoparticles can also be produced by other ways, including reduction of oxides or oxides solid solutions.
The diameters of the nanotubes that are to be grown are related to the size of the metal particles. This can be controlled by patterned or masked deposition of the metal, annealing, or by plasma etching of a metal layer.
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To initiate the growth of nanotubes, two gases are bled into the reactor: a process gas such as ammonia , nitrogen , hydrogen , etc. Nanotubes grow at the sites of the metal catalyst; the carbon-containing gas is broken apart at the surface of the catalyst particle, and the carbon is transported to the edges of the particle, where it forms the nanotubes. This mechanism is still under discussion. The catalyst particles can stay at the tips of the growing nanotube during the growth process, or remain at the nanotube base, depending on the adhesion between the catalyst particle and the substrate.
CVD is a common method for the commercial production of carbon nanotubes. For this purpose, the metal nanoparticles will be carefully mixed with a catalyst support e. One issue in this synthesis route is the removal of the catalyst support via an acid treatment, which sometimes could destroy the original structure of the carbon nanotubes.
However, alternative catalyst supports that are soluble in water have been shown to be effective for nanotube growth. Without the plasma, the resulting nanotubes are often randomly oriented.
Carbon Nanotubes and Related Structures: New Materials for the Twenty-first Century
Under certain reaction conditions, even in the absence of a plasma, closely spaced nanotubes will maintain a vertical growth direction resulting in a dense array of tubes resembling a carpet or forest. There are additional advantages to the CVD synthesis of nanotubes.
Unlike the above methods, CVD is capable of growing nanotubes directly on a desired substrate, whereas the nanotubes must be collected in the other growth techniques.
The growth sites are controllable by careful deposition of the catalyst. Additionally, no other growth methods have been developed to produce vertically aligned nanotubes. Richard Smalley, has concentrated upon finding methods to produce large, pure amounts of particular types of nanotubes. Their approach grows long fibers from many small seeds cut from a single nanotube; all of the resulting fibers were found to be of the same diameter as the original nanotube and are expected to be of the same type as the original nanotube.
Further characterization of the resulting nanotubes and improvements in yield and length of grown tubes are needed. Fullerenes and carbon nanotubes are not necessarily products of high-tech laboratories; they are commonly formed in such mundane places as ordinary flames ,  produced by burning methane,  ethylene,  and benzene,  and they have been found in soot from both indoor and outdoor air. Thus, although they can be used in some applications, they can lack in the high degree of uniformity necessary to meet many needs of both research and industry.
Recent efforts have focused on producing more uniform carbon nanotubes in controlled flame environments.
This method has promise for large scale, low cost nanotube synthesis, though it must compete with rapidly developing large scale CVD production. Determining the toxicity of carbon nanotubes has been one of the most pressing questions in Nanotechnology. Unfortunately such research has only just begun and the data are still fragmentary and subject to criticisms. Preliminary results highlight the difficulties in evaluating the toxicity of this heterogeneous material. Parameters such as structure, size distribution, surface area, surface chemistry, surface charge, and agglomeration state as well as purity of the samples, have considerable impact on the reactivity of carbon nanotubes.
However, available data clearly show that, under some conditions, nanotubes can cross the membrane barriers and suggests that if raw materials reach the internal organs they can induce harmful effects as inflammatory and fibrotic reactions.
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