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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