Structural Phase Transformation Research Articles

Investigation of structural phase transformation of Al metallic glass under uniaxial compression strain by molecular dynamics simulation

Investigation of structural phase transformation of Al metallic glass under uniaxial compression strain by molecular dynamics simulation

Imaging and controlling coherent phonon wave packets in single graphene nanoribbons

The motion of atoms is at the heart of any chemical or structural transformation in molecules and materials. Upon activation of this motion by an external source, several (usually many) vibrational modes can be coherently coupled, thus facilitating the chemical or structural phase transformation. These coherent dynamics occur on the ultrafast timescale, as revealed, e.g., by nonlocal ultrafast vibrational spectroscopic measurements in bulk molecular ensembles and solids. Tracking and controlling vibrational coherences locally at the atomic and molecular scales is, however, much more challenging and in fact has remained elusive so far. Here, we demonstrate that the vibrational coherences induced by broadband laser pulses on a single graphene nanoribbon (GNR) can be probed by femtosecond coherent anti-Stokes Raman spectroscopy (CARS) when performed in a scanning tunnelling microscope (STM). In addition to determining dephasing (~440 fs) and population decay times (~1.8 ps) of the generated phonon wave packets, we are able to track and control the corresponding quantum coherences, which we show to evolve on time scales as short as ~70 fs. We demonstrate that a two-dimensional frequency correlation spectrum unequivocally reveals the quantum couplings between different phonon modes in the GNR.

Effect of temperature on structural phase transition and photoluminescence in organic-inorganic hybrid CH3NH3PbI3-xClx perovskite

Organic-inorganic hybrid (CH3NH3PbI3-xClx) perovskite is a useful light accumulating material. Its characteristic features are found to be very sensitive to synthesis and environmental conditions. We studied its structural phase transformation, inter band transitions, and photoluminescence as a function of temperature, which are key parameters governing its light absorbing characteristics. We found that photoluminescence peaks exhibit several finer features, which are identified, deconvoluted, and discussed in detail. Our analyses revealed the presence of multiple phases, defects (mainly Cl), excitonic binding energies, and radiative recombination of trap states in the perovskite. Based on the results, a schematic energy band diagram has been proposed, and it is concluded that the trap states will become less active at elevated temperatures. Moreover, increased post annealing temperature will decrease the recombination at interfaces due to improved crystallinity and large grain sizes. Control over the surface and interface trapping by using the appropriate working and annealing temperature for the device may be key to increasing the efficiency of perovskite based solar cells.

Electric field-driven modulation in structural and luminescent properties of Eu3+-doped (1 − x)(Na0.5Bi0.5)TiO3 − xBaTiO3 relaxor ferroelectrics

We investigated the structural, electrical, and luminescent properties of Eu3+-doped (1 − x)(Na0.5Bi0.5)TiO3 − xBaTiO3 (NBT-BT:Eu) relaxor ferroelectrics for x = 0.00–0.08. The introduction of Eu3+ generated good emission properties in the samples without weakening their piezoelectric properties. Rietveld refinement structural analysis revealed that the structural phase of NBT-BT:Eu was a mixture of rhombohedral and monoclinic phases with a significant x-dependence. We observed that NBT-BT:Eu underwent a structural phase transformation upon the application of an electric field. Interestingly, the structural changes caused by poling led to a reduction in the emission of Eu3+. The poling-driven emission quenching behavior resulted from the increase in the local structural symmetry around Eu3+ in the poled samples. We also identified light-induced modulation in the emission of our samples, probably attributed to the photochromic behavior. The multifunctionality of NBT-BT:Eu paves the way for the development of a new type of optoelectronic material.

PREDICTION OF THE STRUCTURAL STATE OF THE WORKING LAYER OF LARGE-SIZE ROLLS DURING THERMAL HARDENING

Problem statement. High-carbon steels, which are additionally alloyed with chromium, molybdenum, and vanadium, are used during the production of large-sized rolling rolls for hot deformation. Rolling rolls are subjected to significant loads during operation, so they must have sufficient hardness and resistance to wear. Steels of the type 65Cr3SiMoV, 80Cr3MoV, 80Cr5MoV have recently been used for the production of hot-formed rolling rolls, but despite this, their resistance to wear in harsh operating conditions is insufficient. There is no information in the literature about the peculiarities of the austenite decay kinetics of the specified rolled steels, therefore, this direction requires appropriate comprehensive research. Purpose. Development of a methodology for predicting the structural state of the working layer of large rolling rolls in the process of thermal strengthening of high-carbon alloy steels, taking into account the determining parameters of the manufacturing technology. Results. A methodology for modeling phase-structural transformations in the process of continuous cooling of high-carbon alloy steels was developed. For steels 65Cr3SiMoV, 80Cr3MoV, 80Cr5MoV were constructed thermokinetic diagrams and on their basis, the peculiarities of the formation of the structural state of the working layer of large rolling rolls (support, working) during thermal hardening were investigated. It has been established that the method of hardening by volumetric heating of support rolls made of 65Cr3SiMoV steel ensures the formation of a bainite structure over the entire normalized depth of their working layer. For working rolls made of 80Cr3MoV and 80Cr5MoV steels, the most effective method is hardening with differentiated heating, while in the latter case continuous cooling should be carried out at a slower speed.

Analysis of Vibrational Spectra of Tetrafluoroethane Glasses Deposited by Physical Vapor Deposition.

This paper presents the results obtained in the study of structural phase transitions in thin films of R134A. The samples were condensed on a substrate by physical deposition of R134A molecules from the gas phase. Structural phase transformations in samples were investigated by observing the changes in characteristic frequencies of Freon molecules in the mid-infrared range with the help of Fourier-transform infrared spectroscopy. The experiments were carried out in the temperature range from 12 to 90 K. A number of structural phase states, including glassy forms, were detected. The changes in thermogram curves at fixed frequencies of half-widths of absorption bands of R134A molecules were revealed. These changes indicate a large bathochromic shift of these bands at frequencies of ν = 842 cm-1, ν = 965 cm-1, and ν = 958 cm-1 and a hypsochromic shift of the bands at frequencies of ν = 1055 cm-1, ν = 1170 cm-1, and ν = 1280 cm-1 at temperatures from T = 80 K to T = 84 K. These shifts are related to the structural phase transformations in the samples.

Carbon-Doped Nickle Via a Fast Decarbonization Route for Enhanced Hydrogen Oxidation Reaction in Alkaline Media.

Nickel (Ni) based materials with non-metal heteroatom doping are competitive substitutes for platinum group catalyst toward alkaline hydrogen oxidation reaction (HOR). However, the incorporation of non-metal atom into the lattice of conventional fcc phase Ni can easily trigger a structural phase transformation, forming hcp phase nonmetallic intermetallic compounds. Such tangle phenomenon makes it difficult to uncover the relationship between HOR catalytic activity and doping effect on fcc phase Ni. Herein, taking trace carbon doped Ni (C-Ni) nanoparticles as an example, a new nonmetal doped Ni nanoparticles synthesized by a simple fast decarbonization route using Ni3 C as precursor is presented, which provides an ideal platform to study the structure-activity relationship between alkaline HOR performance and non-metal doping effect toward fcc phase Ni. The obtained C-Ni exhibits an enhanced alkaline HOR catalytic activity compared with pure Ni, approaching to commercial Pt/C. X-ray absorption spectroscopy confirms that the trace carbon doping can modulate the electronic structure of conventional fcc phase nickel. Besides, theoretical calculations suggest that the introducing of C atoms can effectively regulate the d-band center of Ni atoms, resulting in the optimized hydrogen absorption, thereby improving the HOR activity.

Antisite disorder mediated exchange bias effect and spin-glass state in La[formula omitted]Sm[formula omitted]NiMnO[formula omitted

We report the synthesis of La2−xSmxNiMnO6 (x = 0, 0.1, 0.2) polycrystalline compounds through the sol–gel process using all-aqueous precursors. The influence of Sm-substitution on magnetic properties of La2−xSmxNiMnO6 (LSNMO) compounds are investigated through X-ray diffraction (XRD), dc-magnetization (Mdc), ac-magnetic susceptibility measurements, and Raman spectroscopy. XRD analysis confirms that the phase stability of LSNMO is improved significantly upon Sm-substitution as evident from a structural phase transformation of the compound from a biphasic (monoclinic + rhombohedral) into an aphasic (monoclinic) structure. The magnetic measurements indicate a formation of a metastable magnetic state with random ferromagnetic (FM) and antiferromagnetic (AFM) interactions below 100 K. The metastable spin configuration exhibits critical slow-down dynamics and memory effect that signifies a spin-glass (SG) state. The influence of Sm-doping reduces the Curie temperature (Tc) from 284 K → 245 K and enhances saturation magnetization (Ms). These findings are attributed to the combined effect of a reduced degree of antisite disorder upon the addition of Sm. A profound variation (45.5 → 72.9 K) in SG transition temperature (TSG) is also reported. Magnetization loops M(H) confirm the presence of an exchange bias (EB) effect that value suppressed prominently from 120 → 28 Oe upon Sm-substitution. The temperature-dependent magnetization M(T) and frequency-dependent ac-susceptibility χ[ω, T] are used to study the antisite-driven SG phase, spin relaxation dynamics, magnetic memory, and rejuvenation effect. Raman spectroscopy is employed to probe the degree of antisites disorder that reduces after Sm-addition. The phase stability, lower Tc, tunable Ms, and EB in the single-phase compound make LSNMO a potential candidate for its applications in realizing energy-efficient devices.

Accelerated Deactivation of Mesoporous Co3O4-Supported Au–Pd Catalyst through Gas Sensor Operation

High activity of a catalyst and its thermal stability over a lifetime are essential for catalytic applications, including catalytic gas sensors. Highly porous materials are attractive to support metal catalysts because they can carry a large quantity of well-dispersed metal nanoparticles, which are well-accessible for reactants. The present work investigates the long-term stability of mesoporous Co3O4-supported Au–Pd catalyst (Au–Pd@meso-Co3O4), with a metal loading of 7.5 wt% and catalytically active mesoporous Co3O4 (meso-Co3O4) for use in catalytic gas sensors. Both catalysts were characterized concerning their sensor response towards different concentrations of methane and propane (0.05–1%) at operating temperatures ranging from 200 °C to 400 °C for a duration of 400 h. The initially high sensor response of Au–Pd@meso-Co3O4 to methane and propane decreased significantly after a long-term operation, while the sensor response of meso-Co3O4 without metallic catalyst was less affected. Electron microscopy studies revealed that the hollow mesoporous structure of the Co3O4 support is lost in the presence of Au–Pd particles. Additionally, Ostwald ripening of Au–Pd nanoparticles was observed. The morphology of pure meso-Co3O4 was less altered. The low thermodynamical stability of mesoporous structure and low phase transformation temperature of Co3O4, as well as high metal loading, are parameters influencing the accelerated sintering and deactivation of Au–Pd@meso-Co3O4 catalyst. Despite its high catalytic activity, Au–Pd@meso-Co3O4 is not long-term stable at increased operating temperatures and is thus not well-suited for gas sensors.

MODELING OF DIRECTIONAL SOLIDIFICATION/MELTING BY THE ENTHALPY-POROSITY METHOD

The research is focused on the development of mathematical models and software based on them to simulate complex processes of structural-phase transformations for new-generation materials, such as materials with phase transitions (PCM), biomedical materials, materials for additive manufacturing, and materials for the space industry. The mathematical description of the enthalpy-porosity model is performed in this work. The equations of viscous fluid hydrodynamics are used to describe fluid motion in time and space. The analysis of necessary restrictions and assumptions in the model related to consideration of laminar flows and Newtonian fluid model is performed. The computational problem is formulated in terms of the finite volume method and the computational domain and hydrodynamic equations are discretized. The OpenFOAM software, an open integrated platform for numerical simulation of continuum mechanics problems, was used for the computations. The computational algorithm OpenFOAM was developed to analyze the physical state of the system taking into account the initial and boundary conditions in the case of conductive and convective heat transfer. The simulations of gallium melting are performed and the model is verified for the conductive and convective cases. It is shown that in the conductive case the material melting occurs uniformly along the heat sources, while different velocities of convection flows have a significant influence on the formation of the melting boundary. The mathematical models developed in the study, as well as the analytical dependences and the computer simulations are applied to describe real experimental data on crystal growth in supersaturated solutions and supercooled melts.

High-pressure evolution of the magnetic order in LaMnO3

A relationship between evolution of the long-range magnetic order, crystal structure, and lattice distortions in $\mathrm{LaMn}{\mathrm{O}}_{3}$ was studied by a combination of neutron diffraction and Raman spectroscopy at high pressures up to 39 and 50 GPa, respectively, covering the temperature range 5 to 290 K. The Raman spectra reveal a gradual structural phase transformation evolving in the pressure range of 4 to 17 GPa, caused by a modification of the Jahn-Teller (JT) lattice distortions from the static cooperative character to the local one. A presence of residual JT-distorted regions associated with the initial phase is detected up to 32 GPa, where the insulator-metal transition occurs. At higher pressure, the local JT distortions also vanish completely at further compression up to 50 GPa. In the neutron diffraction data, a strong suppression of the A-type antiferromagnetic (AFM) phase is observed over the pressure region of the phase transformation. This is accompanied by a noticeable reduction in magnitude of the ${Q}_{2}$ and ${Q}_{3}$ JT local modes. The effective ordered magnetic manganese moment is reduced about twice at pressures up to 14 GPa. At higher pressures up to 30 GPa, residual regions of the A-type AFM phase coexist with the magnetically disordered phase. In the range 30 to 39 GPa, i.e., during the pressure-induced insulator-metal transition, these regions disappear and, finally, the magnetically disordered metallic phase becomes the only ground state. The possible models of the insulator-metal transition in $\mathrm{LaMn}{\mathrm{O}}_{3}$ are analyzed.

Effect of residual carbon on coal ash melting characteristics in reducing atmosphere

The melting behavior of coal ash at high temperatures directly affects slag discharge in gasifiers. Residual carbon is a key factor that influences the melting and transformation process of coal ash, affecting its melting characteristics, mineral composition, phase transformation, and melt structure. This study examines how residual carbon affects the melting process through mineral transformation, melt structure, phase transformation, atmosphere reaction, dissolution, and collapse of the physical structure of slag. The type and amount of residual carbon have different effects on the melting process. Changes in minerals result in complex changes in the phase transformation process. Residual carbon deprives some Fe and Si elements of the liquid phase, forming new aggregated minerals that compete with others. Residual carbon disrupts the equilibrium and eutectic structure of the system, dissociates the Si-O structure, and reduces the Fe-O structure, forming Fe0 or Fe2+. Free iron atoms form melt structures, such as Fe3C and FeSi.

In situ diffraction studies of phase-structural transformations in hydrogen and energy storage materials: An overview

The paper presents an overview of advanced in situ diffraction studies as a highly valuable tool to probe the structure and reacting mechanisms of hydrogen and energy storage materials. These studies offer benefits from the use of a high flux diffraction beam in combination with high resolution measurements, and allow, even when using very small samples, establishing the mechanism of the phase-structural transformations and their kinetics based on a rapid data collection for the various charge-discharge states at variable test conditions. The applied conditions include a broad range of hydrogen/deuterium pressures, from vacuum to high pressures reaching 1000 bar H2/D2, and temperatures, from cryocooling (2 K) to as high as 1273 K (1000 °C). Simultaneously, various state-of-charge and discharge are probed when studying metal-H/D systems for hydrogen/deuterium gas storage and as anode electrodes of metal hydride batteries. The range of the studied systems includes but is not limited to the AB5, AB2 Laves type, AB, equiatomic ternary ABC intermetallics and Mg-containing layered AB3 structures and composites. Prospects on Li-ion full cells are considered as well. Interrelations between the structure and hydrogen storage performance, including maximum and reversible H storage capacity, hysteresis of hydrogen absorption and desorption, H2 absorption and desorption in dynamic equilibrium conditions, are considered and related to the ultimate goal of optimisation of the H storage behaviours of the advanced H storage materials. Numerous contributions of Dr. Michel Latroche to the field are highlighted, particularly in in situ studies during electrochemical transformations. The paper summarises a long-standing collaboration of the co-authors in the field, which also included 29 joint topical publications with Dr. Michel Latroche, and references 163 original publications.

Electronic excitation induced non-thermal phase transition of tungsten

The electronically excited states of tungsten (W) material tuned by ultrafast laser irradiation cause intriguing jumps between different Born-Oppenheimer surfaces. This may give rise to a structural phase transformation of BCC-to-FCC or BCC-to-HCP on a very short time scale, which is still debated. By performing calculations on W with both density functional theory and a tight-binding approach, a martensitic phase transformation from BCC phase to HCP phase in bulk W is predicted at a threshold (15,500 ± 500 K) of electronic temperatures, where the HCP phase is found to be more stable than both BCC and FCC phases under intense electronic excitations. Additionally, it is proposed that the non-thermal phase transition could potentially be probed through experimental measurements of the c/a ratio of system in the future. The mechanism underlying the BCC-to-HCP transformation in the electronically excited bulk W system is analyzed under the level of electronic structure. It is found that the irradiation-induced changes of the density of states at the Fermi level could drive the structural phase transformation. This provides new insight into the atomic-level non-thermal phase transition process of W under intense electronic excitations.

Development of a Thermal Model of Welding by the Finite Element Method in Software "Bazis"

The paper presents a model and an algorithm for calculating the operational properties of a metal and a structure after its assembly-welding. The model is based on the equations of non-stationary thermal conductivity and mechanical equilibrium, as well as the analysis of structural-phase transformations using thermokinetic diagrams. It is implemented in the original BAZIS software package. Validation of software was carried out when calculating thermal fields and deformations as applied to arc welding, arc deposition, and laser welding of standard joints.

Phonon anharmonicity and equation of state parameters of orthovanadate Mg3(VO4)2: Raman Spectroscopy and X-ray diffraction investigation

Isothermal compression effect is investigated on structure and vibrational properties of orthovanadate Mg3(VO4)2 using Raman spectroscopy and X-ray powder diffraction up to pressures 20.2 and 25.7 GPa respectively. Isobaric temperature effect on the vibrational properties of Mg3(VO4)2 have also been investigated using Raman spectroscopy up to 1273 K. The present investigation indicates stable orthorhombic (Cmca) structure up to the highest pressure achieved and up to near its melting temperature. The high pressure structural behavior is similar to the isostructural compounds Zn3(VO4)2 and Ni3(VO4)2 unlike another isostructural compound Mn3(VO4)2 which is reported to show irreversible structural phase transformation. Experimental equation of state parameters are compared with those reported on the similar compounds and the obtained bulk modulus (141 GPa) of Mg3(VO4)2 matches very well with the empirically calculated value reported earlier. The anharmonicities estimated after combining both studies suggest dominant contribution of three phonon decay process.

Pressure-dependence Raman spectroscopy and the lattice dynamic calculations of Bi2(MoO4)3 crystal

This work reports a pressure-dependent Raman spectroscopic study and the theoretical lattice dynamics calculations of a Bi2(MoO4)3 crystal. The lattice dynamics calculations were performed, based on a rigid ion model, to understand the vibrational properties of the Bi2(MoO4)3 system and to assign the experimental Raman modes under ambient conditions. The calculated vibrational properties were helpful to support pressure-dependent Raman results, including eventual structural changes induced by pressure changes. Raman spectra were measured in the spectral region between 20 and 1000 cm−1 and the evolution of the pressures values was recorded in the range of 0.1–14.7 GPa. Pressure-dependent Raman spectra showed changes observed at 2.6, 4.9 and 9.2 GPa, these changes being associated with structural phase transformations. Finally, principal component analysis (PCA) and hierarchical cluster analysis (HCA) were performed to infer the critical pressure of phase transformations undergone by the Bi2(MoO4)3 crystal.

Atomic scale insight into the decomposition of nanocrystalline zinc hydroxynitrate toward ZnO using Mn2+ paramagnetic probes.

Layered zinc hydroxynitrate (ZHN), with the chemical formula Zn5 (OH)8 (NO3)2·2H2O, exhibits a range of special properties such as anion-exchange and intercalation capacity, as well as biocompatibility, making it attractive for a large variety of applications in fields from nanotechnology to healthcare and agriculture. In this study nanocrystalline ZHN doped with 1,000ppm Mn2+ was prepared by two synthesis methods (coprecipitation and solid state reaction) using similar environment-friendly precursors. The complex morpho-structural [X-ray diffraction, scanning and transmission electron microscopy, textural analysis] and spectroscopic [Fourier transform infrared and electron paramagnetic resonance (EPR)] characterization of the two ZHN nanopowders showed similar crystalline structures with Mn2+ ions localized in the nanocrystals volume, but with differences in their morphological and textural characteristics, as well as in the doping efficiency. ZHN obtained by coprecipitation consists of larger nanoplatelets with more than two times larger specific surface area and pore volume, as well as a dopant concentration than in the ZHN sample obtained by solid state reaction. The thermal stability and the on-set of the structural phase transformation have been investigated at atomic scale with high accuracy by EPR, using Mn2+ as paramagnetic probes. The on-set of the ZHN structural phase transformation toward ZnO was observed by EPR to take place at 110°C and 130°C for the samples prepared by coprecipitation and solid state reaction, respectively, evidencing a manganese induced local decrease of the transformation temperature. Our results contribute to the selection of the most appropriate ZHN synthesis method for specific applications and in the development of new green, cost-effective synthesis routes for Mn2+ doped nano-ZnO.

Polymorphism of Pradofloxacin: Crystal Structure Analysis, Stability Study, and Phase Transformation Behavior.

Pradofloxacin is an important antibiotic with poor physical stability. At present, there is no systematic study on its polymorphic form. The purpose of this study is to develop new crystal forms to improve the stability of Pradofloxacin and systematically study the crystal transformation relationships to guide industrial production. In this work, three solvent-free forms (Form A, Form B and Form C), a new dimethyl sulfoxide solvate (Form PL-DMSO) and a new hydrate (Form PL-H) were successfully obtained and the single crystal data of Form A, Form B and Form PL-DMSO were solved for the first time. Various solid state analysis techniques and slurry experiments have been used to evaluate the stability and determine phase transformation relationships of five crystal forms, the analysis of crystal structure provided theoretical support for the results. The water vapor adsorption and desorption experiences of Forms A, B, C and Form PL-H were studied, and the results show that the new hydrate has good hygroscopic stability and certain development potential. The thermal stability of different forms was determined by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) and the crystal structure shows that there are more hydrogen bonds and C - H···π interactions in form B, which is the reason why Form B is more stable than form A. Finally, the phase transformation relationships of the five crystal forms were systematically studied and discussed. These results are helpful to provide guiding methods in the production and storage of pradofloxacin.

Molecular dynamics study on the structural properties and phase transformation of Cu-Au nanoparticles

The electroreduction of CO2 to carbon-containing products carries considerable significance. Cu-Au alloys have been considered as potential bimetallic catalysts recently. However, the current theoretical study of obtaining Cu-Au alloys that could enhance the catalytic activity is insufficiently thorough. Herein, the structural properties and phase transition rules of Cu-Au nanoparticles are investigated utilizing molecular dynamics. The results indicate that the percentage of disordered atoms in Cu-Au nanoparticles decreases and the melting temperature increases with the growth of particle size. Moreover, the coordination number decreases with increasing radial distance. Cu-Au nanoparticles are coexisting in crystalline and amorphous states during melting. The structural properties of Cu-Au catalysts could be regulated by the phase transition rules, which provided a theoretical basis for the modification of surface activity.