The world's strongest known substance is graphene, a one-atom-thick sheet of carbon atoms arranged hexagonally. When layers of it are piled one on top of another, the result is ordinary pencil graphite that shears apart like a tiny stack of paper. But take a graphene layer and roll it into a tube and it forms a fiber theoretically 100 times stronger than steel and six times lighter: a carbon nanotube.
Not surprisingly, carbon nanotubes (CNTs) have garnered much attention for their strength, which comes from the double bonds that hold their carbon atoms together. Each tube has the potential to be over one million times longer than its diameter (50 thousand times slimmer than a human hair), and inspire visions of a super-strong, super-light-weight material that could enable researchers to engineer the structures and fabrics of the future.
Yet strength is only part of their appeal.
"Most of the interest has been in applications to electronic materials," said Michael Arnold, assistant professor in the Materials Science and Engineering department at the University of Wisconsin-Madison. Arnold's research focuses largely on the production and sorting of CNTs. According to him, CNTs have many appealing electric properties. Depending on their atomic arrangement, they can be either semiconductors or conductors of electricity. Semiconductors are crucial in electronics for making transistors, tiny switches of electric current that convert electric signals to digital data.
However, obstacles exist for practical applications of the technology. The process of making CNTs is difficult and expensive. It involves vaporizing graphite at high temperature and having it reform on metal as the tiny tubes. Currently, CNTs cost a few hundred dollars per gram, although that's down from thousands of dollars only a few years ago.
"Their length is also a limiting factor," Arnold said. Although theoretically one CNT could stretch for miles, the longest ones made so far are only 1 to 2 centimeters. "That's pretty good, considering they're only one nanometer in diameter," he noted. But the tubes will have to be longer and cheaper before they start having a major impact beyond the laboratory, but Arnold is optimistic about the future. "Their properties are so amazing and there's so much interest, that I think eventually people will figure it out," he said.
Out of the many possible applications for carbon nanotubes, here are ten of the more intriguing ones:
1. Space elevator
Sending a payload into space by rocket is expensive ($10,000 per pound) and dangerous. Some folks are proposing a very tall elevator that would stretch from the ground to beyond Earth's atomosphere. Making it a reality requires a long, strong cable tethered to a counterweight in geosynchronous orbit, maintaining a fixed position about 22,000 miles above the earth. CNTs are the only known material up to the task. Among other things, a successful space elevator could create means for safe disposal of nuclear waste, and give life to a space tourism industry.
2. Faster computer chips
The processing speed of a computer chip depends on the number of transistors it has. Today, typical desktop processors using silicon transistors have less than half a billion. Computer chips using CNTs could blow those numbers away. Their small size -- just one nanometer wide -- means many billions of CNT transistors could be packed onto a single processing chip, making for smaller, faster computers and electronics.
3. Better solar cells
Semiconducting materials, when altered with certain impurities, are used in solar cells. When struck by light, these materials release electrons, creating usable electricity. Most of today's solar cells use silicon semiconductors, but that could change. Because they're so tiny, billions of CNTs could be tightly packed onto solar cells and release far more electricity per square inch than silicon. In addition, carbon nanotubes absorb light so well that a professor at Rice University used them to create the darkest ever man-made material.
4. Cancer treatment
CNTs are so small they might one day be used to target and destroy individual cancer cells. By treating CNTs with certain proteins, scientists are developing a method to bind them specifically to cancerous cells. Once attached, the CNTs, which are excellent conductors of heat, could be exposed to infrared light shone through the patient's skin. The light would heat the CNTs to a temperature high enough to destroy the cancer cells while leaving surrounding tissue undamaged. While more research must be done, this method could offer a way to treat certain cancers without harming healthy tissue, a current drawback of treatments like chemotherapy.
5. Better, thinner TVs
Traditional tube TVs essentially work by firing electrons at substances called phosphors to make them glow, creating the colored light of a television picture. This process requires an electron gun in a relatively big picture tube. But new displays, called field emission displays miniaturize the process by using tiny electron emitters positioned behind individual (microscopic) phosphorus dots. An array of CNTs, which are excellent electron emitters, could be used in field emission displays to excite the phosphorus dots, creating bright, high resolution displays that are only millimeters thick and consume less power than plasma and liquid crystal displays.
6. Better capacitors that replace batteries
Instead of storing electricity chemically like a battery, capacitors hold it physically by building a charge on a material called a dielectric. The dielectric's surface area determines how much charge it can hold. CNTs have extraordinarily high surface areas, and using them as the dielectric could increase the storage ability of capacitors to be on par with modern batteries. But if we already have batteries what's the use? Batteries take hours to charge and lose their capacity with time. Capacitors don't have these problems. CNT capacitors might one day be used in instantly rechargeable laptops and electric cars.
7. Flexible displays
The dream of fold-up TVs and computer screens that can fit inside people's pockets has, up until now, been stifled by rigid silicon semiconductors. While some organic semiconductors have used in bendable plastic displays, their performance has been fairly poor. But CNTs, in addition to being very flexible, compare favorably to silicon in terms of performance. Researchers at Purdue and the University of Illinois-Urbana-Champaign are developing carbon nanotube flexible displays which one day could be used for things like electronic newspapers and roll-up handheld devices.
8. Bone healing
Researchers at the University of California-Riverside have discovered that CNTs can act as scaffolds around which bone cells will grow by attracting hydroxyapatites, calcium crystals in the body that are critical for bone formation. The technology might one day help individuals with bone diseases or particularly catastrophic injuries regrow bones.
9. Body armor
Researchers at Cambridge University have figured out how to spin many tiny carbon nanotubes together to create fibers that have the strength of Kevlar, a composite material used in bullet-proof vests. With new techniques rapidly emerging to make longer CNTs, spun fibers using the longer CNTs will soon surpass Kevlar in strength, and weigh less. As CNT prices drop, spun CNT fibers could be the material of choice for better, lighter body armor.
10. Faster flywheels
A flywheel is like a battery in that it stores energy. But unlike a battery, this energy is mechanical and stored via a wheel rotating at high speed (the faster the spin, the more energy it stores). Flywheels offer certain advantages over batteries. But a flywheel, if spun too fast, can shatter because of the strength limits of its material. Because of their strength, CNTs could be used to make faster flywheels that store more energy without shattering. While Flywheels have seen only limited use in amusement rides, race cars and backup power supplies, using CNTs could allow flywheels to become more prevalent in areas like public transportation and hybrid cars.
