水对无定形SiO_2拉伸特性影响的反应分子动力学模拟

Influence of water on the tensile properties of amorphous silica: a reactive molecular dynamics simulation

潮湿对SiO2的强度有重要影响.采用反应场分子动力学模拟方法,研究液态水对无定形SiO2(a-SiO2)准静态拉伸特性的影响.准静态拉伸模拟的结果表明,在干燥条件下,a-SiO2的拉伸强度为9.4 GPa,而在含液态水时则下降为4.7 GPa,表明液态水使得a-SiO2拉伸强度发生显著下降.根据应力-应变曲线分析可知,干燥条件下a-SiO2结构的刚度随着拉伸应变的增加保持稳定,而含液态水的a-SiO2刚度随着拉伸应变的增加而逐步降低,并且应变为16%—20%时的应力-应变曲线类似于金属的屈服现象.通过对拉伸过程的原子图像分析可知,含液态水a-SiO2的拉伸过程并没有发生塑性变形,而是因为应力增大加速了水解反应,使得应力-应变曲线表现出上述塑性现象.

Humidity has an important influence on the strength of the silica（SiO2）. We examine the influence of liquid water on the tensile properties of amorphous silica（a-SiO2） using reactive molecular dynamics simulation. The results of the quasi-static tension show that liquid water reduces the tensile strength of a-SiO2 significantly. The tensile strength of dry a-SiO2 is 9.4 GPa while the tensile strength of a-SiO2 in the presence of liquid water is only 4.7 GPa. The strain-stress curve of dry a-SiO2 indicates that the stiffness of the a-SiO2 structure becomes stable with the increase of strain. On the other hand, the stiffness of the a-SiO2 with liquid water is gradually reduced with the increase of tensile strain. Moreover, the strain-stress curve of a-SiO2 in a strain range of 16% to 20% in the presence of liquid water is similar to the yielding phenomenon of plastic metal. The snapshots of the a-SiO2 in the presence of liquid water during the tension show that no plastic deformation is observed. We propose that the stress-enhanced hydrolysis releases part of the stress for the rupture of the Si—O bonds, so that the stiffness of the a-SiO2 in the presence of liquid water decreases with the increase of strain.