my first paper

CARBON NANO TUBES

 STRUCTURE, PROPERTIES AND APPLICATIONS

         Presented by:

Uday S Y  &  Sandeep G

(Dept. of Electronics & Communication, SIT, Tumkur-572103)

                                                       

“What would be the utility of such machines? Who knows? I cannot exactly see what would happen, but I can hardly doubt about when we have some control of the                                 arrangement  of things on a molecular scale. We will get an enormous greater range of possibility that substances can have, and of the different things we can do”.                                          Richard P. Feynman                                                                                               “There is Plenty of Room at the Bottom” (Dec. 1959)

                                                                                      

Abstract :

         Since their discovery in 1991 by Sumio Iijima, carbon nanotubes have been of great interest, both from a fundamental point of view and for future applications. The most eye catching features of these structures are their electronic, mechanical, and chemical characteristics, which open a way to future applications. With one hundred times the tensile strength of steel, thermal conductivity better than all but the purest diamond, and electrical conductivity similar to copper, but with the ability to carry much higher currents, they seem to be a wonder material. Here we discuss about some of the promising applications of CNTs, also about the challenges that ensue in realizing these applications.

         This report provides an overview of current nanotube technology, with a special focus on structure, properties, applications and future scope of nanotubes.

Keywords : carbon nanotube, SWCNT, MWCNT, chirality, graphene sheet, nanocarbon waste.

INTRODUCTION :

         Nanotechnology is the manufacture and science of materials with at least one dimension in the nanometer scale. Many nano-materials have novel chemical and biological properties and most of them are not naturally occurring. The unique geometric properties of this new allotrope of carbon did not end with graphite, diamond, soccer shaped molecules called fullerene, it was also discovered that carbon atoms can form long cylindrical tubes. These tubes were originally called “buckytubes” but now are better known as carbon nanotubes or CNT in short. Carbon nanotubes are one of the most commonly mentioned building blocks of nanotechnology. Carbon nanotubes (CNTs) are an example of a carbon-based nano-material, which has won enormous popularity in nanotechnology for its unique properties and applications.

         CNTs are allotropes of carbon with a nano-structure that can have a length-to-diameter ratio greater than 1,000,000. These cylindrical carbon molecules have novel properties that make them potentially useful in many applications in nano-technology, electronics, optics and other fields of materials science. They exhibit extra-ordinary strength and unique electrical properties, and are efficient conductors of heat.

        

         Sumio Iijima discovered carbon nanotubes in 1991. He was making C₆₀ molecules with the carbon arc process. In the same soot as the C₆₀ molecules, he found carbon nanotubes.
Carbons are a new form of carbon with unique electrical and mechanical properties. Their interesting electronic structure makes CNTs ideal candidates for making novel molecular devices.

         Recent discoveries of various forms of CNTs have stimulated research on their applications in diverse fields. They are very promising for the development of technological applications, such as batteries, tips for scanning probe microscopy, electrochemical actuators and sensors. The outstanding properties of CNTs such as low-weight, very high aspect ratio, high electrical conductivity, elastic modulus values in the TPa range, and much higher fracture strain values make them attractive candidates for making advanced composite materials with multifunctional features. They hold promise for applications in medicine, drug and gene delivery areas. This paper presents a brief account of their structure, properties and applications of CNTs.

structure :

         Carbons (CNTs) are hollow cylinders of graphite carbon atoms. These tubes are on the nanoscale (10^-9 m), which is so small that 10,000 of them could fit within the diameter of one human hair. This means that they can be about 1/50,000th the thickness of a human hair. A CNT is a hollow tube composed of carbon atoms. Its diameter averages tens of nanometers (10^-9 meters) and its length can vary from nano-meters to centimeters (10^-2 meters). CNTs can be thought of as a rolled-up sheet of hexagonal ordered graphite (graphene sheet,Fig.1(a)) formed to give a seamless cylinder. These cylinders may be composed of single shell single wall carbons (SWCNTs,Fig.1(b)) or of several shells multi-wall carbons (MWCNTs,Fig.1(c)). SWCNTs tend to be stronger and more flexible than MWCNTs.

Figure 1: Carbon Nanotubes are made of single graphene sheets. (a) Cut-out part of a graphite lattice, (b) SWCNT, (c) MWCNT .

 

         The nature of bonding in carbon s(CNTs) is described by applied quantum chemistry, specifically, orbital hybridization. This chemical bonding is composed entirely of sp2 bonds, similar to those of graphite. This bonding structure, which is stronger than the sp3 bonds found in diamond, provides the molecules with their unique strength. Nanotubes naturally align  themselves into "ropes" held together by Van-der-Waals forces. Under high pressure, nanotubes can merge together, trading some sp² bonds for sp³ bonds, giving the possibility of producing strong, unlimited-length wires through high-pressure nanotube linking.

         The electrical properties engineers can altered the tube structure arrangement into shapes called “armchair”(Fig.2(a)) or “zig-zag” (Fig.2(b))  or “chiral” (Fig.2(c)) changing their properties if they need  the tube to behave as a metal or a semiconductor.

         Referring to Fig.2,

§  {n=m} … Will lead to an armchair structure and will make the nanotubes metallic.

§  {n - m}…  If the result of the subtraction is a multiple of 3 the nanotubes is a semiconductor with a very small band gap, if not the nanotubes is a moderate semiconductor.

§  {n >m} … The tube will be a semiconductor.

§  Any other form it’s considered to be a chiral.

                                                                 Figure 2:  Some SWNTs with different chiralities. The difference in structure is easily shown at the open end of

the tubes. a) armchair structure b) zigzag structure c) chiral structure. Courtesy of A. Rochefort, Nano-CERCA, University of Montreal, Canada.

Properties :

         Interest in CNTs has grown at a very rapid rate because of their many exceptional properties, which span the spectrum from mechanical robustness to novel electronic transport properties.

Mechanical and Thermal Properties :

         It has become clear from recent experiments that CNTs could fulfill their promise as the ultimate high strength fibers for use in materials applications. The small diameter of CNTs also has an important effect on their mechanical properties, compared with traditional micron-size graphitic fibers. Perhaps the most striking effect is the opportunity to associate high flexibility and high strength with high stiffness, a property that is absent in graphite fibers. These properties of CNTs open the way for a new generation of high performance composites.

         Theoretical studies on the mechanical properties of CNTs are more numerous and more advanced than experimental measurements, mainly due to the technological challenges involved in the production and in the manipulation of nano-meter-sized objects. However, recent developments in instrumentation (particularly high-resolution transmission electron microscopy (HRTEM) and atomic force microscopy (AFM)), production, process and manipulation techniques for CNTs, have given remarkable experimental results. CNTs are the strongest and stiffest materials on earth, in terms of tensile strength and elastic modulus respectively. This strength results from the covalent sp² bonds formed between the individual carbon atoms. The SWCNT is about 100 times stronger than steel, yet one-sixth of its weight. Its hollow center and chicken-wire-like structure makes it very light, being 1/6th the weight of copper, and about half the weight of aluminum. As for thermal conduction, the CNT surpasses even that of diamond, reaching almost double the value diamond.

Electrical Properties :

         Rolling up a graphene sheet on a nano-meter scale has dramatic consequences on the electrical properties. Because of the symmetry and unique electronic structure of graphene, the structure of a  strongly affects its electrical properties. CNTs are regarded as molecular wires whose electronic properties are largely determined by extended molecular orbits. The chirality and diameter of a CNT is extremely important, because it influences its properties. In electrical terms, chirality and diameter determine whether a CNT will behave as a metal or a semiconductor. Depending on the specific realization, the nanotube may be a true one-dimensional metal or a semiconductor with a gap. For metallic behavior, CNTs can have conductivity up to eight times higher than that of copper. It can carry a current density achievable by any known conventional metallic wire, thus making them as potential candidates as nano-scale wires. By combining metallic and semi-conducting tubes, the whole span of electronic components ranging from wires, bipolar devices to field-effect transistors may be embodied in nanotubes. On the fundamental side, a perfect metallic nanotube is supposed to be a ballistic conductor in which only two one-dimensional sub-band carry the electric current.

Applications :

         Recent discoveries of various forms of carbon nanostructures have stimulated research on their applications in diverse fields.

Hydrogen  storage :

         The advantage of hydrogen as energy source is that its combustion product is water. In addition,

hydrogen can be easily regenerated. For this reason, a suitable hydrogen storage system is necessary, satisfying a combination of both volume and weight limitations. The two commonly used means to store hydrogen are gas phase and electrochemical adsorption.

         Because of their cylindrical and hollow geometry, and nanometer-scale diameters, it has been predicted that carbon nanotubes can store a liquid or a gas in the inner cores through a capillary effect. As a threshold for economical storage, the Department of Energy has set storage requirements of 6.5 % by weight as the minimum level for hydrogen fuel cells. It is reported that SWNTs were able to meet and sometimes exceed this level by using gas phase adsorption (physisorption). Yet, most experimental reports of high storage capacities are rather controversial so that it is difficult to assess the applications potential. What lacks, is a detailed understanding of the hydrogen storage mechanism and the effect of materials processing on this mechanism. Another possibility for hydrogen storage is electrochemical storage. In this case not a hydrogen molecule but an H atom is adsorbed. This is called chemisorption.

Electrochemical  Supercapacitors :

         Supercapacitors have a high capacitance and potentially applicable in electronic devices. Typically, they are comprised two electrodes separated by an insulating material that is ionically conducting in electrochemical devices. The capacity of an electrochemical supercap inversely depends on the separation between the charge on the electrode and the counter charge in the electrolyte. Because this separation is about a nanometer for nanotubes in electrodes, very large capacities result from the high nanotube surface area accessible to the electrolyte. In this way, a large amount of charge injection occurs if only a small voltage is applied. This charge injection is used for energy storage in nanotube supercapacitors. Generally speaking, there is most interest in the double-layer supercapacitors and redox supercapacitors with different charge-storage modes.

Dissipation-less Thermal Conductivity :

         One of the biggest problems with computer chips these days is overheating. Processors in normal PCs require large and noisy fans to stay cool and as we increase in computer ages computers tend to run at higher speeds causing more heat.

Here a situation and its possible solution:

         As we know the electrons travelling through the wires inside the processors experience some friction which leads to heat and today's coolers are not leading the complete heat out of the chip, dissipating heat into other components. By using superconducting metallic single-walled carbon nanotubes as wires this problem would be reduced remarkably. CNTs have great thermal conductivity and could lead the heat out without dissipation.

Strong and conducting plastics :

         Since Plastics are known to be good electrical insulators and they have a lot weaker mechanical properties than metals. Both of these can be changed by loading carbon nanotubes in the plastic mixture.

         Considering a certain percentage X% of  nanotubes into the mixture will result in more than enough conductivity to dissipate static electricity. The good mechanical properties of the nanotubes also make the plastics a lot stronger and stiffer. Parts can be replaceable and durable for the consumers.

Actuators/Artificial muscles :

          An actuator is a device that can induce motion. In the case of a carbon nanotube actuator, electrical energy is converted to mechanical energy causing the nanotubes to move. Two small pieces of "buckypaper," paper made from carbon nanotubes, are put on either side of a piece of double-sided tape and attached to either a positive or a negative electrode. When current is applied and electrons are pumped into one piece of buckypaper and the nanotubes on that side expand causing the tape to curl in one direction. This has been called an artificial muscle, and it can produce 50 to 100 times the force of a human muscle the same size. Applications include: robotics, prosthetics.

Space Elevator :

         The space elevator concept, which sounds like something straight out of science fiction (it was, in fact, popularized by Arthur C. Clarke) involves anchoring one end of a huge cable to the earth and another to an object in space. The taut cable so produced could then support an elevator that would take passengers and cargo into orbit for a fraction of the cost of the rockets used today. Sounds too far out? It has, in fact, been established by NASA as feasible in principle, given a material as strong as SWNTs. The engineering challenges, though, are awesome, so don't expect that 'top floor' button to be taking you into orbit any time soon.

Nano-Medicine :

         Advances in medicine are being contributed to carbon nanotubes in some way. One of the advances that researchers are working on is, that based on carbon NT and their properties it will be possible to carry DNA to cancerous cells and destroy them with minimal damage to surrounding healthy cells through methods such as is nano-shells and quantum dots.

         New nanotechnology proposes that by working with smart nano-devices they can destroy only cancerous cells leaving healthy untouched. These devices will be able to know and identify threats of cancer cells.

Challenges and future scope of CNTs :

         Despite an inevitable element of hype, the versatility of nanotubes does suggest that they might one day rank as one of the most important materials ever discovered. In years to come they could find their way into myriad materials and devices around us and quite probably make some of the leaders in this game quite rich.

         However there has been much discussion about the similarities between CNTs and asbestos within the scientific and regulatory community. This is because some CNTs are similar in shape to asbestos fibers and similar in their ability to persist in the lungs of laboratory animals. Exposure to asbestos can have serious effects on the health of those exposed, including lung fibrosis, lung cancer and mesothelioma. The fundamental question is whether or not, if inhaled, CNTs are also capable of causing these health effects. Researchers are now investigating not only whether nanomaterials are carcinogenic themselves but also whether their manufacturing can create harmful, possibly cancer-causing, by-products.

         Although there is uncertainty about the risks of exposure to CNTs, the regulatory response is to take a precautionary approach. An assessment under the Control of Substances Hazardous to Health Regulations 2002 (as amended) should be carried out for all work involving CNTs and suitable and sufficient risk management measures put in place. Any type of action carried through CNTs may result in nano-carbon-waste, the Environment Agency advises that this type of waste carbon nanotube material should be classified and coded as hazardous waste. Based on current information, they consider high temperature incineration at a hazardous waste incinerator as the preferred disposal method.

“If we are clever enough to make [nanomaterials], we are clever enough to control them”.

REFERENCES  :

1. Collins PG, Avouris P. Nanotubes for electronics. Sci. A 2000; 283: 38 – 45.

2. Rao CNR, Cheetham AK. Science and technology of nano-materials: current status and future prospects. J Mater Chem 11: 2887- 94.

3. Nano Science and Technology Institute. 2006.  28 Mar. 2006.                                                                                                                                                                                                                  (www.nsti.org/news/item.html?id=50)

4. http://en.wikipedia.org/wiki/File:Types_of_Carbon_Nanotubes.png    

         

5. http://www.phys.ttu.edu/~tlmde/thesis/CARBON_NANOTUBES.html