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

Pressure dependence of viscosity and self-diffusion in CaAl_2Si_2O_8 melt: A molecular dynamics study

用分子动力学方法,研究了1999 K下,压力由23 MPa上升到15183 MPa的过程中,CaAl_2Si_2O_8成分熔体的微观结构、剪切粘滞度和粒子自扩散系数的压力效应。在此基础上,探讨压力对剪切粘滞度与粒子自扩散系数之间关系的影响,并将它同微观结构的变化联系起来。结果表明,粒子自扩散系数的压力效应与熔体结构有很强的相关性;压力的挤压效应阻碍了粒子的扩散,而Si-O和Al-O 5次配位体的形成又加速了扩散过程,两种相反的作用相互抵消,造成的结果是在0～5 GPa范围内,Si^(4+),O^(2-)和Al^(3+)等网架形成粒子的自扩散系数随压力变化不明显;当压力继续增大时,挤压效应占了主导,导致自扩散系数值快速减小。Ca^(2+)作为网架修饰粒子,自扩散系数随压力升高单调下降。压力小于5 GPa时,粒子自扩散系数的大小关系是:D_(Ca)>D_(Al)>D_O>D_(Si)。系统粘滞度随压力的变化与熔体中BO的含量密切相关:BO含量小于域值时,一定范围内BO含量的变化不会对粘滞度产生很大的影响,超过域值,BO含量的微小增加会导致粘滞度值迅速增大。有效应用Eyring方程的关键是方程中粒子跳跃距离的确定,本研究发现,Si^(4+)和O^(2-)的跳跃距离可以通过系统中非桥氧的百分含量来获得。这一发现使得我们能够利用系统中NBO的含量。

Molecular dynamics simulations were used to study the pressure dependence of structure, self-diffusion and viscosity in CaAl2Si2O8 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 Si4+ and Al3+ promotes diffusion while the density increase hinders diffusion. The two contradicting effects result in the self-diffusions of Si4+, O2- and Al3+ 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 Ca2+, 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 Si4+ and O2- 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.