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We have carried out an evaluation of the lattice thermal conductivity for 19 different type-I intermetallic clathrate compositions at temperatures near or above their respective Debye temperatures. Although the low temperature data (T < 300 K) show that the thermal transport can vary from glasslike to crystalline depending on the composition, the high temperature lattice thermal conductivity values (T > 300 K) appear to be relatively well described by a simple model that assumes only anharmonic three-phonon scattering processes. The analysis suggests the large number of atoms in the unit cell accompanied by a significant degree of anharmonicity and relatively low Debye temperatures produce low lattice thermal conductivities in these materials above 300 K, regardless of composition or the nature and influence of guest rattling behavior on thermal transport below room temperature. The dominant mechanism underlying the universally low lattice thermal conductivity of type-I intermetallic clathrates (ranging from ~0.6 to ~2 W/m K) in the temperature regime of importance for thermoelectric power generation is therefore clarified. [ABSTRACT FROM AUTHOR]

In its common, thermodynamically stable state, silicon adopts the same crystal structure as diamond. Although only a few alternative allotropic structures have been discovered and studied over the past six decades, advanced methods for structure prediction have recently suggested a remarkably rich low-density phase space that has only begun to be explored. The electronic properties of these low-density allotropes of silicon, predicted by first-principles calculations, indicate that these materials could offer a pathway to improving performance and reducing cost in a variety of electronic and energy-related applications. In this focus review, we provide an introduction and overview of recent theoretical and experimental results related to low-density allotropes of silicon, highlighting the significant potential these materials may have for technological applications, provided substantial challenges to their experimental preparation can be overcome. [ABSTRACT FROM AUTHOR]

Abstract The binary intermetallic clathrates K8-xSi46 (x = 0.4; 1.2), Rb6.2Si46, Rb11.5Si136 and Cs7.8Si136 were prepared from M4Si4 (M = K, Rb, Cs) precursors by spark-plasma route (SPS) and structurally characterized by Rietveld refinement of PXRD data. The clathrate-II phase Rb11.5Si136 was synthesized for the first time. Partial crystallographic site occupancy of the alkali metals, particularly for the smaller Si20 dodecahedra, was found in all compounds. SPS preparation of Na24Si136 with different SPS current polarities and tooling were performed in order to investigate the role of the electric field on clathrate formation. The electrical and thermal transport properties of K7.6Si46 and K6.8Si46 in the temperature range 4–700 K were investigated. Our findings demonstrate that SPS is a novel tool for the synthesis of intermetallic clathrate phases that are not easily accessible by conventional synthesis techniques. [ABSTRACT FROM AUTHOR]

The article looks at the underlying concept of phonon-glass, electron-crystal (PGEC), elucidated by scientist Glen Slack, and its impact on thermoelectric materials research. Topics discussed include glass-like crystals, the filled skutterudites and clathrates proposed by Slack as candidate material systems for PGEC, and about thermoelectric materials and devices.

The article discusses a study on the growth and characterization of germanium-based type 1 clathrate thin films deposited by pulsed laser ablation. It cites use of the pulsed laser ablation technique to grow thin films of type 1 clathrate material. The research has also determined the optimum laser fluence to produce stoichiometric films with the lowest particle density.

We report the synthesis and low-temperature electrical resistivity, Seebeck coefficient, and thermal conductivity of the binary type I clathrate K8Ge44□2 (□=Ge framework vacancy). Electrical resistivity measurements indicated metallic or heavily doped semiconductor behavior, while the Seebeck was relatively high at −77 μV/K at room temperature. The thermal conductivity was very low, on the order of 1 W/m K at room temperature. This work is part of a continuing effort to investigate new compositions in open-structured and guest-framework materials for potential thermoelectric applications. [ABSTRACT FROM AUTHOR]

We report the synthesis and low-temperature electrical resistivity, Seebeck coefficient, and thermal conductivity of the binary type I clathrate K8Ge44□2 (□=Ge framework vacancy). Electrical resistivity measurements indicated metallic or heavily doped semiconductor behavior, while the Seebeck was relatively high at −77 μV/K at room temperature. The thermal conductivity was very low, on the order of 1 W/m K at room temperature. This work is part of a continuing effort to investigate new compositions in open-structured and guest-framework materials for potential thermoelectric applications. [ABSTRACT FROM AUTHOR]

Systematic crystal structure refinements from powder X-ray diffraction data as well as density functional theory calculations demonstrate that the silicon clathrate II Si(cF136) exhibits a lattice contraction as Na is introduced solely into the Si28 cages. When the Si20 cages, in addition, begin to be filled with Na, a contrasting lattice expansion results. The nonmonotonic structural response to filling is an indication of markedly dissimilar guest-framework interactions for Na@Si20 and Na@Si28. [ABSTRACT FROM AUTHOR]

The article presents a study which determines the crystal growth and preparation of Na24Si136 or intermetallic clathrates. The study utilized various conventional synthesis techniques in the preparation and crystal growth of intermetallic clathrates. It determined that spark plasma treatment is the first step for the synthesis and crystal growth of intermetallic clathrate.