New semiconductor material could cut heat emissions

Researchers at West Virginia University have engineered a material that could reduce the amount of heat reduced into the atmosphere by power plants. A team led by Xueyan Song, professor and George B. Berry Chair of Engineering at the Benjamin M. Statler College of Engineering and Mineral Resources, has created an oxide ceramic material that is designed to solve an efficiency problem affecting thermoelectric generators. Those devices generate electricity from heat, including power plant heat emissions, which contribute to global warming.

“We demonstrated the best thermoelectric oxide ceramics reported in the field worldwide over the past 20 years, and the results open up new research directions that could further increase performance,” Song said.

Oxide ceramics are from the same family as materials like pottery, porcelain, clay bricks, cement and silicon, but contain various metallic elements. They’re hard, resistant to heat and corrosion, and well-suited for high-temperature applications in air. They can serve as the material for thermoelectric generator components. However, oxide ceramics have ‘polycrystalline’ structures composed of multiple connected crystals. This presents challenges for large-scale thermoelectric applications for those materials, since the ‘grain boundaries’, the places where those crystals meet, block the current and electron flow that powers thermoelectric generators.

Postdoctoral researcher Romo de la Cruz said Song’s team intentionally added ‘dopants’ or metal ions into the polycrystal ceramics, driving special kinds of dopants to segregate to the grain boundaries. “That’s how we turned the unavoidable and detrimental grain boundaries into electricity-conducting pathways, significantly improving thermoelectric performance,” de la Cruz said.

The research responds to the growing problem of waste heat, a contributor to climate change. When lightbulbs get too hot to touch, they’re giving off waste heat: inefficient extra energy that doesn’t contribute to the light bulb’s primary job of producing light. Waste heat is released into the atmosphere by systems like power plants, home heating systems and automobiles. “Waste heat recovery will play an increasingly key role in balancing growing demand for electricity against the carbon footprint of industrial processes. Thermoelectric oxide ceramics like ours come into play by substantially improving the ability of thermoelectric generators to convert waste heat into electricity,” de la Cruz said.

Thermoelectric generators are a promising technology for waste heat recovery because they are simple to operate and maintain. A powerful thermoelectric generator could capture a significant portion of a power plant’s waste heat. “For the majority of applications, thermoelectric technology is too inefficient to be economical. Thermoelectric’s lack of effectiveness in converting energy severely hampers the development of thermoelectric devices, even though they are desperately needed,” Song said.

Song’s lab addressed this issue by using nanostructure engineering — manipulating the ceramic’s crystal structure on an atomic scale that can only be seen using an electron microscope — to create a dense, textured polycrystalline material that outperformed the single-crystal materials that are currently standard. Although tuning the performance of various materials for thermoelectrics has led to intense theoretical and experimental work for decades, Song believes that for bulk oxide ceramics, the lab is the first to demonstrate an increase in the efficiency of energy generation from heat through the nano- and atomic-scale engineering of grain boundaries between crystals.

“This work is at the cusp for large-scale, high-temperature waste heat recovery. It leads toward a new era for oxide ceramics and could facilitate and accelerate materials design that is magnitudes higher than the current state of the art,” Song said.

The research findings were published in Renewable and Sustainable Energy Reviews.

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