Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session D28: Focus Session: Thermoelectric Materials: Oxides and Complex Crystals |
Hide Abstracts |
Sponsoring Units: DMP FIAP Chair: Ali Shakouri, University of California, Santa Cruz Room: 330 |
Monday, March 16, 2009 2:30PM - 3:06PM |
D28.00001: Thermoelectricity in oxides and weakly coupled single molecules Invited Speaker: Complex oxides have emerged as promising candidate materials for thermoelectric and energy applications. The study of charge and heat transport in these systems is also very interesting and important from the point of view of fundamental physics. We show that oxides in the narrow-bandwidth limit have high values of the thermopower and power factor and also violate the Wiedemann-Franz law yielding high values of the electronic part of the figure of merit. These theoretical results agree with the data on Na$_x$CoO$_2$. We argue that in another oxide Sr$_ {1-x}$La$_x$TiO$_{3-\delta}$, a large effective mass (small bandwidth) band appears due to oxygen vacancies, which can be exploited for thermoelectric applications and comment on recent experiments. Finally, we show that there are commonalities in the thermoelectric behavior of narrow bandwidth oxides and weakly coupled single molecules. The latter systems also offer promise as thermolectric materials due to the possibility of large values of thermopower and we comment on the limiting effect of phonons on their figures of merit. [Preview Abstract] |
Monday, March 16, 2009 3:06PM - 3:18PM |
D28.00002: Magneto-Spectroscopic Measurements of Ca$_{3}$Co$_{4}$O$_{9}$ Thin Films and Single Crystals Jiufeng Tu, Dimitar Dimitrov, Weidong Si, Qiang Li In recent years, the 2D-layered cobaltates have emerged as promising p-type thermoelectric materials due to their unique combinations of high thermo-coefficient and good metallic transport properties at ambient temperatures. These systems show high thermoelectric figure of merit and are ideal candidates as the materials of choice at elevated temperatures. We have carried out far-infrared magneto- spectroscopic studies of Ca$_{3}$Co$_{4}$O$_{9}$ thin films as a function of frequency, magnetic field and temperature with the emphasis on the coupling between the lattice, the charge and the spin degrees of freedom. The spectral response is different with magnetic field perpendicular or parallel to the CoO$_{2}$ layers. Below 20K, hysterisis occurs for perpendicular field but not for parallel field. This indicates that the negative magneto-resistance is due to reduced magnetic scattering when Co spins become aligned. Our results are consistent with co-existence of two types of carriers. [Preview Abstract] |
Monday, March 16, 2009 3:18PM - 3:30PM |
D28.00003: Direct measurement of charge transfer and spin stat transitions in thermoelectric Ca$_{3}$Co$_{4}$O$_{9}$ Guang Yang, Quentin Ramasse, Robert Klie The misfit-layered thermoelectric material Ca$_{3}$Co$_{4}$O$_{9}$ has been the focus of many recent studies due to its high thermal power and good high temperature stability. In particular, it has been suggested that the presence of a mixed valence state in the strongly correlated CoO$_{2}$ layer is essential for the high p-type thermoelectric properties in Ca$_{3}$Co$_{4}$O$_{9.}$ In this study, we combine aberration-corrected scanning transmission electron microscopy (STEM) with electron energy loss spectroscopy (EELS) to study the atomic and electronic structures of Ca$_{3}$Co$_{4}$O$_{9}$. We will show that the position of the O atomic columns in the CoO$_{2}$ layers are highly ordered and can therefore be directly imaged, while the CoO columns in the Ca$_{2}$CoO$_{3}$ rocksalt layer exhibit a strong modulation in the (010) direction. Further, we measure the local Co valence and find significant hole transfer from the rocksalt CoO to the strongly correlated CoO$_{2}$ layers. In addition, we will present the results of our in-situ heating experiments of Ca$_{3}$Co$_{4}$O$_{9}$ [010] at 500 K, which show that the phase transition at 420 K is not accompanied by a structural transition but rather a transition of the Co-ion spin states. [Preview Abstract] |
Monday, March 16, 2009 3:30PM - 3:42PM |
D28.00004: Thermoelectric properties of doubly doped Strontium Titanate thin films Jayakanth Ravichandran, Matthew L. Scullin, Subroto Mukerjee, Joel Moore, R. Ramesh, Arun Majumdar Lanthanum doped Strontium Titanate (SrTiO$_{3})$ is amongst the most promising n-type thermoelectric materials for power generation. We report a double doping method for thin films of SrTiO$_{3}$, grown on (001) oriented LSAT substrates by Pulsed Laser Deposition (PLD), where doping of SrTiO$_{3}$ in the A-site by Lanthanum is accompanied by doping with oxygen vacancies. Based on careful transport measurements, we show that it is possible to obtain enhanced thermoelectric power factor in the limit of high effective mass and large carrier concentration in these thin films. The presence of oxygen vacancies also serves to decrease the thermal conductivity due to effective phonon scattering. The optimized doping concentration leads to a thermoelectric figure of merit, zT $>$ 0.2 at room temperature. [Preview Abstract] |
Monday, March 16, 2009 3:42PM - 3:54PM |
D28.00005: Thermoelectric Properties and Band Structure Calculations of Novel Boron Network Compounds Takao Mori, Toshiyuki Nishimura, Yuri Grin, Toetsu Shishido, Kazuo Nakajima Boron is an interesting element, tending to form atomic networks such as 2D atomic nets and clusters, with some analogy to carbon systems which have been more extensively studied. Boron has one less electron than carbon and thus is electron deficient when forming atomic networks, but this causes it to have a special affinity with the rare earth elements and as a result, many new compounds have recently been discovered [1]. Their potential as viable thermoelectric materials is attracting interest since they are high-temperature materials and possess intrinsic low thermal conductivity, with some compounds exhibiting Seebeck coefficients in excess of 200 $\rm \mu$V/K above 1000 K. The thermoelectric properties and band structure calculations of novel borides such as RB$_{44}$Si$_{2}$, RB$_{17}$CN, RB$_{22}$C$_{2}$N, RB$_{28.5}$C$_{4}$ will be presented. Features in the band structure near the Fermi level indicate large doping effects in these compounds. Various doping experiments were carried out resulting in large increases to the figure of merit. [1] T. Mori, ``Higher Borides,'' in: {\it Handbook on the Physics and Chemistry of Rare Earths}, Vol. 38, (North-Holland, Amsterdam, 2008) p. 105-173. [Preview Abstract] |
Monday, March 16, 2009 3:54PM - 4:06PM |
D28.00006: Electronic structure and high-temperature properties of doped Hf$_{0.5}$Zr$_{0.5}$CoSb phases Jack Simonson, Slade Culp, S. Joseph Poon, Vijayabharathi Ponnambalam, Justine Edwards, Terry Tritt Half-Heusler alloys with compositions of the form Hf$_{0.50}$Zr$_{0.50}$CoSb were synthesized with Mn substituted to one or both of the Hf/Zr and Co sites or with the Sb site doped with Sn. The thermoelectric properties were evaluated from 300 K to 1000 K. The introduction of Mn was performed to investigate modifications to the band structure near the Fermi energy caused by transition metal substitution. Mn substitutions were discovered to increase the electrical resistivity dramatically while having no beneficial impact upon the thermopower. The Sb-doped alloys, on the other hand, exhibited lowered resistivity and thus increased efficiency of high temperature power generation. The results of both substitutions will be discussed in light of recent first-principles electronic structure calculations. In the Sb-doped alloys, ZT was found to reach 0.5 at 1000 K and is projected to increase to 0.6 K at 1100 K, surpassing the industry standard for p-type materials as set by SiGe alloys. [Preview Abstract] |
Monday, March 16, 2009 4:06PM - 4:18PM |
D28.00007: Thermoelectric power generation in ternary skutterudites: a first-principles Wannier-functions' study Dmitri Volja, Marco Fornari, Boris Kozinsky, Nicola Marzari We study from first-principles ternary skutterudites derived from $CoSb_3$, where the pnictogen is substituted with elements from the IVB and VIB groups. We focus on $CoGe_{3/2}S_{3/2}$, $CoGe_{3/2}Te_{3/2}$ and $CoSn_ {3/2}Te_{3/2}$, and compute the structure, electronic structure and vibrational properties from density-functional and density-functional perturbation theory. Since the direct evaluation of transport quantities in the relaxation-time approximation is computationally demanding, we use maximally-localized Wannier functions (MLWFs) for accurate integrations of operators across the Brillouin zone. This MLWFs basis leads to a very efficient and well-conditioned scheme to calculate the thermoelectric transport coefficients and to disentangle and identify the contribution of single bands. In addition, it provides a detailed, transferable picture of bonding in these complex materials. [Preview Abstract] |
Monday, March 16, 2009 4:18PM - 4:30PM |
D28.00008: Thermoelectric Properties in Nanostructured p-type Skutterudites Xiao Yan, Qing Hao, Jian Yang, Hui Wang, Yucheng Lan, Dezhi Wang, Gang Chen, Zhifeng Ren Skutterudites are good examples of phonon glass electron crystal (PGEC), which is proposed to be one of the most desirable materials to maximize the thermoelectric figure of merit. The skutterudite structure has two voids in each unit cell that are large enough to accommodate a variety of atoms, such as La, Ce, Nd, Sm, Yb, etc. These atomic void-fillers rattle about in their oversized cages, thereby drastically reducing thermal conductivity and maximizing ZT. My work on p-type skutterudites is based on compounds of a general formula RFe$_{3.5 }$Co$_{0.5}$Sb$_{12}$ where R stands for a void filler. Besides the influence of rattling of the void fillers, thermal conductivity can be further depressed by the increased phonon scattering at the increased grain boundaries due to nano size grains. [Preview Abstract] |
Monday, March 16, 2009 4:30PM - 4:42PM |
D28.00009: Calculation of electric field gradients and site preference in Ba-Al-Ge clathrates Sergio Rodriguez, Weiping Gou, Joseph Ross Sn, Ge or Si can form cage-like clathrate structures, many of which exhibit enhanced thermoelectric performance. To understand Al substitutional occupation in Ba$_8$Ge$_{46-x-y}$Al$_x\Box_y$ clathrates we performed NMR lineshape simulations for $3 \leq x \leq 24$, and $0 \leq y \leq 3$, where $\Box$ represents a vacancy. The electric field gradient (EFG) was calculated for Al sites assuming an ordered superstructure. To obtain the EFG we used $ab$ $initio$ methods in the Generalized Gradient Approximation as implemented by the WIEN2k code with structural relaxation. Results where used to simulate NMR lineshapes numerically. These were compared to our previously reported NMR lineshapes. For fully occupied Ba$_{8}$Ge$_{30}$Al$_{16}$ we compare different site occupations, obtaining good agreement and thus information about Al site preferences. For reduced-Al samples, WDS measurements indicate the presence of spontaneous vacancies. In the case of the Zintl phase Ba$_{8}$Ge$_{33}$Al$_{12}\Box_1$ we found that Al sites adjacent to the vacancy exhibit a large EFG, while those with the vacancy further away have smaller EFG's. Assuming a larger Knight shift for sites next to vacancies, we obtain good agreement with NMR experimental results for reduced-Al Ba$_{8}$Ge$_{34}$Al$_{12}$. We infer that Al prefers locations close to vacancies rather than random occupation. Supported by Robert A. Welch Foundation (Grant A-1526). [Preview Abstract] |
Monday, March 16, 2009 4:42PM - 4:54PM |
D28.00010: The effects of silicon on the electronic properties of the clathrates A$_{8}$Ga$_{16}$Si$_{x}$Ge$_{30-x}$ (A= Ba, Sr) Emmanuel Nenghabi, Charles Myles We have studied the structural and electronic properties of the Ba and Sr guest-containing type-I semiconductor clathrates Ba$_{8}$Ga$_{16}$Si$_{x}$Ge$_{30-x}$ and Sr$_{8}$Ga$_{16}$Si$_{x}$Ge$_{30-x}$ for Si compositions x = 0, 5 and 15. Our calculations are based on the generalized gradient approximation (GGA). Starting with stable structures of the Ge-based Type I clathrate semiconductors Ba$_{8}$Ga$_{16}$Ge$_{30}$ and Sr$_{8}$Ga$_{16}$Ge$_{30}$ containing no Ga-Ga bonds, we have constructed unit cells of Ba$_{8}$Ga$_{16}$Si$_{x}$Ge$_{30-x}$ and Sr$_{8}$Ga$_{16}$Si$_{x}$Ge$_{30-x}$ by replacing appropriate numbers of the framework Ge atoms with Si. For the values of x that we have considered, we find that the fundamental band gap of Ba$_{8}$Ga$_{16}$Si$_{x}$Ge$_{30-x}$ decreases with increasing x but that the band gap of Sr$_{8}$Ga$_{16}$Si$_{x}$Ge$_{30-x }$ increases with increasing x. We also find that several electronic states near the top of the valence band and near the bottom of the conduction band in both materials are modified by the Si p states. The trends in the structural and electronic properties of these materials as x is varied are discussed, and our results are compared to experiment where possible. [Preview Abstract] |
Monday, March 16, 2009 4:54PM - 5:06PM |
D28.00011: Effects of silicon on the vibrational and thermal properties of the clathrates A$_{8}$Ga$_{16}$Si$_{x}$Ge$_{30-x}$ (A= Ba, Sr) Charles Myles, Emmanuel Nenghabi Using the GGA, we have calculated the vibrational and thermal properties of the clathrates Ba$_{8}$Ga$_{16}$Si$_{x}$Ge$_{30-x}$ and Sr$_{8}$Ga$_{16}$Si$_{x}$Ge$_{30-x}$ for x = 0, 5 and 15. We find that the Ba and Sr guests have localized vibrational modes lying below 80 cm$^{-1}$, which tend to reduce the host acoustic bandwidth. We predict that there is an upshift in the framework optic modes as x increases and that the guest-atom Einstein temperatures vary with x. Our predicted isotropic atomic displacement parameters as functions of temperature for the Ba and Sr guests in these clathrates show that Sr has a larger isotropic displacement parameter than Ba, suggesting that Sr should be more efficient than Ba in lowering the thermal conductivity. We also predict the temperature dependences of the vibrational specific heat, the entropy, and the Helmholtz free energy in these materials. We find that the specific heat increases smoothly with temperature and approaches the Dulong-Petit value at room temperature. We also find that there is a slight x dependence of the heat capacity, free energy, and vibrational entropy. [Preview Abstract] |
Monday, March 16, 2009 5:06PM - 5:18PM |
D28.00012: Large Thermoelectric power factor in CrN Camilo Quintela, Francisco Rivadulla, Jose Rivas We report the electrical resistivitiy and thermoelectric power of stoichiometric and hole-doped chromium nitride (CrN). The results indicate a considerably large power factor, of the order of 1(microW/cmK2) at 400 K, increasing with temperature. Hall effect measurements were used to elucidate the mechanism of electronic transport in this system, in order to optimize its properties. The easy of grow in the form of nanoparticles and thin films, along with a good thermal stability up to 700 K, could make this material interesting for applications at moderate temperatures. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700