Carbon nanotubes go superplastic
In theory, single-walled carbon nanotubes have a maximum tensile strain of around 20%. But by stretching the nanotubes at high temperature, researchers at Boston College, Lawrence Livermore National Laboratory and Massachusetts Institute of Technology, all in the US, have extended nanotubes by almost 280%.
"We discovered the superplasticity phenomena accidentally," Jianyu Huang of Boston College told nanotechweb.org. "When my student Shuo Chen and I were studying the high-bias transport property of individual nanotubes, our scanning tunnelling microscopy (STM) probe was always attracted by the other electrode, which bent the nanotube and affected our measurement." As Chen and Huang pulled the STM probe away from the electrode, they discovered that the nanotube was much more stretchy than expected.
To investigate the phenomenon further, the researchers made a 24 nm-long single-walled carbon nanotube by breaking down a multiwalled nanotube inside a high-resolution transmission electron microscope. They used a piezo manipulator to stretch the nanotube at a bias of 2.3 V. The nanotube failed when it became 91 nm long, a tensile elongation of 280%. Its diameter at failure was 0.8 nm, 15 times smaller than its initial diameter of 12 nm. The scientists reckon that the temperature inside the nanotube was more than 2000°C during deformation.
"This is a new revelation of the intrinsic physical properties of carbon nanotubes," said Huang. "Carbon nanotubes are perceived as rather brittle because of the strong carbon-carbon sp2 bond. Our discovery proves that carbon nanotubes can be extremely ductile at elevated temperatures, which is a completely different landscape to what people thought."
The team believes that kinks and point defects were fully activated at the high temperature, enabling superplastic deformation. Atom diffusion was also an important part of the process, helping to heal defects and prevent crack initiation.
"Our discovery implies that carbon nanotubes can be used as a reinforcement agent to toughen ceramics even at high temperatures," said Huang. "If we incorporate nanotubes into a ceramic to make a composite, the composite could possess both high strength and high ductility. This would make it the ideal material that many scientists are pursuing and it could find applications in many areas, such as a turbo engine blade in an aircraft."
As the defect density increased during deformation, the current flow in the tube dropped from 80 µA to around zero. "We are curious about the electrical properties of such superstretched nanotubes, as they might find important electronic applications," said Huang. "Our initial thought is that the structure of the superstretched nanotubes is highly disordered, meaning that the regular hexagonal honeycomb lattice is completely destroyed and the carbon atoms are randomly arranged. This implies that the electrons are strongly localized."
If that is the case, it might be possible to tune the electronic properties of the nanotubes in an unprecedented way, altering their behaviour from metallic to semiconductor-like. "This means that we could control the diameter and the conductivity of nanotubes as we wish by using the superstretch property," said Huang. "That is a target that has been vigorously pursued by many scientists since the discovery of nanotubes over a decade ago."
The researchers reported their work in Nature.