Researchers at the Georgia Institute of Technology have used a straightforward technique based on mechanical resonance induced by an oscillating electrical voltage to measure the comparative bending strength of tiny carbon nanotubes produced by two competing processes.
(Left) Electric-field-induced mechanical resonance of a carbon nanotube synthesized by catalytically assisted growth. The bending modulus is 35 GPa. (Right) Silicon carbide-silica nanowire showing mechanical resonance induced by applying an oscillating voltage.
The National Science Foundation-sponsored work, which also correlates the strength measurements to observed defects, provides information that will be important in helping materials scientists select the best variety of nanotube for new applications now under development. The high-strength, lightweight and unique electronic properties of multiwalled carbon nanotubes have led to potential applications as diverse as ultralight composites and low-power field-emission displays.
The researchers have used the same technique to study other fiber-like structures that are too small to be measured by conventional testing methods.
"We are able to make a quantitative comparison, with a real number to describe how much the bending modulus differs," said Dr. Z.L. Wang, a Georgia Tech professor of materials science. "This work provides materials scientists the information needed to make a choice, and gives the first set of data for theoretical scientists to model individual nanotubes."
With colleagues Walter de Heer and Zhigang Bai of Georgia Tech, Liming Dai and Mei Gao from CSIRO Molecular Science in Australia, and Ruiping Gao of the Univ. of Science and Technology in Beijing, Wang compared nanotubes produced by traditional high-temperature carbon arc discharge to nanotubes grown through a lower-temperature catalyst-assisted pyrolysis process. In the journal Physical Review Letters, the researchers report on the dramatic strength differences caused by point and volume defects in the catalytically-grown nanotubes.
Researchers had known that the catalytically-grown tubes were weaker than comparable structures grown in carbon arcs, and had made bulk measurements that produced an average strength value. But producing data for individual tubes gives scientists better information to predict performance.
Wang believes the catalytically grown nanotubes may offer advantages for ultra-lightweight composites, where the defects could help interlock the tubes to prevent pulling out of the finished part. On the flip side, however, those defects could cause problems in electronic applications such as field emission electrodes where current flow could cause uneven heating in the narrow regions.
Those considerations must be weighed against the cost of production for both processes, the speed at which the catalytically grown tubes can be "grown like grass on a substrate," and the relatively low yield of the carbon arc method, Wang noted.
"Both types have advantages depending on the specific application," Wang said.
The measurement technique begins by gluing a single nanotube just a few hundredths of a micron in diameter and 5 to 20 µm long to a tiny gold ball in a specially prepared transmission electron microscope (TEM) sample holder. The tube is aligned near another gold ball, and an oscillating electrical voltage is applied. Adjusting the frequency of the voltage allows the researchers to induce a mechanical resonance in the tube that can be observed and measured.
By knowing the outer diameter, inner diameter, length and density of the nanotube under study, the researchers can determine the bending modulus from the frequency at which the tube resonates. Because the oscillating tube can be observed in the TEM, the strength can be correlated to visible defects.
"This is a very simple and straightforward way to measure these properties," Wang said. "It has a wide application as a general technique for making mechanical property measurements of any nanofibers."
Because carbon nanotubes are so light and strong, they could offer significant advantages over conventional carbon fiber reinforcement in the manufacture of composite materials.
"This could be a big gain for space technology," Wang said. "In composites, carbon nanotubes can reduce weight by a factor of 5-10, while increasing the strength by a factor of 5-10 compared to a conventional carbon fiber matrix."
Beyond the carbon nanotubes, Georgia Tech researchers have used the TEM technique to measure the properties of biaxially structured silicon carbide-silica nanowires.
In a paper accepted for publication in Applied Physics Letters, J.L. Gole, Z.R. Dai, Z.G. Bai and Wang, all from Georgia Tech, and R.P. Gao from the Univ. of Science and Technology in Beijing, describe measuring the Young's modulus of a single fiber produced by growing two different materials together. These biaxially structured silicon carbide-silica nanowires could have important applications in nanoelectronics and high-strength composites.
The measurements were made in the Center for Nanoscience and Nanotechnology at Georgia Tech. Support for the work was also provided by the China NSF.