Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session U12: Focus Session: Thermoelectrics Materials II |
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Sponsoring Units: DMP GERA FIAP Chair: David Parker, ORNL Room: 314 |
Thursday, March 21, 2013 11:15AM - 11:51AM |
U12.00001: Anharmonicity and its application in earth abundant thermoelectrics Invited Speaker: Donald Morelli Recently very exciting improvements in the thermoelectric figure of merit have been reported in bulk nanostructured chalcogenides, mostly due to lattice thermal conductivity suppression by nanoscale-level interfaces. A critical issue in these types of structures is maintaining good electrical conductivity while blocking phonon transport. While so-called ``endotaxial'' nanostructuring, for example, can substantially maintain electron transport across interfaces, generally nanocomposite structures display reduced electrical conductivity which can counteract or in some cases overwhelm the improvements in figure of merit due to thermal conductivity reduction. Additionally, the thermal stability of nanostructured materials at operating temperatures at a significant fraction of the melting point is a concern. Here we describe another approach to reducing lattice thermal conductivity based on designing materials with large lattice anharmonicity. Anharmonic phonon vibrations are the source of intrinsic thermal resistivity in solids and manifest themselves in large Gr\"{u}neisen parameters. We show that one class of compounds, those containing antimony atoms with a lone pair configuration, exhibits a strongly anharmonic phonon spectrum that leads to intrinsically small lattice thermal conductivity. We have applied this concept to ternary copper-antimony-chalcogenide semiconductors and find that the family of compounds based on the tetrahedrite crystal structure can exhibit thermoelectric figure of merit rivaling that of conventional materials like PbTe. The tetrahedrite family is the most widespread sulfosalt mineral on Earth and we show that the mineral itself can be used directly as a source material for earth abundant thermoelectrics. This may pave the way for many new, low cost applications of thermoelectrics in waste heat recovery and power generation. [Preview Abstract] |
Thursday, March 21, 2013 11:51AM - 12:03PM |
U12.00002: Phonon lifetime investigation of anharmonicity and thermal conductivity in UO$_2$ Judy Pang, Aleksandr Chernatynskiy, William Buyers, Bennett Larson, Simon Phillpot Understanding low thermal conductivity in UO$_2$ requires a correct accounting for anharmonic phonon-phonon scattering processes. However, over the last five decades there have been remarkably few high-temperature studies of phonon processes in UO$_2$ to underpin its widespread use as a reactor fuel. We have used high-resolution inelastic neutron scattering measurements of individual phonon lifetimes (linewidths) and dispersion at 295 and 1200 K to probe anharmonicity and thermal conductivity in UO$_2$ for individual phonon branches. We found that phonon lifetimes depend strongly on the phonon wave vector and that longitudinal optic phonon modes transport the largest amount of heat, in contrast to recent first principles simulations. The total thermal conductivities calculated using our phonon data demonstrate a quantitative correspondence between microscopic and macroscopic phonon physics. We have also performed density functional theory simulations showing semi-quantitative agreement with phonon lifetimes at 295 K, but larger anharmonicity than measured at 1200 K. These measured phonon dispersion and lifetimes form a benchmark dataset against which numerical simulations including anharmonicity may be assessed. [Preview Abstract] |
Thursday, March 21, 2013 12:03PM - 12:15PM |
U12.00003: Determination of elastic constants via phonon- imaging for crystals with low symmetry Tim Head, Elizabeth Carlisle We report progress toward using group velocity surface projections, rather than group velocity surfaces directly, to find elastic constants for low symmetry crystals. Direct determination of elastic constants is difficult in general because of the multi-valued nature of the group velocity surface and a lack of experimentally accessible information about phonon polarizations. Projection of group velocity surfaces onto a plane depend strongly on the elastic constants. We use Monte-Carlo simulations of phonon-images based on continuum elasticity theory to move toward a best-fit algorithm to find elastic constant values for crystals of low symmetry given phonon-imaging data. [Preview Abstract] |
Thursday, March 21, 2013 12:15PM - 12:27PM |
U12.00004: Thermodynamic Effects of Na on the Morphology of PbTe-PbS Nanostructured Thermoelectrics Jeff Doak, Jiaqing He, Ivan Blum, Steven Girard, Li-Dong Zhao, David Seidman, Mercouri Kanatzidis, Vinayak Dravid, Chris Wolverton, Hui-Qiong Wang, Jin-Cheng Zheng, Gilberto Casillas, Miguel Jose-Yacaman The creation of nanostructures via phase separation provides a mechanism for decreasing the lattice thermal conductivity and increasing the figure of merit of bulk thermoelectric materials like PbTe-PbS. The addition of Na to PbTe-PbS drastically alters the morphology of PbS precipitates in the system. To see if this change in morphology can be attributed to equilibrium thermodynamics, we use first-principles density functional theory (DFT) calculations to study the energetics of Na partitioning between PbTe and PbS and Na segregation at PbTe/PbS interfaces. We calculate a variety of Na defects in PbTe and PbS and find that the lowest energy defect in both PbTe and PbS is Na substituted for Pb. From the Na defect formation energies, we find the solubility limit of Na in PbTe and PbS, as well as the partitioning coefficient between PbTe and PbS. We find that Na partitions to PbS over PbTe, in agreement with experiment. We calculate Na segregation energies by substituting Na for Pb at the PbTe/PbS interface and find that Na segregates at the PbTe-side of the interface, in qualitative agreement with atom-probe tomography analysis. Applying the Gibbs adsorption isotherm to Na segregation, we find a corresponding decrease in interfacial energy leading to a change in morphology. [Preview Abstract] |
Thursday, March 21, 2013 12:27PM - 12:39PM |
U12.00005: Replacement of Ge in GeTe by [Ag$+$Sb] and rare earths: effect on thermoelectric properties E.M. Levin, M. Hanson, R. Hanus, K. Schmidt-Rohr High-efficiency $p$-type Te-Sb-Ge-Ag (TAGS) thermoelectric materials are based on the GeTe narrow-band self-dopant semiconductor where Ge can be replaced by up to 16 at.{\%} [Ag$+$Sb]. To understand the effect of Ge replacement by 4 at.{\%} [Ag$+$Sb] as well as rare earths atoms, we have synthesized and studied XRD, thermopower, electrical resistivity, thermal conductivity, and $^{125}$Te NMR of GeTe and Ag$_2$Sb$_2$Ge$_{\mathrm{46-x}}$R$_{\mathrm{x}}$Te$_{50}$ with R$=$Gd, Dy and $x=$1, 2. At 700 K, GeTe exhibits a thermopower of $+$146 $\mu $VK$^{-1}$ and a large power factor, 42 $\mu $Wcm$^{-1}$K$^{-2}$. Replacement of Ge by [Ag$+$Sb] and rare earths enhances the thermopower, but slightly reduces the power factor due to an increase in electrical resistivity. The thermal conductivity at 300 K of all alloys studied is reduced by a factor of two compared to GeTe. $^{125}$Te NMR spin-lattice relaxation time and resonance frequency reflect changes in carrier concentration. However, decrease of thermal conductivity due to carriers and increase of electrical resistivity are mostly due to a reduction of carrier mobility and indicate strong scattering produced by [Ag$+$Sb] and rare earth atoms. At 700 K, the thermoelectric figure of merit of GeTe is 0.8, whereas that in Ag$_2$Sb$_2$Ge$_{45}$Dy$_1$Te$_{50}$ is much larger, 1.2, due to a reduction in thermal conductivity. Enhancement of thermopower is discussed within a model of energy filtering. [Preview Abstract] |
Thursday, March 21, 2013 12:39PM - 12:51PM |
U12.00006: Primary phase alignment in the Mg-Sb system with a 35T DC magnetic field Seth Imhoff, Thomas Ott, Tim Tucker, Jason Cooley Primary phase alignment behavior in the Mg-Sb system is explored by solidification of samples in a 35 tesla DC magnetic field. Compositions with multiple solidification reaction pathways are found to have different phase alignment characteristics.~ In the current study, the orientation of Mg and Sb primary grains do not appear to be strongly influenced, but the $\alpha $-Mg3Sb2 shows a very strong tendency to align with its long axis perpendicular to the field direction. In comparing two compositions that both first nucleate $\alpha $-Mg3Sb2 from the melt, it is found that the volume fraction involved in the primary reaction is a controlling factor for the total degree of alignment throughout the structure. This volume fraction dependence is interpreted as hindering free rotation in the liquid.~ [Preview Abstract] |
Thursday, March 21, 2013 12:51PM - 1:03PM |
U12.00007: Mapping the Fermi Surface in Nb by Tracking Kohn Anomalies with Neutron Scattering Iyad Al-Qasir, Olivier Delaire, Vickie Lynch, Douglas Abernathy, Matt Stone Electron-phonon interaction in metals is a subject of interest for theoretical and experimental investigations. Phonons in Nb show Kohn anomalies due to the electron-phonon interaction. In this work, we are tracking Kohn anomalies in Nb in the full Brillouin zone experimentally and computationally and relating it to the Fermi surface. We measured the 4-dimensional scattering function, $S(\stackrel{\to}{Q},\omega )$ of Nb as a function of temperature, using time of flight inelastic neutron scattering. The 4D data allow us to map phonon dispersion relations along any direction in the full Brillouin zone. In parallel, density functional theory was used to calculate the electronic band structure and Fermi surface, as well as the phonon dispersion relations and line-widths. We present a quantitative comparison, taking into account experimental resolution. These results point to a new avenue of mapping the Fermi surface and electron-phonon coupling in bulk crystals, complementing existing techniques. [Preview Abstract] |
Thursday, March 21, 2013 1:03PM - 1:15PM |
U12.00008: Theoretical Study of the Properties of the Type II Clathrates AxB$_{136}$(A=alkali atom;B=Si,Ge,Sn0 $\le $ x $\le $ 24) Dong Xue, Craig Higgins, Charley Myles Type II clathrate semiconductors have cage-like lattices in which Group IV atoms are tetrahedrally-coordinated and sp$^{3}$ covalently bonded. The cages can contain ``guest'' atoms; usually alkali or alkaline earth atoms. These materials are of interest because of their thermoelectric properties. Motivated by recent experimental and theoretical interest [1,2] in the x dependence of properties of the Si and Ge-based Type II clathrate materials A$_{\mathrm{x}}$Si$_{136}$ and A$_{\mathrm{x}}$Ge$_{136}$ (A $=$ alkali atom) we are carrying out a systematic theoretical study of the properties of the Type II clathrate systems A$_{\mathrm{x}}$B$_{136}$(A $=$ alkali atom; B $=$ Si, Ge, Sn). Recent powder X-ray diffraction experiments have found the very interesting result that in Na$_{\mathrm{x}}$Si$_{\mathrm{136}}$, for increasing x in the range 0 $\le $ x $\le $ 8 a lattice contraction occurs and that as x is increased further (8 $\le $ x $\le $ 24), a contrasting lattice expansion results. These observations have motivated us to study the behavior of the lattice constant and other properties as a function of guest concentration in several Type II clathrates. We present results of a density functional based study of the properties of A$_{\mathrm{x}}$B$_{136}$ as a function of x. Results are discussed for the x dependence of the lattice constant and for other structural and electronic properties of these materials. [1] S. Stefanoski and G. Nolas, Cryst. Growth Des. 2011, dx.doi.org/10.1021/cg200756r [2] M. Beekman, E. Nenghabi, K. Biswas, C. Myles, M. Baitinger, Y. Grin, G.S. Nolas, Inorg. Chem. 49 2010, DOI: 10.1021/ic1005049 [Preview Abstract] |
Thursday, March 21, 2013 1:15PM - 1:27PM |
U12.00009: Light Si Based Clathrates For Thermal Energy Conversion: A First Principles Study Yuping He, Fan Sui, Susan Kauzlarich, Giulia Galli Clathrates containing light, earth abundant elements, i.e. Si and Al, are promising materilas for thermoelectric applications, due to their low thermal conductivity, about 2 orders of magnitude smaller than that of bulk Si. However existing Si based clathrates [1] have poor electronic properties for efficient thermal energy conversion. We carried out density functional theory calculations to investigate the electronic and vibrational properties of newly synthesized type I clathrate K$_{8}$Al$_{8}$Si$_{38}$[2]. We predicted that while Al site occupancy does not substantially affect the structure of these systems, it has a strong influence on their electronic and optical properties. In particular, Al occupancy greatly influences the location of the K atoms, and the magnitude and character of the electronic gap of the clathrate (e.g. Whether direct or indirect). Our findings suggest that K$_{8}$Al$_{8}$Si$_{38}$ may have much improved electronic properties, compared to several families of clathrates [2] investigated in the recent literature.\\[4pt] [1] C. L. Condron et al. Inorg. Chem. 2008, 47, 8204.\\[0pt] [2] F. Sui et al. Synthesis and characterization of type I clathrate K$_{8}$Al$_{8}$Si$_{38}$ for thermoelectric application (in preparation) [Preview Abstract] |
Thursday, March 21, 2013 1:27PM - 1:39PM |
U12.00010: High pressure effect on structure, electronic structure and thermoelectric properties of MoS$_2$ Huaihong Guo, Teng Yang, Zhidong Zhang We systematically study high pressure effect on the shape of the unit cell, electronic structure and transport properties of 2H-MoS$_2$, based on density functional calculations and the Boltzmann transport theory. Under pressure, the cross-plane lattice size decreases much faster than the in-plane one, due to the van der Waals interaction, and the size reduction becomes more difficult as external pressure exceeds 20 GPa, agreeing with experimental observation. A conversion from van der Waals to covalent bonding is seen in the calculated charge density and obtial projection of the wave functions. Concurrently, the dependence of band structure on pressure shows that a transition from semiconductor to metal occurs at 25 GPa. Band features close to the Fermi level are found to be advantageous for high values of thermopower. Our transport calculations also find pressure-enhanced electrical conductivities, high values of thermopower (up to a few hundred $\mu$V/K), and significant values of the thermoelectric figure of merit (above 0.10 for high pressure and even up to 0.65 at 25 GPa) over a wide temperature range. Our study supplies a new route to improve the thermoelectric performance of MoS$_2$ and of other transition metal dichalcogenides by applying hydrostatic pressure. [Preview Abstract] |
Thursday, March 21, 2013 1:39PM - 1:51PM |
U12.00011: Dimensional crossover and thermoelectric properties in CeTe$_{\mathrm{2-x}}$Sb$_{\mathrm{x}}$ single crystals Jong-Soo Rhyee, Kyung Eun Lee, Jae Nyeong Kim, Ji Hoon Shim, Byeong Hun Min, Yong Seung Kwon Several years before, we proposed that the charge density wave is a new pathway for high thermoelectric performance in In$_{\mathrm{4}}$Se$_{\mathrm{3-x}}$ bulk crystalline materials. (Nature v.459, p. 965, 2009) Recently, from the increase of the chemical potential by halogen doped In$_{\mathrm{4}}$Se$_{\mathrm{3-x}}$H$_{\mathrm{0.03}}$ (H$=$Halogen elements) crystals, we achieved high ZT (maximum ZT 1.53) over a wide temperature range. (Adv. Mater. v.23, p.2191, 2011) Here we demonstrate the low dimensionality increases power factor in CeTe$_{\mathrm{2-x}}$Sb$_{\mathrm{x}}$ single crystals. The band structures of CeTe$_{\mathrm{2}}$ show the 2-dimensional (2D) Fermi surface nesting behavior as well as a 3-dimensional (3D) electron Fermi surface hindering the perfect charge density wave (CDW) gap opening. By hole doping with the substitution of Sb at the Te-site, the 3D-like Fermi surface disappears and the 2D perfect CDW gap opening enhances the power factor up to x $=$ 0.1. With further hole doping, the Fermi surfaces become 3-dimensional structure with heavy hole bands. The enhancement of the power factor is observed near the dimensional crossover of CDW, at x $=$ 0.1, where the CDW gap is maximized. This research was supported by Basic Science Research Program (2011-0021335), Mid-career Research Program (Strategy) (No. 2012R1A2A1A03005174) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology, and TJ Park Junior Faculty Fellowship funded by the POSCO TJ Park Foundation. [Preview Abstract] |
Thursday, March 21, 2013 1:51PM - 2:03PM |
U12.00012: Thickness dependent thermoelectric properties of SrTiO$_{3}$/SrLaTiO$_{3}$ and SrZrO$_{3}$/SrLaTiO$_{3}$ heterostructures Masatoshi Ishii, John Baniecki, Robert Schafranek, Kian Kerman, Kazuaki Kurihara Thermoelectric power generators will be required for future sensor network systems. SrTiO$_{3}$ (STO) [1] is one candidate thermoelectric material due to its non-toxicity and comparable power factor to Bismuth telluride. The energy conversion efficiency of SrTiO$_{3}$--based thermoelectric energy conversion elements has been reported to be enhanced by quantum size effects, such as the two dimensional (2D) electron gas in SrTiO$_{3}$/SrTi$_{0.8}$Nb$_{0.2}$O$_{3}$/SrTiO$_{3}$ [2]. Nevertheless, a complete understanding of the mechanisms for the reported increase in efficiency are missing owing to a lack of understanding of the thickness dependence of the transport properties. In the talk, we will present a study of the thickness dependence of the transport properties of SrTiO$_{3}$/SrLaTiO$_{3}$ and SrZrO$_{3}$/SrLaTiO$_{3}$ heterostructures. The SrZrO$_{3}$/SrLaTiO$_{3}$ interface has a large conduction band off-set of 1.9 eV [3] which can be utilized to confine electrons in a 2D quantum well. Characterization of the thermopower, conductivity, and Hall effect will be presented as a function of the SrLaTiO$_{3}$ thickness down to a few unit cells and the implications of the thickness dependence of the transport properties on carrier confinement and increasing the efficiency STO-based 2DEG quantum well structures will be discussed. [1] J. Baniecki et al, Appl. Phys. Lett. 99, 232111 (2011); [2] H. Otha et al., Nature materials, 6, 129 (2007); [3] R Schafranek et al, J. Phys. D: 45 055303 (2012) [Preview Abstract] |
Thursday, March 21, 2013 2:03PM - 2:15PM |
U12.00013: ABSTRACT WITHDRAWN |
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