Carbon Nanotubes

On this webpage I present information concerning the thermal properties of carbon nanotubes. I have attempted to provide the necessary background information to understand these concepts though no single source can begin to cover all the fundamentals of this topic. If you are interested, I encourage you to look at the Additional References listed at the end of each section as they will provide much more detailed lessons of many of the topics I briefly review.

Carbon

Carbon exists in many different forms such as diamond and graphite. In all of these materials, the carbon atoms form directional covalent bonds, sharing their electrons. Due to the similar energy of the 2s and 2p orbitals, orbital hybridization often occurs. These lead to strong bonds with very small distances between each carbon atom (~1.41 - 1.44 A).

Nanoscale

Structures that range in size from 1 nanometer (10-9 meters) to 100 nanometers are considered to be on the nanoscale. 1 nanometer is only 14 times larger than a single Carbon atom.

Description of Carbon Nanotubes

A carbon nanotube can be thought of as a single layer graphite sheet, one carbon atom thick, that is rolled into a tube. Each graphite sheet is comprised of hexagonal sets of carbon atoms as shown below1.

HexagonalCAtoms.jpg

Carbon nanotubes are described using the (n, m) notation. These numbers correspond to the way the graphite sheet is rolled. One hexagon at the corner of the graphite sheet is designated as the origin (0, 0). This origin is then superimposed onto another hexagon which is n hexagons along the circumference and m hexagons up from the edge2.

Notation.jpg

The way in which a nanotube is rolled affects its properties. There are three types of nanotubes formed depending on the (n, m) values. They are the armchair (n = m) nanotube, the zig-zag (m = 0) nanotube, and the chiral nanotube formed by any other (n, m) pair.
Moreover, if n - m = 3k, where k = 0, 1, 2…, then the nanotube is metallic. If this relation does not hold, then the nanotube is semiconducting. The ability for a nanotube to have different electronic properties based solely on its structure is very interesting.

Carbon nanotubes can be made up of a single rolled graphite layer, which is called a single-walled nanotube (SWNT) or a few layers rolled up together into a series of concentric tubes called a multi-walled nanotube (MWNT).

History of Carbon Nanotubes3

Carbon Nanotubes were discovered by Sumio Iijima of the NEC Corporation in 1991. The nanotubes pictured below are MWNT varying in diameter. The nanotube forming the inner tube of the MWNT in c has a diameter of only 2.2nm.

IijimaEMicrograph.jpg

Fabrication

Today, carbon nanotubes are fabricated using three primary methods4.

The first is the spark method. This involves sending a charge through two graphite rods placed a few millimeters apart. The charge vaporizes the carbon atoms and when the recondense they form nanotubes. This method produces SWNT and MWNT at low yield.

The second method is chemical vapor deposition. In this process, a substrate is heated to 600C and exposed to a Carbon gas such as methane. The carbon atoms are freed and recombine into nanotubes. CVD is capable of producing very high yields of MWNT, but the defect concentration tends to be higher.

The third method uses a laser to produce the same effect as the spark in the first method. This process produces SWNT and can be tuned in order to control nanotube diameter, but it is the most expensive of the three.

These methods often create "mats" consisting of millions of nanotubes5

Nanotubes.jpg

Models

Software is available for free on the internet which allows you to model various nanotube structures. A few in particular are:
http://www.photon.t.u-tokyo.ac.jp/~maruyama/wrapping3/wrapping.html
http://www.ugr.es/~gmdm/contub.htm

Some Properties of Carbon Nanotubes6

Carbon nanotubes have a tensile strength of 45 billion Pascals (22 times greater than high strength steel alloys). A Young's modulus of 1 to 1.8 TPa has been measured (20% better than graphite fibers). The current capacity is estimated to be 1 billion amps/cm2 (1,000 times greater than copper wire).

Additional References

  • Iijima, S., Ichihashi, T., Single-shell Carbon Nanotubes of 1-nm Diameter, Letters to Nature, 1993
  • Endo, M., Iijima, S., and Dresselhaus, M.S., Carbon Nanotubes, 1996
  • http://en.wikipedia.org/wiki/Carbon_nanotube
  • http://www.pa.msu.edu/cmp/csc/ntproperties/
  • Wang, Z. L., Characterization of Nanophase Materials, 2000
  • Baughman, R., et al., Carbon Nanotubes—the Route Toward Applications, Science, 297, 787, 2002
  • Gogotsi, Y., Carbon Nanomaterials, 2006
  • Dai, L., Carbon Nanotechnology, 2006
  • O'Connell, M., Carbon Nanotubes - Properties and Applications, 2006
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