Green Nano Researchers Discover New Material to Produce Clean Energy

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Researchers at the University of Houston have created a new thermoelectric material, intended to generate electric power from waste heat with higher output power than currently available materials.

The material, germanium-doped magnesium stannide, is described in the current issue of the Proceedings of the National Academy of Sciences. Zhifeng Ren, lead author of the article and M.D. Anderson Chair professor of physics at UH, said the new material has a peak power factor of 55, with a figure of merit – a key factor to determine efficiency – of 1.4.

The new material – chemical compound Mg2Sn0.75Ge0.25 – is important in its own right, Ren said. He has formed the company APower (http://apower.com) to commercialize the material, along with Gang Chen of the Massachusetts Institute of Technology and two former students.

Ren said another key point made in the paper is the importance of looking for materials with a high power factor in addition to the traditional focus on a high figure of merit, or efficiency, commonly referred to as ZT.

"The way everybody pursued higher ZT is by reducing thermal conductivity," Ren said. "We were, too. But the reduction of thermal conductivity is limited. We need to increase the power factor. If thermal conductivity remains the same and you increase the power factor, you get higher ZT."

Thermoelectric materials produce electricity by exploiting the flow of current from a warmer area to a cooler area. In the germanium-doped magnesium stannide, the current is carried by electrons.

"Pursuing high ZT has been the focus of the entire thermoelectric community …" the researchers wrote. "However, for practical applications, efficiency is not the only concern, and high output power density is as important as efficiency when the capacity of the heat source is huge (such as solar heat), or the cost of the heat source is not a big factor (such as waste heat from automobiles, steel industry, etc.)"

Germanium-doped magnesium stannide has a fairly standard figure of merit, at 1.4, but a high power factor, at 55, the researchers report. That, coupled with a raw material cost of about $190 per kilogram, according to the U.S. Geological Survey Data Series, makes it commercially viable. Ren, who also is a principal investigator at the Texas Center for Superconductivity at UH, said several competing materials have lower power factors and more expensive raw materials.

The material was created through mechanical ball milling and direct current-induced hot pressing. It can be used with waste-heat applications and concentrated solar energy conversion at temperatures up to 300 degrees Centigrade. Ren said typical applications would include use in a car exhaust system to convert heat into electricity to power the car's electric system, boosting mileage, or in a cement plant, capturing waste heat from a smokestack to power the plant's systems.

Next to Ren, researchers on the paper include Weishu Liu, Hee Seok Kim, Shuo Chen, Qing Jie, Bing Lv and Paul Ching-Wu Chu, all of the UH physics department and the Texas Center for Superconductivity; Mengliang Yao, Zhensong Ren and Cyril P. Opeil of Boston College, and Stephen Wilson of the University of California at Santa Barbara.

* Jeannie Kever, Senior media relations representative at the University of Houston

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