O. B. Sokolov

The variation of the equilibrium of chemical reactions in the process of (Bi2Te3)(Sb2Te3)(Sb2Se3) crystal growth

Methods of computation for thermodynamic and differential thermal analysis were used to study the effect of the variation of the chemical equilibrium in the synthesis of (Bi2Te3)x(Sb2Te3)y(Sb2Se3)z (x+y+z=100%) in an ampoule on the characteristics of crystals grown by vertical zone-melting technique. Measurements of thermoelectric characteristics of reference samples were made. The case study of pseudo-binary systems (antimony telluride—antimony selenide, x=0; bismuth telluride—antimony selenide, y=0; bismuth telluride—antimony telluride, z=0) proves the following. As a result of some components turning into a gas–vapour phase, some unreacted original components may be found in a condensed phase. With a single-stage synthesis the products of side reactions of chemical interactions may be formed in the condensed phase. The case study of (Bi2Te3)25(Sb2Te3)72(Sb2Se3)3; (Bi2Te3)70(Sb2Te3)25(Sb2Se3)5; (Bi2Te3)90(Sb2Te3)5(Sb2Se3)5 shows that both the synthesis temperature and the dopants greatly affect the thermodynamic equilibrium between the condensed and gas–vapour phases through the variation of the content of basic components in a condensed phase. On the basis of the performed research, recommendations on the synthesis conditions and crystal growth by zone-melting technique are given.

Phase relations and thermoelectric properties of alloys in the Bi2Te3-Bi2Se3 system

Using differential thermal analysis and x-ray diffraction, we have shown that the Bi2Te3-Bi2Se3 system contains a continuous series of solid solutions in a narrow temperature range and a compound of composition Bi2Te2Se below the solidus line. The liquidus and solidus lines determined using zone-melted samples differ little from those reported in the literature for equilibrium samples. The Bi2Te3−xSex solid-solution phase extends to ∼-14 mol % Bi2Se3 (Bi2Te2.58Se0.42). The thermoelectric power of the alloys drops sharply near the boundary of the two-phase region. Within the homogeneity range of Bi2Te2Se (33.3 mol % Bi2Se3), the thermoelectric power factor has a minimum, while the thermoelectric power has a small maximum.

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.

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.

Thermodynamic analysis of the composition of the condensed and gas-vapor phases of Bi/sub 2/Te/sub 3/-based solid solutions doped with organic and inorganic haloid compounds

Thermodynamic analysis was made with the help of a personal computer. A thermodynamic method for the identification of characteristics for the equilibrium of heterogeneous systems based on the principle of maximum entropies was taken as a basis for the algorithm of the software complex. Thermodynamic, thermophysical and thermochemical characteristics of individual materials were taken as the basis of the information contained in the database. Thermodynamic calculations of the composition of condensed and gas-vapor phases of Bi/sub 2/Te/sub 3/-based solid solutions for particular conditions of the synthesis with the application of various dopants has been affected. Research has been undertaken into the behaviour of haloid-containing dopants in gas-vapor and condensed liquid and solid phases depending on the type of a dopant: organic haloid compounds, inorganic haloid compounds. The effect of haloid-containing dopants on thermoelectric characteristics has been studied.

Doping with organic halogen-containing compounds the Bi 2 (Te,Se) 3 solid solutions

The paper considers the opportunity for the doping with organic halogen-containing compounds the Bi/sub 2/(Te,Se)/sub 3/ solid solutions. The advantages of their application as compared to that of inorganic halogen-containing compounds are described.

Doping with organic halogen-containing compounds: the Bi/sub 2/Te/sub 3/-Bi/sub 2/Se/sub 3/ solid solutions

Presently, Bi/sub 2/(Te,Se)/sub 3/ solid solutions used as an n-type legs for thermoelectric devices, are doped with inorganic halogen-containing compounds. However, most of these compounds are liable to dissociation when exposed to light, they are hygroscopic and tend to electrisation when poured from one place to another. These factors often give rise to wrong dosing of dopants in conditions of commercial production. This paper describes opportunities for doping of Bi/sub 2/(Te,Se)/sub 3/ solid solutions with organic halogen-containing compounds. For practical application it is proposed to use hexachlorethane (C/sub 2/Cl/sub 6/), hexachlorbenzol (C/sub 6/Cl/sub 6/), dibrombenzol (C/sub 6/H/sub 4/Br/sub 2/) hexabrombenzol (C/sub 6/Br/sub 6/) and iodoform (CHI/sub 3/). The advantages of the latter are shown as compared to inorganic halogen-containing compounds.

Chemical interactions and thermoelectric properties on the Bi/sub 2/Te/sub 3/-Bi/sub 2/Se/sub 3/ section in the interval below 33.3 mole % Bi/sub 2/Se/sub 3/

By methods of differential-thermal analysis and X-ray-phase analysis it is proved that the Bi/sub 2/Te/sub 3/-Bi/sub 2/Se/sub 3/ section is a system with a continuous series of solid solutions, with the formation of Bi/sub 2/Te/sub 2/Se compound below the solidus line. The formation of this compound has been confirmed both in experiment and by thermodynamic analysis. It has been found that after zone melting the temperature values of the liquidus and solidus lines correlate with those obtained for equilibrium systems and mentioned in the literature. The boundary has been identified for the existence of Bi/sub 2/Te/sub 3-x/Se/sub x/ solid solutions, which lies at /spl sim/14 mole % Bi/sub 2/Se/sub 3/ (Bi/sub 2/Te/sub 2.58/Se/sub 0.42/). Thermodynamic analysis shows chemical interactions (reactions) resulting in the formation of Bi/sub 2/Te/sub 2/Se. It has been found that in the two-phase zone, the Seebeck coefficient values dramatically change in a jumping manner, with the points of extremum located on the boundaries of the two-phase zone. In the zone of Bi/sub 2/Te/sub 2/Se compound occurrence (33.3 mole % Bi/sub 2/Se/sub 3/) the values of the Seebeck coefficient and power factor are minimal.

The variation of the equilibrium of chemical reactions in the process of (Bi/sub 2/Te/sub 3/)(Sb/sub 2/Te/sub 3/)(Sb/sub 2/Se/sub 3/) zone-melted material production

Methods of thermodynamic computation and differential thermal analysis were used for research into the effect of the variation of the chemical equilibrium in the synthesis of (Bi/sub 2/Te/sub 3/)/sub x/(Sb/sub 2/Te/sub 3/)/sub y/(Sb/sub 2/Se/sub 3/)/sub z/ (x+y+z=100%) in an ampoule on the characteristics of crystals grown by the vertical zone-melting technique.