On the Design of High-Efficiency Thermoelectric Clathrates through a Systematic Cross-Substitution of Framework Elements By Xun Shi, Jiong Yang, Shengqiang Bai, Jihui Yang,* Hsin Wang, Miaofang Chi, James R. Salvador, Wenqing Zhang,* Lidong Chen,* and Winnie Wong-Ng 1. Introduction World energy consumption is projected to expand at an alarming rate of 50% from 2005 to 2030, and approximately 60% of the energy produced by the burning of fossil fuels is lost, mostly through waste heat. [1] Thermoelectric (TE) technology offers the possibility of a reliable and scalable method for converting waste heat into useful electrical energy for automobiles, locomo- tives, power plants, fuel cells, etc. In addition, TE-based heating and cooling Peltier devices have great potential to augment or replace mechanical compres- sor-based air conditioning systems, leading to significant fossil fuel savings and green house emission reduction. [2,3] The effi- ciency of a TE device is, to a large extent, determined by its materials transport properties. What constitutes an efficient TE material is gauged by the dimensionless TE figure of merit, ZT ¼ S 2 sT/k, where T is the absolute temperature, S the thermopower, s the electrical conductivity, and k the thermal conductivity. Despite more than a half-century-long research effort, the ZT s of today’s commercial TE materials have remained about 1. [2,4] From a fundamental materials perspective, this lack of drastic improvement is due to the challenge of simultaneously enhancing the electrical conductivity and thermopower, while also lowering the thermal conductivity; these objectives usually conflict. Furthermore, in order to make TE technology economic- ally viable, the bandgap (E g ) of the TE materials should be in the range of 10 k B T op , where k B is the Boltzmann’s constant and T op the operating temperature, [5] so that the maximum ZT values fall within the operating temperature range. Recent efforts on complex TE materials have led to many advances. [2,4] Enhanced ZT values have been reported for several classes of materials, including complex chalcogenides, [6] filled and double-filled skutterudites, [7,8] Bi 2 Te 3 /Sb 2 Te 3 , [9] and PbSe 0.98 Te 0.02 /PbTe superlattices, [10] zinc antimonide, [11] nanostructured Bi 2 Te 3 , [12] and AgPb m SbTe 2þm bulk materials, [13] and Tl-doped PbTe. [14] Type I clathrates are a recently discovered class of prospective TE materials for high-temperature power generation applications. [15] Typical type I clathrates A 8 M 16 X 30 (A ¼ Sr, Ba, Eu; M ¼ Al, Ga, In; FULL PAPER www.MaterialsViews.com www.afm-journal.de [*] Dr. J. Yang, X. Shi, J. R. Salvador Materials and Processes Laboratory General Motors R&D Center Warren, MI 48090 (USA) E-mail: jihui.yang@gm.com Prof. W. Zhang, L. Chen, Dr. J. Yang, S. Bai State Key Laboratory of High Performance Ceramics and Superfine Microstructure Shanghai Institute of Ceramics, Chinese Academy of Sciences Shanghai 200050 (China) E-mail: wqzhang@mail.sic.ac.cn; cld@mail.sic.ac.cn Dr. X. Shi Optimal Incorporated Plymouth, MI 48170 (USA) Dr. H. Wang, M. Chi Materials Science and Technology Division Oak Ridge National Laboratory Oak Ridge, TN 37831 (USA) Dr. W. Wong-Ng Materials Science and Engineering Laboratory National Institute of Standards and Technology Gaithersburg, MD 20899 (USA) DOI: 10.1002/adfm.200901817 Type I clathrates have recently been identified as prospective thermoelectric materials for power generation purposes due to their very low lattice thermal conductivity values. The maximum thermoelectric figure of merit of almost all type I clathrates is, however, less than 1 and occurs at, or above, 1000 K, making them unfavorable especially for intermediate temperature applications. In this report, the Zintl–Klemm rule is demonstrated to be valid for Ni, Cu, and Zn transition metal substitution in the framework of type I clathrates and offers many degrees of freedom for material modification, design, and optimization. The cross-substitution of framework elements introduces ionized impurities and lattice defects into these materials, which optimize the scattering of charge carriers by the substitution-induced ionized impurities and the scattering of heat-carrying lattice phonons by point defects, respectively, leading to an enhanced power factor, reduced lattice thermal conductivity, and therefore improved thermoelectric figure of merit. Most importantly, the bandgap of these materials can be tuned between 0.1 and 0.5 eV by adjusting the cross-substitution ratio of framework elements, making it possible to design clathrates with excellent thermoelectric properties between 500 and 1000 K. Adv. Funct. Mater. 2010, 20, 755–763 ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 755