Science Fair Project Encyclopedia
Carbon nanotubes are cylindrical carbon molecules with properties that make them potentially useful in extremely small scale electronic and mechanical applications. They exhibit unusual strength and unique electrical properties, and are efficient conductors of heat. Inorganic nanotubes have also been synthesized.
A simple nanotube has a structure similar to a fullerene, but where a fullerene molecule's symmetry is spherical, a nanotube's is cylindrical, with one end typically being capped with half a fullerene molecule. Their name derives from their size; nanotubes are on the order of only a few nanometres wide (on the order of one ten-thousandth the width of a human hair), and their length can be millions of times greater than their width. There are two main types of nanotubes: single-walled nanotubes (SWNT) and multi-walled nanotubes (MWNT).
Nanotubes are composed entirely of spē bonds, similar to graphite. Stronger than the sp3 bonds found in diamond, this bonding structure provides them with their unique strength. Nanotubes naturally align themselves into "ropes" held together by Van der Waals force. Under high pressure, nanotubes can merge together, trading some sp2 bonds for sp3 bonds, giving great possibility for producing strong, unlimited-length wires through high-pressure nanotube linking. 
While it has long been known that carbon fibers can be produced with a carbon arc, and patents were issued for the process, it was not until 1991 that Sumio Iijima, a researcher with the NEC Laboratory in Tsukuba, Japan, observed that these fibers were hollow. This feature of nanotubes is of great interest to physicists because it permits experiments in one-dimensional quantum physics.
The structure of a SWNT can be conceptualized by wrapping a one-atom-thick layer of graphite (called graphene) into a seamless cylinder. The way the graphene sheet is wrapped is represented by a pair of indices (n,m) called the chiral vector. The integers n and m denote the number of unit vectors along two directions in the honeycomb lattice of graphene. This is often thought of as representing the number of carbon atoms around the circumference of the tube, and the number of atoms down the tube axis. If m=0, the nanotubes are called "zigzag". If n=m, the nanotubes are called "armchair". Otherwise, they are called "chiral". Due to the symmetry and unique electronic structure of graphene, the structure of a nanotube strongly affects its electrical properties. For a given (n,m) nanotube, if 2n + m=3q (where q is an integer), then the nanotube is metallic, otherwise the nanotube is a semiconductor. Thus all armchair (n=m) nanotubes are metallic, and nanotubes (5,0), (6,4), (9,1), etc. are semiconducting. An alternative (equivalent) representation of this condition is if (n - m)/3=integer, then the SWNT is metallic. In theory, metallic nanotubes can have an electrical current density more than 1,000 times stronger than metals such as silver and copper. All nanotubes are expected to be very good thermal conductors along the tube, exhibiting a property known as "ballistic conduction," but good insulators laterally to the tube axis.
Techniques have been developed to produce nanotubes in sizeable quantities, but their cost still prohibits any large scale use of them. Fullerenes and carbon nanotubes are not necessarily products of high-tech laboratories, and are also formed in such mundane places as candle flames. However, these naturally occurring varieties are highly irregular in size and quality, and the high degree of uniformity necessary to meet the needs of research and industry is impossible in such an uncontrolled environment. There are several methods employed to make nanotubes, such as arc discharge, laser ablation , and chemical vapor deposition (CVD). In general, the CVD method has shown the most promise in being able to produce larger quantities of nanotube (compared to the other methods) at lower cost. This is usually done by reacting a carbon-containing gas (such as acetylene, ethylene, ethanol, etc.) with a metal catalyst particle (usually cobalt, nickel, or iron) at temperatures above 600°C.
Nanotubes can be opened and filled with materials such as biological molecules , raising the possibility of applications in biotechnology. They can be used to dissipate heat from tiny computer chips.
The strength and flexibility of carbon nanotubes makes them of potential use in controlling other nanoscale structures, which suggests they will have an important role in nanotechnology engineering. The highest tensile strength an individual SWNT has been tested to is 63 GPa . In Earth's upper atmosphere, atomic oxygen erodes the carbon nanotubes, but other applications of carbon nanotubes rarely need the surface to be protected. Though it is debatable if nanotube materials can ever be made with a tensile strength approaching that of individual tubes, composites may still yield incredible strengths potentially sufficient to allow the building of such things as space elevators, artificial muscles, ultrahigh-speed flywheels, and more. MIT is working on combat jackets utilizing carbon nanotubes for ultrastrong fibers and for monitoring its wearer's condition.
Carbon nanotubes additionally can also be used to produce nanowires of other chemicals, such as gold or zinc oxide. These nanowires in turn can be used to cast nanotubes of other chemicals, such as gallium nitride. These can have very different properties from CNTs - for example, gallium nitride nanotubes are hydrophilic, while CNTs are hydrophobic, giving them possible uses in organic chemistry that CNTs could not be used for.
One use for nanotubes that has already been developed is as extremely fine electron guns, which could be used as miniature cathode ray tubes in thin high-brightness low-energy low-weight displays. This type of display would consist of a group of many tiny CRTs, each providing the electrons to hit the phosphor of one pixel, instead of having one giant CRT whose electrons are aimed using electric and magnetic fields. These displays are known as Field Emission Displays (FEDs). A nanotube formed by joining nanotubes of two different diameters end to end can act as a diode, suggesting the possibility of constructing electronic computer circuits entirely out of nanotubes. Nanotubes have been shown to be superconducting at low temperatures.
One application for nanotubes that is currently being researched is high tensile strength fibers. Two methods are currently being tested for the manufacture of such fibers. A French team has developed a liquid spun system that involves pulling a fiber of nanotubes from a bath which yields a product that is approximately 60% nanotubes. The other method, which is simpler but produces weaker fibers uses traditional melt-drawn polymer fiber techniques with nanotubes mixed in the polymer. After drawing, the fibers can have the polymer burned out of them to make them purely nanotube or they can be left as they are.
Ray Baughman's group from the NanoTech Institute at University of Texas at Dallas produced the current toughest material known in mid-2003 by spinning fibers of single wall carbon nanotubes with polyvinyl alcohol. Beating the previous contender, spider silk, by a factor of four, the fibers require 600J/g to break. In comparison, the bullet-resistant fiber Kevlar is 27-33J/g.
In 2004 Alan Windle's group of scientists at the Cambridge-MIT Institute developed a way to make carbon nanotube fiber continuously at the speed of several centimetres per second just as nanotubes are produced. One thread of carbon nanotubes was more than 100 metres long. The resulting fibers are electrically conductive and as strong as ordinary textile threads.  
High purity (80%) nanotubes with metallic properties can be extracted with electrophoretic techniques. 
In June 2004 scientists from China's Tsinghua University and Louisiana State University demonstrated the use of nanotubes in incandescent lamps, replacing a tungsten filament in a lightbulb with a carbon nanotube one.
Nanomechanical computer storage devices using nanotubes are currently in the prototype stages. Both high speed non-volatile memory which can be used to replace nearly all solid state memory in computers today, and high density storage that may replace hard drives, are being developed. Major limiting factors in the development of nanotubes include their cost and difficulties in orienting the nanotubes, which tend to tangle because of their length.
As of 2003, nanotubes cost from 20 euro per gram to 1000 euro per gram, depending on purity, composition (single-wall, double-wall, multi-wall) and other characteristics.
Japanese manufacturer Mitsui & Company has announced plans to build the world's first mass production facility for Carbon Nanotubes. Using technology developed at their research facility, they expect to reduce the selling price of Nanotubes to less than 100 yen per gram (about $0.76).
In 2004, Nature published a photo of an individual 4 cm long single-wall nanotube (SWNT).
Carbon nanotubes in electrical circuits
Carbon nanotubes have many properties—from their unique dimensions to an unusual current conduction mechanism—that make them ideal components of electrical circuits, and it is exciting to envision, or even to implement, novel transistors, MEMS devices, interconnects, and other circuit elements.
The major hurdles that must be jumped for carbon nanotubes to find prominent places in circuits relate to fabrication difficulties. The carbon nanotube production processes are very different from the traditional IC fabrication process. The IC fabrication process is somewhat like sculpture—films are deposited onto a wafer and pattern-etched away. Carbon nanotubes are fundamentally different from films; they are like atomic-level spaghetti (and every bit as sticky).
Today, there is no reliable way to arrange carbon nanotubes into a circuit. Researchers sometimes resort to manipulating nanotubes one-by-one with the tip of an atomic force microscope in a painstaking, time-consuming process. Perhaps the best hope is that carbon nanotubes can be grown through a chemical vapor deposition process from patterned catalyst material on a wafer. Though such a CVD process has been shown to allow a circuit designer to locate one end of a nanotube, there is no obvious way to control where the other end goes as the nanotube grows out of the catalyst. Another way for the self assembly of the carbon nanotube transistors consist in using chemical or biological techniques to place the nanotubes from solution to determinate place on a substrate.
Even if nanotubes could be precisely positioned, there remains the problem that, to this date, engineers have been unable to control the types of nanotubes—metallic, semiconducting, single-walled, multi-walled—produced. This is a problem that chemical engineers must solve if nanotubes are to find a place in commercial circuits.
- Commercial source of carbon nanotubes NTP-nanotube manufacturer in China
- Ahwahnee Technology Silicon Valley carbon nanotube developer
- The Nanotube site
- The wonderous World of Carbon Nanotubes (Good introduction to nanotubes)
- Jamieson V. "Open secret" New Scientist
- Nantero (developers of nanotube based non-volatile memory)
- University of Cambridge, UK, Research group website (Affordable methods for making carbon nanotubes and using them for gene delivery)
- University of Texas at Dallas NanoTech Institute
- NanoDiamond (nanotubes arranged in a diamond formation yielding a very high strength-to-weight ratio material)
- Carbon Nanotube & Fullerene Models - Vincent Herr, Houston, TX
- Science News - Nanotube Super Fibers
- Nanotube production in 2003
- Columbia University Nanoscale Science and Engineering Center presents "Our Energy Challege" September 23, 2003
- Review of Non-Oil and Gas Research Activities in the Houston-Galveston-Gulf Coast Area
- commercial sources
- Carbon Designs, Inc. http://NanotubeComposites.com/
- "For pure nanotubes add water" article by Eric Smalley 2004-12 "stands of single-wall carbon nanotubes as tall as 2.5 millimeters and 99.98 percent pure. Individual nanotubes range from one to three nanometers in diameter."
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