Thermopower waves are a phenomenon that happens when powerful waves of energy shoot through carbon nanotube wires, creating electricity. The researchers from MIT are responsible for this discovery, thus opening a new area of rare energy research.
Michael Strano, MIT’s Charles and Hilda Roddey Associate Professor of Chemical Engineering reported their discovery in Nature Materials two days ago, along with Wonjoon Choi, a doctoral student in mechanical engineering, the lead author.
A thermal wave (a moving pulse of heat), traveling along a microscopic carbon nanotube wire can drive electrons along, creating an electrical current, just like a collection of flotsam propelled along the surface waves traveling across the ocean.
Carbon nanotubes are submicroscopic hollow tubes, made of a lattice of carbon atoms. The nanometer-scale tubes are part of a family of new carbon molecules, including buckyballs and graphene sheets, and have been extensively been studied over the last 20 years.
The MIT experiments used the carbon nanotubes (which are thermally and electrically conductive) coated with a layer of reactive fuel that can produce heat by decomposing. When they ignited the fuel at one end of the nanotube by using lasers or high-voltage sparks, the scientists provoked a quickly-moving thermal wave, traveling along the carbon nanotube like a flame speeding along the length of a lit fuse.
The explanation of the process is that heat from the fuel goes into the nanotube, where it travels thousands of times faster than in the fuel itself, creating a thermal wave along the nanotube. At the temperature of 3,000 °K, the ring of heat speeds along the nanotube 10,000 times faster than the normal spread of the chemical reaction. The heat produced this way pushes electrons along the tube, creating a substantial electrical current.
Though studied theoretically for more than a century, combustion waves have been first observed to be successfully guided by a nanotube or a nanowire, and Michael Strano was the first to predict this practical application.
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, “lo and behold, we were really surprised by the size of the resulting voltage peak” that propagated along the wire.
Strano’s new experiment could have tremendous effects on power sources, as it puts out an energy proportional to its weight, and which is about 100 times greater than an equivalent weight of a Li-Ion battery, the amount of power greatly surpassing the one predicted by thermoelectric calculations. Although the Seebeck effect may produce fairly good results in semiconductors, it doesn’t work well at all in carbon. “There’s something else happening here,” says Strano. “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, adds the researcher.
Thermopower waves could be used in various applications, including new kinds of ultra-small electronic devices, like environmental sensors that could be scattered like dust in the air, or nano-scale robots that can be injected into the body.
Theoretically, thermopower-based batteries could maintain their power indefinitely until used, unlike batteries, who lose their power when left unused for a period of time. Moreover, individual nanowires could be grouped to scale up the power they produce, competing with classic batteries.
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. That would open 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 devices currently in tests yield a very low efficiency, because a lot of power is lost as heat and light, but the team works on improving that efficiency, and maybe in a few years we might actually see this invention on the market, applied in our electric cars or syringe-implantable nano-devices helping to heal deadly diseases. Or… maybe invisible spy cams watching us…
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