Session L20: Focus Session: Thermoelectric Materials: Skutterudites, Novel and Nanostructured Materials

Synthesis and thermoelectric property of Ca-doped n-type Bi$_{85}$Sb$_{15}$ alloy

Bi$_{1-x}$Sb$_{x}$ (0.09$<$x$<$0.20) alloys are n-type semiconducting materials that exhibit a good thermoelectric property at low temperature, around 80 K. In the present work we investigated the thermoelectric properties of undoped Bi$_{85}$Sb$_{15}$ and different Ca-doped Bi$_{85}$Sb$_{15}$Ca$_{x}$ alloys (x=0.5, 2, and 5) synthesized via arc-melting first and followed by ball milling and hot pressing. Effect of different Ca doping levels on transport properties of Bi$_{85}$Sb$_{15}$ alloys has been investigated. It is found that thermal conductivity decreases with increasing Ca. Electrical transport measurements show that power factor increases with doping level of Ca up to Bi$_{85}$Sb$_{15}$Ca$_{2}$ and then decreases, yielding the maximum value of power factor of 3.8$\times $10$^{-3}$ Wm$^{-1}$K$^{-2}$ and ZT of 0.39 at room temperature for Bi$_{85}$Sb$_{15}$Ca$_{2}$. Properties at below room temperature will also be presented.   

Exploration of Electron Poor Materials and their thermoelectric properties

The Electron Poor Materials (EPM); ZnSb, ZnAs which have an average 3.5 valence electrons are explored via ab initio calculations. These materials are of interest for thermoelectric research. The EPM are then compared to valence balanced zinc-blende materials; InSb, GaSb, ZnSe, and ZnTe. Analysis of binding to assess the interesting electronic properties such as the role of nonclassical four-center bonds and the thermoelectric Seebeck coefficient are discussed. Bandstructure comparisons to a simple tight-binding model (Linear Combination of Atomic Orbitals (LCAO)) are preformed in order to test the effects of the atomic orbitals on the electronic structure.   

Chemical Doping Effect on the Thermoelectric Properties of $T$Ga$_{3 }(T$ = Fe, Ru, Os)

Thermoelectric properties of chemically-doped intermetallic narrow-band semiconductors: $T$Ga$_{3 }(T$ = Fe, Ru, Os) are reported. The parent compounds show semiconductor-like behavior ($E_{g} \quad \sim $ 0.2 eV, $n_{290K} \quad \sim $ 10$^{18 }$cm$^{3})$ with large $n-$type Seebeck coefficients at room temperature ($S_{290K }\sim $ -300 $\mu $V/K). The semiconductor-like FeGa$_{3}$ becomes metallic upon chemical doping (adding electron carriers), but RuGa$_{3}$ and OsGa$_{3}$ remain semiconducting. While the electrical resistivity and the Seebeck coefficients of all the compounds decrease with electron doping, the Seebeck coefficients remain fairly large and $n-$type, which leads to larger power factors than those of the pure samples. The thermal conductivity (\textit{$\kappa $}$_{290K }$= 1.6 W/m K) of electron-doped FeGa$_{3}$ decreases, which increases the room temperature power factor by a large percentage ($S^{2}$\textit{/$\rho $}$_{290K}$ = 60 $\mu $W/m K$^{2})$ over that of pure FeGa$_{3}$. This improvement in the power factor leads to a corresponding enhancement in the thermoelectric figure of merit (\textit{ZT}) -- a factor of 5 increases above undoped polycrystalline FeGa$_{3}$ and two orders of magnitude improvement over that of pure single crystalline FeGa$_{3}$.   

Doping dependence of thermoelectric performance in Mo$_3$Sb$_7$: first principles calculations

Experimental studies have indicated the substantial thermoelectric promise of doped Mo$_3$Sb$_7$, with a figure-of-merit ZT of 0.9 (H. Xu {\it et al}, J. Appl. Phys. {\bf 105}, 053703 (2009)) already achieved at high temperature. However, optimal doping levels have not yet been achieved. We study doping of Mo$_3$Sb$_7$ with transition metals (Ni,Fe,Co,Ru) via first principles calculations, including electronic structure, lattice dynamics and Boltzmann transport. We discuss the selection of dopant and the potential thermoelectric performance of optimally doped Mo$_{3}$Sb$_{7}$.   

Giant Seebeck Coefficient in V-TCNE thin films

The disordered structure of organic conductors results in a naturally low thermal conductivity ($\kappa )$ but their ZT is known to be low because of their low thermopower (S) and electrical conductivity ($\sigma )$. Here we report an exception, with results obtained from 220 to 320K for the thermopower of V-TCNE$_{x}$ (V-(C$_{2}$(CN)$_{4})_{x})$ thin films deposited on a Si wafer (111). At room temperature S=+21.8 mV/K and increases with decreasing temperature. Those values are matched only by very pure semiconductors such as Si at low temperature, Bi nanowires, or strongly correlated electron systems like FeSb$_{2}$. The valence band of V-TCNE has a very high density of states over a very narrow energy range, ascribed mostly to vanadium 3d(t$_{2g})$ orbitals,\footnote{Y-J Yoo et al., Nat. Mat. \textbf{9} 638 2010} which is consistent with the exceptionally large value of S. The dependence of S and $\sigma $ upon illumination will also be shown, alongside preliminary estimates for the ZT.   

Einstein Modes in the Phonon Density of States of the Single-Filled Skutterudite Yb$_{0.2}$Co$_{4}$Sb$_{12}$

Measurements of the phonon density of states by inelastic neutron scattering and specific heat measurements along with first principles calculations, provide compelling evidence for the existence of an Einstein oscillator (\emph{rattler}) at ${\omega}_{E1} \approx$ 5.0 meV in the filled skutterudite Yb$_{0.2}$Co$_{4}$Sb$_{12}$. Multiple dispersionless modes in the measured density of states of Yb$_{0.2}$Co$_{4}$Sb$_{12}$ at intermediate transfer energies (14 meV $\leq$ \emph{$\omega$} $\leq$ 20 meV) are exhibited in both the experimental and theoretical \emph{density of states} of the Yb-filled specimen. A peak at 12.4 meV is shown to coincide with a second Einstein mode at ${\omega}_{E2} \approx$ 12.8 meV obtained from heat-capacity data. The emergence of local modes at intermediate transfer energies is attributed to altered properties of the host CoSb$_{3}$ cage as a result of Yb filling. It is suggested that these modes are owed to a complementary mechanism for the scattering of heat-carrying phonons in addition to the mode observed at ${\omega}_{E1} \approx$ 5.0 meV.   

On the role of nanostructure on the thermal conductivity of skutterudite thermoelectrics

One of the most effective strategies to improve the thermoelectric figure of merit in skutterudites is to reduce thermal conductivity via alloying, filling, or nanostructuring. The latter is most effective when the dimension of the domains is comparable in size to the mean free path of the dominant heat-conducting phonons. In bulk, pristine semiconductors and insulators thermal conductivity and phonons' mean-free paths can nowadays be calculated fully from first-principles from the anharmonic terms in the ionic displacements. We show here our results for the lattice thermal conductivity of several compounds with the skutterudite structure, obtained from the Boltzmann transport equation using phonon lifetimes determined from density functional calculations. We will also discuss the effect of fourth-order terms, albeit as obtained using phenomenological approaches. Last, we comment on the interplay between the different length scales for the nanostructured domains and the relevant heat-carrying phonons.   

Thermoelectric Properties of p-type Yb Filled Skutterudite Yb$_{y}$Fe$_{x}$Co$_{4-x}$Sb$_{12}$

Since the discovery in 1995 of high thermoelectric figure of merit in skutterudite compounds, much work has been done to optimize the thermoelectric properties of these materials. As a result of this effort, n-type skutterudites are available today with figure of merit \textit{ZT} approaching 1.8. By contrast, p-type skutterudites have lagged behind, with the best materials having figure of merit less than unity. In this study, we report the thermoelectric and magnetic properties of p-type Yb-filled skutterudites of nominal composition Yb$_{y}$Fe$_{x}$Co$_{4-x}$Sb$_{12}$ with the aims of extending our knowledge of the filled skutterudite family and enhancing the thermoelectric properties of these p-type materials.   

First principles calculations of the interactions of a filling atom with its neighboring atoms in a skutterudite (LaFe4Sb12)

The room temperature lattice thermal conductivity of filled skutterudites is about a factor of 5 smaller than that of unfilled skutterudites which has caused a great deal of attention to be focused on these materials from a scientific standpoint, as well as a technological one, i.e., thermoelectric applications. In an effort to gain a microscopic understanding of this we have previously used a central force model and Green-Kubo techniques with force parameters heavily based on first principles results [Bernstein et al., Phys Rev. B {\bf 81}, 134301 (2010)]. However, as we had no first principles information on the La-Fe cubic anharmonic parameters in LaFe4Sb12 we have performed new direct method calculations for a larger supercell than the Bravais cell used previously to compute not only the six independent La-Fe cubic anharmonic parameters but numerous other parameters. Atomic forces were computed in various structural configurations differing only by the coordinates of one of the two La positions in the simple cubic supercell. DFT results are compared for LAPW, PAW, and plane-wave pseudopotential methods.   

Thermoelectric Technology for Automotive Waste Heat Recovery

Essential to the long term success of advanced thermoelectric (TE) technology for practical waste heat recovery is fundamental physics and materials research aimed at discovering and understanding new high performance TE materials. Applications of such new materials require their development into efficient and robust TE modules for incorporation into real devices such as a TE generator (TEG) for automotive exhaust gas waste heat recovery. Our work at GM Global R{\&}D includes a continuing investigation of Skutterudite-based material systems and new classes of compounds that have potential for TE applications. To assess and demonstrate the viability of a TEG using state-of-the-art materials and modules, we have designed, fabricated, installed, and integrated a working prototype TEG to recover exhaust gas waste heat from a production test vehicle. Preliminary results provide important data for the operation and validation of the mechanical, thermal, and electrical systems of the TEG in combination with the various vehicle systems (e.g., exhaust bypass valve and controls, thermocouples, gas and coolant flow and pressure sensors, TE voltage and output power). Recent results from our materials research work and our functioning automotive TEG will be presented.   

Bottom-Up Strategy for Thermoelectric Nanocomposites

Thermoelectric (TE) materials that incorporate nano-scale domains offer potential control over electrical and thermal properties simultaneously. A bottom-up strategy may provide cost-effective, scalable, and reproducible processing of TE materials~with improved~TE properties above existing materials. The strategy involves composition and size controlled syntheses of TE materials as nanocrystals by employing facile solution based processes followed by densification into bulk nanocomposite pellets using Spark Plasma Sintering. In this talk an overview of the various solution phase synthesis processes for preparing nanocrystals of different TE materials will be presented. In addition the TE properties after SPS densification will be discussed in relation to composition and grain size within the nanocomposites. Experimental results will be assessed together with theoretical modeling in describing the effect of the nano-scale domains on the TE properties.   

Formation Mechanisms of Embedded Zincblende and Wurtzite Nitride Nanocrystals

Semiconductor nanocomposites have been proposed for high figure of merit thermoelectrics. A promising approach to nanocomposite synthesis is matrix-seeded growth, which involves ion-beam-amorphization of a semiconductor film, followed by nanoscale re-crystallization via rapid thermal annealing (RTA) [1]. In this work, we are studying the formation and evolution of N ion-implanted InAs and GaAs. Low temperature (77K) N ion implantation into InAs leads to the formation of an amorphous layer with crystalline InAs remnants. RTA at up to 550\r{ }C leads to the nucleation of zincblende (ZB) InN nanocrystals (NC). RTA at 600\r{ }C leads to nucleation of both ZB and wurtzite (WZ) InN, with an increase in average NC size. These results are consistent with the predictions of a thermodynamic model for the nanoscale-size-dependence for nucleation of ZB and WZ InN. We are also developing a novel approach to \textit{direct} the seeding of nanostructure arrays, using a combination of focused-ion-beam (FIB) implantation and conventional ion implantation. To date, we have demonstrated the selective positioning of WZ and ZB GaN NCs using 75keV and 100keV N implantation, followed by FIB patterning and 800\r{ }C RTA. [1] X. Weng, et al, \textit{J. Appl. Phys}. \textbf{97,} 64301 (2005).   

Thin film thermocouples for thermoelectric characterization of nanostructured materials

The increased use of nanostructured materials as thermoelectrics requires reliable and accurate characterization of the anisotropic thermal coefficients of small structures, such as superlattices and quantum wire networks. Thin evaporated metal films can be used to create thermocouples with a very small thermal mass and low thermal conductivity, in order to measure thermal gradients on nanostructures and thereby measure the thermal conductivity and the Seebeck coefficient of the nanostructure. In this work we confirm the known result that thin metal films have lower Seebeck coefficients than bulk metals, and we also calibrate the Seebeck coefficient of a thin-film Ni/Cr thermocouple with 50 nm thickness, showing it to have about 1/4 the bulk value. We demonstrate reproducibility of this thin-filmSeebeck coefficient on multiple substrates, and we show that this coefficient does, in fact, change as a function of film thickness. We will discuss prototype measurement designs and preliminary work as to how these thin films can be used to study both Seebeck coefficients and thermal conductivities of superlattices in various geometries. The same technology can in principle be used on integrated circuits for thermal mapping, under the name ``Integrated On-Chip Thermocouple Array'' (IOTA).