Solid-liquid Phase Boundaries Research Articles

Laser-shock compression of magnesium oxide in the warm-dense-matter regime

Magnesium oxide has been experimentally and computationally investigated in the warm-dense solid and liquid ranges from 200 GPa to 1 TPa along the principal Hugoniot. The linear approximation between shock velocity and particle velocity is validated up to a shock velocity of 15 km/s from the experimental data, this suggesting that the MgO B1 structure is stable up to the corresponding shock pressure of ∼350 GPa. Moreover, our Hugoniot data, combined with ab initio simulations, show two crossovers between MgO Hugoniot and the extrapolation of the linear approximation line, occurring at a shock pressures of approximately 350 and 650 GPa, with shock temperatures of 8000 and 14,000 K, respectively. These crossover regions are consistent with the solid-solid (B1-B2) and the solid-liquid (B2-melt) phase boundaries predicted by the ab initio calculations.

Composite cooperative enhancement on the hydrogen desorption kinetics of LiBH4 by co-doping with NbCl5 and hexagonal BN

LiBH4 is an attractive material for hydrogen storage owing to its high hydrogen capacity of 13.8 wt% capacities. However, its high thermodynamic stability and sluggish kinetics limit its practical application as an onboard hydrogen storage medium. In this work, a synergetic effect of NbCl5 and hexagonal BN (h-BN) on notably improving the dehydrogenation properties of LiBH4 was investigated. It is found that the addition of NbCl5 and h-BN co-dopants can significantly enhance the dehydrogenation kinetics of LiBH4, and the catalytic effect of co-dopant is better than that of NbCl5 or h-BN dopant separately. The NbCl5/h-BN co-doped LiBH4 can release 10.78 wt% hydrogen, which is about 13 times and 5 times more than that of the NbCl5 doped LiBH4 and h-BN doped LiBH4 within 10 min at 400 °C, respectively. The major dehydrogenation temperature of NbCl5/h-BN co-doped LiBH4 is reduced to 377 °C, much lower than that of ball-milled LiBH4 (464 °C). The apparent activation energy (Ea) of hydrogen desorption is reduced from 195.81 kJ/mol of LiBH4 to 122.75 kJ/mol of NbCl5/h-BN co-doped LiBH4. The microstructural results reveal that the catalytic effect of NbCl5/h-BN co-dopant on improving the dehydrogenation kinetics of LiBH4 could be ascribed to the in situ formed nano NbH@h-BN, which serves as the heterogeneous nucleation site to reduce the decomposition activation energy barrier of LiBH4 and shortens the distance of the solid–liquid phase boundary movement of LiBH4 decomposition.

Solid−liquid critical behavior of water in nanopores

Nanoconfined liquid water can transform into low-dimensional ices whose crystalline structures are dissimilar to any bulk ices and whose melting point may significantly rise with reducing the pore size, as revealed by computer simulation and confirmed by experiment. One of the intriguing, and as yet unresolved, questions concerns the observation that the liquid water may transform into a low-dimensional ice either via a first-order phase change or without any discontinuity in thermodynamic and dynamic properties, which suggests the existence of solid-liquid critical points in this class of nanoconfined systems. Here we explore the phase behavior of a model of water in carbon nanotubes in the temperature-pressure-diameter space by molecular dynamics simulation and provide unambiguous evidence to support solid-liquid critical phenomena of nanoconfined water. Solid-liquid first-order phase boundaries are determined by tracing spontaneous phase separation at various temperatures. All of the boundaries eventually cease to exist at the critical points and there appear loci of response function maxima, or the Widom lines, extending to the supercritical region. The finite-size scaling analysis of the density distribution supports the presence of both first-order and continuous phase changes between solid and liquid. At around the Widom line, there are microscopic domains of two phases, and continuous solid-liquid phase changes occur in such a way that the domains of one phase grow and those of the other evanesce as the thermodynamic state departs from the Widom line.

Melting and phase transitions of nitrogen under high pressures and temperatures.

Dense nitrogen exhibits fascinating molecular and extended polymorphs as well as an anomalous melt maximum at high temperatures. However, the exact solid-liquid phase boundary is still the subject of debate, as both creating and probing hot dense nitrogen, solid and fluid alike, poses unique experimental challenges. Raman studies of nitrogen were performed to investigate the melting curve and solid-solid phase transitions in the pressure-temperature range of 25 to 103 GPa and 300 to 2000 K. The solid-liquid phase boundary has been probed with time-resolved Raman spectroscopy on ramp heated nitrogen in diamond anvil cell (DAC), showing a melting maximum at 73 GPa and 1690 K. The solid-solid phase boundaries have been measured with spatially resolved micro-confocal Raman spectroscopy on resistively heated DAC, probing the δ-ɛ phase line to 47 GPa and 914 K. At higher pressures the θ-phase was produced upon a repeated thermal heating of the ζ-phase, yet no evidence was found for the ι-phase. Hence, the present results signify the path dependence of dense nitrogen phases and provide new constraints for the phase diagram.

Recent advances in the study of the UO2–PuO2 phase diagram at high temperatures

Recently, novel container-less laser heating experimental data have been published on the melting behaviour of pure PuO2 and PuO2-rich compositions in the uranium dioxide–plutonium dioxide system. Such data showed that previous data obtained by more traditional furnace heating techniques were affected by extensive interaction between the sample and its containment. It is therefore paramount to check whether data so far used by nuclear engineers for the uranium-rich side of the pseudo-binary dioxide system can be confirmed or not. In the present work, new data are presented both in the UO2-rich part of the phase diagram, most interesting for the uranium–plutonium dioxide based nuclear fuel safety, and in the PuO2 side. The new results confirm earlier furnace heating data in the uranium-dioxide rich part of the phase diagram, and more recent laser-heating data in the plutonium-dioxide side of the system. As a consequence, it is also confirmed that a minimum melting point must exist in the UO2–PuO2 system, at a composition between x(PuO2)=0.4 and x(PuO2)=0.7 and 2900K⩽T⩽3000K. Taking into account that, especially at high temperature, oxygen chemistry has an effect on the reported phase boundary uncertainties, the current results should be projected in the ternary U–Pu–O system. This aspect has been extensively studied here by X-ray diffraction and X-ray absorption spectroscopy. The current results suggest that uncertainty bands related to oxygen behaviour in the equilibria between condensed phases and gas should not significantly affect the qualitative trend of the current solid–liquid phase boundaries.

Melting point determination of uranium nitride and uranium plutonium nitride: A laser heating study

Understanding of the behaviour of nuclear material in extreme conditions is essential for the analyses of the operation limits of nuclear fuels, and prediction of possible nuclear reaction accidents. In this context, the high temperature behaviour of uranium nitride and mixed uranium–plutonium nitrides has been studied in the present work by laser heating under controlled atmosphere coupled with fast multi-wavelength pyrometry. Such an approach has allowed performing a thermal arrest analysis and establishing the solid–liquid phase boundaries in the investigated compositions, whereby non-congruent vaporisation was avoided by setting a suitable nitrogen overpressure. In addition, the normal spectral emissivities of the current samples were determined by radiance spectroscopy. Besides revealing a slightly more metallic optical behaviour in plutonium-containing compositions, this latter characterisation led to the determination of the real melting/solidification temperatures of the investigated nitrides. It is confirmed that UN melts congruently at (3120±30) K in a nitrogen pressure of 0.25MPa (2.5bar). The melting/solidification temperatures decrease in plutonium containing samples, reaching (3045±25) K for x(PuN)=0.2, a composition of interest for potential applications of this material as a nuclear fuel. Besides their fundamental importance, the current results are useful for a deeper understanding of the nitride fuel behaviour under accidental conditions, whereby uncontrolled thermal excursions might occur in the nuclear reactor core.

L’état de mal myoclonique post-anoxique est-il épileptique ?

An inverse finite element method is developed for simultaneous solution of multi-dimensional solid-liquid phase boundaries and associated three-dimensional solid phase temperature fields. The technique, applicable to quasisteady phase change problems, fixes element nodes at known temperature locations and uses a coarse, spatially limited mesh. This approach is designed to: (1) reduce direct and overall solution costs, (2) eliminate iterative direct solutions associated with temperature dependent thermophysical properties, (3) limit calculations to the heat affected zone and (4) eliminate ad hoc assumptions concerning the boundary heat flux distribution. The inverse algorithm couples a nonlinear solid phase conduction solver with conjugate gradient minimization. First-order regularization and upwind differencing are implemented to improve solution smoothness and stability and an analog welding experiment is used to investigate the technique's capabilities.

Melting of compressed iron by monitoring atomic dynamics

We present a novel method for detecting the solid–liquid phase boundary of compressed iron at high temperatures using synchrotron Mössbauer spectroscopy (SMS). Our approach is unique because the dynamics of the iron atoms are monitored. This process is described by the Lamb–Mössbauer factor, which is related to the mean-square displacement of the iron atoms. Focused synchrotron radiation with 1meV bandwidth passes through a laser-heated 57Fe sample inside a diamond-anvil cell, and the characteristic SMS time signature vanishes when melting occurs. At our highest compression measurement and considering thermal pressure, we find the melting point of iron to be TM=3025±115K at P=82±5GPa. When compared with previously reported melting points for iron using static compression methods with different criteria for melting, our melting trend defines a steeper positive slope as a function of pressure. The obtained melting temperatures represent a significant step toward a reliable melting curve of iron at Earth's core conditions. For other terrestrial planets possessing cores with liquid portions rich in metallic iron, such as Mercury and Mars, the higher melting temperatures for compressed iron may imply warmer internal temperatures.

Operational principles of electrochemical nanoemitter solar cells for photovoltaic and photoelectrocatalytic applications

The principle design of nanoemitter solar cells is described and their multiple applicability in photoelectrochemical solar cells that operate in the regenerative photovoltaic/photoelectrocatalytic mode as well as in solid state photovoltaics, is emphasized. For sustained operation, photoelectrochemical solar energy conversion systems have to be stabilized against corrosion processes at the reactive electrolyte interface. In the nanoemitter concept, the inclusion of passive films in the cell design allows physical stabilization of photoelectrodes. The individual steps in the preparation of photovoltaic and photoelectrocatalytic electrochemical solar cells with n- and p-type Si are described and the electronic properties of nanoemitter solar cells at the solid–liquid phase boundary are analyzed. The influence of the spatially anisotropical electrostatic potential in the nanoemitter composite structures is outlined and trajectories for the excess minority carriers are deduced that explain observed spectral properties. Routes to overcome the contact potential limitation of p-Si photocathodes for light-induced H 2 evolution are described and parameters that determine the photoelectrochemical current at the employed metallic nanoemitters which form highly local MOS (metal–oxide–semiconductor) structures are identified.

An experimental study of the effects of surface tension in homogenizing perturbations in melt fraction

Static annealing experiments were conducted on fine-grained samples of a partially molten, olivine-rich rock to explore the role of interfacial tension driven flow in redistributing melt within the sample. A sample of fine-grained olivine + 20% chromite was prepared with an initially homogeneously distributed melt fraction of 0.04. When this sample was deformed in torsion, the melt segregated into distinct melt-rich bands uniformly spaced throughout the sample, as demonstrated in prior studies. Portions of this sample were then statically annealed for different lengths of time to observe the homogenization of the melt distribution. The evolution of the melt distribution in experimental samples was compared to numerical models based on formulations for interfacial tension driven flow that do or do not incorporate the effects of dissolution/precipitation coupled with diffusive mass transfer of components of the solid phase through the liquid phase (dissolution/diffusion/precipitation). The results indicate that, at the grain size and perturbation length scales in these samples, dissolution/diffusion/precipitation in response to the chemical potential gradient arising from the curvature of solid–liquid phase boundaries at triple junctions plays a significant role in accommodating interfacial tension driven flow. These experiments provide a valuable test for theory and allow us to place constraints on the homogenization rate of perturbations in melt fraction in rocks with a relatively simple composition. The results contribute to increasing our understanding of the nature of melt-rich, high-permeability pathways that may facilitate melt extraction from the upper mantle.

Crucible-free pulling of germanium crystals

Commonly, germanium crystals are grown after the Czochralski (CZ) method. The crucible-free pedestal and floating zone (FZ) methods, which are widely used for silicon growth, are hardly known to be investigated for germanium. The germanium melt is more than twice as dense as liquid silicon, which could destabilize a floating zone. Additionally, the lower melting point and the related lower radiative heat loss is shown to reduce the stability especially of the FZ process with the consequence of a screw-like crystal growth. We found that the lower heat radiation of Ge can be compensated by the increased convective cooling of a helium atmosphere instead of the argon ambient. Under these conditions, the screw-like growth could be avoided. Unfortunately, the helium cooling deteriorates the melting behavior of the feed rod. Spikes appear along the open melt front, which touch on the induction coil. In order to improve the melting behavior, we used a lamp as a second energy source as well as a mixture of Ar and He. With this, we found a final solution for growing stable crystals from germanium by using both gases in different parts of the furnace. The experimental work is accompanied by the simulation of the stationary temperature field. The commercially available software FEMAG-FZ is used for axisymmetric calculations. Another tool for process development is the lateral photo-voltage scanning (LPS), which can determine the shape of the solid–liquid phase boundary by analyzing the growth striations in a lateral cut of a grown crystal. In addition to improvements of the process, these measurements can be compared with the calculated results and, hence, conduce to validate the calculation.

Ab initio two-phase molecular dynamics on the melting curve of SiO2

Ab initio two-phase molecular dynamics simulations were performed on silica at pressures of 20–160 GPa and temperatures of 2 500–6 000 K to examine its solid-liquid phase boundary. Results indicate a melting temperature (T m) of 5 900 K at 135 GPa. This is 1 100 K higher than the temperature considered for the core-mantle boundary (CMB) of about 3 800 K. The calculated melting temperature is fairly consistent with classical MD (molecular dynamics) simulations. For liquid silica, the O-O coordination number is found to be 12 along the T m and is almost unchanged with increasing pressure. The self-diffusion coefficients of O and Si atoms are determined to be 1.3×10−9–3.3×10−9 m2/s, and the viscosity is 0.02–0.03 Pa·s along the T m. We find that these transport properties depend less on pressure in the wide range up of more than 135 GPa. The eutectic temperatures in the MgO-SiO2 systems were evaluated and found to be 700 K higher than the CMB temperature, though they would decrease considerably in more realistic mantle compositions.

Multiphase equation of state and elastic moduli of solid beryllium from first principles

Based on ab initio calculations, we provide a consistent modeling in pressure and temperature of the solid phases of beryllium, including theoretical phase diagram, multiphase equation of state (EOS), and elastic moduli. The quasiharmonic approximation (QHA) allows us to determine the whole theoretical phase diagram: at room temperature, QHA predicts the hexagonal compact $(\ensuremath{\alpha}\text{-hcp})$ phase as the most stable structure up to 400 GPa, where a transition toward the body-centered-cubic $(\ensuremath{\beta}\text{-bcc})$ phase occurs. However, the QHA does not account for the low-pressure-high-temperature bcc phase found experimentally. Combining frozen phonon and density-functional perturbation theory methods, we show that soft phonon modes as reservoirs of entropy may stabilize the low-pressure bcc phase. However the thermodynamic stability of this phase has still to be established. We provide the QHA multiphase EOS in analytic form and an evaluation of the uncertainties on the QHA solid-solid and solid-liquid phase boundaries. Then, we calculate the density and temperature dependence of elastic constants by determining the free energies of strained structures. Consistency between the polycrystalline (i.e., averaged over the elastic constants) and the EOS isothermal bulk moduli is achieved for both phases. Our results are in fair agreement with the most recent experimental data and permit us to raise questions on some experimental moduli.

Structure and short-time dynamics in suspensions of charged silica spheres in the entire fluid regime

We present an experimental study of short-time diffusion properties in fluidlike suspensions of monodisperse charge-stabilized silica spheres suspended in dimethylformamide. The static structure factor S(q), the short-time diffusion function D(q), and the hydrodynamic function H(q) have been probed by combining x-ray photon correlation spectroscopy experiments with static small-angle x-ray scattering. Our experiments cover the full liquid-state part of the phase diagram, including de-ionized systems right at the liquid-solid phase boundary. We show that the dynamic data can be consistently described by the renormalized density fluctuation expansion theory of Beenakker and Mazur over a wide range of concentrations and ionic strengths. In accordance with this theory and Stokesian dynamics computer simulations, the measured short-time properties cross over monotonically, with increasing salt content, from the bounding values of salt-free suspensions to those of neutral hard spheres. Moreover, we discuss an upper bound for the hydrodynamic function peak height of fluid systems based on the Hansen-Verlet freezing criterion.

10.1007/s11700-008-1022-0

The technique of low temperature synthesis of single-domain magnetite particles at the solid-liquid phase boundary was developed. Spherical magnetite particles 6–50 nm in size were obtained. The phase structure, elemental composition and morphology of the particles were studied using X-ray phase analysis, Auger spectroscopy, and scanning electron and atomic force microscopy.

Properties of iron under Earth’s core conditions: Molecular dynamics simulation with an embedded-atom method potential

Using an earlier proposed embedded-atom method potential, we have performed molecular dynamics simulations of the thermodynamic, structural, and diffusional properties of iron under Earth’s core conditions (temperatures of up to 5000 K and densities of up to 12.5 g/cm3). The results attest to spontaneous crystallization of liquid iron at certain parameters of state. We have derived equations of state for the liquid and solid phases and have calculated their heat capacities at constant pressure and temperature, thermal expansion coefficients, isothermal and adiabatic bulk moduli, and sound velocities under various conditions. In addition, we have calculated the heat of fusion of iron and the change in its molar volume on melting. The results agree with literature data. The self-diffusion coefficient of iron decreases linearly with increasing density, vanishes at the densities corresponding to the liquid-solid phase boundary, and is a roughly linear function of temperature.

Powder materials and technologies for solar cells

The regularities of recrystallisation of initial powders of cadmium and zinc chalcogenides and chalcopyrites in different molten fluxes are studied. It is shown that the capillary phenomena in the solidliquid phase boundary, wetting of the solid phase with the liquid flux phase are the processes that determine the mechanism of recrystallisation. Results indicate to the possibility of manufacturing of powders of complicate semiconductor materials in monograin form and with qualities acceptable for Monograin Layer (MGL) design of solar cells. Influence of several technological processes to parameters of monograin powders and MGL solar cells are studied.

Three-body interactions and solid-liquid phase equilibria: Application of a molecular dynamics algorithm

The effect of three-body interactions on the solid-liquid phase boundaries of argon, krypton, and xenon is investigated via a novel technique that combines both nonequilibrium and equilibrium molecular dynamics. The simulations involve the evaluation of two- and three-body forces using accurate two-body and three-body intermolecular potentials. The effect of three-body interactions is to substantially increase the coexistence pressure and to lower the densities of liquid and solid phases. Comparison with experiment indicates that three-body interactions are required to accurately determine the total pressure. In contrast to vapor-liquid phase equilibria, the relative contribution of three-body interactions to the freezing pressure exceeds the contribution of two-body interactions at all temperatures.

Modeling the Phase Diagram of Carbon

We determined the phase diagram involving diamond, graphite, and liquid carbon using a recently developed semiempirical potential. Using accurate free-energy calculations, we computed the solid-solid and solid-liquid phase boundaries for pressures and temperatures up to 400 GPa and 12 000 K, respectively. The graphite-diamond transition line that we computed is in good agreement with experimental data, confirming the accuracy of the employed empirical potential. On the basis of the computed slope of the graphite melting line, we rule out the hotly debated liquid-liquid phase transition of carbon. Our simulations allow us to give accurate estimates of the location of the diamond melting curve and of the graphite-diamond-liquid triple point.

Crystallisation of a multiparaffinic wax in normal tetradecane under high pressure

The Wax appearance temperature as well as the amount and composition of wax precipitation were determined as a function of pressure in a synthetic complex system tetracosane + (multiparaffinic commercial wax). Measurements were performed by using two different experimental techniques: - A high pressure microscopy apparatus for the solid–liquid phase boundary measurement. - A filtration apparatus coupled to a chromatographic analysis of the partially frozen mixtures for the determination of the solid phase behaviour. Experimental results were compared with the values calculated by a predictive model. The influence of pressure on wax formation is shown to be as important as the influence of temperature and the model used can provide an excellent description of the melting behaviour over a wide pressure range.