Donghai Sun

The cooling rate dependence of crystallization for liquid copper: A molecular dynamics study

The constant-pressure molecular dynamics simulations based on the second-moment approximation of tight-binding scheme have been performed to study the relationship between the resulting crystallization microstructure of the liquid copper and the cooling rate. Below the glass-forming critical cooling rate, the metastable hcp phase and the stable fcc phase can coexist in the resulting configuration with all sorts of proportion and various forms such as layering and phase separation. The sizes and the distributions of the two crystalline phases depend on the cooling rate: the faster the cooling rate the larger percents of the metastable hcp phase and the more easily the layering take place. From the split of peaks of the angular distribution function, for both hcp and fcc phase the faster the cooling rate is the more imperfect they are, which may be considered as the precursor of glass-forming.

Solvatochromic behavior of phenol blue in CO2+ethanol and CO2+n-pentane mixtures in the critical region and local composition enhancement

The UV-Vis spectra of probe phenol blue in CO(2)+ethanol and CO(2)+n-pentane binary mixtures were studied at 308.15 K and different pressures. The experiments were conducted in both supercritical region and subcritical region of the mixtures by changing the compositions of the mixed solvents. On the basis of the experimental results the local compositions of the solvents about phenol blue were estimated by neglecting the size difference of CO(2) and the cosolvents. Then the local composition data were corrected by a method proposed in this work, which is mainly based on Lennard-Jones sphere model. It was demonstrated that the local mole fraction of the cosolvents is higher than that in the bulk solution at all the experimental conditions. In the near critical region of the mixed solvents the local composition enhancement, defined as the ratio of cosolvent mole fraction about the solute to that in the bulk solution, increased significantly as pressure approached the phase boundary from high pressure. The local composition enhancement was not considerable as pressure was much higher than the critical pressure. In addition, in subcritical region the degree of composition enhancement was much smaller and was not sensitive to pressure in the entire pressure range as the concentration of the cosolvents in the mixed solvents was much higher than the concentration at the critical point of the mixtures.