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Tensile Tests Measure Strength of Carbon Nanotubes

St. Louis, Mo., Jan. 25, 2000 - Carbon nanotubes are smaller than the eye can see, yet stronger than steel. But just how strong are these nanoscale materials -- the foundations of what some are calling a new technological order? In a milestone measurement, Rodney S. Ruoff, Ph.D., associate professor of physics in Arts & Sciences, and his nanotechnology research group at Washington University in St. Louis have determined how much force a carbon nanotube can withstand before breaking.

In the experiment performed by Ruoff and his research group, individual multiwalled carbon nanotubes -- rolled sheets of graphite -- were picked up, positioned on a nanometer length scale, firmly attached by a novel method, and tensile loaded (stretched by applying a force) until broken. A readout showed the applied force. In some cases, micro-Newtons of force were needed to break individual nanotubes, a force many times higher than the force that would be needed to break a similar sized nanotube made of high-grade steel, if such a thing existed.

Ruoff, graduate student MinFeng Yu and postdoctoral fellow Oleg Lourie, with co-authors Mark J. Dyer of Zyvex, a start-up company aiming to develop nanotechnology, and Katerina Moloni and Thomas F. Kelly from the University of Wisconsin, report their findings, "Strength and Breaking Mechanism of Multiwalled Carbon Nanotubes Under Tensile Load," in the Jan. 28 issue of Science.

"A milestone measurement has been considered to be the tensile strength -- how much load can these tiny fibers bear before they fail? We are the first to perform and report on this type of experiment. New tools, under development in our laboratory for the past several years, have allowed us to manipulate these nanotubes, which have diameters on the order of 10,000 times smaller than a human hair, and lengths of only several microns, onto opposing, tiny cantilevers, or micro measuring instruments. We video record the entire tensile loading experiment as it takes place inside of a scanning electron microscope," Ruoff said.

"The elastic modulus, which, when multiplied by the specimen cross-sectional area gives the stiffness or the spring constant of the specimen, and the tensile strength of nanotubes have been of tremendous interest because theoretical treatments have suggested that these could be both the stiffest and the strongest fibers ever," Ruoff noted. "The modulus had been measured for the multiwalled carbon nanotubes (MWCNTs) by other groups by applying bending forces with atomic-force microscope tips, or by measuring the vibrational amplitude and frequency of cantilevered nanotubes. But the strength and the stress-strain relationship had never been determined."

MWCNTs break with what Ruoff refers to as a "sword-in-sheath" mechanism. "These are 'Russian Matryoshka doll-like' structures," Ruoff noted. "One nanotube is nested inside of another, which is inside of another, and so on. For the MWCNTs we mechanically loaded, there would be typically 10 to 40 nested cylinders.

"Our method of 'nano-welding' these onto the cantilever tips, which are our 'fingers' for holding and pulling, is to focus the electron beam onto the MWCNT where it is loosely attached by the relatively weak van der Waals forces to the cantilever tips. Doing this causes residual hydrocarbon gases in the electron microscope to be decomposed, and to build up a small carbonaceous deposit. This deposit is the strong attachment that holds the nanotubes in place during the experiment."

The method needs further development, Ruoff said. About one half of the MWCNTs that MinFeng attached in this manner still broke at the attachment site rather than within the loaded nanotube section after the load was applied. But the other half still represented 19 separate MWCNT tensile-loading experiments, which is considered a good data set for analysis.

"Since the attachment is to the outermost shell, and the interaction between these nested nanotubes in a multi-walled nanotube is relatively weak, one might expect that the outermost shell will carry the load and break, with pullout of the inner shells then occurring immediately after the break. This is exactly what we observed," Ruoff noted.

"This allowed us to measure, for the first time, the tensile strength of a single nanotube, namely the outermost shell. And the highest strength value, 63 GPa, exceeds that of any reported value for any type of material. When we take account of the lower density of carbon nanotubes as compared to high-grade steels, the outer shell is about 50 to 60 times stronger. This suggests that there are future applications for very light-weight, high-strength cables and composites, where the carbon nanotubes are the load-carrying element."

Ruoff's group is currently working on mounting and breaking of the still smaller "single-wall" nanotubes, which are ~1.4 nanometers in diameter and up to tens of microns long. These nanotubes have only one cylinder, are not nested like the multi-walled variety, and are now available due to advances in synthesis. "The multiwalled type were discovered in 1991, and the single-walled nanotubes a few years later. But only recently have synthesis methods improved to the point where we have good single-walled samples for our work," Ruoff said.

"There will, obviously, be no 'sword-in-sheath' failure for these single-walled nanotubes -- there is only one cylinder. We are excited about how strong these nanotubes may be, because theory is suggesting strengths several hundred times that of the highest strength steels."

In addition, Ruoff's group is expanding its repertoire of measurements to include determining changes in transport properties as a function of mechanical relaxation. "We think that carbon and also boron nitride nanotubes may show remarkable changes in conductivity when mechanically deformed. They may also be piezoelectric. Piezoelectric materials expand and contract with the application of an electric voltage. This could lead to new applications, such as in nanoscale sensors or in artificial muscle."

Ruoff said that the tools that his group has built are themselves of interest for use in the new and rapidly growing area of research and development in nanotechnology. "While I suppose we should be happy both for taking on a challenging problem and for the fact that we have achieved some important scientific results, there is still much room for further advances with manipulation of matter at the nanometer length scale.

"In this case we relied on the sure hands of MinFeng, Oleg, and our collaborator Mark Dyer at Zyvex," said Ruoff. "I admit to an emotional desire that this 'human element' will always be present, but in the future, it will be the chore of remarkably tiny machines, perhaps having some stiff and strong nanotube components, to carry out the 'pick and place' of material to build other little machines, devices, cars, houses, roads and so on, as pre-programmed. In the future, we will be giving up control, and instruction sets embedded in the nanotechnological devices will be taking over on the local level. It is an absolutely wide-open area for R&D and there is now, justifiably so in my opinion, tremendous interest in pushing nanotechnology R&D in the United States."

Ruoff points to a comment made at the recent Foresight Conference on Nanotechnology held in Santa Clara, Calif., where he presented an invited talk on the results of research on mechanical properties of MWCNTs that also appeared in the journal Science.

"There was one fellow there who, following my talk, came up to me with fire in his eyes and asked me if I understood 'in both your head and heart that nanotechnology is going to be the final technology, because with a fully operational nanotechnology, we will be able to build anything we want with almost any starting material.' This is why our country is now readying itself for substantial investment in this area, as are competitors in Europe, Asia and elsewhere in the world -- the payoff will be very high," Ruoff said.

"There will be applications to virtually every field due to nanotechnology, among them computing, materials engineering, and medical devices and applications."

Contact: Susan Killenberg
Washington University School of Medicine

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