分子动力学模拟钠硼硅酸盐玻璃电子辐照诱导的结构演化效应

钠硼硅酸盐玻璃作为高放射废物玻璃固化体的候选材料之一,已有大量实验对该类玻璃开展了电子或重离子的辐照效应研究.然而,在理论计算与模拟方面的工作却很少,目前主要集中于重离子的辐照效应,对电子的辐照效应的模拟尚未见报道.本文利用分子动力学工具提出一种新的方法,以实现对电子辐照诱导的玻璃结构演化进行模拟.该方法基于实验中玻璃的结构变化特点,即实验中的拉曼结果已经证实:在大剂量的电子辐照后的玻璃中存在分子氧的事实,由于这些分子氧不会与其他粒子发生相互作用,因而可以通过从体系中逐步地移除一定数量氧原子的方式,以达到模拟大剂量电子辐照的情形,进而得到电子辐照后的玻璃的结构信息.模拟结果显示:随着移除氧原子的数量增加,玻璃中的Si—O—Si平均键角逐渐减小;而且玻璃中的小环数量会因氧的逐渐减少而逐渐增加;玻璃中部分BO4]结构会转变为BO3]结构,最终这种转变会达到饱和;大量移除氧之后,玻璃中的钠元素也出现明显的相分离.这些模拟辐照的玻璃结构特性能较好地与实验中的硼硅酸盐玻璃电子辐照诱导的结构变化符合.因此,本文提出的方法有望为通过分子动力学模拟硼硅酸盐玻璃的电子辐照效应提供新思路.

Structure evolution of electron irradiated borosilicate glass simulated by molecular dynamics

Sodium borosilicate (NBS) glass is one of the candidate materials for high-level waste glass immobilization. A large number of experiments are performed to study the effect of irradiation by electrons or heavy ions on this type of glass. However, only a few researches of numerically investigating the effect of irradiated NBS glass have been reported. Furthermore those studies mainly focus on heavy-ion irradiation, and none of them is devoted to simulating the effects of electron irradiation on glass that has been irradiated by electrons, especially for structure evolution. In this paper, we propose a novel method of using molecular dynamics (MD) to simulate structure evolution of electron-irradiated NBS glass with compositions of 67.73% SiO₂, 18.04% B₂O₃ and 14.23% Na₂O, in mol.%. This method is based on the previous experimental results of Raman spectra and mechanism of structure transformation in irradiated glass. The Raman spectra confirm that the peak indicating the existence of molecular oxygen appears at 1550 cm^-1 in irradiated glass. It is assumed that those oxygen atoms do not have any interactions with other adjacent atoms nor participate in the glass network recombination. This assumption is reasonable, for molecular oxygen mainly exists as dissolved oxygen instead of oxygen bubble and is located at interstice of glass network. Thus the presence of molecular oxygen does not have any effect on glass network structure. Then irradiated glass can be obtained by gradually randomly removing a certain number of oxygen atoms from the pristine glass. The glass with removed oxygen atoms is regarded as an irradiated glass which is considered as one irradiated by electrons in experiments. The results derived from MD simulation include average Si—O—Si bond angle, ring size distribution, sodium profile, evolution of BO₄] units, and BO₃] units. With the increase of removed oxygen atoms, the average bond angle of Si—O—Si decreases and the number of small rings gradually increases in irradiated glass. Besides, sodium phase separation is observed obviously after extensively removing oxygen. Moreover, in the process of removing oxygen, some BO₄] units transform into BO₃] units, and the transformation process reaches a saturation state finally. Those effects derived from MD such as decrease of Si—O—Si bond angle, increase of small rings in number, phase separation of sodium and structure change between BO₄] units and BO₃] units, are consistent with those of glass irradiated by electrons in previous experiments. Therefore, the method proposed in this paper will provide a new perspective to understand the mechanism of structure evolution in sodium borosilicate glass after being irradiated by electrons.