The U.S. military uses large amounts of fuel during deployments and battlefield operations. Consequently, the U.S. military has a strong need to develop technologies that increase fuel efficiency and minimize fuel requirements all along the logistics trail and in all battlefield operations. There are additional requirements to reduce and minimize the environmental footprint of various military equipment and operations and reduce the need for batteries (non-rechargeable) in battlefield operations. The tri-agency SERDP (Strategic Environmental Research and Development Program) office is sponsoring a challenging, high-payoff project to develop a lightweight, small form-factor, soldier-portable advanced thermoelectric generator (TEG) system prototype to recover and convert waste heat from a variety of deployed equipment with the ultimate purpose of obtaining additional power for soldier battery charging, advanced capacitor charging, and other battlefield power applications. The project seeks to achieve power conversion efficiencies of 10% (double current commercial TE conversion efficiencies) in a system with ˜1.6-kW power output for a spectrum of battlefield power applications. In order to meet this objective, the project is taking on the multi-faceted challenges of tailoring LAST/LASTT-based thermoelectric (TE) materials for the proper temperature ranges (300 K – 700 K), fabricating these materials with cost-effective hot-pressed and sintered processes while maintaining their TE properties, measuring and characterizing their thermal fatigue and structural properties, developing the proper manufacturing processes for the TE materials and modules, designing and fabricating the necessary microtechnology heat exchangers, and fabricating and testing the final TEG system. The ultimate goal is to provide an opportunity to deploy these TEG systems in a wide variety of current military equipment. This would help the Army in achieving one of the Office of Secretary of Defense’s major strategic objectives to maintain and enhance operational effectiveness while reducing total force energy demands. The presentation will review the progress made on 1) the performance of LAST / LASTT TE materials and tailoring their temperature dependency; 2) evaluating the structural (Elastic modulus, Poisson’s ratio and mechanical strength) properties of these materials, 3) development of the necessary LAST/LASTT-based TE modules, 4) development of the required hot- and cold-side microtechnology heat exchangers, and 5) the overall system designs for 30 kW and 60 kW TQG applications and potential performance pathways/differences for these two TQG cases. This work leverages critical fundamental research performed by the Office of Naval Research in developing LAST/LASTT materials.

Germanium dioxide nanowires have gained significant interest lately due to the the bandgap of 2.44 eV, and high index of refraction, n=1.63. In this paper we aim at investigating the lateral growth of high density metal-catalyzed germanium dioxide nanowires between electrodes. The gaps between two electrodes varied. The catalyst metal was placed on the electrodes, followed by a thermal annealing process, resulting in lateral growth of germanium dioxide whiskers with several microns length and eventually the formation of bridge-like nanostructures. These whiskers have certain unique properties, such as a high surface-to-volume ratio and the capability of connecting two electrodes. Because of these properties, the growth of whiskers from the electrodes has the potential to be developed as electronic devices such as nanosensors. These products are characterized by scanning electron microscopy (SEM), as well as X-Ray diffraction (XRD).

Derivatives of β-K2Bi8Se13 are an interesting series of materials for thermoelectric investigations due to their very low thermal conductivity and highly anisotropic electrical properties. Up to now substitutions on the Bi and alkali metal sites have been studied in order to tune the thermoelectric properties. In this work, the thermoelectric properties of the sulfur-substituted K2Bi8Se13−xSx (0<x<13) are presented with respect to Seebeck coefficient, the electrical and thermal conductivity as a function of temperature. Seebeck coefficient measurements showed the n-type character of all members while electrical conductivity shows higher values compare to the other solid solution series of the same type. The lattice thermal conductivity is affected due to the Se/S disorder. The temperature dependence of the figure-of-merit ZT shows that these materials have potential for high temperatures applications with promising thermoelectric performance.

AgPb10SbTe12 is one member of the cubic family of materials AgPbmSbTem+2, which adopts NaCl structure where Ag, Pb and Sb atoms occupy the Na site and Te atoms occupy the Cl site. Ingots of this compound were prepared by a solid state reaction for thermoelectric measurements. AgPb10SbTe12 is a narrow band gap semiconductor with Eg∼0.26 eV. In order to optimize the ZT of this member, compositions with deficiency of Ag and Bi-substitution were examined and found to exhibit enhanced power factor at 300 K. The Bi-substituted ingot had ZT∼0.39 at 300 K and ZT∼0.68 at 400 K. Carrier concentration and the mobility measurements are reported.

The known mineral Pavonite AgBi3S5 shows a complex structure composed of NaCl type fragments but has not been studied from the thermoelectric point of view. We present initial results on the synthesis and themoelectric properties of synthetic AgBi3S5, which shows n-type metallic conductivity. In addition, the examination of the solid solutions AgSbxBi3-xS5 (x=0.3) is reported.

Recent interest in thermoelectric materials for power generation applications has initiated the development of a measurement system in our laboratory for characterization of materials in the 80K to 800K temperature range. This system has been specifically designed for measuring thermoelectric power and electrical conductivity as needed for determining the power factor of the measured samples. This is a single sample measurement system based on a continuous flow cryostat. Significant effort has gone into the computer controlled data acquisition and PID controlled temperature stabilization. Investigation of the influence of temperature stability on the measured data will be presented along with important aspects of the system design, development, and testing. Data collected on reference materials and new thermoelectric materials of interest will be presented.

Solid solution series of the type K2Bi8-xSbxSe13, K2-xRbxBi8Se13 as well as K2Bi8Se13-xSx were prepared and the distribution of the atoms (Bi/Sb, K/Rb and Se/S) on different crystallographic sites, the band gaps and their thermoelectric properties were studied. The distribution Se/S appears to be more uniform than the distribution of the Sb and Rb atoms in the β-K2Bi8Se13 structure that shows preference in specific sites in the lattice. Band gap is mainly affected by Sb and S substitution. Seebeck coefficient measurements showed n-type character for of all Se/S members. In the Bi/Sb series an enhancement of p-type character was observed. The thermoelectric performance as well as preliminary high temperature measurements suggest the potential of these materials for high temperature applications.

Thermoelectrics is a multidiscipline area of study, rich in condensed matter physics, chemistry, engineering, and material science. The figure of merit used for evaluating individual materials consists of three interdependent material properties. The measurement of these properties should be taken on the same sample for all three measurements, preferably simultaneously. Each of these measurements requires close attention to potential sources of losses for accurate analysis of the materials and testing of theoretical models. For example, relatively simple scanning measurement techniques can be used to gain insight into accurate geometry measurements and influences of contact dimensions. In addition, the field of thermoelectrics spans a wide temperature range, from cryogenic temperatures to > 1000 °C. This requires systems capable of large temperature variations, and/or multiple measurement systems for various ranges of interest. Additional measurements, such as Hall effect, help to gain further insight into the material properties and their optimization. The number and importance of measurements is further extended as the development of devices from these new materials is initiated, where studies of contact resistance and overall device performance must be evaluated. For mechanical robustness of fabricated modules, properties such as the coefficient of thermal expansion, and grain size for the new materials are of interest. Models for device behavior are useful in evaluating the measured results and further extracting material and device properties. In this paper, we review measurements used in evaluating bulk thermoelectric materials some of the information that can be extracted from these measurements, along with a model that can be used in conjunction with these measurements for module design.

By introducing of various equivalents of PbTe into the layered framework of CsBi4Te6, the four new compounds CsPbBi3Te6 (1), CsPb2Bi3Te7 (2), CsPb3Bi3Te8 (3) and CsPb4Bi3Te9 (4), were discovered. The compounds adopt layered structures built up of anionic slabs of progressively increasing thickness. The [PbmBi3Te5+m]- (m = 1, 2, 3, 4) slabs in the four structures can be viewed as fragments excised from the PbTe-type structures with 4-, 5-, 6- and 7-monolayers, respectively. As prepared, these materials are off-stoichiometric and n-type conductors. We present preliminary results of the crystal structures and thermoelectric properties of these materials and classify them as members of the new homologous series CsPbmBi3Te5+m (m = 1 to 4).

The compound types A1+xM3-2xBi7+xSe14, A1−xM3−xBi11+xSe20, A1−xM4−xBi11+xSe21 and A1−xM5−xBi11+xSe22 (A = K, Rb, Cs; M = Sn, Pb) form from reactions involving A2Se, Bi2Se3, M and Se. The single crystal structures reveal that they are all structurally related so that they all belong to the homologous series Am[M6Se8]m[M5+nSe9+n] (M = di- and trivalent metal), whose characteristics are three-dimensional anionic frameworks with tunnels filled with alkali ions. The building units that make up these structures are derived from different sections of the NaCl lattice. In these structures, the Bi and Sn (Pb) atoms are extensively disordered over the metal sites of the chalcogenide network, giving rise to very low thermal conductivity. These phases are all narrow gap semiconductors with 0.25 < Eg< 0.60 eV and many possess physico-chemical and charge transport properties suitable for thermoelectric investigations.

Results on the synthesis and characterization of the solid solutions CsBi4-xSbxTe6, CsBi4Te6-ySey, as well as doping experiments on CsBi4Te6 are reported. We report X-ray structural investigations showing that the Sb or Se atoms in these compounds are not uniformly distributed in the lattice but show preferential occupation of specific crystallographic sites. Thermoelectric properties of selected systems are presented.

We present the structure and thermoelectric properties of the new quaternary selenides K1+xM4–2xBi7+xSe15 (M = Sn, Pb) and K1-xSn5-xBi11+xSe22. The compounds K1+xM4-2xBi7+xSe15 (M= Sn, Pb) crystallize isostructural to A1+xPb4-2xSb7+xSe15 with A = K, Rb, while K1-xSn5-xBi11+xSe22 reveals a new structure type. In both structure types fragments of the Bi2Te3-type and the NaCl-type are connected to a three-dimensional anionic framework with K+ ions filled tunnels. The two structures vary by the size of the NaCl-type rods and are closely related to β-K2Bi8Se13 and K2.5Bi8.5Se14. The thermoelectric properties of K1+xM4-2xBi7+xSe15 (M = Sn, Pb) and K1-xSn5-xBi11+xSe22 were explored on single crystal and ingot samples. These compounds are narrow gap semiconductors and show n-type behavior with moderate Seebeck coefficients. They have very low thermal conductivity due to an extensive disorder of the metal atoms and possible “rattling” K+ ions.

Our exploratory research to identify new promising candidates for next generation thermoelectric applications has produced several interesting new materials which are briefly described here. We present their compositions, solid state structures, properties and charge transport behavior. The compounds CsBi4Te6, β-K2Bi8Se13, Ba4Bi6Se13, Eu2Pb2Bi6Se13, KBi6.33S10, Eu2Pb2Bi4Se10, Ba2Pb2Bi6S13 and K1.25 Pb3.5Bi7.25Se15 are particularly noteworthy.

New Bi-based chalcogenide compounds have been prepared using the polychalcogenide flux technique for crystal growth. These materials exhibit characteristics of good thermoelectric materials. Single crystals of the compound CsBi4Te6 have shown conductivity as high as 2440 S/cm with a p-type thermoelectric power of ≈ +110 μV/K at room temperature. A second compound, β-K2Bi8Se13 shows lower conductivity ≈ 240 S/cm, but a larger n-type thermopower ≈ −200 μV/K. Thermal transport measurements have been performed on hot-pressed pellets of these materials and the results show comparable or lower thermal conductivities than Bi2Te3. This improvement may reflect the reduced lattice symmetry of the new chalcogenide thermoelectrics. The thermoelectric figure of merit for CsBi4Te6 reaches ZT ≈ 0.32 at 260 K and for β-K2Bi8Se13 ZT ≈ 0.32 at room temperature, indicating that these compounds are viable candidates for thermoelectric refrigeration applications.

A solid state chemistry synthetic approach towards identifying new materials with potentially superior thermoelectric properties is presented. Materials with complex compositions and structures also have complex electronic structures which may give rise to high thermoelectric powers and at the same time possess low thermal conductivities. The structures and thermoelectric properties of several new promising compounds with K-Bi-Se, K-Bi-S, Ba-Bi- Te, Cs-Bi-Te, and Rb-Bi-Te are reported.

The compound BaBiTe3 was prepared by the reaction of Ba/Bi/Te at over 700 °C either in K2TE4 or BaTe3 flux and was recrystallized in Ba/Te3 flux. The black rod-shaped polycrystalline material crystallizes in the orthorhombic space group P212121 with a=4.6077(2) A, b=17.0437(8) Å, c=18.2997(8) Å. Its structure is made of interdigitating columnar anionie [Bi4Te10(Te2)] ∞2+ “herring-bone” shaped segments which arrange into layers with Ba ions between them. The electrical conductivity, thermopower, thermal lattice conductivity, infrared absorption properties of this material suggest it is a narrow gap semiconductor.

KBi6.33S10 and K2Bi8S13 were synthesized by the direct combination of K2S/Bi2S3 at high temperature (>700°C). The reaction of K2S/3.3Bi2S3 at 800°C revealed the presence of a new ternary sulfide KBi6.33S10 (I) (92% yield). The structure consists of blocks of Bi2Te3 and Cdl2-type units that are connected to form a three-dimensional network with K+ ions located in the channels that run along the b-axis. The same reaction but with a different ratio, at 750°C, gave the new ternary sulfide K2Bi8S13 (II) (94% yield). The structure of the shiny rod-like crystals is closely related to that of I. As in I, it also consists of Bi2Te3 and Cdl2-type fragments that connect to form K+-filled channels. The two potassium atoms and one bismuth atom are disordered over three sites. Some preliminary electrical and optical properties of these materials are discussed.