It’s not every day that a new way to produce electricity is discovered…although it does seem there is a multitude of approaches. This one involves carbon nanotubes, those jacks-of-all-trades in the nanotech business, nanometer sized tubes of pure carbon. (In this case, think of them as ‘wires’ one-hundred thousandth of the thickness of human hair.) The team of scientists at MIT (Massachusetts Institute of Technology, USA) began working with nanotubes and thermal waves – waves of heat energy – that they sent down the nanotubes like current through wires. To their surprise, what they also got was a relatively large voltage electrical current generated by the thermopower wave.
There was something different about the carbon nanotubes. They were coated with a layer of fuel that can produce heat when it decomposes (burns). The fuel was ignited at one end of a nanotube with a laser or high voltage spark. It was like a fuse, a very fast fuse, traveling as a wave of heat (3000 degrees Kelvin) spreading along the tube 10,000 times faster than a normal chemical reaction. As predicted by mathematical studies, the thermal wave pushed electrons ahead of it, sort of collecting them as a beach wave will collect flotsam from the water, creating an electrical current. This much was expected. What was not predicted by the thermoelectric calculations was the magnitude of the voltage peak.
Normally with carbon, the Seebeck effect, which produces electricity from a heated semiconductor, is very weak. Something else was happening. As the senior author, Dr. Michael Strano (Associate Professor of Chemical Engineering, MIT) puts it:
“We call it electron entrainment since part of the current appears to scale with wave velocity.”
The thermal wave, he explains, appears to be entraining the electrical charge carriers (either electrons or electron holes) just as an ocean wave can pick up and carry a collection of debris along the surface. This important property is responsible for the high power produced by the system, Strano says.
Because this is such a new discovery, he says, it’s hard to predict yet exactly what the practical applications will be. But he suggests that one possible application would be in enabling new kinds of ultra-small electronic devices — for example, a devices the size of grains of rice, perhaps a sensor or treatment device that could be injected into the body. Or it could lead to “environmental sensors that could be scattered like dust in the air,” he says.
After some refinement, the thermopower system produces energy in proportion to its weight about 100 times greater than an equivalent weight lithium-ion battery. However, as would be expected, much of the energy produced is also in the form of heat and light – not exactly what’s needed for most practical energy sources. Packaging and efficiency will be important limitations to overcome. On the other hand, eventually carbon nanotubes will be inexpensive; moreover the nanotubes loaded with the fuel coating can sit in storage for a long time without losing ‘charge’ as typical batteries would do. The combination of ingredients could lead to interesting niche uses for this ‘new’ energy source.