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The ability of carbon nanotubes to withstand repeated stress yet retain their structural and mechanical integrity is similar to the behaviour of soft tissue, according to a new study from Rensselaer Polytechnic Institute.
When paired with the strong electrical conductivity of carbon nanotubes, this ability to endure wear and tear, or fatigue, suggests the materials could be used to create structures that mimic artificial muscles or interesting electro-mechanical systems, researchers said.
The report, “Fatigue resistance of aligned carbon nanotube arrays under cyclic compression,” appears in the July issue of Nature Nanotechnology. Despite extensive research over the past decade into the mechanical properties of carbon nanotube structures, this study is the first to explore and document their fatigue behaviour, said co-author Victor Pushparaj, a senior research specialist in Rensselaer’s department of materials science and engineering.
“The idea was to show how fatigue affects nanotube structures over the lifetime of a device that incorporates carbon nanotubes,” Pushparaj said. “Even when exposed to high levels of stress, the nanotubes held up extremely well. The behaviour is reminiscent of the mechanics of soft tissues, such as a shoulder muscle or stomach wall, which expand and contract millions of times over a human lifetime.”
Pushparaj and his team created a free-standing, macroscopic, two-millimetre square block of carbon nanotubes, made up of millions of individual, vertically aligned, multiwalled nanotubes. The researchers then compressed the block between two steels plates.
The team repeated this process more than 500,000 times, recording precisely how much force was required to compress the nanotube block down to about 25 percent of its original height.
Even after 500,000 compressions, the nanotube block retained its original shape and mechanical properties. Similarly, the nanotube block also retained its original electrical conductance.
In the initial stages of the experiment, the force needed to compress the nanotube block decreased slightly, but soon stabilized to a constant value, said Jonghwan Suhr, an assistant professor of mechanical engineering at the University of Nevada in Reno, who received his doctorate from Rensselaer in 2005, and with Pushparaj contributed equally to this report.
While more promising than polymers and other engineered materials that exhibit shape memory, carbon nanotubes by themselves do not perform well enough to be used as a synthetic biomaterial. But Pushparaj and his fellow researchers are combining carbon nanotubes with different polymers to create a material they anticipate will perform as well as soft tissue. The team is also using results from this study to develop mechanically compliant electrical probes and interconnects.
In addition to Pushparaj and Suhr, other contributing authors of the paper include Pulickel Ajayan, the Henry Burlage Professor of Materials Science and Engineering at Rensselaer; Omkaram Nalamasu, professor of chemistry and materials science and engineering at Rensselaer; Lijie Ci, Rensselaer research associate; Subbalakshmi Sreekala, a research associate in the department of mechanical and aerospace engineering at Princeton University; and X. Zhang, research associate in the school of materials science and engineering at Shanghai Jiao Tong University.
Funding for the project was provided by the Focus Center New York for Interconnects.
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