Researchers Find New Way to Develop Efficient Thermoelectric Nanomaterials

A research team doped a thermoelectric material with very small amounts of sulfur and discovered a new way to improve the efficiency of materials used for waste energy recapture and solid-state cooling and heating.

Image Credit: Rensselaer

This new approach changes the electronic band structure of bismuth telluride selenide, enhancing the “figure of merit,” which is grading used for the performance of a material. This ranking determines efficiency in varied applications and makes room for improved applications of thermoelectric materials to transfer heat wasted from power plants to computer chips.

This is an exciting breakthrough because this allows us to untangle two unfavorably coupled properties that limit thermoelectric performance. Moreover, our approach works for both nanocrystals as well as bulk materials, which is relevant to applications.

Ganpati Ramanath, Professor of Materials Science and Engineering, Rensselaer Polytechnic Institute (RPI)

Thermoelectric materials are capable of transforming a voltage to a thermal gradient and vice-versa, causing one part of a material to become cold or hot. The material’s figure of merit helps to determine the efficiency at which the material transforms a voltage to a thermal gradient. Modern thermoelectric materials lack major efficiency and their use is limited to niche applications, like night vision goggles, car-seat climate control, domestic water heaters, and picnic refrigerators. A considerable enhancement in figure of merit will enable thermoelectric materials to be used for highly advanced applications that include waste heat harvesting in engines and power plants, and cooling computer chips.

Seventy percent of all energy loss is heat. If we can generate even 5 percent more electricity from that waste heat, we’ll be on our way to making a big impact on power production and carbon dioxide emissions reduction. Thermoelectrics could also enable efficient, compact, and modular heat pump systems to revolutionize air conditioning for applications in cars and buildings.

Theo Borca-Tasciuc, Professor of Mechanical Engineering, Rensselaer Polytechnic Institute

A thermoelectric material’s figure of merit relies on three properties that include thermal conductivity (the materials’ ability to conduct heat); Seebeck coefficient (the material’s ability to cross-convert electricity and heat); and electrical conductivity (the material’s ability to conduct electrons). To obtain a higher figure of merit, a material will need to have low thermal conductivity, high Seebeck coefficient, and high electrical conductivity. One drawback to obtain high figure of merit is that Seebeck coefficient and electrical conductivity both have a contrary relationship - as one increases and the other decreases.

“By doping bismuth telluride selenide with hundreds of parts per million of sulfur, we are able to increase both electrical conductivity and the Seebeck coefficient in nanocrystals as well as bulk materials made from the nanocrystals,” Ramanath said. The study shows an up to 80% increase in the figure of merit of the bulk material. “Larger improvements could be possible with higher doping or the use of other dopants.”

“The big challenge in generating power with thermoelectrics is how to get high voltage and low resistance at the same time. Our work shows a new and important way forward: we need to optimize this method and put it into practice,” said David Singh, a University of Missouri professor whose theoretical calculations offer a basis to describe the observed results based on  complex changes in the electronic band structure shape.

A detailed report of research is available in the May 11, 2016, online issue of Advanced Materials in the article “Harnessing topological band effects in bismuth telluride selenide for large enhancements in thermoelectric properties through isovalent doping.” The work is a joint effort of researchers at Rensselaer, University of Missouri, and the Max Planck Institute for Solid State Research in Germany. For this work, Devender — the paper’s first author and a doctoral student of Ramanath — received a Norman Stoloff award for graduate research excellence by the Department of Materials Science and Engineering at Rensselaer. Devender presently works at GlobalFoundries Inc.

The research conducted by Ramanath illustrates the work being performed at The New Polytechnic. It also talks about complex and tough global challenges, the need for true and interdisciplinary collaboration, and the use of state-of-the-art technologies and tools, many of which are manufactured at Rensselaer.

Ramanath’s research concentrates on nanomaterials and interfaces for use in energy and electronics. His investigations cover the development of new varieties of bulk materials and thin films through directed assembly and synthesis. The investigations also include the development of molecularly customized interfaces with unique properties. The recent discoveries of Ramanath include a brand new class of thermoelectric nanomaterials, including this latest alternative of sulfur doped bismuth telluride selenide, developed from assemblies of sculpted nanostructures for electricity harvesting from waste heat and high-efficiency solid-state refrigeration together with nanomolecular layers of “nanoglue” that can link non-sticking materials, increase thermal transport, and prevent  chemical intermixing.

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