In the search for new thermoelectric materials, quasicrystals have been investigated due to their inherently low thermal conductivity. Crystalline phases called approximants which are closely related to the quasicrystals, however, have largely been ignored. These approximants have similar structure, yet the periodicity of a crystal. In this paper, we have investigated an icosahedral i-AlCuFe quasicrystalline phase as well as two of its corresponding approximant phases (ω and β). Electrical and thermal transport properties of these materials are presented. The viability of the i-AlCuFe quasicrystalline system and the ω and β approximant phases as thermoelectric materials is discussed.

Quantitative X-ray powder Rietveld refinements for a series of alloys from the solid solution EuyFe4−xNixSb12, synthesized by argon arc-melting followed by long term annealing, established in all cases isotypism with the partially filled skutterudite-type structure, LaFe4P12. The Eucontent of the samples was determined from the combined data obtained results of Rietveld refinements and electron microprobe measurements. These investigations confirmed a systematic trend for the Eu-occupancy y in the parent lattice, revealing a gradual decrease of the maximum Eu-content from practically full occupancy, y = 0.83, in Eu0.83Fe4Sb12 to y ∼ 0.5 for Eu∼0.5Fe2Ni2Sb12. Eu0.83Fe4Sb12 orders magnetically below 84 K and the transition temperature decreases as a function of Fe/Ni substitution. As a further consequence of the Fe/Ni substitution electronic transport crosses over from a hole conductivity regime into electron dominated behaviour. Concomitantly, the transition metal exchange increases the Seebeck coefficient significantly, hence the figure of merit enhances.

We have prepared (Ca1−x−yMxBiy)3Co4Oz (M = Mg, Sr, and Ba) thin films by a combinatorial approach using a solution process. In the systems of (Ca1−x−yMxBiy)3Co4Oz (M = Mg, Sr, and Ba), solid solution range was determined to be × < 0.8 (M = Sr, y = 0), x < 1.0 (M = Mg, y = 0), x = 0.0 (M = Ba, y = 0), and x < 0.4 (M = Bi, x = 0). No solid solution range was obtained for the substitution of Ba for Ca site. The in-plane compressive stress in the CoO2 sublattice is controllable by the cation substitution for Ca in the (Ca2CoO3) sublattice. With increasing in-plane stress, the magnitude of thermoelectric power and resistivity increased.

The amorphous Si-Ge-Au bulk samples were fabricated with using the melt spinning method for the practical power supply or cooling devices. X-ray diffraction results showed that our samples were amorphous and the thermoelectric properties were measured by DC method. Although the electrical resistivity of the bulk sample was higher than that of the amorphous thin film, the thermoelectric power of the bulk sample was larger. The thermal conductivity of the amorphous Si-Ge-Au bulk sample was almost the same to the conventional crystalline Si-Ge bulk value. Consequently, non-dimensional figure of merit ZT is around 2 (at 600 K, • •=6.5 10λ-1V/K, • =1.9 10 ohm-m, • •= 6 W/mK) that is about ten times higher than the conventional crystalline Si-Ge bulk value.

Multi-temperature (15, 100, 150, 200, 300, 450, 600, 900 K) single crystal neutron diffraction data on the type I clathrate Ba8Ga16Si30 are analyzed with the maximum entropy method to obtain direct space nuclear densities. The nuclear densities suggest that the guest atoms are structurally disordered, and the disorder appears to be temperature dependent with increased host-guest interaction at high temperatures.

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.

Dynamical phenomena during the solid phase epitaxy (SPE) of guest-free Si clathrates (Si34 and Si46) via molecular-dynamics (MD) simulations using the Tersoff potential have been reported. The activation energy of SPE for Si34 has been found to correspond with the experimental value for the cubic diamond phase (c-Si; approximately 2.7eV), while the SPE rates of Si46 are much lower than that of c-Si. The structural transition from Si46 (type-I) to Si34 (type-II) can be also observed during the Si46 [001] SPE. The present results suggest that it is worthwhile to intensify experimental studies concerning crystal growth techniques of clathrate materials and these interesting Si forms may open up a new field in “silicon technologies”.

Our efforts to improve the thermoelectric properties of β-K2Bi8Se13, led to systematic studies of solid solutions of the type β-K2Bi8−xSbxSe13. The charge transport properties and thermal conductivities were studied for selected members of the series. Lattice thermal conductivity decreases due to the mass fluctuation generated in the lattice by the mixed occupation of Sb and Bi atoms. Se excess as a dopant was found to increase the figure-of merit of the solid solutions.

We report 133Cs and 23Na spectra of Cs8NaxGe136 clathrates. Fully loaded Cs8Na16Ge136 shows only ionic signals from both Cs and Na nuclei. In contrast to the Cs8Na16Si136 clathrate, germanium Cs8Na16Ge136 clathrate shows no large Knight or paramagnetic shifts. However, when sodium is removed from Cs8Na16Ge136, the resulting Cs8Ge136 clathrate shows a very large shift in the 133Cs NMR spectrum.

A great potential for further improving the room-temperature figure of merit (Z) has been identified by dispersing a small fraction of Ag in the (Bi0.25Sb0.75)2Te3 alloy. The maximum Z of 3.41×10−3K−1, 17% higher than that of the unadded, is attained at 0.02wt% Ag. Addition of BN, either alone or in combination with Ag, however, does not generate a favorable figure of merit.

Fabrication and characterization of SiGe/Si superlattice microcoolers integrated with thin film resistors are described. Superlattice structures were used to enhance the device performance by reducing the thermal conductivity, and by providing selective emission of hot carriers through thermionic emission. Thin film metal resistors were integrated on top of the cooler devices and they were used as heat load for cooling power density measurement. Various device sizes were characterized. Net cooling over 4.1 K and a cooling power density of 598 W/cm2 for 40 × 40 μm2 devices were measured at room temperature.

Electron emissive thin films that possess the thermionic properties of macroscale dispenser cathodes have been fabricated using standard RF sputter deposition techniques. These films are based on a compositionally modulated structure using W and a Ba-containing ternary oxide. These films exhibit uniform work function values of 1.9 to 2.2 eV with high emission coefficients of 2 to 26 A·cm−2K−2. Electron microscopy shows that W layer coalescence and continued particle growth occurs with prolonged annealing, eventually disrupting the original surface layer. Chemical modification of the oxide during deposition may provide a route to limit the extent of W coalescence and preserve surface metallization layers on these films. These films are designed to be integrated into thermionic micro-devices.

The thermoelectric properties of bismuth telluride based thermoelectric (TE) materials are well-characterized, but comparatively little has been published on the mechanical and thermomechanical properties of these materials. In this paper, we present the initial dynamic mechanical analysis (DMA) data for n-type and p-type bismuth telluride based TE materials. The materials' tan δ values, indicative of viscoelastic energy dissipation modes, approach that of glassy or crystalline polymers and are greater than ten times the tan delta of structural metals. TE samples measured perpendicular to the van der Waals planes have higher tan δ values. Thermal scans in the DMA compressive mode showed changes in mechanical properties versus temperature with clear hysteresis effects. These changes were correlated to differential scanning calorimetry (DSC) thermal transitions. The expected anisotropy was shown in flexural 3-point bending results for one n-type material that showed a storage modulus of 0.10 to 0.45 GPa in the direction parallel to the van der Waals planes and 0.07 to 0.2 GPa in the perpendicular direction.

We present results of our investigation of the synthesis, structural properties and electrical transport properties of lead selenide (PbSe) nanoparticle-derived solids. Stable colloidal solutions containing crystalline PbSe particles with sizes on the order of 5-10 nm were synthesized using an organometallic lyothermal growth method in high-temperature organic solvents (100∼200 °C). The nanoparticle powders have been characterized by X-ray scattering (WAXS/SAXS), electron microscopy and optical absorption. Thin lms were formed by controlled precipitation of the nanoparticles from solution onto insulating substrates. Electrical resistance (R) and Seebeck coecient (S) for conductive PbSe lms from dierent annealing conditions were studied and compared. We were able to obtain conductive PbSe lms from colloids by low temperature annealing which did not disturb the nanoparticle self-assembly.

Recently, there has been a renewed interest in thermoelectric material research. There are a number of different systems of potential thermoelectric (TE) materials that are under investigation by various research groups. Some of these research efforts focus on minimizing lattice thermal conductivity while other efforts focus on materials that exhibit large power factors. An overview of some of the requirements and strategies for the investigation and optimization of a new system of materials for potential thermoelectric applications will be discussed. Some of the newer concepts such as low-dimensional systems and Slack's phononglass, electron-crystal concept will be discussed. Current strategies for minimizing lattice thermal conductivity and also minimum requirements for thermopower will be presented. The emphasis of this paper will be to identify some of the more recent promising bulk materials and discuss the challenges and issues related to each. This paper is targeted more at “newcomers” to the field and does not discuss some of the very interesting results that are being reported in the thin film and superlattice materials. Some of the bulk materials which will be discussed include complex chalcogenides (e.g.CsBi4Te6 and pentatellurides such as the Zr1−XHfXTe5 system), half-Heusler alloys (e.g. TiNiSn1−XSbX), ceramic oxides (NaCo4O2), skutterudites (e.g. YbXCo4−XSb12 or EuXCo4−XSb12) and clathrates (e.g. Sr8Ga16Ge30). Each of these systems is distinctly different yet each exhibits some prospect as a potential thermoelectric material. Results will be presented and discussed on each system of materials.

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.

The Structure I type binary metal clathrates of K/Si, Rb/Si and Cs/Sn have been synthesised and studied by powder X-ray diffraction and solid state NMR. Rietveld analysis shows that in all three materials some of the cages are empty, and that in the Cs/Sn clathrate there are vacancies in the Sn framework. The NMR results yield Knight shifts for 29Si and 39K and confirm that the Cs/Sn clathrate is not conducting. Many of the features of the NMR spectra can be understood in terms of the distributions of atom vacancies.

A non-monotonic character of the dependences of the thermoelectric properties of PbS/EuS/(001)KCl heterostructures on the PbS layer thickness d (d = 2 – 200 nm) was detected. Pronounced extrema at d ∼ 15 nm and less distinct extrema at d ∼ 30 nm and d ∼ 100 nm were observed. It is suggested that the complex character of the dependences is caused by the competition between percolation phenomena and size quantization. The critical exponent for the electrical conductivity (t = 1.6 ± 0.15) is determined.

The potential of Bi and Bi1−x Sbx nanowire arrays for thermoelectric applications is discussed. The advantages of bismuth as a low dimensional thermoelectric material are enumerated and the role of modeling is emphasized. The advantages of using the Sb concentration as well as the wire diameter as materials parameters for optimizing the thermoelectric performance of these nanowires are discussed, with particular emphasis given to the development of a high performance p-type nanowire thermoelectric material.

Quasicrystalline materials have been investigated for application as thermoelectric materials due to their inherently low thermal conductivity. With the discovery of a new stable, binary Cd5.7Yb quasicrystal, thermal and electrical transport measurements have been performed on these materials. It is found that the electronic contribution to the thermal conductivity calculated from the Wiedemann-Franz relationship is comparable to or greater than the total measured thermal conductivity, leaving the appearance of a “negligible lattice contribution.” In addition, no semblance of the lattice contribution appears in the temperature dependence of the thermal conductivity. The thermal conductivity increases linearly with temperature above 75K and proportional to T3/4 between 2 K and 75 K.