Yu. V. Granatkina

Thermoelectric properties of Bi2Te3-Sb2Te3 single crystals in the range 100–700 K

The electrical conductivity, thermoelectric power, and thermal conductivity of Bi2Te3-Sb2Te3 crystals grown by the floating-crucible technique were measured in the temperature range from 100 to 700 K. The thermoelectric figure of merit of the crystals was evaluated. The effect of crystal composition on these properties was analyzed.

Thermoelectric and mechanical properties of the Bi0.5Sb1.5Te3 solid solution prepared by melt spinning

This paper reports the preparation and characterization of pressed microcrystalline materials based on a p-type Bi0.5Sb1.5Te3 solid solution produced from a melt-spun powder. We have examined the effect of melt spinning conditions (temperature, disk rotation rate, and purity of the inert gas in the heat treatment chamber) on the particle size and morphology of the powders and the microstructure and thermoelectric properties of hot-pressed samples and investigated the mechanical properties (compression and bend tests) of materials prepared by various methods. The thermoelectric properties of the materials (thermopower, electrical conductivity, and thermal conductivity) were studied at room temperature and in the range 100–700 K. The highest thermoelectric figure of merit ZT of the materials prepared by pressing the melt-spun powder was 1.3, whereas the ZT of the materials prepared by the other methods did not exceed 1.1. The higher ZT of the materials studied was due to their lower lattice thermal conductivity and slightly higher thermopower.

Nernst-ettingshausen tensor in Sb2Te3 single crystal

On one Sb2Te3 single crystal, the temperature dependences of all three independent components of the Nernst-Ettingshausen tensor (Qikl) are measured in the temperature range of 85–450 K, all three components being negative. Alongside with the Nernst-Ettingshausen effect, the anisotropy of the Hall (Rikl) and Seebeck (Sij) coefficients and the conductivity (σii) is also investigated. The carried-out analysis of the experimental data on the Nernst-Ettingshausen and Seebeck effects indicates that there is the mixed scattering mechanism with the participation of acoustic phonons and impurity ions, the relative contributions of these mechanisms varying with temperature. In the relaxation-time-tensor approximation, the values of the effective scattering parameter (r) are determined. The obtained values point to the dominant scattering at acoustic phonons in the cleavage plane and to the substantial contribution of charged ions to the scattering along the trigonal axis c3. It is shown that it is possible to explain the major features of experimental data on the Nernst-Ettingshausen effect within the two-valence-band model with the participation of several groups of holes in the transport phenomena.

Extruded thermoelectric materials based on Bi2Te3-Bi2Se3 solid solutions

Extruded n-type materials based on Bi2Te3-Bi2Se3 alloys containing 6 to 40 mol % Bi2Se3 have been investigated using microstructural analysis and thermoelectric measurements at room temperature and in the range 100–400 K. Their electrical properties have been compared to those of single-crystal analogs. Compositions have been found at which the extruded materials offer the highest thermoelectric performance in different temperature ranges.

Extruded materials for thermoelectric coolers

We have optimized the compositions of p-type thermoelectric materials based on solid solutions between bismuth and antimony tellurides with high thermoelectric figures of merit in different temperature ranges between 100 and 300 K. The materials have been prepared by extrusion and have been characterized by microstructural analysis. Their thermoelectric properties have been studied in the range 100–400 K.

Thermoelectric materials based on Sb2Te3-Bi2Te3 solid solutions with optimal performance in the range 100–400 K

We have optimized the compositions of thermoelectric materials based on Sb2Te3-Bi2Te3 solid solutions using Czochralski-grown single crystals. The thermoelectric performance of Sb2Te3-Bi2Te3 solid solutions containing 0–100 mol % Bi2Te3 and Bi2Te3-Sb2Te3-Bi2-Bi2Se3 solid solutions containing 2, 4, or 7 mol % Bi2Se3 has been investigated. The Bi2Se3-doped crystals are found to have higher thermoelectric figures of merit compared to the undoped crystals. The optimal crystal compositions are selected for different temperatures in the range 100–400 K.

Structural Defects in Tin-Doped Antimony Telluride Single Crystals

The structural perfection and homogeneity of Czochralski-grown Sb2Te3and Sb1.96Sn0.04Te3crystals were characterized by transmission electron microscopy and electron probe x-ray microanalysis. The Sb, Sn, and Te core-level and valence-band x-ray photoelectron spectra were measured, and the corresponding binding energies were determined. The effect of Sn doping on the electronic structure and transport properties of the crystals was examined. The crystals were shown to be uniform in Sb : Te ratio and to have a relatively perfect structure. The dislocation density in the crystals was determined by electron microscopy and selective etching. The origin of the moiré patterns observed in electron micrographs was discussed. Sn doping of Sb2Te3was shown to produce no significant shifts of the core levels or changes in the electronic density of states in the valence band.

Crystallization and mechanical properties of solid solutions between bismuth and antimony chalcogenides

Using scanning electron microscopy, we have studied how the conditions of preparation of granules of Sb2Te3–Bi2Te3 and Bi2Te3–Bi2Se3 solid solutions through melt solidification in a liquid influence their morphology, fractographs of fracture surfaces of samples prepared by hot-pressing the granules, and the contents of the major components in the samples. The granules are rounded (solidification in water and liquid nitrogen) or platelike (solidification in water under an excess pressure and in liquid-nitrogen-cooled ethanol) in shape. Fracture surfaces of hot-pressed samples prepared from granules comminuted in a ball mill have a uniform, fine microstructure, with faceted grains several microns in size. Characteristically, samples prepared from granules comminuted in a cutting mill have transgranular layered fractures, with layers up to hundreds of microns in thickness. The mechanical properties of the samples (ultimate strength and relative elongation) have been studied using compression tests at temperatures of 300 and 620 K. The samples experience brittle fracture. Their compression strength σc is 55 ± 12 MPa. With increasing temperature, σc varies only slightly, but at 620 K the samples become more plastic and their relative elongation εb increases by a factor of 2–4. The ultimate strength of hot-pressed samples prepared by uniaxial compression is 20% higher than that of samples prepared by biaxial compression.

Melt-spun materials based on an n-type Bi2Te2.7Se0.3 solid solution

Halide-doped n-type samples of the Bi2Te2.7Se0.3 solid solution have been prepared by hot-pressing melt-spun powders. The morphology of the powders and fracture surfaces of the hot-pressed samples have been examined by scanning electron microscopy, and the electrical conductivity, Seebeck coefficient, and thermal conductivity of the samples have been measured at room temperature and in the range 100–700 K. The thermoelectric properties of the samples have been compared to those of a sample prepared by extruding a mechanically ground ingot. The highest thermoelectric figure of merit of the materials obtained in this study is ZT ≃ 0.9.

Anisotropy of plasma reflection of solid solutions (Bi 2 − x Sb x )Te 3 (0 x < 1) in the Temperature Range 78–293 K

We study IR polarized reflection spectra of solid solutions (Bi2−xSbx)Te3 (0 < x < 1) in the temperature range 78–293 K and in the excitation range of plasma oscillations of free charge carriers. We reveal that, as the content of Sb2Te3 in the solid solution increases from 0 to 50%, the anisotropy of both plasma frequencies and magnetic susceptibility vary insignificantly. This indicates that, at 293 K, these crystals do not show significant changes in the composition of groups of free carriers near the chemical potential level. We have found that to describe the observed anisotropy of plasma frequencies, it is necessary to take into account the effect of heavy holes of the valence band. This is also confirmed by the results of the study of the temperature behavior of plasma reflection spectra, which exhibit regularities that are similar to anomalous temperature changes of the Hall coefficient. We show that a decrease in plasma frequencies with increasing temperature cannot be described by an increase in the effective mass of charge carriers in accordance with the expression m* ∼ T0.17, which was obtained upon interpretation of temperature dependences of a number of kinetic coefficients. We use the empirical Moss rule ɛ∞2Eg to estimate the rate of decrease of the thermal band gap dEg/dT = −1.6 × 10−4 eV/deg in a crystal that contains 25% Sb2Te3.