Nearly 200 years ago scientists discovered thermoelectricity: When a heat source and heat sink are connected in a circuit, a difference in temperature can cause electricity to flow. In principle, it’s a simple phenomenon, but practical applications have been rare.
Now Stanford researchers have shown how it might one day be possible to create efficient thermoelectric circuits that could, for instance, turn excess heat from computer chips into useful electricity.
The Stanford material, described in a recent study in Nature Materials, is a variant of the carbon nanotube – cylinders 500 times thinner than a human hair, made by rolling up a sheet of carbon atoms like a tube.
Prior to this study, scientists knew that heat traveled exceptionally well through carbon nanotubes – too well, in fact. Heat zipped through the carbon conduit and bypassed the thermoelectric conversion.
So Stanford researchers, led by postdoctoral scholar Takashi Kodama, built some resistance into the system by stuffing the nanotubes with buckyballs, fullerene molecules that resemble tiny soccer balls. These stuffed carbon nanotubes are aptly named “nanopeapods.” If the nanopeapod could be magnified millions of times, it would look like a roll of chicken wire stuffed with soccer balls.
Stanford mechanical engineer Kenneth Goodson, senior author of the study, explained that stuffing buckyballs into the carbon tube deformed its mesh structure, slowing heat flow and enhancing the thermoelectric effect.
It’s still very, very early days in this field, but his team thinks it may one day be possible to design a bundle of nanopeapods to funnel excess heat away from chips that must be cooled anyway – while putting what would otherwise be wasted energy to work making thermoelectricity.
One of the key breakthroughs was finding a way to collect thermal data for the nanopeapods, structures which have evaded detailed characterization for the nearly 15 years since their discovery. The Stanford researchers did this by searching for individual nanopeapods that were aligned properly in a complex thermal mesh platform, which allowed them to measure heat moving through the structures.
Further engineering will be necessary to improve nanopeapod performance and turn these tiny structures into practical thermoelectric generators. Goodson’s group, which has studied carbon nanotubes and thermoelectric materials in prior papers, believes nanopeapods could be designed to extract power from heat sources under a variety of conditions in the future. They might, for instance, be embedded in clothing to exploit temperature differences between the human body and the ambient air, or situated in buildings near hot water lines or windows.
“A lot of us are excited about finding ways to use the waste heat that is everywhere in society, and this is a provocative finding and important step,” Goodson says. “What’s more, Dr. Kodama’s clever experimental approach will enable us to figure out what is going on with a variety of other innovative thermoelectric nanomaterials – we’ll soon see a wealth of new data and this will move the ball forward for thermoelectric heat recovery.”
Kenneth Goodson chairs the Department of Mechanical Engineering at Stanford School of Engineering and holds the Davies Family Provostial Professorship. Other Stanford contributing authors include Woosung Park and Joonsuk Park. This study was supported financially by grants from the Air Force Office of Scientific Research and the National Science Foundation.