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Traditional manufacturing methods restrict the expansion of thermoelectric technology. Here, we demonstrate a new manufacturing approach for thermoelectric materials. Selective laser melting, an additive manufacturing technique, is performed on loose thermoelectric powders for the first time. Layer-by-layer construction is realized with bismuth telluride, Bi2Te3, and an 88% relative density was achieved. Scanning electron microscopy results suggest good fusion between each layer although multiple pores exist within the melted region. X-ray diffraction results confirm that the Bi2Te3 crystal structure is preserved after laser melting. Temperature-dependent absolute Seebeck coefficient, electrical conductivity, specific heat, thermal diffusivity, thermal conductivity, and dimensionless thermoelectric figure of merit ZT are characterized up to 500 °C, and the bulk thermoelectric material produced by this technique has comparable thermoelectric and electrical properties to those fabricated from traditional methods. The method shown here may be applicable to other thermoelectric materials and offers a novel manufacturing approach for thermoelectric devices.
Double-filled high Fe content skutterudites, BaxYbyFe3CoSb12 (x + y = 1), were synthesized to investigate their high temperature transport properties. Both their phase and stoichiometry were characterized by powder x-ray diffraction and energy dispersive spectroscopy. The Seebeck coefficient, S, and electrical resistivity, ρ, increase with increasing temperature for all specimens over the entire measured temperature range. The thermal conductivity for the two low Ba content specimens decreases with increasing temperature up to 550 K at which point it increases with temperature due to bipolar diffusion. Bipolar diffusion becomes negligible with increasing Ba content. Due to this low bipolar diffusion, the ZT values of the higher Ba content specimens increase linearly with temperature, with the highest ZT value obtained for Ba0.9Yb0.1Fe3CoSb12.
We present a theoretical model for carrier conductivity and Seebeck coefficient of thermoelectric materials composed of nanogranular regions. The model is used to successfully describe experimental data for chalcogenide PbTe nanocomposites. We also present similar calculations for skutterudite CoSb3 nanocomposites. The carrier scattering mechanism is considered explicitly and it is determined that it is a key factor in the thermoelectric transport process. The grain interfaces are described as potential barriers. We investigate theoretically the role of the barrier heights, widths, and distances between the barriers to obtain an optimum regime for the composites thermoelectric characetristics.
Dimensional nanocomposites of PbTe with varying carrier concentrations were prepared from undoped and Ag doped PbTe nanocrystals synthesized utilizing an alkaline aqueous solution-phase reaction. The nanocrystals were densified by Spark Plasma Sintering (SPS) for room temperature resistivity, Hall, Seebeck coefficient, and temperature dependent thermal conductivity measurements. The nanocomposites show an enhancement in the thermoelectric properties compared to bulk PbTe with similar carrier concentrations, thus demonstrating a promising approach for enhanced thermoelectric performance.
Doped lead telluride dimensional nanocomposites were prepared by densifying nanocrystals synthesized employing an alkaline aqueous solution-phase reaction. The nanocrystal synthesis procedure resulted in high product yields of over 2 g per batch. These nanocrystals were then subjected to Spark Plasma Sintering (SPS) for densification. Transport properties were evaluated through temperature dependent resistivity, Hall, Seebeck coefficient, and thermal conductivity measurements. The results for these lead telluride nanocomposites were compared to bulk polycrystalline lead tellurides with similar carrier concentrations.
Preliminary results from an investigation into the synthesis and characterization of silicon and germanium type II clathrates are reported. A series of NaxSi136 (0 < x < 24) clathrates was synthesized and characterized by powder X-ray diffraction and Rietveld analysis. The NaxSi136 lattice parameters are observed to first decrease, then increase with increasing Na content, indicating a non-monotonic structural response to Na filling. New type II Ge clathrate compositions Cs8Na16MyGe136-y (M = Cu, In) utilizing framework substitution are reported. Electrical transport measurements on a Cu substituted specimen indicate framework substitution modifies the transport properties of these materials. The potential type II clathrate phases possess for thermoelectric applications is discussed.
Good thermoelectric materials possess low thermal conductivity while maximizing electric carrier transport. This article looks at various classes of materials to understand their behavior and determine methods to modify or “tune” them to optimize their thermoelectric properties. Whether it is the use of “rattlers” in cage structures such as skutterudites, or mixed-lattice atoms such as the complex half-Heusler alloys, the ability to manipulate the thermal conductivity of a material is essential in optimizing its properties for thermoelectric applications.
In the filled gallium-germanium clathrates, R8Ga16Ge30, where R is Ba, Sr, or Eu, the guests are located in two large cages and are weakly bound to the crystalline clathrate framework. The caged guests exhibit a localized “rattling” vibrational mode that provides an efficient mechanism for reducing the thermal conductivity. Inelastic neutron scattering and nuclear inelastic scattering measurements have yielded the phonon density of states in R8Ga16Ge30; the line width of the localized vibrational modes is found to be an important parameter in determining the lattice thermal conductivity. Neutron diffraction studies on R8Ga16Ge30 have shown that the guests in the larger cage are located off-center, and it was proposed that their jumping about the four off-center locations is responsible for the observed glass-like thermal conductivity at temperatures below 10 K. The detection of such slow guest motion is challenging because the typical time and energy scales involved are ca. 4 ns and 1 µeV, respectively. We have studied the slow europium tunneling dynamics in Eu4Sr4Ga16Ge30 by both Mössbauer and microwave absorption spectroscopy.
We have synthesized the type II silicon clathrates Na1Si136 and Na8Si136, and report on the electrical and thermal transport in these materials. The crystal structure consists of a covalently bonded silicon framework in which sodium guest atoms are encapsulated inside the silicon host framework. Differential scanning calorimetry measurements show the compounds decompose above 600°C to diamond-structure silicon. Temperature dependant electrical resistivity measurements show the specimens to have an insulating character, with magnitudes that decrease with increasing sodium content. For the first time, thermal conductivity measurements on type II sodium-silicon clathrates are presented. The thermal conductivity is very low for both specimens, and for Na8Si136 exhibits a clear dip in the range from 50 to 70 K. These data suggest that the “rattling” behavior observed in type I clathrates may also be present in type II clathrates.
The electronic structure and thermoelectric properties of Yb partially filled CoSb3 skutterudite compounds have been investigated by x-ray photoelectron spectroscopy and band calculation in terms of an itinerant f electron model. In these materials, the significant effect of Yb filling is the large reduction of lattice thermal conductivity, remaining relatively high electron mobility and Seebeck coefficient, resulting in high thermoelectric figure of merit. We discuss the effects of the valence fluctuation between Yb2+ and Yb3+ and the strong hybridization of Yb 4f states with the valence band states on the electronic properties and their relation to thermoelectric properties for Yb partially filled CoSb3 compounds.
Filled skutterudites exhibit properties that comply with the concept of a “phonon-glass electron-crystal”, as proposed by Slack. The optimal filled skutterudite would have filler atoms that exhibit large thermal vibration amplitudes in the voids of the crystal structure. It is desirable that these loosely bound atoms give rise to strong phonon scattering without greatly affecting the essential part of the band structure of the skutterudites. This criterion is difficult to meet. Most attempts have employed charge compensation for filling fractions above 50 %. In this report we present the use of a high-pressure technique for the synthesis of new filled skutterudites. By using our high-pressure synthesis technique CoSb3-based skutterudites filled with group-14 elements (Ge, Sn, and Pb) have been synthesized with up to 100 % filling without charge compensation of the host lattice. The structural analysis reveals that the Sn atoms exhibit very large thermal vibration amplitude, indicative of a large “rattling” motion. The Sn-filled specimens exhibit a low thermal conductivity, lower than that of any previously reported filled skutterudite, while the favorable semiconducting nature of the host lattice is not substantially changed by Sn filling. Tin atoms may therefore be better “rattlers” in the CoSb3 host lattice than lanthanide or actinide atoms.
In a good semiconductor the electrons (or holes) propagate through the lattice structure of well ordered atoms without being scattered by the coherent vibrations of the crystal. Thus semiconductors are good conductors of electrons (or holes) and as such have given rise to modern microprocessors that are revolutionizing the way we live. In the same semiconductors the vibrations of the lattice atoms mainly carry the heat. Due to the covalent nature of the bonding in these materials the thermal conductivity is very large. These are therefore poor materials for thermoelectric applications. If the atomic vibrations, or phonons, can be localized so that the heat transfer is essentially an atom-to- atom propagation, then the thermal conduction can be drastically reduced. A semiconductor can, in principle, have the thermal conductivity of a glass. Amorphous semiconductors are of course very poor conductors of electricity therefore one does not want the electrons to propagate through a glass-like material. Instead one wants the electrons to travel as though they only “see” the well-ordered, periodic structure of a crystal while the phonons are scattered by localized disorder within the covalently bonded lattice. “Open structure” semiconductors do, in fact, exist and recent research has given rise to new thermoelectric materials.
Several new materials in the CsBi4Te6, A2Bi8Se13, (A = K, Rb, Cs), HoNiSb, Ba/Ge/B (B = In, Sn), and AgPbBiQ3 (Q = S, Se, Te) systems have shown promising characteristics for thermoelectric applications. New synthesis techniques are able to produce samples at much higher rates than previously possible. This has led to a persistent challenge in thermoelectric materials research of rapid and comprehensive characterization of samples. This paper presents a description of a new 4-sample transport measurement system and the related measurement techniques. Special features of the system include fully computer-controlled operation (implemented in LabView™) for simultaneous measurement of electrical conductivity, thermo-electric power, and thermal conductivity. This system has been successfully used to characterize several new thermoelectric materials (including some of the above-mentioned compounds) and reference materials exhibiting a wide range of thermal conductivities.
Filled skutterudite compounds possess very low thermal conductivities due to the scattering of a wide range of phonon modes caused by a loosely bound cation incorporated in a cavity of the structure. The inclusion of such a filler cation causes several synthetic difficulties since the desired compounds are thermodynamically unstable with respect to disproportionation. Modulated elemental reactants were used in this study to circumvent these difficulties. SnxCo4Sb12 samples with x=0.5 and nearly 1.0 were synthesized using this method. To prevent nucleation of unwanted binary compounds, the repeat unit made up of elemental layers was less than 20 angstroms 500mg of each sample were produced, allowing for the samples to be hot pressed into a pellet. Structural analysis as well as measurements of the physical properties are presented.
We discuss ongoing work in three areas of thermoelectric materials research: 1) broad band semiconductors featuring anion networks, 2) filled skutterudites, and 3) polycrystalline Bi-Sb alloys. Key results include: a preliminary evaluation of a previously untested ternary semiconductor, KSnSb; the first reported data in which Sn is used as a charge compensator in filled antimonide skutterudites; the finding that Sn doping does not effect polycrystalline Bi1−xSbx as it does single crystal samples.
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