A collective rattling effect in a type of crystalline semiconductor was discovered to block most heat transfer while preserving high electrical conductivity — a rare pairing that could reduce heat buildup in electronic devices and turbine engines. The traits were discovered in a class of materials known as halide perovskites, which are also considered promising candidates for next-generation solar panels, nanoscale lasers, electronic cooling, and electronic displays.
These interrelated thermal and electrical (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. This single-crystal material can also be more easily produced in large quantities than typical thermoelectric materials such as silicon-germanium.
It was 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. The rattling mechanism was not just with the cesium, but was the overall structure rattling. The rattling mechanism is associated with the crystal structure itself, and is not the product of a collection of tiny crystal cages.
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. Because the material is composed of an orderly, single-crystal structure, electrical current can still flow through it despite this collective rattling.
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. The material is highly reactive to air and water, so it requires a protective coating or encapsulation to function in a device.
To measure the thermal conductivity of the material, 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.
They also performed scanning electron microscopy to precisely measure the dimensions of the nanowire. They used these dimensions to provide an exacting measure of the material’s thermal conductivity. The team repeated the experiment with several different nanowire materials and multiple nanowire samples to compare thermoelectric properties and verify the thermal conductivity measurements.
Just as computer chip manufacturers implant a succession of elements into silicon wafers to improve their electronic properties — a process known as “doping” — scientists hope to use similar techniques to more fully exploit the thermoelectric traits of this semiconductor material.