Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session T20: Focus Session: Thermoelectric Materials: LAST/TAGS, Heusler, and Silicides |
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Sponsoring Units: DMP GERA FIAP Chair: Joseph Heremans, Ohio State University Room: D168 |
Wednesday, March 23, 2011 2:30PM - 2:42PM |
T20.00001: Charge Neutral Yukawa Lattice Gas on a FCC Lattice He Huang, S.D. Mahanti Structural phase transitions associated with the ordering of $Ag$ and $Sb$ ions in the quaternary systems, $(AgSbTe_{\rm 2}) _x (PbTe)_{2(1 - x)}$ (of current thermoelectric interest) has been investigated using an anti-ferromagnetic 3-state Ising model on a FCC lattice with screened Coulomb interaction (Yukawa lattice gas (YLG) model). We have carried out Monte Carlo simulations (MCs) to study phase transitions (PT) in YLG. The nature and the strength of PT depend on the screening parameter $\kappa$. The transition is $1^{st}$ order and the transition temperature $T_c$ is a weak function of the concentration x (excepting when $x\sim 0$ or 1), in agreement with earlier work for $\kappa =0$. We find $T_c(x, \kappa)=f(x)g (\kappa)$, where $g(\kappa)\to const$ when $\kappa\to 0$ and $g (\kappa)\to 0$ when $\kappa\to \infty$. For $x=0.5$, there are two special structures, layered and tubular which have the same ground state energy, independent of $\kappa$. This is understood by looking at the connectivity and ordering of ions. Above but near $T_c$, the generation rates of different micro structures have been analyzed using a simple surface energy density picture. MCs results agree with this analysis and show that the energy barriers decide the generation rates of different micro structures. [Preview Abstract] |
Wednesday, March 23, 2011 2:42PM - 2:54PM |
T20.00002: Effect of doping Ag$_{y}$Sb$_{y}$Ge$_{50-2y}$Te$_{50}$ thermoelectric materials with rare earths E.M. Levin, S.L. Bud'ko, K. Schmidt-Rohr The Ag$_{y}$Sb$_{y}$Ge$_{50-2y}$Te$_{50}$ system represents some of the most efficient thermoelectrics, the so-called TAGS materials. In order to understand the effect of doping of Ag$_{6.52}$Sb$_{6.52}$Ge$_{36.96}$Te$_{50}$ (``TAGS-85'') with rare earth atoms on the Ge and Te sites, Ag$_{6.52}$Sb$_{6.52}$Ge$_{36.96-x}$R$_{x}$Te$_{50}$ and Ag$_{6.52}$Sb$_{6.52}$Ge$_{36.96}$R$_{x}$Te$_{50-x}$ materials with R = Gd and Dy (rare earth atoms with large magnetic moments) have been studied by measuring X-ray diffraction (XRD) and $^{125}$Te nuclear magnetic resonance (NMR) at 300 K, thermopower and resistivity at 300-760 K, and the magnetization at 1.8-350 K and in magnetic field 0-55 kOe. XRD and $^{125}$Te NMR show that some rare earth atoms are incorporated into the lattice and enhance the thermopower by $\sim $10{\%}. At 700 K, this yields a power factor of up to 36 $\mu $W$\cdot $cm$^{-1}\cdot $K$^{-2}$, which is $\sim $20{\%} higher than in TAGS-85. All materials studied can be considered as degenerate magnetic semiconductors with non-interacting localized magnetic moments formed by rare earth atoms, with a different effect of rare earths on the Ge and Te sites. Reasons for the thermopower enhancement due to doping with rare earths including magnetic and non-magnetic phenomena are discussed. [Preview Abstract] |
Wednesday, March 23, 2011 2:54PM - 3:06PM |
T20.00003: Nanodopant Induced Band Modulations and Electronic Transport Properties in AgPb$_m$SbTe$_{2+m}$-type Thermoelectric Nanocomposites Yi Zhang, Changfeng Chen, Xuezhi Ke, Jihui Yang, Paul Kent The remarkable performance of many novel thermoelectric materials is attributed to their nanosized inclusions. By extensive first-principles calculations we show the distinct band structure modulation in AgPb$_m$SbTe$_{2+m}$ (LAST)-type nanocomposites. A band gap widening and conduction band minimum splitting process resulting from the nanodopants is discovered for a series of nanocomposites. Boltzmann transport calculations demonstrate that this process leads to a pronounced change in the high temperature electronic transport. The effects of different substitutional elements and atomistic orderings are discussed. Our results provide new understanding of nanosized doping in thermoelectric materials and narrow gap semiconductors. [Preview Abstract] |
Wednesday, March 23, 2011 3:06PM - 3:18PM |
T20.00004: Strong off-stoichiometry and large Gr\"{u}neisen parameter in AgSbTe$_2$: a first principles study Sergey V. Barabash, Vidvuds Ozolins, Michele D. Nielsen, Joseph P. Heremans We use first-principles density-functional theory calculations to study the dynamical and compositional instabilities in AgSbTe$_2$, and compare the theoretical predictions to the results of an experimental investigation. For pure AgSbTe$_2$, some native defects, particularly Ag vacancies, have negative formation energies for a wide range of experimental conditions, thus forming in high concentrations even at low $T$. This leads to large deviations from the formal stoichiometry, in agreement with experimental results. Substantial deviations of the AgSbTe$_2$ phase field away from the isoplethal Ag$_2$Te-Sb$_2$Te$_3$ section may be expected, potentially explaining the contradictions in the low-temperature regions of the previously published phase diagrams. We estimate the defect concentrations and the resulting intrinsic doping levels under various experimental conditions. Finally, we demonstrate that the stoichiometric AgSbTe$_2$ is at the verge of a dynamical instability: the energies of acoustic phonons near the L point depend strongly on volume, changing sign at nearly the experimental volume. This leads to an unusually large value of the Gr\"{u}neisen parameter, in agreement with experiment. [Preview Abstract] |
Wednesday, March 23, 2011 3:18PM - 3:30PM |
T20.00005: Effect of electron correlation on thermoelectric properties of the full-Heusler compound Fe$_2$VAl Dat Do, Mal-Soon Lee, S.D. Mahanti Heusler-type alloys have been studied extensively since they were first discovered by Heusler in 1903. Among those Fe$_2$VAl became interesting when Nishino {\it et. al.}[1] suggested that it might be a heavy fermion system. LDA/GGA calculations have shown that Fe$_2$VAl is a pseudo-gap system with sharp edges in the density of state near the Fermi level. This feature makes it a promising thermoelectric material. Since then electronic properties of nominally pure and doped Fe$_2$VAl have been studied extensively. However the exact nature of the ground state of this system is still not well understood. Since it contains d-electrons one expects electron correlation effects to be important. We have carried out band structure calculations using GGA+U method with several values of the on-site Coulomb interaction parameter U. Using the obtained band structure, transport coefficients were calculated within Boltzmann approach. Electronic structure and thermoelectric properties were studied for different values of U and compared to available experiments.\\[4pt] [1] Y. Nishino {\it et. al., Phys. Rev. Lett. {\bf 79} (10), 1909} (1997). [Preview Abstract] |
Wednesday, March 23, 2011 3:30PM - 3:42PM |
T20.00006: Phase-separated-induced changes in the transport properties of Heusler compounds for thermoelectric applications Tanja Graf, Michael Schwall, Peter Klaer, Hans-Joachim Elmers, Benjamin Balke, Claudia Felser The solid solution Co$_{2}$Mn$_{1-x}$Ti$_{x}$Sn shows a phase separation into two Heusler compounds, Co$_{2}$MnSn and Co$_{2}$TiSn. Only at the edges of the composition range a slight admixture of Mn and Ti to the respective other phase is observed. This phase separation leads to a distinct microstructure which can be altered by the composition of the material. Due to the formed phase and grain boundaries, pronounced changes in the magnetic and electronic properties take place with varying composition. The observed reduction of the thermal lattice conductivity is of particular interest for an optimization of Heusler compounds for thermoelectric applications. Thus, the concept of phase separated materials is transferred to Half-Heusler compounds with an improved thermoelectric performance. [Preview Abstract] |
Wednesday, March 23, 2011 3:42PM - 4:18PM |
T20.00007: Strategies to Bulk Half-Heusler Nanocomposites with Simultaneously Enhanced Power Factor and Reduced Thermal Conductivity Invited Speaker: Among promising thermoelectric materials for power generation, half-Heusler (HH) phases with general compositions TNiSn and TCoSb (T = Ti, Zr, Hf) have attracted tremendous attention not only because they involve abundant and environmentally friendly elements, but also due to their combination of large Seebeck coefficients with moderately low electrical resistivities. However, the ability to synthesize HH based materials with decent figures of merit (ZT$>$1) has been jeopardized by their very large thermal conductivities. Strategies to reduce the thermal conductivity of HH phases focusing on mass fluctuation phonon scattering via solid solution alloying or phonon scattering at grain boundaries and interfaces in HH phases with embedded pre-synthesized nanoparticles have failed to generate materials with high figures of merit due to simultaneous reductions in the power factor. Here, we introduce innovative approaches to revolutionary increases in the figure of merit of HH based materials through simultaneous large enhancement of the power factor and drastic reduction in the thermal conductivity. Our strategy is focused on atomic-scale structural engineering of the HH matrix through the confinement of full-Heusler (FH) inclusion phases on the lattice constant length-scale. Emphasis will be placed on the n-type Zr$_{0.25}$Hf$_{0.75}$Ni$_{1+x}$Sn$_{1-y}$Pn$_{y}$ and Ti$_{0.5}$Zr$_{0.5}$Ni$_{1+x}$Sn$_{1-y}$Pn$_{y}$ as well as the p-type Ti$_{0.5}$Zr$_{0.5}$Co$_{1+x}$Pn$_{1-y}$Sn$_{y}$, (Pn = Sb, Bi) nanocomposites. We will discuss the underlying mechanism for the formation of half-Heusler/full-Heusler (HH/FH) nanocomposites with coherent matrix/inclusion interfaces. The role of synthetic and processing methods; and size, dispersion and mole fraction of the FH inclusions on the thermoelectric performance of bulk HH/FH nanocomposites will be assessed by combining transmission electron microscopy studies with thermal and electronic charge transport data. [Preview Abstract] |
Wednesday, March 23, 2011 4:18PM - 4:30PM |
T20.00008: Thermoelectric Properties of Boron-doped CoSi Bo Yu, Hui Wang, Hongli Gao, Weishu Liu, Xinbing Zhao, Gang Chen, Zhifeng Ren Engineering in density of states $D(E)$ has been found effective in improving the transport properties of thermoelectric materials. As one example, intermetallic CoSi, when doped with boron or other suitable elements, exhibits a good combination of high electrical conductivity (\textit{$\sigma $}) and Seebeck coefficients ($S)$ due to possible sharp structures in $D(E) $near Fermi level. However, despite of its high power factor ($S^{2}$\textit{$\sigma $}), the high thermal conductivity ($\kappa )$ becomes the obstacle for the performance. Here, we present that mechanical alloying and hot press which had been proved successful in many thermoelectric materials, could also reduce the thermal conductivity of boron doped CoSi while keeping its high power factor. [Preview Abstract] |
Wednesday, March 23, 2011 4:30PM - 4:42PM |
T20.00009: The Electrical Contact for Higher Manganese Silicide Thermoelectric Material Xinghua Shi, Zahra Zamanipour, Daryoosh Vashaee The Electrical Contact for Higher Manganese Silicide Thermoelectric Material Xinghua Shi, Zahra Zamanipour, Daryoosh Vashaee Several electrical contact materials for Higher Manganese Silicide (HMS) are introduced. HMS is useful thermoelectric material for medium to high temperature applications. We have investigated several materials including Co, Ni, Cr, Ti, Mo, MnSi, MoSi2, and TiSi2 in search of the best contact material to HMS. The low electrical resistivity and reliability of the contact are two important elements to make a high efficient TE device. Moreover, the contact must maintain its chemical, mechanical, thermal, and electrical properties over a broad range of temperature (20C-700C). The investigated elemental metals failed to make reliable contact in terms of mechanical and chemical stability at high temperature. In contrast, the investigated metal silicides showed superior stability over extended operation at high temperature. The thermal stability and strong mechanical bonding of TiSi2 C54 phase and MnSi were specially observed. Their ohmic contact resistance was also within the range of interest over the whole range of temperature (10$^{-5}$-10$^{-4}\Omega $cm$^{2})$. [Preview Abstract] |
Wednesday, March 23, 2011 4:42PM - 4:54PM |
T20.00010: High Thermoelectric Power Factor in CoSi$_{1-x}$B$_{x}$ Alloys Hui Sun, Donald Morelli CoSi$_{1-x}$B$_{x}$ alloys with x ranging from 0 to 0.1 have been prepared by an arc melting and annealing procedure. X-ray diffraction studies suggest the occurrence of minor CoB phase when x$\ge $0.05. The thermoelectric (TE) properties were measured from 80 to 300K. The samples with x$\le $0.02 showed much lower electrical resistivity than CoSi. The Seebeck coefficient was negative for all samples over the investigated temperature range, indicating dominant transport by electrons in this temperature range. A high TE power factor (70 $\mu $W/K$^{2}$cm at room temperature) was obtained in CoSi$_{0.98}$B$_{0.02}$, which we ascribe to the appropriate tuning of the Fermi level near the pseudogap in the density of states. In optimized samples the dimensionless figure of merit is in excess of 0.13 due to this enhanced power factor. We will also report on our efforts to further increase the figure of merit by thermal conductivity reduction methods. [Preview Abstract] |
Wednesday, March 23, 2011 4:54PM - 5:06PM |
T20.00011: Thermoelectric properties of FeSi and Related Alloys: Evidence for Strong Electron-Phonon Coupling Brian Sales, Olivier Delaire, Michael McGuire, Andrew May The effects of various transition metal dopants on the electrical and thermal transport properties of Fe$_{1-x}$M$_{x}$Si alloys (M= Co, Ir, Os) are reported. The thermoelectric figure of merit ZT is improved from 0.007 at 60 K for pure FeSi to ZT = 0.08 at 100 K for 4{\%} Ir doping. A comparison of the thermal conductivity data among Os, Ir and Co doped alloys indicates strong electron-phonon coupling in this compound. The common approximation of dividing the total thermal conductivity into electronic and lattice components ($\kappa _{Total}=\kappa _{electronic}+\kappa _{lattice})$ fails spectacularly for these alloys. The effects of nanostructuring on thermoelectric properties of Fe$_{0.96}$Ir$_{0.04}$Si alloys are also reported. The thermal conductivity can be lowered by about 50{\%} with little or no effect on the electrical resistivity or Seebeck coefficient. This results in ZT$_{max}$ = 0.125 at 100 K, still about a factor of five too low for solid-state refrigeration applications. Research sponsored by the Materials Science and Engineering Division, Office of Basic Energy Sciences, U.S. DOE. [Preview Abstract] |
Wednesday, March 23, 2011 5:06PM - 5:18PM |
T20.00012: Electron-phonon coupling in FeSi thermoelectrics: inelastic neutron scattering and first-principles simulations Olivier Delaire, Jie Ma, Brian Sales, Paul Kent, Matthew Stone, Karol Marty, Matthew Lucas, Douglas Abernathy, David Mandrus FeSi is a promising thermoelectric material for refrigeration applications, with a Seebeck coefficient over 500$\mu$V/K at 40K. FeSi is a narrow band-gap semiconductor at low temperature (B20 structure), and undergoes a semiconductor-to-metal transition around room temperature. Using inelastic neutron scattering, phonons were measured on both single crystals and powders as a function of composition and temperature. We report a strong coupling between the phonons and the semiconductor-to-metal transition, upon increasing temperature and carrier concentration. Using first-principles electronic structure calculations and ab-initio molecular dynamics, we show that the band gap and the sharp features around the band edges are strongly affected by the thermal disordering induced by phonon excitations. We also report on the effect of heavy impurities (Ir, Os) on the phonons. [Preview Abstract] |
Wednesday, March 23, 2011 5:18PM - 5:30PM |
T20.00013: Carrier concentration optimization and electrical and thermal transport properties of the defect manganese silicide MnSi$_{\delta }(\delta \sim $ 1.74)$^{ \ast }$ V. Ponnambalam, Gloria Lehr, D.T. Morelli Defect manganese silicide MnSi$_{\delta }$ ($\delta \sim $ 1.74) is known for unusually low thermal conductivity. In addition, it also exhibits promising thermoelectric properties. We have substituted MnSi$_{\delta}$ with various elements to optimize the carrier concentration. Electrical and thermal transport properties of the resulting alloys have been studied over a temperature (T) range of 80-300 K. Both resistivity and Seebeck coefficient vary with substitution. Hall measurements suggest that the carrier concentration indeed varies in these alloys. Interestingly, thermal conductivity either remains constant or weakly increases with T in the temperature range 80- 300 K, eventually reaching values $\sim $ 3 W/m K at 300 K. The results will be presented and discussed. *This work was supported as part of the Center for Revolutionary Materials for Solid State Energy Conversion, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001054. [Preview Abstract] |
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