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Quaternary chalcogenides with tetrahedral zinc-blend structure types continue to be of interest for thermoelectrics applications. We report on the synthesis, crystal structure, and high temperature transport properties of Cu2.1Fe0.9SnSe4, Cu2.2Fe0.8SnSe4 and Cu2.2Zn0.2Fe0.6SnSe4. The identity and compositions for each specimen were established using a combination of Rietveld refinement and elemental analysis and indicate that all compositions are homogeneous with the stannite crystal structure. Excess Cu reduces the electrical resistivity, ρ, by an order of magnitude compared with Cu2FeSnSe4 with no significant degradation of the Seebeck coefficient, S. The energy band gaps were estimated from the high temperature S values and indicate that Cu2.1Fe0.9SnSe4 and Cu2.2Fe0.8SnSe4 possess narrow band gaps, 0.18 eV and 0.25 eV, respectively, as compared to most other quaternary chalcogenides. The power factor (PF = S2/ρ) increases with decreasing Fe content. Although Cu2.2Fe0.8SnSe4 possesses a smaller PF than that of Cu2.2Zn0.2Fe0.6SnSe4, a ZT of 0.45 was obtained at 750 K for Cu2.2Fe0.8SnSe4 due to its low thermal conductivity.

Materials with very low thermal conductivities continue to be of interest for a variety of applications. We synthesized CuSbS

The Materials Genome Initiative, a national effort to introduce new materials into the market faster and at lower cost, has made significant progress in computational simulation and modeling of materials. To build on this progress, a large amount of experimental data for validating these models, and informing more sophisticated ones, will be required. High-throughput experimentation generates large volumes of experimental data using combinatorial materials synthesis and rapid measurement techniques, making it an ideal experimental complement to bring the Materials Genome Initiative vision to fruition. This paper reviews the state-of-the-art results, opportunities, and challenges in high-throughput experimentation for materials design. A major conclusion is that an effort to deploy a federated network of high-throughput experimental (synthesis and characterization) tools, which are integrated with a modern materials data infrastructure, is needed.

A suite of complementary high-throughput screening systems for combinatorial films was developed at National Institute of Standards and Technology to facilitate the search for efficient thermoelectric materials. These custom-designed capabilities include a facility for combinatorial thin film synthesis and a suite of tools for screening the Seebeck coefficient, electrical resistance (electrical resistivity), and thermal effusivity (thermal conductivity) of these films. The Seebeck coefficient and resistance are measured via custom-built automated apparatus at both ambient and high temperatures. Thermal effusivity is measured using a frequency domain thermoreflectance technique. This paper will discuss applications using these tools on representative thermoelectric materials, including combinatorial composition-spread films, conventional films, single crystals, and ribbons.

Adaptive radar systems will be required to operate in different frequency bands with spectrum requirements that will likely change both in time and with geographic region. A new algorithm is presented that allows direct optimisation of an amplifier's load reflection coefficient to find a Pareto optimum between the power-added efficiency (PAE) and adjacent-channel power ratio. Comparisons of simulation and measurement results are presented. The new algorithm's results compare well with traditionally acquired data and show consistency between the optimum values for PAE that are obtained from different starting points. Measurement comparison is performed with the previous optimisation algorithm reported by the authors, and results show that up to a 50% reduction in the number of measured experimental queries can be obtained. This translates into a significant time savings in reconfiguring a radar transmitter power amplifier.

The increased research and development of thermoelectric materials for energy conversion applications is driven primarily by the need for improved efficiency in the global utilization of energy resources. To facilitate the search for higher efficiency thermoelectric materials, we have developed a suite of complimentary high-throughput screening systems for performing thermoelectric metrology on combinatorial thin films. These custom capabilities include a facility for combinatorial thin film synthesis and suite of tools for screening the Seebeck coefficient, electrical resistance, and thermal conductivity of combinatorial films. The room temperature Seebeck coefficient and resistance are measured via an automated multiprobe apparatus, thus obtaining the power factor, S2σ (where S = Seebeck coefficient, σ = electrical conductivity). In addition, we are developing a high temperature (>300 K) power factor screening tool. Thermal effusivity (to calculate the thermal conductivity) is measured using a frequency...

Power amplifiers in modern wireless systems must meet increasingly stringent spectral constraints while still operating with high power efficiency. A new peak-search algorithm is demonstrated to find the Pareto optimum of power-added efficiency (PAE) while producing an adjacent channel power ratio (ACPR) value under a specified maximum. A peak-search is first performed for the maximum PAE, and then a small-step steepest descent walk in the ACPR is taken from the PAE maximum toward the ACPR minimum until ACPR requirements are met. This approach reasonably approximates the Pareto optimum for the device. Simulation and measurement data are presented for searches from multiple starting points, and good correspondence is obtained for the Pareto optimum reflection coefficient, as well as the optimum PAE and ACPR values. This measurement-based algorithm is expected to find useful application in real-time adaptive radio and radar transmitters, as well as in bench-top measurements for amplifier design.

The compound Yb6.6Ir6Sn16 was obtained as single crystals in high yield from the reaction of Yb with Ir and Sn run in excess indium. Single-crystal X-ray diffraction analysis shows that Yb6.6Ir6Sn16 crystallizes in the tetragonal space group P42/nmc with a = b = 9.7105(7) A and c = 13.7183(11) A. The crystal structure is composed of a [Ir6Sn16] polyanionic network with cages in which the Yb atoms are embedded. The Yb sublattice features extensive vacancies on one crystallographic site. Magnetic susceptibility measurements on single crystals indicate Curie-Weiss law behavior

Temperature-dependent transport properties of the recently discovered layered bismuth-rich tellurobromides BinTeBr (n = 2, 3) are investigated for the first time. Dense homogeneous polycrystalline specimens prepared for different electrical and thermal measurements were synthesized by a ball milling-based process. While the calculated electronic structure classifies Bi2TeBr as a semimetal with a small electron pocket, its transport properties demonstrate a semiconductorlike behavior. Additional bismuth bilayers in the Bi3TeBr crystal structure strengthens the interlayer chemical bonding thus leading to metallic conduction. The thermal conductivity of the semiconducting compositions is low, and the electrical properties are sensitive to doping with a factor of four reduction in resistivity observed at room temperature for only 3% Pb doping. Investigation of the thermoelectric properties suggests that optimization for thermoelectrics may depend on particular elemental substitution. The results presented are intended to expand on the research into tellurohalides in order to further advance the fundamental investigation of these materials, as well as investigate their potential for thermoelectric applications., This work was supported by the ERASMUS+ ICM WORLDWIDE exchange program funded by the European Union. F.P. and M.R. acknowledge financial support from the Deutsche Forschungsgemeinschaft (DFG) through the Würzburg-Dresden Cluster of Excellence on Complexity and Topology in Quantum Matter—ct.qmat (EXC 2147, Project No. 39085490). G.S.N. acknowledges the support from the U.S. National Science Foundation (NSF) under Grant No. DMR-1748188. D.H. acknowledges the II-VI Foundation Block-Gift Program. H.W. acknowledges the support of the assistant secretary of Energy Efficiency and Renewable Energy and the Materials Program under the Vehicle Technologies Program. Oak Ridge National Laboratory is managed by UT-Batelle LLC under Contract No. DE-AC05000OR22725. I.P.R. acknowledges the support from the Ministry of Education and Science of the Russian Federation within the framework of the governmental program “Megagrants” (State Task No. 3.8895.2017/P220) (for theoretical investigation of thermoelectric properties of Bi3TeBr), Academic D. I. Mendeleev Fund Program of Tomsk State University (Project No. 8.1.01.2018), and by the Russian Science Foundation No. 18-12-00169 (for theoretical investigation of thermoelectric properties of Bi2TeBr); E.V.C. and I.P.R. also acknowledge the support from Saint Petersburg State University (Project No. 15.61.202.2015).

Large-volume, phase-pure synthesis of BC8 silicon ($Ia\overline{3}$, $cI16$) has enabled bulk measurements of optical, electronic, and thermal properties. Unlike previous reports that conclude BC8-Si is semimetallic, we demonstrate that this phase is a direct band gap semiconductor with a very small energy gap and moderate carrier concentration and mobility at room temperature, based on far- and midinfrared optical spectroscopy, temperature-dependent electrical conductivity, Seebeck and heat capacity measurements. Samples exhibit a plasma wavelength near $11\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$, indicating potential for infrared plasmonic applications. Thermal conductivity is reduced by 1--2 orders of magnitude depending on temperature as compared with the diamond cubic (DC-Si) phase. The electronic structure and dielectric properties can be reproduced by first-principles calculations with hybrid functionals after adjusting the level of exact Hartree--Fock (HF) exchange mixing. These results clarify existing limited and controversial experimental data sets and ab initio calculations.

We have developed a custom apparatus for the consecutive measurement of the electrical resistivity, the Seebeck coefficient, and the thermal conductivity of materials between 300 K and 12 K. These three transport properties provide for a basic understanding of the thermal and electrical properties of materials. They are of fundamental importance in identifying and optimizing new materials for thermoelectric applications. Thermoelectric applications include waste heat recovery for automobile engines and industrial power generators, solid-state refrigeration, and remote power generation for sensors and space probes. The electrical resistivity is measured using a four-probe bipolar technique, the Seebeck coefficient is measured using the quasi-steady-state condition of the differential method in a 2-probe arrangement, and the thermal conductivity is measured using a longitudinal, multiple gradient steady-state technique. We describe the instrumentation and the measurement uncertainty associated with each transport property, each of which is presented with representative measurement comparisons using round robin samples and/or certified reference materials. Transport properties data from this apparatus have supported the identification, development, and phenomenological understanding of novel thermoelectric materials.

Imaging from strongly scattering objects requires nonlinear inverse algorithms and multiple scattering can yield improved resolution. However, near resonance, weak scattering inverse methods can still prove useful. We investigate this phenomenon using strongly resonant meta-atoms.

Using finite-element analysis, we have developed a metrology simulation to model errors in the measurement of the Seebeck coefficient. This physical parameter is the constant of proportionality relating the electric potential generated across a conductor to the applied thermal gradient. Its measurement requires careful attention to the electrical and thermal contact interfaces. Furthermore, it is essential that the electric potential and temperature difference be acquired at the same time and at the same location. We have performed Seebeck coefficient measurement simulations to quantitatively explore the effect of temporal perturbation to the voltage and temperature correspondence, by comparing simultaneous and staggered data acquisition techniques under the quasi-steady-state condition. Using a similar method, we have developed an error model to explore the effect of misalignment between the voltage and temperature probes on the measurement of the Seebeck coefficient. This approach enables the exploration of experimentally inaccessible data spaces under ideal conditions.

We have employed finite element analysis to develop computational Seebeck coefficient metrology simulations. This approach enables a unique exploration of multiple probe arrangements and measurement techniques within the same temporal domain. To demonstrate the usefulness of this approach, we have performed these Seebeck coefficient measurement simulations to quantitatively explore perturbations to voltage and temperature correspondence, by comparing simultaneous and staggered data acquisition techniques under the quasi-steady-state condition. The results indicate significant distortions to the Seebeck coefficient and a strong dependence on the time delay, the acquisition sequence, and the probe arrangement.

FeSb 2 nanocrystals were synthesized by employing an ethanol-mediated surfactant-free solvothermal approach and subsequently densified to form dense polycrystalline Pnnm FeSb 2 with nano-grains employing spark plasma sintering (SPS). Structural and low temperature transport properties of nanostructured polycrystalline FeSb 2 were investigated and compared with that of bulk FeSb 2 . The nanostructured FeSb 2 had a much lower thermal conductivity, particularly below 150 K, compared to bulk FeSb 2 with resistivity values that were comparable to that of the bulk at these temperatures. This resulted in enhanced thermoelectric properties at cryogenic temperatures as compared to that of the bulk. The ability to form nanostructured polycrystalline materials by SPS processing of solution-phase synthesized nanocrystals, together with the resulting enhanced thermoelectric properties, represent proof of concept for this approach for low-temperature thermoelectrics applications.

High yield syntheses of size-confined Bi2Te3 and Sb2Te3 nanocrystals and nanoflakes, and their alloys, were carried out by a facile glycol-mediated solvothermal process. Phase purity and good cryst...

The study of thermoelectric materials spans condensed matter physics, materials science and engineering, and solid-state chemistry. The diversity of the participants and the inherent complexity of the topic mean that it is difficult, if not impossible, for a researcher to be fluent in all aspects of the field. This review, which grew out of a one-week summer school for graduate students, aims to provide an introduction and practical guidance for selected conceptual, synthetic, and characterization approaches and to craft a common umbrella of language, theory, and experimental practice for those engaged in the field of thermoelectric materials. This review does not attempt to cover all major aspects of thermoelectric materials research or review state-of-the-art thermoelectric materials. Rather, the topics discussed herein reflect the expertise and experience of the authors. We begin by discussing a universal approach to modeling electronic transport using Landauer theory. The core sections of the review are focused on bulk inorganic materials and include a discussion of effective strategies for powder and single crystal synthesis, the use of national synchrotron sources to characterize crystalline materials, error analysis, and modeling of transport data using an effective mass model, and characterization of phonon behavior using inelastic neutron scattering and ultrasonic speed of sound measurements. The final core section discusses the challenges faced when synthesizing carbon-based samples and the measuring or interpretation of their transport properties. We conclude this review with a brief discussion of some of the grand challenges and opportunities that remain to be addressed in the study of thermoelectrics.

Quantum-confined semiconductors composed of heavy elements hold great promise as thermoelectric materials. An increase in the density of states near the Fermi level due to quantum confinement effects and an increased scattering of boundary phonons due to nanostructuring can lead to an increase in the dimensionless figure of merit, ZT, which is defined as s 2 STk � 1 . [1,2] Here, s is the electrical conductivity, S is the Seebeck coefficient, T is the absolute temperature, and k is the thermal conductivity. To realize the highest ZTquantum-confined material possible from a conventional thermoelectric semiconductor material such as bismuth telluride, meeting several criteria are important. First, it is reasonable to expect that uniform, optimal size quantum domains will lead to the highest ZT at a given doping level. The electronic structure of the semiconductor, which determines the thermoelectric power, depends on the degree of quantum confinement and thus the domain size. Second, the interfacial electrical transport must be optimized. The presence of impurities between the domains/particles typically leads to barriers to transport that reduce the electrical conductivity. Third, while optimizing electrical transport between domains, it is important that the quantum architecture not be destroyed, or the phonon scattering will decrease and the thermal conductivity will increase. Fourth, it is well knownthatproperdopingiscriticaltotheperformanceofbulk

In an effort to develop a Standard Reference Material (SRM™) for Seebeck coefficient, we have conducted a round-robin measurement survey of two candidate materials—undoped Bi2Te3 and Constantan (55 % Cu and 45 % Ni alloy). Measurements were performed in two rounds by twelve laboratories involved in active thermoelectric research using a number of different commercial and custom-built measurement systems and techniques. In this paper we report the detailed statistical analyses on the interlaboratory measurement results and the statistical methodology for analysis of irregularly sampled measurement curves in the interlaboratory study setting. Based on these results, we have selected Bi2Te3 as the prototype standard material. Once available, this SRM will be useful for future interlaboratory data comparison and instrument calibrations.

There are a large number of inorganic compounds with open-framework structures that entrap atoms or molecules within the lattice. Of these, the skutterudites and type I clathrates have gained the greatest attention from the perspective of thermoelectric applications. The crystal structure of these materials can be considered as being open in the sense that they possess voids whereby interstitially placed atoms are bounded loosely, thereby creating localized disorder in an otherwise well-ordered, covalently bonded lattice. The optimum situation occurs if the intrinsic mobility is relatively high due to the well-ordered, periodic structure of the crystal framework while the phonons are scattered by localized disorder. Substantial experimental and theoretical research has been devoted to these two material systems over the past decade. This effort has shown that the physical properties are directly related to their unique crystal structures, as well as the different compositions that can be synthesized in order to modify these physical properties.

A Standard Reference Material (SRM™) for the Seebeck coefficient is critical for inter-laboratory data comparison and for instrument calibration. To develop this SRM™, we have conducted an international round-robin measurement survey of two candidate materials—undoped Bi2Te3 and constantan (55% Cu and 45% Ni alloy). Measurements were performed in two rounds by twelve laboratories involved in active thermoelectric research using a number of commercial and custom-built measurement systems and techniques. We report the results of these measurements and the statistical analysis performed. Based on this extensive study, we have selected Bi2Te3 as the prototype standard material.

In computer science, procrastination and related problems with managing programming projects are viewed as primary causes of student attrition. Unfortunately, the most successful techniques for reducing procrastination (such as courses in study skills) are resource-intensive and do not scale to large classrooms. In this paper, we describe three course interventions that are designed to be scalable for large classrooms and require few resources to implement. Reflective writing assignments require students to consciously consider how their time management choices impact their classroom performance. Schedule sheets force students to actively plan out the time required to solve a programming project. Email alerts inform students of their progress relative to their peers as they work on an assignment, and suggest ways to improve behavior if their progress is found to be unsatisfactory. We implemented these interventions in a junior-level data structures course and analyzed data from 330 students over two semesters. Separate analyses of reflective writing responses, schedule sheet contents, and e-mail alert contents are discussed, along with student opinions about the value and effectiveness of each treatment. We found a statistically significant relationship between the time when work is completed and its quality, with late work being of lower quality. We found that one of the three interventions had a statistically significant effect on reducing late work: e-mail alerts sent to students to make them more aware of how they were doing with respect to expectations were associated with both a reduction in assignments completed late, and an increase in assignments completed at least one day early. This result was found despite the fact that students reported subjectively that e-mail alerts were of marginal utility.

X-ray absorption fine structure spectroscopy at the Ga and Ge K-edges was used to study changes in the Ga and Ge electronic structure and local coordination geometry as a function of composition in Ba(8)Ga(16)Ge(30-x)Si(x) type I clathrates (x = 0, 7.5, 9.1, 10.7, 13.4 and 30). Based on XANES data, the partial density of unoccupied states with p character is modified for both Ga and Ge upon Si substitution. Among the specimens which contain both Ge and Si, we found that the specimen with the highest measured power factor (i.e., x = 7.5) has the lowest density of unoccupied states for both Ga and Ge. Our experimental results are qualitatively consistent with computational results based on density functional theory, indicating that a series of pertinent electronic states are modified by Si p states. This suggests that an increase in the electron density near the Fermi level for an optimal Si substitution leads to an increase in the Seebeck coefficient and consequently in the power factor, according to the Cutler-Mott relation. Based on quantitative analysis of EXAFS spectra, we found that Ga has more Si neighbors than Ge, indicating that Si resides preferentially next to Ga. Both the Ge-Ga/Ge and Ga-Ge/Ga coordination distances remain relatively unchanged (~2.51 Å) regardless of the degree of Si substitution. Furthermore, the Ge-Si and Ga-Si coordination distances remain relatively unchanged at ~2.41 and ~2.45 Å, respectively, regardless of the degree of Si substitution. For the Ba(8)Ga(16)Si(30) specimen, on average, Ga is coordinated with 0.9 Ga and 3.1 Si at roughly the same distance of ~2.50 Å. The number of Ga-Ga bonds is consistent with the fact that Ga is distributed on the framework sites in a way which reduces the number of Ga-Ga bonds relative to that based on a random distribution. An understanding of the underlying physics of the structure-property relationship provides for potential additional routes for tuning the electronic properties of clathrates for thermoelectric applications.

Single crystals of the new title compounds are prepared in high yields from the elements using excess In as a flux (1000 °C, 5—72 h).

The intermetallic compounds YbAu(2)In(4) and Yb(2)Au(3)In(5) were obtained as single crystals in high yield from reactions run in liquid indium. Single crystal X-ray diffraction data of YbAu(2)In(4) showed that it crystallizes as a new structure type in the monoclinic space group P2(1)/m and lattice constants a = 7.6536(19) Å, b = 4.5424(11) Å, c = 9.591(2) Å and β = 107.838(4)°. The YbAu(2)In(4) compound is composed of a complex [Au(2)In(4)](3-) polyanionic network in which the rare-earth ions are embedded. Yb(2)Au(3)In(5) crystallizes in the polar space group Cmc2(1) with the Y(2)Rh(3)Sn(5) type structure and lattice constants a = 4.5351(9) Å, b = 26.824(5) Å, and c = 7.4641(15) Å. The gold and indium atoms define a complex three-dimensional [Au(3)In(5)] network with a broad range of Au-In (2.751(2) Å-3.0518(16) Å) and In-In (3.062(3) Å-3.3024(19) Å) distances. Magnetic susceptibility measurements of YbAu(2)In(4) revealed a transition at 25 K. Below the transition, the susceptibility of YbAu(2)In(4) follows Curie-Weiss behavior with an effective paramagnetic moment of 0.79 μ(B)/Yb. Magnetic susceptibility measurements on Yb(2)Au(3)In(5) show a mixed valent ytterbium and the magnetic moment within the linear region (100 K) of 1.95 μ(B)/Yb. Heat capacity data for YbAu(2)In(4) and Yb(2)Au(3)In(5) give Debye temperatures of 185 and 153 K, respectively.

The influence of incorporating nanoparticulate additions into Ca3Co4O9 (CCO) thin films prepared by pulsed laser deposition using composite targets of CCO and CCO + 3wt% BaZrO3 (BZO) on Si and LaAlO3 substrates is investigated. X-ray data and high-resolution scanning electron microscopy reveal preferred c-axis orientation of the films deposited on Si substrates with the formation of nanoparticles between ∼ 10 – 50 nm. Preliminary thermoelectric behavior shows an enhancement of the power factor α2/ρ at room temperature. The microstructure and thermoelectric behavior of the CCO films are compared to the BZO-doped films.

We present a model describing the carrier conductivity and Seebeck coefficient of thermoelectric nanocomposite materials consisting of granular regions. The model is successfully applied to explain relevant experimental data for PbTe nanocomposites. A key factor is the grain potential boundary scattering mechanism. Other mechanisms, such as carrier-acoustic phonon, carrier-nonpolar optical phonon, and carrier-ionized impurities scattering are also included. Our calculations reveal that by changing the physical characteristics of the grains, such as potential barrier height, width, and distance between the grains, one can increase the mean energy per carrier in order to obtain an optimum power factor for improved thermoelectric performance. The model can be applied to other nanocomposites by incorporating the appropriate electronic structure parameters.

There are a variety of material systems employing different strategies in an effort to establish a new paradigm for thermoelectric materials performance. One approach is the PGEC, or “phonon-glass electron crystal”, approach were research towards optimization of the electrical properties of very low thermal conductivity materials is key. Other efforts focus on materials that exhibit high power factors via quantum-confinement or nano-scale affects. Still others focus on “engineering” metastable phases that possess properties that are distinct, if not unique, to solid state chemistry. All these approaches are valid and provide a fundamental knowledge base whereby present and future scientific materials discoveries will lead to new technological improvements. This paper focuses on bulk materials, in particular those material systems currently under investigation in the novel materials laboratory at the University of South Florida and the requirements and strategies for their optimization towards improved thermoelectric properties.

Resistivity, Seebeck coefficient, and Hall measurements were performed on densified nanocrystalline composite materials of undoped and Ag-doped PbTe nanocrystals to investigate the physical mechanisms responsible for Seebeck coefficient enhancement in nanocrystalline systems. The unique temperature dependence of the resistivity and mobility for these PbTe nanocomposites suggests that grain-boundary potential barrier scattering is the dominant scattering mechanism. We propose that carrier trapping in the grain boundaries forms energy barriers that impede the conduction of carriers between grains, essentially filtering charge carriers with energy less than the barrier height. These nanocomposites therefore demonstrate an enhanced Seebeck coefficient as compared to single crystal or polycrystalline PbTe at similar carrier concentrations.

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.

There are a variety of material systems employing different strategies in an effort to establish a new paradigm for thermoelectric materials performance. One approach is the PGEC, or “phonon-glass electron crystal”, approach were research towards optimization of the electrical properties of very low thermal conductivity materials is key. Other efforts focus on materials that exhibit high power factors via quantum-confinement or nano-scale affects. Still others focus on “engineering” metastable phases that possess properties that are distinct, if not unique, to solid state chemistry. All these approaches are valid and provide a fundamental knowledge base whereby present and future scientific materials discoveries will lead to new technological improvements. This paper focuses on bulk materials, in particular those material systems currently under investigation in the novel materials laboratory at the University of South Florida and the requirements and strategies for their optimization towards improved thermoelectric properties.

Magnetization, static and ac magnetic susceptibility, nuclear forward scattering, and electrical resistivity measurements have been performed on polycrystalline ${\mathrm{Eu}}_{4}{\mathrm{Sr}}_{4}{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$, a type I clathrate that has divalent strontium and europium ions encapsulated within a $\mathrm{Ga}\text{\ensuremath{-}}\mathrm{Ge}$ framework. These data are compared with those of type I clathrates ${\mathrm{Eu}}_{8}{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$ and ${\mathrm{Eu}}_{6}{\mathrm{Sr}}_{2}{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$. The ferromagnetic ordering of these Eu-containing clathrates is substantially altered by the incorporation of strontium, as compared to ${\mathrm{Eu}}_{8}{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$. Ferromagnetism, accompanied by a relatively large negative magnetoresistance, is observed below 15 and $20\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ in ${\mathrm{Eu}}_{4}{\mathrm{Sr}}_{4}{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$ and ${\mathrm{Eu}}_{6}{\mathrm{Sr}}_{2}{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$, respectively. An effective magnetic moment of $7.83\phantom{\rule{0.3em}{0ex}}{\mathrm{\ensuremath{\mu}}}_{B}$ per Eu ion is observed above $30\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ for ${\mathrm{Eu}}_{4}{\mathrm{Sr}}_{4}{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$, a moment which is close to the free-ion moment of $7.94\phantom{\rule{0.3em}{0ex}}{\mathrm{\ensuremath{\mu}}}_{B}$ per europium(II) ion.

A new stannite phase was synthesized and its temperature dependent transport properties were investigated. Cu2CdSnTe4 possesses strong p-type conduction, while the temperature dependence of the thermal conductivity exhibits typical dielectric behavior. Electronic structure calculations allowed for a description of the transport characteristics in terms the energy band structure, density of states, and Fermi surface. The potential for thermoelectric applications is also discussed.

The Seebeck coefficient is the most widely measured property specific to thermoelectric materials. There is currently no consensus on measurement protocols, and researchers employ a variety of techniques to measure the Seebeck coefficient. The implementation of standardized measurement protocols and the use of reliable Seebeck Coefficient Standard Reference Materials (SRMs®) will allow the accurate interlaboratory comparison and validation of materials data, thereby accelerating the development and commercialization of more efficient thermoelectric materials and devices. To enable members of the thermoelectric materials community the means to calibrate Seebeck coefficient measurement equipment, NIST certified SRM® 3451 “Low Temperature Seebeck Coefficient Standard (10 K to 390 K)”. Due to different practical requirements in instrumentation, sample contact methodology, and thermal stability, a complementary SRM® is required for the high temperature regime (300 K to 900 K). The principal requirement of a SR...

Combinatorial metrology has evolved as a useful approach to rapidly determine the composition-structure-property relationships for solid solution systems in a far more efficient way than the traditional one composition at a time approach. The success of this method applied in thermoelectric (TE) research relies on screening tools to evaluate the TE properties for a combinatorial library. We report here on a thermoelectric screening tool capable of performing temperature dependent measurement of the Seebeck coefficient and electrical resistivity from 300 K to 800 K. The validity of the apparatus is demonstrated by screening the power factor of a filled skutterudite combinatorial film at room temperature and at elevated temperatures as well. The accuracy of the measurement is verified using the low temperature Seebeck coefficient Standard Reference Material (NIST SRM 3451) and a heavily doped SiGe specimen for high temperature comparison. Several important parameters, such as measurement atmosphere, film substrate, and probe configuration, are identified that directly affect the Seebeck coefficient measurement in this and other similar apparatus.

In Seebeck coefficient metrology, the present diversity in apparatus design, acquisition methodology and contact geometry has resulted in conflicting materials data that complicate the interlaboratory confirmation of reported high efficiency thermoelectric materials. To elucidate the influence of these factors in the measurement of the Seebeck coefficient at high temperature and to identify optimal metrology protocols, we measure the Seebeck coefficient as a function of contact geometry under both steady-state and transient thermal conditions of the differential method, using a custom developed apparatus capable of in situ comparative measurement. The thermal gradient formation and data acquisition methodology, under ideal conditions, have little effect on the measured Seebeck coefficient value. However, the off-axis 4-probe contact geometry, as compared to the 2-probe, results in a greater local temperature measurement error that increases with temperature. For surface temperature measurement, the dominant thermal errors arise from a parasitic heat flux that is dependent on the temperature difference between the sample and the external thermal environment, and on the various thermal resistances. Due to higher macroconstriction and contact resistance in the 4-probe arrangement, the measurement of surface temperature for this contact geometry exhibits greater error, thereby overestimating the Seebeck coefficient.

Several polycrystalline p-type skutterudites with compositions BaxYbyCo4−zFezSb12, with varying filler concentrations x and y, and z = 1 to 2, were synthesized by reacting the constituents and subsequent solid state annealing, followed by densification by hot-pressing. Their thermoelectric properties were evaluated from 300 to 820 K. The Yb filling fraction increased with Fe content while the amount of Fe substitution had little influence on the Ba filling fraction. High purity specimens were obtained when the Fe content was low. Bipolar conduction contributed to the thermal conductivity at elevated temperatures. A maximum ZT value of 0.7 was obtained at 750 K for the specimen with the highest Fe content and filling fraction. The potential for thermoelectric applications is also discussed.