B. Lenoir

Transport properties of Bi-rich Bi-Sb alloys

This study is focused on the investigation of the transport properties of Bi-Sb alloys. Electrical resistivity, thermoelectric power and thermal conductivity were measured in a direction perpendicular or parallel to the trigonal axis within the temperature range 4.2 - 300 K on various alloy compositions containing up to 18.2 at.% antimony. The temperature dependences of the three coefficients are described in detail. Low temperature behaviour depends strongly on the crystal purity, particularly in the semiconducting range. A qualitative explanation of these results is given in terms of an impurity band which merges with the conduction band. Thermoelectric properties are discussed and compared with previous studies.

Effect of antimony content on the thermoelectric figure of merit of Bi1−xSbx alloys

Abstract The electrical resistivity, thermoelectric power and thermal conductivity have been measured between 4.2 and 300 K on three Bi1−xSbx alloys of different composition (x = 0.144, 0.165 and 0.181) prepared by the travelling heater method. The temperature dependences of these parameters follow the same general trends as those of semiconducting alloys having lower Sb contents. The antimony dependence of the figure of merit for semiconducting alloys has been determined for various temperatures. The dependence presents two maxima for Sb contents of x ≈ 0.09 and 0.16 around 80 K. These features have been qualitatively explained from the band structure and interband hole scattering mechanisms.

Solar thermolectric generator based on skutterudites

A numerical simulation based on the optimal control theory was developed to assess the performances, dimensions and weight characteristics of skutterudite-based solar thermoelectric generators (STG) for satellite missions at distances close to the Sun. It is shown that the STG designs based on these advanced thermoelectric materials are potentially attractive for this type of application and could advantageously replace the standard power sources such as the solar (cells) panel or the radioisotope thermoelectric generators.

Numerical simulation of the 3ω method for measuring the thermal conductivity

A four-pad 3ω method was developed for determining the thermal conductivity of bulk and thin film materials. The heat conduction in the sample geometry was investigated by numerical simulation employing the finite volume technique. As a result, the conditions (i.e., sample geometry, frequency range) can be predicted when conventional analytical formulas are suited to determine the thermal conductivity of bulk and thin film material from the data measured. Experiments were performed that confirm the validity and accuracy of the numerical calculations.

Growth of Bi1 − xSbx alloys by the traveling heater method

This study describes for the first time the preparation of single crystal bismuth-antimony alloys by the traveling heater method, which requires a precise knowledge of the binary phase diagram of the alloys. Differential scanning calorimetry measurements were performed on homogeneous samples to determine both the liquidus and solidus lines on the Pi-rich side. The microhomogeneity and macrohomogeneity of an ingot with the specific composition Bi0.92Sb0.08 are discussed in detail. In addition, the figures of merit parallel and perpendicular to the trigonal axis were determined from 6 to 300 K and compared with literature values.

Preparation and transport properties of polycrystalline Bi and Bi–SiO2 nanocomposites

Bismuth–silica nanocomposites and polycrystalline bismuth were prepared via powder metallurgy in order to study the influence of silica inclusions on the thermoelectric properties of bismuth. Bi–SiO2 powders containing from 0.5 to 15 vol. % of silica and pure Bi powders were produced by an arc-plasma processing. Transmission electron microscopy investigations revealed the presence of a nanometric silica shell around the Bi grains. The powders were cold pressed and sintered close to the melting temperature of bismuth. The bulk microstructures are very different for the bismuth and the Bi–SiO2 nanocomposites because silica, which is primarily dispersed at grain boundaries, inhibits the grain growth during sintering. The electrical resistivity was measured from 5 to 300 K, while the thermoelectric power and the thermal conductivity were measured from 65 to 300 K on both polycrystalline bismuth and Bi–SiO2 samples containing 0.5, 4, and 15 vol. % of silica, respectively. The transport properties are mainly di...

Mechanical alloying of BiSb semiconducting alloys

Abstract Polycrystalline Bi 100− x Sb x ( x =7, 10, 12, 15, 22) semiconducting alloys were synthesized by mechanical alloying in order to achieve performant homogeneous thermoelectric materials. The influence of the milling parameters on the final homogeneity and morphology of the powders, such as ball-to-powder weight ratios (BPR=10:1, 40:1, 92:1) and balls sizes (diameters of 30 and 20 mm), was investigated. Powder samples were characterized by X-ray diffraction, differential scanning calorimetry, scanning electron microscopy and electron microprobe analysis. The results show that the samples obtained with the 10:1 BPR and four balls of 30 mm in diameter present the best homogeneity whatever the alloy composition.

X-ray Characterization, Electronic Band Structure, and Thermoelectric Properties of the Cluster Compound Ag2Tl2Mo9Se11

We report on a detailed investigation of the crystal and electronic band structures and of the transport and thermodynamic properties of the Mo-based cluster compound Ag2Tl2Mo9Se11. This novel structure type crystallizes in the trigonal space group R3̅c and is built of a three-dimensional network of interconnected Mo9Se11 units. Single-crystal X-ray diffraction indicates that the Ag and Tl atoms are distributed in the voids of the cluster framework, both of which show unusually large anisotropic thermal ellipsoids indicative of strong local disorder. First-principles calculations show a weakly dispersive band structure around the Fermi level as well as a semiconducting ground state. The former feature naturally explains the presence of both hole-like and electron-like signals observed in Hall effect. Of particular interest is the very low thermal conductivity that remains quasi-constant between 150 and 800 K at a value of approximately 0.6 W·m(-1)·K(-1). The lattice thermal conductivity is close to its minimum possible value, that is, in a regime where the phonon mean free path nears the mean interatomic distance. Such extremely low values likely originate from the disorder induced by the Ag and Tl atoms giving rise to strong anharmonicity of the lattice vibrations. The strongly limited ability of this compound to transport heat is the key feature that leads to a dimensionless thermoelectric figure of merit ZT of 0.6 at 800 K.

Electrical performance of skutterudites solar thermoelectric generators

The paper focuses on the electrical performance of skutterudites based solar thermoelectric generators (STG). Basics of STG mathematical modeling are reported. Two STG concepts, namely the flat plate model with identical collector and radiator areas and the STG configuration with concentrators have been considered. Their performance is assessed for different solar intensities. The idea is to understand how such systems would perform if they were on board a spacecraft on journey towards the Sun.

Production of ultrafine powders of BiSb solid solution

Investigations of the synthesis and consolidation of bismuth-antimony alloys submicrometric powders were performed. Powders containing from 6 to 20 atomic percent of antimony were prepared by evaporation of Bi-Sb pre-alloyed targets using arc energy under argon-hydrogen atmosphere. Chemical analyses, EDS analyses, morphological and microstructural studies were performed on the product powders. The influence of the evaporation parameters on the powder generation yield and the local chemical composition of the powders was studied. We show that the control of the powder permits adjustment in the powder particle size from 25 nm to 0.8 μm. Finally, we present the influence of the ultrafine powder morphology on the microstructure of bulk alloys prepared by cold pressing powders.