Low-density Phase Research Articles

Pressure promoted low-temperature melting of metal-organic frameworks.

Metal-organic frameworks (MOFs) are microporous materials with huge potential for chemical processes. Structural collapse at high pressure, and transitions to liquid states at high temperature, have recently been observed in the zeolitic imidazolate framework (ZIF) family of MOFs. Here, we show that simultaneous high-pressure and high-temperature conditions result in complex behaviour in ZIF-62 and ZIF-4, with distinct high- and low-density amorphous phases occurring over different regions of the pressure-temperature phase diagram. In situ powder X-ray diffraction, Raman spectroscopy and optical microscopy reveal that the stability of the liquid MOF state expands substantially towards lower temperatures at intermediate, industrially achievable pressures and first-principles molecular dynamics show that softening of the framework coordination with pressure makes melting thermodynamically easier. Furthermore, the MOF glass formed by melt quenching the high-temperature liquid possesses permanent, accessible porosity. Our results thus imply a route to the synthesis of functional MOF glasses at low temperatures, avoiding decomposition on heating at ambient pressure.

Third family of compact stars within a nonlocal chiral quark model equation of state

A class of hybrid compact star equations of state is investigated that joins by a Maxwell construction a low-density phase of hadronic matter, modeled by a relativistic meanfield approach with excluded nucleon volume, with a high-density phase of color superconducting two-flavor quark matter, described within a nonlocal covariant chiral quark model. We find the conditions on the vector meson coupling in the quark model under which a stable branch of hybrid compact stars occurs in the cases with and without diquark condensation. We show that these hybrid stars do not form a third family disconnected from the second family of ordinary neutron stars unless additional (de)confining effects are introduced with a density-dependent bag pressure. A suitably chosen density dependence of the vector meson coupling assures that at the same time the $2~$M$_\odot$ maximum mass constraint is fulfilled on the hybrid star branch. A twofold interpolation method is realized which implements both, the density dependence of a confining bag pressure at the onset of the hadron-to-quark matter transition as well as the stiffening of quark matter at higher densities by a density-dependent vector meson coupling. For three parametrizations of this class of hybrid equation of state the properties of corresponding compact star sequences are presented, including mass twins of neutron and hybrid stars at 2.00, 1.39 and 1.20 $M_\odot$, respectively. The sensitivity of the hybrid equation of state and the corresponding compact star sequences to variations of the interpolation parameters at the 10% level is investigated and it is found that the feature of third family solutions for compact stars is robust against such a variation. This advanced description of hybrid star matter allows to interpret GW170817 as a merger not only of two neutron stars but also of a neutron star with a hybrid star or of two hybrid stars.

Hard-Core Radius of Nucleons within the Induced Surface Tension Approach

We review the recent approach to model the hadronic and nuclear matter equations of state using the induced surface tension concept, which allows one to go far beyond the usual Van der Waals approximation. Since the obtained equations of state, classical and quantum, are among the most successful ones in describing the properties of low density phases of strongly interacting matter, they set strong restrictions on the possible value of the hard-core radius of nucleons, which is widely used in phenomenological equations of state. We summarize the latest results obtained within this novel approach and perform a new detailed analysis of the hard-core radius of nucleons, which follows from hadronic and nuclear matter properties. Such an analysis allows us to find the most trustworthy range of its values: the hard-core radius of nucleons is 0.3–0.36 fm. A comparison with the phenomenology of neutron stars implies that the hard-core radius of nucleons has to be temperature and density dependent. Such a finding is supported when the eigenvolume of composite particles like hadrons originates from their fermionic substructure due to the Pauli blocking effect.

The effect of boundaries and impurity on a system with non-local hop dynamics

We study a one-dimensional lattice model of particles which interact with each other via hard-core repulsion, and which can make long hops in addition to the usual nearest neighbour hopping dynamics. The parameter p gives the probability for long hops and the p = 0 limit leads to the well studied totally asymmetric simple exclusion process (TASEP). The first part of this study describes the combined effect of open boundaries and long hops on the steady state of the system. Apart from the usual low density (LD), high density (HD) and maximum current (MC) phases, the introduction of a finite p leads to a new possibility-an empty road (ER) phase with particles clearing out faster than they enter. The variation in the phase diagram with p is interesting, with the LD and MC phases vanishing at large values of p while the ER and HD phases divide the parameter space into 1/2. In the second part of this study, we look at the combined effect of long hops and static/dynamic impurities. The boundaries are taken to be periodic for simplicity. We see that the system with a static impurity and with 0 < p < 1 shows a phase transition from a shock phase characterised by finite densities on either side of the shock (HD–LD phase) to a phase where the density on one side of the shock is zero (HD-ER phase). We also study the effect of a dynamic impurity, introduced via a slow particle. Here, the system undergoes a phase transition from a homogeneous phase to a shock phase depending on the density, the speed of the particle, as well as the probability p. The shock phase dominates the phase diagram at large values of p. All our studies involve numerical simulations which are supported by mean field theory arguments. The mean field approximation works well qualitatively and correctly identifies the possible phases, while the quantitative agreement with numerics varies, depending on parameter values.

Phase Transitions in Phospholipid Monolayers: Theory Versus Experiments

The Doniach lattice gas (DLG) represents a ternary-mixture statistical model, whose components, water molecules (w), ordered-chain lipids (o), and disordered-chain lipids (d)-the latter carrying a high degenerescence ω ≫ 1-are located at each site of a two-dimensional lattice. The DLG model was introduced to describe phospholipid Langmuir films at the air-water interface and can be mapped into a spin-1 model, with the single-site states s i = 0, +1, and -1 representing the three types of molecules in the system (w, o, and d), respectively. The model allows lipid-density fluctuations and has been analyzed at the mean-field approximation (Guidi, H. S.; Henriques, V. B. Phys. Rev. E 2014, 90, 052705) as well as at the pair approximation (de Oliveira, F. O.; Tamashiro, M. N. Phys. Rev. E 2019, 99, 012147). In this work, we focus on performing an explicit comparison of the theoretical predictions obtained for the DLG model at the pair approximation with isothermal monolayer compression experiments (Nielsen, L. K.; Bjørnholm, T.; Mouritsen, O. G. Langmuir 2007, 23, 11684) for the two most commonly studied saturated zwitterionic phospholipids, DMPC (1,2-dimyristoyl- sn-glycero-3-phosphocholine) and DPPC (1,2-dipalmitoyl- sn-glycero-3-phosphocholine). The model parameters obtained by fitting to the experimental data yield phase diagrams that are qualitatively consistent with the observed phase transitions on DMPC and DPPC monolayers, with the absence of a low-density gas phase. Quantitative agreement, however, was less significant partially because of the challenging reproducibility of Langmuir monolayer compression experiments, claimed in the literature to be influenced by kinetic effects.

Temperature-dependent density profiles of dipolar droplets

Recently, trapped dipolar gases were observed to form high density droplets in a regime where mean field theory predicts collapse. These droplets present a novel form of equilibrium where quantum fluctuations are critical for stability. So far, the effect of quantum fluctuations have only been considered at zero temperature through the local chemical potential arising from the Lee--Huang--Yang correction. Here, we extend the theory of dipolar droplets to non-zero temperatures using Hartree--Fock--Bogoliubov theory (HFBT), and show that the equilibrium is strongly affected by temperature fluctuations. HFBT, together with local density approximation for excitations, reproduces the zero temperature results, and predict that the condensate density can change dramatically even at low temperatures where the total depletion is small. Particularly, we find that typical experimental temperatures ($T \sim $ 100 nK) can significantly modify the transition between low density and droplet phases.

Estimate of 3D power wall loads due to Neutral Beam Injection in EU DEMO ramp-up phase

Heating and current drive systems such as high energy Neutral Beam Injection (NBI) are being considered for pulsed EU DEMO (“DEMO1”) pre-conceptual design. Their aim is to provide auxiliary power, not only during flat-top, but also during transient phases (i.e. plasma current ramp-up and ramp-down).In this work, NBI fast particle power loads on DEMO1 first wall, due to shine-through and orbit losses, are calculated for the diverted plasma ramp-up phase. Numerical simulations are performed using BBNBI and ASCOT Monte Carlo codes. The simulations have been done using a complete 3D wall geometry, and implementing the latest DEMO NBI design, which foresees NBI at 800 keV particle energy. Location and power density of NBI-related power loads at different ramp-up time steps are evaluated and compared with the maximum tolerable heat flux taken from ITER case. Since NBI shine-through losses (dominant during low density phases) depend mainly on the beam energy, plasma density and volume, DEMO has a more favourable situation than ITER, enlarging NBI operational window. Using ITER criteria, DEMO NBI at full energy and power could be switched on during ramp-up at <ne> ~ 1.3 × 1019 m-3. This increases the appeal of neutral beam injectors as auxiliary power systems for DEMO.

Dynamical transition in the TASEP with Langmuir kinetics: mean-field theory

We develop a mean-field theory for the totally asymmetric simple exclusion process (TASEP) with open boundaries in order to investigate the so-called dynamical transition. The latter phenomenon appears as a singularity in the relaxation rate of the system toward its non-equilibrium steady state. In the high-density (low-density) phase, the relaxation rate becomes independent of the injection (extraction) rate at a certain critical value of the parameter itself, and this transition is not accompanied by any qualitative change in the steady-state behavior. We characterize the relaxation rate by means of bounds, becoming tight in the thermodynamic limit. These results are generalized to the TASEP with Langmuir kinetics, where particles can also bind to empty sites or unbind from occupied ones in the symmetric case of equal binding/unbinding rates. The theory predicts that a dynamical transition will occur in this case as well.

Dynamical transitions in a driven diffusive model with interactions

We study the dynamics of an asymmetric simple exclusion process with open boundaries and local interactions using a pair approximation which generalizes the 2-node cluster mean field theory and the Markov chain approach to kinetics and shares with these approaches the property of reproducing exact results for the bulk current-density relation and the steady-state phase diagrams. We find that the relaxation rate exhibits a dynamical transition, with no static counterpart, analogous to that found without interactions. Remarkably, for some values of the model's parameters, we find 2 dynamical transitions in the same low-density phase. We study the dynamics of relaxation to the steady state on both sides of these transitions and make an attempt at providing a physical interpretation for this phenomenon. Results from numerical approaches and a modified Domain Wall Theory confirm the picture provided by the pair approximation.

Phase transformation as the mechanism of mechanical deformation of vertically aligned carbon nanotube arrays: Insights from mesoscopic modeling

Vertically aligned carbon nanotube (VACNT) arrays or “forests” behave mechanically as foams when compressed, exhibiting a characteristic nonlinear stress – strain response. However, the fiber structure of VACNT forests is unlike that of cellular foams, and the microscopic mechanisms of the deformation are quite different. While numerous studies have addressed the mechanical response of VACNT forests undergoing uniaxial compression, the underlying deformation mechanisms are not yet fully established. In this paper, we report the results of large-scale mesoscopic simulations of the uniaxial compression of a VACNT forest composed of 2-μm-long carbon nanotubes (CNTs) as well as three structurally distinct forests composed of 0.6-μm-long CNTs. The simulations reveal that the compressive deformation proceeds as a phase transformation from an original low-density phase composed of vertically aligned CNT bundles to a densified phase with horizontal alignment of CNTs. The two phases are separated by a well-defined interfacial layer, which advances during the compressive deformation through localized bending and folding of nanotubes. For the 2-μm-tall forest, the folding involves correlated displacements of multiple CNT bundles, hinting on the origin of the collective buckling behavior observed in experiments. The characteristic three-stage stress-strain dependence (an initial "elastic" peak followed by an extended plateau region and a sharp rise of stress in the densification regime), commonly observed in experimental probing of the mechanical properties of VACNT forests, is reproduced in all of the simulations, suggesting that the heterogeneous propagation of densification front may be the general mechanism of the mechanical deformation of VACNT forests.

Mechanism of the spontaneous formation of plant protein microcapsules in aqueous solution

Aqueous pea protein isolate (PPI) solutions were investigated as a function of the pH (6–8) and the protein concentration (C = 1–4 wt%). Phase separation was observed below a critical pH and C-pH phase diagrams were determined at different temperatures between 5 and 40 °C. The morphology of the solutions after setting the pH was investigated with confocal laser scanning microscopy. It is shown that heating microphase separated systems induces both dispersion of the dense protein phase and formation of permanent crosslinks between the proteins. The relative kinetics of these two processes determines the morphology of the system after heating. Different morphologies, amongst which microcapsules, were obtained depending sensitively on the pH, protein concentration and heating protocol. A mechanism for the microcapsule formation is proposed in terms of nucleation and growth of the low density phase within the dense microdomains, with full redispersion inhibited by the formation of permanent crosslinks.

Spatiotemporal evolution of runaway electrons from synchrotron images in Alcator C-Mod

In the Alcator C-Mod tokamak, relativistic runaway electron (RE) generation can occur during the flattop current phase of low density, diverted plasma discharges. Due to the high toroidal magnetic field (B0 = 5.4 T), RE synchrotron radiation is measured by a wide-view camera in the visible wavelength range (λ ≈ 400–900 nm). In this paper, a statistical analysis of over one thousand camera images is performed to investigate the plasma conditions under which synchrotron emission is observed in C-Mod. In addition, the spatiotemporal evolution of REs during one particular discharge is explored in detail via a thorough analysis of the distortion-corrected synchrotron images. To accurately predict RE energies, the kinetic solver CODE (Landreman et al 2014 Comput. Phys. Commun. 185 847–855) is used to evolve the electron momentum-space distribution at six locations throughout the plasma: the magnetic axis and flux surfaces q = 1, 4/3, 3/2, 2, and 3. These results, along with the experimentally-measured magnetic topology and camera geometry, are input into the synthetic diagnostic SOFT (Hoppe et al 2018 Nucl. Fusion 58 026032) to simulate synchrotron emission and detection. Interesting spatial structure near the surface q = 2 is found to coincide with the onset of a locked mode and increased MHD activity. Furthermore, the RE density profile evolution is fit by comparing experimental to synthetic images, providing important insight into RE spatiotemporal dynamics.

Impact of effective injection rates on density and phase diagram at the branching point

In this paper, the effects of different effective injection rates at the branching point on density profiles and phase diagram have been investigated by theoretical calculation and computer simulation. At the branching point, a single-lane is divided into two sub-lanes. The variables p and q which satisfy p+q = 1 and p [Formula: see text] q are used to describe the percent of currents which enter two sub-channels, respectively. With different effective injection rates, the phase diagram includes three regions and two transition lines. Obviously, the sub-lane 2 is always in the low-density (LD) phase because the small p leads to fewer particles moving through sub-lane 2. However, sub-lane 3 is easy in the high-density (HD) phase (namely, traffic jam) because of the large q. In addition, the current of lane 1 is often in the maximal current (MC) when entrance rate [Formula: see text] [Formula: see text] 0.5. With the increase of variable q, the regions of LD/LD/LD and MC/LD/LD phases shrink dramatically while the region of the HD/LD/HD phase only expands. This result implies that the changes of the phase diagram are determined by the variable q, meanwhile, the large q will likely lead to traffic congestion.

Nano-scale mechanical properties investigation of C-S-H from hydrated tri-calcium silicate by nano-indentation and molecular dynamics simulation

Calcium silicate hydrate (C-S-H) is the most important hydration product in cementitious material. Due to its complexity, the investigation on its mechanical properties is still in progress. In this study, synthesized C3S was utilized to produce relatively pure C-S-H gels with two different water/solid ratios. Then mechanical properties of the C-S-H were measured and characterized by nano-indentation and SEM/EDX/BSE. Two types of C-S-H with Ca/Si ratio around 1.8 were observed through statistical analysis of the indentation modulus (Er) and hardness (H). The looser one possessing lower Er around 25–30 GPa while the denser one possesses higher Er of 35.9 GPa. Both phases have higher stiffness than those of high density (HD) and low density (LD) phase obtained from cement pastes. Furthermore, molecular dynamics provides mechanical understanding of C-S-H gel at nano-scale. Based on the crystal structure of tobermorite 11 Å, 64 models with Ca/Si ratios from 1.3 to 2.0 were constructed for uniaxial tension test. The stiffness and cohesive strength of the C-S-H gel are greatly influenced by Ca/Si ratio, water content and nano-porosity. With increasing Ca/Si ratio from 1.3 to 2.0, long silicate chains were broken to shorter ones and more water molecules penetrated into defect regions. The destroyed silicate skeleton and the “hydrolytic weakening” effect resulted in the reduction of the mechanical properties of C-S-H gels. Concurrently considering the intrinsic porosity and chemical compositions, the appropriate micromechanical models are utilized to upscale the molecular properties to higher level.

Vaporlike phase of amorphous SiO2 is not a prerequisite for the core/shell ion tracks or ion shaping

When a swift heavy ion (SHI) penetrates amorphous $\mathrm{Si}{\mathrm{O}}_{2}$, a core/shell (C/S) ion track is formed, which consists of a lower-density core and a higher-density shell. According to the conventional inelastic thermal spike (iTS) model represented by a pair of coupled heat equations, the C/S tracks are believed to form via ``vaporization'' and melting of the $\mathrm{Si}{\mathrm{O}}_{2}$ induced by SHI (V-M model). However, the model does not describe what the vaporization in confined ion-track geometry with a condensed matter density is. Here we reexamine this hypothesis. While the total and core radii of the C/S tracks determined by small angle x-ray scattering are in good agreement with the vaporization and melting radii calculated from the conventional iTS model under high electronic stopping power $({S}_{e})$ irradiations (g10 keV/nm), the deviations between them are evident at low-${S}_{e}$ irradiation (3--5 keV/nm). Even though the iTS calculations exclude the vaporization of $\mathrm{Si}{\mathrm{O}}_{2}$ at the low ${S}_{e}$, both the formation of the C/S tracks and the ion shaping of nanoparticles (NPs) are experimentally confirmed, indicating the inconsistency with the V-M model. Molecular dynamics (MD) simulations based on the two-temperature model, which is an atomic-level modeling extension of the conventional iTS, clarified that the ``vaporlike'' phase exists at ${S}_{e}\ensuremath{\sim}5$ keV/nm or higher as a nonequilibrium phase where atoms have higher kinetic energies than the vaporization energy, but are confined at a nearly condensed matter density. Simultaneously, the simulations indicate that the vaporization is not induced under 50-MeV Si irradiation $({S}_{e}\ensuremath{\sim}3$ keV/nm), but the C/S tracks and the ion shaping of nanoparticles are nevertheless induced. Even though the final density variations in the C/S tracks are very small at the low stopping power values (both in the simulations and experiments), the MD simulations show that the ion shaping can be explained by flow of liquid metal from the NP into the transient low-density phase of the track core during the first \ensuremath{\sim}10 ps after the ion impact. The ion shaping correlates with the recovery process of the silica matrix after emitting a pressure wave. Thus, the vaporization is not a prerequisite for the C/S tracks and the ion shaping.

Full Phase Diagram of Active Brownian Disks: From Melting to Motility-Induced Phase Separation.

We establish the complete phase diagram of self-propelled hard disks in two spatial dimensions from the analysis of the equation of state and the statistics of local order parameters. The equilibrium melting scenario is maintained at small activities, with coexistence between active liquid and hexatic order, followed by a proper hexatic phase, and a further transition to an active solid. As activity increases, the emergence of hexatic and solid order is shifted towards higher densities. Above a critical activity and for a certain range of packing fractions, the system undergoes motility-induced phase separation and demixes into low and high density phases; the latter can be either disordered (liquid) or ordered (hexatic or solid) depending on the activity.

Glassy dynamics due to a trajectory phase transition in dissipative Rydberg gases

The physics of highly excited Rydberg atoms is governed by blockade or exclusion interactions that hinder the excitation of atoms in the proximity of a previously excited one. This leads to cooperative effects and a relaxation dynamics displaying space-time heterogeneity similar to what is observed in the relaxation of glass-forming systems. Here we establish theoretically the existence of a glassy dynamical regime in an open Rydberg gas, associated with phase coexistence at a first-order transition in dynamical large deviation functions. This transition occurs between an active phase of low density in which dynamical processes take place on short timescales, and an inactive phase in which excited atoms are dense and the dynamics is highly arrested. We perform a numerically exact study and develop a mean-field approach that allows to understand the mechanics of this phase transition. We show that radiative decay --- which becomes experimentally relevant for long times --- moves the system away from dynamical phase coexistence. Nevertheless, the dynamical phase transition persists and causes strong fluctuations in the observed dynamics.

General framework for nonclassical nucleation

A great deal of experimental evidence suggests that a wide spectrum of phase transitions occur in a multistage manner via the appearance and subsequent transformation of intermediate metastable states. Such multistage mechanisms cannot be explained within the realm of the classical nucleation framework. Hence, there is a strong need to develop new theoretical tools to explain the occurrence and nature of these ubiquitous intermediate phases. Here we outline a unified and self-consistent theoretical framework to describe both classical and nonclassical nucleation. Our framework provides a detailed explanation of the whole multistage nucleation pathway showing in particular that the pathway involves a single energy barrier and it passes through a dense phase, starting from a low-density initial phase, before reaching the final stable state. Moreover, we demonstrate that the kinetics of matter inside subcritical clusters favors the formation of nucleation clusters with an intermediate density, i.e. nucleation precursors. Remarkably, these nucleation precursors are not associated with a local minimum of the thermodynamic potential, as commonly assumed in previous phenomenological approaches. On the contrary, we find that they emerge due to the competition between thermodynamics and kinetics of cluster formation. Thus, the mechanism uncovered for the formation of intermediate phases can be used to explain recently reported experimental findings in crystallization: up to now such phases were assumed a consequence of some complex energy landscape with multiple energy minima. Using fundamental concepts from kinetics and thermodynamics, we provide a satisfactory explanation for the so-called nonclassical nucleation pathways observed in experiments.

The Ideal Crystal Structure of Cristobalite X-I: A Bridge in SiO2 Densification

On compression of α-cristobalite SiO2 to pressures above approximately 12 GPa, a new polymorph known as cristobalite X-I forms. The existence of cristobalite X-I has been known for several decades; however, consensus regarding its exact atomic arrangement has not yet been reached. The X-I phase constitutes an important step in the silica densification process, separating low-density tetrahedral framework phases from high-density octahedral polymorphs. It is the only nonquenchable high-density SiO2 phase, which reverts to the low-density form on decompression at ambient temperature. Recently, an experimental study proposed an octahedral model of SiO2 X-I with intrinsic structural defects involving partial Si site occupancies. In contrast, our new single-crystal synchrotron X-ray diffraction experiments have shown that the ideal structure of this phase should instead be described by a defect-free model, which does not require partial occupancies. The structure of cristobalite X-I consists of octahedral chai...

Determination of Benzotriazole and Its Derivatives in Aqueous Sample with Air-assisted Liquid-Liquid Microextraction Followed by High-performance Liquid Chromatography

Abstract Benzotriazole and its derivatives are persistent contaminants and ubiquitous in aquatic environments. Here, a simple, reliable method based on air-assisted liquid-liquid microextraction coupled with high-performance liquid chromatography with a UV detector for determining 1H-benzotriazole, 5-methyl-1H-benzotriazole and 5-chloro-1H-benzotriazole in aqueous samples was developed. In this procedure, the analytes quickly entered into the extraction solvent via the bolus flow formed during the movement of the plunger without disperser solvent. 1-Hexanol was used as the extractant, thus avoiding chlorinated solvents, and a narrow-neck tube with round bottom was self-made to facilitate collection of the low-density extract phase and centrifugation at high speed. Those main parameters, including sample volume, type and volume of extraction solvent, number of suction-injection cycle, salt addition, and the pH were optimized. Under the optimized conditions, the pretreatment could be completed within two minutes with a good linearity (r > 0.9996). Satisfactory intra-day (3.0%–4.3%, n = 5) and inter-day (5.9%–7.8%, n = 5) precisions were observed. Further, the method was applied in analysis of real samples including tap water, lake water, river water, and municipal waste water with good recoveries (73%−116%).