Session A17: Focus Session: Thermoelectrics - Nanostructured and Oxide TE

Thermoelectric properties of filling-controlled zinc-antimonides with layer structure

Thermoelectric properties have been investigated for polycrystalline samples of layer-structured $R_{1-x}A_x$ZnSbO ($R$=La, Ce; $A$=Ca, Sr) as two-dimensional analogues of a conventional thermoelectric semiconductor ZnSb[1]. By substituting $A^{2+}$ for $R^{3+}$ in the charge-reservoir layers, carrier concentration can be successfully controlled without lowering the carrier mobility. The hole doped materials showed low thermal conductivity and moderately high thermopower, whose temperature- and doping-dependence were well explained by theoretical calculation. The values of dimensionless figure of merit $ZT$ were found to increase without showing any sign of saturation up to 390 K, and even higher values can be expected along the conducting ZnSb layers for a single crystal. These results indicate the potential of the hole-doped $R$ZnSbO as a good thermoelectric material. This work was in part supported by FIRST program on \lq\lq Quantum Science on Strong Correlation\rq\rq \ from JSPS. \\[4pt] [1] T. Suzuki, M. S. Bahramy, R. Arita, Y. Taguchi, and Y. Tokura, Phys. Rev. B 83, 035204 (2011)   

Enhancing thermoelectric properties of FeSb$_{2}$ by altering stoichiometry and nanostructure

FeSb$_{2}$ is a strongly correlated semiconductor that has been shown to have an extraordinarily large Seebeck coefficient in single crystal samples. Bentien \textit{et al.} report a Seebeck Coefficient of -45000 microV/K at 10K. The peak value of the dimensionless figure of merit (ZT$_{max})$ for single crystal samples is calculated to be approximately 0.005 at 10 K and is constrained by its relatively high thermal conductivity. In our previous studies, we find that nanocomposites (NC) tend to decrease thermal conductivity substantially by introducing phonon mismatches between crystal grains. Given that the Seebeck coefficient on the FeSb$_{2}$ system is quite sensitive to carrier concentration, we focus on the effects of stoichiometric changes that heighten thermoelectric properties of FeSb$_{x}$ where x=2.04, 2.00, 1.96, 1.92. By tuning the stoichiometry and using the nanocomposite method, the peak value of ZT$_{max}$ was found to be 0.0123 at 43K. Carrier concentration and Hall-mobility measurements will also be discussed.   

Local Structure of Thermoelectric Clathrates

We present local structure results from extended X-ray absorption fine structure (EXAFS) analysis of two new kinds of clathrates: type I clathrates with a light 'rattler' atom and type VIII clathrates. The rattler atom is extremely important for the thermoelectric properties as it is what strongly scatters phonons and lowers the thermal conductivity without affecting the electrical conductivity. In most thermoelectric clathrates, the rattler atom is a heavy atom, typically Ba or Eu, and such type I clathrates have been discussed extensively in the literature; a smaller rattler atom such as K results in more free space inside the cage. Type VIII clathrates have the same chemical formula as type I clathrates, but have a different, more distorted, cage structure resulting in differing electronic properties. Recent studies indicate a thermoelectric figure of merit at least as high as 1.2 at 400 K is attainable. For both kinds of clathrates, we collect and analyze temperature-dependent data over the range 10-300 K and compare the results with our models.   

Nanostructured Thermoelectrics and the New Paradigm

A comprehensive and stable energy strategy would require proportionate attention to all three legs of the ``energy stool''; supply (sources), demand (efficiency) and storage/transport (delivery). Thermoelectric materials, that convert waste thermal energy into useful electrical energy, have an important role to play in any and all these three legs. The efficacy and efficiency of thermoelectrics is reflected in the figure of merit ZT, which is directly proportional to the power factor (comprising electrical conductivity and Seebeck coefficient) and inversely proportional to thermal conductivity (comprising carrier and lattice contributions). The recent emergence of nanostructured thermoelectrics has ushered in a new era for bulk thermoelectrics, which show considerable promise to enhance the ``contra-indicating'' parameters of high electrical conductivity and low thermal conductivity. This is achieved by introducing nanostructures in bulk thermoelectric host materials to significantly reduce lattice thermal conductivity via effective scattering of heat carrying phonon through hierarchical architecture of nanostructured thermoelectrics. The presentation will cover recent developments, current research in our EFRC and future prospects for high performance bulk materials. Systems based on lead chalcogenides (e.g., PbTe, PbSe, PbS) present key science challenges with promising properties and are given particular emphasis. We have achieved excellent control of synthesis and crystal growth of such materials resulting in record enhancements in the figure of merit. These enhancements derive from very large reductions in lattice thermal conductivity possible with nanostructuring. We have experimentally realized concurrent synergistic effect of phonon blocking and charge transmission via the endotaxial placement of nanocrystals in thermoelectric material host. In particular, we have shown that the enhanced performance is due to nanostructuring of thermoelectric host matrix, with a compelling influence of hierarchy of length-scales associated with these systems. The presentation will outline possible future strategies for enhancing the thermoelectric figure of merit of bulk thermoelectric materials.   

Assessing the thermoelectric properties of sintered compounds via high-throughput ab initio calculations

In order to identify promising thermoelectric materials, we study several thousand compounds from the ICSD database. In particular, we consider nano-grained sintered power thermoelectric compounds with the high-throughput {\it ab initio} \textsf{AFLOW} framework (http://aflowlib.org and http://materials.duke.edu/aflow.html). By regression analysis, we find that the power factor is positively correlated to electronic band gap, carrier effective mass, and the number of atoms per unit cell. This work illustrates the important role that experimental and theoretical databases can play in the development of novel materials.   

Enhancement of the figure-of-merit in strongly correlated multilayers

A theory of charge and heat transport in inhomogeneous multilayers (ML) with correlated electrons is presented. We consider the device which consists of several strongly correlated metallic planes (channel) sandwiched between two semi-infinite Mott insulators (barriers). A driving field is applied parallel to the ML planes and the gate voltage perpendicular to the ML planes. The device is described by the Falicov-Kimball model and transport coefficients are calculated by the linear response theory using the DMFT. The device is tuned by i) increasing the correlation strength so as to enhance the slope of the local density of states in the channel-planes at the chemical potential and ii) changing the gate voltage so as to shift the position of the chemical potential with respect to the band-edge. Both effects have a large impact on the thermoelectric properties and are used to optimize the figure-of-merit, ZT, of the device. The effect of the number of planes is discussed as well. The results for the electrical conductivity, the Seebeck coefficient, the power factor, the Lorenz number and ZT are presented. Optimal tuning gives ZT $\gg$ 1.   

Thermoelectric and Structural Properties of the Chemically Doped Ca$_{3}$Co$_{4}$O$_{9}$ System

The Cu and Y doped thermoelectric oxide system [Ca$_{2}$CoO$_{3}$][CoO$_{2}$]$_{1.61}$, also referred to as Ca$_{3}$Co$_{4}$O$_{9}$, was prepared by solid state reaction followed by annealing under oxygen. The temperature dependent thermoelectric properties, including resistivity ($\rho )$, Seebeck coefficient (S) and thermal conductivity ($\kappa )$, were measured on Cu doped [Ca$_{2}$Co$_{1-x}$Cu$_{x}$O$_{3}$][CoO$_{2}$]$_{1.61}$ and Y doped [Ca$_{2-x}$Y$_{x}$CoO$_{3}$][CoO$_{2}$]$_{1.61}$. In order to understand the origin of the changes in ZT with doping, local (XAS) and long range (XRD) structural measurements as a function of doping were conducted. Identification of the locations of the doping sites and the impact on ZT will be discussed. This work is supported by DOE Grant DE-FG02-07ER46402. The Physical Properties Measurements System was acquired under NSF MRI Grant DMR-0923032 (ARRA award).   

The effects of increased Co-ion spin states on the Seebeck coefficient in thermoelectric Ca$_{3}$Co$_{4}$O$_{9}$

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 1,000 K. It was previously suggested that the Seebeck-coefficient can be further improved by stabilizing an increased Co-ion spin state in the CoO$_{2}$ layers. Here we report a significant increase in the room-temperature in-plane Seebeck coefficient of 40 nm thick CCO films grown by pulsed laser deposition on SrTiO$_{3}$ substrates. We combine aberration-corrected Z-contrast imaging, atomic-column resolved electron energy-loss spectroscopy, and density-functional calculations to show that the increase is caused by CoO$_{2}$ stacking faults with Co$^{4+}$-ions in a higher spin state compared to that of bulk CCO. The higher Seebeck coefficient makes the CCO system suitable for many high-temperature waste-heat-recovery applications. The role of dopants, such as Bi and Ti will also be explored.   

Anisotropic thermopower and magnetothermopower in a misfit-layered calcium cobaltite

An unusual anisotropy of thermopower and magnetothermopower has been observed in the powerful thermoelectric Ca$_3$Co$_4$O$_{9+\delta}$ single crystal. The in-plane thermopower is about twice as big as the out-of-plane thermopower. Combining {\em ab initio} band structure calculation with semi-classical model analysis, we understand this anisotropy with band structure effects and especially with anisotropic Fermi surface. We find that a strong anisotropy in the topology of Fermi surface leads to the anisotropy of (magneto)thermopower. This study may also shed light on anisotropic properties of other layered cobalt oxides.   

The Origin of Enhanced High Temperature Electron Transport in Thermoelectric Ca$_{3}$Co$_{4}$O$_{9}$

Temperature dependent measurements of resistivity, crystal structure and heat capacity reveal significant hysteresis occurring near 400 K in Ca$_{3}$Co$_{4}$O$_{9}$. The largest changes in structure occur in the CoO$_{2}$ layer associated with electron transport: manifested mainly by $b_{2}$ axis changes. Application of magnetic fields up to 8 T reduces the area of the resistivity hysteresis loop with saturation at $\sim $4 T. Reduced resistivity associated with this first order phase transition from metallic to semiconducting behavior enhances the thermoelectric properties at high temperatures and points to the metal-insulator transition as a mechanism for improved ZT in high temperature thermoelectric oxide. This work is supported by DOE Grant DE-FG02-07ER46402. The Physical Properties Measurements System was acquired under NSF MRI Grant DMR-0923032 (ARRA award).   

Electrical and Thermal Transport Properties of Bi$_{2}$Sr$_{2}$Co$_{2}$O$_{9-\delta }$ Single Crystals and Thin Films

Layered Bi$_{2}$Sr$_{2}$Co$_{2}$O$_{9-\delta }$ possesses rich physical properties, promising for thermoelectric applications. We have successfully synthesized Bi$_{2}$Sr$_{2}$Co$_{2}$O$_{9-\delta }$ in both single crystal and epitaxial thin film forms by applying various oxygen pressures. We found that their electrical and thermal transport properties are sensitive to the oxygen content, suggesting that the oxidation state of Co plays an important role in thermoelectric properties. Comparison of power factor between single crystals and thin films will be presented.   

Transport Properties of La- doped SrTiO$_{3}$ Ceramics Prepared Using Spark Plasma Sintering

In this work, thermoelectric transport properties of La-doped SrTiO$_{3}$ ceramics prepared using conventional solid state reaction and spark plasma sintering have been investigated. Room temperature power factor of single crystal strontium titanate (SrTiO$_{3})$, comparable to that of Bi$_{2}$Te$_{3}$, has brought new attention to this perovskite-type transition metal-oxide as a potential n-type thermoelectric for high temperature applications. Electronic properties of this model complex oxide, SrTiO$_{3}$ (ABO$_{3})$, can be tuned in a wide range through different doping mechanisms. In addition to A site (La-doped) or B site (Nb-doped) substitutional doping, introducing oxygen vacancies plays an important role in electrical and thermal properties of these structures. Having multiple doping mechanisms makes the transport properties of these perovskites more dependent on preparation parameters. The effect of these synthesis parameters and consolidation conditions on the transport properties of these materials has been studied.   

Transition-metal-based perovskite oxides for enhanced thermopower

Due to the enhancement of thermopower by spin and orbital degrees of freedom, transition-metal-based perovskite oxides are good candidates for stable and nontoxic materials with a large thermoelectric figure of merit ZT. We have investigated the most promising Mn-, Co- and Ti-based perovskite oxides. Electron doping of SrMnO$_{3}$ materials on either Mn or Sr sites induces a rapid decrease in both electrical resistivity and thermopower with the doping level due to the introduction of itinerant charge carriers. The thermopower of electron-doped SrTiO$_{3}$ materials satisfy the basic Heikes description, however, no additional enhancement is observed. The hole-doped $R$CoO$_{3}$ perovskites exhibit limited solubility of alkaline earth's for small rare earth ion sizes. The dependence of thermopower on charge doping and temperature appears to follow the extended Heikes formulation only at low doping and below 300 K, which indicates that Co$^{3+}$ and Co$^{4+}$ exist in several spin states beyond that range. Among all investigated compounds the largest ZT$\sim $0.3 values were observed for 3-8{\%} Nb-substituted SrTiO$_{3}$ materials at about 700 K. Supported by the U.S. DOE-BES DE-AC02-06CH11357.