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The Wondrous World of Carbon Nanotubes

March 2004

Notes from Powerpoint Presentation

Overview

  • Introduction

  • General Properties

  • Structure

  • Synthesis and Purification

  • Deformation Processes

  • Applications

  • Conclusion
     

Introduction

  • Discovered in 1991 by Iijima

  • Unique material properties

  • Nearly one-dimensional structures

  • Single (SWNT) and multi (MWNT)

Understanding the Scale

Nanotubes are measured on a nanometer scale
1nm = 10^-9m = 0.000000001m

The nanometer is referring to the diameter of the nanotube

They range from 1 to 50 nm
Multiwall (MWNT): 5-50 nm diameter, 10 mm long
single-wall (SWNT): 1-10 nm diameter, 1 mm long

They are up to 10 micrometers long

Leads to an aspect ratio (length/diameter) of 1000
 

Brief History

1970's - While doing his PhD work, Morinobu Endo grew carbon fibre 5nm in length.  However these were not recognized as nanotubes, and were not studied

1991 - With the air of high-resolution transmission electron microscopy, Sumio Liijma observer nanotubes reulting from the arc-evaporation synthesis of fullerenes

1993 - To allow for the production of single-walled nanotubes the arc-evaporation process was changed to add other metals to the electrodes

1996 - Dr. Rick Smalley, at Rice University, synthesized the first bundle of single-wallled carbon nanotubes.

General Physical Properties

  • CNTs are 10 to 100 times stronger than steel

  • Good thermal conductivity

  • CNTs can be metals or semiconductors

  • Average diameter of SWNT = 1.2-1.4 nm

  • Average density of a SWNT = 1.36 g/cm3

  • All armchair tubes are metallic

  • Electronic properties of CNTs depend on chiral symmetry of the molecular fiber

  • Mechanical properties in elastic domain are believed to be independent of the chirality
     

Basic Structure

  • CNTs exist as a macro-molecule of carbon, analogous to a sheet of graphite rolled into a cylinder

  • Graphite resembles a sheet of chicken wire

  • When coiled, the carbon arrangement becomes very strong

  • CNTs have a hemispherical "cap" at each end of the cylinder

  • CNTs are light, flexible, thermally stable, and are chemically inert

  • CNTs have the ability to be either metallic or semi-conducting, depending on the "twist" of the tube
     


 

Nanotube Structure

  • Roll a graphite sheet in a certain direction

  • Defects result in bends and transitions
     

Armchair structure



Zigzag structure

Chiral structure

Defining The Type Of Structure

  • Imagine that the nanotube is unraveled into a planar sheet

  • Draw two lines (the blue lines) along the tube axis where the separation takes place

  • Find any point on one of the blue lines that intersects one of the carbon atoms (point A)

  • Draw the Armchair line (the thin yellow line), which travels across each hexagon, separating them into two equal halves

  • Now that you have the armchair line drawn, find a point along the other tube axis that intersects a carbon atom nearest to the Armchair line (point B)

  • Now connect A and B with our chiral vector, R (red arrow)

  • The wrapping angle (not shown) is formed between R and the Armchair line

  • If R lies along the Armchair line (θ =0°), then it is called an "Armchair" nanotube

  • If θ =30°, then the tube is of the "zigzag" type

  • Otherwise, if 0°< θ <30°, then it is a "chiral" tube

  • The vector a1 lies along the "zigzag" line. The other vector a2 has a different magnitude than a1, but its direction in a reflection of a1 over the Armchair line

  • When added together, they equal the chiral vector R

     


 

Structures with Flaws

  • Deformations are introduced by replacing a hexagon with a heptagon or pentagon

  • Deformations can be inward or outward

  • Electrical properties are seriously changed by these deformations

  • Another class of defects is caused by impurities that are built in during or after the CNT synthesis

 

Representation of Defects

  • Gradual transition from a large diameter to a smaller one

  • No defects in the wall of the cone, fewer pentagons in the end cap

  • Defects can also result in various new structures such as Y-branches T-branches or SWNT junctions

  • Under certain circumstances, these defects can be introduced in a "controlled" way

Special Properties of CNTs

  • Electronic, molecular, and structural properties of CNTs are determined by their one-dimensional structure

  • The most important properties of CNTs:
  • Chemical reactivity

  • Electrical conductivity

  • Mechanical strength

  • Optical activity

Special Properties of CNTs - Chemical Reactivity

  • Comparable to a graphite sheet

  • Enhanced as a direct result of the curvature of the surface

  • Reactivity is directly related to the pi-orbital mismatch caused by an increased curvature

  • A distinction must be made between the sidewall and the end caps of a nanotube

  • A smaller nanotube diameter results in increased reactivity
     

Special Properties of CNTs - Chemical Reactivity

  • Comparable to a graphite sheet

  • Enhanced as a direct result of the curvature of the surface

  • Reactivity is directly related to the pi-orbital mismatch caused by an increased curvature

  • A distinction must be made between the sidewall and the end caps of a nanotube

  • A smaller nanotube diameter results in increased reactivity

Special Properties of CNTs - Electrical Conductivity

  • Depending on their chiral vector, CNTs are either semi-conducting or metallic

  • Conducting properties are caused by the molecular structure that results in a different band structure and different band gap

  • Differences in conductivity can easily be derived from the graphite sheet properties

  • The resistance to conduction is determined by quantum mechanical aspects and was proved to be independent of the nanotube length

Special Properties of CNTs - Optical

  • Theoretical studies have revealed that the optical activity of chiral CNTs disappear when size becomes larger

  • Expected that other physical properties are influenced by these parameters

  • Optical activity might result in optical devices in which CNTs play an important role

Special Properties of CNTs - Mechanical Strength

  • Very large Young modulus in their axial direction

  • Very flexible as a whole because of the great length

  • Potentially suitable for applications in composite materials that need anisotropic properties

Synthesis: Growth Mechanism

  • Not exactly known

  • Several theories exist

  • What is known:

  • Carbide particles (C2) are formed on the surface of a metal catalyst particle

  • Due to metastability, a rod-like carbon is formed rapidly

  • Graphitization then occurs at the particles’ wall

 

Possible theory:

  • Metal catalysts are floating or are supported on graphite or another substrate

  • Deposition takes place on one-half of the surface

  • Carbon particles diffuse and precipitates on the opposite half

  • CNTs can then form by either Root Growth or Tip Growth
     

Root Growth and Tip Growth
 

 

Synthesis: Main Techniques

Main techniques for synthesis:

  • Arc discharge

  • Laser ablation (or vaporization)

  • Chemical vapour deposition (CVD)
     

Synthesis: Arc Discharge

  • Easiest way to produce CNTs

  • Produces mixture that requires separation from CNTs

  • Apparatus:

  • Two carbon rods separated by 1mm

  • Enclosure filled with inert gas @ low pressure

  • Direct current of 50-100A from 20V supply

Arc Discharge: Improving Quality

 

SWNTs

MWNTs

  • Mixture of inert gases (Helium and Argon)
  • Inert gas pressure
  • Modifying the concentration of catalyst
  • Improving oxidation resistance
  • Synthesis in liquid nitrogen (very economical)
  • Magnetic field synthesis
  • Plasma rotating arc discharge

Improving oxidation resistance

 

Plasma rotating arc discharge
 

 

 

Laser Ablation

  • Comparatively expensive technique

  • Results in higher yield for SWNT synthesis

  • Technique provides good diameter control

  • Apparatus:

  • Continuous or pulsed laser

  • Graphite target

  • 1200°C oven filled with helium or argon

Method

  • Laser targets the graphite target in the oven, producing very hot vapour

  • Very hot vapour containing carbon and catalyst molecules forms as a result

  • Carbon molecules condense to form larger clusters

  • Catalysts condense and attach to carbon clusters

  • Tubular molecules grow into SWNTs
     

Apparatus

TEN Image of SWNT

 

Laser Ablation: Improving Quality

  • Because of good quality of SWNTs, scientists are trying to scale up laser ablation for greater productivity

  • Newest developments:

  • Ultra fast pulses from a free electron laser (FEL) method

  • Continuous wave laser-powder method

Continuous wave laser-powder method:

Apparatus
 

Resultant Cross Section

Synthesis: Chemical Vapour Deposition

  • Produces SWNTs with excellent alignment

  • Control over diameter

  • Growth rate of SWNTs can be maintained

  • Apparatus:

  • Gaseous carbon source

  • Resistively heated coil

  • Catalyst coated substrate (quartz boat)

Method

  • Gaseous carbon enters gas inlet

  • Energy source “cracks” the molecules into reactive atomic carbon

  • Carbon diffuses towards substrate , forming CNTs
     

Apparatus

TEM of Highly Aligned SWNT's

 

Chemical Vapour Deposition: Improving Quality

  • Several innovative techniques for CVD have been developed:

  • Thermal chemical CVD

  • Vapour phase growth

  • Aero gel-supported CVD

  • Laser-assisted CVD

  • Alcohol catalytic CVD

  • Plasma enhanced CVD

Plasma Enhanced CVD

 

Synthesis: Comparison

 

Method

Arc-discharge

CVD

Laser ablation

Typical Yield

30 to 90%

20 to 100%

Up to 70%

SWNT Diameter

-Short tubes                   --Diameter=0.6–1.4 nm

-Long tubes

-Diameter=0.6-4 nm

-Long bundles of tubes

-Individual diameter=1-2 nm

MWNT Diameter

-Short tubes        

-Inner diameter=1-3 nm

-Outer diameter= 10 nm

-Long tubes

-Diameters=10-240 nm

-Not usually synthesized using this technique

Pros

-Easily produces

SWNTs and MWNT

-SWNT has few structural defects

-MWNT requires no catalyst, not too expensive

-Easiest to scale up to industrial production

-Long length

-Simple process

-SWNT diameter is controllable

-Quite pure

-Good diameter control

-Few defects

-Quality very high

Cons

-Tubes tend to be short with random sizes and directions

-Needs purification

-MWNTs usually develop, often riddled with defects

-Costly technique

-Expensive lasers

-High power requirement

 

Purification

Research being done to find more suitable techniques:

  • Oxidation

  • Acid treatment

  • Annealing

  • Ultrasonication

  • Micro filtration

  • Ferromagnetic separation

  • Cutting

  • Functionalization

CNT Mechanics

 

CNT strength can be illustrated by bending the CNT back and forth several times without breaking
 

 

 

 

Applications

  • Structural Materials

  • Thermal Materials

  • Electrical Conductive Materials

  • Energy Storage

  • Others

CNT Composites

 

  • By incorporating CNTs into composites, nanoscopic properties can be translated into macroscopic properties

  • Properties of CNT composites:

  • Light weight

  • Strong

  • Electrical conductivity

  • Thermal conductivity

  • Composite performance is dependent on dispersion, orientation, interfacial bonding, and CNT deformation within matrix

  • Known theories for polymer/carbon fibre reinforcements cannot predict the expected behaviour

  • Theoretical calculations indicate that properties of CNT composites will result in superior properties

 

Associated Problems

 

  • High surface area and chemical resistance

  • Manipulation is difficult

  • Costly with many impurities

  • Cannot interface properly with different materials

  • Need to purify and disperse the naturally forming bundles

Construction

  • Several Different techniques

  • Stacking single-molecule layers of CNTs and polymer on top of each other

  • Dipping into CNT solution, then into a polymer solution, causing one layer of CNT to stick to surface

  • Can be made stronger by attaching chemical groups into CNT

Testing

  • Deformation tests in uniaxially aligned MWNT in a thermoplastic matrix.

  • Well dispersed without significant aggregation

  • Degree of alignment determined by x-ray diffraction and TEM

  • Better load transferring efficiencies under compression than in tension

Buckling

  • Used to estimate the onset strain from buckling and critical fracture strain

  • Sharp bends in MWNTs

  • Reduction in diameter
     

 

  • Strain is measured as the ratio of outer radius and radius of curvature.

  • Under moderate strain, bucking is reversible (up to 8% strain)

  • Restores itself to original shape upon heating the polymer matrix by electron irradiation

  • Buckling occurs with strain over 4.7%
     

 

 

Fracture Strain

 

Highest strain was 18% (lower limit for fracture strain)

Contact and adherence of the polymer to CNT

Results

  • Load transfer to CNT was not sufficient for fracture

  • The failure of composites arise from pullout of the CNT and mechanical fracture of the polymer matrix

  • Different techniques are being investigated

 

Other Composite Applications

  • Addition of CNT into polymers can significantly improve the electrical conductivity

  • Thermal conductivity of the polymer increases

  • Smart materials that can act as sensors

Field Emission Display Device

  • CNT’s can be made to emit electrons from their tips

  • Useful as the electron source for Flat Panel Display with lower power consuming and high voltage circuit is unneeded

Other Applications

  • Nanoprobes and sensors

  • Nano tweezers

  • Supersensitive Sensors

  • Hydrogen storage

  • Energy storage

  • Electrical circuits and transistors

  • Biological applications

  • Space Elevators

  • Nano gears and levers

Conclusions

  • CNTs have unique properties.

  • Lots of research

  • Synthesis and Purification has to be developed to reduce cost, increase output, and control properties

  • Controlling properties is the key to future applications

  • Electronic applications will be seen before structural applications

  • Application are still new and need to be developed in order to bring it to mass market