Melting Of Clusters Research Articles

Flame Front Model with the Clusters Condensation

The processes model in a flame during the n-alkanes air mixture combustion initiation is proposed, taking into account the supramolecular structures formation possibility in the peroxide clusters form. This approach is justified by the n-alkanes melting temperatures correlation with their autoignition temperatures and anti-knock indexes. The condensation possibility is provided for such high molecular structures. Boiling temperatures values at flame front pressures characteristic were evaluated. To predict the peroxide clusters melting temperatures, a formula developed earlier for the hydrocarbons condensed state was used, which takes into account the length and molecular weight of modeled clusters. Expected peroxide clusters melting temperatures were predicted for conditions of dimeric and tetrameric structures. A linear dependence was used to recalculation the obtained values in boiling temperatures. It is shown that the calculated clusters phase transitions characteristic temperatures can be realized in the flame front preparatory zone. Based on the condensation theory, the flame front thickness and the minimum non-extinguishing sphere radius during ignition were estimated: the obtained data closely coincide with these parameters known values.

Laser-Induced Melting and Coalescence of Nanoparticle Clusters Captured in Real Time

Laser-Induced Melting and Coalescence of Nanoparticle Clusters Captured in Real Time

Assessment of Anharmonicities in Clusters: Developing and Validating a Minimum-Information Partition Function.

Precise thermodynamic calculations are essential for understanding the dynamics of cluster systems and new particle formation. However, the widely employed harmonic statistical mechanical approach often falls short in terms of accuracy. In this study, we present an improved statistical model that incorporates vibrational anharmonicity via a novel partition function that requires only one additional system-specific input parameter. In addition to considering vibrational aspects, we also account for anharmonicity related to the configurational space. The role of anharmonicities is thoroughly examined in the case of general clusters, where the complete sets of conformers, mechanically stable spatial arrangements, are known up to clusters composed of 14 monomers. By performing consistent Monte Carlo simulations on these systems, we benchmark the statistical model's efficacy in reproducing key thermodynamic properties (formation free energy and potential energy) in the classical limit. The model exhibits exceptional alignment with simulations, accurately reproducing free energies within a precision of 2kBT and reliably capturing cluster melting temperatures. Furthermore, we demonstrate the significance and applicability of the model by reproducing thermodynamic barriers in homogeneous gas-phase nucleation of larger clusters. The transferability of our developed approach extends to more complex molecular systems and bears relevance for atmospheric multicomponent clusters, in particular.

Pd8 Cluster: Too Small to Melt? A BOMD Study.

The question of whether a solid-liquid phase transition occurs in small clusters poses a fundamental challenge. In this study, we attempt to elucidate this phenomenon through a thorough examination of the thermal behavior and structural stability of Pd8 clusters employing ab initio simulations. Initially, a systematic global search is carried out to identify the various isomers of the Pd8 cluster. This is accomplished by employing an ab initio basin-hopping algorithm and using the PBE/SDD scheme integrated in the Gaussian code. The resulting isomers are further refined through reoptimization using the deMon2k package. To ensure the structural firmness of the lowest-energy isomer, we calculated normal modes. The structural stability as a function of temperature is analyzed through the Born-Oppenheimer molecular dynamics (BOMD) approach. Multiple BOMD trajectories at distinct simulated temperatures are examined with data clustering analysis to determine cluster isomers. This analysis establishes a connection between the potential energy landscape and the simulated temperature. To address the question of cluster melting, canonical parallel-tempering BOMD runs are performed and analyzed with the multiple-histogram method. A broad maximum in the heat capacity curve indicates a melting transition between 500 and 600 K. To further examine this transition, the mean-squared displacement and the pair-distance distribution function are calculated. The results of these calculations confirm the existence of a solid-liquid phase transition, as indicated by the heat capacity curve.

Relevance of the Wigner–Seitz Cell Approximation for the Coulomb Clusters

A molecular dynamics simulation of a system of massive charged particles on a compensating homogeneous background confined by a spherical surface has been carried out. A crystallized cluster is a set of nested spherical shells of almost the same structure and a core. It is shown that cluster melting is a combination of shell and core melting. It is found that the values of the Coulomb coupling parameter Γ corresponding to these two types of melting do not depend on the cluster size. Methods for determining Γ based on the Wigner–Seitz cell model are discussed. It is shown that the estimate based on the root-mean-square deviation of a particle from the center of its cell is unreliable due to the self-diffusion of particles. A relation is proposed that defines Γ in terms of the root-mean-square velocity and acceleration of the particle and does not include the root-mean-square deviation of the particle from its average position. It is shown that this relation is satisfied with high accuracy not only for the crystallized, but also for the liquid state. Thus, it has been demonstrated that the Wigner–Seitz cell model is applicable to the strongly inhomogeneous system under consideration.

Cluster Mechanism of the Explosive Processes Initiation in the Matter

The relationship between substance characteristic temperatures: autoignition, melting, flash, boiling is demonstrated and analyzed. Based on the oscillatory and step changes presence, a conclusion was made about the supramolecular structures presence and periodicity in the n-alkanes homologous series. A method for modeling equivalent lengths of peroxide supramolecular structures for predicting the explosion and fire hazard parameters of n-alkanes is proposed. An approximation dependence was developed for predicting autoignition temperatures tai of n-alkanes. It is shown that stoichiometric concentrations of the various supramolecular peroxide structures formation accord to different flammability and explosion limits. A correlation between tai and Anti-Knock Index (AKI) was established. An approximation dependence was developed for predicting n-alkanes AKI. The detonation propensity index КD was introduced based on cluster supramolecular structures modeling and melting temperatures. It is shown that КD indicator correlates with the n-alkanes AKI and with the explosives detonation velocity. The possibility of taking into account during calculations the supramolecular structures presence at the combustion stage confirms their existence.

Temperature-Dependent Circularly Polarized Luminescence of a Cholesteric Copolymer Doped with a Europium Complex

The design of new materials for non-contact temperature sensors is an important task for scientists working in the fields of chemistry, physics, and materials science. In the present paper, a novel cholesteric mixture based on a copolymer doped with a highly luminescent europium complex was prepared and studied. It was found that the spectral position of the selective reflection peak strongly depends on temperature and a shift towards shorter wavelengths is observed upon heating with an amplitude of more than 70 nm, from the red to green spectral range. This shift is associated with the existence and melting of clusters of smectic order, as confirmed by X-ray diffraction investigations. The extreme temperature dependence of the wavelength of selective light reflection provides a high thermosensitivity of the degree of circular polarization of the europium complex emission. The highest values of the dissymmetry factor are observed when the peak of selective light reflection fully overlaps with the emission peak. As a result, the highest sensitivity of 65%/K for luminescent thermometry materials was obtained. In addition, the ability of the prepared mixture to form stable coatings was demonstrated. The obtained experimental results, i.e., the high thermosensitivity of the degree of circular polarization, and the ability to form stable coatings allow us to consider the prepared mixture as a promising material for luminescent thermometry.

Melting of two-dimensional electron clusters in a magnetic field

The molecular dynamics method is applied to simulate the behavior of two-dimensional electron clusters of various shapes containing about 100 particles. The influence of a magnetic field on the effects of ordering and disordering (melting) in such clusters is studied. The field dependence of the rms displacements of particles in clusters is determined, and the trajectories of electron motion are found as functions of temperature and magnetic field. Ordering and disordering in clusters were fixed by the appearance of characteristic features in the rms displacements of electrons in the system. Hysteresis was observed in the field dependence of these displacements at a fixed temperature. The obtained data are compared with the results for root-mean-square displacements of electrons in a two-dimensional electronic infinite crystal. The qualitative agreement was found with these results; at the same time, the field dependence of root-mean-square displacements in an unbounded crystal is much weaker than in the clusters under study. The influence of the cluster shape on ordering transitions is studied, and it is found that the shape of a defect-free cluster does not affect the transition conditions. The distribution of electron velocities in clusters is studied for various magnetic fields.

Stability of FePt, FePt3 Nanoclusters of Different Habits

Calculations of the total energy of Fe-Pt nanoclusters, corresponding in the phase diagram to the compositions of FePt, FePt3 intermetallics and possessing either characteristic structures L10 and L12, respectively, or non-characteristic disordered structure A1, as well as various particle habits (cuboctahedra, icosahedra) are carried out by molecular dynamics for the first time. The dependences of cluster stability on their size and temperature are plotted, along with the schemes of temperature transformations of cluster morphology and the dependence of the melting points of the clusters with these structures and habits on their size. The size range (2–8 nm) corresponds to the sizes of particles observed by high-resolution electron microscopy. It is shown that the species play an essential part in the phase transformations proceeding under heating in the nanostructured system Fe-Pt and leading to the formation of nanocrystals with highly ordered L10 structure possessing giant coercivity are cubic nanoclusters with the ordered structures L10 and L12. With an increase in cluster size, their stability and melting points increase, tending to saturation of the dependencies within the size range above 10 nm. The least stable clusters are those of intermetallics with non-characteristic disordered structure A1 and icosahedral habit.

Ultrafast atomic view of laser-induced melting and breathing motion of metallic liquid clusters with MeV ultrafast electron diffraction

Under the irradiation of an ultrafast intense laser, solid materials can be driven into nonequilibrium states undergoing an ultrafast solid-liquid phase transition. Understanding such nonequilibrium states is essential for scientific research and industrial applications because they exist in various processes including laser fusion and laser machining yet challenging in the sense that high resolution and single-shot capability are required for the measurements. Herein, an ultrafast diffraction technique with megaelectron-volt (MeV) electrons is used to resolve the atomic pathway over the entire laser-induced ultrafast melting process, from the initial loss of long-range order and the formation of high-density liquid to the progressive evolution of short-range order and relaxation into the metastable low-density liquid state. High-resolution measurements using electron pulse compression and a time-stamping technique reveal a coherent breathing motion of polyhedral clusters in transient liquid aluminum during the ultrafast melting process, as indicated by the oscillation of the interatomic distance between the center atom and atoms in the nearest-neighbor shell. Furthermore, contraction of interatomic distance was observed in a superheated liquid state with temperatures up to 6,000 K. The results provide an atomic view of melting accompanied with internal pressure relaxation and are critical for understanding the structures and properties of matter under extreme conditions.

Mid-Cretaceous Wake seamounts in NW Pacific originate from secondary mantle plumes with Arago hotspot composition

The geochemistry of oceanic intraplate (primarily oceanic island and seamount) lavas can provide essential information on the composition and evolution of their mantle source and geodynamics. Due to very limited rock sampling of the many mid-Cretaceous Wake seamounts in NW Pacific, the mantle source lithology and petrogenesis of their lavas, and the geodynamic mechanism responsible for generating the lavas have not been fully delineated. In order to help resolve these issues, here we present whole-rock major-trace element and Sr-Nd-Pb-Hf isotopic data for twenty-one lava samples collected from Lamont, Dacheng, Xufu, Penglai, Niulang, Zhinyu, and Zhanlu seamounts in the Southern Wake seamount trail (WST). These lava samples are silica-undersaturated alkali basalt and basanites/nephelinite. They have high CaO, FeOT and TiO2 contents and CaO/Al2O3 ratios, consistent with their derivation from partial melting of a carbonated peridotite or reaction between carbonated MORB-eclogite-derived silicate melts and fertile peridotite. High Zr/Hf and negative Zr-Hf-Ti anomalies in the most mafic lavas further suggest a contribution from carbonated components in their mantle source. These lavas show FOZO (focal zone)-HIMU (high μ = 238U/204Pb)-like Sr-Nd-Pb-Hf isotopic compositions (e.g., (206Pb/204Pb)i = 19.36–20.72), falling within the Arago (also known as “Young Rurutu” or “Atiu”) hotspot field. Combined with the sparse previous age and geochemical data, we propose that WST lavas were most likely derived from partial melting of secondary plume clusters emanating from the top of Arago mantle plume trapped at the mantle transition zone. Alternatively, WST lavas could have come from a number of secondary plumelets emanating from the top of the Pacific Large Low Shear Velocity Province (LLSVP). The simultaneously upwelling secondary plumes or plumelets generated chronologically overlapping, compositionally similar and closely-spaced Wake seamounts atop the moving Pacific plate.

Scale Symmetry of Stochastic Surface Clustering under Plasma Influence in Fusion Devices

Titanium, tungsten, carbon, lithium, and beryllium surface structure were analyzed after plasma irradiation in fusion devices. Exceptional extreme high-temperature plasma load in fusion devices leads to specific surface clustering. It is strictly different from any other conditions of material’s clustering. The hierarchical granularity with cauliflower-like shape and surface self-similarity have been observed. Height’s distribution is deviated from the Gaussian function. The relief roughness differs qualitatively from the ordinary Brownian surface and from clustering under other conditions. In fusion devices, the specific conditions regulate material surface clustering faced to plasma. Ions and clusters melt on the surface and move under the effect of stochastic electromagnetic field driven by the near-wall turbulent plasma. In such a process, long-term correlations lead to the growth of surface with a self-similar structure. The multiscale synergistic effects influence the self-similarity–fractal growth from nanometers to millimeters. Experimental results illustrate universality of stochastic clustering of materials irradiated with plasma in fusion devices.

Light clusters in warm stellar matter: calibrating the cluster couplings

The abundances of light clusters within a formalism that considers in-medium effects are calculated using several relativistic mean-field models, with both density-dependent and density-independent couplings. Clusters are introduced as new quasiparticles, with a modified coupling to the scalar meson field. A comparison with experimental data from heavy ion collisions allows settling the model dependence of the results and the determination of the couplings of the light clusters to the meson fields. We find that extra experimental constraints at higher density are needed to convincingly pin down the density associated to the melting of clusters in the dense nuclear medium. The role of neutron rich clusters, such as $^6$He, in asymmetric matter is discussed.

Melting of different types of 2D electron clusters

The molecular dynamics method is used to study the melting of two-dimensional clusters that can form in electronic systems above the surface of superfluid helium with the help of triangular, square, hexagonal and round holding electrodes. Clusters with a fixed surface density of 108 cm−2, but different numbers of particles (from 3 to 406), are considered. It is shown that in defect-free clusters, the transition to a disordered state occurs abruptly and the melting point, Тmelt, is easy to determine. The dependence of Тmelt on the cluster size is observed for clusters with N < 400 particles. In square clusters with N < 50, the ordering of particles is imposed by the boundary conditions, whereas for N > 100, a triangular lattice with defects near the boundary is observed. In round clusters, electrons begin to move easily in the angular direction at rather low temperatures, but the “radial melting” temperature is consistent with the melting point of small square clusters. In the presence of defects, the transition to a disordered state begins from the defect region and gradually spreads to the entire cluster. There is no clear definition of the melting point in this scenario.

First-principles melting of krypton and xenon based on many-body relativistic coupled-cluster interaction potentials

The solid-to-liquid phase transition for krypton and xenon is studied by means of parallel-tempering Monte Carlo simulations based on an accurate description of the atomic interactions within a many-body ansatz using relativistic coupled-cluster theory. These high-level data were subsequently fitted to computationally efficient extended Lennard-Jones and extended Axilrod-Teller-Muto types of interaction potentials. Solid-state calculations demonstrate that the many-body decomposition of the interaction energy converges well for the heavier rare gas solids, leading to solid-state properties in good agreement with experiment. The results show that it suffices to include two- and three-body interactions only for the melting simulation. The melting of the bulk is simulated for cells with cubic periodic boundary conditions, as well as within a finite cluster approach. For the latter, melting of spherical magic number clusters with increasing cluster size is studied, and the melting temperatures are obtained from extrapolation to the bulk. The calculated melting temperatures for the cluster extrapolation (the periodic approach values corrected for superheating are set in parentheses) are ${T}_{m}=113.7$ K (110.9 K) and ${T}_{m}=160.8$ K (156.1 K) for krypton and xenon, respectively. Both are in very good agreement with corresponding experimental values of 115.75 and 161.40 K.

The Manifestation of Hysteresis in the Thermal Properties of Nanosystems Based on the Example of Supercooled Water Clusters in Wet Sephadex of the G Type

Abstract—The research cycle of thermal properties of water clusters in polysaccharide–water systems (starch and Sephadex G-100) with a low content of freezing water was continued based on the example of another modified polysaccharide, Sephadex G-25. This Sephadex has a more rigid three-dimensional structure compared with Sephadex G-100. The doublet structure of the melting curve of water clusters, which indicated the bimodal nature of their size distribution, was the main peculiarity of the data obtained for Sephadex G-25 by differential scanning calorimetry. The observed decrease in the temperature and heat of melting and crystallization of water clusters with a decrease in the humidity of Sephadex G-25, as in the case of other polysaccharides, was a typical manifestation of the size effect for nanosystems. At the same time, hysteresis occurred between the temperatures $$\Delta T = {{T}_{{{\text{mel}}}}} - {{T}_{{{\text{cr}}}}}$$, and the heats $$\Delta Q = {{Q}_{{{\text{mel}}}}} - {{Q}_{{{\text{cr}}}}}$$ of these transitions, which is also typical for small systems. It was found that the transformation processes (recrystallization, reorganization, and size change) in the system of clusters of crystallized supercooled water in Sephadex G-25 with low humidity during heating could occur both below the melting interval and inside it, which indicated the nonequilibrium state of the original set of nanoclusters. It was shown that the decrease of Vheat, as well as annealing within the melting interval led to a redistribution of clusters in size and, according to the revealed size effect, to an increase in the melting heat. These facts taken together indicated that the nonequilibrium state of the initially formed clusters during cooling and, as a consequence, their ability to transform with increasing temperature certainly play an important, if not a decisive, role in the manifestation of hysteresis in the thermal properties of water nanoclusters.

The Structure and Crystallization Process of Amorphous Iron Nanoparticles

This paper studies the crystallization process and structure of amorphous iron nanoparticles by molecular dynamics method. The study shows that amorphous iron nanoparticles could not be crystallized at 300 K and 500 K. Iron nanoparticle, annealed at 900 K over a long time, was crystallized into a BCC crystal structure. The structure of crystallized iron nanoparticle at 900 K was analyzed through the pair radial distribution function and the number of crystal atoms upon various regions in nanoparticles. The simulation revealed that the first nuclei was formed most frequently in the area near the surface of the nanoparticle. Then the crystal cluster grew toward the centre of the nanoparticle. The completely crystallized nanoparticle had two components: the core with a BCC crystal structure and surface with an amorphous structure. As for the amorphous nanoparticle at 300 or 500 K, crystal-clusters were too small to grow large enough to crystallize the nanoparticle.
 
 Keywords
 Iron nanoparticle, crystallize, annealing, crystal atom, crystal cluster.
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Atomic-resolution imaging of surface and core melting in individual size-selected Au nanoclusters on carbon

Although the changes in melting behaviour on the nanoscale have long attracted the interest of researchers, the mechanism by which nanoparticles melt remains an open problem. We report the direct observation, at atomic resolution, of surface melting in individual size-selected Au clusters (2–5 nm diameter) supported on carbon films, using an in situ heating stage in the aberration corrected scanning transmission electron microscope. At elevated temperatures the Au nanoparticles are found to form a solid core-liquid shell structure. The cluster surface melting temperatures, show evidence of size-dependent melting point suppression. The cluster core melting temperatures are significantly greater than predicted by existing models of free clusters. To explore the effect of the interaction between the clusters and the carbon substrate, we employ a very large-scale ab initio simulation approach to investigate the influence of the support. Theoretical results for surface and core melting points are in good agreement with experiment.

Evidence for melting of HD and D2 clusters in solid neon below 1 K

We report on an electron spin resonance study of H and D atoms stabilized in solid mixtures of neon, molecular deuterium, and hydrogen deuteride. We observed that H and D atoms can be stabilized in pure HD and ${\mathrm{D}}_{2}$ clusters formed in pores of solid Ne as well as in a Ne environment. Raising temperature from 0.1 to $1.3\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ results in a rapid recombination of a significant fraction of both H and D atoms in HD and ${\mathrm{D}}_{2}$ clusters. The recombination rate appears to be five and seven orders of magnitude faster than in solid bulk samples of HD and ${\mathrm{D}}_{2}$, respectively. We explain this recombination rate enhancement by melting of clusters of molecular hydrogen isotopes, similar to what has been observed for atomic hydrogen in ${\mathrm{H}}_{2}$ clusters [Sheludiakov et al., Phys. Rev. B 97, 104108 (2018)]. Our observations do not provide evidence for a superfluid behavior of these clusters at temperatures of $0.1--1.3\phantom{\rule{0.28em}{0ex}}\mathrm{K}$.

The Relationship between Crystallization of Water Clusters Dispersed in Sephadex Resins in their Wet Form and the Polymeric Matrix Glass Transition Temperature

Abstract—Differential scanning calorimetry (DSC) analysis was used to study the dependences of the main parameters of crystallization and melting of small water clusters dispersed in Sephadex resins on the amount of water (30–55%) in the wet polymer. The temperatures and heats of the processes were shown to depend on the frozen water (FW) concentration in Sephadex, with the dependence being considered as a manifestation of the size effect. Hysteresis between FW melting and crystallization was detected in a certain Sephadex humidity range; i.e., Tm was higher than Tcr and Qm higher than Qcr. The two findings reflected the basic properties of low-dimensional systems. Sephadex and amorphous starch, which was examined previously, were compared as two polysaccharide systems with different structural organizations, and a significant difference was observed in FW crystallization in the systems. Nucleation and crystallite growth, which determine water crystallization, were found to occur separately in time in Sephadex with a low humidity under these experimental conditions. Nucleation occurred during cooling, while growth was observed mainly during heating. Both of the processes occurred during cooling and partly overlapped only at a high humidity, far from Tg of Sephadex. In contrast, the two processes always occurred during cooling in amorphous starch regardless of its humidity. The difference in FW crystallization was assumed to arise because water mobility differs between the systems as a result of their difference in molecular mobility of the biopolymer matrix proper near the glass transition region.