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Thursday, 6 August 2020

An Overview on Carbon Nanotubes

Materials of which single unit size is in between 1 to 1000 nanometers but usually is 1 to 100 nm are termed as the nanomaterials. Carbon Nanotubes ( a form of carbon allotrope) has been emerged as an amazing material of the 21 st century and attracted tremendous research interest due to its very exceptional properties. Still, rigorous research is being carried out on CNTs and CNTs based composites in almost all the spheres of science and engineering.

Carbon Nanotubes | Texpedia

Historical Development
History of CNTs is perhaps as fascinating as the nanotubes themselves. In actuality, carbon nanotubes had been discovered 30 years earlier but had not been fully appreciated at that time.

Year
Scientist
Discovery
Late 1950s
Roger Bacon
A straight & hollow tubes of graphitic carbon layers.
In 1970s
Morinobu Endo
Single-layer tubes of rolled-up graphite.
In 1991
Sumio Iijima
Multi-wall carbon nanotubes in a carbon arc discharge.
In 1993
Iijima and Bethune
Single-wall nanotubes (Buckytubes).

Structure of Carbon Nanotubes:
Carbon nanotubes are cylindrical molecules that consist of a rolled-up sheet of graphene while graphene is a 2 D monoatomic hexagonal lattice of a carbon allotrope. They are the basic structural element of other allotropes including graphite charcoal carbon nanotubes and fullerenes.

Graphene sheet | Texpedia


Classification of CNTs:

Carbon nanotubes can be in a variety of forms based on the no of constituent graphene layers. If carbon nanotubes consist of a single layer of graphene, it is called single-wall carbon nanotubes, SWCNTs while multiple sheets of graphene form the multi-walled carbon nanotubes MWCNTs.



Single-walled Carbon Nanotubes, SWCNTs:
SWCNTs are made of a single graphene sheet that is rolled into a cylindrical shape.

Single-walled Carbon Nanotube | Texpedia
Forming SWCNTs from a single Graphene sheet

Double-walled Carbon Nanotubes, DWCNTs: 
In this structure exactly two coaxial concentric graphene layers are wrapped inside each other.
Double-walled Carbon Nanotubes | Texpedia
Forming DWCNTs from 2 Graphene sheets

Triple walled Carbon Nanotubes, TWCNTs: 
This structure is the next step in approaching MWCNTs and is characterized by the presence of three concentric layers of graphene sheets.
Triple walled carbon nanotubes | Texpedia
Forming TWCNTs from 3 Graphene sheets

Multi-walled Carbon Nanotubes MWCNTs: 

Multi-walled carbon nanotubes, MWCNTs consist of multiple rolled layers of graphene.

Multi-walled Carbon Nanotubes | Texpedia
Forming MWCNTs from several Graphene sheets

Carbon Allotropes
Carbon Allotropes | Texpedia

Structural Features of Carbon Allotropes:

Isomer
Fullerene(C60)
Nanotube
Graphite
Diamond
Dimension
0D
1D
2D
3D
Bond shape
sp2
sp2
sp2
sp3
Density (g/cm3)
1.72
1.2-2.0
2.26
3.515
Bond length (nm)
1.40 (C=C)
1.44 (C=C)
1.42 (C=C)
1.54 (C-C)
Electronic Properties
Semi-conductor
Metal or Semi-conductor
Semi-metal
Insulator

Synthesis of Carbon Nanotubes (CNTs:

Arc Discharge Method | Texpedia
Arc Discharge Method


Laser Ablation method | Texpedia
Laser Ablation Method


CVD method | Texpedia
Chemical Vapour Deposition Method

Miscellaneous Synthesis Methods:
CNTs can be synthesized by-
👉Ball milling,
👉Flame synthesis,
👉SiC decomposition,
👉Graphene scrolling Dipping hot graphite in cold water.

Merits-Demerits of CNTs Synthesis Methods:


Method
Arc Discharge
Chemical Vapor Deposition
Laser Ablation
Merits
-Can easily produce SWNT, MWNTs. 
-SWNTs have few structural defects; 
-MWNTs without catalyst, 
-not too expensive, 
-open-air synthesis possible
-Easiest to scale up to industrial production;
-long length,
-simple process,
-SWNT diameter controllable,
-quite pure
-Primarily SWNTs, with good diameter control and few defects.
-The reaction product is quite pure.
Demerits
-Tubes tend to be short with random sizes and directions;
-Often needs a lot of purification. 
-CNTs are usually MWNTs and often riddled with defects
-The costly technique, because it requires expensive lasers and high power requirement, but is improving


Typical defects in an SWCNTs: 

👉(A) Five or seven-membered rings in the carbon framework instead of the normal six-membered ring leads to a bend in the tube. 

👉(B) sp3 hybridized defects (R=H and OH). 

👉(C) Carbon framework damaged by oxidative conditions, which leaves a hole lined with COOH groups. 
👉(D) Open-end of the SWNT terminated with COOH groups.
Defects in Carbon nanotubes | Texpedia

Purification: 
Determining the purity degree of nanotube material is still controversial as no unified standard has been established yet. SWCNTs purification usually employs various- 
👉Oxidation steps, 
👉Thermal treatments, 
👉Sonication treatments, 
👉Chemical Acid treatments.


Properties of Carbon Nanotubes (CNTs): 

1. Mechanical Properties 

👉Strong Like Steel 
👉Light Like Aluminum 
👉Elastic Like Plastic 

2. Thermal Properties 
👉High thermal conductivity ~2000W/m/K. 
👉Very good thermal conductors along the tube. 
👉The temperature stability is up to 2800℃ in a vacuum and about 750℃ in air. 

3. Optical Properties 
👉Stronger Raman signal. 
👉Every nanotube –colourful. 
👉Bulk nanotube samples –black. 
👉Inelastic scattering of monochromatic light. 
👉Nanotube absorbs light & show infrared luminescence. 

4. Electrical Properties 
👉Exhibit both metallic and semiconducting behaviour. 
- Armchair structured CNTs show metallic properties. 
- Chiral structured CNTs show semiconducting properties. 
👉Very high current carrying capacity. 
- Metallic CNTs can carry an electrical current density of 4×10^9A/cm^2 which is almost than 1,000 times greater than metals such as copper. 

Applications of Carbon Nanotubes (CNTs): 
👉Current Applications of Carbon Nanotubes 
- Lithium Batteries. 
- Anti-static Packaging. 
- Structural Reinforcement. 
- Enhance Fiber Composites. 
- Electrically Conductive Fillers in Plastics. 
- Bulk Composite Materials and Thin Films. 
- Control or enhance conductivity in polymers. 
- Sporting Goods like tennis rackets, baseball bats, and bicycle frames. 

👉Potential Applications of CNTs 
- Weight-sensitive applications such as kevlar. 
- Semiconducting Properties: replace existing computer chips. 
- Electrically Conductive: cost-effective replacement for metal wires. 
- Environmentally-friendly, flame-retardant additive to plastics and paints.

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