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Critical Phase Transition Leading a New Route into Ultrahigh Thermoelectrics

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Solid-state thermoelectric technology uses electrons or holes as the working fluid for heat pumping and power generation. There are huge opportunities of adopting the technology in harvesting solar heat and conversing waste industrial heat into electricity, and pumping out operational heat in solid-state electronics. Electronic industry focuses specially on the potential to rapidly cool microprocessors and sensors by the technology within a relatively narrow temperature range around or slightly above room temperature.

Scientists from Shanghai Institute of Ceramics, Chinese Academy of Sciences, in collaboration with scientists from University of Science and Technology of China, Shanghai Institute of Applied Physics, and the University of Michigan, have developed a new route to realize ultrahigh thermoelectric performance by using the critical electrical and thermal transports during a continuous 2nd-order phase transition. In the material described in a paper published in Advanced Materials, the critical electron and phonon scattering provides a unprecedented example for the subtle tuning of thermoelectric transports, leading to a significantly improved performance. The research potentially offers a cutting-edge opportunity of studying abnormal thermoelectric transports relevant to continuous phase transition, an innovative strategy for developing high-efficiency thermoelectric materials, and a new way of applications especially for electronic heat pumping.

Current bulk materials with figure of merit zT above 2 have only been reported at fairly high temperatures near 1000K in a few nano-structured materials. The commercial room-temperature Bi2Te3-based materials have a maximum zT less than 2.  There is an urgent need to develop high performance thermoelectric materials for near room temperature applications.

Many groups have been developing various strategies for enhancing the performance of thermoelectric materials. The most notable results reported were the large reduction of thermal conductivity, without severely affecting electrical transports.  In principle, there is a limit, i.e. the amorphous limit, which the lattice thermal conductivity can reach in solid materials. For the current state-of-the-art thermoelectric materials, the lattice thermal conductivities are very low and some of them have approached the amorphous limit. In this case, any further enhancement in zT must come from improvement in the Seebeck coefficient.

“Our approach of using phase transition induced critical thermoelectric transports is a new strategy to increase Seebeck coefficient and decrease thermal conductivity” said the Scientist from Shanghai. “It not only reduces the ‘mean free path’ of lattice phonons and charge carriers, but also significantly enhances the Seebeck coefficient.”

The Seebeck coefficient were observed to be significantly increased by the critical scattering of electrons during the phase transition in Cu2Se, leading to the value of thermoelectric figure of merit dramatically improved to 2.3 within a range of dozens of degrees around room temperature. This is the highest value reported in bulk samples near room temperature, providing a great flexibility for applications such as in cooling of microprocessors.

The work for the first time reported the thermoelectric performance enhancement by continuous phase transitions. “The use of phase-transition-induced critical scattering in thermoelectrics goes beyond the traditional ideas and approaches of improving thermoelectric performance, predominantly based on static structures.” With the rich phenomena happening during the phase transition, especially the dynamic nature of fluctuating microstructures, the work probably open a window of investigating abnormal thermoelectric transports in dynamic systems, especially those relevant to continuous phase transition. In the same time, “Element doping can easily shift the temperatures of critical thermoelectrics,” said by the Scientist from Shanghai Institute of Ceramics, “We also expect a proliferation of applications where these new phase transition thermoelectrics would serve as powerful solid-state heat pumps or power generators within a narrow (a few tens of degrees) temperature range around room temperature.”

This research was funded by National Basic Research Program of China, National Natural Science Foundation of China, ‘Hundred-talent program’ of Chinese Academy of Sciences and U.S. Department of Energy. 

Links: http://onlinelibrary.wiley.com/doi/10.1002/adma.201302660/abstract