Journal of Phase Equilibria and Diffusion最新文献

The paper analyzes the works of the last decade on the study of ionic conductivity, thermoelectric, photovoltaic, photocatalytic, optical, etc. properties of the argyrodite family compounds (prototype Ag₈GeS₆), as well as phases and composite materials based on them. Considered works allow us to characterize them as valuable environmentally friendly functional materials with great potential for practical application. The main approaches which have been used in these works to enhance functional properties, as well as to improve the application capabilities of this class of substances have been analyzed. The importance of further systematic investigations on their design based on the “composition-structure-property” relationship has been noted. In the present review, special attention is paid to the analysis of works on phase equilibria in the corresponding systems since the information accumulated in phase diagrams is extremely important for optimizing the properties of compounds through targeted variation of composition and structure. Available data on the phase equilibria and thermodynamic properties of ternary and quaternary systems forming Cu/Ag argyrodite compounds and solid solutions based on them are presented and analyzed. It has been shown that anionic and Cu ↔ Ag substitutions in compounds lead to a strong decrease in their polymorphic phase transition temperatures and an expansion of the temperature-composition ranges of the existence of high-temperature ion-conducting phases up to room temperature and below. The possibility of replacing part of the chalcogen atoms in the compounds with halogens, and the Cu(Ag) atoms with elements of the zinc subgroup, which expands the range of argyrodite phases is also shown. The significance of expanding studies of phase equilibria and thermodynamic properties of these systems, especially five-component and more complex systems possessing a big possibility of the formation of high-entropy argyrodite phases, which have thermodynamic stability in a wide temperature-composition range and better applied characteristics has also been specified.

Abstract

In this study, a continuous and dense FeAl/Al₂O₃ gradient coating was prepared on the surface of 316L stainless steel by hot-dipping aluminizing, vacuum annealing and low-oxygen pressure oxidation. The results showed that when a hot-dipping pure aluminum was applied, the alloy compound layer was composed of (Fe, Cr, Ni)₂Al₅ and (Fe, Cr, Ni)Al₃ phases. When the Si content in the bath was higher than 2.5 wt.%, a new intermetallic compound, Al₇(Fe, Cr)₂Si, appeared at the interface, and the thickness of the metal compound layer decreased continuously upon increasing the silicon content in the melt pool. When the hot-dipped sample was annealed in vacuum at 900 °C for 3 h, the brittle (Fe, Cr, Ni)₂Al₅ phase was transformed into a ductile (Fe, Cr, Ni)Al phase. As low-oxygen pressure oxidation progressed, FeAl/Al₂O₃ gradient coatings were formed on the surface layer. The oxide film formed by the Al-2.5 Si wt.% sample was flatter and denser, which significantly improved its high-temperature oxidation resistance. Reducing the oxygen partial pressure promoted the external oxidation of Al element and the formation of Al₂O₃ oxide film.

Heavy p-elements-rare earth element tellurides are important multifunctional materials with thermoelectric, magnetic, optical, topological insulating, and other properties. This paper presents the results of a study of phase equilibria in the Tl₂Te-TlBiTe₂-TlGdTe₂ system and the magnetic properties of the TlBi_1−xGd_xTe₂ solid solutions. A diagram of solid-phase equilibria of the indicated system has been constructed. According to the data obtained, it is characterized by the formation of wide areas of homogeneity based on the initial compounds of the TlBiTe₂-TlGdTe₂ boundary system and a continuous series of solid solutions along the Tl₉BiTe₆-Tl₉GdTe₆ section (δ-phase). It has been shown that the homogeneity regions of the TlBiTe₂ (β₁) and TlGdTe₂ (β₂) compounds form a narrow (1-2 mol.%) band extending to 35 and 10 mol.% TlGdTe₂ and TlBiTe₂, respectively, along the boundary TlBiTe₂-TlGdTe₂ system while the homogeneity area of the δ-phase significantly extends into the compositions region rich in Tl₂Te. Based on powder diffraction patterns, the phase compositions of the alloys and the crystallographic parameters of the identified solid solutions were determined. The work also presents several polythermal sections, isothermal sections at 760 and 800 K of the phase diagram, as well as a projection of the liquidus and solidus surface of the Tl₂Te-Tl₉BiTe₆-Tl₉GdTe₆ subsystem. Magnetic properties of TlBi_1−xGd_xTe₂ samples were investigated by magnetization and Electron Paramagnetic Resonance (EPR) measurements. It was found that these samples behave as a Curie-Weiss paramagnet containing Gd³⁺ ions with no indications of magnetic ordering or freezing down to 2 K.

Using our new program code, we have calculated the temperature dependence of the Seebeck coefficient ((S-T)) in the linearized Boltzmann transport equation with a constant relaxation time (LBT-CRT) for Fe₂VAl (cF16) and its quaternary compounds for the range from − 263 °C (10 K) to 727 °C (1000 K). We revealed that Fe₂VAl compound free from any defects exhibited the Seebeck coefficient with a negative sign at odds with experimental data with a positive sign. However, this dilemma could be removed after the introduction of Al/V near neighbor inversion defects into the perfect Fe₂VAl. A key point in developing a reliable temperature-dependent Seebeck coefficient software lies in how precisely we calculate the density of states times square of the group velocity ({left|{{text{v}}}_{x}right|}^{2}) along the direction (x) of thermal gradient. The present method is contrasted to the Fourier Transform Interpolation method in BoltzTraP developed by Madsen and Singh (2006). Nevertheless, both could reproduce the experimental data of Fe₂VAl once the inversion effect was taken into account. Our new software allows us to seek the origin of characteristic behaviors in the (S-T) curve by decomposing the electronic parameter above into sub-bands and analyzing the sub-band dependence of the energy spectrum (Aleft(varepsilon right)) in the LBT-CRT equation.

The equilibrium T − x space of the Ag-Ga-Te-AgBr system in the part Ag₂Te-GaTe-Te-AgBr-Ag₂Te below 600 K has been divided into separate phase regions using the electromotive force (EMF) method. Accurate experimental data were obtained using the following electrochemical cells (ECs): (−) IE | NE | SSE | R{Ag⁺} | PE | IE (+), where IE is the inert electrode (graphite powder), NE is the negative electrode (silver powder), SSE is the solid-state electrolyte (glassy Ag₃GeS₃Br), PE is the positive electrode, R{Ag⁺} is the region of PE that is contact in with SSE. At the stage of cell preparation, PE is a non-equilibrium phase mixture of the well-mixed powdered compounds Ag₂Te, GaTe, Ga₂Te₃, AgBr, and tellurium, taken in ratios corresponding to two or three different points of interest for each of the phase regions. The equilibrium set of phases was formed in the R{Ag⁺} region at 600 K for 48 h with the participation of the Ag⁺ ions. Silver cations, displaced for thermodynamic reasons from the NE to the PE of ECs, acted as catalysts, i.e., small nucleation centers of equilibrium phases. The spatial position of the established phase regions relative to the position of silver was used to express the overall reactions of synthesis of the binary Ga₂Te₅, Ga₇Te₁₀, Ga₃Te₄, ternary AgGa₅Te₈, and quaternary Ag₃Ga₁₀Te₁₆Br, Ag₃Ga₂Te₄Br, Ag₂₇Ga₂Te₁₂Br₉ compounds in the PE of ECs. The values of the standard thermodynamic functions (Gibbs energies, enthalpies, and entropies) of these compounds were determined based on the temperature dependencies of the EMF of the ECs.

Thermodynamic descriptions of the Bi-Se and Bi-Te systems have been developed using the CALculation of PHAse Diagrams (CALPHAD) method based on the experimental data available in the literature. The liquid phases were described by the associated solution model for the Bi-Se and Bi-Te systems with Bi₂Se₃ and Bi₂Te₃ as associates, respectively. The intermetallics Bi₂Se₃, Bi₃Se₄, Bi₈Se₉, BiSe, Bi₈Se₇, Bi₄Se₃, Bi₃Se₂, Bi₄Te₅, Bi₈Te₉, BiTe, Bi₄Te₃, Bi₂Te and Bi₇Te₃ were treated as stoichiometric compounds while Bi₂Te₃ was modeled by the sublattice model based on its homogeneity range and crystal structure. A set of self-consistent thermodynamic parameters for the Bi-Se and Bi-Te systems was obtained. Comparisons between the calculated results and experimental data available in the literature show that the most reliable experimental information can be satisfactorily accounted for by the present modeling.

The Cr–Ti system was investigated by several experimental methods and first-principles calculations. The thermodynamic activity of the body-centered cubic solid solution was measured by Knudsen effusion mass spectrometry. The stability of all three polymorphic structures of the Laves phase (C14, C15, and C36) was determined by differential thermal analysis, and the equilibrium tie-lines with the solid solution were obtained by combining results from diffusion couples and equilibrated alloys. The enthalpy of formation of the Laves phases with the corresponding end-members were calculated using density functional theory and the obtained values were integrated in the models. The experimental and computed data available in the literature was reviewed and the binary system was assessed by the Calphad method. The present evaluation results in an improved thermodynamic description, which can describe the experimentally observed activity in a large temperature range. The temperatures of the invariant reactions between the C15 and the C36 phase with the Cr-rich and the Ti-rich bcc solid solution were significantly modified. The difference of the temperature of transformation between the C15 and the C36 polytypes on both sides of the Laves phase is much smaller than reported previously.