Nanostructuring greatly improves thermoelectric material

Energy

April 3, 2008

False-color transmission electron micrograph showing the random orientation of the grains in the nanocrystalline thermoelectric material. (Courtesy of Boston College, MIT, GMZ Energy, and Nanjing University, China.)

A nanocrystalline form of a standard thermoelectric material shows significantly improved performance, report US and Chinese researchers, setting the scene for low-cost cooling and power-generation devices based on the thermoelectric effect [Poudel et al., Sciencexpress (2008), doi: 10.1126/science.1156446].

The prospect of generating electrical power from waste heat is a tantalizing one. Similarly, the thermoelectric effect could be used for new air-conditioning products, refrigeration, or cooling electronics.

For such devices to be competitive, it is generally agreed that the dimensionless figure of merit, ZT, of the thermoelectric materials should be >1. Yet peak ZT values of commonly used Bi₂Te₃-based materials have remained stubbornly at ~1 for decades. Although other materials have been developed for high-temperature applications, Bi₂Te₃ and its alloys still dominate near room temperature.

The team from Boston College, GMZ Energy, Massachusetts Institute of Technology (MIT), and Nanjing University tried a new tack. They synthesized a p-type nanocrystalline Bi_xSb_2–xTe₃ alloy by ball milling the bulk material and hot pressing the resulting nanoparticles into ingots.

This results in a nanostructured material with highly crystalline, randomly oriented grains. Strong phonon scattering at the interfaces gives a significant reduction in the thermal conductivity compared with the bulk alloy, and is largely responsible for the high peak ZT value of 1.4 at 100°C.

“We have found a way to improve an old material by breaking it up and then rebuilding it in a composite of nanostructures in bulk form,” explains Zhifeng Ren of Boston College. The method of manufacture is simple, can be scaled up for mass production, and most importantly, is cheap.

The researchers have demonstrated the potential of the new materials by constructing cooling devices that are able to produce temperature differences of 86°C, 106°C, and 119°C with the temperature of the hot side set at 50°C, 100°C, and 150°C, respectively.

“The amazing part is that the process they introduce is a combination of ‘low-tech’, scalable, and cost-effective engineering,” says Qiang Li of Brookhaven National Laboratory. “Yet it produces a bulk nanocomposite of a well-known industrial material with superior thermoelectric properties.”

The team will work on further improvements in the materials’ properties, Ren told Materials Today. “At the same time, we will push efficient cooling devices and power-generation systems to market as soon as possible through GMZ Energy, a company we founded in 2007.”

Jonathan Wood