Molecular dynamics study on superheating of Pd at high heating rates

Molecular dynamics simulations are employed here to study the melting and superheating behaviors of bulk Palladium at high heating rates. Quantum Sutton-Chen many body potential is used for these simulations. Being heated, the superheating and melting behavior is found to be strongly affected by the heating rate, and heating rate induced randomization during non-equilibrium heating processes is found to be the main driving force for phase transformation, and it eliminates the energy barrier for nucleation. Not only Pd crystals but also Pd crystals with defects are studied. And the upper limit of heating rate induced superheating is determined to be around 2100?K.     

Melting curve of NaCl0.5Br0.5 up to 4.5 GPa

Abstract

The melting curve of NaCl_0.5Br_0.5 has been measured under pressure up to 4.5 GPa. The melting temperatures of Ag and NaCl have been used to determine the pressure in the sample at its melting temperature.     

Melting Curve of Iron: The Never-Ending Story?

For the better general understanding of melting behavior at high pressure, we investigated the influence of both crystallographic and electronic structure, and compressibility on melting temperatures for a large class of materials. In particular, we have established a large data base for melting of transition metals to megabar pressures. In general, bcc metals ( e.g. W, Ta, Mo, V, Cr) have very flat melting curves, and the initially steeper melting curves of fcc metals (Fe, Co, Ni) flatten significantly at high pressure. We also observed this trend for the more complicated alkaline earth, and rare earth metals. The flattening of the melting curves is due to the similarity of the solid and liquid structures and volumes. For iron there may exist an additional complication which may explain the reported results on both melting temperatures and structures. Due to the similarity in the free energies of its high pressure structures, these may coexist over a large pressure range. This phenomenon has been recently documented for noble gases with similar structures.     

Melting curve of copper measured to 16 GPa using a multi-anvil press

The melting temperature, T _m, of copper has been determined from ambient pressure to 16 GPa using multi-anvil techniques. The melting curve obtained (T _m=1355(5)+44.5(31)P?0.61(21)P ², with T _m in Kelvin and P in GPa) is in good agreement with both the previous experimental studies and with recent ab initio calculations.     

β-MODIFICATION OF ISOTACTIC POLYPROPYLENE: PREPARATION,STRUCTURE, PROCESSING,PROPERTIES, AND APPLICATION

The methods of preparation and formation of supermolecular structures in quiescent and sheared melts and the properties of the β-modification of isotactic polypropylene (β-iPP) are reviewed. The introduction of selective β-nucleants is the most reliable method for preparation of samples rich in β-modification or of pure β-iPP. The advantages and drawbacks of the known β-nucleating agents are summarized. It is emphasized that pure β-iPP can be prepared under laboratory and processing conditions in the presence of highly active and selective β-nucleants. Nevertheless, there are no literature data—apart from that of the author's groups—which evidenced unambiguously the formation of pure β-iPP. It hints at the insufficient selectivity of β-nucleants used or at the inappropriate crystallization or melting conditions applied by other scientists. The structure formation during the high-temperature hedritic crystallization is discussed comprehensively and illustrated by polarized light microscopy and scanning electron microscopy micrographs. Some specific features of β-iPP, namely the high- and low-temperature growth transition, the restricted temperature range of the formation of pure β-iPP, and the unique melting and recrystallization characteristics (melting and annealing memory effect) are summarized. It was emphasized that impact strength and toughness of β-iPP markedly exceed those of α-iPP. Processing of β-nucleated iPP and application of β-nucleated iPP is described briefly.     

Melting transitions in metallic elements correlated with shear modulus and atomic volume

We show that two quite recent treatments of dislocation-mediated melting transitions result in the thermal energy associated with the melting temperature, T _m, being expressed as a product of a volume factor and a combination of elastic constants times a lattice structure-dependent factor. We further show that the result for the latent heat of fusion L _m obtained in one of these studies leads to the ratio L _m/GΩ, where G is the shear modulus at melting and Ω the atomic volume, being a constant. Since the ratio of the vacancy formation energy to GΩ is also found to be roughly constant, we suggest that the factor GΩ at melting is crucial in determining the melting temperature, the latent heat of fusion and the vacancy formation energy and we comment on the reasons why this should be so.     

Molecular dynamic study of the melting temperature in MgF2 with the fluorite structure

The melting temperature of MgF₂ with cubic fluorite structure has been calculated by the constant temperature and pressure molecular dynamics (MD) simulation using the well-tested effective pair-wise potentials, which consist of the Coulomb, dispersion, and repulsion interaction by varying temperature from 300 to 2500 K. It is found that the potential parameters for MgF₂ derived from ab initio periodic Hartree-Fock calculations are very successful in reproducing accurately the DFT-GGA combined with quasi-harmonic Debye model calculated volumes of the cubic fluorite-type MgF₂ over a wide range of temperature at room pressure. Our simulated melting temperature of cubic fluorite-type MgF₂ is very close to the actual melting temperature 1539 K. Meanwhile, the radial distribution functions of Mg-Mg, F-F, and Mg-F ion pairs near the melting temperature are investigated from the isobaric and isothermal ensemble.     

Conversion of batch to molten glass, II: Dissolution of quartz particles

Quartz dissolution during the batch-to-glass conversion influences the melt viscosity and ultimately the temperature at which the glass forms. Batches to make a high-alumina borosilicate glass (formulated for the vitrification of nuclear waste) were heated at 5 K min^− 1 and quenched from temperatures of 400 to 1200 °C at 100 K intervals. The batches contained quartz as a silica source, with particles ranging from 5 to 195 μm in diameter. The content of unreacted quartz in the samples was determined with X-ray diffraction. Most of the fine quartz dissolved during the early batch reactions (at temperatures < 800 °C), whereas coarser quartz dissolved mostly in a continuous glass phase via diffusion. The mass-transfer coefficients were assessed from the data as functions of the initial particle sizes and the temperature. A series of batches were also tested that contained nitrated components and additions of sucrose, known to accelerate melting. While sucrose addition had no discernible impact on quartz dissolution, nitrate batches melted somewhat more slowly than batches containing carbonates and hydroxides in addition to nitrates.     

Sol-gel synthesis of a new composition of bioactive glass in the quaternary system SiO2-CaO-Na2O-P2O5: Comparison with melting method

New sol-gel experimental conditions were tested to prepare a new SiO₂-based bioactive glass with high Na₂O content. The aim of this work is to investigate the real influence of the synthesis route (sol-gel versus melting) on the glass intrinsic properties and then, later, on the glass behavior and particularly on bioactivity. The obtained glass and its melt derived counterpart were characterized from structural and morphological (porosity, specific surface area) point of view. It could be noticed that the synthesis mode has no significant influence on glass structure. Conversely, the synthesis mode greatly influences the glass texture. The sol-gel derived glass exhibits a greatly higher specific surface area and pore volume than melt derived glass. This parameter may be a key factor of glass bioactivity.