Abstract

The distribution of sodium and diffusion mechanism in sodium-silicate melt with various compositions are investigated via molecular dynamics simulation. The microstructure and dynamical characteristic have been studied with the help of Voronoi polyhedron, simplex and Si-O subnet, oxygen-cluster. The simulation results reveal that Na atoms tend to be in the O polyhedrons and not in Si ones. Moreover, the Na atoms are mainly located in non-bridging oxygen (NBO) and free oxygen (FO) polyhedrons. The Voronoi volume of bridging oxygen (BO) or NBO weakly depends on the number of Na located in polyhedron which indicates the strong Si-O bond compared to Na-O bond. The structure of sodium silicate melt consists of two separate regions: the Na-poor regions of Si-BO subnets and a Na-rich region of NBO-FO cluster. The density of sodium in the NBO-FO cluster is by 3–6 times larger than the one of Si-O subnets. This largest NBO-FO cluster represents a diffusion channel for sodium in sodium-silicate. Furthermore, we find that each BO polyhedron contains 0 or 1 Na atom meanwhile each NBO polyhedron contains 1 or 2 Na atoms. Thereby, the BO and NBO polyhedron contains 1 and 2 sites, respectively. The energy for Na atom located in BO site is larger than that in NBO site. The transition energy for Na atom moving from present site to neighboring NBO site is smaller than that to neighboring BO site. So, Na atoms move frequently to neighboring polyhedrons of NBO, and rarely to the BO ones.

Graphical abstract

中文翻译：

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硅酸钠成分的扩散机理及扩散依赖性

摘要

通过分子动力学模拟研究了各种组成的硅酸钠熔体中钠的分布和扩散机理。借助Voronoi多面体，单纯形和Si-O子网，氧簇对微结构和动力学特性进行了研究。仿真结果表明，Na原子倾向于存在于O多面体中，而不存在于Si中。此外，Na原子主要位于非桥接氧（NBO）和游离氧（FO）多面体中。桥接氧（BO）或NBO的Voronoi体积微弱地取决于位于多面体中的Na的数量，这表明与Na-O键相比，Si-O键很强。硅酸钠熔体的结构由两个单独的区域组成：Si-BO子网的Na贫乏区域和NBO-FO簇的Na富集区域。NBO-FO簇中钠的密度是Si-O子网之一的3-6倍。最大的NBO-FO簇代表钠在硅酸钠中的扩散通道。此外，我们发现每个BO多面体均包含0或1个Na原子，而每个NBO多面体均包含1个或2个Na原子。因此，BO和NBO多面体分别包含1和2个位点。位于BO位置的Na原子的能量要大于NBO位置的Na原子的能量。Na原子从当前位置向邻近的NBO位置移动的跃迁能小于向邻近的BO位置的跃迁能量。因此，Na原子经常移至NBO的相邻多面体，而很少移至BO的多面体。我们发现每个BO多面体包含0或1个Na原子，而每个NBO多面体包含1或2个Na原子。因此，BO和NBO多面体分别包含1和2个位点。位于BO位置的Na原子的能量要大于NBO位置的Na原子的能量。Na原子从当前位置向邻近的NBO位置移动的跃迁能小于向邻近的BO位置的跃迁能量。因此，Na原子经常移至NBO的相邻多面体，而很少移至BO的多面体。我们发现每个BO多面体包含0或1个Na原子，而每个NBO多面体包含1或2个Na原子。因此，BO和NBO多面体分别包含1和2个位点。位于BO位置的Na原子的能量要大于NBO位置的Na原子的能量。Na原子从当前位置向邻近的NBO位置移动的跃迁能小于向邻近的BO位置的跃迁能量。因此，Na原子经常移至NBO的相邻多面体，而很少移至BO的多面体。

图形概要