Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session W20: Focus Session: Thermoelectric Materials: Clathrates and Oxides |
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Sponsoring Units: DMP GERA FIAP Chair: Donald Morelli, Michigan State University Room: D168 |
Thursday, March 24, 2011 11:15AM - 11:27AM |
W20.00001: The Atomic Structure of Ga and Ge in Ba-Ga-(Si,Ge) Clathrate A.N. Mansour, J.B. Martin, W. Wong-Ng, G.S. Nolas Compression studies on Sr$_{8}$Ga$_{16}$Ge$_{30}$ type I clathrate revealed a 3 fold increase in ``ZT'' [J. F. Meng et al., J. Appl. Phys., 89, 1730 (2001)]. Substitution of Si for Ge in Ba-Ga-Ge clathrate could mimic the effect of bulk compression, and subsequently enhance ``ZT''. Recent studies on Si substituted Ba-Ga-Ge clathrate have shown a decrease in the lattice constant and an increase in the power factor with Si substitution. However, the effects of Si on the electronic and local atomic structures of Ga and Ge have not been investigated in detail. We have used XAS to characterize the electronic and local atomic structures of Ga and Ge for a number of samples with varying degree of Si substitution. Analysis of Ga and Ge K-edge XANES spectra revealed that the partial density of $p$-states was modified for both Ga and Ge with Si substitution with the changes being more pronounced in the case of Ga. Comparisons of Fourier transforms of EXAFS spectra revealed that the local structure of Ga is significantly changed with Si substitution while changes in the local structure of Ge with Si substitution are moderate. [Preview Abstract] |
Thursday, March 24, 2011 11:27AM - 11:39AM |
W20.00002: Examining Lattice Disorder in Type-I Clathrate Ba$_{8}$Ga$_{16}$Sn$_{30}$ using EXAFS Scott Medling, Michael Kozina, Frank Bridges, Koichiro Suekuni, Toshiro Takabatake Semiconducting type I clathrates, such as Ba$_{8}$Ga$_{16}$Ge$_{30}$ (BaGaGe) and Ba$_{8}$Ga$_{16}$Sn$_{30}$ (BaGaSn), have a cage-like crystal structure with ``rattler" atoms (Ba) located near the center of cages (Ga-Ge/Sn). Such compounds have a low thermal conductivity which is attributed mainly to vibrations of the ``rattler" atoms inside the cages which strongly scatter phonons. BaGaSn has a surprising lower thermal conductivity than BaGaGe. To better understand why, we studied samples of BaGaSn using Extended X-ray Absorption Fine Structure (EXAFS). The analysis shows that the average Ga-Sn distance is shorter and the average Sn-Sn distance is longer than the distances found from diffraction; also, the Ba-Ga and Ba-Sn distances have greatly increased disorder. This suggests that the cage-like structure is severely distorted, in contrast to BaGaGe; such a large distortion will strongly scatter phonons, decreasing the thermal conductivity, but unfortunately also will reduce the electrical conductivity. We compare our results for BaGaSn with earlier results for BaGaGe and discuss them in light of recent transport measurements. [Preview Abstract] |
Thursday, March 24, 2011 11:39AM - 11:51AM |
W20.00003: Structural, Electronic and Vibrational Properties of Na$_{x}$Si$_{136 }$(0 $<$ x $<$ 24) Clathrates Craig Higgins, Emmanuel Nenghabi, Charles Myles, Koushik Biswas, Matt Beekman, George Nolas CRAIG HIGGINS, EMMANUEL NENGHABI$^{\dag }$, CHARLES W. MYLES, Texas Tech U.; KOUSHIK BISWAS, Oak Ridge National Lab; MATT BEEKMAN, U. of Oregon; GEORGE S. NOLAS, U. of South Florida - Na$_{x}$Si$_{136}$ is a Type II clathrate with important thermoelectric properties. It's face-centered cubic lattice contains polyhedral ``cages'' of silicon atoms with Na atom ``guests'' in the cages. This material is very interesting because powder X-ray diffraction experiments$^{1 }$for differing Na content x have shown that, for increasing x in the range 0 $<$ x $<$ 8, lattice contraction occurs. After all Si$_{28}$ cages in the unit cell are filled (x = 8) and x is increased further, causing a filling of the Si$_{20}$ cages, a contrasting lattice expansion results. Using the local density approximation, we have calculated the x dependences of the structural, electronic and vibrational properties of Na$_{x}$Si$_{136}$. Results are presented for the x dependences of the lattice constant, electronic bands, and vibrational modes. Our results for the x dependence of the lattice constant are in agreement with our X-ray data$^{1}$. $^{\dag }$Deceased. $^{1}$M. Beekman, E.N. Nenghabi, K. Biswas, C.W. Myles, M. Baitinger, Y. Grin, G.S. Nolas, Inorg. Chem. 49, 5338--5340 (2010). [Preview Abstract] |
Thursday, March 24, 2011 11:51AM - 12:03PM |
W20.00004: NMR relaxation and phonon rattling in type-I Ba$_{8}$Ga$_{16}$Sn$_{30}$ clathrates Xiang Zheng, Sergio Y. Rodriguez, Joseph H. Ross, Jr. The atomic motion of guest atoms inside clathrate cages has been considered as one of the important reasons for the observed glasslike thermal behavior. $^{69}$Ga and $^{71}$Ga Nuclear Magnetic Resonance (NMR) studies of type-I Ba$_{8}$Ga$_{16}$Sn$_{30}$ clathrates show a clear low temperature spin-lattice relaxation peak attributed to the influence of Ba rattling dynamics on the framework-atom resonance. Analysis indicates that the quadrupolar relaxation is the leading contribution. The data are analyzed using a two-phonon Raman process, according to a recent theory involving localized one dimensional anharmonic oscillators. Excellent agreement is obtained using this model, with the parameters corresponding to a double well with very large anharmonicity. We have extended the theory to include a two dimensional anharmonic well, with similar parameters providing the best fit to the data. We also examine the Einstein type peak observed in heat capacity using this model, and compare to previous reported results obtained using different models for the anharmonic oscillator. This work is supported by Robert A. Welch Foundation (A-1526). [Preview Abstract] |
Thursday, March 24, 2011 12:03PM - 12:15PM |
W20.00005: Zintl stabilization and site preference in Ba-Cu-Ge clathrates Sergio Y. Rodriguez, Xiang Zheng, Laziz Saribaev, Joseph H. Ross, Jr. Sn, Ge and Si clathrates have cage-like structures, and many exhibit enhanced thermoelectric performance. To understand Cu substitution and Zintl stabilization in Ba$_8$Cu$_x$Ge$_{46-x}$ type-I clathrates with $4 \leq x \leq 6$, we performed NMR measurements coupled with first principles calculations. The $^{63}$Cu NMR resonance exhibits a lineshape characteristic of Cu occupation of the high-symmetry 6$c$ site. All electron computational results showed that the lowest energy configuration is the one with all the Cu atoms located in the 6$c$ site, in agreement with NMR and crystallographic analysis. From bandstructure calculations we find that the preferred structure is a semiconductor, consistent with the observed Zintl stabilization in this material. A direct band gap of about 0.27 eV is found within the Generalized Gradient Approximation formalism. The preferred compositions follow quite closely the valence-counting scheme. From an Atoms In Molecules analysis it is seen that the Ba charge is less than 2$^+$, even though the simple electron-counting argument works well. The framework is seen to be composed of a $sp^3$ bonded network, with strongly polar bonds for Cu. Ba atoms in the large cages are least strongly bond in the plane of the hexagonal faces, corresponding to ease of rattling as seen for other clathrates. This work is supported by Robert A. Welch Foundation (Grant A-1526). [Preview Abstract] |
Thursday, March 24, 2011 12:15PM - 12:27PM |
W20.00006: Thermoelectric properties of low-dimensional clathrates from first principles Deepa Kasinathan, Helge Rosner Type-I inorganic clathrates are host-guest structures with the guest atoms trapped in the framework of the host structure. From a thermoelectric point of view, they are interesting because they are semiconductors with adjustable bandgaps. Investigations in the past decade have shown that type-I clathrates $X_{8}$Ga$_{16}$Ge$_{30}$($X$ = Ba, Sr, Eu) may have the unusual property of ``phonon glass-electron crystal'' for good thermoelectric materials. Among the known clathrates, Ba$_{8}$Ga$_{16}$Ge$_{30}$ has the highest figure of merit (ZT~1). To enable a more widespread usage of thermoelectric technology power generation and heating/cooling applications, ZT of at least 2-3 is required. Two different research approaches have been proposed for developing next generation thermoelectric materials: one investigating new families of advanced bulk materials, and the other studying low-dimensional materials. In our work, we concentrate on understanding the thermoelectric properties of the nanostructured Ba-based clathrates. We use semi-classical Boltzmann transport equations to calculate the various thermoelectric properties as a function of reduced dimensions. We observe that there exists a delicate balance between the electrical conductivity and the electronic part of the thermal conductivity in reduced dimensions. Insights from these results can directly be used to control particle size in nanostructuring experiments. [Preview Abstract] |
Thursday, March 24, 2011 12:27PM - 1:03PM |
W20.00007: Nanostructured Oxides and Sulfides for Thermoelectrics Invited Speaker: Thermoelectric power generation can be applied to various heat sources, both waste heat and renewable energy, to harvest electricity. Even though each heat source is of a small scale, it would lead to a great deal of energy saving if they are combined and collected, and it would greatly contribute to reducing carbon dioxide emission. We have been engaged in developing novel thermoelectric materials to be used for energy saving and environmental protection and are currently developing nanostructured ceramics for thermoelectric conversion. We have demonstrated a quantum confinement effect giving rise to two dimensional electron gas (2DEG) in a 2D superlattice, STO/STO:Nb (STO: strontium titanate), which could generate giant thermopower while keeping high electrical conductivity. One unit-cell thick Nb-doped well layer was estimated to show ZT=2.4 at 300K. Then, a ``synergistic nanostructuring'' concept incorporating 2DEG grain boundaries as well as nanosizing of grains has been applied to our STO material and 3D superlattice ceramics was designed and proposed. It was verified by numerical simulation that this 3D superlattice ceramics should be capable of showing ZT=1.0 at 300K which is comparable to or even higher than that of conventional bismuth telluride-based thermoelectrics. We have recently proposed titanium disulfide-based misfit-layered compounds as novel TE materials. Insertion of misfit-layers into the van der Waals gaps in layer-structured titanium disulfide thus forming a natural superlattice gives rise to internal nanointerfaces and dramatically reduces its lattice thermal conductivity. ZT value reaches 0.37 at 673 K even without optimization of electronic properties. Our challenge to further increase ZT by controlling their electronic system and superlattice structures will be presented. [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:15PM |
W20.00008: Electronic Structure Determination of the Thermoelectric CuRh$_{1-x}$Mg$_{x}$O$_{2}$ using Soft X-Ray Spectroscopies Eric Martin, Paolo Vilmercati, Christine Cheney, Takao Sasagawa, Norman Mannella Magnesium-doped rhodium oxides with formula unit CuRh$_{1-x}$Mg$_{x}$O$_{2}$ and delafossite-type structure exhibit a high thermoelectric figure of merit at elevated temperatures. The electronic structure of CuRh$_{1-x}$Mg$_{x}$O$_{2}$ has been studied with x-ray emission spectroscopy (XES), x-ray absorption spectroscopy (XAS), and photoemission spectroscopy (PES). The data reveal that the states at the Fermi level are Rh-derived. Measurements carried out by changing the orientation of the linear photon polarization further indicate that the Rh states have a more localized character along the c-axis, consistent with the layered crystal structure. Given the similarity of the electronic configurations of Co and Rh, these data provide solid experimental evidence that the orbital degrees of freedom of the d$^{6 }$ionic configuration of the states rooted in transport are key for explaining the thermoelectric properties of oxide materials. [Preview Abstract] |
Thursday, March 24, 2011 1:15PM - 1:27PM |
W20.00009: Thermal Conductivity of Aluminum Oxide from First Principles Moses Ntam, Jianjun Dong, Bin Xu Alumina (Al$_{2}$O$_{3})$ is a well-known ceramic material. First-principles study of lattice thermal conductivity can assist our understanding in extreme conditions that are difficult to achieve experimentally, as well as analyze the fundamental difference between other materials. We combine density functional theory and the Peierls--Boltzmann transport theory to predict the temperature and pressure dependencies of lattice thermal conductivity of the corundum phase. We use a real space super cell method to extract second force constants and third order lattice anharmonicity tensors. These are then used to directly evaluate the phonon scattering rates due to lattice anharmonicity. Our preliminary results show that at a density of 4.23 g/cm$^{3}$ Al$_{2}$O$_{3}$ has thermal conductivities of 14.8Wm$^{-1}$K$^{-1}$ at 300K and 5.31Wm$^{-1}$K$^{-1}$ at 1000K. Moreover, we calculated the thermodynamic properties such as thermal expansion coefficient, bulk modulus and heat capacity, which are in excellent agreement with available measurements and previous theoretical calculations. [Preview Abstract] |
Thursday, March 24, 2011 1:27PM - 1:39PM |
W20.00010: Thermoelectric properties of Ca$_{3}$Co$_{4}$O$_{9}$ thin film Robert Klie, Qiao Qiao, Ahmet Gulec, Tadas Paulauskas, Stanislaw Kolesnik, Bogdan Dabrowski, Cihat Boyraz, Mehmet Ozdemir, Dipanjan Mazumdar, Arun Gupta Thermoelectric oxides have attracted increasing attention due to their high thermal power and temperature stability. In particular, Ca$_{3}$Co$_{4}$O$_{9 }$(CCO), a misfit layered structure consisting of single layer hole-doped CoO$_{2}$ sandwiched between insulating Ca$_{2}$CoO$_{3}$ rocksalt layers, exhibits a high Seebeck coefficient at 1000 K. It was suggested that the Seebeck-coefficient can be further increased by growing doped thin films with controlled defects structures. This study combines pulsed layer deposition thin film synthesis of pristine CCO on several oxide substrates, as well as CCO thin films doped with Ti, Bi or La, with aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy (EELS) to examine the effects of interfacial strain and doping on the atomic and electronic structures of CCO. The thermoelectric properties will be measured and correlated to the local changes in the atomic and electronic structures. We will further evaluate the role of CoO$_{2}$ stacking faults, as well as film thickness on the thermoelectric properties of CCO. [Preview Abstract] |
Thursday, March 24, 2011 1:39PM - 1:51PM |
W20.00011: Improvement of the thermoelectric properties of substituted SrTiO$_{3}$ by synthesis conditions S. Kolesnik, S. Boona, B. Dabrowski, K. Swierczek, K. Wojciechowski We have studied thermoelectric properties of polycrystalline Sr$_{1-x}$La$_{x}$TiO$_{3}$ and SrTi$_{1-x}$Nb$_{x}$O$_{3}$ (x$<$=0.2) synthesized by a solid state synthesis method in a H$_{2}$/Ar atmosphere. The incorporation of La and Nb into the crystal structure was confirmed by x-ray diffraction and energy dispersive x-ray spectroscopy. By increasing the synthesis temperature (up to $\sim $1570$^{o}$C) and decreasing the partial pressure of oxygen, we were able to optimize the thermoelectric properties of the studied materials. The determined values of the thermoelectric figure of merit ZT$\sim $0.1 at 400 K and $\sim $0.3 at 800 K are comparable to those of single crystals of La- and Nb- substituted compounds. Our results show that the synthesis conditions play a crucial role in tailoring of the thermoelectric properties of substituted strontium titanates. [Preview Abstract] |
Thursday, March 24, 2011 1:51PM - 2:03PM |
W20.00012: Thermoelectric Properties of a-InGaZnOx D.S. Williams, Stephan Piotrowski, B.E. White Primarily known as an active layer in thin film transistors, the electrical and thermal properties of a-InGaZnOx indicate promise as a thermoelectric material. In contrast to most phonon-blocking, electron-transmitting thermoelectric materials, a-InGaZnOx is a structurally amorphous material that maintains relatively high electron mobility (10-50 cm2/V-s) and optical transparency. Here we report on the electrical conductivity, thermal conductivity, and Seebeck coefficient of this material as a function of charge carrier concentration. Carrier concentration is modulated through thin film annealing in a reducing ambient. Room temperature thermal conductivity is found to be 0.35 W/m-K with a Seebeck coefficient of approximately 200$\mu $V/K. These data suggest room temperature thermoelectric figures of merit in the range of 0.1-0.3 are achievable with these materials, offering the possibility of transparent thermoelectric energy generation. [Preview Abstract] |
Thursday, March 24, 2011 2:03PM - 2:15PM |
W20.00013: ABSTRACT WITHDRAWN |
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