Crystal Growth From Ionic Melts Studied by Molecular Dynamics Simulation

通过分子动力学模拟研究离子熔体的晶体生长

基本信息

批准号：
11640589
负责人：
OKADA Isao
金额：
$ 1.92万
依托单位：
Sophia University
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (C)
财政年份：
1999
资助国家：
日本
起止时间：
1999 至 2001
项目状态：
已结题

项目摘要

Molecular dynamics simulation has been perfonned to study crystal growth from ionic melts. The periodic boundary condition was imposed in the x-, y- directions while in the z-direction the top of the cell was open to a vacuum. For NaC1, crystal growth on (100), (110) and (111) planes has been studied. After melting the ions at 6-14 layers, crystal growth was started by keeping all the ions at a given temperature except for the bottom layer. The crystal grew in the [100] direction not only in the (100) system but also in the (110) system. Quite a similar tendency has been found also for MgO. The growth rate as a function of temperature is found to have a maximum.For CaC1_2 having a distorted rutile structure crystal growth has been studied on (001), (100) and (110) planes. In this case, crystal grew only on the (001) plane within an MD time of about 1 ns, presumably because charge neutrality is maintained on this plane. However, its growth rate was slower than that of NaCl-type by a factor of 100.On a molecular level, it seems that crystal grows in a small charge neutral unit such as a cationanion pair. In the case of NaC1-type, cation-anion pair can become such unit. On the other hand, in CaCl_2, one Ca^<2+> plus two C1^<-'>s are needed to fonn such a unit, in which a defect can be easily generated. The defects will, in turn, retard further crystal growth, and therefore crystal growth is slow. For CaSiO_3 having the perovskite structure, two kinds of cations compete the position of one layer for crystallization. Further, since the viscosity is high the growth rate is very slow even in the (001) plane.A video film (ca. 52 min. for the running time) has also been made on crystallization from MgO melt.

分子动力学模拟已得到经过精心研究，以研究离子熔体的晶体生长。周期性边界条件施加在x，y方向上，而在z方向上，细胞的顶部向真空开放。对于NaC1，已经研究了（100），（110）和（111）平面的晶体生长。在6-14层融化离子后，通过将所有离子保持在给定温度以外的温度来开始晶体生长。晶体不仅在（100）系统中，而且在（110）系统中生长。对于MGO也发现了相似的趋势。发现生长速率作为温度的函数最大。对于CAC1_2，在（001），（100）和（110）平面上研究了金红石结构扭曲的结构晶体生长。在这种情况下，晶体仅在大约1 ns的MD时间内在（001）平面上生长，这大概是因为电荷中立性保持在该平面上。但是，其生长速率比NaCl型的生长速率慢100倍。在分子水平上，晶体似乎在小电荷中性单元（例如阳离子对）中生长。在NAC1型的情况下，阳离子 - 阳离子对可以成为这样的单位。另一方面，在cacl_2中，需要一个ca^<2+>加上两个C1^<--'> S来fonn这样的单位，其中很容易生成缺陷。这些缺陷反过来会延迟进一步的晶体生长，因此晶体生长很慢。对于具有钙钛矿结构的casio_3，两种阳离子竞争一层的位置进行结晶。此外，由于粘度很高，即使在（001）平面中，增长率也非常慢。视频膜（约52分钟，运行时间）也是通过MGO熔体结晶而制作的。