Thermodynamic Phase Stability Research Articles

Materials Design by Constructing Phase Diagrams for Defects

AbstractPhase transformations and crystallographic defects are two essential tools to drive innovations in materials. Bulk materials design via tuning chemical compositions is systematized using phase diagrams. It is shown here that the same thermodynamic concept can be applied to manipulate the chemistry at defects. Grain boundaries in Mg–Ga system are chosen as a model system, because Ga segregates to the boundaries, while simultaneously improving the strength and ductility of Mg alloys. To reveal the role of grain boundaries, correlated atomic‐scale characterization and simulation to scope and build phase diagrams for defects are presented. The discovery is enabled by triggering phase transformations of individual grain boundaries through local alloying, and sequentially imaging the structural and chemical changes using atomic‐resolution scanning transmission electron microscopy. Ab initio simulations determined the thermodynamic stability of grain boundary phases, and found out that increasing Ga content enhances grain boundary cohesion, relating to improved ductility. The methodology to trigger, trace, and simulate defect transformation at atomic resolution enables a systematic development of defect phase diagrams, providing a valuable tool to utilize chemical complexity and phase transformations at defects.

Probing the thermodynamics of charged Gauss Bonnet AdS black holes with the Lyapunov exponent

In this paper, we investigate the thermodynamic properties of the charged AdS Gauss–Bonnet black holes and their associations with the Lyapunov exponent. The chaotic features of the black holes and the isobaric heat capacity characterized by the Lyapunov exponent are studied to reveal the thermodynamic stability of the black hole phases. By considering both the timelike and null geodesics, we find that the relationship between the Lyapunov exponent and the Hawking temperature can accurately represent the features of the Small/Large phase transition and even the triple point. We also reveal the properties of the difference in the Lyapunov exponent as an order parameter. It is demonstrated that there is a negative correlation between the Lyapunov exponent and the size of the black hole shadow, which can be used to bridge the thermodynamic properties and the shadow of black holes.

Competition between surficial and volumetric diffusion in sintering TiO2 polymorphs by molecular dynamics simulation

AbstractThe competition between surficial and volumetric diffusion during the sintering process for ceramic polymorphism at different sintering temperatures is worth deeply exploring. In the current work, various sintering cases of mixed TiO2 nanoparticles with rutile and anatase types were investigated and the effects of crystalline phase, sintering temperature, and particle size on the sintering process were systematically investigated. The difference in thermodynamic stability and surficial activity of the crystalline phase could promote the decomposition and densification of the nanoparticles. Comparing the mixed‐phase sintering process at different sintering temperatures, the results showed that the particle binding process at low temperature relied mainly on surficial diffusion, however, volumetric diffusion played a crucial role at high temperature. The internal occurrence of volumetric diffusion inhibited the surficial diffusion, resulting in a smaller diameter of the sintering neck and a different sintering rate. In the case of sintering three nanoparticles with rutile and anatase types, it could be found that the decomposition of nanoparticles is more uniform in mixed‐phase sintering at high temperatures and the sintering rate is not significantly influenced by the sintering temperature. This work demonstrates the advantage of a mixed‐phase sintering strategy for TiO2 nanoparticles, which could provide insight into ceramic materials with polymorphism.

Balancing thermodynamic stability, dynamics, and kinetics in phase separation of intrinsically disordered proteins.

Intrinsically disordered proteins (IDPs) are prevalent participants in liquid-liquid phase separation due to their inherent potential for promoting multivalent binding. Understanding the underlying mechanisms of phase separation is challenging, as phase separation is a complex process, involving numerous molecules and various types of interactions. Here, we used a simplified coarse-grained model of IDPs to investigate the thermodynamic stability of the dense phase, conformational properties of IDPs, chain dynamics, and kinetic rates of forming condensates. We focused on the IDP system, in which the oppositely charged IDPs are maximally segregated, inherently possessing a high propensity for phase separation. By varying interaction strengths, salt concentrations, and temperatures, we observed that IDPs in the dense phase exhibited highly conserved conformational characteristics, which are more extended than those in the dilute phase. Although the chain motions and global conformational dynamics of IDPs in the condensates are slow due to the high viscosity, local chain flexibility at the short timescales is largely preserved with respect to that at the free state. Strikingly, we observed a non-monotonic relationship between interaction strengths and kinetic rates for forming condensates. As strong interactions of IDPs result in high stable condensates, our results suggest that the thermodynamics and kinetics of phase separation are decoupled and optimized by the speed-stability balance through underlying molecular interactions. Our findings contribute to the molecular-level understanding of phase separation and offer valuable insights into the developments of engineering strategies for precise regulation of biomolecular condensates.

Achieving the Superior Abrasive Wear Resistance in ZTAP/Fe Composites from the Theoretical Calculations Guided Designing of Metallurgical Interface Transition Layer

Using the first-principles calculations and phonon QHA method, the thermodynamic stability, mechanical and thermophysical properties of binary Ni-Ti phases are obtained. The most thermodynamically feasible binary Ni3Ti phase with a superior elasticity and mechanical tensile strength is designed as the interfacial transition layer between ZTA ceramic particles and iron matrix. A high-Cr white cast iron matrix composite reinforced with ZTA particles is fabricated via infiltration casting process. The microstructures and three-body abrasive wear resistance of the composite are investigated. The obtained ZTAP/Fe composite has a compact metallic Ni3Ti metallurgical transition layer with a small amount of AlNi2Ti and TiO phases in the interfacial region, leading to a strongly adhesion between iron matrix and ZTA ceramic particles due to the presence of covalent Ti-O, Fe-O and Zr-O bonds. The composite exhibits the relative wear resistance 12.9 times higher than that of reference Cr15 specimen. The wear mechanism of the ZTAP/Fe composite is mainly attributed to the removal of iron matrix under the SiO2 abrasives, as revealed from the secondary electron images and laser scanning confocal topography.

Roles of Refractory Solutes on the Stability of Carbide and Boride Phases in Nickel Superalloys

Nickel-base superalloys contain high amounts of solutes like Cr, Mo, W, Nb, Ti, etc. These solutes promote the formation of different types of carbide and boride phases that may contain multiple elements. Researchers have mostly discussed the roles of primary elements responsible for the formation of a given carbide/boride phase, often ignoring the role of other solutes on its stability. In the present work, thermodynamic stability of carbide and boride phases in seven commercial superalloys, namely, Alloy 625, Alloy 690, Alloy 718, MAR M246, Rene 100, Udimet 710 and Nimonic 80A, has been studied using the CALPHAD based Thermo-Calc software. The aim of the study was to understand the role of different alloying elements on temperature stability and chemical compositions of equilibrium phases in superalloys. As the accuracy of CALPHAD based predictions depends upon the database used, a detailed examination of its inadequacies has also been carried out to ascertain the limitations of the predicted data. From the calculated equilibrium chemical compositions, major and minor constituents promoting the formation of carbides and borides have been identified. The individual effect of a given solute as well as the synergistic effect of two solutes on the relative thermodynamic stability of carbide/boride phases has been identified using property diagrams and isothermal sections of the temperature-composition diagrams. Most of the simulated results have been found to be consistent with the experimental data available in the literature. From a comparison of the experimental literature and the simulated data of the stable carbide and boride phases in the studied alloys, the interplay of different solutes has been deduced to define conditions under which these phases form, within the limitations of the database used. This study has helped in better understanding of general tendencies of solutes to form different carbide and boride phases in nickel-based superalloys.

Exploring potential of lead-free KSn0.5Ge0.5I3 based perovskite solar cell: A combined first-principle DFT and SCAPS-1D study

Perovskite solar cells present a promising alternative to conventional silicon or lead-based solar cells, offering a synergistic blend of cost-effectiveness and potential for elevated power conversion efficiency. We investigated the potential of an innovative KSn0.5Ge0.5I3-based perovskite solar cell (PSC) using density functional theory (DFT) and SCAPS-1D simulation studies. Computational analysis via DFT probed the crystal structure, elastic, mechanical, and optoelectronic properties of KSn0.5Ge0.5I3. Tolerance factor computation (0.94) and negative formation energy (-3.16 eV) affirmed the thermodynamic stability of the orthorhombic phase in KSn0.5Ge0.5I3. Optical variables including refractive index, absorption coefficient, dielectric function, optical loss, and reflectivity, were computed to explore KSn0.5Ge0.5I3 viability in PV applications. Band structure analysis, density of states (DOS), and projected density of states (PDOS) have been used to determine the band gap (∼1.87 eV), effective masses, and mobility of e-/h+. We have conducted SCAPS-1D simulation for diverse PSCs configurations using various holes transport layers (HTLs) and electron transport layers (ETLs), identified IGZO (Eg = 3.05 eV) as a proficient ETL while CuSbS2 (Eg = 1.58 eV) as an effective HTL. A comprehensive investigation of parameters such as absorber layer thickness, shunt and series resistance, donor and acceptor density, absorber defect density, back contact metal, temperature, and interface characteristics was undertaken, and the impact on PV device performances was assessed through current-voltage (IV) and quantum efficiency (Q.E) characteristics curves. These findings underscore the potential of KSn0.5Ge0.5I3 as a promising material for PSCs, offering a sustainable and eco-friendly avenue for future renewable energy.

Bromine Incorporation Affects Phase Transformations and Thermal Stability of Lead Halide Perovskites.

Mixed-cation and mixed-halide lead halide perovskites show great potential for their application in photovoltaics. Many of the high-performance compositions are made of cesium, formamidinium, lead, iodine, and bromine. However, incorporating bromine in iodine-rich compositions and its effects on the thermal stability of the perovskite structure has not been thoroughly studied. In this work, we study how replacing iodine with bromine in the state-of-the-art Cs0.17FA0.83PbI3 perovskite composition leads to different dynamics in the phase transformations as a function of temperature. Through a combination of structural characterization, cathodoluminescence mapping, X-ray photoelectron spectroscopy, and first-principles calculations, we reveal that the incorporation of bromine reduces the thermodynamic phase stability of the films and shifts the products of phase transformations. Our results suggest that bromine-driven vacancy formation during high temperature exposure leads to irreversible transformations into PbI2, whereas materials with only iodine go through transformations into hexagonal polytypes, such as the 4H-FAPbI3 phase. This work sheds light on the structural impacts of adding bromine on thermodynamic phase stability and provides new insights into the importance of understanding the complexity of phase transformations and secondary phases in mixed-cation and mixed-halide systems.

Studying the Thermodynamic Phase Stability of Organic-Inorganic Hybrid Perovskites Using Machine Learning.

As an important photovoltaic material, organic-inorganic hybrid perovskites have attracted much attention in the field of solar cells, but their instability is one of the main challenges limiting their commercial application. However, the search for stable perovskites among the thousands of perovskite materials still faces great challenges. In this work, the energy above the convex hull values of organic-inorganic hybrid perovskites was predicted based on four different machine learning algorithms, namely random forest regression (RFR), support vector machine regression (SVR), XGBoost regression, and LightGBM regression, to study the thermodynamic phase stability of organic-inorganic hybrid perovskites. The results show that the LightGBM algorithm has a low prediction error and can effectively capture the key features related to the thermodynamic phase stability of organic-inorganic hybrid perovskites. Meanwhile, the Shapley Additive Explanation (SHAP) method was used to analyze the prediction results based on the LightGBM algorithm. The third ionization energy of the B element is the most critical feature related to the thermodynamic phase stability, and the second key feature is the electron affinity of ions at the X site, which are significantly negatively correlated with the predicted values of energy above the convex hull (Ehull). In the screening of organic-inorganic perovskites with high stability, the third ionization energy of the B element and the electron affinity of ions at the X site is a worthy priority. The results of this study can help us to understand the correlation between the thermodynamic phase stability of organic-inorganic hybrid perovskites and the key features, which can assist with the rapid discovery of highly stable perovskite materials.

Theory of strain phase equilibria and diagrams

Chemical phase equilibria and phase diagrams are well established and have served as indispensable guides for designing materials through chemical tuning. However, a general thermodynamic theory for strain phase equilibria remains elusive despite extensive studies which have revealed dramatic impacts of mechanical strain on the stability of phases and domain states in solid-state materials. Here, we establish the thermodynamic theory of strain equilibria and a general framework for efficiently constructing multidomain and multiphase diagrams of arbitrarily strained solids under incoherent conditions. As examples, we obtain temperature-strain phase diagrams of ferroelectric PbTiO3, strongly correlated VO2, and unconventional ferroelectric Hf0.5Zr0.5O2. We reveal the analogs of the Gibbs phase rules, multiple- and multi-critical points, common-tangent construction, and level rule in strain equilibria to those in the familiar chemical phase equilibria. Our strain equilibria theory offers a powerful framework for predicting the thermodynamic stability of structural phases and domains in strained solids and for guiding the strain engineering of their functional properties.

Improving the mechanical properties of Cantor-like alloys with Bayesian optimization

The search for better compositions in high entropy alloys is a formidable challenge in materials science. Here, we demonstrate a systematic Bayesian optimization method to enhance the mechanical properties of the paradigmatic five-element Cantor alloy in silico. This method utilizes an automated loop with an online database, a Bayesian optimization algorithm, thermodynamic modeling, and molecular dynamics simulations. Starting from the equiatomic Cantor composition, our approach optimizes the relative fractions of its constituent elements, searching for better compositions while maintaining the thermodynamic phase stability. With 24 steps, we find Fe21Cr20Mn5Co20Ni34 with a yield stress improvement of 58%, and with 72 steps, we find Fe6Cr22Mn5Co32Ni35 where the yield stress has improved by 74%. These optimized compositions correspond to Ni-rich medium entropy alloys with enhanced mechanical properties and superior face-centered-cubic phase stability compared to the traditional equiatomic Cantor alloy. The automatic approach devised here paves the way for designing high entropy alloys with tailored properties, opening avenues for numerous potential applications.

Predictions of Boron Phase Stability Using an Efficient Bayesian Machine Learning Interatomic Potential.

Thermodynamic phase stability of three elemental boron allotropes, i.e., α-B, β-B, and γ-B, was investigated using a Bayesian interatomic potential trained via a sparse Gaussian process (SGP). SGP potentials trained with data sets from on-the-fly active learning achieve quantum mechanical level accuracy when employed in molecular dynamics (MD) simulations to predict wide-ranging thermodynamic, structural, and vibrational properties. The simulated phase diagram (500-1400 K and 0-16 GPa) agrees with experimental measurements. The SGP-based MD simulations also successfully predicted that the B13 defect is critical in stabilizing β-B below 700 K. At higher temperatures, the entropy becomes the dominant factor, making β-B the more stable phase over α-B. This letter demonstrates that SGP potentials based on a training set consisting of defect-free-only systems could make correct predictions of defect-related phenomena in solid-state crystals, paving the path to investigate crystal phase stability and transitions.

In-situ fabrication and characterization of W-ZrC composites via pressureless reaction sintering

This research presents a study on the fabrication and characterization of tungsten‑zirconium carbide (W-ZrC), an ultra-high temperature ceramic, using an in-situ reaction sintering method. Tungsten carbide (WC) and zirconia (ZrO2) powders were used as precursors to synthesize composites with three different ratios. The thermodynamic phase stabilities of the precursor materials were analyzed, and the reaction products were determined based on the total pressure within the system. The fabricated composites were characterized by their microstructures, porosities, hardnesses, and thermal conductivities. The results show that the composites exhibited distinct crystal phases, including those of tungsten, tungsten carbide (W2C), zirconium carbide (ZrC), and tetragonal zirconia (t-ZrO2). The reaction pathways were determined based on the composition ratios and thermodynamic stability. The composites demonstrate good densification and interparticle bonding owing to the in-situ reaction sintering process, which simplifies the fabrication and lowers processing costs. The composites also exhibited desirable properties, such as high hardness and thermal conductivity (maximum 36.4 W·m−1·K−1), making them suitable for high-temperature applications. The findings of this study contribute to the understanding of the synthesis process and properties of W-ZrC composites and provide insights for their potential applications to industries such as aerospace, defense, and nuclear energy.

First-principles study of the discharge electrochemical and catalytic performance of the sulfur cathode host Fe0.875M0.125S2 (M = Ti, V).

Lithium-sulfur batteries (LSBs) are one of the most promising energy storage devices with high energy density. However, their application and commercialization are hampered by the slow Li-S redox chemistry. Fe0.875M0.125S2 (M = Ti, V), as the sulfur cathode host, enhances the Li-S redox chemistry. FeS2 with Pa3̄ is transformed into Li2FeS2 with P3̄m1 after discharge. The structure changes and physicochemical properties during Fe0.875M0.125S2 discharge process are further investigated to screen out the sulfur cathode host materials with the best comprehensive properties. The discharge structure of Fe0.875M0.125S2 is verified by the thermodynamic stability of Li-deficient phases, voltage and capacity based on Monte Carlo methods. Fe0.875M0.125S2 with Pa3̄ is transformed into Li2Fe0.875M0.125S2 with P3̄m1 after discharge. Using the first-principles calculations, the physicochemical properties of Li2Fe0.875M0.125S2 are systematically investigated, including the formation energy, voltage, theoretical capacity, electrical conductivity, Li+ diffusion, catalytic performance and Li2S oxidation decomposition. The average redox voltage of Li2Fe0.875V0.125S2 is higher than that of Li2Fe0.875Ti0.125S2. Li2Fe0.875M0.125S2 shows metallic properties. Li2Fe0.875V0.125S2 is more beneficial to the reduction reaction of Li2S2 and Li2S oxidation decomposition. Fe0.875V0.125S2 has more potential as the sulfur cathode host than Fe0.875Ti0.125S2 in LSBs. A new strategy for the selection of the sulfur cathode host material for LSBs is provided by this work.

The synthesis and application of crystalline-amorphous hybrid materials.

Crystalline-amorphous hybrid materials (CA-HMs) possess the merits of both pure crystalline and amorphous phases. Abundant dangling bonds, unsaturated coordination atoms, and isotropic structural features in the amorphous phase, as well as relatively high electronic conductivity and thermodynamic structural stability of the crystalline phase simultaneously take effect in CA-HMs. Furthermore, the atomic and bandgap mismatch at the CA-HM interface can introduce more defects as extra active sites, reservoirs for promoted catalytic and electrochemical performance, and induce built-in electric field for facile charge carrier transport. Motivated by these intriguing features, herein, we provide a comprehensive overview of CA-HMs on various aspects-from synthetic methods to multiple applications. Typical characteristics of CA-HMs are discussed at the beginning, followed by representative synthetic strategies of CA-HMs, including hydrothermal/solvothermal methods, deposition techniques, thermal adjustment, and templating methods. Diverse applications of CA-HMs, such as electrocatalysis, batteries, supercapacitors, mechanics, optoelectronics, and thermoelectrics along with underlying structure-property mechanisms are carefully elucidated. Finally, challenges and perspectives of CA-HMs are proposed with an aim to provide insights into the future development of CA-HMs.

Machine learning interatomic potentials in engineering perspective for developing cathode materials

Machine learning interatomic potentials (MLIPs) predict thermodynamic phase stability and structural parameters like density functional theory (DFT) but are much faster, making them valuable for engineering applications.

Growth and Structural Characterization of h-LuMnO3 Thin Films Deposited by Direct MOCVD.

In this work, we investigated the MOCVD conditions to synthesize thin films with the hexagonal P63cm h-LuMnO3 phase as a potential low-band gap ferroelectric material. The main parameters investigated were the ratio of organometallic starting materials, substrate temperature, and annealing effect. Two different substrates were used in the study: fused silica (SiO2) glass and platinized silicon (Pt\Ti\SiO2\Si(100)). In order to investigate the thermodynamic stability and quality of the developed phases, a detailed analysis of the crystal structure, microstructure, morphology, and roughness of the films was performed by X-ray diffractometer, scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), Raman spectroscopy, and piezoelectric force microscopy (PFM). Molar compositions in the film within 0.93 < |Lu|/|Mn| < 1.33 were found to be suitable for obtaining a single-phase h-LuMnO3. The best films were obtained by depositions at 700 °C, followed by thermal treatments at 800 °C for long periods of up to 12 h. These films exhibited a highly crystalline hexagonal single phase with a relatively narrow direct band gap, around 1.5 eV, which is within the expected values for the h-LuMnO3 system.

Structural, elastic, mechanical, electronic, and magnetic properties of In2NbX6 (X = Cl, Br) variant perovskites

AbstractThis article is about the study of structural, elastic, mechanical, electronic, and magnetic properties of two variant perovskites In2NbX6 (X = Cl, Br) using density functional theory. To the best of our information all calculations were carried out for the first time. The results established the cubic state stability of the present compounds in the FM phase. The thermodynamic and perovskites phase stabilities are respectively confirmed from negative values of formation energies and tolerance factor. The calculated elastic constants C11, C12, and C44 confirmed the mechanical stability of In2NbX6 compounds. The mechanical properties confirmed ductile nature of In2NbCl6 and brittle nature of In2NbBr6. The electronic properties showed that these compounds were metallic in spin up state and semiconductor in spin down state. The magnetic moments were calculated as 1 μB for both the compounds majorly associated with the Nb atom. The spin dependent electrical conductivity was calculated via BoltzTrap code. Besides half metallic nature, high spin dependent electrical conductivities reveals that In2NbX6 variant perovskites compounds are suitable for spintronic applications.

Improvement of Thermodynamic Phase Stability and High-Rate Capability of Li Layered Oxides.

The use of elemental doping in lithium cobalt oxide (LCO) cathode material at high cutoff voltage is a widely adopted technique in the field of rechargeable batteries to mitigate multiple unfavorable phase transitions. However, there is still a lack of fundamental understanding regarding the rationality of each doping element implemented in this method, specifically considering the various thermodynamic stability and phase transitions. Herein, we investigated the effect of Ti doping on an O2 phase LCO (LCTO) cathode material that exhibited enhanced rate performance. We suggest that the incorporation of Ti into an O2 phase LCO results in the mitigation of multiple-phase transitions and the improvement of phase stability, thereby yielding a high-rate-capable cathode material. Through a combination of experimental and computational calculations, we demonstrate that Ti doping improves the thermodynamic stability and kinetics of Li-ions during the cycling process.

Prediction of thermodynamic stability of actinide compounds by machine learning model

The thermodynamic phase stability plays a crucial role as it serves as a fundamental parameter governing the synthesizability of materials and their potential for degradation under specific operating conditions. In this study, two machine learning (ML) models, random forest (RF) and neural network (NN), were used to predict the thermodynamic phase stability of actinide compounds using a dataset consisting of 62204 DFT-calculated energies. Our study utilizes a comprehensive range of properties that do not contain structural information, making them applicable to materials composed of any number of constituent elements. Notably, the trained models achieve an approximation that closely aligns with the error obtained from DFT calculations, while drastically reducing computational time by several orders of magnitude. Moreover, we extended our analysis to predict binary phase diagrams of Generation IV nuclear fuels using the trained models. To address the limitations of a single model for predicting certain compounds and enhance model robustness, a simple ensemble learning approach, i.e., the multi-component learner was employed. By synergistically combining prediction outcomes from RF and NN models, the ensemble learning approach excels in accurately predicting phase diagrams of actinide compounds. Utilizing the compound components forecasted by the model as a foundation, we embarked on an extensive series of structural searches and conducted thorough phonon dispersion studies. The outcomes unequivocally affirm the model's efficacy in accurately predicting stable compound compositions.