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Thermopower Creating Waves

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CAMBRIDGE, Mass., March 10, 2010 – Like a flame speeding along the length of a lit fuse, a powerful wave of energy can be created by using a laser to ignite a reactive-fuel-coated carbon nanotube. It could lead to a new way of producing electricity, said the MIT team that discovered the previously unknown phenomenon.

Described as thermopower waves, the phenomenon "opens up a new area of energy research, which is rare," said Michael Strano, MIT's Charles and Hilda Roddey associate professor of chemical engineering. 

As with a collection of flotsam propelled along the surface by waves traveling across the ocean, it turns out that a thermal wave – a moving pulse of heat – traveling along a microscopic wire can drive electrons along, creating an electrical current.

The key ingredient in the recipe are carbon nanotubes – submicroscopic hollow tubes made of a chicken-wirelike lattice of carbon atoms. These tubes, just a few billionths of a meter (nanometers) in diameter, are part of a family of novel carbon molecules, including buckyballs and graphene sheets, that have been the subject of intensive worldwide research over the past two decades.

A previously unknown phenomenon

In the new experiments, each of these electrically and thermally conductive nanotubes was coated with a layer of a reactive fuel that can produce heat by decomposing. The 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. 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 3000 K, this ring of heat speeds 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.

A carbon nanotube (shown in illustration) can produce a very rapid wave of power when it is coated by a layer of fuel and ignited by a laser or high-voltage spark, so that heat travels along the tube. (Graphic: Christine Daniloff)

Combustion waves – such as this pulse of heat hurtling along a wire – "have been studied mathematically for more than 100 years," Strano said, 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 said, when they wired up the carbon nanotubes with their fuel coating to study the reaction, "lo and behold, we were really surprised by the size of the resulting voltage peak" that propagated along the wire.

After further development, the system now puts out energy, in proportion to its weight, about 100 times greater than an equivalent weight of lithium-ion battery.

The amount of power released is much greater than that predicted by thermoelectric calculations, he said. Although 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,' he said. "We call it electron entrainment, since part of the current appears to scale with wave velocity."

The thermal wave 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 said.

Exploring possible applications

Because this is such a new discovery, it's hard to predict exactly what the practical applications will be. But he suggests that one possible application would be in enabling new kinds of ultrasmall electronic devices; for example, devices the size of grains of rice, perhaps with biomedical sensors or treatment devices that could be injected into the body. Or it could lead to "environmental sensors that could be scattered like dust in the air."

In theory, such devices could maintain their power indefinitely until used, unlike batteries whose charges leak away gradually as they sit unused. And although the individual nanowires are tiny, Strano suggests that they could be made in large arrays to supply significant amounts of power for larger devices.

The researchers also plan to pursue another aspect of their theory: that by using different kinds of reactive materials for the coating, the wavefront could oscillate, thus producing an alternating current. That would open up a variety of possibilities 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," Strano said.

Also, the present versions of the system have low efficiency because a great deal of power is being given off as heat and light. The team plans to work on improving that next.

The research, described in the March 7 issue of Nature Materials (Stano was senior author; lead author was Wonjoon Choi, a doctoral student in mechanical engineering), was funded by the US Air Force Office of Scientific Research and the National Science Foundation.

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Mar 2010
The movement of particulate material by flowing gas or liquid.
batterybiomedicalBiophotonicscarbon nanotubeCombustionelectricityelectronicentrainmentenvironmental sensorMichael StranoMITnanoNature MaterialsResearch & TechnologySensors & Detectorsthermal wavethermoelectricultrasmalllasers

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