The previously unknown phenomenon, described as thermopower waves, "opens up a new area of energy research, which is rare," says Michael Strano, an associate professor of chemical engineering at MIT.
Strano is the senior author of a paper detailing the discovery that appeared Sunday in the journal "Nature Materials." The lead author is Wonjoon Choi, a an MIT doctoral student in mechanical engineering.
The new system puts out energy about 100 times greater than an equivalent weight of a lithium-ion battery.
Strano envisions thermopower waves that could enable ultra-small electronic devices, no larger than a grain of rice, perhaps a sensor or treatment device that could be injected into the body.
Or they might be used in "environmental sensors that could be scattered like dust in the air," he says.
While the individual nanowires are tiny, Strano suggests that they could be made in large arrays in order to supply significant amounts of power for larger devices.
In theory, says Strano, such devices could maintain their power indefinitely until used, unlike batteries whose charge leaks away gradually as they sit unused.
A carbon nanotube can produce a very rapid wave of power when it is coated by a layer of fuel and ignited. (Illustration by Christine Daniloff courtesy MIT)
The key ingredient is carbon nanotubes - submicroscopic hollow tubes made of a lattice of carbon atoms. These tubes, just a few billionths of a meter in diameter, are part of a family of novel carbon molecules that have been the subject of intensive worldwide research over the last two decades.
In the new experiments, each of these electrically and thermally conductive nanotubes was coated with a layer of a highly reactive fuel that can produce heat by decomposing.
This fuel was then ignited at one end of the nanotube using either a laser beam or a high-voltage spark, and the result was a fast-moving thermal wave traveling along the length of the carbon nanotube like a flame speeding along the length of a lit fuse.
Heat from the fuel goes into the nanotube where it travels thousands of times faster than in the fuel itself.
As the heat feeds back to the fuel coating, a thermal wave is created that is guided along the nanotube.
With a temperature of 3,000 kelvins (2,726 degrees Celsius or 4,940 degrees Fahrenheit) this ring of heat speads along the tube 10,000 times faster than the normal spread of this chemical reaction.
The heating produced by that combustion, it turns out, also pushes electrons along the tube, creating a substantial electrical current.
Dr. Michael Strano (Photo courtesy MIT)
Combustion waves like this pulse of heat "have been studied mathematically for more than 100 years," Strano says, but he was the first to predict that such waves could be guided by a nanotube or nanowire and that this wave of heat could push an electrical current along that wire.
In the group's initial experiments, Strano says, when they wired up the carbon nanotubes with their fuel coating in order to study the reaction, the scientists were surprised by the size of the resulting voltage peak that propagated along the wire.
The amount of power released, he says, is much greater than that predicted by thermoelectric calculations. While many semiconductor materials can produce an electric potential when heated, through something called the Seebeck effect, that effect is very weak in carbon.
"There's something else happening here," Strano says. "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 property is responsible for the high power produced by the system, Strano says.
The researchers' theory predicts that using different kinds of reactive materials for the coating could make the wave front oscillate, producing an alternating current.
That opens up a variety of possibilities, Strano says, because alternating current is the basis for radio waves such as cell phone transmissions, but present energy-storage systems all produce direct current.
"Our theory predicted these oscillations before we began to observe them in our data," he says.
The present versions of the system have low efficiency, because much of the power generated is being given off as heat and light. The team plans to work on improving that efficiency.
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