Carbon Nanotubes
The Carbon Nanotube (CNT) is an innovative nano/micro
structural material, which was discovered by electron microscopist Sumio Iijima
at NEC-Japan in 1991. It is a self-assembled carbon structure with a
diameter as small as 1 nm. One CNT is 10,000 times thinner than a
human hair! Its length can range from a few nanometers to several microns or longer (one micron is equal to 1,000 nanometers). Therefore, their
aspect ratios (defined as the length divided by the diameter) are typically very large, usually on the order of thousands. CNTs have existed
naturally since the beginning of our world, but because of their infinitesimal size, went unnoticed. Methods to
produce them in useful quantities and ways to use them have preoccupied researchers since their recent discovery.
Basically, the CNT is formed almost entirely, except for impurities, from simple carbon atoms (C). The structure of the carbon is a hexagonal lattice (“graphene
sheet”) rolled into a cylindrical tube, sometimes with a
"cap" at each end of the cylinder (see first picture
below). Cylinders may exist inside other cylinders like the layers
of a leek vegetable, with a gap of 0.34 nm between cylinders (see
second
picture below). Some pentagonal (five-sided) and heptagonal (seven-sided) defects can cause cylinders
to bend, change diameter, or twist.
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| Single-walled CNT (top), Multi-walled CNT (bottom) |
The CNT has been crowned the king of nanotechnology due to its uniqueness and surprising properties. In addition to its
unique microstructure, the CNT demonstrates extremely useful structural, electrical, thermal, and chemical properties and enables a variety
of products to be lighter, stronger, cheaper, cleaner, more efficient, and precise in their function. Simply speaking, two different types
of electrical conduction are possible, based on the orientation of the hexagonal rings in the graphene tube (see picture below). The “armchair”
type has the characteristics of a metal; the “zigzag” type has properties that change depending on the tube diameter – one-third
have the characteristics of a metal and two-thirds of those behave as a semiconductor; the “spiral” or “chiral” type
also has one-third metallic and two-thirds semiconducting nanotubes depending on the orientation and size of the graphene sheet comprising each nanotube.
| TYPE | Armchair |
Zig-zag |
Chiral |
|---|
| Chiral indices, (n,m) [example for those shown] |
(10, 10) |
(17, 0) |
(15, 4) |
| Side view (perspective turned off, all atoms visible) |
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| View down tube (perspective turned on) |
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| Side view (perspective turned on, back-side atoms obscured) |
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| Comments |
Note Symmetry |
Asymmetrical (has ‘handedness’) |
| General electronic behavior (naturally occurring)1 |
All metallic |
1/3 are metallic (when n divisible by 3); 2/3 are semi-conducting (remainder) |
1/3 are metallic (when n – m = 3q, where q = integer); 2/3 are semi-conducting (remainder) |
CNTs exhibit extraordinary mechanical properties as well. For example, the Young’s modulus is typically over 1 Tera-Pascal
along its axis, which is as stiff as diamond. The estimated tensile strength is about 200 GPa,
which is an order of magnitude higher than that of any other
material (100 times stronger and 6 times lighter than steel).
A variety of engineering property advantages of CNTs are shown in the tables2 below, compared to other material options. Clearly, CNTs
are superior in a multitude of properties.
| Properties
| Nanotubes
| By Comparison |
|---|
| Size (in diameter) |
SWCNT: 0.6 – 1.8 nm (1.4 nm typical)
MWCNT: 20 – 50 nm |
Electron beam lithography can create lines 50 nm wide, a few nm thick |
| Current carrying capacity |
Estimated at 1 billion amps per square centimeter
MWCNT: 20 – 50 nm |
Copper wires burn out at about 1 million A/cm |
| Field Emission |
Can activate phosphors at 1 – 3 volts if electrodes are spaced 1 micron apart |
Molybdenum tips require fields of 50 – 100 V/µm and have very limited lifetimes |
| Heat Transmission |
Predicted to be as high as 6,000 watts per meter per Kelvin at room temperature |
Nearly pure diamond transmits heat at 3,320 W/m·K |
| Temperature Stability |
Stable up to 2,800 degrees Celsius in vacuum, 750 degrees C in air |
Metal wires in microchips melt at 600 – 1,000 °C |
| Material |
Elastic modulus (GPa) |
Strain(%) |
Yield strength (GPa) |
Density (g/cm3) |
|---|
| SWCNT |
1210† |
4 |
65.0 |
1.4 |
| MWCNT |
1260† |
1.5 |
2.7 |
1.8 |
| IM-7 / 977-3 (graphite fiber) |
152 |
1.2 |
2.1 |
1.6 |
| Titanium |
103 |
15 |
0.9 |
4.5 |
| Aluminum (2024) |
69 |
16 |
0.5 |
2.7 |
| Steel (1050 |
207 |
9 |
0.8 |
7.8 |
As a result of these extraordinary properties, CNTs promise ‘a tiny revolution.’ Their unique and extreme properties allow them to be
used in a variety of engineering disciplines:
-
Carbon nanotubes - stronger than steel, faster than
silicon and much lighter than Aluminum"3
-
Construction Engineering (Form good load-bearing reinforcements in composite materials)
-
Mechanical, Automotive, and Aerospace Engineering (Auto/air/space body parts)
-
Electrochemical Engineering (Rechargeable batteries and fuel cells)
-
Biomedical Engineering (Nanoprobes and sensors)
-
Electrical Engineering (Transistors, Logic Gates, Microelectronics, Gas Discharge Tubes)
-
and many more …
In fact, several products are already selling, made with rival’s CNTs. (See Ahwahnee Image Gallery.) For example:
_____________________________
1 Sources: Carbon Nanotubes and Related Structures, P.J.F. Harris, Cambridge Universtiy Press, 1999
2 Sources: Scientific American, December 2000
3 Sources: Current Science & Technology Center, Museum of Science, Boston
† Sources: A. Zettl / U.C. Berkeley 2004
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