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John O | August 2017

Berkeley Lab researchers discover semiconductor that beats the heat

researchers from the u.s. department of energy lawrence berkeley national laboratory and the university of california at berkeley discovered a rattling effect in halide perovskites, a crystalline semiconductor, which blocks most heat transfer without significantly impacting electrical conductivity.


rattling structures of halide perovskites: cesium tin iodide (left) and cesium lead iodide (right).
(berkeley lab/uc berkeley)


these unusual thermoelectric properties were found in nanoscale wires of cesium tin iodide (cssni3) that was observed to have one of the lowest levels of thermal conductivity among materials with continuous crystalline structure, according to a report from berkeley lab.


it is also a material that can be produced in large quantities easier than typical thermoelectric materials, such as silicon-germanium. researchers believe this material can be used to reduce heat buildup in electronic devices.


“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,” the article explained. “such rattling can serve to disrupt heat transfer in a material.”


instead, researchers observed that the rattling came from the overall structure of the crystal, calling it a “group atomic motion.”


the article continued, “within the material’s crystal structure, the distance between atoms is shrinking and growing in a collective way that prevents heat from easily flowing through.


“but because the material is composed of an orderly, single-crystal structure, electrical current can still flow through it despite this collective rattling. picture its electrical conductivity is like 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 drawback for this material is that it is highly reactive to air and water, requiring a coating to make applicable for real-world functions.


“to measure the thermal conductivity of the material,” the article added, “researchers bridged two islands of an anchoring material with a cesium tin iodide nanowire. the nanowire was connected at either end to micro-islands that functioned as both a heater and a thermometer. researchers heated one of the islands and precisely measured how the nanowire transported heat to the other island.”


scientists will continue to experiment with additives and doping of the material to enhance its thermoelectric properties.


the research was recently published in proceedings of the national academy of science (pnas). the abstract read:


“controlling the flow of thermal energy is crucial to numerous applications ranging from microelectronic devices to energy storage and energy conversion devices. here, we report ultralow lattice thermal conductivities of solution-synthesized, single-crystalline all-inorganic halide perovskite nanowires composed of cspbi3 (0.45 ± 0.05 w·m−1·k−1), cspbbr3 (0.42 ± 0.04 w·m−1·k−1), and cssni3 (0.38 ± 0.04 w·m−1·k−1).


“we attribute this ultralow thermal conductivity to the cluster rattling mechanism, wherein strong optical–acoustic phonon scatterings are driven by a mixture of 0d/1d/2d collective motions. remarkably, cssni3 possesses a rare combination of ultralow thermal conductivity, high electrical conductivity (282 s·cm−1), and high hole mobility (394 cm2·v−1·s−1).


“the unique thermal transport properties in all-inorganic halide perovskites hold promise for diverse applications such as phononic and thermoelectric devices.


“furthermore, the insights obtained from this work suggest an opportunity to discover low thermal conductivity materials among unexplored inorganic crystals beyond caged and layered structures.”

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