Aug 01, 2017

Researchers at the Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) discovered unique properties in certain halide perovskites: they are able to block heat transfer while preserving high electrical conductivity - a rare pairing that could reduce heat buildup in electronic devices and turbine engines, among other possible applications.

Unique thermoelectric properties found in perovskites image

These thermoelectric properties were found in nanoscale wires of cesium tin iodide (CsSnI3). The material was observed to have one of the lowest levels of heat conductivity among materials with a continuous crystalline structure. The material can also be more easily produced in large quantities than typical thermoelectric materials, such as silicon-germanium, researchers said.

"Its properties originate from the crystal structure itself. It's an atomic sort of phenomenon," said a researcher at Berkeley Lab. Researchers earlier thought that the material's thermal properties were the product of "caged" atoms rattling around within the material's crystalline structure, as had been observed in some other materials. Such rattling can serve to disrupt heat transfer in a material. Researchers at the Massachusetts Institute of Technology then performed some theory work and computerized simulations that helped to explain what the team had observed. Researchers also used Berkeley Lab's Molecular Foundry, which specializes in nanoscale research, in the study.

"We believe there is essentially a rattling mechanism, not just with the cesium. It's the overall structure that's rattling; it's a collective rattling," the team said. "The rattling mechanism is associated with the crystal structure itself," and is not the product of a collection of tiny crystal cages. "It is group atomic motion," they added.

Since the material is composed of an orderly, single-crystal structure, electrical current can still flow through it despite this collective rattling. The team likened the material's electrical conductivity to a submarine traveling smoothly in calm underwater currents, while its thermal conductivity is like a sailboat tossed about in heavy seas at the surface.

The team said two major applications for thermoelectric materials are in cooling, and in converting heat into electrical current. For this particular cesium tin iodide material, cooling applications such as a coating to help cool electronic camera sensors may be easier to achieve than heat-to-electrical conversion.

A challenge is that the material is highly reactive to air and water, so it requires a protective coating or encapsulation to function in a device.

"A next step is to alloy this (cesium tin iodide) material," the researchers said. "This may improve the thermoelectric properties". Also, the scientists hope to use similar techniques to more fully exploit the thermoelectric traits of this semiconductor material. This is relatively unexplored territory for this class of materials.