Carbon Nanotubes

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Chapter 6 : Carbon Nanotubes

Carbon Nanotubes (CNT) arrow_upward

  • A Carbon Nanotube (CNT) can be modeled as a hexagonal lattice (graphene) “rolled up” into a cylinder.
  • This is a nanoscopic structure made of carbon atoms in the shape of a hollow cylinder.
  • The cylinders are typically closed at their ends by semi-fullerene-like structures.
  • A nanotube may consist of one tube of graphite, a one-atom thick called Single Wall CNT (SWCNT) or a number of concentric tubes called Multi Wall CNT (MWCNT).
  • When viewed with a transmission electron microscope these tubes appear as planes.
  • Whereas single walled nanotubes appear as two planes, and in multi walled nanotubes more than two planes are observed, and can be seen as a series of parallel lines.

  • Single Wall CNT arrow_upward

  • The SWCNT has only a single wall.
  • The structure of SWCNT can be conceptualized by wrapping a one-atom-thick layer of graphite (called graphene) into a seamless cylinder.
  • The diameter of a SWCNT varies from 0.3 nm to 3 nm, while its length varies from a several microns to a few millimeters.
  • Theoretically SWCNT can be infinite in length.

  • Multi Wall CNT arrow_upward

  • MWCNTs are characterized by formation of nanotubes within nanotubes.
  • MWCNTs are formed with concentric graphitic shells.
  • The diameter of a MWCNT varies from 5 nm to 80 nm, and they are a few microns long.
  • In general, the MWCNTs are shorter and wider than the SWCNTs but their structures are more prone to defects.
  • When the diameter of the MWCNT is in the range of a few microns, the defects are more pronounced and the material is referred to as carbon fibers.
  • A CNT is characterized by its chiral
  • vector.
  •  Chiral angle with respect to the zig-zag axis,

    n, m are integers,

    ,  are unit vectors.

  • Depending on how the graphene plane is 'cut' before it’s rolled up, three types of carbon nanotubes are obtained:
    • Armchair
    • Zig-zag
    • Chiral (helical)

  • Rolling up the sheet along one of the symmetry axis gives either a zig-zag (m = 0) tube or an armchair (n = m) tube.
  • Armchair CNT have (n, n) and a chiral angle of 30.
  • Zig-zag CNT corresponds to (n, 0) or (0, m) and have a chiral angle of 0°.
  • Chiral CNT have general (n, m) values and a chiral angle of between 0° and 30°.
  • It is also possible to roll up the sheet in a direction that differs from a symmetry axis to obtain a chiral nanotube.
  • The diameter of the nanotubes (n, m) depends on the values of n and m.

  • CNT Fabrication Processing Technique arrow_upward

  • There are three methods for CNT fabrication:
    • Arc Discharge Method,
    • Chemical Vapor Deposition,
    • Laser Ablation or Pulsed Laser Vaporization.

    Arc Discharge Method arrow_upward

  • The Arc Discharge Method produces a number of carbon nanostructures such as fullerenes, whiskers, soot and highly graphitized carbon nanotubes from high temperature plasma that approaches
  • 3700° C.
  • In an Arc Discharge Method, a carbon is vaporized between two carbon electrodes, the anode and the cathode in a noble gas (helium or argon) environment.
  • Schematic representation of a typical arc discharge unit is presented in the figure below:
  • It has conveniently been used to produce both SWNTs and MWNTs as revealed by Transmission Electron Microscope (TEM) analysis.
  • An arc discharge with a cathode containing metal catalysts (such as cobalt, iron or nickel) mixed to graphite powder results in a deposit containing SWCNTs.
  • An arc is struck between two pure graphite electrodes in a gas atmosphere produces MWCNTs.

  • Chemical Vapor Deposition arrow_upward

  • In this technique, carbon nanotubes grow from the decomposition of hydrocarbons at a temperature range of 500 to 12000 C.
  • They can grow on substrates such as carbon, quartz, silicon etc. or on floating fine catalyst particles, e.g. Fe, Ni, Co etc. from numerous hydrocarbons e.g. benzene.
  • Thermochemical decomposition of organic material at elevated temperature deposits carbon (as soot) and carbon nanotubes on reactor wall.
  • SWCNTs use carbon monoxide (CO) or methane () for a carbon source and a much higher growth temperature (900 - 1200°C).
  • MWCNTs use acetylene gas for the carbon source and a growth temperature between 600 - 800°C.

  • Laser Ablation (Pulsed Laser Vaporization) arrow_upward

  • Laser Ablation technique involves the use of a laser beam to vaporize a target of a mixture of graphite and metal catalyst such as cobalt or nickel at a temperature approximately 12000 C in a flow of controlled inert gas (argon) and pressure.
  • The nanotube deposits are recovered at water cooled collector at much lower and convenient temperature.
  • This method was used in early days to produce ropes of SWNTs with remarkably uniform narrow diameters ranging from 5-20 nm, and high yield with graphite conversion greater than 70-90%.

  • Properties of CNT arrow_upward

  • The strongest and most flexible molecular material because of C-C
  •     covalent bonding.
  • Very high current carrying capacity.
  • Strain  much higher than any material.
  • Thermal conductivity  in the axial direction with small values in the radial direction.
  • Small size offers faster switching speeds (100GHz) and low power.
  • Young’s Modulus is over 1 Tera Pascal.

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