Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session X23: Thermoelectrics Theory IIFocus
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Sponsoring Units: DMP GERA FIAP Chair: Heiner Linke, Lund University Room: 322 |
Friday, March 18, 2016 8:00AM - 8:36AM |
X23.00001: Thermal and Electronic Transport in Inorganic and Organic Thermoelectric Materials Invited Speaker: Zhiting Tian In this talk, we will first talk about first-principles calculations of phonon and electron transport in inorganic thermoelectric materials. We will start with rocksalt PbTe and PbSe, and move on to wurtzite ZnO. We will emphasize the strategies to reduce the lattice thermal conductivity. Then we apply first-principles calculations to organic thermoelectric materials. The thermoelectric properties of doped polypyrrole (PPy) will be discussed. In addition, we will cover the chain confinement effects observed in amorphous polymer thin films using molecular dynamics simulations, which highlights the fundamental difference in heat conduction between crystalline polymers and amorphous polymers [Preview Abstract] |
Friday, March 18, 2016 8:36AM - 8:48AM |
X23.00002: \textbf{Electron-phonon coupling and thermal transport in thermoelectric compound Mo}$_{\mathrm{\mathbf{3}}}$\textbf{Sb}$_{\mathrm{\mathbf{7-x}}}$\textbf{Te}$_{\mathrm{\mathbf{x}}}$ Dipanshu Bansal, Chen Li, Ayman Said, Douglas Abernathy, Jiaqiang Yan, Olivier Delaire Complex interactions between solid-state excitations, such as phonon-phonon, phonon-electron, and phonon-magnon couplings are often responsible for unusual material properties. In this presentation, we report on our investigations of phonon propagation and thermal transport in thermoelectric Mo$_{\mathrm{3}}$Sb$_{\mathrm{7-x}}$Te$_{\mathrm{x}}$. We have performed extensive inelastic neutron and x-ray scattering measurements of phonons in Mo$_{\mathrm{3}}$Sb$_{\mathrm{7-x}}$Te$_{\mathrm{x}}$, mapping the phonon dispersions and density of states, as function of temperature and composition. Our first-principles density functional theory simulations, coupled with experimental measurements, reveal the importance of electron-phonon coupling, which dominates the scattering rates over alloy disorder scattering. Doping with Te shifts the Fermi surface near the top of the valence band, suppressing screening and causing phonons to stiffen markedly. Our measurements of acoustic dispersions and linewidths, coupled with DFT simulations and models of phonon scattering enable us to quantify the impact of the electron-phonon coupling on the thermal conductivity. [Preview Abstract] |
Friday, March 18, 2016 8:48AM - 9:00AM |
X23.00003: Vibrational Dynamics of Filled Skutterudites Susmita Basak, Christian Carbogno, Matthias Scheffler Skutterudites are promising candidates for thermoelectric applications, since their cage like structure can be filled with guest atoms to tune the electronic and vibrational properties and so to optimize the thermoelectric transport coefficients. Various conflicting phenomenological models (e.g., incoherent rattling, coherent coupling [1]) have been proposed to explain the interaction between guest and host, but the exact mechanisms are still topic of debate. To clarify this question, we determine the temperature dependence of the geometric, electronic, and vibrational properties for a set of skutterudites (CoSb$_3$, CoAs$_3$) and guests (Ga, In, Sn, etc.) using density-functional theory in the quasi-harmonic approximation. We find different coupling mechanisms to be active depending on the guest, which leads to drastically different dynamics ranging from localized to coherent phonon modes. These modes, which are robust against doping and defy the common assumption that the guest's mass primarily determines the coupling, also largely influence the structural and electronic properties. Finally, we discuss the implications of our findings for the control and optimization of the thermoelectric efficiency. [1] M. M. Koza {\it et al.}, Nat. Mat. {\bf 7}, 805, (2008). [Preview Abstract] |
Friday, March 18, 2016 9:00AM - 9:12AM |
X23.00004: Low-energy phonon dispersion in LaFe$_4$Sb$_{12}$ Andreas Leithe-Jasper, Martin Boehm, Hannu Mutka, Michael M. Koza We studied the vibrational dynamics of a single crystal of LaFe$_4$Sb$_{12}$ by three-axis inelastic neutron spectroscopy. The dispersion of phonons with wave vectors $q$ along $[xx0]$ and $[xxx]$ directions in the energy range of eigenmodes with high amplitudes of lanthanum vibrations, i.e., at $\hbar \omega < 12$\,meV is identified. Symmetry-avoided anticrossing dispersion of phonons is established in both monitored directions and distinct eigenstates at high-symmetry points and at the Brillouin-zone center are discriminated. The experimentally derived phonon dispersion and intensities are compared with and backed up by {\textit ab initio} lattice dynamics calculations. results of the computer model match well with the experimental data. [Preview Abstract] |
Friday, March 18, 2016 9:12AM - 9:24AM |
X23.00005: First-principles investigations of phonon anharmonicity and electronic instability in thermoelectric SnSe Jiawang Hong, Chen W. Li, A. F. May, D. Bansal, S. Chi, T. Hong, G. Ehlers, Olivier Delaire The promising thermoelectric material SnSe exhibits ultra-low and strongly anisotropic thermal conductivity. By combining first-principles calculations and inelastic neutron scattering measurements, we have investigated the phonon dispersions and phonon scattering mechanisms, and probed the origin of the large anharmonicity in SnSe.\footnote{C.W. Li*, J. Hong*, A.F. May, D. Bansal, S. Chi, T. Hong, G. Ehlers, and O. Delaire, “Orbitally driven giant phonon anharmonicity in SnSe”, Nature Physics, (2015)} We will discuss the connection between the phonon properties and the high-temperature structural phase transition, and how the electronic structure leads to large anharmonic phonon interactions in SnSe. The present results provide a microscopic picture connecting electronic structure and phonon anharmonicity in SnSe, which could help design materials with ultralow thermal conductivity. [Preview Abstract] |
Friday, March 18, 2016 9:24AM - 9:36AM |
X23.00006: Phonons Near Lattice Instabilities in Thermoelectric SnSe, SnTe, and PbTe Olivier Delaire, Chen Li, Jiawang Hong, Jie Ma, Andrew May, Dipanshu Bansal, Georg Ehlers, Songxue Chi, Tao Hong A number of high-performance thermoelectric materials are found in the vicinity of lattice instabilities, including PbTe, SnTe, SnSe, tetrahedrites, Cu$_2$Se, among others. The large phonon anharmonicity found in such compounds suppresses the lattice thermal conductivity, a key aspect of their thermoelectric efficiency. In this presentation, we will discuss results from our recent investigations of phonons in these materials using inelastic neutron scattering and first-principles simulations, focusing on anharmonic effects near lattice instabilities. Commonalities will be highlighted, including connections between strong anharmonicity and the electronic structure and bonding. [Preview Abstract] |
Friday, March 18, 2016 9:36AM - 9:48AM |
X23.00007: Pressure intrinsically induced thermoelectric enhancement in the SnSe crystal Yongsheng Zhang, Shiqiang Hao, Li-Dong Zhao, Christopher Wolverton, Zhi Zeng SnSe is an excellent thermoelectric material due to its high $ZT$ value ($\sim$2.6 along the $b$ direction) at high temperature $\sim$923 K. However, in the temperature range of 300$-$773K, the $ZT$ values are just 0.1$-$0.9. To make this material more efficient, its thermoelectric properties should be large in the entire temperature range. Here, we use DFT calculations to show how pressure intrinsically enhances the thermoelectric properties below 700 K along the three directions ($a$, $b$ and $c$) of the crystal (the low-$T$ SnSe-$Pnma$ phase) due to a significant boost in electrical transport. The estimated $ZT$ values of $p$-type along the $b$ and $c$ directions can reach as high as 3.3 and 2.1 at 6 GPa and 700 K, respectively. At 6 GPa, the $a$ direction shows $n$-type properties and its $ZT$ value is 1.9 at 600 K. It is significant that high-performance both $n$-type and $p$-type conductors could be available in SnSe just through applying pressure. Our work on SnSe under pressure sheds light on a new mechanism for screening high efficiency thermoelectric materials. [Preview Abstract] |
Friday, March 18, 2016 9:48AM - 10:00AM |
X23.00008: Broadband Phonon Scattering in PbTe-based Materials Driven Near the Peierls Phase Transition by Strain or Alloying Ivana Savic, Ronan Murphy, Eamonn Murray, Stephen Fahy Efficient thermoelectric energy conversion is highly desirable as 60\% of the consumed energy is wasted as heat. Low lattice thermal conductivity is one of the key factors leading to high thermoelectric efficiency of a material [1]. However, the major obstacle in the design of such materials is the difficulty in efficiently scattering phonons across the frequency spectrum [2]. Using first principles calculations, we predict that driving PbTe materials close to a Peierls-like phase transition could be a powerful strategy to solve this problem. We illustrate this concept by applying tensile [001] strain to PbTe and its alloys with another rock-salt IV-VI material, PbSe; and by alloying PbTe with a IV-VI Peierls-distorted material, GeTe. This induces extremely soft optical modes, which increase acoustic-optical phonon coupling and decrease phonon lifetimes at all frequencies. We show that PbTe, Pb(Se,Te) and (Pb,Ge)Te alloys driven near the phase transition in the described manner could have the lattice thermal conductivity considerably lower than that of PbTe. The proposed concept may open new opportunities for the development of more efficient thermoelectric materials. [1] G. J. Snyder and E. S. Toberer, Nature Mater. 7, 105 (2008). [2] K. Biswas et al. Nature 489, 414 (2012). [Preview Abstract] |
Friday, March 18, 2016 10:00AM - 10:12AM |
X23.00009: Understandng of phonon anharmonicity in thermoelectric clathrates Katsumi Tanigaki, Jiazhen Wu, Hidekazu Shimotani, Khuong Huynh, Kazuto Akagi Anharmonicity in phonons, apart from the conventional Einstein- or Debye- mode harmonic phonons, is frequently observed for amorphous or glass-like materials. A frontier topic relating to anharmonic phonons revolves around the fact that they are also observed in a single crystal with a void of cage structure. Although the origin of the phonon anharmonicity has been the center of scientific debate for many years, a clear understanding has not yet been achieved. In the present study, we show that the anharmonic oscillations in thermoelectric clathrates can successfully be rationalized in terms of a single unified exponential line for a variety of clathrates by employing a new parameter associated with the freedom of space. The intrinsic nature of phonon anharmonicity is described based on the unified picture with a help of first principles calculations. Although the origin of the anharmonicity appearing in disordered materials is complex to understand due to the missing information on the real structure, the present unified picture gives important information applicable to other systems. [Preview Abstract] |
Friday, March 18, 2016 10:12AM - 10:24AM |
X23.00010: First-principles Study of Guest Impurities of Na and Ba on Lattice Thermal Conductivity of Type-I Si Clathrate Yi Xia, Fei Zhou, Vidvuds Ozolins The type-I clathrate compounds, known as good thermoelectric materials, exhibit phonon-glass electron-crystal properties with their open cages filled with guest atoms. We present a first-principles study of the intrinsic impact of these rattlers on the lattice thermal conductivity in type-I Si clathrate compounds. We observe both coherent and incoherent coupling between guest and framework acoustic modes which could be ascribed to the difference in atomic radius, confirming the ``avoided crossing'' behavior. Our calculated lattice thermal conductivities for Si$_{46}$ (37.64 W/(m K)), Na$_{8}$Si$_{46}$ (2.75 W/(m K)) and Ba$_{8}$Si$_{46}$ (1.37 W/(m K)) are in good agreement with experimental measurements and simulations at room temperature. Significant reductions in both phonon lifetime and group velocity due to guest atoms are responsible for the reduction in lattice thermal conductivity. The energy widths of acoustic modes of both empty and filled silicon clathrates are beyond the resolution limit of 0.2 meV of inelastic x-ray scattering and thus cannot be used to exclude the Umklapp scatterings as main cause of reduction in lattice thermal conductivity. In addition, the Umklapp scatterings of acoustic modes are directly mediated by the guest modes. [Preview Abstract] |
Friday, March 18, 2016 10:24AM - 10:36AM |
X23.00011: First-Principles Study of Guest-Host Bonding in the Type-II Clathrate Compounds A$_{\mathrm{x}}$Si$_{\mathrm{136}}$ (A $=$ Na, K, Rb, Cs; 0 $\le $ x $\le $ 24) Charles Myles, Dong Xue The Type II clathrate-based materials are interesting due to their potential thermoelectric applications. Recently, a synthesis and characterization of Na$_{\mathrm{x}}$Si$_{\mathrm{136\thinspace }}$for various x has been reported [1]. Powdered X-Ray diffraction (XRD) data and density functional theory (DFT) studies of Na$_{\mathrm{x}}$Si$_{\mathrm{136}}$ have found a lattice contraction as x increases for 0 \textless x \textless 8 and an expansion as x increases for x \textgreater 8. This is explained by XRD data showing that, as x increases, the 28-atom Si cages are filled first for x \textless 8 and the 20-atom Si cages are then filled for x \textgreater 8. We report results of first-principles calculations focused on analyzing the underlying mechanisms relevant to guest-host interactions and to understanding the role of the guest atom to host atom size ratio in this material. We have also studied the compounds A$_{\mathrm{x}}$Si$_{\mathrm{136}}$ (A $=$ Na, K, Rb, Cs; 0 $\le $ x $\le $ 24). Our LDA calculations for K$_{\mathrm{x}}$Si$_{\mathrm{136}}$ (0 \textless x \textless 16) predict that this material should exhibit a non-monotonic structural response similar to that in Na$_{\mathrm{x}}$Si$_{\mathrm{136}}$; the lattice should contract and then expand as x increases. We find that the heavier guests (Rb, Cs) vibrate nearer to the center of the Si$_{\mathrm{28}}$ cages than do Na and K. Our results also show that Na and K are both strongly coupled to the Si framework in A$_{\mathrm{x}}$Si$_{\mathrm{136}}$ (A $=$ Na, K; x $=$ 4, 8). [1] S. Sefanoski, C.D. Malliakas, M.G. Kanatzidis, G.S. Nolas, \underline {\textit{Inorg. Chem.}} \underline {51}, 8686 (2012). [Preview Abstract] |
Friday, March 18, 2016 10:36AM - 10:48AM |
X23.00012: Thermoelectric property of a new silicon crystal Kisung Chae, Seon-Myeong Choi, Duck Young Kim, Young-Woo Son We present ab initio calculations on thermoelectric properties of a recently synthesised allotrope of silicon crystal [1]. A new silicon crystal with 24 Si atoms per unit cell has open channels along the specific crystallographic direction and shows a quasidirect energy gap of 1.3 eV. Using various first-principles calculation techniques for electrical and thermal conductivity as well as Seebeck coefficient, we find large suppression of thermal conductivity and relatively large Seebeck coefficient in the new silicon crystal, thus demonstrating a competitive thermoelectric figure of merit. \vphantom{.} \noindent [1] D. Y. Kim et al, Nat. Mat. 14, 169 (2015). [Preview Abstract] |
Friday, March 18, 2016 10:48AM - 11:00AM |
X23.00013: First Principles Study of the Properties of the Type II Clathrate Alloy Si$_{\mathrm{136-x}}$Ge$_{\mathrm{x\thinspace }}$(x $=$ 8, 32, 96) Dong Xue, Charles Myles The Type-II clathrate materials based on Si, Ge, and Sn have ``open-framework'' lattices consisting of large ``cages'' of atoms covalently bonded together. Due primarily to their potential thermoelectric applications, there has been considerable research on these materials with various guest atoms in the cages and with various substitutional atoms on the lattice framework. Also of interest are the pure Type II clathrates M$_{\mathrm{136}}$ (M $=$ Si, Ge, Sn) with neither framework substitution nor guest atoms in the cages. A fundamental understanding of the intrinsic properties of these ``guest-free'' clathrates is therefore also needed. Mixtures or ``alloys'' of two different Type II clathrate materials are also potentially interesting. For example, Moriguchi \textit{et al. }[1] have reported the successful synthesis of Type II clathrates with mixtures of Si and Ge on the framework lattice. Motivated by these experiments, we have carried out a computational and theoretical study the properties of the Type II clathrate ``alloy'' Si$_{\mathrm{136-x}}$Ge$_{\mathrm{x}}$. We report the results of DFT-based first-principles calculations of the structural, electronic, vibrational, and thermal properties of Si$_{\mathrm{136-x}}$Ge$_{\mathrm{x\thinspace }}$for x $=$ 8, 32, 96. Our calculations have assumed that the ideal lattice symmetry is unaffected by the mixing of Si and Ge. Among other results, we predict that Si$_{\mathrm{136-x}}$Ge$_{\mathrm{x}}$ should have a direct band gap ranging from 1.2 to 2.0 eV. [1]. K. Moriguchi, S. Munetoh, A. Shintani, \underline {\textit{Phys. Rev}}$.$ B \underline {62}, 7138 (2000). [Preview Abstract] |
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