Electronic, vibrational and transport properties of pnictogen substituted ternary skutterudites

First principles calculations are used to investigate electronic band structure and vibrational spectra of pnictogen substituted ternary skutterudites. We compare the results with the prototypical binary composition CoSb$_3$ to identify the effects of substitutions on the Sb site, and evaluate the potential of ternary skutterudites for thermoelectric applications. Electronic transport coefficients are computed within the Boltzmann transport formalism assuming a constant relaxation time, using a new methodology based on maximally localized Wannier function interpolation. Our results point to a large sensitivity of the electronic transport coefficients to carrier concentration and to scattering mechanisms associated with the enhanced polarity. The ionic character of the bonds is used to explain the detrimental effect on the thermoelectric properties

Films of Ni–7 at% V, Pd, Pt and Ta–Si–N as diffusion barriers for copper on Bi2Te3

Films of Ni–7 at% V, Pt, Pd, and Ta40Si14N46, each approximately 100 nm thick, were magnetron-deposited and interposed between about 250 nm thick copper overlayers and Bi2Te3 single-crystalline substrates. The samples were then annealed in vacuum up to 350 degrees C. The performance of the metal and the tantalum-silicon-nitride films as diffusion barriers for in-diffusion of Cu and out-diffusion of Bi and Te was evaluated by 2.0 MeV 4He backscattering spectrometry and x-ray diffraction. The Ni–7 at% V, Pd and Pt films all fail to prevent interdiffusion of Cu and Bi2Te3 after a few hours of annealing at 200 degrees C. However, the Ta40Si14N46 barrier preserves the integrity of the contact after 250 degrees C for 50 h and 350 degrees C for 1 h anneals. These results confirm the superior characteristics of the metal-silicon-nitride films as diffusion barriers

Topological insulators in filled skutterudites

We propose new topological insulators in cerium filled skutterudite (FS) compounds based on ab initio calculations. We find that two compounds CeOs4As12 and CeOs4Sb12 are zero gap materials with band inversion between Os-d and Ce-f orbitals, which are thus parent compounds of two and three-dimensional topological insulators just like bulk HgTe. At low temperature, both compounds become topological Kondo insulators, which are Kondo insulators in the bulk, but have robust Dirac surface states on the boundary. This new family of topological insulators has two advantages compared to previous ones. First, they can have good proximity effect with other superconducting FS compounds to realize Majarona fermions. Second, the antiferromagnetism of CeOs4Sb12 at low temperature provides a way to realize the massive Dirac fermion with novel topological phenomena.Comment: 4 page, 3 figure

Skutterudites: An Update

Abstract Materials with the skutterudite crystal structure possess attractive transport properties and have a good potential for achieving ZT values substantially larger than for state-of-theart thermoelectric materials. Studies conducted at JPL on CoAs 3 , RhAs 3 , CoSb 3 , RhSb 3 and IrSb 3 have shown that ptype conductivity samples are characterized by carriers with low effective masses and very high mobilities, low electrical resistivities and moderate Seebeck coefficients. The carrier mobilities of n-type samples are about an order of magnitude lower, but low electrical resistivities and relatively large Seebeck coefficients can still be obtained at high doping levels. The room temperature lattice thermal conductivities of these binary skutterudites was found to be 7 to 10 times larger than that of Bi 2 Te 3 . This results in low ZT values at 300K, though very heavily doped n-type CoSb 3 samples can achieve ZT~1 at 600 o C. Several research groups, mostly in the U.S., are now working on understanding and optimizing the transport properties of skutterudites. Most of the efforts are focusing on reducing the lattice thermal conductivity by filling the empty octant cages in the skutterudite structure with rare earth atoms. Additional approaches have also been pursued at JPL, in particular the formation of solid solutions and alloys, and the study of novel ternary skutterudite compounds. Recent experiments have demonstrated that ternary compounds such as Ru 0.5 Pd 0.5 Sb 3 and filled skutterudites such as CeFe 4 Sb 12 had much lower lattice thermal conductivity. High ZT values have been obtained for several filled skutterudites in the 500-700 o C temperature range, but figures of merit at 300K are still low. This paper reviews recent experimental and theoretical results on skutterudites with a particular emphasis on the transport properties of ternary compounds and filled compositions. The latest results obtained at JPL are presented and the possibility of obtaining high ZT values near room temperature is discussed

Testing of Milliwatt Power Source Components

A milliwatt power source (MPS) has been developed to satisfy the requirements of several potential solar system exploration missions. The MPS is a small power source consisting of three major components: a space qualified heat source (RHU), a thermopile (thermoelectric converter or TEC) and a container to direct the RHU heat to the TEC. Thermopiles from Hi-Z Technology, Inc. of San Diego and the Institute of Thermoelectricity of Chernivtsi Ukraine suitable for the MPS were tested and shown to perform as expected, producing 40 mW of power with a temperature difference of about 170°C. Such thermopiles were successfully life tested for up to a year. A MPS container designed and built by Swales Aerospace was tested with both a TEC simulator and actual TEC. The Swales unit, tested under dynamic vacuum, provided less temperature difference than anticipated, such that the TEC produced 20 mW of power with heat input equivalent to a RHU

Some Properties of Re 2 Te 5 -based Materials

Abstract Re2Te5 is a semiconducting compound with an energy band gap of about 0.8 eV. It has a relatively complex crystal structure with 84 atoms per unit cell. Initial results obtained on p-type polycrystalline samples showed that they possess large Seebeck coefficient values but large electrical resistivity values. They also exhibit very low thermal conductivity with a room temperature value of 13 mW/cmK. Another attracting feature of Re2Te5 is the possibility to insert a variety of atoms in the large voids (2.8 Å in diameter) of the crystal structure to form Re6M2Te15 filled compositions. The void fillers could act as phonon scattering centers, further reducing the thermal conductivity in these materials. As part of an effort to evaluate the potential of Re2Te5-based materials for thermoelectric applications, we are currently exploring the synthesis and properties of filled compositions as well as n-type Re2Te5 samples. We present and discuss in this paper initial results obtained on Fe and Ag doped Re2Te5 samples

First Principles Study of Zn-Sb Thermoelectrics

We report first principles LDA calculations of the electronic structure and thermoelectric properties of $\beta$-Zn$_{4}$Sb$_{3}$. The material is found to be a low carrier density metal with a complex Fermi surface topology and non-trivial dependence of Hall concentration on band filling. The band structure is rather covalent, consistent with experimental observations of good carrier mobility. Calculations of the variation with band filling are used to extract the doping level (band filling) from the experimental Hall number. At this band filling, which actually corresponds to 0.1 electrons per 22 atom unit cell, the calculated thermopower and its temperature dependence are in good agreement with experiment. The high Seebeck coefficient in a metallic material is remarkable, and arises in part from the strong energy dependence of the Fermiology near the experimental band filling. Improved thermoelectric performance is predicted for lower doping levels which corresponds to higher Zn concentrations.Comment: 5 pages, 6 figure

Thermal properties of electrodeposited bismuth telluride nanowires embedded in amorphous alumina

3 pages, 3 figures.Bismuth telluride nanowires are of interest for thermoelectric applications because of the predicted enhancement in the thermoelectric figure-of-merit in nanowire structures. In this letter, we carried out temperature-dependent thermal diffusivity characterization of a 40 nm diameter Bi2Te3 nanowires/alumina nanocomposite. Measured thermal diffusivity of the composite decreases from 9.2×10–7 m2 s–1 at 150 K to 6.9×10–7 m2 s–1 at 300 K and is lower than thermal diffusivity of unfilled alumina templates. Effective medium calculations indicate that the thermal conductivity along nanowires axis is at least an order of magnitude lower than thermal conductivity of the bulk bismuth telluride.G.C. would like to acknowledge financial support from JPL and DOE. M.S.M.G. acknowledges a fellowship awarded by the MCYT (Spain) in the Ramon y Cajal Program.Peer reviewe

Miniaturized Radioisotope Solid State Power Sources

Abstract. Electrical power requirements for the next generation of deep space missions cover a wide range from the kilowatt to the milliwatt. Several of these missions call for the development of compact, low weight, long life, rugged power sources capable of delivering a few milliwatts up to a couple of watts while operating in harsh environments. Advanced solid state thermoelectric microdevices combined with radioisotope heat sources and energy storage devices such as capacitors are ideally suited for these applications. By making use of macroscopic film technology, microgenerators operating across relatively small temperature differences can be conceptualized for a variety of high heat flux or low heat flux heat source configurations. Moreover, by shrinking the size of the thermoelements and increasing their number to several thousands in a single structure, these devices can generate high voltages even at low power outputs that are more compatible with electronic components. Because the miniaturization of state-of-the-art thermoelectric module technology based on Bi 2 Te 3 alloys is limited due to mechanical and manufacturing constraints, we are developing novel microdevices using integrated-circuit type fabrication processes, electrochemical deposition techniques and high thermal conductivity substrate materials. One power source concept is based on several thermoelectric microgenerator modules that are tightly integrated with a 1.1W Radioisotope Heater Unit. Such a system could deliver up to 50mW of electrical power in a small lightweight package of approximately 50 to 60g and 30cm 3 . An even higher degree of miniaturization and high specific power values (mW/mm 3 ) can be obtained when considering the potential use of radioisotope materials for an alpha-voltaic or a hybrid thermoelectric/alpha-voltaic power source. Some of the technical challenges associated with these concepts are discussed in this paper