L. I. Petrova

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.

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.

Physicochemical interaction at the contact of higher manganese silicide with chromium

Diffusion processes taking place at the contact of higher manganese silicide MnSi1.71−1.75 with chromium at elevated temperatures are considered. The microstructures of cast and annealed samples show a diffusion region that evolves at the HMS/Cr interface during the HMS melt crystallization on chromium flakes. As the temperature and time of annealing of HMS/Cr samples increase, the diffusion region grows thicker and new phases appear. It is shown that the diffusion is of reaction character. The diffusion coefficients of magnesium, chromium, and silicon in the solid solution based on manganese and chromium monosilicides and in the solid solution based on the Cr3Si compound are calculated. It is also shown that, when the temperature of the HMS used as a thermoelement positive leg is optimal, the depletion time of a 10-µm-thick chromium layer exceeds 15000 h. Intermetallic phases forming in the diffusion region between the HMS and Cr are of metallic conduction and exhibit a high thermal conductivity, which minimizes energy losses in the contact layers of thermoelements.

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.

Materials based on bismuth and antimony chalcogenides for thermoelectric cooler stages

To enhance the efficiency of multistage thermoelectric coolers, extruded materials based on p- and n-type solid solutions of bismuth and antimony chalcogenides have been optimized for particular temperatures in the range 100–300 K. We have studied the effect of selenium doping on the thermoelectric efficiency of the p-type materials. We have fabricated pilot micromodules and compared calculated and experimentally determined characteristics (maximum temperature difference and thermoelectric figure of merit) of the modules in the range 100–300 K.

Physicochemical interaction at the MnSi1.71–1.75/Mo interface

Diffusion processes at the interface between higher manganese silicide (HMS) MnSi1.71–1.75 and Mo at elevated temperatures have been studied by microstructural analysis and X-ray microanalysis. The results demonstrate the formation of a reaction diffusion zone at the HMS/metal interface. The compositions of the phases identified in intermediate layers are consistent with phase equilibria in the ternary system Mn-Mo-Si, and their electrical and thermal conductivity is high enough not to create an energy barrier in the contact zone with the semiconductor. The thermal expansion mismatch between the phases in contact may degrade the bonding between the layers.

Graded carrier concentration materials for thermoelectric coolers

We have optimized the compositions of p-and n-type graded thermoelectric materials based on solid solutions between bismuth and antimony chalcogenides with high thermoelectric efficiency in the temperature range 100–400 K. Czochralski-grown graded single crystals have been used to fabricate graded legs 2.5 mm in height and 1.4 × 1.4 mm2 in cross-sectional area for different stages of thermoelectric coolers. Pilot thermoelectric modules have been fabricated using homogeneous and graded legs, and the maximum temperature difference across the modules has been measured from 100 to 300 K. The efficiency of the modules with graded legs is shown to exceed that of the modules with homogeneous legs, especially at low temperatures.