2. TYPES OF CARBON NANO TUBES:
Single walled CNT (SWCNT)
Multi walled CNT (MWCNT)
Can be metallic or semi conducting
3. single walled
-Most single-walled nanotubes (SWNTs) have a diameter of 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
4. multi walled
• Multi-walled nanotubes (MWNTs) consist of
multiple rolled layer( concentric tubes) of
graphene
5. • The way the graphene sheet is wrapped is represented by a pair of
indices (n,m) called the chiral vector.
• The integers n and m denote the number of unit vectors along two
directions in the honeycomb crystal lattice of graphene
• If m = 0, the nanotubes are called "zigzag". If n = m, the nanotubes
are called "armchair". Otherwise, they are called "chiral".
6. CNT: Rolling-up a graphene sheet to
form a tube
Schematic
of a CNT
SEMimage
of CNT
9. 3 Electric-Arc Method – Experimental Devices
Sketch of an electric arc reactor. It consists
of a cylinder of about 30 cm in diameter
and about 1m in height.
After the triggering of the arc
between two electrodes, a
plasma is formed consisting
of the mixture of carbon
vapor, the rare inert gas
(helium or argon), and the
vapors of catalysts.
The vaporization is the
consequence of the energy
transfer from the arc to the
anode made of graphite
doped with catalysts.
10. ARC DISCHARGE
• CNT production requires 3 elements ,
I. Carbon feed
II. Metal catalyst
III. Heat
a) Two Graphite electrodes placed in an inert Helium atmosphere
.
b) When DC current is passed anode is consumed and material
forms on cathode.
c) For SWNT mixed metal catalyst is inserted into anode
d) Pure iron catalyst + Hydrogen-inert gas mixture gives 20 to
30cm long tube.
e) The nanotubes were initially discovered using this technique, it
has been the most widely-used method of nanotube synthesis.
11. Arc evaporation technique involves evaporation of
graphite anode rod and condensation of the deposit on
the cathode rod under inert atmosphere.
A plasma is achieved by making a gas to conduct
electricity by providing a potential difference across two
electrodes
Electrodes are made of conducting materials
In arc discharge method, two high purity graphite
electrodes as anode and cathode are held a short
distance apart under a helium atmosphere.
12. Inert atmosphere/Gas
Inert gas is meant for cooling / condensation of
the sample.
The chamber must be connected both to a
vacuum line with a diffusion pump and to
helium supply.
13. Important parameters of gas
• Pressure Material
1000 torr Soot/ Carbon onions
500 torr Carbon nanotubes
100 torr Fullerenes
• 20 torr Amorphous carbon/soot
Therefore pressure & type of inert gas used
determines the structure of carbons to be
obtained.
14. Electrodes:
In arc evaporation method, graphite rods are
used as electrodes.
Graphite rods with 99.99% purity are used.
Major impurities in graphite are sulphur atoms
as Sulphur changes the morphology of CNTs.
The anode is a long rod of 6mm diameter & the
cathode is a short rod of 9mm diameter
15. Cooling of Electrode
Efficient cooling of the electrodes & the chamber
are essential to produce good quality nanotubes
and also to avoid excessive sintering.
Without proper cooling-sintering occurs-with a
hard deposit of mass
With proper cooling-sintering does not occurs-
forms a uniform deposit i.e., Homogeneous
deposit with aligned bundles of nanotubes.
16. Current & Voltage
DC power supply is given in the range of 50-100mA
and the discharge voltage is between 20 – 50 V.
DISTANCE BETWEEN ELECTRODES
The position of the anode can be adjustable from outside
the chamber.
Distance between the electrodes must be constant to obtain
a stable current.
The two electrodes are maintained at constant distance for
obtaining CNTs
When two electrodes are in contact / not at a particular
distance - fullerenes can be formed.
17. ARC DIS CHARGE PROCESS
• It 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.
18. Arc Discharge Method Parameters of SWNTs:
In the arc discharge production method sulfur functions
as a SWNTs growth promoter and surfactant when
added together with Ni/Fe/Co, Ni/Co, Ni/Y/Fe or
Ni/Ce/Fe catalysts into the anode.
The metal-sulfur interactions change surface tension
and melting point of small droplets of metals. This can
support the creation of SWNTs for metals which in pure
form catalyze badly.
The highest yield of web product containing the smallest
concentration of metals was obtained for the sample C
where the composition of the anode is Fe:Y:S:C
at6.6%:1.1%:1.6%:90.7%.
19.
20. SEM images of the samples which were synthesized
(a) in an air atmosphere at 300 Torr and
(b) in the helium atmosphere at 500 Torr
21. SEM images of MWNT’s produced by arc discharge
(Kunsan National Univ)
22. Synthesis of Carbon Nanotube
1 Laser Ablation – Experimental Devices
- graphite pellet
containing the catalyst put
in an inert gas filled quartz
tube;
-oven maintained at a
temperature of 1,200 ◦C;
-energy of the laser beam
focused on the pellet;
-vaporize and sublime the
graphiteSketch of an early laser vaporization apparatus
The carbon species are there after deposited as soot in different regions:
water-cooled copper collector, quartz tube walls.
23. 2 Synthesis with CO2 laser
Fig. 3.10 Sketch of a synthesis reactor with a
continuous CO2 laser device
Vaporization of a target at a
fixed temperature by a
continuous CO2 laser beam (λ =
10.6μm). The power can be varied
from 100Wto 1,600 W.
The synthesis yield is controlled
by three parameters: the
cooling rate of the medium
where the active, secondary
catalyst particles are formed,
the residence time, and the
temperature (in the 1,000–
2,100K range) at which SWNTs
nucleate and grow.
24.
25. 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 the laser ablation process, a pulsed laser vaporizes a graphite
target in a high-temperature reactor while an inert gas is bled
into the chamber.
Nanotubes develop on the cooler surfaces of the reactor as the
vaporized carbon condenses.
A water-cooled surface may be included in the system to collect
the nanotubes.
The laser ablation method yields around 70% and produces
primarily single-walled carbon nanotubes with a controllable
diameter determined by the reaction temperature.
it is more expensive than either arc discharge or chemical vapor
deposition.
26. Chemical Vapor Deposition
• Carbon is in the gas phase
• Energy source transfers
energy to carbon molecule
• Common Carbon Gases
– Methane
– Carbon monoxide
– Acetylene (C2H2)
http://neurophilosophy.files.wordpress.com/2006/08/multiwall-large.jpg
27. Chemical Vapor Deposition
• Carbon is in the gas phase
• Energy source transfers energy to
carbon molecule
• Usually a silicon plate coated with
iron particles is the substrate.
• Common Carbon Gases
– Methane
– Carbon monoxide
– Acetylene
28. Chemical Vapor Deposition
• After energy transfer, the
carbon molecule binds to the
substrate
• Temperature between
~1300⁰F
• Carbon nanotubes stick to
each other due to Vander
walls force.
• When tubes are extracted ,
cling on to each other and pull
each other out of substrate
• Yield is usually about 30%
• One of the most common
methods of carbon nanotube
synthesis
29.
30. Chemical vapor deposition (cvd):
During CVD, a substrate is prepared with a layer of metal catalyst articles, most
commonly nickel, cobalt, iron, or a combination.
The diameters of the nanotubes that are to be grown are related to the size of
the metal particles.
The substrate is heated to approximately 700°c.
To initiate the growth of nanotubes, 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.
31. Chemical Vapor Deposition
Advantages
Disadvantages
• Easy to increase scale to
industrial production
• Large length
• Simple to perform
• Pure product
http://endomoribu.shinshu-u.ac.jp/research/cnt/images/cat_cnt.jpg
• Defects are common
32.
33. Arc Discharge Method Chemical Vapor Deposition Laser Ablation (Vaporization)
Connect two graphite rods to
a power supply, place them
millimeters apart, and throw
switch. At 100 amps, carbon
vaporizes in a hot plasma.
Place substrate in oven, heat
to 600 C, and slowly add a
carbon-bearing gas such as
methane. As gas decomposes
it frees up carbon atoms,
which recombine in the form
of NTS
Blast graphite with intense
laser pulses; use the laser
pulses rather than electricity
to generate carbon gas from
which the NTS form; try
various conditions until hit on
one that produces prodigious
amounts of SWNTS
Can produce SWNT and
MWNTs with few structural
defects
Easiest to scale to industrial
production; long length
Primarily SWNTS, with a large
diameter range that can be
controlled by varying the
reaction temperature
Tubes tend to be short with
random sizes and directions
NTS are usually MWNTS and
often riddled with defects
By far the most costly, because
requires expensive lasers
