Simulation of the melting and crystallization processes in monocrystalline silicon exposed to nanosecond laser radiation

Numerical simulation of the melting and crystallization processes of monocrystalline silicon exposed to the nanosecond radiation of a ruby laser was carried out with the kinetics of the phase transformations accounted for on the basis of Kolmogorov equations. A two-dimensional mechanism of nucleation and growth of the new phase was invoked to describe the phase transitions. It was shown that the temporal dependences of monocrystal overheating and liquid phase supercooling in the melting and crystallization stages, respectively, are nonmonotonic and determined by the kinetics of the phase transitions. The maximum values of the overheating and supercooling were ∼100 K.     

The role of the “Casimir force analogue” at the microscopic processes of crystallization and melting

Melting (crystallization), a phase transition from a crystalline solid to a liquid state, is a common phenomenon in nature. We suggest a new factor, “the Casimir force analogue”, to describe mechanisms of melting and crystallization. The Casimir force analogue is a force occurring between the surfaces of solid and liquid phases of metals caused by different energy density of phonons of these phases. It explains abrupt changes in geometry and thermodynamic parameters at a melting point. “The Casimir force analogue” helps to estimate latent melting heat and to gain an insight into a solid–liquid transition problem.     

The peculiarities of crystallization of low-temperature eutectic in magnesium-lead system

Density of low-temperature eutectic in magnesium-lead system (83.06 at. % Pb) has been measured by gamma-raying of the samples with narrow beam from cesium-137 isotope over the temperature range 293–1000 K of solid and liquid states. Approximation density dependences have been obtained and data of this work and other authors have been compared. Reference tables of temperature dependences of the alloy thermal properties have been compiled for the entire range of measurements and their errors estimated. It is shown that differences in the values of volumetric changes obtained during melting and crystallization are bound with the metastable β′-phase formation.     

Desorption and crystallization kinetics in nanoscale thin films of amorphous water ice

The amorphous to crystalline ice phase transition is studied by measuring the water desorption rate from nanoscale thin films of water vapor deposited on Au(111) and Ru(001) single crystal metallic substrates. The desorption kinetics are substrate dependent and suggest strongly that the film morphology is governed by the hydrophilicity of the substrate. The crystallization kinetics are independent of substrate but depend strongly on both temperature and film thickness and are consistent with a spatially random nucleation and isotropic growth model.     

Study of Raman spectra in the processes of glass formation,crystallization, and melting of strontium diborate

The behavior of the Raman spectra of SrB₄O₇ (SBO) and Li₂B₄O₇ crystals upon melting and glass formation is studied by high-temperature Raman spectroscopy (HTRS). The character of formation of boron–oxygen fragments in different aggregate states of SBO is discussed based on an analysis of the Raman spectra in the crystalline, glassy, and molten states.     

Ultrasound-assisted extraction of water-soluble polysaccharides from the roots of valerian (Valeriana officinalis L.)

The insoluble plant residues, obtained after preparation of medicinal tinctures from the roots of valerian (Valeriana officinalis L.) by classical and ultrasound-assisted extraction with aqueous ethanol in a pilot plant, were subsequently treated with hot water to isolate the accessible polysaccharide cell wall components. At almost equal amounts of the hot-water extractable material, the yields of the recovered polysaccharides were lower in the ultrasonical experiment. This is due to the fact that a part of accessible polysaccharides were already solubilised by the aqueous ethanol and recoverable from the medicinal tincture. Therefore, the net yield of extracted polysaccharides was enhanced in the ultrasonical procedure. This fact as well as the sugar composition and structural features of the isolated polysaccharides suggest that ultrasonication have attacked the integrity of cell walls, released and degraded its most accessible polysaccharides (pectic polysaccharides and starch) and increased also the extractibility of its less accessible components--xylan, mannan and glucan. The water-soluble polysaccharide fractions from both the conventional and ultrasonical experiments exhibit significant immunostimulatory activities in mitogenic and comitogenic thymocyte tests.     

Interfacial melting of ice in contact with SiO(2)

The physical behavior of condensed matter can be drastically altered in the presence of interfaces. Using a high-energy x-ray transmission-reflection scheme, we have studied ice-SiO2 model interfaces. We observed the formation of a quasiliquid layer below the bulk melting temperature and determined its thickness and density as a function of temperature. The quasiliquid layer has stronger correlations than water and a large density close to rho(HDA)=1.17 g/cm(3) of high-density amorphous ice suggesting a structural relationship with the postulated high-density liquid phase of water.     

Thermal hysteresis during the melting and crystallization of macroobjects

Different thermal hysteresis processes associated with macroobjects have been analyzed using heating and cooling thermograms with regard to liquid–solid phase transformations. Two types of thermal hysteresis for equilibrium and nonequilibrium crystallizations have been revealed. The paths of Gibbs energy variation are shown for both cases.     

Structural properties of complex (dusty) plasma upon crystallization and melting

A change in the local order of a bounded complex (dusty) plasma in the process of its crystallization and melting has been examined by molecular dynamics simulations. The dynamics of microparticles is considered in the framework of a Langevin thermostat, the pair interaction between charged particles is described by a screened Coulomb potential (Yukawa potential) with the hard wall potential as a confinement. It has been shown that the beginning of the crystallization of such a system is accompanied by the formation of clusters with the hexagonal close packed (hcp) structure; a noticeable number of these clusters are then transformed to the face centered cubic (fcc) phase. A plasma crystal formed after crystallization consists of the metastable hcp phase, fcc clusters, and a small number of clusters with a body centered cubic (bcc) crystal lattice. Beginning with a certain threshold value of the thermostat temperature, the number of fcc/bcc clusters decreases sharply with increasing temperature, which is an important signature of the beginning of the melting of the plasma crystal.     

利用胶体研究晶体的熔化和结晶

晶体的熔化和结晶尽管有上百年的研究历史,但还缺乏基础层面上的理论,实验上则难以看到晶体内部单分子的运动.近二十年来,均匀微米大小的胶体粒子为研究这些问题提供了一个良好的实验平台.通过光学显微镜可以直接观察胶体粒子组成的晶体的表面和内部,研究相变在最初小尺度上的成核过程,并用图像处理得到单个粒子的运动轨迹,为复杂的结晶和熔化过程提供了丰富的微观信息.     

利用胶体研究晶体的熔化和结晶

晶体的熔化和结晶尽管有上百年的研究历史,但还缺乏基础层面上的理论,实验上则难以看到晶体内部单分子的运动。近二十年来,均匀微米大小的胶体粒子为研究这些问题提供了一个良好的实验平台。通过光学显微镜可以直接观察胶体粒子组成的晶体的表面和内部,研究相变在最初小尺度上的成核过程,并用图像处理得到单个粒子的运动轨迹,为复杂的结晶和熔化过程提供了丰富的微观信息。     

On reasons for the hysteresis of melting and crystallization of nanoparticles

The specific features of the EPR spectra of Tm³⁺ impurity ions in synthetic forsterite have been studied by continuous-wave EPR spectroscopy in the frequency range of 270–310 GHz at a temperature of 4.2 K in weak magnetic fields. Narrow resonance signals unrelated to the modulation of the resonance conditions of EPR under the modulation of the external magnetic field have been discovered in measurements at frequencies corresponding to the zero field splitting between the ground and first excited singlet electron states of Tm³⁺ ions in zero magnetic field. The origin of these narrow lines is discussed.     

Correlation between the size-dependent melting and crystallization temperatures of metal nanoparticles

The correlation between the melting and crystallization temperatures of metal nanoparticles is investigated by means of the thermodynamic approach. Size-dependent variations in the melting temperature of aluminum, tin, and copper nanoparticles are calculated with allowance for the corresponding size dependences of surface tensions in solid and liquid phases and interfacial tension. Size-dependent variations in crystallization temperature are determined under the assumption that a certain effective surface layer (skin-layer) arises before melting.     

Peculiarities of melting and crystallization of n-decane in a porous glass

Melting and crystallization of n-decane embedded into porous glass with the mean pore size of about 6.4 nm were studied using acoustic and DSC methods. Smearing of the phase transitions, decrease of melting and freezing temperatures, pronounced hysteresis between melting and crystallization were revealed by both methods. In DSC measurements for the pore filling factors 70% and higher double peaks were observed upon cooling while only single peaks were present upon heating. Also a high reduction of the corresponding phase transition heats was revealed. Melting and freezing intervals determined by acoustic and DSC methods strongly differed from each other. A model which qualitatively explains the observed anomalies is proposed. It supposes the formation of liquid layers on the surface of the pores.     

Diffuse X-ray scattering by ice in the vicinity of the melting point

The X-ray pattern of ice recorded at −10°C reveals, along with the reflexes of a hexagonal phase, intense diffuse X-ray scattering, testifying to the presence of a noncrystalline phase in the sample. Heating of ice to a temperature close to the melting point leads to almost complete decomposition of the crystalline phase. As this takes place, intense diffuse X-ray scattering with a maximum at 2Θ of 23°C appears in the diffraction pattern, which is typical for a metastable amorphous phase. The first maximums of the radial distribution function for the metastable amorphous phase of ice appear to be close in their positions to the first radii of the hexagonal phase coordination spheres.     

低密度Ar熔化及结晶的分子动力学模拟

采用有位移力的ＬｅｎｎａｒｄＪｏｎｅｓ（１２－６）势对微正则系综下低密度Ａｒ系统（简约数密度为ρ＝０．８５）的一级相变过程进行了细致的分子动力学模拟,发现Ａｒ系统的熔化过程是原子的崩塌过程,结晶过程和理想的完整晶体不同,是一活化过程：形核长大过程随温度的降低进行,原来均匀分布在系统中的自由体积呈集中分布,由此系统达到了更稳定的结晶状态。     

Size effects in the melting and crystallization temperatures of copper chloride nanocrystals in glass

The size dependence of the crystallization temperature T _c and melting temperature T _m of copper chloride nanocrystals in glasses is studied in the size range R _a from 1 to 30 nm. Jumps are found in the temperature dependence of T _c at R _a =13 nm and R _a =1.8 nm, a jump is found in the temperature dependence of T _m at R _a =2.2 nm, and the supercooling of the melt is found to vanish in the case of crystallization at R _a ⩽1.4 nm. It is conjectured that the jump in T _c with large R _a occurs for a nano-crystal radius equal to that of a crystalline nucleus in the melt, and the jumps in T _c and T _m for smaller values of R _a are due to a change in the shape (faceting) of the nanocrystals. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 7, 481–486 (10 October 1997)     

Temperature dependence of the kinetics rate of the melting and crystallization of aluminum

Using the molecular dynamics method, heterogeneous melting–crystallization of aluminum is simulated under conditions when the phase front propagates over the overheated-overcooled phase. The dependence of the phase front velocity on the temperature deviation from the equilibrium melting temperature is determined. The dependence obtained from atomistic simulation is used as an approximating function to obtain the temperature dependence of the kinetic rate in the analytical form. The steady-state temperature dependence of the kinetic rate v(T _s? ) for extreme values of aluminum overheating-overcooling is constructed for the first time.