GROWTH OF CARBON NANOTUBES
SUMIT KUMAR
20120959
MANGALAYATAN UNIVERSITY
Carbon nanotubes ???• Discovered in 1991 by Lijima• It has Unique material properties• They are nearly One-dimensional
structures• There are two types Single-walled and
Multi-walled
Index
• Definition• Types• Synthesis• Properties• Applications
Definition• Carbon nanotubes are nothing but
rolled up graphene sheets in armchair structure, zigzag structure and chiral structure.
• Carbon nanotubes, composed of interlocking carbon atoms, are 1000x thinner than an average human hair – but can be 200x stronger than steel.
• Carbon nanotubes are hexagonally shaped arrangements of carbon atoms that have been rolled into tubes.
• These tiny straw-like cylinders of pure carbon have useful electrical properties. They have already been used to make tiny transistor and one-dimensional copper wire.
If:m=0 , the nanotubes are called zigzagn=m ,the nanotubes are called armchairOtherwise ,they are called chiral.
The upper view of folded sheets• Armchair
• Zigzag
• Chiral
Types
• Single Wall CNT (SWCNT)
• Multiple Wall CNT (MWCNT)
• Can be metallic or semiconducting depending on their geometry.
Single Wall CNT(SWCNT)
• Most single-walled nanotubes (SWNTs) have a diameter close to 1 nanometer, with a tube length that can be many millions of time longer.
• The structure of a SWNTs can be conceptualized by wrapping a one-atom-thick layer of graphite called graphene in to a seamless cylinder.
How to get SWCNT
Multiple Wall CNT (MWCNT)
Multi-walled nanotubes (MWNTs) consist of multiple rolled layer( concentric tubes) of graphene
• In the Russian Doll model, sheets of graphite are arranged in concentric cylinders, e.g., a (0,8) single-walled nanotube (SWNT) within a larger (0,17) single-walled nanotube.
• In the Parchment model, a single sheet of graphite is rolled in around itself, resembling a scroll of parchment or a rolled newspaper.
• The interlayer distance in multi-walled nanotubes is approximately 3.4 Å. Its individual shells can be described as SWNTs, which can be metallic or semiconducting.
Comparison
• Single –walled CNTs exhibit electric properties that are not shared by the multi-walled CNTs.
• SWNTs is useful in the development of the first intramolecular field effect transistors (FET).
Synthesis
• Chemical Vapor Deposition (CVD)• Arc-Discharge• Laser Ablation
Chemical Vapor Deposition• A substrate is prepared with a layer of
metal catalyst particles, most commonly nickel, cobalt, iron , or a combination.
• The substrate is heated to approximately 700°C.
• Two gases are bled into the reactor: a process gas (such as ammonia , nitrogen or hydrogen ) and a carbon-containing gas (such as acetylene , ethylene , ethanol or methane ).
• 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.
Laser Ablation
• Use of very strong laser• Expensive (energy costs)• Commonly applied
Another method to grow SWNTs using laser ablation was demonstrated in 1996 by Smalley's group and has prompted a lot of interest.
The synthesis could be carried out in a horizontal flow tube under a flow of inert gas at controlled pressure.
In this set-up the flow tube is heated to ~1200°C by a tube furnace. Laser pulses enter the tube and strike a target consisting of a mixture of graphite and a metal catalyst such as Co or Ni.
SWNTs condense from the laser vaporization and are deposited on a collector outside the furnace zone.
Working Process
Arc-Discharge
• Relatively cheap• Many side-products
Arc-Discharge Process
• The carbon arc discharge method, is the most common and perhaps easiest way to produce 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 CNTs from the soot and the residual catalytic metals present in the crude product.
• This method creates 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.
• A direct current of 50 to 100A, 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 CNTs in high yield depends on the uniformity of the plasma arc, and the temperature of the deposit forming on the carbon electrode.
Purification• The main impurities :graphite (wrapped
up) sheets, amorphous carbon, metal catalyst and the smaller fullerenes…
• Rules : - separate the SWNTs from the impuritiesgive a more homogeneous diameter or size distribution.
• The techniques that will be discussed are oxidation, acid treatment, annealing, ultrasonication, micro filtration, ferromagnetic separation, cutting, functionalisation and chromatography techniques.
Properties
• CNTs have High Electrical Conductivity • CNTs have Very High Tensile Strength • CNT are Highly Flexible- can be bent
considerably without damage • CNTs are Very Elastic ~18% elongation
to failure • CNTs have High Thermal Conductivity • CNTs have a Low Thermal Expansion
Coefficient • CNTs are Good Electron Field Emitters • CNTs have a High Aspect Ratio (length
= ~1000 x diameter
Applications• Thermal Conductivity of CNTs • Field Emission of CNTs • Conductive Plastics with CNTs • Energy Storage using CNTs • Conductive Connectors with CNTs • Molecular Electronics based on CNTs • Thermal Materials with CNTs • Structural Composites with CNTs • Fibers and Fabrics with CNTs
Summary
• Carbon nanotubes have very different properties compared to the other carbon allotropes- these unique properties offer huge potential in product development.
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