Editorial pubs.acs.org/cm
Virtual Issue on Thermoelectric Materials
T
lattice thermal conductivity.7 Similar phenomena are found in several papers on controlling the electronic structure of SnTe with Ca, Mg, and In alloying.8−10 Doping can be also performed on the nano- and macroscale, which results in the formation of thermoelectric nanocomposites that incorporate nanoparticle precipitates within a bulk matrix. In such materials heat-carrying phonons are scattered on the interfaces, which may be “transparent” for charge carriers. Effective examples of applying this approach to classical TE materials are PbTe- and AgSnBiTe-based heterogeneous nanocomposites.8,11−13 Nanostructrual discontinuities can be also introduced on a significantly smaller scale, as shown in the example of natural and engineered nanolattices formed in layered chalcogenides.14,15 Nanostructuring materials has led to significant increases in ZT, but revolutions in thermoelectric performance will require new bulk materials. Thus, novel bulk materials are in high demand in the field of TE. In the search for new thermoelectric materials, we see both traditional, experimentally led searches, e.g., MgAgSb,16 and high throughput, computationally led efforts.2 Analysis of the large computational data set generated in the second paper2 identifies chemical trends related to cation valence that facilitate good thermoelectric performance. The “phonon glass−electron crystal” (PGEC) approach suggests to look for new bulk thermoelectrics among narrow band semiconductors having cage-like structures with inclusion atoms or molecules trapped inside the cages. The rattling of guest atoms will provide effective scattering of heat carrying phonons, thus decreasing the lattice thermal conductivity while charge carrier transport will occur through the covalent framework. Thus, the thermal and electric transport properties of PGEC phases may to some extent be optimized independently. This concept is actively explored nowadays with respect to cage-like compounds with cationic guests.17−21 Beyond rattling, anisotropic bonding is explored in CdSb as a strategy to enhance phonon−phonon scattering through anharmonic bonding.22 Another, more traditional approach, to achieve low lattice thermal conductivity is to increase structural complexity for compounds containing heavy elements. Those compounds often can tolerate a significant degree of doping or incorporation of interstitial atoms which allows for optimization of the charge transport properties.23−25 Local disorder plays an important role in the reduction of lattice thermal conductivities of those compounds. Our intention with this issue is to represent the diversity and complexity of the thermoelectric field. As evidenced by the affiliations of our authors the thermoelectrics research lies at the interface of solid state chemistry, condensed matter physics, materials science, and engineering. Collaboration between theoretical and experimental groups within each of those disciplines and between the disciplines is the key to success in thermoelectric research.
he escalating demand for alternative clean energy sources requires the development of new and effective materials for energy recovery, conversion, storage, and transfer. A survey conducted by the Lawrence Livermore National Lab shows that 59% of all energy generated in 2014 in the US was lost to the environment in the form of heat. While the exergetic value of much of this heat is low, there are nevertheless opportunities for waste heat recovery. Thermoelectric (TE) materials convert heat into electrical energy and vice versa and, as such, are promising materials for waste heat reduction or recovery. Further advances in thermoelectric materials could enable stand-alone solid state heat engines. Today, applications of TE materials range from portable refrigerating bags and outdoor cell phone chargers to modern units designed for power generation in space utilizing heat from nuclear sources to power solar system exploration missions. Such thermoelectric generators depend upon the Seebeck effect, in which a temperature gradient applied across p-type and n-type materials generates an electrical voltage as a result of the diffusive flow of charge carriers. A similar phenomenon is found in thermoelectric coolers, in which external voltage bias drives charge carriers and induces a temperature gradient. The dimensionless figure-of-merit, ZT, describing the efficiency of TE devices, is expressed as ZT = (S2·T·σ)/κ, where S is the Seebeck coefficient, T is the absolute temperature, σ is the electrical conductivity, and κ is the thermal conductivity. Thus, a TE material should be a good electrical conductor (high σ) and induce high voltage in response to a temperature gradient (high S) but should be a poor heat conductor to maintain the applied temperature gradient (low κ). An optimization of TE properties is compromised in most materials because S, σ, and κ are strongly coupled. In this virtual issue (http://pubs.acs.org/page/vi/thermoelectric-materials.html) we selected 25 recent papers (Table 1) representing the current trends and approaches toward improving the efficiency of thermoelectric materials. Computing the temperature dependent heat and charge transport properties is not a trivial task and requires state-of-the-art methods to be applied that are still quite computationally expensive. Despite of these limitations, theory provides useful guidelines in both the optimization of known thermoelectric materials and the identification of potential thermoelectrics among known compounds for which TE properties are not yet characterized.1,2 Doping, defects, and electronic structure control is a major theme in the recent literature. For example, deleterious defects are found in Cu2−ySe1−xBrx that compensate the dopants and lead to Fermi level pinning.3 On the other hand, modifications to SrTiO3 and BaGa2Sb2 through defect chemistry engineering yield performance enhancements.4−6 Together, these efforts highlight the lack of intuition we have for defects in materials and the need for defect design strategies in thermoelectric materials. This trend continues as codopants and alloys are explored. For example, codoping in the tetrahedrite Cu12Sb4S13 allows for simultaneous band structure engineering and reduced © 2016 American Chemical Society
Published: April 26, 2016 2463
DOI: 10.1021/acs.chemmater.6b01002 Chem. Mater. 2016, 28, 2463−2465
Chemistry of Materials
Editorial
Table 1. Articles Selected for the Virtual Issue on Thermoelectric Materialsa 1
Naoki Sato, Hideyasu Ouchi, Yoshiki Takagiwa, and Kaoru Kimura
2
Prashun Gorai, Philip Parilla, Eric S. Toberer, and Vladan Stevanović
3
Tristan W. Day, Kai S. Weldert, Wolfgang G. Zeier Bor-Rong Chen, Stephanie L. Moffitt, Ulrike Weis, Klaus P. Jochum, Martin Panthöfer, Michael J. Bedzyk G. Jeffrey Snyder, and Wolfgang Tremel ́ Javier Macias, ́ Philipp Aleksey A. Yaremchenko, Sascha Populoh, Sónia G. Patricio, Thiel, Duncan P. Fagg, Anke Weidenkaff, Jorge R. Frade, and Andrei V. Kovalevsky Zhilun Lu, Huairuo Zhang, Wen Lei, Derek C. Sinclair, and Ian M. Reaney
Influence of Compensating Defect Formation on the Doping Efficiency and Thermoelectric Properties of Cu2−ySe1−xBrx
6
Umut Aydemir, Alex Zevalkink, Alim Ormeci, Zachary M. Gibbs, Sabah Bux, and G. Jeffrey Snyder
Thermoelectric Enhancement in BaGa2Sb2 by Zn Doping
7
Xu Lu, Donald T. Morelli, Yi Xia, and Vidvuds Ozolins
Increasing the Thermoelectric Figure of Merit of Tetrahedrites by Co-Doping with Nickel and Zinc
8
Gangjian Tan, Wolfgang G. Zeier, Fengyuan Shi, Pengli Wang, G. Jeffery Snyder, Vinayak P. Dravid, and Mercouri G. Kanatzidis
9
Rabih Al Rahal Al Orabi, Nicolas A. Mecholsky, Junphil Hwang, Woochul Kim, Jong-Soo Rhyee, Daehyun Wee, and Marco Fornari
High Thermoelectric Performance SnTe−In2Te3 Solid Solutions Enabled by Resonant Levels and Strong Vacancy Phonon Scattering Band Degeneracy, Low Thermal Conductivity, and High Thermoelectric Figure of Merit in SnTe−CaTe Alloys
10
Ananya Banik, U. Sandhya Shenoy, Shashwat Anand, Umesh V. Waghmare, and Kanishka Biswas
Mg Alloying in SnTe Facilitates Valence Band Convergence and Optimizes Thermoelectric Properties
11
Oliver Falkenbach, Andreas Schmitz, Torben Dankwort, Guenter Koch, Lorenz Kienle, Eckhard Mueller, and Sabine Schlecht
12
13
Hongchao Wang, Junphil Hwang, Matthew Loren Snedaker, Il-ho Kim, Chanyoung Kang, Jungwon Kim, Galen D. Stucky, John Bowers and Woochul Kim Ruth A. Downie, Ronald I. Smith, Donald A. MacLaren, and Jan-Willem G. Bos
Tin Telluride-Based Nanocomposites of the Type AgSnmBiTe2+m (BTST-m) as Effective Lead-Free Thermoelectric Materials High Thermoelectric Performance of a Heterogeneous PbTe Nanocomposite
14
Priyanka Jood, Michihiro Ohta, Oleg I. Lebedev, and David Berthebaud
15
Devin R. Merrill, Duncan R. Sutherland, Jeffrey Ditto, Sage R. Bauers, Matthias Falmbigl, Douglas L. Medlin, and David C. Johnson
Metal Distributions, Efficient n-Type Doping, and Evidence for in-Gap States in TiNiMySn (M = Co, Ni, Cu) half-Heusler Nanocomposites Nanostructural and Microstructural Ordering and Thermoelectric Property Tuning in Misfit Layered Sulfide [(LaS)x]1.14NbS2 Kinetically Controlled Site-Specific Substitutions in HigherOrder Heterostructures
16
Pingjun Ying, Xiaohua Liu, Chenguang Fu, Xianqiang Yue, Hanhui Xie, Xinbing Zhao, Wenqing Zhang, and Tiejun Zhu
High Performance α-MgAgSb Thermoelectric Materials for Low Temperature Power Generation
17
Fan Sui, Hua He, Svilen Bobev, Jing Zhao, Frank E. Osterloh, and Susan M. Kauzlarich
18
Masahiro Kanno, Takahiro Yamada, Hisanori Yamane, and Hideaki Nagai
Synthesis, Structure, Thermoelectric Properties, and Band Gaps of Alkali Metal Containing Type I Clathrates: A8Ga8Si38 (A = K, Rb, Cs) and K8Al8Si38 Synthesis, Crystal Structure, and Thermoelectric Properties of Na2+xAl2+xSn4−x (x = −0.38, − 0.24)
19
Robin Lefèvre, David Berthebaud, Olivier Perez, Denis Pelloquin, Sylvie Hébert, and Franck Gascoin
Polar Transition-Metal Chalcogenide: Structure and Properties of the New Pseudo-Hollandite Ba0.5Cr5Se8
20
Juli-Anna Dolyniuk, Jian Wang, Kathleen Lee, and Kirill Kovnir
21
Baoli Du, Yuta Saiga, Kousuke Kajisa, and Toshiro Takabatake
22
Shanyu Wang, Jiong Yang, Lihua Wu, Ping Wei, Jihui Yang, Wenqing Zhang, and Yuri Grin
Twisted Kelvin Cells and Truncated Octahedral Cages in the Crystal Structures of Unconventional Clathrates, AM2P4 (A = Sr, Ba; M = Cu, Ni) Thermoelectric Properties of p-Type Clathrate Ba8.0Ga15.9ZnySn30.1 Single Crystals with Various Carrier Concentrations Anisotropic Multicenter Bonding and High Thermoelectric Performance in Electron-Poor CdSb
23
Kurt Silsby, Fan Sui, Xiaowei Ma, Susan M. Kauzlarich, and Susan E. Latturner
Thermoelectric Properties of Ba1.9Ca2.4Mg9.7Si7: A New Silicide Zintl Phase with the Zr2Fe12P7 Structure Type
24
Nasrin Kazem, Julia V. Zaikina, Saneyuki Ohno, G. Jeffrey Snyder, and Susan M. Kauzlarich
Coinage-Metal-Stuffed Eu9Cd4Sb9: Metallic Compounds with Anomalous Low Thermal Conductivities
25
Yufei Hu, Jian Wang, Airi Kawamura, Kirill Kovnir, and Susan M. Kauzlarich
Yb14MgSb11 and Ca14MgSb11New Mg-Containing Zintl Compounds and Their Structures, Bonding, and Thermoelectric Properties
4
5
a
Glass-like Lattice Thermal Conductivity and Thermoelectric Properties of Incommensurate Chimney-Ladder Compound FeGeγ Computational Exploration of the Binary A1B1 Chemical Space for Thermoelectric Performance
Boosting Thermoelectric Performance by Controlled Defect Chemistry Engineering in Ta-Substituted Strontium Titanate High-Figure-of-Merit Thermoelectric La-Doped A-Site-Deficient SrTiO3 Ceramics
Chem. Mater. 2016, 28 (2), 529−533 Chem. Mater. 2015, 27 (18), 6213−6221 Chem. Mater. 2015, 27 (20), 7018−7027 Chem. Mater. 2015, 27 (14), 4995−5006 Chem. Mater. 2016, 28 (3), 925−935 Chem. Mater. 2015, 27 (5), 1622−1630 Chem. Mater. 2015, 27 (2), 408−413 Chem. Mater. 2015, 27 (22), 7801−7811 Chem. Mater. 2016, 28 (1), 376−384 Chem. Mater. 2015, 27 (2), 581−587 Chem. Mater. 2015, 27 (21), 7296−7305 Chem. Mater. 2015, 27 (3), 944−949 Chem. Mater. 2015, 27 (7), 2449−2459 Chem. Mater. 2015, 27 (22), 7719−7728 Chem. Mater. 2015, 27 (11), 4066−4072 Chem. Mater. 2015, 27 (3), 909−913 Chem. Mater. 2015, 27 (8), 2812−2820 Chem. Mater. 2016, 28 (2), 601−607 Chem. Mater. 2015, 27 (20), 7110−7118 Chem. Mater. 2015, 27 (12), 4476−4484 Chem. Mater. 2015, 27 (5), 1830−1836 Chem. Mater. 2015, 27 (3), 1071−1081 Chem. Mater. 2015, 27 (19), 6708−6716 Chem. Mater. 2015, 27 (21), 7508−7519 Chem. Mater. 2015, 27 (1), 343−351
The superscript numbers in the text refer to the numbers given in this table. 2464
DOI: 10.1021/acs.chemmater.6b01002 Chem. Mater. 2016, 28, 2463−2465
Chemistry of Materials
Editorial
Kirill Kovnir, Guest Editor Department of Chemistry, University of California, Davis, California
Eric S. Toberer, Guest Editor
■
Department of Physics, Colorado School of Mines, Golden, Colorado
AUTHOR INFORMATION
Notes
Views expressed in this editorial are those of the author and not necessarily the views of the ACS.
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DOI: 10.1021/acs.chemmater.6b01002 Chem. Mater. 2016, 28, 2463−2465
