Thermodynamic Equilibrium Phase Diagram Research Articles

Calculation of steady-state dynamical phase diagram in U-Mo binary system under irradiation

The steady-state dynamical phase diagram of U-Mo system under irradiation is studied by the thermodynamic model and effective free energy model. In the effective free energy model, we consider the irradiation-enhanced diffusion and ballistic mixing caused by recoil implantation and cascade collision. The effective free energy and diffusivity of each phase in U-Mo binary system at different temperatures were calculated. The dynamical phase diagrams of U-Mo alloys under different irradiation intensities were calculated and the differences between the dynamical phase diagram and thermodynamic equilibrium phase diagram was discussed. The calculated results show that, due to the irradiation-induced ballistic mixing, the high-temperature stable γ(U,Mo) phase can be stabilized near room temperature, which agree well with the experimental reports.

Calculation of Dynamical Phase Diagram in U-Zr Binary System Under Irradiation

Abstract By taking into account the ballistic mixing caused by recoil implantation and cascade collision, a model that describing effective free energy for alloys under irradiation from the perspective of diffusion equilibrium was used to study the effect of irradiation-induced defects on phase stability in U-Zr binary system. Based on the effective free energy model and the thermodynamic model, dynamical phase diagrams of U-Zr alloy under different irradiation intensities were calculated, and the differences between the dynamical phase diagram and thermodynamic equilibrium phase diagram (TEPD) were discussed. The obtained results indicate that, under irradiation, the high-temperature stable γ(U, Zr) phase can be stabilized resulting in two invariant reactions at low temperature, which could be ascribed to the domination of irradiation-induced ballistic mixing.

Steady-State Dynamic Phase Diagram Calculation of U-Ti and U-V Binary System under Irradiation

In this paper, by considering the irradiation-enhanced diffusion, a combination of effective free energy model and the thermodynamic model was used for studying the phase relationships in the U-Ti and U-V binary system under irradiation. The steady-state dynamical phase diagrams of U-Ti and U-V binary alloys under different irradiation intensities were calculated and compared with the conventional thermodynamic equilibrium phase diagram. The calculated results show that under irradiation the high-temperature stable (βTi, γU) and (γU,V) phases were stabilized at relatively low temperature resulting in invariant reactions at relatively low temperature. In addition, with the increase of the irradiation intensity, the temperature of the invariant reactions increased, and the phase regions of the (βTi, γU) and (γU,V) also increased.

FTIR and Mössbauer Spectroscopy Investigations of Ag/FexAl2−xO3 Nanocomposites

An investigation of Ag/(hematite-alumina) solid solutions (Ag/FexAl2−xO3, 0.5 ≤ x ≤ 2) was completed via x-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform Infrared spectroscopy (FTIR) and Mossbauer spectroscopy. Results reveal that the materials obtained by combustion synthesis are multi-phase nanocomposites and silver metal is supported on hematite-rich and alumina-rich nanoparticles for x < 2. For the iron-rich hematite phase, the experimental results indicated a beginning transition to the superparamagnetic state. They also show that the distribution of iron and aluminum cations in the product phases strongly depends on the starting concentrations and that their compositions closely correspond to those of the equilibrium phase diagram at about 1250°C. Mossbauer signal intensities of samples with x = 0.5, 1.0 and 1.5 could consistently be explained by coexistence of phases of approximate composition Fe0.2Al1.8O3 and Fe1.8Al0.2O3 , which closely corresponds to the thermodynamic equilibrium phase diagram of Fe2O3-Al2O3 at about 1250°C. Finally, the Mossbauer spectra reveal line shape changes for the samples with x < 2 which are characteristic for the onset of superparamagnetic behavior.

Steady-state dynamical phase diagram calculation of U–Nb binary system under irradiation: Ballistic effect

A kinetic model involved in ballistic effects has been used for studying the steady-state configuration under irradiation, with considering the irradiation-enhanced diffusion. By applying this model to all the solid phases, the dynamical phase diagram of U–Nb system under irradiation is calculated and critically compared with the assessed thermodynamic equilibrium phase diagram. The calculated results indicate that the irradiation has little effect on the phase relationships of U–Nb system at high temperatures. However, it leads to the close of the miscibility gap of U–Nb system at relatively low temperatures. Meanwhile, the high-temperature stable γ(U,Nb) phase is stabilized by the irradiation, which is in agreement with the available experimental results.

On the Definition of a Monte Carlo Model for Binary Crystal Growth

We show that consistency of the transition probabilities in a lattice Monte Carlo (MC) model for binary crystal growth with the thermodynamic properties of a system does not guarantee the MC simulations near equilibrium to be in agreement with the thermodynamic equilibrium phase diagram for that system. The deviations remain small for systems with small bond energies, but they can increase significantly for systems with large melting entropy, typical for molecular systems. These deviations are attributed to the surface kinetics, which is responsible for a metastable zone below the liquidus line where no growth occurs, even in the absence of a 2D nucleation barrier. Here we propose an extension of the MC model that introduces a freedom of choice in the transition probabilities while staying within the thermodynamic constraints. This freedom can be used to eliminate the discrepancy between the MC simulations and the thermodynamic equilibrium phase diagram. Agreement is achieved for that choice of the transition probabilities yielding the fastest decrease of the free energy (i.e., largest growth rate) of the system at a temperature slightly below the equilibrium temperature. An analytical model is developed, which reproduces quite well the MC results, enabling a straightforward determination of the optimal set of transition probabilities. Application of both the MC and analytical model to conditions well away from equilibrium, giving rise to kinetic phase diagrams, shows that the effect of kinetics on segregation is even stronger than that predicted by previous models.

A Nanothermodynamic Analysis of Cubic Boron Nitride Nucleation upon Chemical Vapor Deposition

AbstractBased on a boron nitride thermodynamic equilibrium phase diagram, a nanothermodynamic analysis was proposed, with respect to the effect of nanometer‐size‐induced additional pressure on the Gibbs' free energy of critical nuclei and the phase transition of hexagonal boron nitride (h‐BN) to cubic boron nitride (c‐BN), to elucidate nucleation of c‐BN during chemical vapor deposition (CVD). Notably, the analysis showed that the effect of surface tension induced by the nanometer‐size curvature of critical nuclei could drive the metastable phase region of c‐BN nucleation into the stable phase region of the boron nitride phase diagram; consequently, c‐BN nucleation would occur prior to h‐BN nucleation in the competing growth of c‐BN and h‐BN upon CVD.

Thermodynamics of diamond nucleation on the nanoscale.

To have a clear insight into the diamond nucleation upon the hydrothermal synthesis and the reduction of carbide (HSRC), we performed the thermodynamic approach on the nanoscale to elucidate the diamond nucleation taking place in HSRC supercritical-fluid systems taking into account the capillary effect of the nanosized curvature of the diamond critical nuclei, based on the carbon thermodynamic equilibrium phase diagram. These theoretical analyses showed that the nanosize-induced interior pressure of diamond nuclei could drive the metastable phase region of the diamond nucleation in HSRC into the new stable phase region of diamond in the carbon phase diagram. Accordingly, the diamond nucleation is preferable to the graphite phase formation in the competing growth between diamond and graphite upon HSRC. Meanwhile, we predicted that 400 MPa should be the threshold pressure for the diamond synthesis by HSRC in the metastable phase region of diamond, based on the proposed thermodynamic nucleation on the nanoscale.

A Nanoscaled Thermodynamic Approach in Nucleation of CVD Diamond on Nondiamond Surfaces

Based on the carbon thermodynamic equilibrium phase diagram, a nanoscaled thermodynamic approach, taking the effect of nanosize-induced additional pressure on Gibbs free energy of diamond critical nuclei and phase transition probability from graphite to diamond into account, was performed to elucidate diamond nucleation on nondiamond surfaces upon chemical vapor deposition (CVD). The theoretical analysis showed that the capillary effect of the nanosize curvature of diamond critical nuclei could drive the metastable phase region of nucleation of CVD diamond into a new stable phase region in the carbon diagram. Consequently, diamond nucleation is preferable to graphite phase formation in competing growth of diamond and graphite upon CVD.

Nucleation stability of diamond nanowires inside carbon nanotubes: A thermodynamic approach

Aiming at synthesis diamond nanowires, a simple thermodynamic approach was performed with respect to the effect of nanosize-induced additional pressure on the Gibbs free energy of critical nuclei to elucidate diamond nucleation inside carbon nanotubes upon chemical vapor deposition, based on the carbon thermodynamic equilibrium phase diagram. Notably, these analysis showed that the diamond nucleation would be preferable inside a carbon nanotube due to the effect of surface tension induced by the nanosize curvature of the carbon nanotube and diamond critical nuclei, compared with diamond nucleation on the flat surface of a silicon substrate. Meanwhile, the metastable phase region of diamond nucleation would be driven into a new stable phase region in the carbon thermodynamic equilibrium phase diagram by the effect of nanosize-induced additional pressure. Eventually, we predicted that carbon nanotubes would be an effective path to grow diamond nanowires by chemical vapor deposition.

Nucleation Thermodynamics of Cubic Boron Nitride upon High-Pressure and High-Temperature Supercritical Fluid System in Nanoscale

Based on the boron nitride thermodynamic equilibrium phase diagram, a thermodynamic analysis in nanoscale was proposed, with respect to the effect of nanosize-induced additional pressure on Gibbs f...

The Influence of Microstructural Aspects on the Service Behaviour of Advanced Power Plant Steels.

There is a world-wide substantial demand to increase the application temperature and design stresses of advanced creep resistant ferritic-martensitic 9-12 % Cr steels to increase the efficiency of thermal power plants. They show, compared to austenite grades, favourable physical properties-like good thermal conductivity and low coefficient of thermal expansion coupled with better service behaviour. In the frame work of the European research action COST 501 intensive investigations of these types of steels were performed. These alloys show during service conditions pronounced microstructural changes in the aged as well as in the stressed conditions. Therefore an extrapolation of short term creep tests to long times-i.e. 100 kh-using conventional methods is not successful because of changes in the acting creep mechanism governed by microstructural development. Therefore an intensive investigation and modelling of the microstructure of W- and Mo-alloyed advanced 9-12 % Cr steels, mainly G-X12 CrMoVWNbN 10-1-1, was applied to understand the mechanisms. Hardness tests, optical microscopy, SEM, TEM methods and thermodynamic equilibrium phase diagram calculations were performed. Very promising is the use of the EFTEM (Energy filtering TEM) method to quantify the precipitation sequence of the different types of precipitate: M23C6, MX, Laves Phase and Z-Phase as a function of time, temperature and straining. A new approach (Graz Model) was proposed for the improvement of the creep resistant alloys in terms of the α/γ-transformation and the Curie temperature respectively, which influence the microstructural stability. Both were found as important influencing factors. Experimentally it was found, that the alloys showing the highest creep resistance i.e. the Japanese grade NF616/P92 and the alloy B2, one of the COST 501 activity, show the highest degree of microstructural stability after creep exposure. Further investigations and modelling is necessary to develop an optimised alloy composition of advanced 9-12% Cr steels.

Preparation and structure of magnetic superlattices (invited) (abstract)

Magnetic multilayers that exhibit long-range structural coherence perpendicular to the layers (i.e., superlattices) have been prepared using sputtering and molecular-beam epitaxy. The structure was characterized using x-ray diffraction and electron microscopy. A newly developed microcleavage technique has been applied to directly image the transverse cross section of the layers. The low-angle x-ray diffraction is directly related to the chemical modulation of the multilayer. The high-angle x-ray diffraction peaks due to the superlattice modulation are found to be different depending on the presence or absence of large lattice spacing modulation. The in-plane structure, the sharpness of the interface, and the mosaic spread are determined by lattice matching and/or the binary thermodynamic equilibrium phase diagram. A possible relation between the epitaxial growth temperature and characteristic temperatures of the phase diagram will also be discussed. If time permits, a comparison between the chemical and the magnetic modulation in Ni/Mo superlattices will also be presented.