Page 472 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Inorganic Polymers 435

nanotubes, C giving zigzag structures, and C giving helical forms of nanotubes. Figure 12.11
70 80
contains representations of these three fullerenes.

This difference in structure also influences the electrical conductivity with the armchair form being
conductive or metal-like in its conductivity and most of the other forms act as semiconductors.
Two other forms of CNTs are recognized. The fullerite is a highly incompressible form with a
diamond-like hardness. The torus is a doughnut-shaped nanotube. These circular nanotubes have
extremely high magnetic moments and outstanding thermal stability. The particular properties are
dependent on the radius of the tube.
One of the major reasons for the intense interest in CNT is their extreme and varied properties.
CNTs are among the strongest and stiffest materials known. Tensile strengths to about 65 gigapas-
2
cals (GPa) have been found. This translates to a cable of 1 mm cross section capable of holding
about 3,200 tons. A general density of CNTs is about 1.4 g/cc. Its specific strength is more than 300

times as great as steel. Under stress, tubes can undergo permanent deformation (plastic deforma-
tion). Interestingly, because the tubes are hollow, they are not nearly as strong as diamonds buck-
ling under compression, torsion, and bending stress. Nanotubes conduct and transport along the
lengthwise direction resulting in them often being referred to as being one dimensional. Table 12.8
contains a comparison between SWCNTs and competitive materials/techniques. Another reason for
the intense interest is that CNTs are produced from readily available inexpensive materials, they are
being considered for use as both bulk materials, such as in composites and clothing, and as compo-
nents in computers, electrical devices, and so on.
The number of potential and real uses where an essential ingredient is CNTs is growing almost
daily. We are able to control the length and kind of tubes formed. Because the starting material is so
plentiful and inexpensive, this is an area where most of the limits are ones we impose on ourselves.
Following is a discussion of some of the application areas for CNTs.
Electrical—Nanotubes can be metallic or semiconducting, depending on their diameters and heli-
cal arrangement. Armchair (n = m) tubes are metallic. For all other tubes (chiral and zigzag),
when tube indices (n + m)/3 is a whole number integer the tubes are metallic, otherwise they are
semiconducting. CNTs can in principle play the same role as silicon does in electronic circuits,
but on a molecular scale where silicon and other standard semiconductors cease to work. Single

TABLE 12.8
Comparison between Selected Properties of Single-Walled CNTs and Competitive
Materials/Techniques
Property Single-Walled Carbon Nanotubes Comparison
Size 0.6–1.8 nm in diameter Electron beam lithography can create lines 50 nm
wide and a few nm thick
Density 1.33–1.40 g/cc Aluminum has a density of 2.7 g/cc and titanium
has a density of 4.5 g/cc
Tensile strength ca 45 billion Pascals High-strength steel alloys break at about 2 billion
Pascals

Resilience Can be bent at large angles and Metals and carbon fibers fracture at grain
restraightened without damage boundaries
Current carrying Estimated at 1 billion A/cc Copper wires burn out at about 1 million A/cc
capacity
.
.
Heat transmission Predicted to be as high as 6,000 W/m K Diamond transmits 3,320 W/m K

Field emission Can activate phosphors at 1–3 V if Molybdenum tips require fields of 50–100 V/µm
electrodes are spaced 1 μm apart and have limited lifetimes
Temperature stability Stable up to 2,800 C in vacuum, 750 C Metal wires in microchips melt at 600 C–1,000 C
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