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A Comprehensive Vision
“Nanotubes are a very versatile material with absolutely fascinating physical properties, all the way from ballistic conduction to really interesting mechanical behavior,” says Pulickel Ajayan, the Henry Burlage Professor of Materials Science and Engineering and a world-renowned expert in fabricating nanotube materials. “I don’t think we have ever come across a material with such a wide range of possibilities.”
Rensselaer researchers are exploiting this broad portfolio of properties across a variety of fields, beginning with the fundamental building blocks of matter and working up to devices and systems with a multiplicity of applications.
The vision is fast becoming a reality, as Rensselaer scientists and their collaborators continue to report significant advances in the field.
In a 2005 paper published in Science, researchers from Rensselaer, the University of Hawaii at Manoa, and the University of Florida showed that films of vertically aligned carbon nanotubes can act like a layer of “super-compressible” mattress springs, flexing and rebounding in response to a force. But unlike a mattress, which can sag and lose its springiness, these nanotube foams maintain their resilience even after thousands of compression cycles, opening the door to foam-like materials for just about any application where strength and flexibility are needed, from disposable coffee cups to the exterior of the space shuttle.
The foams are just the latest in a long line of nanotube-based materials that have been produced through collaborations with Ajayan’s lab, including tiny brushes with bristles made from carbon nanotubes. The brushes, which were described in Nature Materials, already have been tested in a variety of tasks that range from cleaning microscopic surfaces to serving as electrical contacts, and they eventually could be used in a whole host of electronic, biomedical, and environmental applications, Ajayan says.
Carbon nanotubes can carry large amounts of electrical current without losing heat, making them ideal materials for nanoscale wires. Along with colleagues in Germany, Mexico, the U.K., and Belgium, Rensselaer researchers have reported a way to weld these tiny tubes together end-to-end, overcoming a major obstacle to realizing nano-tube-based electronic devices. By passing a high current through a thin film with nanotubes dispersed across its surface, they generated visible flashes of lightsimilar to the familiar arc from a welder’s torch. Further investigation revealed that the flashes occur at junctions where overlapping carbon nanotubes are welded together. Current methods to make nanowires require bombarding the surface with electrons or other charged particles, which may not be easily scalable. The team, which is led by Ganapathiraman Ramanath, associate professor of materials science and engineering, suggests that their new technique could provide a viable tool for producing nanowires cost-effectively.
In collaboration with researchers from Banaras Hindu University in India, Rensselaer scientists have devised a simple method to produce carbon nanotube filters that can efficiently remove micro- to nano-scale contaminants from water and heavy hydrocarbons from petroleum. Made entirely of carbon nanotubes, the filters are easily manufactured using a new method for controlling the cylindrical geometry of the structure. While activated carbon has long been the standard for removing organic contaminants from drinking water, the new research suggests that carbon nanotubes have significant potential as better materials for water purification.
Researchers also are branching out into new projects that are just starting to yield results. For example, with a $1.15 million grant from the NSF, a team led by Toh-Ming Lu, the R.P. Baker Distinguished Professor of Physics at Rensselaer, is exploring the potential of nanomechanical systems by making and testing springs, rods, and beams on the nanoscale.
The past decade has seen an explosion of interest in electronic devices at the molecular level, but less attention has been paid to nanoscale mechanical systems, according to Lu. Yet these devices may have as important an impact as nanoelectronics, representing a potential multibillion-dollar high-technology industry that will save energy and improve the quality of lives, he says. Lu envisions a wide range of applications for these devices, including much more efficient light emitters and solar cells, extremely sensitive chemical and biological sensors, and super-high-density three-dimensional magnetic memory.
Some fundamental issues, however, have kept researchers from realizing the full potential of nanotubes. “There is a lot of hype in this field, and it has been difficult to live up to,” says Nikhil Koratkar, associate professor of mechanical, aerospace, and nuclear engineering. “Researchers have not been able to get the 10- to 20-fold increases in strength and stiffness that have been touted over traditional composites and materials.”
One of the biggest engineering challenges comes when nanotubes are combined with other materials to make composites, according to Koratkar. The interface between the materials is not as strong as one might expect because it is difficult to disperse nanotubes in an orderly way. Single-walled nanotubes are particularly hard to disperse, since they tend to form clusterslike ropes where only the nanotubes on the outside layer come in contact with the other material. Ajayan and Koratkar are partnering with researchers across the campusand around the worldto address some of these challenges.
Though much of the research has focused on improving the strength and stiffness of nanomaterials, Koratkar and his colleagues have directed their attention to another important property: damping, or the ability of a material to dissipate energy. They have found that dispersing nanotubes throughout traditional materials creates new composites with vastly improved damping capabilities. And in a recent paper published in the journal Nano Letters, the researchers have also shown for the first time that these damping properties are enhanced as the temperature increases. Traditional damping polymers perform poorly at elevated temperatures, so the new nanocomposites could fill an important gap for any kind of structure that is exposed to vibration, from high-performance parts for spacecraft and automobile engines, to golf clubs that don’t sting and stereo speakers that don’t buzz.
In 2004, Koratkar received an NSF Faculty Early Career Development Award to fund the development of these new materials, and during the next phase of the grant he plans to move into “hybrid” systems. These structures will combine the high stiffness of carbon fiber composites with the damping properties of nanotubes, leading to a class of materials that truly offers “the best of both worlds.”
Meanwhile, Linda Schadler, professor of materials science and engineering, is leading a group that is working to improve the optical and mechanical behavior of polymers for packaging materials by filling them with nanoparticles. And as part of a joint research project with the University of Florida, Schadler and her colleagues are developing a new generation of synthetic lubricant coatings for aircraft and spacecraft. The coatings, which are made of thin layers of carbon nanotubes, polymers, and ceramics, will be sensitive to changes in the environment that a spacecraft experiences, with the potential to reduce the rate of wear by 1,000 times or more.
Other teams are working to exploit another interesting property of nanotubes called “field emission.” When a voltage is applied to certain materials, electrons are pulled out from the surface, making these materials useful in electronic displays. “Nanotubes are very good field emitters because they have a low threshold for emission and they produce high currents,” says Swastik Kar, a post-doctoral researcher in materials science and engineering. Kar and a team of researchers from Rensselaer, Northeastern, and New Mexico State have developed a new process to make flexible, conducting “nano skins” based on field emission for a variety of applications, from electronic paper to sensors for detecting chemical and biological agents. The materials can be bent, flexed, and rolled up like a scroll, all while maintaining their ability to conduct electricity, which makes them ideal materials for flexible electronics, according to the researchers. Nanotube arrays normally are held together by weak forces that don’t always maintain their shape when transferred, but the team has developed a new procedure that allows them to transfer arrays of nanotubes into a soft polymer matrix without disturbing the shape, size, or alignment of the nanotubes.
Ajayan, working with researchers at the University of Akron, is using a similar process to mimic the agile gecko, with its uncanny ability to run up walls and across ceilings. Last year, the team reported a process for creating artificial gecko feet with 200 times the sticking power of the real thing, using nanotubes to imitate thousands of microscopic hairs on a gecko’s footpad.
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