Putting the heat on nanotubes
Characterization
January 2, 2008
 |
| The temperature profiles of the nanotube and thermal test platform beneath are shown at a point just past the melting point of the Au nanoparticles. (Courtesy of the Zettl Research Group.) |
Multiwalled carbon nanotubes (MWNTs) are more thermally stable than either diamond or graphite, researchers from the University of California, Berkeley and Lawrence Berkeley National Laboratory have found.
The researchers have developed a new platform for probing the thermal properties of individual nanoscale systems up to extreme temperatures in the transmission electron microscope (TEM) [Begtrup et al., Phys. Rev. Lett. (2007) 99, 155901].
Investigating the thermal properties of nanostructures is difficult for a number of technical reasons. Even measuring local temperature at the nanoscale is problematic.
Alex Zettl and coworkers have come up with a new approach to this problem. Their thermal test platform consists of a thin Si3N4 membrane that allows TEM observation onto which the sample is placed.
Electrical contacts are added so that the sample is heated by Joule heating as a current is driven through it.
Finally, small Au nanoparticles are deposited on the sample and membrane. The nanoparticles act as single-shot nanothermometers by melting at a known temperature. As the whole platform heats up, a nanoparticle melting front progresses out from the sample, allowing temperature change to be mapped out.
The group applied this approach to single MWNTs. When a bias is applied to a MWNT held between electrodes on the thermal platform, the Au nanoparticles melt in a front fanning out from the centre of the nanotube. The Si3N4 membrane then begins to disintegrate, until eventually the nanotube fails.
“[MWNTs] turn out to be stable to 3200 K, a record for carbon-based materials and better than diamond or graphite,” says Zettl.
Finite element analysis, and the knowledge that 6 nm Au nanoparticles melt at 1275 K and Si3N4 at 2173 K, allows the thermal conductivity of the nanotubes to be deduced, as well as the local temperature throughout the experiment.
Even just prior to failure, MWNTs retain 10% of their peak thermal conductivity and carry a current of 1.7 x 108 A/cm2.
“They are exceptional electrical and thermal conductors even at extreme temperatures,” comments Zettl. This has profound implications for MWNT applications in extreme environments, he believes.
The thermal platform also allowed the group to determine the melting points of Au nanoparticles of different diameters down to ~3 nm.
“The system is very versatile and can be applied to virtually any nanomaterials, electrically conducting or not,” adds Zettl.
Jonathan Wood