Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session R23: Thermoelectrics-NanostructuresFocus
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Sponsoring Units: DMP GERA DCMP Chair: Matthew Grayson, Northwestern University Room: 322 |
Thursday, March 17, 2016 8:00AM - 8:12AM |
R23.00001: Thermoelectric properties of Mg$_{\mathrm{2}}$(Ge, Sn) solid solutions Jifeng Sun, David J. Singh Intermetallic compounds Mg$_{\mathrm{2}}$X (X $=$ Si, Ge, Sn) and their solid solutions have attracted much attention as they are composed of environmental friendly and naturally abundant constituent elements and can be promising thermoelectric materials at intermediate temperature range (500 K - 1000 K). The figure of merit (ZT) of n-type Mg$_{\mathrm{2}}$X solid solutions can reach up to 1.5. However, the p-type materials have much lower ZT values up to 0.38. In this talk, we will present the evolution of the thermoelectric properties of Mg$_{\mathrm{2}}$(Ge, Sn) solid solutions with a typical composition of Mg$_{\mathrm{2}}$Ge$_{\mathrm{0.5}}$Sn$_{\mathrm{0.5}}$ using first principles calculations combining with the experimental data. The ZT was optimized with respect to both doping concentrations and operating temperatures. Importantly, at 500 K - 1000 K temperature range we find ZT values up to 2 are possible for optimized n-type material within 10$^{\mathrm{19}}$ cm$^{\mathrm{-3}}$ to 10$^{\mathrm{20}}$ cm$^{\mathrm{-3}}$ carrier concentrations. But the p-type counterparts show inferior performance with ZT values ranging from 0.2 to 0.7. [Preview Abstract] |
Thursday, March 17, 2016 8:12AM - 8:24AM |
R23.00002: \textbf{Ab initio calculations of the vibrational and dielectric properties of PbSnTe alloys} Luisa Scolfaro, A.R. Rezende Neto, H.W. Leite Alves, J.E. Petersen, T.H. Myers, P.D. Borges Thermoelectric devices have promise in dealing with the challenges of the growing demand for alternative clean energy and Te-based materials well-known candidates for them. Recently [1], we have shown that the high values for the dielectric constant, together with anharmonic LA-TO coupling, reduces the lattice thermal conductivity and enhances the electronic conductivity in PbTe. Also, it was shown that by alloying this material with Se, the electronic conductivity of the alloys is also enhanced [2]. But, it is not clear if the same occurs when alloying with Sn. We show, in this work, our ab initio results for the vibrational and dielectric properties of Pb$_{\mathrm{1-x}}$Sn$_{\mathrm{x}}$Te alloys. The calculations were carried out by using the Density Functional Theory, and the alloys were described by both the Virtual Crystal Approximation and Cluster Expansion Method. Our results show that the anharmonic LA-TO coupling enhances and reach its maximum for Sn concentration values of 0.75, corresponding to the maximum value for the dielectric constant, which is higher than that obtained for PbTe. [1] H. W. Leite Alves, et al., Phys. Rev. B87, 115204 (2013). [2] Y. Pei, et al., Nature 473, 66 (2011). [Preview Abstract] |
Thursday, March 17, 2016 8:24AM - 8:36AM |
R23.00003: Band Degeneracy, Low Thermal Conductivity, and High Thermoelectric Figure of Merit in SnTe-CaTe Alloys R. Al Rahal Al Orabi, N. Mecholsky, J. P. Hwang, W. Kim, J. S. Rhyee, D. Wee, M. Fornari Pure lead-free SnTe has limited thermoelectric potentials because of the low Seebeck coeffcients and the relatively large thermal conductivity. In this study, we provide experimental evidence and theoretical understanding that alloying SnTe with Ca greatly improves the transport properties leading to ZT of 1.35 at 873 K, the highest ZT value so far reported for singly doped SnTe materials. The introduction of Ca (0-9{\%}) in SnTe induces multiple effects: (1) Ca replaces Sn and reduces the hole concentration due to Sn vacancies, (2) the energy gap increases limiting the bipolar transport, (3) several bands with larger effective masses become active in transport, and (4) the lattice thermal conductivity is reduced of about 70{\%} due to the contribution of concomitant scattering terms associated with the alloy disorder and the presence of nanoscale precipitates. An effciency of 10{\%} (for $\Delta $T $=$ 400 K) was predicted for high temperature thermoelectric power generation using SnTe-based n- and p-type materials. [Preview Abstract] |
Thursday, March 17, 2016 8:36AM - 9:12AM |
R23.00004: Mode resolved modeling of phonon-structure interactions in semiconductor nanocomposites Invited Speaker: Joseph Feser Introducing nanoscale inhomogeneities into semiconductor alloys is a known route to enhance the scattering of long wavelength phonons and to subsequently reduce thermal conductivity. For key applications such as thermoelectric energy conversion materials, this must be done efficiently to avoid harming electronic functionality. Thus, key questions arise such as what type (i.e. contrast mechanisms), shape, size, and number density of particles should be used. This talk presents two new theoretical developments in this area from our group: (1) The use of continuum mechanics to analytically calculate exact phonon scattering cross sections of cylindrical and spherical shaped elastic discontinuities across the Mie regime, and their subsequent use in Boltzmann transport models of thermal transport and (2) the development of a new frequency-domain atomistic approach to simulate the scattering cross section of nanoparticles of arbitrary complexity for wavevectors spanning the entire Brillouin zone, and which can accommodate very large atomistic systems. Its advantages compared to Atomistic Green's functions and molecular dynamics will be discussed. [Preview Abstract] |
Thursday, March 17, 2016 9:12AM - 9:24AM |
R23.00005: Thermal Conductivity of Nanocrystalline Silicon Prepared by Plasma-Enhanced Chemical-Vapor Deposition Battogtokh Jugdersuren, Xiao Liu, Brian Kearney, Daniel Queen, Thomas Metcalf, James Culbertson, Christopher Chervin, Michael Katz, Rhonda Stroud Nanocrystallization by ball milling has been used successfully to reduce the thermal conductivity of silicon-germanium alloys (SiGe) and turn them into useful thermoelectric materials at a temperature of a few hundred degrees C. Currently the smallest grain sizes in nanocrystalline SiGe are in the 10 nm range. Germanium is added to scatter short wavelength phonons by impurity scattering. In this work, we report a record low thermal conductivity in nanocrystalline silicon prepared by plasma-enhanced chemical-vapor deposition. By varying hydrogen to silane ratio, we can vary the average grain sizes from greater than 10 nm down to 3 nm, as determined by both the high resolution transmission electron microscopy and X-ray diffraction. The values of thermal conductivity, as measured by the 3$\omega $ technique, can be correspondingly modulated from that of ball-milled nanocrystalline SiGe to a record low level of 0.3 W/mK at room temperature. This low thermal conductivity is only about 1/3 of the minimum thermal conductivity limit of silicon. Possible causes of such a large reduction are discussed. [Preview Abstract] |
Thursday, March 17, 2016 9:24AM - 9:36AM |
R23.00006: Thermal Investigations of Periodically Nanoporous Si Films --- The Impact of Structure Sizes and Pore-Edge Amorphization Dongchao Xu, Hongbo Zhao, Qing Hao In recent years, nanoporous Si films have been intensively studied as promising thermoelectric materials, which mainly benefits from their dramatically reduced lattice thermal conductivity $k_{L}$ and bulk-like electrical properties.$^{1,2}$ Despite many encouraging results, challenges still exist in the theoretical explanation of the observed low $k_{L}$.$^{3}$ Existing studies mainly attribute the low $k_{L}$ to 1) phonon bandstructure modification by coherent phonon processes in a periodic structure (phononic effects), and/or 2) pore-edge defects. In this work, temperature-dependent $k_{L}$ is measured for nanoporous Si films with different pore sizes and spacing to compare with model predictions. For systematic studies, two fabrication techniques are used to drill the nanopores: 1) reactive ion etching, and 2) a focus ion beam to introduce more pore-edge defects. The results from this work will provide guidance for phonon engineering in general materials with periodic interfaces or boundaries. References: 1. Tang et al., \textit{Nano Letters} \textbf{10}, 4279-4283 (2010). 2. Yu et al., \textit{Nature Nanotechnology} \textbf{5}, 718-721 (2010). 3. Cahill et al., \textit{Applied Physics Reviews} \textbf{1}, 011305/1-45 (2014) Nanoscale thermal transport. II. 2003--2012. [Preview Abstract] |
Thursday, March 17, 2016 9:36AM - 9:48AM |
R23.00007: Thermoelectric Power of Nanocrystalline Silicon Prepared by Hot-Wire Chemical-Vapor Deposition Brian Kearney, Xiao Liu, Battogtokh Jugdersuren, Daniel Queen, Thomas Metcalf, James Culbertson, Christopher Chervin, Rhonda Stroud, William Nemeth, Qi Wang Although doped bulk silicon possesses a favorable Seebeck coefficient and electrical conductivity, its thermal conductivity is too large for practical thermoelectric applications. Thin film nanocrystalline silicon prepared by hot-wire chemical-vapor deposition (HWCVD) is an established material used in multijunction amorphous silicon solar cells. Its potential in low cost and scalable thermoelectric applications depends on achieving a low thermal conductivity without sacrificing thermoelectric power and electrical conductivity. We examine the thermoelectric power of boron-doped HWCVD nanocrystalline silicon and find that it is comparable to doped nanostructured silicon alloys prepared by other methods. Given the low thermal conductivity and high electrical conductivity of these materials, they can achieve a high thermoelectric figure of merit, ZT. [Preview Abstract] |
Thursday, March 17, 2016 9:48AM - 10:00AM |
R23.00008: \textbf{Anharmonicity Rise the Thermal Conductivity in Amorphous Silicon} Wei Lv, Asegun Henry We recently proposed a new method called Direct Green-Kubo Modal Analysis (GKMA) method, which has been shown to calculate the thermal conductivity (TC) of several amorphous materials accurately. A-F method has been widely used for amorphous materials. However, researchers have found out that it failed on several different materials. The missing component of A-F method is the harmonic approximation and considering only the interactions of modes with similar frequencies, which neglect interactions of modes with large frequency difference. On the contrary, GKMA method, which is based on molecular dynamics, intrinsically includes all types of phonon interactions. In GKMA method, each mode's TC comes from both mode self-correlations (autocorrelations) and mode-mode correlations (crosscorrelations). We have demonstrated that the GKMA predicted TC of a-Si from Tersoff potential is in excellent agreement with one of experimental results. In this work, we will present the GKMA applications on a-Si using multiple potentials and gives us more insight of the effect of anharmonicity on the TC of amorphous silicon. [Preview Abstract] |
Thursday, March 17, 2016 10:00AM - 10:12AM |
R23.00009: Designing SiGe superlattices and alloys for minimum thermal conductivity Jihui Nie, Pawel Keblinski We use equilibrium molecular dynamics simulations to study the thermal conductivity of SiGe alloys to design the structures for minimum thermal conductivity, which is desired, e.g., for better thermoelectric properties or thermal barriers coatings. We explore how a combination of layered/superlattice structures with a degree of random alloying is capable of effective scattering of both low frequency (long wavelength) and high frequency (short wavelength) phonons thus greatly reducing thermal conductivity. We will discuss strategies towards guided search for arrangements of alloy constituents that minimizes thermal conductivity. [Preview Abstract] |
Thursday, March 17, 2016 10:12AM - 10:24AM |
R23.00010: Thermal Conductivity of Quantum Wires with Surface Roughness Selman Hershfield, Khandker Muttalib Quantum wires have been shown to have greatly reduced thermal conductivity compared to bulk systems because of the increased role of surface scattering. The lattice thermal conductance and conductivity is calculated in the harmonic approximation for a long quantum wire placed between two heat baths using the Landauer formula for phonons and a recursive Green function technique to compute the transmission probabilities. The width of the wires is varied in the transverse direction so as to have a root mean square value $\sigma $ and correlation length $L$. As observed experimentally, we find that the thermal conductance is decreased with increasing $\sigma$ and increased as $L$ increases. The full scaling of the thermal conductance as a function of $\sigma$, $L$, the width and the length of the sample is discussed. The simulations are also compared to approximate techniques such as modeling the surfaces as having diffusive scattering. [Preview Abstract] |
Thursday, March 17, 2016 10:24AM - 10:36AM |
R23.00011: Probing the low thermal conductivity of single-crystalline porous Si nanowires Yunshan Zhao Pore-like structures provide a novel way to reduce the thermal conductivity of silicon nanowires, compared to both smooth-surface VLS nanowires and rough EE nanowires. Because of enhanced phonon scattering with interface and decrease in phonon transport path, the porous nanostructures show reduction in thermal conductance by few orders of magnitude. It proves to be extremely challenging to evaluate porosity accurately in an experimental manner and further understand its effect on thermal transport. In this study, we use the newly developed electron-beam based micro-electrothermal device technique to study the porosity dependent thermal conductivity of mesoporous silicon nanowires that have single-crystalline scaffolding. Based on the Casino simulation, the power absorbed by the nanowire, coming from the loss of travelling electron energy, has a linear relationship with it cross section. The relationship has been verified experimentally as well. Monte Carlo simulation is carried out to theoretically predict the thermal conductivity of silicon nanowires with a specific value of porosity. These single-crystalline porous silicon nanowires show extremely low thermal conductivity, even below the amorphous limit. These structures together with our experimental techniques provide a particularly intriguing platform to understand the phonon transport in nanoscale and aid the performance improvement in future nanowires-based devices. [Preview Abstract] |
Thursday, March 17, 2016 10:36AM - 10:48AM |
R23.00012: Effects of Ordered Stacking Faults on Electrical Transport Properties in Silicon Nanowires Marc Collette, Oussama Moutanabbir, Alexandre Champagne Lattice defects in silicon nanowires (SiNWs) allow the exploration of the fundamental physics governing transport mechanisms. We study charge transport in SiNW transistors with stacking faults in the 3C sequence, producing local hexagonal ordering. This structure leads to polytype SiNWs with distinct properties for novel applications in thermoelectronics. Since charge carrier and phonon behavior depend on crystal structure, these planar defects affect the transport properties of the nanowire. We grow our SiNWs using a VLS method, with stacking faults induced during growth. Structural characterization of each SiNW is done with Raman spectroscopy to quantify hexagonality. Individual nanowires are located and contacted using different metals to understand the Schottky barrier of the contacts at the SiNWs. We suspend 2 $\mu$m-long SiNW devices using a wet oxide etch to uncouple the SiNW from the substrate. We study the electrical properties by \textit{I-V} measurements across the FET device while modulating the applied back gate voltage. Our initial data show that the presence of stacking faults causes an increase in resistivity by two orders of magnitude, thus greatly hindering charge transport through the SiNW. [Preview Abstract] |
Thursday, March 17, 2016 10:48AM - 11:00AM |
R23.00013: Electronic structures and related thermoelectric properties of Pb$_{7}$Bi$_{4}$Se$_{13}$ using first principle calculations and Boltzmann transport theory Mohammed Benali Kanoun The electronic structure, optical and thermoelectric properties of Pb$_{7}$Bi$_{4}$Se$_{13}$ have been investigated using a combination of Density functional theory and Boltzmann transport theory. We applied the generalized gradient approximation as exchange-correlation energy functional added to the Coulomb energy (U Hubbard term). The existence of Bi and Pb has required the spin-orbit coupling. The intensity data for Pb$_{7}$Bi$_{4}$Se$_{13}$ were measured at 100 K and 300 K leading to consider Pb$_{7}$Bi$_{4}$Se$_{13}$ in two phases. The valence band maximum emerges predominantly from Se-$p$ state with admixture of Bi$-p$ and Pb$-p$ states, while the conduction band minimum comes from Se-$d$ states. The optical absorption shows the possibility of smaller multiple direct and indirect inter-band transitions in the visible region. We computed Seebeck coefficient, electrical and thermal conductivities, figure of merit and power factor, as function of temperature using the Boltzmann transport theory. Pb$_{7}$Bi$_{4}$Se$_{13}$ is a potential shielding material that can be used at visible and UV region for thermoelectric devices. The present results were validated by comparison with the available experimental measurements. [Preview Abstract] |
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