CheapTubes.com Nanotubes-CNTs 101 Presentation

CheapTubes.com is "The Source for High Quality, Low Cost Carbon Nanotubes-CNTs! Please see our Carbon Nanotubes-CNTs 101 presentation below for information about Carbon Nanotubes-CNTs Synthesis, Properties, Nanotube Products, Markets & Uses.

 


Nanotubes-101 Presentation

A practical guide to understanding carbon nanotubes-synthesis, properties, applications, markets and utility.

 

 

The intent of this page is to convey a general understanding of what Nanotubes -CNTs) are, how they are produced, their many unique and interesting properties, markets, and applications.

 

industrial grade multi walled carbon nanotubes-CNTs 10-30nm od

  

History of Carbon Nanotubes -CNTs

    In 1980 we knew of only three forms of carbon, namely diamond, graphite, and amorphous carbon. Today we know there is a whole family of other forms of carbon. The first to be discovered was the hollow, cage-like buckminsterfullerene molecule - also known as the buckyball, or the C60 fullerene. There are now thirty or more forms of fullerenes, and also an extended family of linear molecules, carbon nanotubes. C60 is the first spherical carbon molecule, with carbon atoms arranged in a soccer ball shape. In the structure there are 60 carbon atoms and a number of five-membered rings isolated by six-membered rings. The second, slightly elongated, spherical carbon molecule in the same group resembles a rugby ball, has seventy carbon atoms and is known as C70. C70’s structure has extra six-membered carbon rings, but there are also a large number of other potential structures containing the same number of carbon atoms. Their particular shapes depend on whether five-membered rings are isolated or not, or whether seven-membered rings are present. Many other forms of fullerenes up to and beyond C120 have been characterized, and it is possible to make other fullerene structures with five-membered rings in different positions and sometimes adjoining one another.  [“Nanotechnology: Basic Science and Emerging Technologies”, M. Wilson et al, (2002)]

     The important fact for nanotechnology is that useful dopant atoms can be placed inside the hollow fullerene ball. Atoms contained within the fullerene are said to be endohedral. Of course they can also be bonded to fullerenes outside the ball as salts, if the fullerene can gain electrons.  [“Nanotechnology: Basic Science and Emerging Technologies”, M. Wilson et al, (2002)]

     Endohedral fullerenes can be produced in which metal atoms are captured within the fullerene cages. Theory shows that the maximum electrical conductivity is to be expected for endohedral metal atoms, which will transfer three electrons to the fullerene. Fullerenes can be dispersed on the surface as a monolayer. That is, there is only one layer of molecules, and they are said to be mono dispersed. Provided fullerenes can be placed in very specific locations, they may be aligned to form a fullerene wire. Systems with appropriate material inside the fullerene ball are conducting and are of particular interest because they can be deposited to produce bead-like conducting circuits. Combining endohedrally doped structures with non-doped structures changes the actual composition of a fullerene wire, so that it may be tailored in-situ during patterning. Hence within a single wire, insulating and conducting regions may be precisely defined. One-dimensional junction engineering becomes realistic with fullerenes.  [“Nanotechnology: Basic Science and Emerging Technologies”, M. Wilson et al, (2002)]

      Possibly more important than C60 fullerenes are Carbon Nanotubes (CNTs), which are related to graphite. The molecular structure of graphite resembles stacked, one-atom-thick sheets of chicken wire - a planar network of interconnected hexagonal rings of carbon atoms. In conventional graphite, the sheets of carbon are stacked on top of one another, allowing them to easily slide over each other. That is why graphite is not hard, but it feels greasy, and can be used as a lubricant. When graphene sheets are rolled into a cylinder and their edges joined, they form Carbon Nanotubes (CNTs). Only the tangents of the graphitic planes come into contact with each other, and hence their properties are more like those of a molecule.  [“Nanotechnology: Basic Science and Emerging Technologies”, M. Wilson et al, (2002)]

 

Types of Carbon Nanotubes Available from CheapTubes.com

 

       Carbon Nanotubes (CNTs) come in a variety of diameters, lengths, and functional group content. Carbon Nanotubes (CNTs) today are available for industrial applications in bulk quantities up metric ton quantities from Cheap Tubes.  Several CNT manufacturers have >100 ton per year production capacity for multi walled nanotubes.

       Carbon Nanotubes (CNTs) may consist of one tube of graphite, a one-atom thick single walled nanotubes (SWNTs) a two atom thick double walled carbon nanotubes (DWNTs) or a number of concentric tubes called multiwalled nanotubes (MWNTs). When viewed with a transmission electron microscope these tubes appear as planes. Whereas single walled nanotubes (SWNTs) appear as two planes, in multi walled nanotubes more than two planes are observed, and can be seen as a series of parallel lines. There are different types of Carbon Nanotubes (CNTs), because the graphitic sheets can be rolled in different ways. The three types of Carbon Nanotubes (CNTs) structure are Zigzag, Armchair, and Chiral. It is possible to recognize zigzag, armchair, and chiral Carbon Nanotubes (CNTs) just by following the pattern across the diameter of the tubes, and analyzing their cross-sectional structure.  [“Nanotechnology: Basic Science and Emerging Technologies”, M. Wilson et al, (2002)]

      Multi walled nanotubes (MWNTs) can come in an even more complex array of forms, because each concentric nanotube can have different structures, and hence there are a variety of sequential arrangements. The simplest sequence is when concentric layers are identical but different in diameter. However, mixed variants are possible, consisting of two or more types of concentric Carbon Nanotubes (CNTs) arranged in different orders. These can have either regular layering or random layering.  The structure of the carbon nanotube (CNTs) influences its properties - including electrical and thermal conductivity, density, and lattice structure. Both type and diameter are important. The wider the diameter of the carbon nanotube (CNTs), the more it behaves like graphite. The narrower the diameter of the Carbon Nanotubes (CNTs), the more its intrinsic properties depends upon its specific type.

 

Production of Carbon Nanotubes

     There are a number of methods of making Carbon Nanotubes (CNTs) and C60 Fullerenes.  C60 Fullerenes were first observed after vaporizing graphite with a short-pulse, high-power laser, however this was not a practical method for making large quantities. Carbon Nanotubes (CNTs) have probably been around for a lot longer than was first realized, and may have been made during various carbon combustion and vapor deposition processes, but electron microscopy at that time was not advanced enough to distinguish them from other types of tubes. The first method for producing Carbon Nanotubes (CNTs) and C60 Fullerenes in reasonable quantities – was by applying an electric current across two carbonaceous electrodes in an inert gas atmosphere. This method is called plasma arcing. It involves the evaporation of one electrode as cations followed by deposition at the other electrode. This plasma-based process is analogous to the more familiar electroplating process in a liquid medium. C60 Fullerenes and Carbon Nanotubes (CNTs) are formed by plasma arcing of carbonaceous materials, particularly graphite. The C 60 Fullerenes appear in the soot that is formed, while the Carbon Nanotubes (CNTs) are deposited on the opposing electrode. Another method of Carbon Nanotubes (CNTs) synthesis involves plasma arcing in the presence of cobalt with a 3% or greater concentration. As noted above, the Carbon Nanotubes (CNTs) product is a compact cathode deposit of rod like morphology. However when cobalt is added as a catalyst, the nature of the product changes to a web, with strands of 1mm or so thickness that stretch from the cathode to the walls of the reaction vessel. The mechanism by which cobalt changes this process is unclear, however one possibility is that such metals affect the local electric fields and hence the formation of the five-membered rings.   [“Nanotechnology: Basic Science and Emerging Technologies”, M. Wilson et al, (2002)]

 

Arc Method Carbon Nanotubes

    The carbon arc discharge method, initially used for producing C60 fullerenes, is the most common and perhaps easiest way to produce Carbon Nanotubes (CNTs), as it is rather simple. However, it is a technique that produces a complex mixture of components, and requires further purification - to separate the Carbon Nanotubes (CNTs) from the soot and the residual catalytic metals present in the crude product.  This method creates Carbon Nanotubes (CNTs) through arc-vaporization of two carbon rods placed end to end, separated by approximately 1mm, in an enclosure that is usually filled with inert gas at low pressure. Recent investigations have shown that it is also possible to create Carbon Nanotubes (CNTs) with the arc method in liquid nitrogen. A direct current of 50 to 100 A, driven by a potential difference of approximately 20 V, creates a high temperature discharge between the two electrodes. The discharge vaporizes the surface of one of the carbon electrodes, and forms a small rod-shaped deposit on the other electrode. Producing Carbon Nanotubes (CNTs) in high yield depends on the uniformity of the plasma arc, and the temperature of the deposit forming on the carbon electrode.  [“Nanotechnology: Basic Science and Emerging Technologies”, M. Wilson et al, (2002)]

 

 Laser Method Carbon Nanotubes

     In 1996 Carbon Nanotubes (CNTs) were first synthesized using a dual-pulsed laser and achieved yields of >70wt% purity. Samples were prepared by laser vaporization of graphite rods with a 50:50 catalyst mixture of Cobalt and Nickel at 1200oC in flowing argon, followed by heat treatment in a vacuum at 1000oC to remove the C60 and other fullerenes. The initial laser vaporization pulse was followed by a second pulse, to vaporize the target more uniformly. The use of two successive laser pulses minimizes the amount of carbon deposited as soot. The second laser pulse breaks up the larger particles ablated by the first one, and feeds them into the growing nanotube structure. The material produced by this method appears as a mat of “ropes”, 10-20nm in diameter and up to 100um or more in length. Each rope is found to consist primarily of a bundle of single walled nanotubes, aligned along a common axis. By varying the growth temperature, the catalyst composition, and other process parameters, the average nanotube diameter and size distribution can be varied.  Arc-discharge and laser vaporization are currently the principal methods for obtaining small quantities of high quality CNTs. However, both methods suffer from drawbacks. The first is that both methods involve evaporating the carbon source, so it has been unclear how to scale up production to the industrial level using these approaches. The second issue relates to the fact that vaporization methods grow CNTs in highly tangled forms, mixed with unwanted forms of carbon and/or metal species. The Carbon Nanotubes (CNTs) thus produced are difficult to purify, manipulate, and assemble for building Carbon Nanotubes (CNTs)-device architectures for practical applications. [“Nanotechnology: Basic Science and Emerging Technologies”, M. Wilson et al, (2002)]

  

 Chemical Vapor Deposition of Carbon Nanotubes

    Chemical vapor deposition of hydrocarbons over a metal catalyst is a classical method that has been used to produce various carbon materials such as carbon fibers and filaments. for over twenty years. Large amounts of Carbon Nanotubes (CNTs) can be formed by catalytic CVD of acetylene over Cobalt and iron catalysts supported on silica or zeolite. The carbon deposition activity seems to relate to the cobalt content of the catalyst, whereas the Carbon Nanotubes (CNTs) selectivity seems to be a function of the pH in catalyst preparation. Fullerenes and bundles of single walled carbon nanotubes (SWNTs) were also found among the multi walled carbon nanotubes (MWNTs) produced on the carbon/zeolite catalyst.  Some researchers are experimenting with the formation of Carbon Nanotubes (CNTs) from ethylene. Supported catalysts such as iron, cobalt, and nickel, containing either a single metal or a mixture of metals, seem to induce the growth of isolated single walled carbon nanotubes (SWNTs) or single walled carbon nanotube (SWNTs) bundles in the ethylene atmosphere. The production of single walled carbon nanotubes (SWNTs), as well as double-walled carbon nanotubes (DWNTs), on molybdenum and molybdenum-iron alloy catalysts has also been demonstrated. CVD of carbon within the pores of a thin alumina template with or without a Nickel catalyst has been achieved. Ethylene was used with reaction temperatures of 545oC for Nickel-catalyzed CVD, and 900oC for an uncatalyzed process. The resultant carbon nanostructures have open ends, with no caps. Methane has also been used as a carbon source. In particular it has been used to obtain ‘nanotube chips’ containing isolated single walled carbon nanotubes (SWNTs) at controlled locations. High yields of single walled carbon nanotubes (SWNTs) have been obtained by catalytic decomposition of an H2/CH4 mixture over well-dispersed metal particles such as Cobalt, Nickel, and Iron on magnesium oxide at 1000oC. It has been reported that the synthesis of composite powders containing well-dispersed Carbon Nanotubes (CNTs) can be achieved by selective reduction in an H2/CH4 atmosphere of oxide solid solutions between a non-reducible oxide such as Al2O3 or MgAl2O4 and one or more transition metal oxides. The reduction produces very small transition metal particles at a temperature of usually >800oC. The decomposition of CH4 over the freshly formed nanoparticles prevents their further growth, and thus results in a very high proportion of single walled carbon nanotubes (SWNTs) and fewer multi walled carbon nanotubes (MWNTs).  This process also includes the production of single walled carbon nanotubes (SWNTs) or multi walled carbon nanotubes (MWNTs) arrays on SI or other substrate materials. [“Nanotechnology: Basic Science and Emerging Technologies”, M. Wilson et al, (2002)]

 

Ball Milling or Carbon Nanotubes

        Ball milling and subsequent annealing is a simple method for the production of Carbon Nanotubes (CNTs). Although it is well established that mechanical attrition of this type can lead to fully nano porous microstructures, it was not until a few years ago that Carbon Nanotubes (CNTs) and boron nitride nanotubes (BNNTs) were produced from these powders by thermal annealing. Essentially the method consists of placing graphite powder into a stainless steel container along with four hardened steel balls. The container is purged, and argon is introduced. The milling is carried out at room temperature for up to 150 hours. Following milling, the powder is annealed under an inert gas flow at temperatures of 1400oC for six hours. The mechanism of this process is not known, but it is thought that the ball milling process forms nanotube nuclei, and the annealing process activates nanotube growth.  Research has shown that this method produces more multi walled carbon nanotubes (MWNTs) and few single walled carbon nanotubes (SWNTs) [“Nanotechnology: Basic Science and Emerging Technologies”, M. Wilson et al, (2002)]

 

 Other Methods of Carbon Nanotubes Production

    Carbon Nanotubes (CNTs) can also be produced by diffusion flame synthesis, electrolysis, use of solar energy, heat treatment of a polymer, and low-temperature solid pyrolysis. In flame synthesis, combustion of a portion of the hydrocarbon gas provides the elevated temperature required, with the remaining fuel conveniently serving as the required hydrocarbon reagent. Hence the flame constitutes an efficient source of both energy and hydrocarbon raw material. Combustion synthesis has been shown to be scalable for high-volume commercial production.  There has recently been new innovations in carbon nanotubes (CNTs) synthesis allowing for the production of Single Walled Carbon Nanotubes (SWNTs) with no metal catalyst.  The inventor of this process is Jeanette Benavides. [“Nanotechnology: Basic Science and Emerging Technologies”, M. Wilson et al, 

  

Purification of Carbon Nanotubes

     Purification of Carbon Nanotubes (CNTs) generally refers to the separation of Carbon Nanotubes (CNTs) from other entities, such as carbon nanoparticles, amorphous carbon, residual catalyst, and other unwanted species. The classic chemical techniques for purification have been tried, but they have not been found to be effective in removing the undesirable impurities. Three basic methods have been used with varying degrees of success, namely gas-phase, liquid-phase, and intercalation methods. 

    Generally, a centrifugal separation is necessary to concentrate the single walled carbon nanotubes (SWNTs) in low-yield soot before the micro filtration operation, since the nanoparticles easily contaminate membrane filters. The advantage of this method is that unwanted nanoparticles and amorphous carbon are removed simultaneously and the Carbon Nanotubes (CNTs) are not chemically modified. However 2-3 mol nitric acid is useful for chemically removing impurities.   

     It is now possible to cut Carbon Nanotubes (CNTs) into smaller segments, by extended sonication in concentrated acid mixtures or by using an extrusion system.  The resulting Carbon Nanotubes (CNTs) form a colloidal suspension in solvents. They can be deposited on substrates, or further  manipulated in solution, and can have many different functional groups attached to the ends and sides of the Carbon Nanotubes (CNTs) [“Nanotechnology: Basic Science and Emerging Technologies”, M. Wilson et al,

  

Gas Phase Carbon Nanotubes Purification:

   The first successful technique for purification of Carbon Nanotubes (CNTs) was developed by Thomas Ebbesen and coworkers.  Following the demonstration that Carbon Nanotubes (CNTs) could be selectively attached by oxidizing gases these workers realized that nanoparticles, with their defect rich structures might be oxidised more readily than the relatively perfect Carbon Nanotubes (CNTs). They found that a significant relative enrichment of Carbon Nanotubes (CNTs) could be achieved this way, but only at the expense of losing the majority of the original sample. [“Carbon Nanotubes”, T. W. Ebbesen, Ann. Rev. Mater. Sci. 24, 235 (1994); Physics Today 381, 678 (1996)]

     A new gas-phase method has been developed at the NASA Glenn Research Center to purify gram-scale quantities of single walled carbon nanotubes (SWNTs). This method, a modification of a gas-phase purification technique previously reported by Smalley and others, uses a combination of high-temperature oxidations and repeated extractions with nitric and hydrochloric acid. This improved procedure significantly reduces the amount of impurities such as residual catalyst, and non-nanotube forms of carbon within the Carbon Nanotubes (CNTs), increasing their stability significantly. 

 

Liquid Phase Carbon Nanotubes Purification:

The current liquid-phase purification procedure follows certain essential steps:

  • preliminary filtration- to get rid of large graphite particles;

  • dissolution- to remove fullerenes (in organic solvents) and catalyst particles (in concentrated acids)

  • centrifugal separation-

  • microfiltration- and

  • chromatography

It is important to keep the Carbon Nanotubes (CNTs) well-separated in solution, so the Carbon Nanotubes (CNTs) are typically dispersed using a surfactant prior to the last stage of separation.

 

 Intercalation Carbon Nanotubes Purification:

     An alternative approach to purifying multi walled carbon nanotubes (MWNTs) was introduced in 1994 by a Japanese research group. This technique made use of the fact that nanoparticles and other graphitic contaminants have relatively “open” structures and can therefore be more readily intercalated with a variety of materials that can close carbon nanotubes (CNTs). By intercalating with copper chloride, and then reducing this to metallic copper, the research group was able to preferentially oxidize the nanoparticles away, using copper as an oxidation catalyst. Since 1994, this has become a popular method for purification of Carbon Nanotubes (CNTs). “The first stage is to immerse the crude cathodic deposit in a molten copper chloride and potassium chloride mixture at 400oC and leave it for one week. The product of this treatment, which contains intercalated nanoparticles and graphitic fragments, is then washed in ion exchanged water to remove excess copper chloride and potassium chloride. In order to reduce the intercalated copper chloride-potassium chloride metal, the washed product is slowly heated to 500oC in a mixture of Helium and hydrogen and held at this temperature for 1 hour. Finally, the material is oxidized in flowing air at a rate of 10oC/min to a temperature of 555oC. Samples of cathodic soot which have been treated this way consist almost entirely of carbon nanotubes (CNTs). A disadvantage of this method is that some amount of nanotubes are inevitably lost in the oxidation stage, and the final material may be contaminated with residues of intercalates. A similar purification technique, which involves intercalation with bromine followed by oxidation, has also been described.  [“Carbon Nanotubes and Related Structures : New Materials for the Twenty-first Century”, P. F. Harris, Cambridge University Press (1999) ISBN 0-521-55446-2 page 49]

 

   

Dispersion of Carbon Nanotubes

 

 

Cheap Tubes Inc exclusively uses the SONICS VCX750 ultrasonic equipment.  We routinely disperse Carbon Nanotubes (CNTs) using the following process. 

The Carbon Nanotubes (CNTs) solution is composed of either Single Walled Carbon Nanotubes (SWNTs), Double Walled Carbon Nanotubes (DWNTs) or Multi Walled Carbon Nanotubes (MWNTs), PVP (or other surfactant), and water, in the proportions of 10 parts Carbon Nanotubes (CNTs): ~1-2 parts PVP: 2,000 parts water or other solvents. The required dispersion (sonication) time is ~2 to 8 minutes with an interruption of 10 seconds every 30 seconds at full or high amplitude.  If the power of your ultrasonic equipment is less than that of the SONICS VCX750 unit then you must prolong the carbon nanotubes sonication time accordingly.  For Dispersing Carbon Nanotubes (CNTs), we recommend a pulsed sonication method for 40 minutes at reduced amplitude.  We typically pulse our sonicator on for 30 seconds & off for 10 seconds, then we repeat that step until we have reached the desired sonication time. We have found that to effectively disperse Single Walled Carbon Nanotubes (SWNTs) that we need to set the amplitude at 40% for much longer times to break apart the Van der Waals physical bonds which make the Single Walled Carbon Nanotubes (SWNTs) agglomerate into bundles.  Since Single Walled Carbon Nanotubes (SWNTs) are such a fine particle, the agglomerated bundles are harder to disperse.

 

     Although both probe style and bath style ultrasonic systems can be used for dispersing Carbon Nanotubes (CNTs), it is widely believed that the probe style ultrasonic systems work better for dispersing Carbon Nanotubes (CNTs).  It is also widely known that adding a dispersing reagent (surfactant) into the solution will accelerate the dispersion effect. The reagent Polyvinyl Pyrrolidone (PVP) is a good dispersion agent. Some people like to use the reagent Sodium Dodecyl Benzene Sulfonate (SDBS) or Poly Vinyl Alcohol (PVA) as well.  We have found that the dispersing reagent and proportions listed above do change when using different solvents.   When working on a new dispersion in IPA, Acetone, or any solvents other than DI water, we usually start with 500mg Carbon Nanotubes (CNTs), 125 mgs dispersing reagent, into 500 mls of solvent and use the ultrasonic process detailed above.  In our experience, much less reagent is used for dispersing in DI water than other solvents.  We believe this is due to the high polarity of water compared to other solvents.  Typically, it is a question of chemistry to achieve a stable dispersion.  A stable dispersion will last for days, weeks, or months with little to no settling.  

 

       In some applications, achieving a stable dispersion can require other agents in the solution to prevent the Carbon Nanotubes (CNTs) from falling out of solution over time.  Emulsifier T-60 (also known as Tween 60) is commonly used with Di water or Isopropyl Alcohol.  Organic Titanates can be used with Acetone and Xylene.   The specific application determines whether these agents remain in the solution when further processing, or if they need to be removed.  Some organic titanates can be removed by heating the solution above 2500C.  The addition of the OH and COOH functional groups assists the carbon nanotubes (CNTs) dispersing in DI water and other solvents as well as the chemical bonding to other materials during further processing.   

 

 © 2005-2009, Cheap Tubes Inc

Functionalization of Carbon Nanotubes

     Pristine Carbon Nanotubes (CNTs) are unfortunately insoluble in many liquids such as water, polymer resins, and most solvents.  Thus they are difficult to evenly disperse in a liquid matrix such as epoxies and other polymers. This complicates efforts to utilize the Carbon Nanotubes (CNTs) outstanding physical properties in the manufacture of composite materials, as well as in other practical applications which require preparation of uniform mixtures of Carbon Nanotubes-CNTs -CNTs with many different organic, inorganic, and polymeric materials.

    To make Carbon Nanotubes-CNTs more easily dispersible in liquids, it is necessary to physically or chemically attach certain molecules, or functional groups, to their smooth sidewalls without significantly changing the Carbon Nanotubes (CNTs) desirable properties. This process is called functionalization. The production of robust composite materials requires strong covalent chemical bonding between the filler particles and the polymer matrix, rather than the much weaker van der Waals physical bonds which occur if the Carbon Nanotubes -CNTs are not properly functionalized. 

    Functionalization methods such as chopping, oxidation, and “wrapping” of the Carbon Nanotubes-CNTs (CNTs) in certain polymers can create more active bonding sites on the surface of the Carbon Nanotubes-CNTs (CNTs).  For biological uses, Carbon Nanotubes-CNTs (CNTs) can be functionalized by attaching biological molecules, such as lipids, proteins, biotins, etc. to them. Then they can usefully mimic certain biological functions, such as protein adsorption, and bind to DNA and drug molecules. This would enable medially and commercially significant applications such as gene therapy and drug delivery.  In biochemical and chemical applications such as the development of very specific biosensors, molecules such as carboxylic acid (COOH), poly m-aminobenzoic sulfonic acid (PABS), polyimide, Amines (NH2) and polyvinyl alcohol (PVA) have been used to functionalize Carbon Nanotubes (CNTs), as have amino acid derivatives, halogens, and compounds.  Some types of functionalized Carbon Nanotubes-CNTs (CNTs) are soluble in water and other highly polar, aqueous solvents.

 

Properties of Carbon Nanotubes-CNTs

There are many useful and unique properties of Carbon Nanotubes-CNTs and some of them are detailed below.

 

The list includes

  

Electrical Conductivity of Carbon Nanotubes-CNTs

     Carbon Nanotubes (CNTs) can be highly conducting, and hence can be said to be metallic. Their conductivity has been shown to be a function of their chirality, the degree of twist as well as their diameter. Carbon Nanotubes (CNTs) can be either metallic or semi-conducting in their electrical behavior.  Conductivity in multi walled carbon nanotubes (MWNTs) is quite complex. Some types of “armchair”-structured Carbon Nanotubes (CNTs) appear to conduct better than other metallic Carbon Nanotubes (CNTs). Furthermore, interwall reactions within multi walled carbon nanotubes (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 carbon nanotubes (SWNTs). The behavior of the ropes of semi-conducting single walled carbon nanotubes (SWNTs) is different, in that the transport current changes abruptly at various positions on the Carbon Nanotubes (CNTs). 

     The conductivity and resistivity of ropes of single walled carbon nanotubes (SWNTs) has been measured by placing electrodes at different parts of the Carbon Nanotubes (CNTs). The resistivity of the single walled carbon nanotubes (SWNTs) ropes was of the order of 10–4 ohm-cm at 27°C. This means that single walled carbon nanotube (SWNTs) ropes are the most conductive carbon fibers known. The current density that was possible to achieve was 10-7 A/cm2, however in theory the single walled carbon nanotube (SWNTs) ropes should be able to sustain much higher stable current densities, as high as 10-13 A/cm2. It has been reported that individual single walled carbon nanotubes (SWNTs) may contain defects. Fortuitously, these defects allow the single walled carbon nanotubes (SWNTs) to act as transistors. Likewise, joining Carbon Nanotubes (CNTs) together may form transistor-like devices. A carbon nanotube (CNTs) 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 single walled carbon nanotubes (SWNTs) can route electrical signals at speeds up to 10 GHz when used as interconnects on semi-conducting devices.

 

Strength and Elasticity of Carbon Nanotubes-CNTs   

         The carbon atoms of a single 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,Carbon Nanotubes (CNTs) are expected to be the ultimate high-strength fibers.  Single walled carbon nanotubes (SWNTs) are stiffer than steel, and are very resistant to damage from physical forces. Pressing on the tip of a Carbon Nanotubes (CNTs) will cause it to bend, but without damage to the tip.  When the force is removed, the Carbon Nanotubes (CNTs) returns to its original state. This property makes Carbon Nanotubes (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 atomic force microscopy, the unanchored ends of a freestanding Carbon Nanotubes (CNTs) can be pushed out of their equilibrium position, and the force required to push the Carbon Nanotubes (CNTs) can be measured. The current Young’s modulus value of single walled carbon nanotubes (SWNTs) is about 1 TeraPascal, but this value has been widely 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 single walled carbon nanotubes (SWNTs), ranging from 1.22 Tpa to 1.26 Tpa. They have calculated a value of 1.09 Tpa for a generic carbon nanotube (CNTs). However, when working with different multi walled carbon nanotubes (MWNTs), others have noted that the modulus measurements of multi walled carbon nanotubes (MWNTs) using AFM techniques do not  strongly depend on the diameter. Instead, they argue that the modulus of the multi walled carbon nanotubes (MWNTs) correlates to the amount of disorder in the carbon nanotube (CNTs) walls. Not surprisingly, when multi walled carbon nanotubes (MWNTs) break, the outermost layers break first. 

 

Thermal Conductivity and Expansion of Carbon Nanotubes-CNTs

      Carbon Nanotubes (CNTs) have been shown to exhibit superconductivity below 20o 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 carbon - carbon bonds make them exceptionally strong and stiff against axial strains. The almost zero in-plane thermal expansion but large inter-plane expansion of single walled carbon nanotubes (SWNTs) implies strong in-plane coupling and high flexibility against non-axial strains.

     Many applications of Carbon Nanotubes (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 Carbon Nanotubes (CNTs)-polymer composites have also appeared. These measurements suggest modest enhancements in strength characteristics of Carbon Nanotubes (CNTs)-embedded matrixes as compared to bare polymer matrixes. Preliminary experiments and simulation studies on the thermal properties of Carbon Nanotubes (CNTs) show very high thermal conductivity. It is expected, therefore, that Carbon Nanotubes (CNTs) reinforcements in polymeric materials may also significantly improve the thermal and thermomechanical properties of the composites. 

 

Field Emission of Carbon Nanotubes-CNTs

       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 Carbon nanotubes (CNTs) is very favorable for field emission. Even for moderate voltages, a strong electric field develops at the free end of supported Carbon Nanotubes (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 multi walled carbon nanotubes (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.  [B.Q. Wei, et al, Appl. Phys. Lett. 79 1172 (2001)].

 

High Aspect Ratio of Carbon Nanotubes-CNTs

    Carbon Nanotubes (CNTs) represent a very small, high aspect ratio conductive additive for plastics of all types. Their high aspect ratio means that a lower loading of Carbon Nanotubes (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.  Carbon Nanotubes (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.

  

High Absorbency of Carbon Nanotubes-CNTs

   The large surface area and high absorbency of Carbon Nanotubes (CNTs) make them ideal candidates for use in air, gas, and water filtration.  A lot of research is being done in replacing activated charcoal with Carbon nanotubes (CNTs) in certain ultra high purity applications. 

 

 Applications of Carbon Nanotubes-CNTs

   The special nature of carbon combined with the molecular perfection of single-walled carbon nanotubes (SWNTs) 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 bonds vibrations provide an intrinsic thermal conductivity higher than even diamond.  In most conventional 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. Carbon Nanotubes (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 Carbon Nanotubes (CNTs).   Carbon Nanotubes (CNTs) are an example of true nanotechnology: they are under 100 nanometers 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.  Carbon Nanotubes (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 the tremendously rich chemistry of carbon. This provides opportunity to modify their structure, and to optimize their solubility and dispersion. Very significantly, Carbon Nanotubes (CNTs) are molecularly perfect, which means that they are normally free of property-degrading flaws in the Carbon Nanotubes (CNTs) structure. Their material properties can therefore approach closely the very high levels intrinsic to them. These extraordinary characteristics give Carbon Nanotubes (CNTs) potential in numerous applications.
 

Field Emission of Carbon Nanotubes-CNTs

      Carbon Nanotubes (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.  The smaller the tip’s radius of curvature, the more concentrated the electric field will be, leading to increased field emission. 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. Carbon Nanotubes (CNTs) can carry an astonishingly high current density.  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 Carbon Nanotubes (CNTs)-based displays there is a separate Carbon Nanotubes (CNTs) electron gun for each individual pixel in the display. Their high current density, low turn-on and operating voltages, and steady, long-lived behavior make Carbon Nanotubes (CNTs) very attractive field emitters in this application. Other applications utilizing the field-emission characteristics of Carbon Nanotubes (CNTs) include general types of low-voltage cold-cathode lighting sources, lightning arrestors, and electron microscope sources.

 

Conductive or Reinforced Plastics with Carbon Nanotubes-CNTs

       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 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 the filler particles, the lower the loading required to achieve a given level of conductivity.

     Carbon Nanotubes (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 Carbon Nanotubes (CNTs) include EMI/RFI shielding composites; coatings for enclosures, gaskets, and other uses such as electrostatic dissipation; antistatic materials, transparent conductive coatings; and radar-absorbing materials for stealth applications. 

    A lot of automotive plastics companies are using Carbon Nanotubes (CNTs) as well.  Carbon Nanotubes (CNTs) have been added into the side mirror plastics on automobiles in the US since the late 1990s.  I have seen forecasts predicting that GM alone will consume over 500 pounds of Carbon Nanotubes (CNTs) masterbatches in 2006 for using in all areas of automotive plastics.  Masterbatches normally contain 20 wt% Carbon Nanotubes (CNTs)which are already very well dispersed.  Manufacturers then need to perform a “let down” or dilution procedure prior to using the masterbatch in production

  

Energy Storage with Carbon Nanotubes-CNTs

     Carbon Nanotubes (CNTs) have the intrinsic characteristics desired in material used as electrodes in batteries and capacitors, two technologies of rapidly increasing importance. Carbon Nanotubes (CNTs) have a tremendously high surface area, good electrical conductivity, and very importantly, their linear geometry makes their surface highly accessible to the electrolyte.   

     Research has shown that Carbon Nanotubes (CNTs) have the highest reversible capacity of any carbon material for use in lithium ion batteries.  [B. Gao, Chem. Phys. Lett. 327, 69 (2000)].   In addition, Carbon Nanotubes (CNTs) are outstanding materials for super capacitor electrodes [R.Z. Ma, et al., Science in China Series E-Technological Sciences 43 178 (2000)] and are now being marketed for this application.  Carbon Nanotubes (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. Because of their high electrical conductivity, they may also be used in gas diffusion layers, as well as current collectors.  Carbon Nanotubes (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.  CheapTubes.com has a new product coming out that is a carbon nanotubes (CNTs) based conductive additive specifically for Li Ion Batteries.

  

Conductive Adhesives and Connectors with Carbon Nanotubes-CNTs

     The same properties that make Carbon Nanotubes (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, coaxial cables, and other types of connectors.
 

 

Molecular Electronics with Carbon Nanotubes-CNTs

    The idea of building electronic circuits out of the essential building blocks of materials - molecules - has seen a revival the past few 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.

     There are already companies such as Nantero from Woburn, MA that are already making Carbon Nanotubes (CNTs) based non-volitle random access memory for PC’s.  A lot of research is being done to design Carbon Nanotubes (CNTs) based transistors as well. 

 

 Thermal Materials with Carbon Nanotubes-CNTs

    The record-setting anisotropic thermal conductivity of Carbon Nanotubes (CNTs) is enabling many applications where heat needs to move from one place to another. Such an application is found in electronics, particularly heat sinks for chips used in advanced computing, where uncooled chips now routinely reach over 100oC.  The technology for creating aligned structures and ribbons of Carbon Nanotubes (CNTs) [D.Walters, et al., Chem. Phys. Lett. 338, 14 (2001)]  is a step toward realizing incredibly efficient heat conduits. In addition, composites with Carbon Nanotubes (CNTs) have been shown to dramatically increase their bulk thermal conductivity, even at very small loadings.

 

Structural Composites with Carbon Nanotubes-CNTs    

       The superior properties of Carbon Nanotubes (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.

  

Fibers and Fabrics with Carbon Nanotubes-CNTs

    Fibers spun of pure Carbon Nanotubes (CNTs) have recently been demonstrated [R.H. Baughman, Science 290, 1310 (2000)] and are undergoing rapid development, along with Carbon Nanotubes (CNTs) composite fibers. Such super-strong fibers will have many applications including body and vehicle armor, transmission line cables, woven fabrics and textiles.

 

  Carbon Nanotubes-CNTs Catalyst Supports

   Carbon Nanotubes (CNTs) intrinsically have an enormously high surface area; in fact, for single walled carbon nanotubes (SWNTs) every atom is not just on one surface - each atom is on two surfaces, the inside and the outside of the Carbon Nanotubes (CNTs)! Combined with the ability to attach essentially any chemical species to their sidewalls this provides an opportunity for unique catalyst supports. Their electrical conductivity may also be exploited in the search for new catalysts and catalytic behavior

  

Carbon Nanotubes-CNTs Ceramics

    A ceramic material reinforced with carbon nanotubes (CNTs) 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 Carbon Nanotubes (CNTs).  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 (CNTs) 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 Carbon Nanotubes (CNTs). It also has interesting thermal properties, conducting heat in one direction, along the alignment of the Carbon Nanotubes (CNTs), but reflecting heat at right angles to the Carbon Nanotubes (CNTs), making it an attractive material for thermal barrier coatings.

 

Biomedical Applications of Carbon Nanotubes-CNTs

        The exploration of Carbon Nanotubes (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 Carbon Nanotubes (CNTs), so they appear to have no toxic effect. The cells also do not adhere to the Carbon Nanotubes (CNTs), potentially giving rise to applications such as coatings for prosthetics and surgical implants.  The ability to functionalize the sidewalls of Carbon Nanotubes (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 Carbon Nanotubes (CNTs, which can then be successfully inserted into a cell; this has potential applications in gene therapy.

  

Air, Water, and Gas Filtration with Carbon Nanotubes-CNTs

       Many researchers and corporations have already developed Carbon Nanotubes (CNTs) 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 Carbon Nanotubes (CNTs) have already been commercialized and products are on the market now.  Someday Carbon Nanotubes (CNTs) may be used to filter other liquids such as fuels and lubricants as well. 

    A lot of research is being done in the development of Carbon Nanotubes (CNTs) based air and gas filtration.  Filtration has been shown to be another area where it is cost effective to use Carbon Nanotubes (CNTs)  already.  The research I’ve seen suggests that 1 gram of Multi Walled Carbon Nanotubes (MWNTs) can be dispersed onto 1 sq ft of filter media.  Manufacturers can get their cost down to $0.95 cents per gram of purified Multi Walled Carbon Nanotubes (MWNTs) when purchasing ton quantities.

 

Other Applications of Carbon Nanotubes-CNTs

      Some commercial products on the market today utilizing Carbon Nanotubes (CNTs) include stain resistant textiles, Carbon Nanotubes (CNTs) reinforced tennis rackets and baseball bats.  Companies like Kraft foods are heavily funding cnt based plastic packaging.  Food will stay fresh longer if the packaging is less permeable to atmosphere.  Coors Brewing company has developed new plastic beer bottles that stay cold for longer periods of time.  Samsung already has Carbon Nanotubes (CNTs) based flat panel displays on the market.  A lot of companies are looking forward to being able to produce transparent conductive coatings and phase out ITO coatings.  Samsung uses aligned Carbon Nanotubes (SWNTs) in the transparent conductive layer of their display manufacturing process. 

 

    In closing, Carbon Nanotubes (CNTs) have many unique and desirable properties.  Although many applications may take significant investments of time and money to develop to reach commercial viability, there are plenty of applications today in which Carbon Nanotubes (CNTs) add significant benefits to existing products with relatively low implementation costs.  Most of these applications are in the polymer, composite materials, batteries, paints, plastics, ceramics, automotive, and textiles industries. 

 

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References:

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21. [B.Q. Wei, et al, Appl. Phys. Lett. 79 1172 (2001)].

22. [R.H. Baughman, Science 290, 1310 (2000)]

23. [D.Walters, et al., Chem. Phys. Lett. 338, 14 (2001)]

24. [B. Gao, Chem. Phys. Lett. 327, 69 (2000)]

25. [R.Z. Ma, et al., Science in China Series E-Technological Sciences 43 178 (2000)]

 

 

 

 

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