H. Scherrer

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.

Thermoelectric properties of (BixSb1-x)2Te3 single crystal solid solutions grown by the T.H.M. method

Abstract From the experimental determination of the Te-rich comer of the Bi-Sb-Te phase diagram, single crystal solid solutions with compositions Bi 8 Sb 32 Te 60 , Bi 9 Sb 31 Te 60 and Bi 10 Sb 30 Te 60 have been grown using the Traveling Heater Method (T.H.M.). A thermoelectric characterization of samples of these p -type solid solutions is carried out as a function of stoichiometric deviations. Highly pure samples were prepared from single crystalline ingots using an annealing saturation technique at temperatures between 510 and 570°C. With the help of measurements of electrical and thermal conductivities and the Seebeck coefficient at room temperature, a maximum in the figure of merit Z = 3.2 × 10 −3 K −1 was determined for the solid solution with a composition Bi 9 Sb 31 Te 60 .

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.

Study of the Bi-Sb-Te ternary phase diagram

Abstract A study of the Bi-Sb-Te ternary phase diagram is carried out by Differential Scanning Calorimetry measurements to obtain both liquids and solidus temperatures of the Bi 2 Te 3 −Sb 2 Te 3 pseudo-binary section. The same technique is used for liquidus temperature determination of the tellurium-rich field. The liquid-solid equilibria (tie-lines) are also determined for Bi 8 Sb 32 Te 60 and Bi 10 Sb 30 Te 60 solid solutions by an equilibrium annealing technique and for temperatures ranging between 510°C and the solid solution melting points. All the collected data will be critical for the growth of single crystals well defined from a thermodynamic point of view.

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...

Transport properties analysis of single crystals (BixSb1 − x)2Te3 grown by the traveling heater method

Abstract A modeling of the transport properties of p -type (Bi x Sb 1− x ) 2 Te 3 single crystals is presented. Reasonably good agreement between calculated and experimental transport coefficients was achieved by considering a single valence band and acoustic phonon and ionized impurity scattering for holes. The anisotropy of the transport coefficients, in directions perpendicular (11) and parallel (33) to the trigonal c-axis of the crystals, was also taken into account in the model. The effective masses m ii , the isotropic and anisotropic energy independent relaxation time factors τ oac and τ oion ii for the main directions ( ii = 11 or 33) were determined for two solid solutions with x = 0.2 and 0.25. The lattice thermal conductivities were estimated for the two solid solutions. The maximum room temperature figure of merit was estimated at 3.2 × 10 −3 K −1 for a solid solution with x = 0.225 in good agreement with the experimental results.

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.

Thermal properties of high quality single crystals of bismuth telluride—Part II: Mixed-scattering model

Abstract From the preceding experimental study on the thermoelectric properties of Bi2Te3, a mixedscattering model was used to fit the sharp variations of the various parameters with stoichiometric deviations. First a one-band model was developed: the Fermi level and the main scattering mechanism were determined and values for the Hall factor, the Lorenz number, the effective mass and the relaxation time constant found for both n- and p-type material. Using a two-band model permitted the fitting of the behaviour of the thermoelectric parameters in the vicinity of the change in type. The optimum Fermi level for the maximum of the figure of merit was found just at the bottom of the conduction band.

Chapter 4 An overview of recent developments for BiSb Alloys

Publisher Summary This chapter reviews the recent developments for bismuth–antimony (Bi–Sb) alloys. It describes the main physical properties of Bi–Sb alloys and their synthesis. The chapter discusses the salient results pertaining to thermoelectric properties for both single crystalline and polycrystalline materials. The chapter focuses on the Bi–Sb alloys in the Bi-rich region. Transport properties are only described qualitatively, because the band structure is strongly correlated to antimony content and temperature. Although the thermomagnetic effects present considerable interest in Bi–Sb solid solutions, the chapter discusses the zero-field coefficients. The chapter summarizes the basic features of the band structure of the pure elements and alloys pertinent to the understanding of the transport properties. The small distortion from the simple cubic lattice induces that bismuth and antimony are semimetals characterized by a small overlap of the fifth and sixth bands, leading to the presence of a small equal number of electrons ( n ) and holes ( p ) at all temperatures. In isotropic monovalent metals or in extrinsic semiconductors, where only one type of cartier is present, the carrier density n and mobility μ may be easily experimentally determined by using the Hall coefficient, R H. The knowledge of cartier density and mobility in the temperature range 80–200 K is of the greatest interest from the point of view of the use of Bi–Sb alloys in thermoelectric devices. In Bi–Sb alloys, the thermal conductivity λ is principally the sum of two terms: an electronic contribution λ E (including both unipolar and bipolar terms) and a contribution associated with the lattice λ L .