Peering through the lab window to a limitless world of uses in industry is an amazing new material that some day could be as commonplace as plastic.
By Don Wood
When Sumio Iijima of NEC Laboratories in Japan discovered carbon nanotubes in 1991, he set off a worldwide gold rush in academic, government and industrial labs to find practical uses for these extraordinary molecules.
In these cylindrical, microscopic bits of pure carbon, engineers saw the potential for hundreds, if not thousands, of applications. Here was a new material, never before seen, that promised to bring nanotechnology to the real world.
Researchers scrambled to learn more about carbon nanotubes and their fantastic properties. The gold rush turned into a stampede as engineers conjured up ever more potential uses for the tiny, spaghetti-shaped molecule. Lighter and stronger aircraft, spacecraft and cars! Entirely new types of electronic devices! Incredible new fabrics! Portable X-ray machines! Artificial muscles! Nanotubes became the hottest thing in engineering since semiconductors.
But blocking the path to these wonderful dreams stood two major hurdleswhich, now 15 years on, still frustrate progress into exploiting one of the most versatile molecules ever discovered.
First, nanotubes proved difficult to manufacture and purify in quantity, making them unbelievably expensivepossibly, weight for weight, the most expensive substance on Earth. A single gram of the best quality, highest purity nanotubes can cost $2,000and a gram is the weight of a dollar bill. Even nanotubes of lesser quality and purity can cost hundreds of dollars per gram.
Second, the molecules are difficult to work with. They come as an extremely light, fine, dry powder. An errant sneeze can make a jarful disappear into the air as the world’s most expensive dust. And at the molecular level, they tend to stick together in tangled clumps while engineers would prefer them to disperse evenly through a medium (such as a solution) as more well-behaved molecules do.
Despite these problems, nanotubes are beginning to show up in a few commercial products. They lend their strength and lightness to high-end sporting goods such as golf clubs, tennis rackets, baseball bats and bicycle frames. And they turn up in over half the batteries used in cell phones and laptop computers (they make the batteries last longer).
Engineers have whittled away at the processing and production problems so that technology prognosticators expect explosive growth in commercial applications for carbon nanotubes during the next five to 10 years.
One of the leaders in this emerging field is Ben Wang, director of the Florida Advanced Center for Composite Technologies at the FAMU-FSU College of Engineering. Wang’s research is focused on “buckypaper,” a flat, foldable material made of pure nanotubes. As it turns out, this product turns one of the molecule’s key disadvantages into an advantage.
In the 1990s researchers realized that if they dispersed nanotubes into a liquid suspension (no mean feat in itself) and then passed the liquid through a fine mesh filter, the nanotubes would collect on the filter. The tubes’ tendency to stick to each otherusually considered a problemin this case allowed the nanotubes to form a thin film. The resultdubbed “buckypaper”was a small disk made entirely of nanotubes.
Wang first encountered buckypaper in 2000 when an engineering colleague, Les Kramer of Lockheed, showed him a sample fabricated at Rice University by Nobel laureate Richard Smalley. It wasn’t much more than a laboratory curiositya lightweight, black disk only an inch in diameterbut Wang examined it with an engineer’s eye.
“I was fascinated,” Wang said. “All the nice properties of carbon nanotubes are measured in individual, single nanotubes. But we as engineers are interested in bulk materialssomething we can process, something we can make products out of. Now, the million dollar question is, can these wonderful properties be realized in a bulk material?”
So Wang, along with the other faculty and students in his lab, began an ambitious research and development program to make, study and find uses for buckypaper. Wang’s center was the first to characterize and study the material’s mechanical and electrical properties, which indeed proved wonderful. They developed methods for making the stuff in large sizes and for processing it into useful materials.
Another key innovation was inventing a way to process buckypaper inside the bore of a powerful magnetconveniently located at the National High Magnetic Field Laboratory just down the street from Wang’s lab. Magnetic fields make nanotubes all line up and point in the same direction rather than being scattered about in random orientations, a technique that dramatically enhances buckypaper’s properties for certain applications.
Wang’s efforts have resulted in a number of strong, lightweight buckypaper composites with mechanical, electrical and thermal properties that make them particularly suited for dozens of applications. Collectively, Wang calls them “Nanotubes Optimized for Lightweight Exceptional Strength,” with the felicitous acronym NOLES.
Strength in Numbers
CompositesWang’s team’s specialtyare combinations of two or more materials that together produce structural or functional properties not present in the materials by themselves. One example is carbon fiber composite, a current favorite in the high-tech aerospace industry.
Available commercially from dozens of suppliers, carbon fiber comes as a fabric. When properly processed with an epoxy resin, an astonishing transformation takes place. The composite, resembling a piece of hard plastic, becomes incredibly strong.
“These composites are the kinds of materials that the Air Force, the Navy and the Army use for fighter jets and future armored vehicles. The composites are lighter than metal and in some ways much better than metal. For instance, they don’t corrode,” Wang said.
Every composite consists of a reinforcement and a glue-like matrix. In the case of the carbon fiber composite, the fiber, in the form of the fabric, is the reinforcement. The matrix is an epoxy that holds the reinforcement together. In NOLES, buckypaper is the reinforcement, and the matrix is selected for the particular application.
Success with the NOLES composites hasattracted the attention of both the armed forces and industryto the tune of $8 million in research grants in just the last three years. Major supporters include the Air Force Office of Scientific Research, the Air Force Research Lab, the Army Research Lab, Lockheed Martin Corp. and Boeing.
Leader of the Pack
Wang’s lab is hardly the only research team pursuing the buckypaper dream, but Wang has reasons to believe his group is far ahead of the competition.
“Our process is unique. The one thing that separates our research from the rest of the pack is our know-how in processing nanotubes and making buckypaper. We can make it in large sizes and we can align the nanotubes using magnetic force.”
While the competition is still making small pieces of the material measuring a couple of inches wide, Wang’s center consistently produces large swatches of the paper.
In 2003, Wang and his colleagues made the world’s largest sheet of single-walled, magnetically aligned buckypaper. The sheet measured 4 inches wide and 15 inches long, giving 60 square inches of useable area. (The sheet is still the record holder; the center routinely makes 7-inch by 7-inch pieces for research purposes.)
The difference between random and aligned buckypaper shows up dramatically in photographs taken with a scanning electron microscope. In random paper, ropy strands of nanotubes form a tangled mass resembling a plate of spaghetti. But in the aligned paper, nanotubes line up in smooth parallels like fur on the back of a well-petted lap cat. The difference in orientation creates differences in electrical properties of the two types of paper, making each useful for different applications.
For example, one potential application is protecting airplanes from lightning strikes, a perpetual problem for all aircraft. At present, airplane manufacturers use a network of copper or aluminum foils covering the wings and fuselage to provide lightning strike protection.
A main drawback of copper, of course, is its weight. Buckypaper weighs only 15 percent as much as copper, and only thin strips are required, Wang said, so the immediate payoff for airplane designers is a tremendous savings in weight.
“Aligned buckypapers will distribute all the current throughout the aircraft so it dissipates,” Wang said. “The high voltage goes away, so it will not create a hole in the aircraft.”
The novel material offers other advantages, as well, such as shielding against electromagnetic interference, Wang said. All electrical and electronic equipment generates electromagnetic waves, which radiate away in all directions. These waves can disrupt sensitive electronics such as computers. So manufacturers typically install such devices inside a cage of copper chicken wire to keep the interfering waves out.
Once again, buckypaper can replace copper for a great savings in weight. And this time, it’s the random buckypaper that works best.
“The other side of the coin is, buckypaper can keep the electromagnetic waves from going out of the box,” Wang said. This feature becomes important in designing stealth aircraft and other systems for the military. “It reduces the electromagnetic signature of the systems, so they will not be detected by the enemy’s radar. This is part of weapon signature management, which is a huge issue in stealth technology.”
Another part of stealth technology is thermal managementcontrolling the amount of tell-tale heat pouring from an aircraft engine. A heat-seeking missile has a tracking system that can detect high temperature and then follow the heat path for a sure kill.
Aligned buckypaper has a thermal conductivity almost as high as that of brass, Wang said. The material can play a significant role in cutting an airplane’s thermal signature.
Better Wings for Weapons
One application for buckypaper composites now under consideration by the Air Force involves a missile called (in militarese) the Low Cost Autonomous Attack System (LOCAAS). “Basically it is a cruise missile,” Wang explained. “The conventional design was fixed wing, so once the missile is deployed, the wing stayed the same. But the Air Force wants to add more range to the missile without adding weight to it. So they came up with an extended wing design.”
In the new design, once the missile is launched, an additional section pops out of the end of each wing. Measuring only 3.5 inches by 5 inches, the extensions increase the glide range of the missile by 23 percent, at least in computer tests.
As part of a research contract with the Air Force Research Lab Munitions Directorate, Wang and his engineers stretched a thin layer of buckypaper composite over a foam core, producing a tough, strong wing extender that’s light as a featherand probably the largest single-walled carbon nanotube structure ever made.
“This is a perfect application. It has almost no weight, and it has shielding from electromagnetic interference, offers lightning strike protection and also has very good thermal conductivity.”
The Air Force likes the buckypaper wing and has used it as a demo piece to show the practicality of buckypaper as an aerospace structural material, but actual production for use in missiles or any other aircraft will have to await manufacturing facilities to produce buckypaper in quantity.
But not all of buckypaper’s promise lies in the aerospace and defense sectorsfar from it, in fact. Wang believes that one of the biggest markets for buckypaper eventually will be in consumer electronics. When designers have to cram a lot of electronics into a small space (as in a laptop computer), getting rid of the heat becomes a major problem. Buckypaper’s thermal properties make it an ideal candidate to serve as a heat sink, bleeding away the heat without adding much weight to the design.
Those crowded electronics can interfere with each other, too. Buckypaper’s ability to provide electromagnetic interference shielding can help solve that problem as well.
Wang’s group is also working on ways to exploit buckypaper’s electrical properties for television screens and computer displays, with the potential for energy and weight savings and a more uniform level of brightness over today’s top-of-the-line liquid crystal technology.
From the Lab to the Real World
Before buckypaper-made products can make it to store shelves, scientists and engineers are going to have to come up with a production facility to make the material in bulk to bring the price down.
“Buckypaper is relatively expensive, compared to steel, and making it is very time consuming,” Wang said. “With the best technology we have right now, it takes several hours to make a single sheet of buckypaper.”
So, Wang’s team is tackling that problem, too. They’ve already built a prototype production machine that makes buckypaper ribbons 1.5 inches wide and 36 inches long. The next step is automating the process, and Wang is confident he and his team can overcome that hurdle in the near future, cutting the time required to make buckypaper from hours down to a minute or two.
“Our processing technology will make buckypaper affordable,” Wang said, smiling. “This is really like a dream material for future applications.”