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. “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 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.
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