A paper published in Nature Materials this week details a new method for using nanotubes to generate significant amounts of power, at least for their size. When multi-walled carbon nanotubes are covered with a material that produces an exothermic reaction, the nanotubes help conduct the heat in one focused direction. To the apparent surprise of the researchers, this created an electrical pulse, a quick surge of power, that could be put to a number of uses.

When you couple a heat-activated material with exothermically-reactive chemicals, it's theoretically possible to create self-propagating waves of heat. However, there are a couple of problems with implementing systems like these. The waves generally propagate in all directions, which is not terribly efficient for heat- or power-generating purposes. Furthermore, materials that both prevent the wave of the pulse from scattering and can stand up to a large amount of heat are fairly rare.

To turn the theory into practice, scientists needed a material that was heat-proof up to a few thousand Kelvin and took a hard line against heat scattering, preferably concentrating it in a single direction. They also needed it to be very small—the waves would propagate best, and at increasing velocities, if the phonons, or lattice vibration modes, had a mean free path about the same as the length of the material they propagated through.

Enter the carbon nanotube—a structure that is particularly apt at minimizing scattering. When energy is given to them, it escapes, or dissipates, at very low rates. Carbon nanotubes also have a high thermal conductivity and heat tolerance, making them ideal for functions that require them to be surrounded by very hot chemicals. Being nano-, they are also small. The team realized that if they could combine the carbon nanotube with the necessary exothermic reactions, they could create a very powerful system.

For the chemical reaction, scientists coated arrays of multi-walled carbon nanotubes with a thin layer of cyclotrimethylene trinitramine. They then ignited the coated tubes on one end, with either laser irradiation or high-voltage electrical discharge, expecting that the coating would react to the heat, the reaction would propagate along the length of the nanotubes, and the nanotubes inside would help hold onto the heat generated by the reaction.

When the coating was ignited, scientists found that the waves generated by the reaction on the nanotubes moved 10,000 times faster than if the chemicals were burning in bulk. The carbon nanotubes concentrated the heat, allowing the reaction to sweep along their length very quickly, which generated a sizeable amount of heat in a very short time and in a small space.

The rapid transit of the reaction down the nanotubes appeared to pull the electrons within it along. This appears to be something that wasn't predicted by theory, since the authors describe it by writing that they need to "introduce a new phenomenon that results from their effect on carrier propagation." (Of course, if it was completely unexpected, why measure current at all?) They refer to the combined reaction/heat/electrical pulse as a thermopower wave.

The maximum power the system could generate was 7 kilowatts per kilogram, greater than high-performance lithium ion batteries, and at a much higher discharge rate. While this is significant, researchers found that this relationship decreased as the devices got bigger, and keeping them smaller helped preserve the larger power-to-mass ratios.

For generating thrust, the reactions on the nanotube had a similarly impressive metric: the impulse produced 300 Newton-seconds per kilogram. Scaling this down to the size and weight of carbon nanotubes could make this number seem unimpressive, but the impulse was up to 100 times stronger than other microthruster systems.

While the power generated by these setups are large when measured next to a comparatively large unit of mass, the fact that they are necessarily small suggests their usage might be quite limited—at least, they probably won't be powering rockets off the earth anytime soon. The systems are also single-use only since, once the reactive chemical is burned away, the nanotubes would have to be recoated to generate another reaction and more power. Because of this, the system might be better suited for use in high-tech combustion applications. Nonetheless, these coated carbon nanotubes could be an extremely powerful addition to many micro-mechanical systems, even in tiny doses.

Nature, 2010. DOI: 10.1038/NMAT2714 (About DOIs).