Pressure dependence of viscosity and self-diffusion in CaAl2Si2O8 melt: A molecular dynamics study
钙长石成分熔体粘滞度和自扩散系数压力效应的分子动力学研究

Molecular dynamics simulations were used to study the pressure dependence of structure, self-diffusion and viscosity in CaAl_2Si_2O_8 melt from 23 MPa to 15183 MPa at 1999 K. The change with pressure of the relationship between self-diffusion and shear viscosity is also studied and related to microscopic structure of the melt. There exists strong correlation between self-diffusion and melt structure. With the increase of pressure, the formation of fivefold coordination of Si~(4 ) and Al~(3 ) promotes diffusion while the density increase hinders diffusion. The two contradicting effects result in the self-diffusions of Si~(4 ), O~(2-) and Al~(3 ) vary slowly with pressure at first. Then as pressure exceeds 5 GPa, the effect of density increase surpasses that of the formation of fivefold coordination, so there is a rapid decrease in self-diffusion. The self-diffusion of Ca~(2 ), a network-modifying ion, decouples with other ions and decreases monotonously with the increase of pressure. The self-diffusions of these ions are in the order D_(Ca) > D_(Al) > D_O > D_(Si) at pressures below 5 GPa. Viscosity is closely related to the BO(Bridging Oxygen) concentration in the melt. The variation of the concentration of BO below a threshold value almost can not influence viscosity, while as it exceeds the threshold value, little increase of the concentration of BO could result in dramatic increase of viscosity. The value of jump distance in Eyring equation is the key to apply the equation to silicate melt. We found that the jump distances of Si~(4 ) and O~(2-) can be calculated from the quantity of NBO(Non-Bridging Oxygen) in the melt. This makes it possible to calculate self-diffusion of oxygen and silicon from shear viscosity or vise versa, by measuring the NBO% of the melt. The method transforms derivation of dynamic properties to the measurement of melt static property.