“Liquid-like” thermoelectrics: The blue spheres represent selenium atoms forming a crystal lattice. The orange regions in between represent the copper atoms that flow through the crystal structure like a liquid. This liquid-like behavior is what gives the selenium-copper material its unique thermoelectric properties. Credit: Jeff Snyder and Lance Hayashida, Caltech.

The April 2012 issue of the ACerS Bulletin (just posted online) is all about thermoelectric materials and areas where exciting thermoelectric advances are being made. Conveniently, new research, led by scientists from the Chinese Academy of Science’s Shanghai Institute of Ceramics, has just been published in Nature Materials concerning a selenium-copper crystalline structure with unusual “liquid-like” properties that appear to make it a remarkable and “ideal” high-efficiency thermoelectric material at high temperatures.

Just to review, thermoelectric materials can work in two ways: first, they can convert waste heat into usable electricity, or conversely, they can convert electricity into solid-state cooling. The overall efficiency of a thermoelectric material is described by a dimensionless figure of merit, “ZT.” Discussions of ZT typically include the temperature at which the ZT was calculated, because it can vary significantly as temperature changes. ZT values above one at a desired temperature are considered to be “good” (although even that is somewhat relative depending on whether its a bulk material or highly engineered one- or two-dimensional nanoproduct). To operate, thermoelectric materials must be able to maintain a temperature gradient (the heat is relatively stationary) while allowing electrons to flow.

The new thermoelectric properties discovered by the Chinese group (all with researchers from Caltech, Brookhaven National Laboratory and the University of Michigan) relate to a Cu2−xSe material that forms a rigid face-centered cubic lattice. The material, itself, is not new, and according to a Caltech news release, “NASA engineers first used this copper-selenium material roughly 40 years ago for spacecraft design… . But its liquid-like properties — which were not understood at the time — made it difficult to work with.

What researchers understand now, and what makes the material interesting, is that the selenium atoms create a stable sublattice while, according to the paper, “Copper Ion Liquid-like Thermoelectrics” (doi:10.1038/nmat3273), “the Cu ions are highly disordered around the Se sublattice and are superionic with liquid-like mobility.”

The authors go on to say, “This extraordinary “liquid-like” behavior of copper ions around a crystalline sublattice of Se in Cu2−xSe results in an intrinsically very low lattice thermal conductivity which enables high ZT in this otherwise simple semiconductor. This unusual combination of properties leads to an ideal thermoelectric material. The results indicate a new strategy and direction for high-efficiency thermoelectric materials by exploring systems where there exists a crystalline sublattice for electronic conduction surrounded by liquid-like ions.”

Most importantly, they report the Cu2−xSe “reaches a ZT of 1.5 at 1,000 K, among the highest values for any bulk materials.”

In the news release, one of the paper’s authors, Caltech’s Jeff Snyder, tries to paint a picture of what’s going on in the Cu2-xSe material. He says, “It’s like a wet sponge. If you have a sponge with very fine pores in it, it looks and acts like a solid. But inside, the water molecules are diffusing just as fast as they would if they were a regular liquid. That’s how I imagine this material works. It has a solid framework of selenium atoms, but the copper atoms are diffusing around as fast as they would in a liquid.”

According to the release, “the team found that because heat-carrying vibrations in a liquid can travel only via longitudinal waves, a material with liquid-like properties is less thermally conductive. Therefore, a liquid-like material that’s also good at conducting electrically should be more thermoelectrically efficient than traditional amorphous materials, Snyder says. In the case of the copper-selenium material that the researchers studied, the crystal structure of the selenium helps conduct electricity, while the free-flowing copper atoms behave like a liquid, damping down thermal conductivity.”

“Hopefully, the scientific community now has another strategy to work with when looking for materials with a high thermoelectric figure of merit,” Snyder says.

The other authors  Huili Liu, Xun Shi, Lidong Chen, Fangfang Xu, Linlin Zhang, and Wenqing Zhang (of SIC); Qiang Li (Brookhaven National Lab); Citrad Uher (UMich) and Tristan Day (Caltech).