Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session Y20: Focus Session: Thermoelectric Materials: Chalcogenides and 1D/2D Systems |
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Sponsoring Units: DMP FIAP GERA Chair: Bhanu Mahanti, Michigan State University Room: D168 |
Friday, March 25, 2011 8:00AM - 8:36AM |
Y20.00001: Resonant Energy Levels and the Thermoelectric Figure of Merit Invited Speaker: Distortions of the electronic density of states are a potent mechanism to increase the thermopower and ZT of thermoelectric semiconductors. Band-structure engineering approaches will be reviewed that can be used to do this, namely quantum size effects, hybridization effects in strongly correlated electron systems, and resonant impurity levels. The properties of known resonant impurities for PbTe, SnTe, Bi$_{2}$Te$_{3}$ and GaSb will also be reviewed. They can increase the thermoelectric power through 2 mechanisms, (1) the increase in density of states, and (2) resonant scattering. The first increases the thermopower in a nearly temperature-independent way; the second results in an electron energy filtering effect that increases the thermopower, but only at cryogenic temperatures where the electron-phonon interactions are weaker. An analysis of the thermomagnetic tensor components makes it possible to dissociate the two contributions experimentally. [Preview Abstract] |
Friday, March 25, 2011 8:36AM - 8:48AM |
Y20.00002: The Effect of Sintering on the Thermoelectric Properties of Chemically Synthesized Nano-Bulk Bi$_{2-x}$Sb$_{x}$Te$_{3}$ J.S. Dyck, S. Dorroh, B.D. Mao, J.W. Wang, C. Burda Considerable research effort has gone into improving the performance of traditional thermoelectric (TE) materials such as Bi$_{2-x}$Sb$_{x}$Te$_{3}$ through a variety of nanostructuring approaches. Bottom-up, chemical approaches have the potential of producing very small nanoparticles ($<$ 50 nm) with narrow size distributions and controlled shape. For this study, nanocrystalline powder of Bi$_{2-x}$Sb$_{x}$Te$_{3}$ with x = 0 -- 1.5 has been synthesized using a ligand- assisted chemical method, and consolidated into bulk pellets with cold pressing followed by sintering. These materials have the interesting property that a wide range of carrier concentrations are accessible through different Bi/Sb ratios, with low values of x being n-type and higher values becoming p-type. In this work, we present the thermoelectric transport measurements from 6 -- 300 K as a function of sintering temperature, and a beneficial effect is found. The samples are also characterized by Hall effect, XRD, and compositional analysis. We will present results on the structure-property relations, and discuss strategies for optimization of this class of TE materials for high performance. [Preview Abstract] |
Friday, March 25, 2011 8:48AM - 9:00AM |
Y20.00003: Coherent Phase Stability of IV-VI Rocksalt Semiconductor Alloys Jeffrey Doak, Chris Wolverton The creation of nanoscale precipitates via phase separation provides a mechanism for decreasing the lattice thermal conductivity of some bulk thermoelectric materials. The IV-VI semiconductor alloy systems may phase separate by either a spinodal decomposition or nucleation and growth mechanism. To better understand these phase transformations, we use first-principles density functional theory (DFT) calculations to investigate the coherent and incoherent phase stability of a series of IV-VI rocksalt semiconductor alloys (IV=Pb, Sn, Ge and VI = S, Se, Te). We use mixing enthalpies derived from calculations of special quasirandom structures (SQS), along with coherency strain energies to model the thermodynamic driving forces for incoherent and coherent phase separation. By fitting these inputs to a sub-regular mixing enthalpy model and including an ideal mixing entropy term, we calculate incoherent and coherent phase diagrams. We show the incorporation of coherency strain energies cause large depressions of the coherent spinodals for each system. The depressions are large enough that at realistic processing temperatures, the dominant precipitation mechanism of phase separation is nucleation and growth. [Preview Abstract] |
Friday, March 25, 2011 9:00AM - 9:12AM |
Y20.00004: Electronic structure of PbTe doped with K and Na Mal-Soon Lee, S. D. Mahanti PbTe is a well-known thermoelectric which shows excellent thermoelectric performance (for both {\it p}- and {\it n}-type) in the temperature range between ambient and 600$^{\circ}$C. Thermopower (S) of PbTe can be enhanced with proper doping. Hermann {\it et al}. have found the figure of merit ZT=1.5 at 773~K with 2~\% Tl doping in PbTe. They ascribe this to the enhancement of the density of states (DOS) caused by Tl-induced resonance level in the valence band. This is in agreement with the {\it ab initio} studies of Ahmad {\it et al}., who also found an enhanced DOS associated with K defects in PbTe. Recently Androulakis {\it et al}. have looked for resonant states in the valence band associated with Na/K impurities in PbTe. Although they observe an increase in power factor at high temperature, they do not find any evidence of resonant states. We have reexamined this issue by carrying out detailed band structure calculations in the presence of K and Na defects in PbTe using 64-atom supercells. The question of the existence of resonant states and the origin of the enhanced DOS near the valence band maximum will be discussed. [Preview Abstract] |
Friday, March 25, 2011 9:12AM - 9:24AM |
Y20.00005: Ho Doped $\mathbf{Bi_xSb_y}$ Nanopolycrystalline Alloys K.C. Lukas, G. Joshi, Dezhi Wang, Z.F. Ren, C.P. Opeil Department of Physics, Boston College, Chestnut Hill, Massachusetts, 02467. Bismuth-Antimony alloys have been shown to have high ZT values below room temperature, especially for single crystals. For polycrystalline samples, impurity doping and magnetic field have proven to be powerful tools in the search for understanding and improving thermoelectric performance. Nanopolycrystalline $\mathrm{Bi_xSb_y}$ doped with 1 and 3$\%$ Ho were prepared by ball milling and dc hot pressing technique. Electrical resistivity, Seebeck coefficient, thermal conductivity, carrier concentration, mobility, and magnetization are measured in a temperature range of 5-350 K and in magnetic fields up to 9 Tesla. The effects of Ho doping on the thermoelectric properties of $\mathrm{Bi_xSb_y}$ in magnetic field will be discussed. [Preview Abstract] |
Friday, March 25, 2011 9:24AM - 9:36AM |
Y20.00006: Thermoelectric properties of Quintuple Layer Bi$_2$Te$_3$ Roger Lake, Ferdows Zahid Motivated by recent experimental results,\footnote{D. Teweldebrhan, V. Goyal and A. A. Balandin, Nano Lett. 10, 1209 (2010); D. Teweldebrhan, V. Goyal, M. Rahman, and A. A. Balandin, Appl. Phys. Lett. 96, 053107 (2010); Y. Zhang et al., Nat. Phys. 6, 584 (2010). } we derive the thermoelectric parameters of a Bi$_{2}$Te$_{3}$ film of one quintuple layer thickness. Our results show approximately ten times increase in the figure of merit (ZT) for the thin film (ZT = 7.2) compared to that for the bulk (ZT = 0.68). The large enhancement in ZT results from the change in the distribution of the valence band density of modes brought about by the quantum confinement in the thin film. Our theoretical model uses ab initio electronic structure calculations as implemented in the VASP software package combined with a Landauer approach for calculating the linear-response transport coefficients. We employ two fitting parameters: a rigid shift of the conduction and valence bands to match the known bulk bandgap (i.e. a `scissors operator'), and an energy independent electron mean free path for the phonon scattering inside the device. With these two fitting parameters, our results show excellent agreement with the known experimental values for bulk Bi$_{2} $Te$_{3}$. [Preview Abstract] |
Friday, March 25, 2011 9:36AM - 9:48AM |
Y20.00007: Phase diagram of thermoelectric Bi2S3-Bi2Se3-Bi2Te3 system Weishu Liu, Qinyong Zhang, Qian Zhang, Bo Yu, Gang Chen, Zhifeng Ren It is well known that the highest ZT value, at an optimized carrier concentration, is mainly determined by a material parameter $\beta =\mu $(m*/m0)3/2/$\kappa $lat, where $\mu $(m*/m0)3/2 and $\kappa $lat are the weighted carrier mobility and lattice thermal conductivity, respectively. In order to explore some new compositions in Bi2S3-Bi2Se3-Bi2Te3 system, we propose a compositional thermoelectric phase diagram (TPD), including weighted carrier mobility, lattice thermal conductivity, and material parameter, for the 1{\%} copper doped Bi2S3-Bi2Se3-Bi2Te3 solid solution fabricated by MA-HP method. Here, the $\mu $(m*/m0)3/2 and $\kappa $lat values could be deduced from the measured electrical resistivity, Seebeck coefficient, and thermal conductivity. The alloying effect on the thermoelectric phase diagram will be discussed from varying atomic size, chemical bond, lattice structure, etc. [Preview Abstract] |
Friday, March 25, 2011 9:48AM - 10:00AM |
Y20.00008: Exploration of effects of various impurities in Bismuth and its extension to Bismuth-Antimony alloys Hyungyu Jin, Katherine Whitehouse, Joseph Heremans While Te and Se are known donors and Pb and Sn known acceptors in elemental bismuth, little is known about other possible dopants. The effect of various impurities on thermoelectric properties of elemental bismuth is investigated here. Impurities investigated encompass the transition metals, group III and IV elements, and the chalcogens. The thermoelectric power, electrical resistivity and Hall coefficients of Bi samples doped with these impurities are measured from room temperature to 2K. Indium is found to be an acceptor, which is surprising because it is mostly trivalent. A calculation of the band structure subsequently performed at the AGH University of Science and Technology in Cracow reveals that In gives an excess density of states in the valence band. This finding in elemental Bi is extended to the case of bismuth-antimony alloys which have superior thermoelectric efficiency at cryogenic temperatures. [Preview Abstract] |
Friday, March 25, 2011 10:00AM - 10:12AM |
Y20.00009: Thermoelectric properties of Sn1-xEuxTe Yibin Gao, Joseph Heremans SnTe has potential in thermoelectric application for intermediate temperature [1]. However, the figure of merit ZT of SnTe is limited because that it always has a high hole concentration owing to Sn vacancies. As a result, the Seebeck coefficient of SnTe is low and it is very difficult to get SnTe to the optimized doping level required to get a good figure of merit. SnTe also has heavy valence band close to the light valence band edge. We know from theoretical calculations that degenerate bands are preferable than bands separated by an energy difference for thermoelectric application. EuTe has a much higher band gap than SnTe. Recent results [2] show that in Sn1-xEuxTe films prepared by hot-wall epitaxy, the direct L-point bandgap first closes with x, and then opens. In this presentation, we report on the synthesis of bulk Sn1-xEuxTe samples, and report on their Seebeck coefficient, Hall coefficient, resistivity and thermal conductivity. A simplified model is proposed to explain the experimental data. The results confirm the results of the previous study, and point towards the possibility of finding a high-ZT formulation in these compounds. The work is supported by ZTPlus. \\[4pt] [1] V. P. Vedeneev et al., Semiconductors, 32, 241 (1998) \\[0pt] [2] Akihiro Ishida et al., J. Appl. Phys. 107, 123708 (2010) [Preview Abstract] |
Friday, March 25, 2011 10:12AM - 10:24AM |
Y20.00010: Incipient Ferroelectricity in Thermoelectric Lead Telluride Emil Bozin, Christos Malliakas, Petros Souvatzis, Thomas Proffen, Nicola Spaldin, Mercouri Kanatzidis, Simon Billinge PbTe, is the parent compound of currently the most important thermoelectric (TE) materials in applications just above room temperature [1]. It has an anomalously low thermal conductivity resulting in a rather high TE figure of merit. Our neutron total scattering and atomic pair distribution function analysis shows the existence of a novel paraelectric state at and above room temperature. However, on cooling the structural dipoles do not order, but disappear resulting in an undistorted rock-salt ground-state. We suggest that new thermoelectrics should be sought among materials that, like PbTe [2], are close to a ferroelectric instability.\\[4pt] [1] Z.H. Dughaish, Physica B v.322, pp205 (2002).\\[0pt] [2] E.S. Bozin et al, Science (to be published). [Preview Abstract] |
Friday, March 25, 2011 10:24AM - 10:36AM |
Y20.00011: Enhancement of thermoelectric figure-of-merit in a wide temperature range in In$_4$Se$_{3-x}$Cl$_y$ bulk crystals Jong-Soo Rhyee, Kyunghan Ahn, Kyu Hyoung Lee, Ji Hoon Shim, Jae Hoon Kim Recently, we proposed that the charge density wave is a new pathway for high thermoelectric performance in bulk crystalline materials [1,2]. Through the quasi one-dimensional lattice distortion (Peierls distortion) in In$_4$Se$_{3-x}$ bulk single crystals, we have achieved a high thermoelectric figure-of-merit {\it ZT} of 1.48 at 705 K. From the Boltzman transport calculation, it was confirmed that the reported {\it ZT} could be further increased if we could increase the chemical potential of the In$_4$Se$_{3-x}$ crystals. Here we report the significant increase of {\it ZT} over a wide temperature range from 50 $^{\circ}$C to 425 $^{\circ}$C by chlorine doping in the In$_4$Se$_{3-x}$, which comes from the improvement of crystal quality and increase of chemical potential, resulting in the power factor enhancement and the thermal conductivity reduction. \\[4pt] [1] J.-S. Rhyee et al., J. Appl. Phys. {\bf 105}, 053712 (2009). \\[0pt] [2] Jong-Soo Rhyee et al., Nature (London) {\bf 459}, 965 (2009). [Preview Abstract] |
Friday, March 25, 2011 10:36AM - 10:48AM |
Y20.00012: Thermoelectricity in the ultra-thin limit Jayakanth Ravichandran, Pim Rossen, Vincent Wu, Arun Majumdar, R. Ramesh Hicks and Dresselhaus [1] predicted an enhanced thermoelectric power factor due to quantum confinement. In the past, superlattices have been employed to demonstrate this effect but the results have remained controversial. Sustained efforts on surface termination and treatment of single crystalline oxide substrates has enabled growth of high quality thin films using techniques like pulsed laser deposition and molecular beam epitaxy. In this work, we explore the nature of thermoelectric response for ultra thin layers ($\sim $ 1 -- 100 nm) of model thermoelectric oxides such as doped SrTiO$_{3}$ and Bi$_{2}$Sr$_{2}$Co$_{2}$O$_{y }$grown by pulsed laser deposition. Thermopower, resistivity and Hall measurements were carried out as a function of thickness to understand the role of quantum confinement and other extraneous effects like surface depletion etc. on the thermoelectric response. References: [1] L.D. Hicks and M. S. Dresselhaus, Phys. Rev. B, 47, 12727 (1993). [Preview Abstract] |
Friday, March 25, 2011 10:48AM - 11:00AM |
Y20.00013: Zero-dimensional nanostructured material with metallic bismuth nanoparticles: a new route for thermoelectrics Roland Benoit, Mona Treguer, Marie-Louise Saboungi The thermoelectric figure of merit ZT has so far not exceeded the value ZT=3 need to compete with mechanical energy conversion systems. However, theoretical work has shown that it is possible to reach values of ZT higher than this. One of the most promising routes is nanostructured materials, which offer the opportunity to tailor physical properties such as electrical and heat transport, due to the effects of electron filtering and phonon confinement. Dresselhaus \textit{et al. (ref.?)} were among the first to show that 2D and 1D structures are capable of reaching ZT values higher than 2. The thermoelectric materials of current interest are in the form of nanotubes, nanodots and, more generally, superlattices composed of a matrix and nanoparticles. In our work we synthesize a periodic network of bismuth nanoparticles in a matrix of mesoporous SiO$_{2}$. We find that in this form bismuth transforms from a rhombohedral to a cubic structure, with improved filtering of electrons and phonons. [Preview Abstract] |
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