Application and sintering
of nanotubes
Lecture 10
MTX9100
Nanomaterials
OUTLINE
-What are NT good for?
- Where can NT be used?
- Can NT-based transistors exist?
- How to make nanotubes?
1
Carbon nanotubes
Fantastic Mechanical properties
• stiffness &strength due to Sp2 bond
• Much stronger structure than diamond
• May replace steel in the future
Construction purposes like using nanocomposites -bridges and
dams
Light- Low mass. Aerospace industry
to build aircrafts and satellites.
Thermal and Electrical conductivity
•
2
Electrical devices as interconnects
transistors , CMOS industry, Nanowires
Schematic illustrations of the structures
3
Applications of CNT
• Carbon nanotubes can be inert and can have a
high aspect ratio, high tensile strength, low
mass density, high heat conductivity, large
surface area, and versatile electronic behavior
including high electron conductivity.
• Their secondary structures such as ropes,
fibers, papers, thin films with aligned tubes,
etc., have their own specific properties.
• High quality single shell carbon nanotubes can
cost 50–100 times more than gold.
Current applications
4
– Near field microscope probes
– Field-emission based devices
– Chemical sensors
– Catalyst support
Electrical properties
5
Nanoelectronics
CNTs have been sought as the new generation of
interconnect structures as well as field-effect
transistors.
Interconnects, which carry the electrical signals
between transistors, are currently made of copper,
but as electronic circuits continue to shrink, copper
interconnects will suffer from overheating.
Electronic transport in metallic SWNTs occurs
ballistically along the nanotube length, allowing the
conduction of high currents with no heating.
Due to the lack of phonon and/or impurity scattering
perpendicular to the tube, CNTs behave as 1-D
ballistic conductors.
6
Electronic applications
CNT transistor
7
Field-effect transistor components
The current flows through a CNT
with semiconductor properties
along a path called the channel.
When a small voltage is applied to
the silicon substrate, which acts as
a gate in FETs, the conductivity of
the CNT can change by more than a
million times, allowing a FET to
amplify a signal.
By controlling the current, the
top CNTs can be made to contact
the bottom CNTs, producing a
metal-semiconductor junction that
acts as a switch.
8
SWNT transport properties
• SWNTs are ideal quantum wires.
• Metallic SWNTs can transport huge current
densities (max. 109 A/cm2) without being damaged, i.
e. about three orders of magnitude higher than in Cu.
• SWNTs are model systems to study one-dimensional
charge transport phenomena.
• Visible light is strongly absorbed. It has been
observed that flash illumination with a broadband
light can lead to spontaneous burning of a macroscopic
sample of agglomerated (i. e., ropes) carbon nanotubes
in air and room temperature.
9
• SWNT thermal conductivity is comparable to that
of high purity diamond.
Fuel cells and batteries
CNTs have been sought to store hydrogen, particularly for
automotive applications, where hydrogen should be contained in small
volumes and weights, yet enabling reasonable driving distances
10
Still in the area of fuel cells, there is currently an increasing
interest in using CNTs as a support material for the catalyst
nanoparticles present in the cathode and anode electrodes of a fuel
cell. The introduction of CNTs is expected to enhance the
conductivity of the support and reduce the mobility of the catalyst
nanoparticles.
Supercapacitors
CNTs exhibit high porosity,
large specific surface area,
high electrical conductivity,
and chemical stability.
In a conventional capacitor, energy is
typically stored by the transfer of
electrons from one metal electrode to
another one separated by an insulating
material. The capacitance depends on
the separation distance and the
dielectric material inserted between
electrodes. In the case of a
supercapacitor, there is instead an
Each layer contains a highly porous
electrical double layer.
electrode suspended within an electrolyte.
An applied potential on the positive electrode attracts the negative ions,
whereas the potential on the negative electrode attracts the positive ions. A
dielectric between the electrodes prevents the charges from crossing
between two electrodes. If the electrodes are made of CNTs, the effective
charge separation is about a nm, compared with separations on the order of
micrometers for ordinary capacitors. This small separation, combined with a
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large surface area, is responsible for the high capacitance of these devices
Nanotube nanocomposites
Due to the outstanding modulus and tensile strength
resulting from the covalent bonds between the carbon
atoms, CNTs are one of the strongest materials known. In
addition, CNTs exhibit a high aspect ratio.
Therefore, CNTs are ideal as a reinforcement phase.
Attaching chemical groups – benzyne –
nanotubes in
polyepoxide
12
soldier’s armour
Epoxy glue 3 times harder
Carbide strength – alternative layers
polymer, nanotubes
Thin layer of composite on other
materials, nanocomposite paint, grease
Water filtration
Limited water source ,water contamination, toxic pollutants and hazardouse
chemicals is a great danger for animals and human .
Dye =toxic polutant (textile, paper and carpet industries waste water )
cause environmental contamination they accumulate in nature and cause
hight toxicity chronic effect on human health (concentration and
time of exposure)
f-MWNT can remove dyes
High aspect ratio (surface to volume) it has high porosity and ability to
adsorb substances .
f-MWNT are hydrophilic and they have the ability to form bonds with
organic materials c=c.
Adsorption is considered superior technique for filtration purposes:
Low cost, availability, simplicity and ease of operation.
13
Even after oxidation with nitric acid CNTs showed exceptional enhanced
adsorption.
Comparison of other
materials to CNT
Material
Young’s
Modulus (GPa)
(Modulus of
Elasticity)
Yield Strength
(Gpa)
Concrete, High
Strength
30
0.04 ?
Aluminum
69
0.095
Titanium Alloy
105-120
0.73
Si
170
?
Steel
200
0.69
Diamond
1050-1200
?
SWCNT/MWCNT
1050/1200 (same
as diamond)
~200
CNT cable
Super strong, light weight
14
NT new structures
AY-branch, the defects are marked in blue.
Introduction of defects can also result in
various new structures such as Y-branches Tbranches or SWNT junctions.
Under certain circumstances, these defects
15can be introduced in a ‘controlled’ way.
A transition from a
metallic to a
semiconducting SWNT.
The change is made by
insertion of pentagons
and heptagons.
Images of NT
multiwall nanotubes
from batch arc reactor
HRTEM image of 3 Ao CNT formed
inside MWNT.
The diameter of 3 Ao CNT and the
interlayer space of MWNT, 3:4 Ao,
are indicated by a pair of lines
marked by arrows. Each end is
marked by two horizontal arrows, A
and A’, and is capped by half of a
C12 cage containing two tetragons.
16Smallest Carbon Nanotube Is 3 Ao in Diameter – Physical Review Letters (92, 12, March 2004) X. Zhao,1,* Y. Liu,2 S. Inoue,1 T. Suzuki,1 R.O. Jones,2 and Y.
Ando1 1Department of Materials Science and Engineering, 21st Century COE Program ‘‘Nano Factory,’’ Meijo University, Nagoya 468-8502, Japan
Making nanotubes
Electric arc - batch reactor scale up - continuous reactor
cathode deposit
batch reactor operation
17
Arc discharge method
In arc discharge, a vapour is created by an arc discharge between two
carbon electrodes with or without catalyst.
The carbon arc discharge method is
the most common and perhaps
easiest way to produce CNTs.
It is a technique that produces a
complex mixture of components, and
requires purification - to separate
the CNTs from the soot and the
residual catalytic metals present in
the crude product. This method
creates CNTs through arcvaporization 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.
Arc discharge in hydrogen
Recent investigations have shown
that it is also possible to create
CNTs with the arc method in
liquid nitrogen. 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
HREM image of multiwalled carbon
deposit on the other electrode.
nanotube (MWNT) prepared by arc
Producing CNTs in high yield
discharge in hydrogen. Note a single wall depends on the uniformity of the
3 Å diameter carbon nanotube
plasma arc, and the temperature
AA’ inside MWNT
of the deposit forming on the
[X. Zhao et al., Phys. Rev. Lett. 92 (2004) 125502]
carbon electrode.
Laser production
In the laser ablation technique, a
high-power laser beam impinges on a
volume of carbon –containing
feedstock gas (methane or carbon
monoxide).
Now laser ablation produces a very
small amount of pure nanotubes,
while an arc‐discharge method
produces in general large amounts of
the impure material.
20
Chemical vapour deposition (CVD)
Acetylene over iron
Also can use suspended catalyst
nanoparticles to form suspended NT
nanoparticles 700OC
forms MWNT covered
with amorphous C on outer
layer
Ethylene, hydrogen +
methane over Co, Ni, Fe
nanoparticles at 1000OC
forms 70-80% SWNT
uncapped
2CO -> C + CO2 also forms
NT
Low stiffness, strength of NT compared with those from high T arc
production
21
CVD results in MWNTs or poor quality SWNTs.
The SWNTs produced with CVD have a large diameter range, which can be
poorly controlled. But on the other hand, this method is very easy to scale up,
what favours commercial production.
CVD
CVD process has mostly two main steps which
first is preparing substrate by sputtering
then to use thermal annealing to have catalyst
nanoparticles on the substrate .
Large scale production and high yield production
Low cost
Continuous production instead of batch production
Control of the quality and CNT
Ability to manipulate
No separation of unwanted by-products
22
Difficulties in NT production
An issue that is important in the fabrication of CNTs is the
large concentration of impurities that remain
embedded inside the CNT network after processing.
As a consequence, the powder needs to be filtered to
reduce the amount of impurities present. This is normally
achieved by acid oxidation, gas oxidation, or
filtration.
However, these methods may dissolve some of the CNTs,
cause structural damage to CNTs, or be unable to remove
large particle aggregates.
In addition, these purity-driven techniques tend to be very
expensive.
23
Purification techniques (1)
Oxidation
By oxidation we can partially purify CNTs from impurities ( time
and temperature of exposure of the process are very important)
While oxidation –COOH or –OH groups are generated which help
the attachment of organic or inorganic material to increase
solubility.
Best way is to mildly oxidize them with H2O2 and H2SO4 which
only causes oxide defects.
Acid treatment
By acid treatment mostly metal catalysts are removed by the
reaction with Nitric acid or Sulfuric acid or a mixture of both.
Annealing
In this method a high temperature is applied (800-1800 °C) in a
vacuum atmosphere which caused CNT atoms to rearrange and
form a perfect CNT, metal is melted and also can be removed
from the reaction
24
Purification techniques (2)
Ultrasonication
Ultrasonic created a low pressure and high
pressure waves in the liquid and it improves
the reaction and causes reactants to be
mixed. It forces particles to vibrate and
disperse in the liquid evenly.
Magnetic Purification
Microfiltration by using a membrane.
25
Endohedral functionalization
Modification of CNT by putting nanoparticles inside the tube.
Change the hydrophobic structure to hydrophilic and make them as
solvents.
Filling Nanotubes with nanoparticles to add the characteristics of the
Nanoparticles inside the Carbon Nanotubes to fantastic phenomenal of
CNT.
This method itself is sub categorized to two methods:
1. Putting CNT inside the suspension containing nanoparticles so that it can
penetrate the tube internal site and stay inside the CNT
Depends on surface energy (surface tension ) of the liquid.
Experiments show that if surface tension of the liquid is more than
200 mN/m, liquid can fill the Nanotubes
2. Filling with a material which reacts with it and then produces
26
nanoparticles which are trapped
Exohedral functionalization
Modification of external part of CNTs like side walls.
This method itself is subcategorized into three main methods :
1. Covalent Exohedral functionalization - Defects
defect in CNT = best place for functionalization.
2. Covalent Exohedral functionalization -Functional groups
Side wall functionalization & to attach more functionalized group.
3. Noncovalent exohedral functionalization- Polymer wrapping
wrapping CNT in polymer, surfactants and peptides (smaller amino
27
acids in length).
By wrapping the polymer around the CNT there is a phenomena called
Pi stacking.
Pi stacking is when the P orbitals of CNT and functionalized group
interact with each other and cause less stability.
In this type of functionalization electrical and optical properties of
CNTs are not damaged and perturbed but because of poor interaction
of p orbitals, stability is quite low.
As experiments show there is an improved electrical property of
the polymer