Carbon Nanotubes – One of the Greatest Nanotech innovations
Abstract :
The paper contains information on carbon nanotubes: their origin, structure, manufacturing techniques, properties, applications(both current and future).
Structure: Various structures of carbon nanotubes, the carbon carbon bonding and the geometry of the bonds, and some additional information about bond energy, bond length etc.
Manufacturing techniques: Regarding different methods of production of both single-walled and multi-walled nanotubes using different carbon containing elements as raw materials.
Properties: The electrical, thermal, and mechanical properties of both single-walled and multi-walled carbon nanotubes and their variation with structure, particularly chirality.
Applications: Current use of carbon nanotubes, future applications of nanotubes so as to exploit their properties to maximum for multiple uses.
Table of contents :
Introduction
Origin(History) of carbon nanotubes
Structure of nanotubes
Manufacturing techniques of nanotubes
Properties of single-walled and multi-walled nanotubes
Applications of nanotubes
Introduction :
Carbon nanotubes are the members of fullerene family that were discovered much later after some other members such as Buckminster fullerene(Bucky balls). Nanotubes are just a rolled version of a single sheet of graphene, but have properties that are much different than graphene. They are called nanotubes as their average diameter is around 1nm i.e., which is 100 times thinner than a human hair. These materials have some very useful properties due to their orientation and their unique length to diameter ratio.Their properties include very high mechanical strength, high electrical and thermal conductivity. The nanotubes are hollow cylinders with one atom thick walls.
Where can these nanotubes be used?
Can they replace traditional copper wires in electrical applications?
Can they replace steel in various industrial uses?
These are some of the questions arising in the mind of every material scientist in this century. If the answer to the questions is “yes”, it can cause a major breakthrough in every field.
The content ahead aims to find some best possible answers to these questions considering both advantageous and disadvantageous properties of Carbon nanotubes.
History of carbon nanotubes
The history of carbon nanotubes dates back to 1952 when some scientists found hollow carbon fibres of about 50nm in diameter. A major discovery was made when Roger Bacon(at Union Carbide) found MWNT(Multi Walled Nanotubes) while studying carbon near it’s triple point.
The synthesising of carbon nanotubes was first demonstrated by Japanese scientist, Sumio Iijima. Inspite of many disputes going on regarding who should be given the credit, at present Sumio Iijima is recognised as the one who found carbon nanotubes. He used electric arc discharge technique to synthesize nanotubes. He led a group at NEC(Nippon Electric Company) which discovered Single Walled Nanotubes after two years of his first foundings.
Structure of carbon nanotubes
Graphene sheet (b) SWNT(Single Walled Nano-Tube)(c) MWNT(Multi Walled Nanotube)
As can be seen, each carbon atom is bonded to 3 other carbon atoms.These are bonds, which are stronger than bonds.
The bond length of carbon-carbon bonds, for (5,5) nanotube is around 1.41 to 1.44 .
The bond energy is around 348 KJ/mol.
Assume that a nanotube is made by rolling a graphene sheet in a particular direction, let that direction be given by a vector called Chirality vector, . Assume two vectors and aligned as shown in the figure below. The vector can be represented by it’s components along and .
Let .
As this chirality vector defines the structure of nanotubes, the factors (n,m) are used to generally refer to chirality of nanotube. Varying n and m varies the structure and properties of a nanotube.
For Example, if m=0, the nanotube has a zig-zag structure and if n=m, the nanotube has armchair structure.
This structural classification also helps to determine the diameter of the nanotube. It is given by,
3. Manufacturing Techniques of nanotubes
There are many manufacturing techniques for producing carbon nanotubes, here we will discuss two of the important ones.
3.a. Electric arc discharge:
A container is filled with deionised water(due to it’s great cooling capability and as it provides insulation from atmospheric oxygen).
Anode: Graphite electrode of 7mm diameter.
Cathode: Graphite electrode of 20mm diameter.
DC current of 100 to 200 A at a voltage of 20 to 30 V is supplied.
MWNTs are produced at a current voltage combination of 50 A and 20 V.
On varying current voltage combination, nanotubes of different diameter can be observed, in a range of 15 to 150nm.
3.b Chemical Vapour Deposition:
Carbon containing gas: Acetylene
Catalyst: Nickel
A layer of nickel is made on a 1cm by 1cm glass substrate, and this glass substrate is inserted into a quartz tube which is in turn kept into a ceramic boat and this ceramic boat is then kept into a furnace.
The furnace is heated upto 750 along with the flow of Hydrogen and Argon
Once the temperature reaches 750, flow of Argon is stopped and Acetylene is introduces into the furnace. Acetylene flow is kept on for about 30 mins after which argon flow is started again.
The furnace is allowed to cool down to room temperature.
4. Properties of nanotubes
Nanotubes are a topic of high research interest due to their incredible properties. Nanotubes have some properties that are highly useful for both industrial and domestic purposes.
4.a Electrical Properties of nanotubes:
Carbon nanotubes have great conductivity, in some particular structural arrangements. According to the structural classification discussed earlier, if n=m, i.e., Armchair nanotube, it will be metallic, if
n-m is divisible by 3, along with nm and nm0 then the nanotube will be quasi-metallic(i.e., it will have a very small band gap). In all other cases, the nanotube will be a poor semiconductor.
If the nanotube is conducting, it only allows electron movement along the axial direction, which is the reason for it’s high conductivity. Motion restriction of electron only along axial direction reduces scattering which leads to lower resistance.
To understand the conductivity, let’s compare it with copper:
Mean free path(Average distance an electron travels without collision with obstacles such as impurities etc.) of electrons in copper at room temperature is about 40 and that of a carbon nanotube is about 1, which leads to way more efficient electron transfer.
Lowest resistivity of carbon nanotubes is measured to be for SWNT and x for MWNT, and that for copper is x. Resistivity being comparable, there may be a two fold advantage in using nanotubes due to the density at which these values were measured, Density of SWNT is MWNT is and that of copper is . A material with same conductivity can be produced with much lesser density(lesser by a factor of 6.89) which implies light weight conductors. The combination of these two factors alone can revolutionise the entire electrical industry.
To further quantify the conductance, let’s define a quantity called Conductance Quantum, which is the conductance of quantum channels when the probability of electron transfer is 1 i.e., the channel allows transfer of every electron entering into it without scattering. If the probability of electron transfer is <1, the conductance of that material is lower than conductance quantum.
Conductance quantum,
The conductance of carbon nanotubes is twice that of conductance quantum.
4.b Mechanical Properties
Carbon nanotubes excel many conventional metals used in our day to day life in this field. It is around 4 times stronger than steel along with being much much lighter than steel, but only when stretched along it’s axial direction. The strength along the axial direction is due to the orientation of bonds in the hollow cylindrical structure. Nanotubes are bonded to each other by Van Der Waals force, which are weaker than the bonds. Thus to increase the thickness of a material made with nanotubes, they need to dipped into a epoxy or a polymer matrix.
Tensile Modulus(axial) of carbon nanotube ranges from 300 to 950GPa(according to the structure of the nanotube) while that of steel is 200GPa. Again, density of SWNT is only and that of steel is 8.05.
4.c Thermal Properties
Thermal conductivity of carbon nanotube(along axial direction) exceeds that of the best known heat conductors. For SWNT, it is 3500 and for MWNT it is 3000 while for copper it is, 400. This high value of thermal conductivity is also due the very high surface area of nanotubes.
4.d Field Emission
Carbon nanotubes are great field emitters . This is due to two reasons:
High conductivity(and thus high stable current density(as high as ) carrying capability)
Sharpness of their tip(only 1nm diameter), sharper the tip, greater the emission along with emission at low voltages(which implies low turn-on and operating voltages).
4.e Surface Area
Nanotubes have incredibly high surface area due to their single atom thick cylindrical structure.
Moreover they are hollow and thus the inner surface area may also contribute in some applications.
5. Applications
Field emission property of CNTs are used in building low-power electrical devices that need field emitters. CNTs are used in field emission flat panel displays. These displays have an electron gun for each pixel on the screen whereas the normal CRT(Cathode Ray Tube) displays use only one electron gun for the entire screen.
The mechanical properties of carbon nanotubes are being exploited by some selective companies such as those manufacturing sports equipments such as professional racing bicycles, tennis rackets etc. Nanotube’s stiffness despite being lightweight makes them eligible for such applications.
The mechanical properties are also being exploited for building light weight vehicles for both terrestrial and aquatic transport. Unmanned vehicles, drones etc. use these CNTs dipped in some polymer matrix for light weight parts.
The higher surface area property is used in components such as capacitors and batteries. Due to higher surface area, it makes it easier for electrolytes to interact with.
6. Potential Applications
The compact size and great electrical conductivity of CNTs can be applied for use in miniaturisation. These tubes can be used as internal connectors within the some electronic components and thus the scale of some opponents can be made to be much smaller.
Properties such as high thermal conductivity, low-weight and smaller size can be exploited for use into the cooling equipments required in computer chips and boards. The nanotubes allow very rapid transfer of heat and thus they are ideal for use in circuits that need cooling.
New composites can be generated with some other materials to make the resultant material to gain one or properties of CNTs. As CNTs are light and small, the composite would gain a new property without addition of much weight or without a significant increment in size.
Very high surface area can be used to hold on catalysts onto the surface of nanotubes which can make a reaction significantly faster and feasible than earlier.