Session R16: Thermoelectrics

Fabrication and Characterization of Bi2Te3 Nanoparticles for Thermoelectric Applications

Bi2Te3 nanoparticles with diameters of 10-30 nm have been successfully synthesized via a hydrothermal method. The as-prepared nanoparticles were characterized by X-ray diffractometer (XRD, Cu Kalpha, Bruker AXS), field emission scanning electron microscope (SEM, JEOL-6340F) and transmission electron microscope (TEM/HRTEM, JEOL-2010F) equipped with an energy-disperse X-ray spectrometer (EDS). The densification of Bi2Te3 nanopowders was conducted in two ways: plasma pressure compaction (P2C) and hot pressing. The density of the as-pressed pellet sample was about 98-99 {\%} of theoretical density (7.7 g/cm3). The Seebeck coefficient, electrical and thermal conductivities were further investigated.   

Characterization and thermoelectric properties of Si-Ge nanocomposite

Low dimension is one of the most promising directions to search for high-ZT thermoelectric materials. It has been predicted by theory and proved by experiments that structures such as quantum well and superlattice can increase ZT by several times vs the corresponding bulk materials. However, it is very difficult to manufacture those low dimensional structures in large scale for bulk applications. To realize those principles, we have designed and synthesized successfully a new structure so called Si-Ge nanocomposite---nano Silicon particles in SiGe alloy matrix. The Si-Ge nanocomposite was made of nano silicon and germanium particles by a unique hot-press procedure. The samples are being characterized by TEM, SEM, XRD and thermoelectric property measurements. The results will be reported in detail.   

Prospects for nanoscale thermoelectric Bi(1-x)Sb(x) alloys

Bi(1-x)Sb(x) alloys are the materials with the highest thermoelectric figure of merit at cryogenic temperatures around 100 K. Recently, Thonhauser et al. (Appl. Phys. Lett. \textbf{85} 588 2004) calculated that heavily-doped pure Bi could be an excellent thermoelectric material at 300 K. Bi doped $p$-type with 1{\%} Sn to a hole concentration of about 15x10$^{21}$cm$^{-3}$ and a Fermi level 250 meV below that of the pure semimetal could reach \textit{ZT}=1.44. Unfortunately, no $p$-type doping has been achieved experimentally in semimetallic Bi at room temperature, not only because of the solubility limit of Sn in Bi, but mostly because the band structure is very temperature-dependent, and the energy overlap between conduction and valence band is much larger at 300 K than at 4 K. Two new approaches are suggested here. Firstly, one can attempt to use the model in dilute Bi(1-x)Sb(x) alloys. Secondly, one should be able to increase the power factor by adding nanoprecipitates to a Bi or Bi(1-x)Sb(x) alloy matrix, following a precipitation anneal similar to that used on PbTe:Pb by Heremans et al. (J. Appl. Phys. \textbf{98} 063703 2005). Model calculations, and the result of some preliminary experiments, will be given.   

Quantum Confinement and Surface State Effects in the Thermopower of Bismuth Nanowires.

Because of the increased density of states arising from one-dimensional confinement, it is anticipated that bismuth quantum wires will exhibit superior thermoelectric properties. Recently, angle-resolved photoemission spectroscopy (ARPES) studies have shown that Bi supports surface states that have not been considered in current models of quantum confinement. Studies of the Fermi surface, employing the Shubnikov-de Haas (SdH) method, in arrays of 30- to 80-nm bismuth nanowires partially corroborates ARPES findings. We have studied the thermopower of arrays of 50-nm Te- and Sn-doped Bi nanowires and we discuss these experimental results in terms of a Bi nanowire conduction model based on ARPES and SdH results..   

The Order Disorder phase transition in Zn$_{4}$Sb$_{3}$

Zn$_{4}$Sb$_{3}$ has been widely investigated not only for its promise as a thermoelectric material, [1] but also for the unique transport properties it exhibits. Zn$_{4}$Sb$_{3}$ undergoes two distinct phase transitions (i) from $\alpha $ to $\beta $-phase at T$_{C} \quad \approx $ 250 K and (ii) from $\beta $ to $\gamma $-phase at T$_{C} \quad \approx $ 765 K. [2] We have performed electronic and thermal transport measurements exploring the structural phase transition at T $\approx $ 250 K from the ordered $\alpha $, to the disordered $\beta $ phase. The well-known $\alpha $ to $\beta $ phase transition manifests itself in anomalies in the resistivity, thermopower and specific heat measurements at T $\approx $ 250 K as well as a change in slope in the thermal conductivity, leading to a reduction in thermal conductivity as Zn$_{4}$Sb$_{3}$ changes phase from the ordered to the disordered state. Moreover, measurements of the elastic properties using resonant ultrasound spectroscopy (RUS) reveal dramatic changes at the order-disorder transition. \newline \newline [1] Snyder \textit{et al}., Nature Materials \textbf{3}, 458-463, (2004) \newline [2] Vuillard, G. \textit{et al.}, C.R. Seances Acad. Sci., Ser. C: Sci. Chim. \textbf{263}, 1018-1021, (1966)   

Presence of $\alpha $ phase domains in the phonon-glass thermoelectric $\beta $-Zn$_{4}$Sb$_{3}$ from atomic pair distribution function (PDF) analysis

The promisingly high \textit{ZT} of $\beta $-Zn$_{4}$Sb$_{3}$ between 450 K and 670 K is known to be due to its exceptionally low thermal conductivity. The discovery of significant Zn interstitial disorder in $\beta $-Zn$_{4}$Sb$_{3}$ provides foundation for better understanding of the origin of the glass-like thermal conductivity[1]. Furthermore, it has been reported that Zn interstitial atoms become ordered in $\alpha $-phase[2]. We report on the \textit{local} structural study of $\beta $-Zn$_{4}$Sb$_{3}$ using PDF technique, which has been successfully applied to solve the structures of crystallographically challenged materials[3,4]. Prominent diffuse scattering is found both in neutron and x-ray data. The PDF analysis suggests that the average $\beta $-structure consists of locally $\alpha $-structure like domains where Zn interstitial atoms are ordered. This provides an important local structural insight into not well understood $\alpha $ to $\beta $ order-disorder phase transition in Zn$_{4}$Sb$_{3}$ at 260 K. [1] G. J. Snyder \textit{et al}. Nat. Mater. \textbf{3}, 458 (2004) [2] J. Nyl\'{e}n \textit{et al}. J. Am. Chem. Soc. \textbf{126}, 16306 (2004) [3] T. Egami and S. J. L. Billinge, \textit{Underneath the Bragg peaks}, Pergamon Press, Elsevier, Oxford, England, 2003 [4] S. J. L. Billinge and M. G. Kanatzidis, Chem. Commun., 749 (2004)   

Theoretical study of thermal properties of Si clathrates

Pristine silicon clathrate (Si$_{136})$ is an ``expanded volume'' allotrope of Si that is metastably available at ambient conditions, and it exhibits a significant decrease in lattice thermal conductivity. We have theoretical studied vibrational, thermodynamic, and transport properties of Si$_{136}$ and compared its results with those of the ground state diamond-structured Si. The equilibrium temperature-pressure phase boundary between the two phases occurs in the negative pressure regime. Despite obvious differences in the energetics and lattice vibrational modes for the two polymorphic forms of Si, our calculations indicate that their heat capacities are quite similar. We further predict that Si$_{136}$ has a region of negative thermal expansion below T=140K. Our \textit{ab initio} prediction shows that thermal expansion in the guest-free Si$_{136}$ clathrate is significantly smaller than the previously reported data of guest encapsulated clathrates. In contrast to similar thermal properties in the two phases, our calculations of lattice thermal conductivities reveal some dramatically different features in the phases. We will discuss the origin of the predicted ``oscillation'' in the current-current correlation functions.   

Optical and thermoelectric properties of Tl-filled CoSb$_3$ skutterudites from first-principles

Filled skutterudite antimonides have attracted much interest as a new class of thermoelectric materials.\footnote{B.C. Sales, D. Mandrus, and R. K. Williams, Science \textbf{272}, 1325 (1996).} We have determined the electronic structures, optical and thermoelectric propertes of Tl-filled skutterudite CoSb$_3$ by using the highly precise full-potential linearized augmented plane wave (FLAPW) method\footnote{Wimmer, Weinert, Krakauer, Freeman, Phys. Rev. B \textbf{24}, 864 (1981).} within the Perdew-Burke-Ernzerhof (PBE)\footnote{Perdew, Burke, Ernzerhof, Phys. Rev. Lett. \textbf{77}, 3865 (1996).} form of the generalized gradient approximation (GGA) to density functional theory. In contrast to the small-gap semiconducting CoSb$_3$, Tl-filled CoSb$_3$ is calculated to be metallic with Tl-$sp$ bands stongly hybridized with all the other elements over the entire energy region. The thermoelectric properties, \textit{e.g.} the Seebeck coefficient, are evaluated and discussed in terms of the diagonal terms of the optical matrix elements.   

Low-temperature Thermodynamic Properties of Eu-filled Skutterudites

It is well known that the presence of a ?rattling? atom in filled skutterudite antimonides leads to glasslike thermal properties, complemented by an unusual thermodynamic behavior that indicates the presence of low-energy vibrational modes in addition to the normal acoustic phonons. The current work focuses on a study of EuFe$_{4}$Sb$_{12}$, which combines ``rattling'' with magnetic ordering below T$_{c}$= 90 K. The elastic moduli have been measured as a function of temperature and magnetic field, using resonant ultrasound spectroscopy (RUS). The temperature-dependence of the elastic response is dominated by two phase transitions: the well-known magnetic ordering at 90 K, as well as a second transition at 40 K. In addition, specific heat measurements have been carried out for the same compound. Together these measurements provide us with an extensive set of data, probing the complex thermodynamic behavior of this material.   

Effects of Substitution and Grain Size on the High Temperature Thermoelectric Properties of MNiSn Phases.

The merit of n-type Sb doped MNiSn (M=Ti, Zr, Hf) half-Heusler phase, as a promising material for use in high temperature power generation, is exhibited by large thermopower, and small, semimetallic, resistivity values. It has been observed that the level of Sb doping on the Sn site plays a fundamental role in the determining the temperature at which the material will achieve maximum thermoelectric efficiency. Meanwhile, the high thermal conductivity found in ternary MNiSn, can be reduced via mass fluctuations and strain field effects induced through substitution at the M and Ni sites. In addition, the effects of grain size modification, through supplemental synthesis techniques, on the lattice thermal conductivity will be discussed. The combination of substitutions and grain size modification in the ternary half-Heusler system results in a complex thermoelectric material with a figure of merit on the order of ZT=1.   

Promising thermoelectric properties in some p-type half-Heuslers

N-type half-Heuslers are well-known due to their potential thermoelectric properties, however, our study shows that p-type half-Heusler alloys can be prepared with promising values. A series of Hf$_{x}$Zr$_{1-x}$Co$_{y}$Pt$_{1-y}$Sn$_{z}$Sb$_{1-z}$ samples have been synthesized, and thermoelectric properties have been measured. Our results show that the thermopower (S) increases and resistivity decreases in a good amount. In addition to the high power factors, thermal conductivity ($\kappa )$ kept low values, which indicate that we can get promising value by optimizing the combination of elements.   

Thermoelectric properties of some p-type half-Heusler alloys

We have synthesized a series of multi-component p-type half Heusler alloys by simultaneously substituting suitable elements at different crystallographic sites. Both electrical and thermal transport properties of these alloys are studied up to 1000K to evaluate their thermoelectric potential. At 300K, typical thermopower(S) and resistivity($\rho )$ values are around $\sim $ 80 $\mu $V/K and $\sim $1m$\Omega $-cm respectively. Due to heavy substitution/doping, ($\rho )$-T behavior is similar to that of a degenerate semiconductor and the resistivity increases with increasing T with $\rho $ values in the range of few m$\Omega $-cm at 1000K. Thermopower also increases with increasing T and reaches a maximum of hundreds of $\mu $V/K at 1000K. Room temperature thermal conductivity ($\kappa )$ values are about $\sim $ 4.5 W/m-K and these $\kappa $ values are low compared to those of ternary half-Heusler alloys. In well- optimized compositions, these values could improve even further.   

Growth, structural, and transport properties of epitaxial NaxCoO$_{2}$ thin films

Layered cobaltate NaxCoO$_{2}$ has attracted much attention recently due to its superconductivity and exceptionally high thermoelectric power. Here we report structural, electrical, and thermopower properties of epitaxial and topotaxial Na$_{x}$CoO$_{2}$ thin films on (0001) sapphire substrate. Topotaxial Na$_{x}$CoO$_{2}$ films were prepared by converting an epitaxial Co$_{3}$O$_{4}$ film to Na$_{x}$CoO$_{2 }$by annealing in Na vapor and epitaxial Na$_{x}$CoO$_{2 }$films were obtained by pulsed laser deposition. Structural analysis showed the films are $c$-axis oriented. For topotaxial films, annealing in different Na vapor pressures resulted in films with different Na concentrations, which showed distinct transport properties. For directly deposited epitaxial films by pulsed laser deposition, deposition parameters are found to control the Na concentration and hence the film properties. The largest thermoelectric power of the samples made by different methods is found to be similar in the range of 70-100 $\mu $V/K at room temperature.   

On the Thermoelectric Properties of Layered Cobaltates

A study on the thermoelectric properties of layered cobaltates is presented, based on the dynamic mean field theory for strongly correlated electron systems. Electron correlation results in a crossover from coherent quasi-particle excitation at low temperature to incoherent excitation at high temperatures in cobaltates. With an extremely narrow quasi-particle bandwidth (\textit{h$\omega $}$_{c} \quad \sim $ 50 meV), the thermal destruction of Fermi-liquid occurs at the moderate crossover temperature $T_{M}$ ($\sim $ 200 K), and suggests a new scaling for thermoelectric power $S$ of cobaltates ($S \quad \sim $ \textit{kT}/\textit{h$\omega $}$_{c} \quad \sim \quad T$/$T_{M}$ ) at low temperatures. At high temperatures, the dominating incoherent excitation leads to a weak temperature dependent $S$, and electric resistivity \textit{$\rho $ }approaches the Mott-limit \textit{ha}/$e^{2} \quad \sim $ a few m$\Omega \cdot $cm for cobaltates, where $a$ is a lattice constant.   

Microstructure and Growth Mechanism of Ca$_{3}$Co$_{4}$O$_{9}$ Thin Films on Si and Glass Substrates

It has been discovered recently that cobaltates have very large thermoelectric power, which shows that cobaltates hold great promise to be potential integrated heating spreading solution, such as thermal management of microprocessors. Among the cobaltates, Ca$_{3}$Co$_{4}$O$_{9}$ is exhibiting best thermoelectric properties. We have successfully grown highly c-axis orientated Ca$_{3}$Co$_{4}$O$_{9}$ thin films using Pulsed Laser Deposition (PLD) technique on amorphous substrates, such as glass. High-resolution electron microscopy (HREM), electron energy-loss spectroscopy (EELS) and dispersive x-ray spectrometry (EDS) have been used to study the chemical composition and microstructure of the films. The detailed microstructure and growth mechanism of Ca$_{3}$Co$_{4}$O$_{9}$ thin films will be discussed.