Journal of Phase Equilibria and Diffusion最新文献

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.

First-principles calculations based on density functional theory (DFT) were employed to investigate the Tb-Ni binary system, and its thermodynamic characterization was reassessed utilizing the CALPHAD (CALculation of PHAse Diagram) methodology. The liquid solution is described by the Redlich-Kister polynomials model, while the binary compounds are treated as stoichiometric phases. The predicted formation enthalpies of all intermediate compounds in the Tb-Ni binary system were used to support the optimization. Leveraging the Thermo-Calc software, a self-consistent set of thermodynamic parameters was obtained. The calculated phase diagram aligns well with experimental phase equilibrium data from the literature, and the resulting thermodynamic properties exhibit greater reasonability. The trend in thermodynamic information across rare earth (RE)-Ni systems is highlighted, showing that with an increase in the RE atomic number, both the enthalpies of mixing of liquid alloys and the enthalpies of formation of intermetallic compounds become more negative.

In the present work, phase equilibria in the Li(_2)O–Al(_2)O(_3) system were experimentally studied and calorimetric measurements were performed. Based on obtained results and data from literature, thermodynamic parameters of the system were assessed. The solid solution phases were modeled using Compound Energy Formalism (CEF) and liquid phase was described by two-sublattice partially ionic liquid model. The experimental investigations for selected compositions of isothermally heat-treated samples were performed using x-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). Differential Thermal Analysis (DTA) and Differential Scanning Calorimetry (DSC) were used to measure the temperature of the reactions as well as the heat capacities, respectively. After DTA, the microstructure was analyzed using SEM. Temperature of peritectic melting of h-LiAl(_5)O(_8) was determined to be 2222 K and temperature of eutectic reaction Liq (leftrightarrow) (gamma)-LiAlO(_2) + h-LiAl(_5)O(_8) to be 1965 K. Heat capacity of LiAlO(_2) and LiAl(_5)O(_8) was measured in the temperature range of 100-1300 K. The degree of inversion of spinel phase (Al(^{+3}), Li(^{+1}))(_1^T):(Al(^{+3}), Li(^{+1}), Va)(_2^O):O(_4) was modelled assuming Al(^{+3}) and Li(^{+1}) can occupy tetrahedral (T) and octahedral (O) cationic sublattices while its composition extension in Al(_2)O(_3) enriched region was described by introducing vacancies in octahedral sites. Thermodynamic description derived in the present study reproduces the degree of inversion close to that of the high-temperature spinel phase, which means that Al(^{+3}) ions preferentially occupy the tetrahedral sites. The calculated phase diagram satisfactorily agrees with the experimental results. Available experimental thermodynamic data are also reproduced within uncertainty limits.

Systematic studies on the lattice expansion and order-disorder phase equilibria are attempted for the A-B binary alloy on the two-dimensional square lattice. The atomic pair potentials are described by the Morse potential and the configurational entropy is formulated within the pair approximation of the Cluster Variation Method (CVM). The lattice expansion of the uniformly deformable lattice is enhanced by introducing lattice vibration effects through the Debye–Grüneisen model. The introduction of the local lattice distortion by continuous displacement CVM (CDCVM) further increases the lattice expansion. The transition temperature obtained for a uniformly deformable lattice is reduced by the thermal vibration effects, which is interpreted as the curvature effects of atomic pair potentials. The local lattice relaxation further reduced the transition temperature, which is ascribed to the additional freedom of distributing atomic pairs over a wide range of distances.

Many different types of phases can form within alloys, from highly-ordered intermetallic compounds, to structurally-ordered but chemically-disordered solid solutions, and structurally-disordered (i.e. amorphous) metallic glasses. The different types of phases display very different properties, so predicting phase formation is important for understanding how materials will behave. Here, we review how first-principles data from the AFLOW repository and the aflow++ software can be used to predict phase formation in alloys, and describe some general trends that can be deduced from the data, particularly with respect to the importance of disorder and entropy in multicomponent systems.

During powder-pack boronizing, an alloy is enveloped by a mixture comprising a source of boron, a diluent and an activator. If the activity of B in boriding media exceeds that in alloys, then this difference is a driving force, which, at elevated temperatures, may result in single- or multi-layered protective coatings. If it is intended to model and optimize this diffusion-controlled process, then an ability to calculate and control boron activity in both currently employed and prospective diluent-source-activator blends would be indispensable. In literature, such calculations are seldom performed, which likely reflects a reluctance or trepidation to deal with systems routinely containing five or more components. Consequently, it is commonly yet mistakenly believed that the activity of B can be smoothly changed by gradually changing a fraction of its source. In reality, a multiphase nature of boronizing powders is manifested in intricate concentration dependencies of boron activity with intervals where it remains constant. Another puzzling feature is the constancy of boron activity in phase fields which ostensibly have a non-zero number of degrees of freedom. In this work, a seeming inconformity with Gibbs' phase rule is addressed. Although it is not unreasonable to expect that an equilibrium phase assemblage at high temperatures would differ from ingredients mixed at room temperature, it is instructive to realize how drastically dissimilar they can be in some cases. If a boriding medium is utilized several times (a routine industrial practice), then its evolution caused by multiple heating and cooling cycles should not be overlooked.