Based on physics first observed over 200 years ago, thermoelectric devices can convert thermal energy into electrical energy and vice versa. But in all that time, thermoelectric phenomena had never been observed in an all-liquid system. That is, until researchers recently observed thermoelectricity at the interface between two liquid metals.
It’s an important observation: Liquid thermoelectrics could be used to create new devices for scavenging energy from waste heat, and insights from the research could help improve the design of liquid-metal batteries. The researchers, based at the École Normale Supérieure (ENS) in Paris, published their results today in the journal Proceedings of the National Academy of Sciences.
“Studying the thermoelectric effect at an interface between two liquid metals is one of those ideas that’s so intuitive and elegant that it seems obvious in retrospect,” says Douglas Kelley, a mechanical engineer at the University of Rochester, in N.Y. “But to my knowledge, nobody has done it before,” adds Kelley, who was not involved with the research.
Christophe Gissinger, a physicist at ENS, studies the basic physics of liquid metals and their applications in batteries. He says scientists know almost nothing about how temperature gradients affect the flow of electrical currents in conductive liquids. Gissinger says it occurred to him that the conductive layers in liquid-metal batteries were similar to thermoelectric devices. So he decided to look for thermoelectricity in liquid metals.
Gissinger says he thinks scientists haven’t observed thermoelectricity in liquids before because it’s challenging to measure—and because they weren’t looking.
Gissinger and his colleagues chose two metals that are liquid at room temperature: gallium and mercury. The experiments were done in a cylinder with refrigerated walls. In the center of the cylinder, the researchers placed a smaller cylindrical heater. The researchers poured dense liquid mercury into the outer cylinder, then topped it with a layer of lighter liquid gallium. They heated the liquids from the interior and cooled the cylinder’s outer walls, creating a temperature gradient along the interface between the two metals. Wires dipping into the liquid metals measured the resulting electric fields.
The researchers saw a complex, turbulent electrical current in response to the temperature difference. Current looped up through the mercury from the hot side of the chamber to the cold, then crossed over the interface into the gallium. From there, the current flowed from the cold side of the gallium to the hot side, then back into the mercury, and so on, in a loop. There are multiple such loops at the interface between the metals, says Gissinger.
This simple looping current is similar to what’s seen in solids, but it’s not the whole picture. There are also stagnation points, places where there is no current density, along the interface. That doesn’t happen in solids, says Gissinger, and it is likely due to the turbulent, nonlinear flow of heat in liquids. And the current density is very large compared to what it would be in a system made of solid metals—which suggests to Gissinger that this liquid thermoelectricity could be harnessed in new, highly efficient devices that convert waste heat into electricity.
Gissinger says he thinks scientists haven’t observed thermoelectricity in liquids before because it’s challenging to measure—and because they weren’t looking. At the high temperatures needed to liquify most metals, it’s more challenging to precisely control temperature gradients. And it’s harder to measure the microscale voltages induced by thermoelectric effects when the heat is on. Studying metals that are liquid at room temperature was key. Besides, thermoelectricity has traditionally been the realm of solid-state physicists, not researchers who study fluids.
“Studying the thermoelectric effect at an interface between two liquid metals is one of those ideas that’s so intuitive and elegant that it seems obvious in retrospect.” —Douglas Kelley, University of Rochester
Kelley says the phenomenon may be significant in the context of liquid-metal batteries. When Gissinger’s group placed the experimental chamber inside a coil and applied a magnetic field, the liquid started rotating in the tank. “In battery designs where the liquid metal is free to flow, its motion is important,” says Kelley. Violent motion can rupture the electrolyte layers and short the battery. Gentler motion can lead to stirring, which is helpful because it reduces side reactions in the battery, allowing for faster charging and higher power, and lengthening the life of the battery.
Further study is needed to determine how thermoelectricity might already be impacting liquid-metal battery performance, and how it might be harnessed, says Kelley. Gissinger hopes to test these ideas in a battery prototype.
The findings may also have implications for planetary science. “When we discovered this effect we tried to imagine in what situations you have an interface between two conductive liquids, and a temperature difference along it,” says Gissinger. His team hypothesizes that these effects might be contributing to Jupiter’s magnetic field. The planet’s core is surrounded by a large region of metallic hydrogen, which is covered by an atmosphere of liquid molecular hydrogen. The planet’s equator is warmer than its poles, creating a temperature gradient along the metallic-liquid hydrogen interface. “We expect this generates electric current that creates part of the planet’s magnetic field via the thermoelectric effect,” says Gissinger.Katherine Bourzac is a freelance journalist based in San Francisco, Calif. She writes about materials science, nanotechnology, energy, computing, and medicine—and about how all these fields overlap. Bourzac is a contributing editor at Technology Review and a contributor at Chemical & Engineering News; her work can also be found in Nature and Scientific American. She serves on the board of the Northern California chapter of the Society of Professional Journalists.
