By Marlene Cimons, National Science Foundation
Scientists have found a new way to produce electricity, a discovery that could make today’s batteries obsolete and lead to electronic devices as tiny as a grain of rice, as well as provide another source of environmentally-friendly renewable energy.
While still in the earliest stages of development, a team of researchers at the Massachusetts Institute of Technology has found that fast-moving high-temperature pulsing waves of heat can create a substantial current when traveling along a carbon nanotube, the key ingredient in the reaction.
“We discovered a new phenomenon that can convert chemical energy into electrical energy,” said Michael Strano, an associate professor of chemical engineering at MIT.
The researchers set out to examine chemical reactions that release heat, and were especially curious about what might happen near a carbon nanotube, an excellent heat conductor. But, as is often the case with science, their experiments prompted an unexpected finding.
“This thermowave pushes electrons...and it can release a lot of power very quickly,” Strano said. “We were surprised by the size of the voltage that came out. It was very large. That’s when we started to view this as a new energy conversion. Maybe this could form new kinds of batteries, fuels, even new mechanisms that could work in a car.”
The most important components in the experiment were the carbon nanotubes, which are submicroscopic hollow tubes made of a chicken-wire-like lattice of carbon atoms. The tubes, just a few billionths of a meter in diameter, belong to a family of novel carbon molecules, and have been widely studied for much of the last 20 years.
For the experiment, each of the nanotubes was coated with a layer of a reactive fuel that can produce heat by decomposing. Scientists then ignited the fuel at one end of the tube using either a laser beam or a high-voltage spark, which resulted in a fast-moving thermal wave that moved along the length of the tube like a flame speeding along the length of a lit fuse. Heat from the fuel also went speeding inside the tube. The heating produced by the combustion pushed electrons along the tube, creating an electrical current.
“The fuel is coated on the nanotube, wrapped around the tube like a jacket,” Strano explained. “If you add a little bit of heat, like a spark, once the fuel starts to heat, it goes inside the nanotube. The heat is inside, but the reaction takes place outside. You make a thermowave. It moves at about one meter per second along the nanotube. We wanted to document the speed of the waves, so we attached it between two electrodes hoping to get a signal when the reaction and started and stopped. We noticed we got a pulse--sometimes a positive pulse, sometimes a negative pulse, depending on which end. This meant the wave was actually driving the current, which told us it pushes the electrons, but doesn’t push them randomly.”
Combustion waves have been studied mathematically for more than a century, but Strano’s team was the first to predict that such waves could be guided by a nanotube or nanowire, and that these heat waves can push an electrical current along the wire, according to MIT.
The amount of power released is much greater than that predicted by thermoelectric calculations, Strano said. While many semiconductor materials can produce an electric potential when heated, a process known as the Seebeck effect, that effect is very weak in carbon. “There’s something else happening here,” Strano 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 much 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.
These heat waves, however, do not entirely resemble ocean waves. “The kind of waves generated on nanotubes are very special,” Strano said. “They are solitons. Ocean waves crash against the shore and die out. They dissipate. They diffuse their energy. Solitons don’t. They travel infinitely. When you launch a soliton, it continues to travel. This means you can launch a thermowave that has a wave front that does not change, and continue to generate power.”