Carbon Nanotubes

General Fact

Graphite vs. DiamondsWhat's the difference between graphite and diamonds?

Both materials are made of carbon, but both have vastly different properties. Graphite is soft; diamonds are hard. Graphite conducts electricity, but diamonds are insulators and can't conduct electricity. Graphite is opaque; diamonds are usually transparent.

Graphite and diamonds have these properties because of the way the carbon atoms bond together at the nanoscale.

A carbon nanotube is a Nano-size cylinder of carbon atoms. Imagine a sheet of carbon atoms, which would look like a

sheet of hexagons. If you roll that sheet into a tube, you'd have a carbon nanotube.

Carbon nanotube properties depend on how you roll the sheet.

In other words, even though all carbon nanotubes are made of carbon, they can be very different from one another based on how you align the individual atoms.

With the right arrangement of atoms, you can create a carbon nanotube that's hundreds of times stronger than steel, but six times lighter

Engineers plan to make building material out of carbon nanotubes, particularly for things like cars and airplanes.

Lighter vehicles would mean better fuel efficiency, and the added strength translates to increased passenger safety.

Carbon nanotubes can also be effective semiconductors with the right arrangement of atoms.

Scientists are still working on finding ways to make carbon nanotubes a realistic option for transistors in microprocessors and other electronics.

Carbon Nanotube• 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

Introduction: nanotube structure

Roll a Graphene sheet in a certain direction:• Armchair structure• Zigzag structure• Chiral structure

• Single-walled carbon nanotubes exist in a variety of structures corresponding to the many ways a sheet of Graphene can be wrapped into a seamless tube.

• Each structure has a Wrapping Angle say (a).

Explanation for structure• The analogy of a rolled sheet of graphite is however used to

define the many important electrical and mechanical properties that carbon nanotubes exhibit. Using a De Heer abacus the various configurations of carbon nanotubes can be defined by their “roll up” vector. The rolling angle lies between 0 and 30 degrees with the two extremes referred to as “zigzag” and “armchair” respectively. Any nanotube with a roll angle in-between is referred to as “chiral”.

• This vector can be defined as a linear combination of base vectors a and b of the hexagon, i.e.

r =n a +m b

A upper view of folded sheet

Rolling a graphene sheet to get SWNT

Working model of SWNT

Armchair

• The “armchair” structures, with a = 30°, have metallic character

Zigzag

• The “zigzag” tubes, for which a = 0°, can be either semimetallic or semiconducting, depending on the specific diameter.

Chiral

• Nanotubes with chiral angles intermediate between 0 and 30° include both semimetals and semiconductors. (“Armchair” and “zigzag” refer to the pattern of carbon–carbon bonds along a tube’s circumference.)

Special properties

• Difference in chemical reactivity for end caps and side wall

• High axial mechanical strength• Special electrical properties:

• Metallic• Semi conducting

Properties of nanotubes

                        • 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

MWNT

Working model of MWNT

Synthesis: overview• Commonly applied techniques:

• Chemical Vapor Deposition (CVD)• Arc-Discharge• Laser ablation

• Techniques differ by:• Type of nanotubes (SWNT / MWNT )• Catalyst used• Yield• Purity

CVD

•Gas phase deposition•Large scale possible•Relatively cheap

Chemical Vapour Deposition

Essentials for a chemical vapour deposition

Applications

• CVD is used to grow a thin layer of advanced materials on the surface of a substrate.

• Applications may be found in the areas of:

integrated circuits, optoelectronic devices and sensors

catalysts micromachines, and fine metal and

ceramic powders protective coatings

Laser ablation

• Use of very strong laser• Expensive (energy costs)• Commonly applied

Laser ablation process

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 simulation of Laser ablation

Arc discharge

• Relatively cheap• Many side-products

Arc discharge Apparatus

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

• Contaminants:• Catalyst particles• Carbon clusters

• Smaller fullerenes: C60 / C70

• Demerits in purification of Nanotubes:• Completely retain nanotube structure• Single-step purification • Only possible on very small scale:

• Isolation of either semi-conducting SWNTs

Purification techniques• Removal of catalyst:

• Acidic treatment (+ sonication)• Thermal oxidation• Magnetic separation (Fe)

• Removal of small fullerenes• Micro filtration• Extraction with CS2

• Removal of other carbonaceous impurities• Thermal oxidation• Selective functionalisation of nanotubes• Annealing

Applications

• Thermal Conductivity of CNTs • Field Emission of CNTs • Conductive Plastics with CNTs • Energy Storage using CNTs • Conductive Adhesives and Connectors with CNTs • Molecular Electronics based on CNTs • Thermal Materials with CNTs • Structural Composites with CNTs • Fibers and Fabrics with CNTs • Catalyst Supports using CNTs • Biomedical Applications of CNTs • Air and Water Filtration using CNTs • Ceramic Applications with CNTs • Other Applications

By,Gopinathan rm