Thinking Big by Thinking Small
You have probably never heard of them. And in all likelihood, you will never see one since they can be viewed only under an electron microscope. Nonetheless, one of the hottest new discoveries in science -- the nanotube -- may soon change the world around you as surely as the Beatles changed music and Jackie Robinson changed baseball.
Peter Eklund, with a tinkertoy-like model of a nanotube
Nanotubes are straw-like, cylindrical structures measuring only a few atoms in circumference. They are so small that they are measured in nanometers -- billionths of a meter. Attempting to put their size in perspective, two scientists described it this way: A chain of "nanotubes sufficient to span the 250,000 miles between the Earth and the moon at perigee could be loosely rolled into a ball the size of a poppy seed."
Their diminutive size belies another fact, however. Nanotubes are stronger than steel. Their unusual structure mimics the atomic arrangement of the buckyball -- a hollow sphere with 60 symmetrically arranged carbon atoms. Nanotubes differ only in that they are strands rather than spheres.
These molecular structures captivate scientists in other ways as well. Although they are made of carbon, they can conduct electricity. And because they are hollow, their shells may someday be used as storage devices for a variety of purposes.
It is this eclectic collection of properties and possibilities which intrigues four UK scientists exploring various aspects of nanotubes and nanotechnology, the study of functional structures at the atomic level.
Sumio Iijima of NEC Corporation first discovered nanotubes in 1991, and since that time one of the most daunting challenges facing researchers has been to obtain sufficient quantities of the molecular structures.
Quantities of nanotubes will also be essential if they are to be used for practical applications. However, these tiny structures are still expensive to make. The carbon itself is cheap by scientific standards, but the process of turning garbage carbon into useful carbon nanotubes is expensive.
But it's becoming less so. Two UK researchers -- Peter Eklund, a professor of physics, and Robert Haddon, a professor of chemistry and physics -- believe they have solved this problem and now have a patent pending for a process to produce nanotubes.
Their research into mass producing nanotubes started in earnest in 1993 when an inauspicious plastic bag containing what looked like charcoal-colored soot arrived at Eklund's office. The bag held a sample the UK scientist had requested from Richard Smalley, a Nobel Prize winner at Rice University who had just discovered a method of producing material containing 90 percent nanotubes and only 10 percent junk carbon. The sample was the first time that Eklund had access to any significant amount of nanotubes.
"In 1993, we [Eklund's research team] duplicated the process for making nanotubes in small yields," Eklund says. "We published a few papers, but then pretty much set the work aside because the actual material that the nanotubes were in was the overwhelming constituent. In other words, the nanotubes were a few percent and the rest was 98 percent junk -- carbon junk, nanodust.
"It was difficult to do a wide range of experiments, but we did those experiments that we could do. That was essentially to pick out the signal coming from the nanotubes and reject the signal coming from the nanodust."
Still, Eklund recognized the tremendous potential for nanotubes and encouraged Haddon to join him at UK so they could combine their respective physics and chemistry expertise to collaboratively research the fascinating structures.
The scientists still needed a reliable method to mass produce nanotubes, however. Sources were not abundant nor reliable. About that time, a group in France published a report saying that pure nanotube samples could be made using Eklund's 1993 method if the catalyst were changed.
"We tried that, made some important modifications, and moved the French development forward," Eklund says. "That is the basis for the process improvement patent." It is also the basis for their continued work into nanotubes, including a company the pair formed to work on a variety of commercial applications for nanotubes. The company is called CarboLex.
Robert Haddon says nanotubes could be very useful in aerospace, the automobile idustry and pharmaceuticals.
"We are not able to mass produce tubes yet, but we are moving in that direction," Haddon says. "We are able to make several grams of nanotubes an hour compared with the milligrams per hour produced by the French."
"We are really just beginning," Eklund says. "We haven't fiddled around with the catalyst, the chemical composition of the catalyst or the amount of the catalyst. We have simply tried to understand how the process works and explore how to maximize the yield for this particular catalyst. We have also worked hard to automate the process."
"We would like to do chemistry on the objects to expand their role as structural materials and also as conductors," Haddon adds. "If we can truly produce a high enough quantity, they could become engineered materials. They could be very useful in a number of different industries such as aerospace, the automobile industry, and pharmaceuticals."
Testing Nanotube Performance
Another UK researcher investigating the nanotube is Susan Sinnott, an assistant professor in materials science engineering. By looking at the mechanical properties of nanotubes, Sinnott helps other scientists decide whether the end result will be worth the expense for any number of applications.
Using computational nanotechnology, Sinnott simulates nanotube structures and tests nanotubes in various situations. For example, scientists are concerned that the process used to create a composite from nanotubes might degrade them and cause them to deteriorate. Sinnott's computational work thus far has found that while the properties degrade slightly, overall the nanotubes continue to perform well. The structures have also proven to hold up well when tested for strength. When striking a stronger substance, nanotubes bend but do not break.
Coupled with mechanical properties of the nanotube, these findings offer exciting possibilities. Because they are hollow, for example, nanotubes may be used in the future as storage devices. It might be possible to store liquid hydrogen inside the tubes and release the hydrogen on demand. This would eliminate the need for the massive tanks of liquid hydrogen now required for the space shuttle to escape the Earth's atmosphere. Likewise, the hollow tubes might be used as membranes for pharmaceuticals or as a support for catalysts.
"There are some people who still think that this (nanotechnology) is pretty far out, but I think everyone realizes that we have made strides we never thought were possible," Sinnott says. "I think that the detractors' point of view is that, so far, no one can point to any useful application that has emerged from this. But right now we are still learning to walk. Once we gain control, it is only a matter of time before we can create appropriate devices."
Susan Sinnott says that UK researchers are working to make the next generation of composites with much smaller fibers -- nanotubes that are much stronger.
Currently, Sinnott and her research team are undertaking several different projects related to nanotubes.
"We are investigating the use of nanotubes as probes to see how they respond when they are crashed into various surfaces," she says. "We are also studying nanotubes that are embedded into a polymer matrix. Right now, carbon composites are made of polymer matrices with graphite fibers distributed throughout to toughen the material. We are trying to make the next generation of composites with much smaller fibers -- nanotubes that are much stronger."
Making Nanotubes Useful in the Everyday World
Sinnott's theoretical and computational work is complemented by experimental work being done by Eric Grulke, chair of the Department of Chemical & Materials Engineering. Grulke is developing the engineering science needed to use nanotubes in a variety of applications.
"Nanotubes may follow conventional engineering science theories or may not," Grulke says. "We don't know. We are doing the work to either demonstrate that they follow the conventional theories or discover what theories they do follow."
Grulke will take the results of the work done by other scientists and determine ways to disperse the nanotubes in solutions, in polymer melts (polymers above their flow temperatures) and polymer systems (mixtures containing polymers, fillers and/or other additives such as solvents and stabilizers). He is also looking at processing and forming those systems into useful products as well as testing the properties of the products.
"We want to try them in liquid systems, similar to today's polymer systems," says Grulke. "We expect that nanotubes can be combined with commercial polymers to provide systems that can be highly oriented with great strength. Our challenge is to determine whether we can use conventional polymer processing equipment to achieve the high tensile strength and the unusual properties that people expect from other materials."
Grulke's work in this area is just beginning, made possible in large part by the work done by Eklund and Haddon to produce gram quantities of the tubes.
Eric Grulke says that nanotubes have some immediate applications in high-strength composite fibers and laminates and that there may be some novel applications in terms of coatings.
"We're doing rudimentary experiments on simple properties like viscosity, and we're getting surprising results," he says. "These molecular-size fibers have not really been available for test, for research. So these are new and unique."
The results of this work could lead to any number of applications.
"Because they have such high tensile properties, nanotubes have some immediate applications in high-strength composite fibers and laminates, and there may be some novel applications in terms of coatings," Grulke says. "Because we expect them to be expensive relative to normal carbon fibers, we may find that we have to use a coating of nanotubes dispersed in a continuous phase rather than generate a bulk solid. So one focus of our experiments is to learn how to work with these nanotubes in coatings systems.
"For example, it is quite possible that because they can be oriented and strong, they may impart good wear properties to thin films. We are looking at methods for dispersing the nanotubes in coating materials, in delivering the coating materials to an object, orienting the nanotubes on the object, and setting the coating in place."
Someday, this could help scientists create composites that are lighter but stronger than current materials. Composites such as this could find their way into aircraft bodies and into a myriad of military uses. They may be useful for pharmaceuticals and in the space program. A few years from now, the cars we drive may even be coated with a nanotube composite. Whatever their eventual applications, it is clear that the small but mighty nanotube ushers in a new wave of possibilities, not just for scientists but for all of society.
For more on nanotube projects at the University of Kentucky:
Advanced Carbon Materials Center Established At UK