Washington March 8:
A team of scientists at MIT has discovered a previously unknown phenomenon that can cause powerful waves of energy to shoot through minuscule wires known as carbon nanotubes, a discovery that could lead to a new way of producing electricity.
The phenomenon, described as thermopower waves, “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, who was the senior author of a paper describing the new findings.
Like 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 is carbon nanotubes — submicroscopic hollow tubes made of a chicken-wire-like lattice of carbon atoms.
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 travelling along the length of the carbon nanotube like a flame speeding along the length of a lit fuse.
According to Strano, in the group’s initial experiments, 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. 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.
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. We call it electron entrainment since part of the current appears to scale with wave velocity,” Strano said. 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 explained.
“This important property is responsible for the high power produced by the system,” Strano said. Strano suggests that one possible application would be in enabling new kinds of ultra-small electronic devices. Or it could lead to “environmental sensors that could be scattered like dust in the air,” he said.