Phase Stability Calculations Research Articles

First-principles calculations of phase stability, electronic structure, mechanical properties and thermal conductivities of TMH2 (TM=V, Nb, Ta) metal hydrides

TMH2 (TM = V, Nb, Ta) metal hydrides can be used as potential hydrogen storage materials. However, there have been no available data on the mechanical and thermodynamic properties of these hydrides, especially TaH2. Herein, based on first-principles calculations, the phase stability, electronic structure, mechanical properties and thermal conductivities of TMH2 (TM = V, Nb, Ta) hydrides are investigated. The results show that VH2, NbH2, and TaH2 are thermodynamically and dynamically stable. The calculated electronic structures show that these three hydrides possess electron orbital hybridization among atoms and strong chemical bonding. The chemical bonds in TMH2 are mainly ionic and partially covalent. According to the calculated Poisson's ratios, Pugh’s ratio and Cauchy pressures, VH2, NbH2, and TaH2 are ductile. Moreover, these three hydrides have elastic anisotropy and good damage tolerance. Finally, the predicted thermodynamic properties indicate that TMH2 can be used as potential thermal insulating materials.

Effect of Sintering Temperature on Phase Formation and Mechanical Properties of Al–Cu–Li Alloy Prepared from Secondary Aluminum Powders

Aluminum and its alloys are very versatile materials used in a wide range of applications due to the initial characteristics of pure aluminum and the combination of properties obtained from its blend with other elements. Considering that aluminum is the second-most-produced metal after steel, and that its production will increase over time based on the demand to produce products through conventional and additive methodologies, this will lead to an increase in the energy consumed as well as the footprint of carbon generated. It is for this reason that the generation of competitive aluminum alloys must be approached from secondary sources (recycling). To address these environmental issues, in this work, 2070 aluminum alloy (AA2070) samples were manufactured using secondary aluminum powder and compared with the primary aluminum source. The samples were compacted at 700 MPa and sintered at a different range of temperatures between 525 °C and 575 °C. The study includes thermodynamic modeling, microstructure, and mechanical characterization. Microstructure and phases characterization were carried out via scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis, respectively, whereas the mechanical characterization comprised relative density evaluation, hardness, and flexion tests. Results were compared with the calculation of phase stability using Thermo-Calc software 2020a. Based on the results obtained, it can be concluded that the secondary AA2070 optimal sintered temperature, where the components raised the highest mechanical properties and effective relative density range, is 575 °C. Furthermore, the recycled alloys have similar relative densities and flexural strengths than the corresponding alloys made from primary aluminum powder.

TWIP-assisted Zr alloys for medical applications: Design strategy, mechanical properties and first biocompatibility assessment

This study proposes a novel strategy for the design of a new family of metastable Zr alloys. These alloys offer improved mechanical properties for implants, particularly in applications where conventional stainless steels and Co-Cr alloys are currently used but lack suitability. The design approach is based on the controlled twinning-induced plasticity (TWIP) effect, significantly enhancing the ductility and strain-hardenability of the Zr alloys. In order to draw a “blueprint” for the compositional design of biomedical TWIP (Bio-TWIP) Zr alloys—using only non-toxic elements, the study combines d-electron phase stability calculations (specifically bond order (Bo) and mean d-orbital energy (Md)) with a systematic experimental screening of active deformation mechanisms within the Zr-Nb-Sn alloy system. This research aids in accurately identifying the TWIP line, which signifies the mechanism shift between TWIP and classic slip as the primary deformation mechanism. To demonstrate the efficacy of the TWIP mechanism in enhancing mechanical properties, Zr-12Nb-2Sn, Zr-13Nb-1Sn, and Zr-14Nb-3Sn alloys are selected. Results indicate that the TWIP mechanism leads to a significant improvement of strain-hardening rate and a uniform elongation of ∼20% in Zr-12Nb-2Sn, which displays both {332}<113> mechanical twinning and dislocation slip as the primary deformation mechanisms. Conversely, Zr-14Nb-3Sn exhibits the typical mechanical properties found in stable body-centered cubic (BCC) alloys, characterized by the sole occurrence of dislocation slip. Cell viability tests confirm the superior biocompatibility of Zr-Nb-based alloys with deformation twins on the surface, in line with existing literature. Based on the whole set of results, a comprehensive design diagram is proposed.

Unified flash calculations with isenthalpic and isochoric constraints

In the unified flash procedure, a persistent set of unknowns and equations are solved in equilibrium calculations, allowing for simultaneous phase stability and split calculations. For fluids in a subcritical thermodynamic state characterized by pressure and temperature, modeling both liquid and gas phases with inequality conditions for phase fractions has been shown to incorporate the tangent-plane criterion and results in a consistent formulation of compositions for both present and absent phases. However, applications such as high-enthalpy systems in subsurface flow require a state definition using other state variables than pressure and temperature, as well as the capability to represent supercritical phases. Furthermore, the robustness of the flash across a wide range of state values is required if equilibrium dynamics are to be included in a flow and transport problem.This work introduces constraints in terms of enthalpy and volume to allow pressure and temperature to vary in the unified setting. The constraints are shown to arise from equilibrium conditions for the relevant state functions to be minimized. To increase the range of applicability, a modified extension procedure for the compressibility factor was devised, as well as procedures for the flash initialization.The unified formulation is extendable to allow isenthalpic and isochoric flash calculations. The initialization was devised using methodologies from the negative flash and Rachford–Rice equations. Extensive numerical tests with multiple equilibrium definitions in terms of state variables were performed. Gas–liquid, binary and a multicomponent mixture using Peng–Robinson EoS showed consistency of results which are verified using third-party code.

First-principles calculation of phase stability and elastic properties of CrxMoNbTiV refractory high-entropy alloys

Compared with traditional alloys, high-entropy alloys (HEAs) have been widely studied because of their unique phase formation rules and excellent physical properties. This work used the first-principle calculation method to study the effect of Cr content on the phase formation, stability, and mechanical properties of MoNbTiV refractory HEAs (RHEAs). The structural model of CrxMoNbTiV (x = 0.00, 0.25, 0.50, …, 2.00) RHEAs was constructed by the virtual crystal approximation method. The structural model was geometrically optimized using the Cambridge Sequential Total Energy Package code, and the structures’ binding energy, enthalpy of formation, and elastic constants were calculated. The results show that the CrxMoNbTiV RHEAs can form a stable body-centered cubic structure, and the addition of Cr significantly impacts the lattice constant, elastic constant, plastic toughness, and elastic anisotropy of the alloy. At the same time, the three-dimensional surface map of Young’s modulus anisotropy is also drawn.

Phase stability analysis and phase equilibrium calculations in reactive and nonreactive systems using new hybrids of Pelican and Gorilla troops algorithms

Phase stability and equilibrium calculations are very significant in the process industries, ranging from process simulation, equipment design, operation and design. These problems involve several systems of complex and nonlinear equations, which are mathematically challenging. An alternative solution is formulating the problems into a global optimisation problem. A robust and reliable optimisation technique is required to solve these global optimisation problems. In this study, two recently developed stochastic global optimisation algorithms, POA and GTO were applied to phase stability and equilibrium problems. The need to enhance their performance necessitated the development of two modified algorithms, PGOA1 and PGOA2, which are the hybridisation of POA and GTO. All the algorithms' abilities were explored for solving computationally challenging and multidimensional phase stability and equilibrium problems, and their performances were compared. The implementation of POA and GTO was good in solving most problems, though GTO slightly outperformed POA. The hybridisation of both to develop PGOA1 and PGOA2 was worthwhile as both outperformed the POA and GTO. PGOA2 is more robust and reliable in solving phase stability and equilibrium, and its performance superseded the performances of the other algorithms.

Hybrid 2D/3D Graphitic Carbon Nitride‐Based High‐Temperature Position‐Sensitive Detector

Ultraviolet position‐sensitive detectors (PSDs) are expected to undergo harsh environments, such as high temperatures, for a wide variety of applications in military, civilian, and aerospace. However, no report on relevant PSDs operating at high temperatures can be found up to now. Herein, we design a new 2D/3D graphitic carbon nitride (g‐C3N4)/gallium nitride (GaN) hybrid heterojunction to construct the ultraviolet high‐temperature‐resistant PSD. The g‐C3N4/GaN PSD exhibits a high position sensitivity of 355 mV mm−1, a rise/fall response time of 1.7/2.3 ms, and a nonlinearity of 0.5% at room temperature. The ultralow formation energy of −0.917 eV atom−1 has been obtained via the thermodynamic phase stability calculations, which endows g‐C3N4 with robust stability against heat. By merits of the strong built‐in electric field of the 2D/3D hybrid heterojunction and robust thermo‐stability of g‐C3N4, the g‐C3N4/GaN PSD delivers an excellent position sensitivity and angle detection nonlinearity of 315 mV mm−1 and 1.4%, respectively, with high repeatability at a high temperature up to 700 K, outperforming most of the other counterparts and even commercial silicon‐based devices. This work unveils the high‐temperature PSD, and pioneers a new path to constructing g‐C3N4‐based harsh‐environment‐tolerant optoelectronic devices.

Phase stability of Fe from first principles: Atomistic spin dynamics coupled with ab initio molecular dynamics simulations and thermodynamic integration

The calculation of free energies from first principles in materials is a formidable task which enables the prediction of phase stability with high accuracy; these calculations are complicated in magnetic materials by the interplay of electronic, magnetic, and vibrational degrees of freedom. In this work, we show the feasibility and accuracy of the calculation of phase stability in magnetic systems with ab initio methods and thermodynamic integration by sampling the magnetic and vibrational phase space with coupled atomistic spin dynamics-ab initio molecular dynamics (ASD-AIMD) simulations [Stockem et al., PRL 121, 125902 (2018)], where energies and interatomic forces are calculated with density functional theory (DFT). We employ the method to calculate the phase stability of Fe at ambient pressure from 800 K up to 1800 K. The Gibbs free energy difference between fcc and bcc Fe at zero pressure as a function of temperature is calculated carrying out thermodynamic integration over temperature on the energies at the DFT level from ASD-AIMD, using a reference free energy difference calculated in the paramagnetic state at temperatures much higher than the magnetic transition temperatures with thermodynamic integration over stress-strain variables with disordered local moment (DLM)-AIMD simulations. We show the importance of the magnetic ordering temperature of bcc Fe on the $\alpha$ to $\gamma$ structural transition temperature, whereas the $\gamma$ to $\delta$ transition is well reproduced independently of the exchange interactions. The Gibbs free energy difference between the two structures is within 5 meV/atom from the CALPHAD estimate, and both transition temperatures are reproduced within 150 K. The present work paves the way to free energy calculations in magnetic materials from first principles with accuracy in the order of 1 meV/atom.

A first principles study of structural, electronic, elastic, thermal and optical properties of SiZr and GeZr

In the present work, the full-potential linearized augmented plane wave method is implemented to study the structural, electronic, elastic, thermal, and optical properties of semiconductor-based cubic perovskite compounds of SiZr and GeZr. The magnetic phase stability calculations are performed to identify the most stable magnetic state. SiZr and GeZr are predicted to be stable in anti-ferromagnetic phase. The ground state properties such as lattice parameter, elastic constants, bulk modulus, and pressure derivative are calculated. From the band structure calculations, these compounds exhibit narrow band gaps, which are semiconducting in nature. From the values of bulk to shear modulus B/G ratio (Pugh’s ratio), the compounds are found to be ductile. The values of Poisson’s ratio and Zener’s anisotropic index suggest that these compounds show covalent bonding and weak isotropy. A larger Debye temperature implies that compounds have strong interatomic bonding, greater hardness, high melting temperature, and higher mechanical wave velocity. The optical properties such as optical conductivity, reflectivity, and absorption coefficient suggest that SiZr could be a more suitable compound for solar cell applications.

Composition and property optimization of rare-earth-free Mn-Al-C magnet by phase stability and magnetic behavior analysis

Mn-Al-C system offers a possibility of rare-earth-free permanent magnets with the reduced temperature-dependent deterioration of magnetic properties. The Mn-Al-C alloy composition and magnetic properties have been optimized via calculations of electronic structures and phase stability of L 1 0 -ordered ferromagnetic τ-phase (Mn 0 . 5 Al 0 . 5 ) 100−x C x . The WIEN2k program package, Vienna Ab-initio simulation package (VASP), and Alloy Theoretic Automated Toolkit (ATAT) were used to calculate and identify the optimal carbon content for the most stable τ-phase of the L 1 0 -structured Mn-Al-C. We used the Brillouin function and Callen-Callen semiempirical relation to obtain the saturation magnetization and magnetocrystalline anisotropy constant at elevated temperatures. It was found that a carbon content of 2.33 at% gives the most stable τ-phase L 1 0 (Mn 0 . 5 Al 0 . 5 ) 100−x C x that has the lowest formation energy and highest saturation magnetization among the studied carbon contents (x = 0–3.03 at %). The magnetocrystalline anisotropy constant at 0 K increases with increasing the carbon content. Therefore, the carbon-doped Mn-Al becomes magnetically harder than the pure Mn 50 Al 50 . However, the anisotropy constant decreases at 300 K as the carbon content increases. The Curie temperature decreases to 590 K at x = 2.33 from 685 K at x = 0.0. The estimated saturation magnetization was approximately 130 emu/g at 300 K, leading to 18 MGOe under B s = B r and H ci > B r /2. Therefo r e, it is highly probable that Mn-Al-C potentially fills the gap between 10 and 30 MGOe magnets. The results in this study quantify and explain the reason for widely studying the approximately 2 at% carbon-doped Mn-Al systems. • A rare-earth-free, ferromagnetic L10 τ-phase (Mn0.5Al0.5)100-xCx was designed by first-principle calculations. • The calculated carbon content of 2.33 at% gives the lowest formation energy. • Theoretically optimized carbon content was in good agreement with experimental results. • The results explain the reason for widely studying the approximately 2 at% carbon-doped Mn-Al systems. • Rare-earth-free Mn-Al-C potentially fill the gap between 10 and 30 MGOe magnets.

Novel inorganic crystal structures predicted using autonomous simulation agents

We report a dataset of 96640 crystal structures discovered and computed using our previously published autonomous, density functional theory (DFT) based, active-learning workflow named CAMD (Computational Autonomy for Materials Discovery). Of these, 894 are within 1 meV/atom of the convex hull and 26826 are within 200 meV/atom of the convex hull. The dataset contains DFT-optimized pymatgen crystal structure objects, DFT-computed formation energies and phase stability calculations from the convex hull. It contains a variety of spacegroups and symmetries derived from crystal prototypes derived from known experimental compounds, and was generated from active learning campaigns of various chemical systems. This dataset can be used to benchmark future active-learning or generative efforts for structure prediction, to seed new efforts of experimental crystal structure discovery, or to construct new models of structure-property relationships.

Machine learning with knowledge constraints for process optimization of open-air perovskite solar cell manufacturing

Machine learning with knowledge constraints for process optimization of open-air perovskite solar cell manufacturing

Vibrational properties of CuInP2S6 across the ferroelectric transition

In order to explore the properties of a two-sublattice ferroelectric, we measured the infrared and Raman scattering response of CuInP2S6 across the ferroelectric and glassy transitions and compared our findings to a symmetry analysis, calculations of phase stability, and lattice dynamics. In addition to uncovering displacive character and a large hysteresis region surrounding the ferroelectric transition temperature T_C, we identify the vibrational modes that stabilize the polar phase and confirm the presence of two ferroelectric variants with opposite polarizations. Below TC, a poorly understood relaxational or glassy transition at Tg is characterized by local structure changes in the form of subtle peak shifting and activation of low frequency out-of-plane Cu- and In-containing modes. The latter are due to changes in the Cu/In coordination environments and associated order-disorder processes. Moreover, Tg takes place in two steps with another large hysteresis region and significant underlying scattering. Combined with imaging of the room temperature phase separation, this effort lays the groundwork for studying CuInP2S6 under external stimuli and in the ultra-thin limit.

First Principles Calculation of Common Intermediate Phase Stability and Mechanical Properties of G13 Steel

First Principles Calculation of Common Intermediate Phase Stability and Mechanical Properties of G13 Steel

A reconsideration on the resolution of phase stability analysis using stochastic global optimization methods: Proposal of a reliable set of benchmark problems

Phase stability analysis is an important thermodynamic calculation in the context of process systems engineering of multiphase units (e.g., separation systems, reactors). It implies the minimization of the Tangent plane distance function (TPDF), which has been recognized as a challenging global optimization problem. Despite the importance of this thermodynamic calculation, the characterization and classification of resolution complexity of phase stability problems that are commonly used to test and compare global optimization methods have not been addressed in literature. This paper reports the first step to propose a set of benchmark TPDF problems to assess and identify the advantages and limitations of stochastic optimization methods used in phase stability calculations. A detailed review of TPDF problem reported in literature was performed and the most representative ones were studied and classified in terms of their resolution complexity via the analysis of several performance metrics associated to both reliability and efficiency to find the global TPDF minimum. Differential Evolution, Simulated Annealing, Harmony Search, Tabu Search, Particle Swarm Optimization and Genetic Algorithm were selected as the metaheuristics to perform the global minimization of selected TPDF problems. 35 phase stability problems were classified in low, medium and high difficulty TPDF problems. Results showed that, although a wide variety of phase stability problems has been reported and used for phase stability calculations, a significant number of them corresponded to global optimization problems of easy resolution. Therefore, some authors have reported biased conclusions on the performance of stochastic optimization methods. A set of 23 TPDF was proposed as benchmark problems to obtain a reliable analysis of the numerical performance of metaheuristics employed in phase stability calculations. This benchmark set contains problems with different resolution difficulties and characteristics that are considered appropriate to study and assess new methods to resolve the global TPDF optimization, as well as to improve the existing methods. This paper highlights the relevance of a continuous development and improvement of stochastic global optimizers for solving, robustly and efficiently, the phase stability analysis in multicomponent systems. TPDF minimization can be still considered a challenging problem to be faced in the context of applied thermodynamics.

A soft computing method for rapid phase behavior calculations in fluid flow simulations

To describe the flow of hydrocarbons in the reservoir and in pipelines upstream petroleum engineers utilize compositional simulation models which allow for the accurate description of complex phase behavior phenomena by means of EoS models and detailed compositional characterization. To speed up the conventional time-consuming phase stability and phase split calculations, soft computing has been recently utilized by means of methods such as proxy models and on-the-fly tabulation of phase behavior results.

SGTPy: A Python Code for Calculating the Interfacial Properties of Fluids Based on the Square Gradient Theory Using the SAFT-VR Mie Equation of State.

In this work, we showcase SGTPy, a Python open-source code developed to calculate interfacial properties (interfacial concentration profiles and interfacial or surface tension) for pure fluids and fluid mixtures. SGTPy employs the Square Gradient Theory (SGT) coupled to the Statistical Associating Fluid Theory of Variable Range employing a Mie potential (SAFT-VR-Mie). SGTPy uses standard Python numerical packages (i.e., NumPy, SciPy) and can be used under Jupyter notebooks. Its features are the calculation of phase stability, phase equilibria, interfacial properties, and the optimization of the SGT and SAFT parameters for vapor-liquid, liquid-liquid and vapor-liquid-liquid equilibria for pure fluids and multicomponent mixtures. Phase equilibrium calculations include two-phase and multiphase flash, bubble and dew points, and the tangent plane distance. For the computation of interfacial properties, SGTPy incorporates several options to solve the interfacial concentration, such as the path technique, an auxiliary time function, and orthogonal collocation. Additionally, the SGTPy code allows the inclusion of subroutines from other languages (e.g., Fortran, and C++) through Cython and f2py Python tools, which opens the possibility for future extensions or recycling tested and optimized subroutines from other codes. Supporting Information includes a review of the theoretical expressions required to couple SAFT-VR-Mie equation of state with the SGT. The use and capabilities of SGTPy are illustrated through step by step examples written on Jupyter notebooks for the cases of pure fluids and binary and ternary mixtures in bi- and three- phasic equilibria. The SGTPy code can be downloaded from https://github.com/gustavochm/SGTPy.

Face‐Centered Cubic Platinum Hydride and Phase Diagram of PtH

Recent research on superconductivity of high‐pressure hydrides generated many phase stability calculations with a lack of their experimental verification; a typical example is Pt–H system. The stability of eight PtH structures was predicted, while the experiments revealed the existence of only hexagonal close‐packed (hcp) and trigonal PtH. Face‐centered cubic (fcc) PtH was predicted to be nearly isoentalpic to the hcp PtH and stable near 100 GPa, but never observed experimentally. Here we report the first synthesis of the fcc PtH using laser‐heated diamond anvil cell. It was found to occupy a high‐temperature area of the phase diagram in a wide pressure range of 20–100 GPa, being metastable at room temperature. Our results look promising for uncovering weak approximations in current high‐pressure hydrides stability ab initio calculations.

Determination of phase stability, half metallicity, mechanical and thermal behavior of Fe based quaternary Heusler alloys

Structural, magnetic, mechanical, and thermal characteristics of equiatomic quaternary Heusler alloys (EQHA) FeTiVZ (Z = Ga, In) and FeZrVZ (Z = Ga, In) were examined by employing full potential – linearized augmented plane wave (FP-LAPW) scheme. The calculations for structural optimization were performed for three different types of structures based on atomic positions named as Type I (TI), Type II (TII) and Type III (TIII). Geometry optimization results confirmed that TI is the most optimized structure as compared to other two types. The phase stability calculations confirmed the ferromagnetic (FM) nature of these Heusler alloys (HA). The magnetic characteristics have shown that the findings of band structures are agreed to the density of states (DOS). All these Fe-based HA have clear band gap (Eg) in spin down (S↓) channel which confirmed their half metallic (HM) nature and 100% spin polarization (SP). The S↓Eg of FeTiVGa, FeTiVIn, FeZrVGa, and FeZrVIn is 0.33eV, 0.35eV, 0.90eV, and 0.84eV respectively. The elastic constants and other calculated mechanical parameters verify the mechanical stability of compounds except FeTiVIn which needs further investigation. Based on elastic constants, the calculated Debye temperature (θD), and melting temperature (Tm) proves the thermodynamic stability of FeZrVZ (Z = Ga, In) and FeTiVGa. The obtained results have predicted that these alloys may be utilized in spintronic applications.

Solid–Liquid–Vapor Equilibrium Model Applied for a CH4–CO2 Binary Mixture

A low-temperature process involving the formation of dry ice to separate carbon dioxide (CO2) from a CO2-containing mixture is proved to be energy efficient. An accurate prediction of the phase equilibria of the components is necessary for the development of such a process. The current work extended a model for simultaneous calculation of phase stability and flash computations for a multicomponent and multiphase system to a system involving the formation of dry ice. This study derived composition-independent correlations of solid–vapor and solid–liquid and introduced these correlations into a Gibbs energy minimization algorithm with the stability variable of phase in predicting the phase diagram of pure CO2 and the solid–fluid-phase equilibria of a methane (CH4)–CO2 binary mixture. A CO2 solid-phase equation of state (EoS) based on Gibbs free energy and a fluid-phase EoS based on Helmholtz free energy were used in this study. Experimental data were compiled for a CH4–CO2 mixture involving the solid–vapor,...