By doping thermoelectric materials with minute amounts of sulfur, researchers have found a new approach to materials for solid-state heating and cooling, and waste energy recapture. Credit: Image courtesy of Rensselaer Polytechnic Institute (RPI)
By doping a thermoelectric material with minute amounts of sulfur, a team has found a new path to large improvements in the efficiency of materials for solid-state heating and cooling and waste energy recapture. This approach profoundly alters the electronic band structure of the material – bismuth telluride selenide -improving the “figure of merit,” a ranking of a material’s performance that determines efficiency in applications and opening the door to advanced applications of thermoelectric materials to harvest waste heat from power plants to computer chips.
“This is an exciting breakthrough because this allows us to untangle 2 unfavorably coupled properties that limit thermoelectric performance,” said Ganpati Ramanath. “Moreover, our approach works for both nanocrystals as well as bulk materials, which is relevant to applications.”
Thermoelectric materials can convert a voltage to a thermal gradient – causing one side of a material to become hot or cold – and vice-versa. The efficiency with which a material is able to convert a voltage to a thermal gradient is mainly determined by the material’s figure of merit. State-of-the-art thermoelectric materials are not very efficient, limiting their use to niche applications such as picnic refrigerators, domestic water heaters, car-seat climate control and night vision goggles. With substantial improvement in figure of merit, thermoelectric materials could be used for more advanced applications, such as harvesting waste heat in power plants and engines, and cooling computer chips.
“70% of all energy loss is heat. If we can generate even 5% 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,” said Prof. Borca-Tasciuc. “Thermoelectrics could also enable efficient, compact, and modular heat pump systems to revolutionize air conditioning for applications in cars and buildings.”
The figure of merit of a thermoelectric material depends on 3 properties and a higher figures needs high electrical conductivity, high Seebeck coefficient (ability to cross-convert electricity and heat), and low thermal conductivity. One barrier to achieving high figure of merit is that electrical conductivity and Seebeck coefficient have an inverse relationship.
“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 research demonstrates 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 provide a basis to explain the observed results in terms of complex changes in the electronic band structure shape. http://news.rpi.edu/content/2016/06/16/new-approach-building-efficient-thermoelectric-nanomaterials
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