Restructuring Bismuth Material’s Layers Boosts Efficiency Of Heat-To-Electricity Conversion | Chemical & Engineering News
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Web Date: December 20, 2012

Restructuring Bismuth Material’s Layers Boosts Efficiency Of Heat-To-Electricity Conversion

Energy Conversion: The trick is pulling apart and putting back together single layers of the thermoelectric material
Department: Science & Technology
News Channels: Nano SCENE, Materials SCENE, JACS In C&EN
Keywords: thermoelectrics, bismuth selenide, composite, single layers, ZT value

Materials that convert heat into electricity find use in temperature-controlled car seats and in power generators on spacecraft. If these materials, called thermoelectrics, converted heat more efficiently, they could generate electricity from heat wasted by computers, cars, and smokestacks. Researchers in China have found a new way to restructure a well-known thermoelectric material that increases its efficiency eightfold (J. Am. Chem. Soc., DOI: 10.1021/ja3102049).

Scientists measure how well thermoelectrics convert heat to electricity with what they call a figure of merit, ZT. For a material to have a high ZT, it must conduct electricity well but conduct heat poorly. Unfortunately, in bulk materials, it is hard to increase electrical conductivity without also increasing thermal conductivity.

One way around the dilemma is to create nanoplates, nanowires, or nanocrystals of a material. Such nanosized materials have many boundaries that scatter heat-carrying particles called phonons and thereby decrease thermal conductivity. But such nanomaterials also tend to scatter electrons more than researchers desire, lowering electrical conductivity, says Yi Xie, a chemistry professor at the University of Science and Technology of China.

Xie and her colleagues wanted to find a different approach to improve the figure of merit. They chose to work with the thermoelectric material bismuth selenide because of its layered structure. They knew from their previous theoretical studies that single layers of the material would have twice the electrical conductivity of the bulk crystal. Its improved conductivity stems from electrons’ ease of motion.

To make single layers of bismuth selenide, the researchers vigorously mixed bismuth selenide and lithium carbonate in a solution. Lithium ions insert themselves between single layers of bismuth selenide, which are five atoms thick. Exposing a solution of the crystals to ultrasound waves ripped apart the single layers, which the researchers collected and dried after removing lithium.

Next, the researchers pressed together the single layers and heated them to 350 °C to coalesce them into compressed pellets. The interfaces between layers in the resulting composite scatter phonons, decreasing thermal conductivity. The ZT of the pelletized material, 0.35, is eight times higher than that of bulk bismuth selenide crystals. It is also higher than the figures of merit of other forms of nanostructured bismuth selenide, Xie says.

However, the composite’s figure of merit is still too low for practical applications, Xie says, because bismuth selenide has an inherently low ZT. Thermoelectric materials in practical use have figures of merit of at least 0.7. The research team is pursuing composites of single layers of other bulk materials that have higher figures of merit.

This work is the first to make increased-ZT thermoelectric composites of single layers, says Qihua Xiong, a physics professor at Nanyang Technological University, in Singapore. Past efforts to make layered composites used much thicker flakes.

Ganpati Ramanath, a materials science and engineering professor at Rensselaer Polytechnic Institute, finds the work interesting. But, he adds, “if they can translate this to materials like lead telluride that have higher ZT, then it would be really exciting.”

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