Right sizes of nano- and microstructures for high-performance and rigid bulk thermoelectrics

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 111; no. 30; pp. 10949 - 10954
• Main Authors: Wang, Hongchao, Bahk, Je-Hyeong, Kang, Chanyoung, Hwang, Junphil, Kim, Kangmin, Kim, Jungwon, Burke, Peter, Bowers, John E., Gossard, Arthur C., Shakouri, Ali, Kim, Woochul
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 29.07.2014; National Acad Sciences; National Academy of Sciences, Washington, DC (United States)
• Subjects: Annealing; Conductivity; electrical conductivity; Electrical resistivity; Figure of merit; Grain size; guidelines; Hardness; Materials; Melting; microstructure; nanomaterials; Nanostructured materials; Nanostructures; Phonons; Physical Sciences; Quenching; Scattering; solar (photovoltaic), solid state lighting, phonons, thermoelectric, bio-inspired, energy storage (including batteries and capacitors), electrodes - solar, defects, charge transport, materials and chemistry by design, optics, synthesis (novel materials), synthesis (self-assembly), synthesis (scalable processing); strength (mechanics); Theoretical analysis; Thermal conductivity; Thermoelectric materials; Thermoelectric power generation; Wavelengths; energy harvesting; waste heat recovery
• ISSN: 0027-8424; 1091-6490; 1091-6490
• DOI: 10.1073/pnas.1403601111

In this paper, we systematically investigate three different routes of synthesizing 2% Na-doped PbTe after melting the elements: (i) quenching followed by hot-pressing (QH), (ii) annealing followed by hot-pressing, and (iii) quenching and annealing followed by hot-pressing. We found that the thermoelectric figure of merit, zT , strongly depends on the synthesis condition and that its value can be enhanced to ∼2.0 at 773 K by optimizing the size distribution of the nanostructures in the material. Based on our theoretical analysis on both electron and thermal transport, this zT enhancement is attributed to the reduction of both the lattice and electronic thermal conductivities; the smallest sizes (2∼6 nm) of nanostructures in the QH sample are responsible for effectively scattering the wide range of phonon wavelengths to minimize the lattice thermal conductivity to ∼0.5 W/m K. The reduced electronic thermal conductivity associated with the suppressed electrical conductivity by nanostructures also helped reduce the total thermal conductivity. In addition to the high zT of the QH sample, the mechanical hardness is higher than the other samples by a factor of around 2 due to the smaller grain sizes. Overall, this paper suggests a guideline on how to achieve high zT and mechanical strength of a thermoelectric material by controlling nano- and microstructures of the material.