Thermoelectric materials based on Sb<Subscript>2</Subscript>Te<Subscript>3</Subscript>-Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> 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.     

Extruded thermoelectric materials based on Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>-Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript> 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.     

Structural transformations in thin Ge<Subscript>2</Subscript>Sb<Subscript>2</Subscript>Te<Subscript>5</Subscript> films

Structural transformations in thin Ge₂Sb₂Te₅ films for phase-change memory applications have been studied by differential scanning calorimetry. As-grown, amorphous films have been shown to undergo structural transitions to a cubic and then to a hexagonal phase. A reproducible endothermic peak has been detected, which had not been reported earlier. A mechanism for the underlying process has been proposed.     

Phase relations and thermoelectric properties of alloys in the Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>-Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript> 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 Bi₂Te_3?x Se _x solid-solution phase extends to ～-14 mol % Bi₂Se₃ (Bi₂Te_2.58Se_0.42). 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.     

Luminescence properties of CdTe-based solid solutions in the CdTe-Ga<Subscript>2</Subscript>Te<Subscript>3</Subscript> and CdTe-GaTe systems

The extent of the cadmium-telluride-based solid solution along the CdTe-GaTe join at 737°C has been determined from the composition dependence of its unit-cell parameter. The cathodoluminescence spectra of CdTe-based solid solutions in the CdTe-GaTe and CdTe-Ga₂Te₃ systems have been measured. The 895-nm band observed in the 78-K spectrum of the Ga₂Te₃-containing solid solution is due to the [Ga_Cd^· V _Cd] radiative defect complex. The spectra of the CdTe-GaTe solid solutions seem to show a superposition of two lines close in position, attributable to the [Ga_Cd^· V _Cd] and [Ga_Cd^′ V _Te] defect complexes.     

Lithium ion conductivity of LiLaO<Subscript>2</Subscript>-Li<Subscript>2</Subscript>ZrO<Subscript>3</Subscript> solid solutions

The limits of the LiLaO₂-and Li₂ZrO₃-based solid solutions in the LiLaO₂-Li₂ZrO₃ system have been determined: 0–10 mol % Li₂ZrO₃ and 0–5 mol % LiLaO₂, respectively. We have studied the transport properties (electronic conductivity, temperature and composition dependences of conductivity and activation energy) of lithium lanthanate and the solid solutions in the LiLaO₂-Li₂ZrO₃ system. Conduction in LiLaO₂ is likely due to lithium ion transport through a polyhedral network.     

Electrical conductivity of Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>-Tm<Subscript>2</Subscript>O<Subscript>3</Subscript>-Nb<Subscript>2</Subscript>O<Subscript>5</Subscript> solid solutions

New solid solutions, Bi_2?x?y Tm _x Nb _y O_3+δ, with tetragonal and cubic structures have been synthesized in the Bi₂O₃-Tm₂O₃-Nb₂O₅ system, and their electrical conductivity has been measured at temperatures from 670 to 1020 K. The 1020-K conductivity of the tetragonal solid solution Bi_1.8Tm_0.15Nb_0.05O_3+δ is comparable to that of Bi_1.75Tm_0.25O₃, the best conductor in the Bi₂O₃-Tm₂O₃ system.     

Phase equilibria and properties of solid solutions in the Tl<Subscript>5</Subscript>Te<Subscript>3</Subscript>-Tl<Subscript>9</Subscript>BiTe<Subscript>6</Subscript>-Tl<Subscript>5</Subscript>Te<Subscript>2</Subscript>I system

Phase equilibria in the Tl₅Te₃-Tl₉BiTe₆-Tl₅Te₂I system, containing ternary structural analogs of Tl₅Te₃, have been studied by differential thermal analysis, X-ray diffraction, microhardness tests, and emf measurements using concentration cells with a thallium electrode. A number of T-x sections, the 760- and 800-K sections, and the liquidus and solidus projections of the phase diagram are constructed, and the composition dependences of lattice parameters, microhardness, and emf are obtained. The results demonstrate that, despite the presence of a liquid-liquid miscibility gap, the system contains a continuous series of solid solutions isostructural with Tl₅Te₃.     

Electric conductivity of a bulk composite based on Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>/SiO<Subscript>2</Subscript> core-shell nanoparticles

Composite Bi₂Te₃/SiO₂ nanoparticles of the core-shell type have been synthesized for the first time with a view to creating bulk composites possessing high thermoelectric figure of merit (conversion efficiency). It is suggested that bulk composited based on Bi₂Te₃/SiO₂ nanoparticles will provide a combination of low lattice heat conduction due to SiO₂ insulator and rather high electric conduction due to charge-carrier tunneling via dielectric spacers between adjacent Bi₂Te₃ semiconductor grains. The electric resistance of the composite increases with increasing temperature in the range of 130–300 K. This temperature dependence can be described in terms of a tunneling conduction model.     

CuInSe<Subscript>2</Subscript>-In<Subscript>2</Subscript>Te<Subscript>3</Subscript> system

A diagram of the CuInSe₂-In₂Te₃ system state is constructed using the methods of physicochemical analysis. The system is quasi-binary with limited areas of solid solutions based on both source components. The investigations of thermal conductivity of the (CuInSe₂)_0.99(In₂Te₃)_0.01 solid solution demonstrate a complex nature of the heat transfer attributed to both a multicomponent structure of the composition and a defect crystalline structure.     

Stress reduction during phase change in Ge<Subscript>2</Subscript>Sb<Subscript>2</Subscript>Te<Subscript>5</Subscript> by capping TiN film

It has been one of the most important issues to minimize the stress reduction during phase change in GST (Ge₂Sb₂Te₅) alloy for PRAM (Phase-change Random Access Memory) applications, because the alloy has been reported to face the significant stress during the phase change. We fabricated GST/oxide/substrate as a basic structure, and then added two more structures by capping an adhesion layer (Ti) or a barrier metal (TiN) on GST layer, respectively. We report that TiN-capped structure shows about 40% stress reduction during the phase change compared with that of the basic structure. The stress reduction is considered to be due to the intrinsic compressive stress in TiN film itself.     

Synthesis and structure of layered compounds in the PbTe-Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> and PbTe-Sb<Subscript>2</Subscript>Te<Subscript>3</Subscript> systems

A number of tetradymite-like layered compounds in the PbTe-Bi₂Te₃ and PbTe-Sb₂Te₃ systems were prepared by a vertical Bridgman process, and their crystal structures were investigated by x-ray diffraction using single-crystal cleaved specimens. The atomic positional parameters and bond distances in PbBi₄Te₇ were determined. In the pseudobinary system PbTe-Sb₂Te₃, two new compounds were identified, PbSb₂Te₄ and PbSb₄Te₇, isostructural with PbBi₂Te₄ and PbBi₄Te₇, respectively. The positional parameters, site occupancies, and bond distances in the structure of PbSb₂Te₄ were determined. The room-temperature thermoelectric power of the grown ingots was measured along their length. All of the Sb compounds studied were found to be p-type, in contrast to the compounds of the PbTe–Bi₂Te₃ system.Translated from Neorganicheskie Materialy, Vol. 40, No. 12, 2004, pp. 1440–1447.Original Russian Text Copyright © 2004 by Shelimova, Karpinskii, Svechnikova, Avilov, Kretova, Zemskov.     

Bi<Subscript>0.5</Subscript>Sb<Subscript>1.5</Subscript>Te<Subscript>3</Subscript>-4 mol % Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript> Single Crystals with Graded Carrier Concentration

A procedure is developed for the preparation of Bi_0.5Sb_1.5Te₃-4 mol % Bi₂Se₃ single crystals with a graded longitudinal carrier concentration profile. The thermoelectric power gradient in the crystals is 40–70 μV/K over a distance of 1–1.5 mm. The shape of the growth interface is determined. Graded legs are produced that have optimal carrier concentrations for different working temperatures of thermoelectric coolers (the thermoelectric power at the hot end is from 220 to 270 μV/K). Regions with low carrier concentration (α > 220 μV/K) are shown to contain Te-based eutectic precipitates along the cleavage plane. __________ Translated from Neorganicheskie Materialy, Vol. 41, No. 10, 2005, pp. 1200–1205. Original Russian Text Copyright ? 2005 by Ivanova, Granatkina, Petrova, Korzhuev.     

Rare-earth-doped PbTiO<Subscript>3</Subscript>-PbZrO<Subscript>3</Subscript> solid solutions

(Pb_{1 ? x} Ln _x )(Zr_0.53Ti_0.47)O₃ and (Pb_{1 ? x} Ln _x )(Zr_0.65Ti_0.35)O₃ (x = 0.02, 0.06; Ln = La, Pr, Gd, Yb) solid solutions have been prepared by modified solid-state synthesis using organic-ligand precursors. The solid solutions have been characterized by thermal analysis, IR spectroscopy, x-ray powder diffraction, and atomic force microscopy. All of them have a rhombohedrally distorted perovskite structure (sp. gr. R3c).     

Morphology and optical properties of Sb<Subscript>2</Subscript>O<Subscript>3</Subscript>

The chemical and phase compositions of antimony(III) oxide samples prepared under various conditions have been determined. The size and shape of their particles have been assessed by scanning electron microscopy, and the effect of UV irradiation on their diffuse reflectance spectra has been studied. The results are used to determine the whiteness index, integrated reflectance, and lightfastness of the samples.     

Homogeneity ranges of Tl<Subscript>5</Subscript>Te<Subscript>2</Subscript>Cl and Tl<Subscript>5</Subscript>Te<Subscript>2</Subscript>Br

This paper systematizes phase-diagram data for the Tl-TlCl(Br)-Te systems and presents their 500-K subsolidus phase diagrams. Tl₅Te₂Cl and Tl₅Te₂Br (Tl₅Te₃ structure) are shown to be nonstoichiometric compounds with wide homogeneity ranges, which have been accurately determined using emf measurements, x-ray diffraction, and microhardness tests. Using emf data for reversible concentration cells with a thallium electrode, we have evaluated the partial thermodynamic functions of the thallium in the alloys studied, the standard thermodynamic functions of formation of Tl₅Te₂Cl and Tl₅Te₂Br, and their standard entropies. The crystal chemistry of these phases of variable composition is discussed in relation to the Tl₅Te₃ structure.     

Phase equilibria in the Ag<Subscript>2</Subscript>Se-As<Subscript>2</Subscript>Se<Subscript>3</Subscript>-Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript> system

We have studied phase equilibria in the pseudoternary system Ag₂Se-As₂Se₃-Bi₂Se₃ and constructed the 300-, 600-, and 800-K isothermal sections, a number of partial phase diagrams, and the liquidus projection of this system. The AgAsSe₂-AgBiSe₂ and As₂Se₃-AgBiSe₂ joins are shown to be pseudobinary, and the Ag₃AsSe₃-AgBiSe₂ and AgAs₃Se₅-AgAsSe₂ joins are pseudobinary below the liquidus. Several in- and univariant peritectic, eutectic, and eutectoid equilibria and a broad region of AgBiSe₂-based solid solutions are identified. The homogeneity region of the AgBiSe₂-based phase has the largest extent along the AgAsSe₂-AgBiSe₂ join: 40 mol % (650 K) for the high-temperature form of AgBiSe₂ and 20 mol % (300 K) for its low-temperature form.     

Crystallization from high-temperature solutions in the K<Subscript>2</Subscript>O-P<Subscript>2</Subscript>O<Subscript>5</Subscript>-V<Subscript>2</Subscript>O<Subscript>5</Subscript>-Bi<Subscript>2</Subscript>O<Subscript>3</Subscript> system

We have studied general trends of crystallization from high-temperature solutions in the K₂O-P₂O₅-V₂O₅-Bi₂O₃ system at P/V = 0.5?2.0, K/(P + V) = 0.7?1.4, and Bi₂O₃ contents from 25 to 50 wt % and identified the stability regions of BiPO₄, K₃Bi₅(PO₄)₆, K₂Bi₃O(PO₄)₃, and K₃Bi₂(PO₄)_{3 ? x} (VO₄) _x (x = 0?3) solid solutions. The synthesized compounds have been characterized by X-ray powder diffraction and IR spectroscopy, and the structure of two solid solutions has been determined by single-crystal X-ray diffraction (sp. gr. C ₂/c): K₃Bi₂(PO₄)₂(VO₄), a = 13.8857(8), b = 13.5432(5), c = 6.8679(4) Å, β = 114.031(7)°; K₃Bi₂(PO₄)_1.25(VO₄)_1.75, a = 13.907(4), b = 13.615(2), c = 6.956(2) Å, β = 113.52(4)°.     

Integral isoconversional method for evaluating crystallization parameters of thin films of Ge<Subscript>2</Subscript>Sb<Subscript>2</Subscript>Te<Subscript>5</Subscript> phase change memory materials

We propose a method for evaluating kinetic parameters for the crystallization of thin films of phase change materials. Its basic principle is to jointly use model-free and model isoconversional methods in analyzing differential scanning calorimetry results. Using this method, we have identified the reaction model and evaluated the activation energy for crystallization and pre-exponential factor as a function of the degree of conversion for Ge₂Sb₂Te₅-based thin films.