Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session X20: Focus Session: Thermoelectric Materials: Theory |
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Sponsoring Units: DMP FIAP GERA Chair: Marco Fornari, Central Michigan University Room: D168 |
Thursday, March 24, 2011 2:30PM - 3:06PM |
X20.00001: Atomistic simulations of heat transport in nanostructures Invited Speaker: Engineering materials at the nanoscale allows for tuning several of their properties over a broad range. These holds particularly for thermoelectric performances of group IV semiconductors, such as silicon and germanium. Experiments [1,2] suggest that improvements of the thermoelectric figure of merit in nanostructured silicon are mostly related to a drop in the thermal conductivity of about two orders of magnitude with respect to the bulk. In spite of success of macroscopic empirical approaches, we argue that atomistic simulations are necessary to provide the correct physical behavior and achieve significant understanding of a complex phenomenon such as thermal transport at the nanoscale ($\sim 10$nm). By means of atomistic simulation methods, we address the issue of lattice thermal transport in silicon and SiGe nanostructures and nanostructured materials, e.g. nanowires, nanoporous and amorphous silicon thin films. We have reviewed and compared several simulation approaches (equilibrium and non-equilibrium molecular dynamics and anharmonic lattice dynamics), and developed a new method for large scale simulations, based on the scattering approach. We have identified strength, weaknesses and possible artifacts for each method, and established reliable simulation procedures to compute thermal transport properties. Our results shed light on the cooperative effects of dimensionality reduction, nanostructuring and disorder, in reducing the thermal conductivity of silicon-based nanostructured materials, stemming from prominent changes of lattice vibrational properties and enhancement of phonon scattering [3]. \\[4pt] [1] A. I. Hochbaum, et al. Nature (London) {\bf 451}, 163 (2008).\\[0pt] [2] J.-K. Yu, et al. Nature Nanotech. {\bf 5}, 718 (2010).\\[0pt] [3] D. Donadio and G. Galli, Phys. Rev. Lett. {\bf 102}; 195901 (2009), Nano Lett. {\bf 10}, 847 (2010); M. K. Y. Chan, et al. Phys. Rev. B {\bf 81}, 174303 (2010). [Preview Abstract] |
Thursday, March 24, 2011 3:06PM - 3:18PM |
X20.00002: Thermoelectric performance of Si-Ge heterostructured nanowires from first-principles Arash Mostofi, Matthew Shelley We present calculations of the thermoelectric figure of merit $ZT$ of both pristine and axially heterostructured Si/Ge nanowires as a function of their compositional disorder, growth direction and diameter. Our method is based on density-functional theory (DFT). Both charge and transport properties are calculated within the Landauer-Buttiker formalism. We compute $ZT$ for realistic nanowires (ca.~10,000 atoms and 100~nm in length) by using maximally-localized Wannier functions to map large-scale DFT calculations onto short-ranged model Hamiltonians with negligible loss of accuracy. The approach is fully automated and robust, such that large numbers of configurations of the system can be explored with high throughput and efficiency. While we focus here on their application to thermoelectric nanowires, the algorithms we have developed are generally applicable to other classes of disordered quasi-one-dimensional nanostructures such as DNA, carbon nanotubes and graphene nanoribbons. [Preview Abstract] |
Thursday, March 24, 2011 3:18PM - 3:30PM |
X20.00003: Thermoelectric Properties of Ultra Narrow Silicon Nanowires from Atomistic Calculations Neophytou Neophytos, Hans Kosina The progress in nanomaterials' synthesis allows the realization of thermoelectric devices based on 1D nanowires (NWs). In these confined systems the electrical and thermal conductivities, and the Seebeck coefficient can be designed to some degree independently, providing enhanced ZT values as compared to the bulk material's value. We calculate the electrical conductivity, the Seebeck coefficient, and the electronic part of the thermal conductivity of scaled Si NWs. We use the atomistic sp3d5s*-spin-orbit-coupled tight-binding model and linearized Boltzmann transport. Our calculations include up to 5500 atoms, a task still computationally affordable within this model. We examine n-type and p-type NWs of diameters between 3nm and 12nm for [100], [110] and [111] transport orientations, at different doping levels. Using experimentally measured values for the lattice thermal conductivity, the expected ZT values of the nanowires are estimated. We further provide directions for power factor optimization with structure confinement. [Preview Abstract] |
Thursday, March 24, 2011 3:30PM - 3:42PM |
X20.00004: Interface scattering and thermal conductivity in Si/SiGe alloy superlattices Zlatan Aksamija, Irena Knezevic $Si/Si_{1-x}Ge_{x}$ alloy superlattices (SLs) show promise for application as efficient thermoelectrics because of their low thermal conductivity, below that of the bulk $Si_{1-x}Ge_{x}$ alloy. Lattice thermal conductivity in these superlattices is dominated by scattering from the rough interfaces between layers, even at room temperature. Therefore, interface properties, such as roughness, orientation, and composition, are expected to play a significant role in thermal transport and offer additional degrees of freedom to control the thermal conductivity in semiconductor nanostructures based on superlattices. In this paper, we demonstrate the sensitivity of the lattice thermal conductivity in SLs to the interface properties, using a momentum-dependent model for scattering of phonons from rough material interfaces. Our results show excellent agreement with experimental data and explain the measured thickness and temperature dependence, as well as anisotropy of thermal conductivity in superlattices. [Preview Abstract] |
Thursday, March 24, 2011 3:42PM - 3:54PM |
X20.00005: Molecular dynamics simulation of the thermal transport across Si/Al interfaces Woon Ih Choi, Kwiseon Kim, Sreekant Narumanchi Efficient heat dissipation is critical for power electronics where the device package consists of several layers of different materials. Conventional thermal interface materials are bottlenecks in heat removal. Detailed understanding of interfacial heat resistance would benefit efforts to improve the device design. We have chosen Si/Al interfaces for this thermal transport study. We construct Si-Al MEAM interatomic potential parameters based on the density functional theory (DFT) calculations. We generate various interface structures using the first-principles molecular dynamics (MD) simulations. Using the direct method to compute the thermal conductance, we investigated various interface structures. We will discuss the effect of the inter-diffused layers and roughness of the interfaces on the thermal boundary conductance. We will also compare our result with limited data in the literature. [Preview Abstract] |
Thursday, March 24, 2011 3:54PM - 4:06PM |
X20.00006: Thermal Boundary Resistance at Ideal Gas Solid-Fluid Interfaces Sanghamitra Neogi, Gerald Mahan We study the thermal boundary resistance at the interface between an ideal gas solid and another ideal gas fluid. In the solid side, heat is mostly carried by phonons, and thermal resistance occurs due to the partial reflection of phonons at the interface. In the fluid side, the sound waves can carry diffusive heat from the interface into the bulk of the liquid. We include both longitudinal and transverse sound modes of the fluid in the theory. The sound modes in the fluid and the reflected phonons in the solid have the same frequency as the phonon incident at the interface from the solid side. The wave vector for the sound modes is then calculated using the knowledge of the fluid pair distribution function in the bulk. The pair distribution function near the interface is modified due to the presence of the solid atoms. We solve coupled equations of motion for the atoms at the interface to obtain the phonon reflection coefficients. The Kapitza resistance is then obtained using the knowledge of these reflection coefficients. The calculation provides a method for extending the Young-Maris theory to the fluid-solid substances. [Preview Abstract] |
Thursday, March 24, 2011 4:06PM - 4:18PM |
X20.00007: Micro to Nano Scale Heat Conduction in Thermoelectric Materials Martin Maldovan Understanding and controlling heat transfer in solids is very important for increasing the efficiency of thermoelectric materials such as skutterudites, clatharates, superlattices, nanowires, and quantum dots. Although the mechanisms governing the thermal conductivity have been understood for years, a comprehensive theoretical method to calculate heat transfer, particularly at small scales, has not been available. This is mainly due to the complexity of anharmonic processes and phonon boundary scattering. We present a comprehensive theoretical model to calculate the thermal conductivity of thermoelectric materials at small length scales. The approach involves an exact calculation of the reduction of the phonon mean free paths due to boundary scattering and removes the need to solve the Boltzmann equation or to use adjustable terms as in the Callaway or Holland models. The analysis is based on the kinetic theory of transport processes and considers general expressions for dispersion relations, phonon mean free paths, and surface specularity parameters. The results show an excellent agreement with experiments for thin films, nanowires, and superlattices over a wide range of temperature and across multiple length scales. The theoretical approach can further be applied to a wide variety of problems involving the conduction of heat in micro/nanostructured thermoelectrics. This research was funded by the MIT Energy Initiative. [Preview Abstract] |
Thursday, March 24, 2011 4:18PM - 4:30PM |
X20.00008: Quantum Thermoelectric Effects on the Nanoscale Justin Bergfield, Charles Stafford An exact expression for the heat current in a nanostructure coupled to multiple metallic electrodes is derived, including both electron-electron and electron-phonon interactions. We use this formalism to investigate quantum effects on the flow of charge and entropy, and find an enormous quantum enhancement of thermoelectric effects in the vicinity of higher-order interferences in the transmission spectrum of a nanoscale junction. A nonequilibrium quantum analysis of a single-molecule junction based on 3,3'-biphenyldithiol demonstrates a maximum operating efficiency of 27\% of the Carnot limit. Nonlocal quantum corrections to thermoelectric transport coefficients in multiterminal geometries are predicted. [Preview Abstract] |
Thursday, March 24, 2011 4:30PM - 4:42PM |
X20.00009: Thermal transport in Si-based disordered systems: amorphous silicon and silicon germanium alloys Yuping He, Ivana Savic, Giulia Galli, Davide Donadio Understanding and modeling heat transport in structurally and mass disordered semiconductors (e.g. amorphous silicon--a-Si and SiGe alloys) have long been a challenging problem in solid state physics. Using a combination of techniques (equilibrium and non- equilibrium molecular dynamics and lattice dynamics), we analyze the nature of vibrations and compute the thermal conductivities (k) of a-Si, bulk and nanoporous SiGe. We find that in amorphous and mass disordered systems, two types of modes are present, phonons and diffusive modes. In a-Si, phonons ( who are only 3 \% of the total vibrations) contribute to approximately half of k [1]. The value of k critically depends on the morphology of the system [2], for example it considerably dereases if thin films or samples with nano-holes are considered. A discussion of how mean free paths and lifetimes change as a function of morphology and disorder will be presented, together with results showing the effect,on k, of disorder at pores or film surfaces.Work supported by grant DOE DE-FC02-06ER25777.\\[4pt] [1] Y.He, D.Donadio and G.Galli (submitted, 2010).\\[0pt] [2] Y. He, D. Donadio, Joo-H. Lee, J. C. Grossman and G. Galli (submitted, 2010) [Preview Abstract] |
Thursday, March 24, 2011 4:42PM - 4:54PM |
X20.00010: Atomistic study of heat transport in SiGe alloys Ivana Savic, Yuping He, Davide Donadio, Giulia Galli Semiconductor alloys, e.g. SiGe, are considered as promising materials to build efficient thermoelectric devices [1], and atomistic modeling of heat transport in these systems may help complement and guide experiments in optimizing their efficiency. We analyze strengths and weaknesses of several atomistic approaches in modeling the thermal conductivity of SiGe alloys, and we analyze in detail their range of validity. In particular, we focus on equilibrium molecular dynamics [2], an approach based on the solution of the Boltzmann transport equation [3] and Green function techniques [4]. Applications to both bulk and nanostructured SiGe will be presented. \\[4pt] [1] A. J. Minnich, M. S. Dresselhaus, Z. F. Ren, and G. Chen, Energy Environ. Sci. 2, 466 (2009). [2] See e.g. D. Donadio and G. Galli, Phys. Rev. Lett. 102, 195901 (2009); Nano Lett. 10, 847 (2010). [3] See e.g. J. E. Turney, E. S. Landry, A. J. J. McGaughey, and C. H. Amon, Phys. Rev. B, 79, 064301 (2009). [4] See e.g. I. Savic, N. Mingo, and D. A. Stewart, Phys. Rev. Lett. 101, 165502 (2008). [Preview Abstract] |
Thursday, March 24, 2011 4:54PM - 5:06PM |
X20.00011: Scattering of charge carriers and phonons in thermoelectric devices Giuseppe Romano, Lee Joo-Hyoung, Jeffrey Grossman We investigate the effects of the scattering of charge carriers and phonons on the figure of merit of thermoelectric devices. Despite many efforts devoted to the optimization of the figure of merit ZT, the commercial diffusion of such systems is still limited due to their low efficiency. The main problem behind the engineering of ZT is the interdependency between the Seebeck coefficients, electrical conductivity and thermal conductivity. ZT could be maximized by either increasing the Seebeck coefficient or decreasing the thermal conductivity. While the first approach involves the distortion of the electronic density of states [1], the thermal conductivity can be lowered by inserting nonporous in the bulk materials [2]. Recent works have shown a detailed comparison between np-Ge and np-Si material and investigated the effect of the porosity on ZT [3]. Here we couple classical molecular dynamics and continuous simulation to study the phonon-phonon, phonon-pore, electron-phonon and phonon-boundary scattering and their effects on the electrical and thermal conductivities. The knowledge gained about material properties is then used to perform simulations of thermoelectric devices. \\[4pt] [1] PRL \textbf{104}, 016602 (2010)\\[0pt] [2] PRB \textbf{80}, 155327 (2009)\\[0pt] [3] APL \textbf{95}, 013106 (2009) [Preview Abstract] |
Thursday, March 24, 2011 5:06PM - 5:18PM |
X20.00012: Many-body effects in frequency-dependent charge and thermal transport Jesus Cruz, James Freericks Recently, Shastry has proposed that thermoelectric properties (thermopower, Lorenz number, and figure of merit) can be determined accurately in strongly correlated materials by examining their high frequency behavior. He also has derived a sum rule similar to the f-sum rule in optical conductivity, for the frequency dependent thermal conductivity. We examine these ideas within the context of an exactly solvable model (the Falicov-Kimball model) with dynamical mean-field theory. We see that the low-frequency and high-frequency limits are not so close in this system. We also discuss the thermal conductivity sum rule. These results are important in trying to understand strong electron correlation effects in thermoelectrics. [Preview Abstract] |
Thursday, March 24, 2011 5:18PM - 5:30PM |
X20.00013: Effective medium theory for thermoelectrics Paul Haney We report on the application of effective medium theory to binary compound thermoelectric materials. We find a range of parameters for the conductivity and thermopower of the constituent elements such that the compound has an enhanced power factor. The results of effective medium theory are compared to full numerical simulations of an ensemble of disordered systems, and good qualitative agreement is found between the two calculations. The effect of various tailored geometries are explored in the direct numerical solution of the compound thermoelectrics. [Preview Abstract] |
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