Bottom-up Nanostructuring and Novel Materials and Concepts to Develop Viable Thermoelectrics
1National Institute for Materials Science (NIMS), MANA, Japan
2 Graduate School of Pure and Applied Sciences, University of Tsukuba, Japan
Thermoelectric power generation which represents the direct solid-state conversion of waste heat to electricity is a promising field challenging to reach wide-scale application [1,2]. There are intrinsic paradoxes in the requirements of physical properties leading to high thermoelectric performance. Namely, high electrical conductivity s but low thermal conductivity k, and large Seebeck coefficient a but high electrical conductivity s are required. These have made difficult straightforwardly enhancement of the performance, represented by ZT = a2sk-1T. I will present several new principles and materials we are developing to overcome the traditional limitations.
Several novel bottom-up nanostructuring methods have proved to be very effective. Previously, telluride thermoelectric nanosheets were fabricated by bottom-up wet processes and found to have a ZT enhancement . An alternative particularly powerful ZT enhancement (100% enhancement to ZT~1.6 in “empty” rare earth-free skutterudites) was discovered through creating nano-micropores in the target material which functioned to have strong phonon selective scattering . Porous materials had usually always been considered low performance thermoelectrics, with penalty of poor s outweighing the benefit of lower k. The surprising controlled and effective porosity in the materials were achieved through co-doping the Sb cages of CoSb3 and utilizing phase diagrams to create a secondary phase in material which was then annealed and removed to create the nano-micropores . This method can be utilized for other thermoelectric materials. Regarding power factor (=a2s), formation of nanocomposites with a metallic conductive partial network was demonstrated to lead to a large enhancement . The creation of such networks is likely an easier and more widely applicable method compared to modulation doping  and energy filtering  in which band-matching, etc. is required.
As another novel principle, utilizing magnetic and carrier-magnon interactions have also been proposed to be a way to enhance the thermoelectric power factor [8,9]. This was illustrated well for CuFeS2 , while magnetic Mn doping into a nonmagnetic thermoelectric material CuGaTe2 yielded up to a 100% enhancement in a2s . Excellent thermoelectric properties were also realised for one of the thiospinel phases CuCr2S4 which were previously considered to be low performance materials, by unconventional doping and through its intrinsic disorder and heavy mass as a magnetic semiconductor . Striking power factor enhancement of 30 times was also obtained for the Sm phase of RB66, wherein the mixed valency of Sm was proposed to play a role . I will describe in detail these thermoelectric enhancement principles we have been working on, and also advanced measurement techniques , together with a module fabrication example of all novel materials. This work is supported by CREST, JST.
 K. Koumoto and T. Mori, Thermoelectric Nanomaterials Springer Series in Material Science 182 (Springer, 2013), pp. 1-387.
 T. Mori, Small, 13, 1702013 (2017).
 C. Nethravathi, et al., J. Mat. Chem. A 2, 985 (2014).
 A. U. Khan, K. Kobayashi, D. Tang, Y. Yamauchi, K. Hasegawa, M. Mitome, Y. Xue, B. Jiang, K. Tsuchiya, D. Golberg, Y. Bando, and T. Mori, Nano Energy 31 152 (2017).
 T. Mori and T. Hara, Scr. Mater. 111, 44 (2016).
 M. Zebarjadi, G. Joshi, G.H. Zhu, B. Yu, A.J. Minnich, Y.C. Lan, X.W. Wang, M.S. Dresselhaus, Z.F. Ren, G. Chen, Nano Lett. 11, 2225 (2011).
 A. Shakouri, C. Labounty, P. Abraham, J. Piprek, J.E. Bowers, Mater. Res. Soc. Proc. 545, 449 (1999).
 R. Ang, A. U. Khan, N. Tsujii, K. Takai, R. Nakamura, and T. Mori, Angew. Chem. Int. Ed. 54, 12909 (2015).
 F. Ahmed, N. Tsujii and T. Mori, J. Mater. Chem. A, 5, 7545 (2017).
 A. U. Khan, R. A. R. A. Orabi, A. Pakdel, J. B. Vaney, B. Fontaine, R. Gautier, J. F. Halet, S. Mitani, and T. Mori, Chem. Mater., 29, 2988 (2017).
 A. Sussardi, T. Tanaka, A. U. Khan, L. Schlapbach, and T. Mori, J. Materiom. 1, 196 (2015).
 Y. Kakefuda, et al., APL Materials 5, 126103 (2017).