Solid-liquid Phase Boundaries Research Articles

Stability mechanism of crystalline CO2 and Xe.

We explore the phase behaviors of simple molecular crystals in order to investigate the molecular basis of the stability mechanism relative to their liquid counterparts. The free energies of the face centered cubic crystals of Xe and CO2 are calculated as a collection of oscillators, and those of the liquids are from an equation of state via molecular dynamics simulations. The vibrational free energy in the solid is separated into the harmonic and anharmonic terms. The harmonic free energies decrease harshly with the expansion of the volume manifested as the large positive Grüneisen parameters, but the anharmonic free energies are positive and increase with volume, both of which originate from the deviation of the potential surface from the parabolic curve. The anharmonic free energies, though less significant in magnitude and destabilize the solids thermodynamically, serve to enhance their mechanical stability. The solid-liquid phase boundaries cannot be settled correctly without the exquisite balance between the two opposing contributions. A sharp contrast regarding the solid free energy is found in low-pressure ice, where the harmonic free energy does not decrease monotonically with volume and its anharmonic free energy is negative.

First-principles molten salt phase diagrams through thermodynamic integration.

Precise prediction of phase diagrams in molecular dynamics simulations is challenging due to the simultaneous need for long time and large length scales and accurate interatomic potentials. We show that thermodynamic integration from low-cost force fields to neural network potentials trained using density-functional theory (DFT) enables rapid first-principles prediction of the solid-liquid phase boundary in the model salt NaCl. We use this technique to compare the accuracy of several DFT exchange-correlation functionals for predicting the NaCl phase boundary and find that the inclusion of dispersion interactions is critical to obtain good agreement with experiment. Importantly, our approach introduces a method to predict solid-liquid phase boundaries for any material at an abinitio level of accuracy, with the majority of the computational cost at the level of classical potentials.

Effects of mushy-zone parameter on simulating melting and solidification phenomena inside a spherical capsule

The research on latent heat storage technology is beneficial for the large-scale popularization and application of energy storage technology. In order to solve the difficulties in the latent thermal energy storage (LTES) technology, numerical studies of the solid–liquid phase-change process of LTES units using the enthalpy-porosity method have become research hotspots. Among them, the importance of studying the mushy-zone parameter has been neglected. In this paper, a two-dimensional numerical model is created based on the enthalpy-porosity method to investigate the effects of different mushy-zone parameters within a wide range [104–108 kg/(m3·s)] on the melting and solidification processes of the spherical phase change material capsule. A comprehensive examination of the fluid flow and heat transfer characteristics during two phase-change processes is conducted. Meanwhile, the morphology of the solid–liquid phase boundary and the evolution of the mushy zone under different mushy-zone parameters are discussed in detail. It is worth mentioning that a new analytical perspective is innovated by proposing the “eccentricity phenomenon,” and the eccentricity law is further explored. The results show that the influence mechanisms of the mushy-zone parameters on the melting and solidification processes differ greatly. This paper emphasizes the significance of exploring the mushy zone and provides adequate guidance for future simulation studies to determine the mushy-zone parameter.

First-principles study of thermoelasticity and structural phase diagram of CaO

We present pressure $(P)$ and temperature $(T)$ variations of elastic and anisotropic properties, solid-solid (rocksalt, B1 to cesium chloride, B2) and solid-liquid structural phase transitions of CaO. We employed first-principles density functional theory supplemented with an anharmonic contribution to phonon dynamics. Good agreement is obtained for all properties up to the pressure and temperature range relevant to the Earth's mantle and outer core. Born elastic criteria are generalized for arbitrary stress to elaborate the structural phase diagram from a thermoelastic viewpoint. We propose a self-consistent computational scheme to incorporate the effect of thermal hysteresis into Lindemann's melting law. This improvised melting law exhibits quantitative agreement with reported findings for the high-$P$ melting curve. We find the triple point (i.e., the coexistence of B1, B2, and liquid phases) at 23 GPa and 4600 K. By examining the pressure-term included elastic properties, we can show that the solid-solid transition is mechanical in a pressure range of 0--200 GPa and temperature up to 3000 K. The softening of shear elastic constant ${C}_{44}$ drives the B1-B2 phase transition. This assertion is corroborated by examining the phonon dispersion curve and mode Gr\"uneisen parameter at different pressures for both B1 and B2 phases. The solid-liquid phase boundary can be treated accurately through the temperature-dependent thermodynamic Gr\"uneisen parameter. Furthermore, in this paper, we predict a negative melting slope $>140\phantom{\rule{0.28em}{0ex}}\mathrm{GPa}$ with a peak temperature of 7800 K, suggesting a smaller molar volume on the liquid side than that of the solid phase. This finding is supported by electronic band structure calculation. It is proposed that the hybridization of the empty $3d$ band of the cation with $s$ orbitals lowers the conduction band to cross the Fermi energy at the Brillouin zone center and leads the insulator-to-metal/semimetal phase transition $\ensuremath{\sim}200\phantom{\rule{0.28em}{0ex}}\mathrm{GPa}$. Different electronic states on solid and liquid sides make the liquid phase more compressible than the solid phase, eventually reducing the melting temperature with pressure.

Interfacial free energy at the liquid-solid phase boundary calculated by the method of capillary fluctuations

The interfacial free energy at the liquid-solid phase boundary was calculated for some close packed metals (Au, Cu, Al) and silicon. For calculations, the method of capillary fluctuations at temperatures close to the melting point of the studied materials was used. The formation of the two-phase system at the atomic level was carried out using the method of molecular dynamicssimulations by means of the LAMMS classical molecular dynamics package. The interfacial stiffness for materials with different crystallographic orientation of the phase boundary was determined from the analysis of the fluctuation spectrum, which in turn was obtained by Fourier transformation of the oscillations of the interphase boundary. It was shown that the value of the free energy increases in the series Al→Au→Cu→Si

Rayleigh–Bénard instability in the presence of phase boundary and shear

We study the two-dimensional Rayleigh–Bénard instability subject to the combined effects of a solid–liquid phase boundary and shear using linear stability theory and energy analysis. We consider two thermal states of the solid (isothermal and conducting), and two types of shear that can arise in different contexts. When the melting temperature is equal to that held at the top boundary, three instability modes can arise with the increase of Reynolds number Re that characterizes the shear intensity: a boundary mode, a mixed boundary–bulk mode and a bulk flow mode. When the melting temperature lies between the top and the bottom boundaries, the introduction of Couette flow, independent of its intensity, always leads to the mixed mode, whereas the instability with Poiseuille flow is dominated by the bulk flow mode once Re exceeds a critical value, below which the mixed mode dominates. The energy analysis suggests that there exist two mechanisms by which the shear flow affects the system: one is by inhibiting the upward heat flux and another is by absorbing energy from the perturbed hydrodynamic field. These two mechanisms can play totally different roles in different cases. Results in the high-Re regime indicate that, when Re exceeds its classical threshold, i.e. Re = 5772.2 for Poiseuille flow, the Tollmien–Schlichting instability will be dominant in the present system.

Revisiting the Role of the Triple‐Phase Boundary in Promoting the Oxygen Reduction Reaction in Aluminum–Air Batteries

AbstractThe decomposition, electron transfer, and protonation of oxygen molecules are typically assumed to be the rate‐limiting steps of the oxygen reduction reactions (ORR), and the activation energy barriers of these reactions can be surmounted using catalysts. In this study, the physical rate‐limiting step of the ORR consists of the adsorption of gaseous oxygen molecules at the liquid–solid phase boundary, indicating that the formation of a gas–liquid–solid triple‐phase boundary (TPB) is important for accelerating the ORR kinetics. This is experimentally confirmed by analyzing the ORR in aluminum–air batteries. Moreover, the formation of a TPB using the hierarchical pores of sparked reduced graphene oxide is demonstrated, which serve as the cathode, and the remarkable electrochemical performance of the fabricated battery is presented. These findings can be used to accelerate the ORR kinetics by maximizing the TPB, particularly in aluminum–air batteries.

Direct evaluation of the phase diagrams of dense multicomponent plasmas by integration of the Clapeyron equations.

Accurate phase diagrams of multicomponent plasmas are required for the modeling of dense stellar plasmas, such as those found in the cores of white dwarf stars and the crusts of neutron stars. Those phase diagrams have been computed using a variety of standard techniques, which suffer from physical and computational limitations. Here we present an efficient and accurate method that overcomes the drawbacks of previously used approaches. In particular, finite-size effects are avoided as each phase is calculated separately; the plasma electrons and volume changes are explicitly taken into account; and arbitrary analytic fits to simulation data as well as particle insertions are avoided. Furthermore, no simulations at "uninteresting" state conditions, i.e., away from the phase coexistence curves, are required, which improves the efficiency of the technique. The method consists of an adaptation of the so-called Gibbs-Duhem integration approach to electron-ion plasmas, where the coexistence curve is determined by direct numerical integration of its underlying Clapeyron equation. The thermodynamics properties of the coexisting phases are evaluated separately using Monte Carlo simulations in the isobaric semigrand canonical ensemble (NPTΔμ). We describe this Monte Carlo-based Clapeyron integration method, including its basic physical and numerical principles, our extension to electron-ion plasmas, and our numerical implementation. We illustrate its applicability and benefits with the calculation of the melting curve of dense carbon-oxygen plasmas under conditions relevant for the cores of white dwarf stars and provide analytic fits to implement this new melting curve in white dwarf models. While this work focuses on the liquid-solid phase boundary of dense two-component plasmas, a wider range of physical systems and phase boundaries are within the scope of the Clapeyron integration method, which had until now only been applied to simple model systems of neutral particles.

Solid–Liquid Equilibria of a 30 wt % Aqueous Monoethanolamine Solution Containing Urea and Monoethylene Glycol

The solid–liquid phase boundaries on new innovative solvents for the carbon capture and storage (CCS) technology have been measured in this work. The solvents have been formulated with vapor reduct...

Amine Polymerization is a New Observed Molecular Behavior in Co2 Capture

Understanding the solid-liquid phase boundaries for CO2 capture solvents are important for the prevention of solid formation doing process operation. Solid formation in e.g. absorber, and stripper may lead to costly shut-downs. The objectives of this study has been to determine the solid-liquid equilibrium (SLE) of 2-amino-2-methyl-1-propanol (AMP)-water and AMP-CO2-water and to create a deeper understanding of the interactions between AMP and water and AMP, CO2, and water. The aim is to generate a deeper understanding of the AMP-water and the AMP-CO2-water interactions in order to create better thermodynamic modelling of the CO2 capture process. The presented results is based on newly published data [1] for the aqueous AMP system and additional unpublished results for the AMP-CO2-water system. We have used experimental techniques such as powder x-ray diffraction (PXRD), freezing point depression, and solid phase boundary equilibrium determinations. We found a new crystal structure, AMP tetrahydrate (AMP·4H2O) and several eutectic points in the AMP-water system. We extended our work compared to the recent publication, saturating a 30 wt% aqueous AMP solution with CO2. We found a new crystal structure using powder x-ray diffraction (PXRD). The new structure is an AMPH-HCO3 hydrate.

Pre-nucleation cluster formation upon ethyl acetate addition to an aqueous solution of an anionic hydrotrope

Hydrotropes are used to solubilize poorly soluble solutes. Here the general features of the solution behaviour of the very common anionic hydrotrope sodium salicylate is investigated. With the use of density and scattering measurements, it is demonstrated that around the critical point neither pre-Ouzo structuring nor an Ouzo effect are present. By contrast, in a domain near the liquid-solid phase boundary, X-ray light and neutron scattering combined with a density excess determination prove the presence of dynamic aggregates. These aggregates are stabilized without any surfactants, but by selective solvation of ethyl acetate, reminiscent of stable liquid pre-nucleation aggregates. And most surprising: in water saturated by ethyl acetate (20% wt), sodium salicylate micellizes like a medium chain classical surfactant.

Automatic systems supporting research on modern materials and industrial bonding technologies

Industrial bonding technologies, including soldering and brazing, operate on solid-liquid phase boundaries. Commonly applied in many areas of industry (such as electronics, metallurgy or aviation), these bonding processes rely on the wetting of joining surfaces using a liquid bonding material. Understanding the wetting process and quantitative description of its parameters enables the design of new materials and joining technologies, as well as the optimization of existing methods. From the primary interfacial impact parameters, i.e. the contact angle and surface tension, the dynamics of the wetting process can be determined, including adhesive tension. This article investigates two independent measurement systems for wetting parameters in industrial brazing technologies: the sessile drop method and the Wilhelmy plate method. Both methods can be applied effectively in research on wetting processes. The measurement systems are compared in an example study.

High-pressure melt curve of shock-compressed tin measured using pyrometry and reflectance techniques

We have developed a new technique to measure the melt curve of a shocked metal sample and have used it to measure the high-pressure solid-liquid phase boundary of tin from 10 to 30 GPa and 1000 to 1800 K. Tin was shock compressed by plate impact using a single-stage powder gun, and we made accurate, time-resolved radiance, reflectance, and velocimetry measurements at the interface of the tin sample and a lithium fluoride window. From these measurements, we determined temperature and pressure at the interface vs time. We then converted these data to temperature vs pressure curves and plotted them on the tin phase diagram. The tin sample was initially shocked into the high-pressure solid γ phase, and a subsequent release wave originating from the back of the impactor lowered the pressure at the interface along a constant entropy path (release isentrope). When the release isentrope reaches the solid-liquid phase boundary, melt begins and the isentrope follows the phase boundary to low pressure. The onset of melt is identified by a significant change in the slope of the temperature-pressure release isentrope. Following the onset of melt, we obtain a continuous and highly accurate melt curve measurement. The technique allows a measurement along the melt curve with a single radiance and reflectance experiment. The measured temperature data are compared to the published equation of state calculations. Our data agree well with some but not all of the published melt curve calculations, demonstrating that this technique has sufficient accuracy to assess the validity of a given equation of state model.

Technology development of high purity germanium crystals for radiation detectors

Abstract High Purity Germanium (HPGe) crystals are indispensable for radiation detection and fundamental research. Among other applications they will be used in the Large Enriched Germanium Experiment for Neutrinoless double beta Decay (LEGEND) experiment for the search of neutrinoless double beta decay of 76 Ge. Ongoing research activities are done in collaboration with members of the existing GERDA experiment. In this context the Leibniz-Institut fur Kristallzuchtung (IKZ) is engaged in the development of HPGe Czochralski crystal growth including purification and characterization of the material. The Ge starting material is purified in a horizontal multi-zone furnace. Two inch Ge single crystals were grown by the Czochralski method and analyzed with respect to electrically active impurities by means of Hall-effect measurements and Photo-thermal Ionization Spectroscopy. The dislocation density is assessed by combining defect etching and optically microscopy imaging. Lateral Photovoltage Scanning is applied to visualize doping striations, which bear information on the shape of the solid–liquid phase boundary during growth.

Activity modelling of the solid–liquid equilibrium of deep eutectic solvents

Abstract Compared to conventional solvents used in the chemical industry, deep eutectic solvents (DESs) are considered as promising potentially sustainable solvents. DESs are binary mixtures and the resulting liquid mixture is characterized by a large melting point depression with respect to the melting temperatures of its constituents. The relative melting point depression becomes larger as the two components have stronger attractive interactions, resulting in non-ideal behavior. The compositional range over which such binary mixtures are liquids is set by the location of the solid–liquid phase boundary. Here we present experimental phase diagrams of various recent and new DESs that vary in the degree of non-ideality. We investigate whether thermodynamic models are able to describe the solid–liquid equilibria and focus on relating the parameters of these models to the non-ideal behavior, including asymmetric behavior of the activity coefficients. It is shown that the orthogonal Redlich–Kister-like polynomial (OP) expansion, including an additional first order term, provides an accurate description. This theory can be considered as an extension of regular solution theory and enables physical interpretation of the fit parameters.

An effective way to determine the melting curve

A new ‘two-phase’ simulation method with which to accurately predict the melting curve is proposed. The method requires, as a starting configuration, generating a two-phase coexistence state by employing a suitable ensemble. Examining a change in volume ratio of the two phases upon varying temperature (pressure) under a fixed pressure (temperature) allows us to determine the phase transition point. The Clausius–Clapeyron relationship can then be implemented as a guide to predict the nearby phase transition point. The method was applied to determine the solid–liquid phase boundary of the modified Lennard–Jones system. A better accuracy, as that achieved by the non-equilibrium relaxation method (Asano Y, Fuchizaki K. J Phys Soc Jpn. 2017;86:025001), was obtained but with much less computational cost.

Heat transfer and diffusion-controlled kinetics of liquid–solid phase in titanium matrix composite during selective laser melting

This study describes a self-consistent theoretical model of simulating diffusion-controlled kinetics on the liquid–solid phase boundary during high-speed solidification in the melt pool after the selective laser melting (SLM) process for titanium matrix composite based on Ti–TiC system. The model includes the heat transfer equation to estimate the temperature distribution in the melt pool and during crystallization process for some deposited layers. The temperature field is used in a micro region next to solid–liquid boundary, where solute micro segregation and dendrite growth are calculated by special approach based on transient liquid phase bonding. The effect of the SLM process parameters (laser power, scanning velocity, layer thickness and substrate size) on the microstructure solidification is being discussed.

Electrochemical Study of a Pendant Molten Alumina Droplet and Its Application for Thermodynamic Property Measurements of Al-Ir

Limited knowledge of the thermodynamic and transport properties of refractory materials in the liquid state remains a key challenge limiting their application. Using alternating current (AC) and direct current (DC) techniques, the electrochemical kinetics of oxygen evolution and metal deposition was investigated in a pendant droplet of molten alumina (Al2O3) with three iridium (Ir) electrodes in a thermal imaging furnace. For the first time, the direct electrolytic decomposition of molten Al2O3 to oxygen gas and aluminum (Al) metal (alloyed with Ir) was observed, confirming the ionic nature of molten Al2O3. The decomposition potential of molten Al2O3 was measured with high precision using AC voltammetry, and the results demonstrated remarkable sensitivity to variation in temperature enabling measurement of chemical potential and entropy of Al at the Ir-rich solid-liquid phase boundary for the first time. The results were in remarkably close agreement with the most recent thermodynamic assessment of the Al-Ir system.

Melting along the Hugoniot and solid phase transition for Sn via sound velocity measurements

It is very important to determine the phase boundaries for materials with complex crystalline phase structures to construct their corresponding multi-phase equation of state. By measuring the sound velocity of Sn with different porosities, different shock-induced melting pressures along the solid-liquid phase boundary could be obtained. The incipient shock-induced melting of porous Sn samples with two different porosities occurred at a pressure of about 49.1 GPa for a porosity of 1.01 and 45.6 GPa for a porosity of 1.02, based on measurements of the sound velocity. The incipient shock-induced melting pressure of solid Sn was revised to 58.1 GPa using supplemental measurements of the sound velocity. Trivially, pores in Sn decreased the shock-induced melting pressure. Based on the measured longitudinal sound velocity data, a refined solid phase transition and the Hugoniot temperature-pressure curve's trend are discussed. No bcc phase transition occurs along the Hugoniot for porous Sn; further investigation is required to understand the implications of this finding.

Shock Response and Phase Transitions of MgO at Planetary Impact Conditions.

The moon-forming impact and the subsequent evolution of the proto-Earth is strongly dependent on the properties of materials at the extreme conditions generated by this violent collision. We examine the high pressure behavior of MgO, one of the dominant constituents in Earth's mantle, using high-precision, plate impact shock compression experiments performed on Sandia National Laboratories' Z Machine and extensive quantum calculations using density functional theory (DFT) and quantum MonteCarlo (QMC) methods. The combined data span from ambient conditions to 1.2TPa and 42 000K, showing solid-solid and solid-liquid phase boundaries. Furthermore our results indicate that under impact the solid and liquid phases coexist for more than 100GPa, pushing complete melting to pressures in excess of 600GPa. The high pressure required for complete shock melting has implications for a broad range of planetary collision events.