S. 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 .

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.

Study of the n-type Bi2Te2.7Se0.3 doped with bromine impurity

Abstract Solid solutions with the composition of Bi2Te2.7Se0.3, whether doped with bromine impurity or not, have been grown using the Bridgman method. Structural and thermoelectric characterizations of samples cut along the as-grown ingots of these solid solutions are carried out. With the help of electrical and thermal conductivities and the Seebeck coefficient at room temperature, the point defects responsible of the p-type conductivity in non-doped solid solutions are discussed. The analysis of the experimental results concerning the doped solution with bromine atoms leads to obtain the segregation coefficient of Br nearly equal to one. The electrical activity of this impurity is discussed when substituted for Te and when an atom of bromine may be a single donor.

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.

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.

Anisotropy of the selenium diffusion coefficient in bismuth telluride

Abstract We present results for the anisotropic diffusion coefficient of selenium, isoelectronic impurity, in Bi 2 Te 3 along the solidus. The single crystals were prepared by travelling heater method and they have a high crystalline quality. The experimental conditions permitted us to obtain a definite stoichiometric deviation, and diffusion profiles were obtained by SIMS. We propose a mechanism of atomic diffusion by antisite defects and thermal vacancies taking into account of the anisotropy of the diffusion coefficients.

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.

Bi-Sb alloys: an update

This paper is devoted to a survey of the principal studies performed on Bi-Sb thermoelectric materials over the last forty years. Bi-Sb alloys are still the best n-type materials for refrigeration at low temperatures nowadays. After a brief introduction of their physical characteristics (crystallography and band structure), the elaboration, characterization and transport properties (thermoelectric and galvanomagnetic) of the Bi-Sb alloys will be presented. The results obtained on bulk materials, in single crystalline and polycrystalline forms, and thin films will be discussed.

Crystal growth and thermodynamics of n-type Bi2(Te1−xSex)3 single crystal solid solutions (x ⩽ 0.05)

Abstract Bi 2 (Te 1− x Se x ) 3 single crystal solid solutions with x = 0.025 and x = 0.05 have been grown using the travelling heater method. The first step of this work was the study of the BiTeSe ternary phase diagram on the Bi 2 Te 3 -rich side and with excess tellurium. The isoconcentration lines for these two compositions have been determined for temperatures ranging from 550 °C to their melting points. The knowledge of this phase diagram allows us to grow homogeneous ingots of high crystalline quality. The solidus line on the Te-rich side for these compositions has been determined by Hall effect measurements. This study shows that it is possible to grow thermodynamically well-defined single crystals in a reproducible way.