Nanotubes reach flash point
Apr 25, 2002
First-year undergraduate Andres de la Guardia got more than he bargained for when the flash he was using to photograph carbon nanotubes set them alight. But his supervisor at the Rensselaer Polytechnic Institute in the US, Pulickel Ajayan, quickly realized the potential of the accidental effect and assembled a team of researchers to investigate. They found that a flash of light heats carbon nanotubes to a surprisingly high temperature, and could be used to ignite combustion reactions from a safe distance (P Ajayan et al 2002 Science 296 705).
Carbon nanotubes – tiny rolled sheets of graphite – have a host of unusual electronic and mechanical properties, which are the focus of research in Ajayan’s group. During an experiment, de la Guardia tried to photograph single-walled nanotubes using a conventional flash, which has a spectrum similar to that of sunlight, but without the ultraviolet light.
Ajayan’s team – which included researchers working in Mexico, France and UK – repeated this process on single- and multiwalled nanotubes. After packing samples of nanotubes to different densities, they exposed them to light pulses with a range of intensities and recorded the results on video. They later used an electron microscope to inspect the remains of the nanotubes.
Immediately after a flash, the researchers found that ‘hot spots’ appeared on the single-walled nanotubes and then spread through the sample until it was completely burnt. Samples packed to higher densities needed a more powerful flash to ignite them because the samples contained less oxygen, which supports combustion. The multiwalled nanotubes did not burn at all.
The team also tried the experiment in different environments. When the nanotubes were exposed to the flash in air, the carbon they contained burnt and escaped as carbon monoxide and carbon dioxide, leaving a residue of oxidized nickel and iron, which are used in the production of nanotubes. But when the nanotubes were ‘flashed’ in an atmosphere of helium – which does not support combustion – the team found that the carbon rearranged itself into single-layered structures with conical tips, known as ‘nanohorns’.
An intriguing property of carbon nanotubes is their ability to harbour heat. Scientists know that nanotubes oxidize at temperatures of around 900 kelvin, but Ajayan and colleagues believe that the ‘hot spots’ seen in their experiment must have reached about 1800 kelvin to produce such dramatic changes in the structure of the nanotubes.
When the link between light and combustion is better understood, Ajayan believes that the effect could be used in devices such as remote triggers. “Combustion reactions could be started by adding nanotubes to the reaction mixtures and exposing them to light,” he told PhysicsWeb.
De le Guardia – who is now doing a Masters degree at Rensselaer – was very surprised by his discovery. “I'm glad that I was alert and curious, and that I called Professor Ajayan right away,” he told PhysicsWeb.
About the author
Katie Pennicott is Editor of PhysicsWeb