1:30pm - 2:00pmInvited
Molecularly-tailored Synthesis and Metallization of Doped Inorganic Nanothermoelectrics with Superior Properties
Materials Science and Engineering Department, Rensselaer Polytechnic Institute, United States
Realizing high figure-of-merit (ZT) thermoelectric nanomaterials, and tailoring their interfaces with metals, are crucial for emerging solid-state refrigeration packaging and waste-heat harvesting technologies. I will show that doping-induced electronic structure changes and nanostructuring can result in up to 250% ZT increases due to the untangling of unfavorably coupled properties. I will also demonstrate the use of molecular nanolayers to obtain multifold enhancements in the thermal and electronic conductivities at metal-thermoelectric interfaces. Doped nanocrystals pnictogen chalcogenides and oxides prepared by microwave synthesis, and bulk nanostructured pellets made from them, exhibit unusual increases in electrical conductivities and Seebeck coefficients. This approach is applicable to thin films as well. Electron spectroscopy and density functional theory calculations reveal that key mechanisms include subtle, but profound, effects such as doping-induced anti-site defect suppression and multifold increase in the density-of-states effective mass. I will then describe that introducing molecular nanolayers at metal-thermoelectric interfaces can yield large increases in thermal and electronic contact conductivities. Electron and ion beam spectroscopy and X-ray diffraction show that the property enhancements are due to nanolayer-induced alterations to the inorganic interface chemistry and structure. Key mechanisms include strong bonding, interfacial oxide scavenging, diffusion curtailment, and phase formation, which can be controlled by appropriate choice of molecular termini, length and backbone chemistry.
Adv. Mater. 28, 6436 (2016); Nature Mater. 12, 118 (2013); Nature Mater. 11, 233 (2012); Nano Lett. 12, 4523 (2012); Nano Lett. 11(10), 4337 (2011); ACS Nano 4, 5055 (2010); Nano Lett. 10, 4417-22 (2010); Nature 447, 299 (2007); Appl. Phys. Lett. 104, 053903 (2014); J. Vac. Sci. Technol., A 33, 020605 (2015); Scripta Mater. 121, 42-44 (2016); ACS Appl. Mater. Interf. 8, 4275 (2016); ACS Appl. Mater. Interf. (2017); Appl. Phys. Lett. 109, 173904 (2016).
2:00pm - 2:30pmInvited
Thermal and Thermoelectric Transport in Perovskites
University of Southern California, United States
Perovskites are an interesting class of ternary materials, with structural formula ABX3, where A and B are cations and X is an anion. The most prototypical perovskites are the oxides, especially CaTiO3, after which the structure is named after. In this talk, I will review the efforts on developing high thermoelectric figure of merit n-type oxides based on cubic perovskite, SrTiO3 and advanced thermal transport studies based on perovskite oxide superlattices. These thermal transport studies have clarified one of the long-standing predictions on the crossover of thermal transport from incoherent to coherent mechanisms. Finally, I will review the development of a new class of perovskite sulfides, which can provide a framework for intermediate temperature high performance thermoelectric materials.
 J. Ravichandran, J Mater Res 32, 183 (2017).
 S. Niu, H. Huyan, Y. Liu, M. Yeung, K. Ye, L. Blankemeier, T. Orvis, D. Sarkar, D. J. Singh, R. Kapadia, and J. Ravichandran, Adv. Mater. 1604733 (2017).
 J. Ravichandran, A. K. Yadav, R. Cheaito, P. B. Rossen, A. Soukiassian, S. J. Suresha, J. C. Duda, B. M. Foley, C.-H. Lee, Y. Zhu, A. W. Lichtenberger, J. E. Moore, D. A. Muller, D. G. Schlom, P. E. Hopkins, A. Majumdar, R. Ramesh, and M. A. Zurbuchen, Nature Materials 13, 168 (2014).
 J. Ravichandran, W. Siemons, D.-W. Oh, J. T. Kardel, A. Chari, H. Heijmerikx, M. L. Scullin, A. Majumdar, R. Ramesh, and D. G. Cahill, Phys Rev B 82, 165126 (2010).
2:30pm - 2:45pmOral
Structure of Donor-Doped SrTiO3
1University College London, United Kingdom; 2School of Materials Science & Engineering, Nanyang Technological University, Singapore
Among oxides, donor-substituted strontium titanate (STO) shows great promise as an n-type thermoelectric material due to its specific electronic structure, which can be tuned by the introduction of structural defects. This modifies the lattice contribution to the thermal conductivity, enabling enhancements in phonon scattering by substitution and/or micro/ nano-engineering approaches. In typical perovskite structured ABO3 type materials, partial A-site substitution by rare-earth elements and B-site substitution by certain transition metal cations serve as donor additives in STO to attain reasonable electrical conductivity. In previous studies, Nb-doped STO epitaxial films exhibited a high ZT value of ca 0.37 and a large power factor (PF) of 1.5 mW m-1K-2 at 1000K3 . In addition, a ZT of 0.4 at 1100K is realized in Nb-doped STO bulk materials. However, reports of STO-based thermoelectric materials are inconsistent and contradictory with respect to the doping mechanisms, solid‑solution limits, and crystal symmetry, due to the sensitivity of STO samples to processing variables such as temperature, time and atmosphere. Very recent work reported optimized ZT values of 0.41 at 973K for La doped, A-site-deficient STO (Sr1-3x/2LaxTiO3-β) ceramics. This work highlighted the importance of A-site vacancies in the oxygen-loss mechanism and suggested that the formation of oxygen vacancies is conducive to the improvement of their n-type thermoelectric properties. Here we report a detailed structural study of doped STO systems employing a suite of X-ray techniques, specifically, X-ray absorption spectroscopy, X-ray total scattering (PDF) and XPS techniques.
2:45pm - 3:15pmInvited
Low Dimensional Cr-Based Thermoelectric Material
Chongqing University, China
Thermoelectricity is one of the simplest technologies applicable to energy conversion. The efforts of increasing the conversion efficiency have recently attracted great interest among interdisciplinary scientists and engineers. Hicks and Dresselhaus has point out that the reduced dimensionality would play a vital role in the applications of thermoelectric not only in reducing the thermal conductivity but also in enhancing the power factor. Since then, low dimensionality has been envisioned as a new direction for independently tuning S, σ and κ and much efforts have been paid on fabricating the low dimensional material. However, materials possessed with natural low dimensional crystal are less focused. Here, in this work, we present two Cr-based thermoelectric materials with natural low dimensional crystal. Our work indicate that the new two Cr-based compound are the potential thermoelectric materials in the middle temperature area.
 Mildred S. Dresselhaus, Gang Chen, Ming Y. Tang, Ronggui Yang, Hohyun Lee, Dezhi Wang, Zhifeng Ren, Jean-Pierre Fleurial and Pawan Gogna. Adv. Mater. 2007, 19, 1043-1053.
3:15pm - 3:30pmOral
Anomalous Sign Change in the Seebeck Coefficient of Few Layer MoS2
1Institute of Materials Research and Engineering, Agency for Science, Technology and Research (A*STAR), Singapore; 2Department of Electrical Engineering, National University of Singapore, Singapore
Thermoelectrics provides a means to harvest energy sustainably from the environment and also plays an important role in materials science. Such environmentally friendly energy conversion sources are considered to be promising for supplementing future global energy demands. In recent years, low dimensionality (one-dimensional and two-dimensional) has opened up new routes to achieve high-efficiency thermoelectric devices. High mobility two-dimensional (2D) transition metal dichalcogenides semiconductors represent a new class of thermoelectric materials due to their enhanced density of states of confined carriers, as well as their large effective masses and valley degeneracies.
In most cases, the Seebeck coefficient is determined by a normal energy-dependent electronic density of states near the Fermi level and the charge carrier type (electrons or holes). Thus in previous studies, only a negative Seebeck coefficient due to conduction electrons was observed in MoS2 based devices. In this study, we observe for the first time, a positive Seebeck coefficient in n-doped six-layer MoS2 at temperatures below ~70K, which indicates that the Seebeck effect originates from the change in energy dependence of the charge-carrier relaxation times. The measured mobility undergoes a concomitant change in the slope at the same temperature, which corroborates a change in the relaxation time as a function of temperature and results in a sizable positive addition to the Seebeck coefficient. At low temperatures, we observe a large positive Seebeck (~ 1.5 mV/K at 50K). This new finding advances the study of thermoelectric physics in 2D materials and demonstrates a new avenue for superior thermoelectric performance by tuning the energy-dependent relaxation time.
3:30pm - 3:45pmOral
Large Thermoelectric Response of Nb Doped Ba0.7Eu0.3TiO3
National University of Singapore, Singapore
Thermoelectric materials are attracting significant attention in recent years due to their potential applications in solid-state cooling, power generation and waste-heat recovery. The thermoelectric performance of a material is usually evaluated by figure of merit ZT (= S2σT/κ, where S, s, k and T are Seebeck coefficient, electrical conductivity, thermal conductivity and absolute temperature, respectively). While most alloys such as Bi2Te3, Pb2Te3 show fascinating thermoelectric performance , they are usually toxic, environment unfriendly and chemically unstable. Recently, oxide semiconductors which are nontoxic and chemically stable at high temperature have drawn attention as a new class of thermoelectric materials.[2,3] However, their ZT values are not high enough compare to those for thermoelectric alloys. Therefore, to find effective ways for improving electrical conductivity with low thermal conductivity is essential for oxide-based thermoelectric materials.
We report thermoelectric properties of polycrystalline Ba0.7Eu0.3Ti1-xNbxO3 (x = 0.003 – 0.10). BaTiO3, a well-known ferroelectric material shows large Seebeck coefficient but poor electrical and thermal conductivities. In order to reduce thermal conductivity and enhance electrical conductivity of BaTiO3, we substitute Eu2+ for Ba2+ site and Nb5+ for Ti4+ site, respectively. The large ZT value is found for x = 0.01 compound (ZT ~ 0.06 (0.12) at T = 300 K (390 K)). The observed ZT value at room temperature is comparable to that of Sr(La)TiO3 single crystal.
R. Mahendiran and Km Rubi thank MOE (Singapore) for supporting this work under the grant. No. R144-000-349-112.
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