Thermopower waves
Carbon Nanotubes: These are cylindrical nanostructures that consist of rolled up sheets of carbon hexagons. Their name is derived from their size, since the diameter of a nanotube is on the order of a few nanometers (approximately 1/50,000th of the width of a human hair), while they can be up to several millimeters in length. Nanotubes are categorized as single-walled nanotubes (SWNTs) and multi-walled nanotubes (MWNTs). Single-wall carbon nanotubes (SWCNTs) can be considered to be formed by the rolling of a single layer of graphite (called a graphene layer) into a seamless cylinder. A multiwall carbon nanotube (MWCNT) can similarly be considered to be a coaxial assembly of cylinders of SWCNTs, one within another; the separation between tubes is about equal to that between the layers in natural graphite.
Thermopower waves
A team of scientists at MIT have discovered a previously unknown phenomenon that can cause powerful waves of energy to shoot through carbon nanotubes. This phenomenon is described as thermopower waves. These thermal waves are moving pulses of heat traveling along a carbon nanotube that can drive electrons along, creating an electrical current. This discovery was based on a prediction by Michael Strano, Associate Professor of Chemical Engineering. Strano was the first to predict that thermal waves could be guided by a nanotube or nanowire and that this wave of heat could push an electrical current along that wire. Strano is one of the lead researchers of the team.
In their experiment a carbon nanotube was coated with a layer of a 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. 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, 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.
According to Strano 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. He says this important property is responsible for the high power produced by the system.
The researchers claim the system now puts out energy, in proportion to its weight, about 100 times greater than an equivalent weight of lithium-ion battery. Strano says the amount of power released, is much greater than that predicted by thermoelectric calculations. While many semiconductor materials can produce an electric potential when heated, through the Seebeck effect, that effect is very weak in carbon. “There’s something else happening here,” he says. “We call it electron entrainment, since part of the current appears to scale with wave velocity.”
The researchers also plan to pursue another aspect of their theory: that by using different kinds of reactive materials for the coating, the wave front could oscillate, thus producing an alternating current.
Ray Baughman, director of the Nanotech Institute at the University of Texas at Dallas, who was not involved in this work, calls the research “stellar.”
The work, Baughman says, “started with a seminal initial idea, which some might find crazy, and provided exciting experimental results, the discovery of new phenomena, deep theoretical understanding, and prospects for applications.” Because it uncovered a previously unknown phenomenon, he says, it could open up “an exciting new area of investigation.”
Wonjoon Choi, a doctoral student in mechanical engineering is another one of the lead researchers of the team.
Source: http://web.mit.edu/newsoffice/2010/thermopower-waves-0308.html
March 9, 2010