The tip temperatures of succinonitrile dendrites growing in space and on earth

The theoretically estimated tip temperatures of “freely” growing, high purity succinonitrile dendrites, based on the widely used Ivantsov paraboloidal approximation, are shown to exceed the known equilibrium melting point of the material. It is suggested that these discrepancies are due to side-branching activity, and that future experimental work be devoted to an accurate determination of the tip temperatures, in a material with an known amount of impurity, or solute.     

A Lennard—Jones embedded—atom potential and its application to the study of melting

The nearest-neighbour Lennard-Jones potential from the embedded-atom method is extended to a form that includes more than nearest neighbours.The model has been applied to study melting with molecular dynamics.The calculated melting point,fractional volume change on melting,heat of fusion and linear coefficients of thermal expansion are in good agreement with experimental data.We have found that the second and third neighbours influence the melting point distinctly.     

Theoretical study of a melting curve for tin

The melting curve of Sn has been calculated using the dislocation-mediated melting model with the `zone-linking method'. The results are in good agreement with the experimental data. According to our calculation, the melting temperature of γ -Sn at zero pressure is about 436~K obtained by the extrapolation of the method from the triple point of Sn. The results show that this calculation method is better than other theoretical methods for predicting the melting curve of polymorphic material Sn.     

一种新的熔化理论——位错运动的量子化模型

 本文提出了一种新的熔化理论，从位错运动的量子化模型出发导出了林德曼熔化定律，得到了与实验结果相符的熔点计算值。     

Generalized bond-energy model for cohesive energy of small metallic particles

A generalized bond-energy model has been developed to calculate the cohesive energy of nanoparticles by considering the different contributions of face-, edge- and corner-atoms. The model is adapted for metallic particles in a large size range from several atoms to infinity, studying their morphology, phase stability and melting point, etc.     

Die elastischen Moduln von Eis-Einkristallen

The velocity of sound has been measured in ice with temperatures from the melting point to ?140°C, using two different supersonic pulse methods. The complete set of the five elastic moduli has been determined from the measurements for the above mentioned temperature range. To explain various results for piezoelectricity, density and elastic moduli a model of an aging process is discussed.     

Mean square displacements of atoms in cubic metals at melting point

The ratio of the mean square displacement of thermal vibration and the square of interatomic separation at the melting point has been calculated for several cubic metals using the frequency distribution derived from Bhatia's three-force constant model. It is found that this ratio has roughly the same value for metals of certain groups of the periodic table, but exhibits wide variation from one group to another. Lindemann's melting law is shown to be inadequate.We are grateful to the Council of Scientific and Industrial Research and to the University Grants Commission for financial assistance.     

Combined effect of nanochitosan and succinonitrile on structural, mechanical, thermal, and electrochemical properties of plasticized nanocomposite polymer electrolytes (PNCPE) for lithium batteries

A sequence of novel plasticized polymer nanocomposite electrolyte systems based on polyethylene oxide (PEO) as polymer host, LiCF₃SO₃ as salt, and a variety of concentrations of nanochitosan as inert filler, succinonitrile as a solid non-ionic plasticizer has been prepared. The prepared membranes were subjected to X-ray diffraction, FT-IR, tensile strength, morphological studies, thermal analysis, AC ionic conductivity measurement, and interfacial analyses. The combined effect of succinonitrile and nanochitosan on the electrochemical properties of polymer electrolytes has been studied, and it was confirmed that the ionic conductivity is significantly increased. The maximum ionic conductivity of the plasticized nanocomposite polymer electrolytes are found to be in the range of 10^?2.8?S/cm. Besides, the interfacial stability also shows a significant improvement. The tensile measurement and thermal analysis results illustrate that the electrolytes based on that polymer host possess good mechanical and thermal stabilities.     

The melting behaviors of the Nb(1 1 0) nanofilm: a molecular dynamics study

By using molecular dynamics (MD) and the modified analytic embedded atom method (MAEAM), we have studied the melting point, the melting mechanism and the correspondingly dynamical behaviors of a Nb(1 1 0) nanofilm. Firstly, in accordance to the MD time dependence of the potential energy, the melting point of this nanofilm has been roughly estimated. Then, the melting mechanism of the nanofilm have been analyzed in detail with the application of the structure factor. The results clearly indicate that the melting transition of the 8th, 9th, and 10th atomic layer of the nanofilm has been characterized by the exponential, polynomial and linear sequence respectively when the thickness of the quasiliquid film attains to about 1.3 nm. Thirdly, the dynamical behaviors of the nanofilm melting, such as the melting front propagation velocity and the kinetic coefficient, which have also been analyzed, demonstrate that the melting front propagation velocity has linearly increased with the incremental temperature and the evaluated kinetic coefficient has approximately equaled 1.43m/(sK). Finally, by extrapolating the melting front propagation velocity to zero, we can accurately deduce the melting point of the Nb(1 1 0) nanofilm to 2568.3 K, which is much lower than the counterpart (2740 K) of the bulk niobium.     

Second law optimization of a latent heat storage system with PCMS having different melting points

The exergy efficiency, as well as the charging and discharging rates, in a latent heat storage system can be improved by use of the PCMs having different melting points. The melting point distribution of the PCMs has substantial effects on the exergy efficiency. The optimum melting point distribution of the PCMs has been estimated from numerical simulations and also from simple equations. The fast charging or discharging rate leads to high exergy efficiency.     

Hall coefficient and electrical resistivity of liquid lithium

The Hall coefficient and the electrical resistivity of Li have been measured below and above the melting point. Whereas the electrical resistivity changes by a factor of two at the melting point, the Hall coefficient shows only a rather small change. In the liquid state close to the melting point the experimental Hall coefficient (?13.6 ± 0.6 × 10^?11m³/As) is in good agreement with the free-electron model assuming one conduction electron per atom. We discuss this result in the light of recent theoretical developments.     

The effect of van der Waals-type interactions in metals: A pseudopotential model

A model has been developed for the long-range part of the ion-ion interaction of simple metallic many-particle systems. The investigation is based on the local pseudopotential theory. The essential point in the examination is the consideration of interactions of van der Waals-type in the determination of the direct ion-ion potential and the ion-electron pseudopotential. Numerical calculations performed for liquid rubidium near the melting point show that these interactions are distinctly reflected in the long-range part of the ion-ion potential. The reference potential used in the study had been calculated without parameters by means of the experimentally determined pair correlation function.     

New theory of undercooling during rapid solidification: application to pulsed laser heated silicon

A new theory of undercooling has been developed based on the Fickian form of heat flux expressed in terms of a gradient in enthalpy density. It is shown that heat diffusion is more fundamental than Fourier’s law of heat conduction. During the solid–liquid phase change, diffusion requires continuity of the enthalpy density across the phase-change boundary, which has two important consequences. First, the calculated melt-depth during rapid heating is reduced, and second, during cooling the rate of loss of heat is independent of both the rate of release of latent heat and the temperature of the interphase layer. The cause of undercooling is diffusive heat loss without recrystallisation, which simply causes the temperature of the liquid to drop. Both the nature of melting and classical nucleation have been examined and shown to support diffusion over conduction. In particular, it is established that the literature supports a model of melting in which phase fluctuations at the melting point occur, and that these phase fluctuations must inevitably lead to thermal transport at uniform temperature. Hitherto unrecognized inconsistencies in classical nucleation theory also support the diffusive framework. The model is applied to pulsed laser melting and amorphisation of (111) silicon. PACS  64.70.Dv; 66.30.Xj; 68.18.Jk     

Szintillationslichtausbeute und ihre Richtungsabhängigkeit als Funktion der Temperatur von Anthrazen-, trans-Stilben- und para-Terphenyl-Kristallen bei Beschuß mitα-Strahlen

The scintillation light yield of thin trans-stilbene, para-terphenyl and anthracene crystals and an organic plastic scintillator bombarded with α-particles of ThB has been measured as a function of temperature betweent=20° C and the melting point. The light yield of all the specimens investigated has been found to be reduced with increasing temperature. In all cases at the melting point the light yield will be zero. The scintillation anisotropy of the organic crystal scintillators also has been investigated over the range of temperature mentioned above, and there it is independent of temperature and retains the value measured at room temperature.     

Atomic mechanism of homogeneous melting of bcc Fe at the limit of superheating

Atomic mechanism of homogeneous melting of bcc Fe is studied via monitoring spatiotemporal arrangements of the liquid-like atoms, which are detected by the Lindemann criterion of melting, during the heating process. Calculations are performed by molecular dynamics (MD) simulations. Calculations show that liquid-like atoms occur randomly in the crystalline matrix at temperature far below the melting point due to local instability of the crystalline lattice. Number of liquid-like atoms increases with increasing temperature and they have a tendency to form clusters. Subsequently, a single percolated liquid-like cluster is formed in the crystalline model and at the melting point 99% atoms in the model become liquid-like to form a liquid phase. Melting is also accompanied by the sudden changes in various static and thermodynamic quantities. However, total melting is reached just at the point above the melting one. Three characteristic temperatures of the homogeneous melting of bcc Fe are determined.     

Dynamics of ultrafine particles embedded in a crystal of organic molecules

The dynamics of ultrafine particles embedded in crystalline hexadecane and decaline has been studied. At temperatures slightly below the melting point of the decaline very broad components appear in the Mössbauer spectrum. This is interpreted to be due to Brownian oscillations. In the hexadecane-based sample such broad components are not observed. In both cases the absorption area of the Mössbauer spectrum decreases rapidly when the melting point is approached.     

Molecular dynamics simulations on the melting of gold nanoparticles

Molecular dynamics is employed to study the melting of bulk gold and gold nanoparticles. PCFF, Sutton-Chen and COMPASS force fields are adopted to study the melting point of bulk gold and we find out that the Sutton-Chen force field is the most accurate model in predicting the melting point of bulk gold. Consequently, the Sutton-Chen force field is applied to study the melting points of spherical gold nanoparticles with different diameters. Variations of diffusion coefficient, potential energy and translational order parameter with temperature are analyzed. The simulated melting points of gold nanoparticles are between 615～1115 K, which are much lower than that of bulk gold (1336 K). As the diameter of gold nanoparticle drops, the melting point also descends. The melting mechanism is also analyzed for gold nanoparticles.     

Growth dynamics and thermal stability of Ni nanocrystalline nanowires

The growth dynamics and the thermal stability of a Ni nanocrystalline nanowire (NNW) model system fabricated using electrochemical deposition has been investigated using X-ray diffraction, scanning and transition electron microcopy, and differential scanning calorimetry (DSC). It has been found that the thermal stability of the Ni-NNW is dominated by the microstructure movement and the grain boundary rotation mechanism at temperature ranging from 400 to 600 °C. The Ni-NNW experiences the Rayleigh instability at temperature approaching the melting point. The observed fragment separation in the Rayleigh phase-transition is much greater than that expected theoretically.     

区域熔化点掺杂的数学分析

本文对区域熔化点掺杂过程进行了详细的数学分析。得到了点掺杂区熔后杂质沿锭长分布的一系列普遍公式。在本文研究的范围内,利用这些公式,不难写出任意次区熔后锭条上的杂质分布,而不受区熔次数和掺杂点数目之限制。作为一个特例,具体计算了k=0.35,四点掺杂,三次区熔后杂质沿锭长的分布情况,得到:当各掺杂点之位置与掺杂量选取适当时,在总锭长为21.5个熔区长度的料锭中,其16个熔区长度范围内的杂质浓度相对起伏可小于±3.0％。     

纳米团簇熔化过程的分子动力学模拟

本文采用分子动力学结合嵌入原子多体势,模拟了不同半径的Ni纳米团簇的升温熔化过程,研究团簇尺寸对熔点和表面能的影响.模拟结果表明:团簇的熔点显著低于体材料的熔点.团簇熔化的过程首先是在团簇的表面出现预熔,然后向团簇内部扩展,直到整个团簇完全熔为液态.在模拟的纳米尺度范围内,团簇的熔点与团簇尺寸基本成线性关系.团簇的表面能随着团簇尺寸的增大而减小,而且表面能均高于体材料的表面能.