理想拉伸/剪切应变对U_(3)Si_(2)化学键键长及电荷密度分布影响的第一性原理研究

First-principles study of effect of ideal tensile/shear strain on chemical bond length and charge density distribution of U_(3)Si_(2)

2011年福岛核事故之后,U_(3)Si_(2)作为可代替UO_(2)的核燃料被预测为重要的耐事故燃料.近年来的研究结果表明,U_(3)Si_(2)作为耐事故燃料的候选材料,其在微观尺度上进行的模拟还不够深入.在宏观尺度上不足以建立燃料数据库和模型来有效预测U_(3)Si_(2)的一些性能.因此,采用第一性原理计算U_(3)Si_(2)核燃料的一些物理化学数据受到了广泛关注.在之前的工作中,我们采用第一性原理计算拉伸/剪切实验(FPCTT/FPCST)的方法预测了U_(3)Si_(2)在几个低指数晶面/晶向上的理想强度.然而,并未对U_(3)Si_(2)的断裂行为进行过多的解释.因此,本论文通过论述理想拉伸/剪切应变对U_(3)Si_(2)化学键键长及电荷密度分布影响,分析了U_(3)Si_(2)在这几个低指数晶面/晶向上的断裂行为.结果表明:U_(3)Si_(2)在理想拉伸应变的作用下,晶体的破坏主要受化学键变化的影响,而在剪切应变的作用下应变能或应力的突然下降,可能与U_(3)Si_(2)的应变诱导结构相变有关.

After the Fukushima nuclear accident in 2011,U_(3)Si_(2) was predicted to be an important accident tolerant fuel that can replace UO_(2).The results of recent studies have shown that the simulation at the micro-scale of U_(3)Si_(2) serving as a candidate for accident tolerant fuel is not deep enough.It is not sufficient to build fuel databases and models at a macro-scale to effectively predict some properties of U_(3)Si_(2).Therefore,employing the first principles to calculate some physicochemical data of U_(3)Si_(2) nuclear fuel has received extensive attention.In previous work,we predicted the ideal strength of U_(3)Si_(2) in several low-index crystal planes/directions by the first-principles computational tensile/shear test(FPCTT/FPCST)approach.However,the fracture behavior of U_(3)Si_(2) has not been explained much.Therefore,in this work,the effects of ideal tensile/shear strain on the chemical bond length and charge density distribution of U_(3)Si_(2) are discussed to analyze the fracture behaviors of U_(3)Si_(2) in these low-index crystal planes/directions.The effect of strain is achieved by using the incremental simulation elements in the specified crystal plane/direction.The crystal structures of U_(3)Si_(2) under different strains are optimized by using the first principles based on density functional theory.The variation ranges of chemical bond length and the charge density distributions of U_(3)Si_(2) under different ultimate strains are summarized and calculated respectively.The results show that the elongation of the U—U bond is the main contributor to the tensile deformation of U_(3)Si_(2) in the[100]crystal direction under tensile load.The toughness of U_(3)Si_(2) in the[001]crystal direction is mainly due to the elongation of the U—Si bond and U—U bond.However,the tensile deformation produced in the[110]crystal direction of U_(3)Si_(2) is mainly related to the elongation of the Si—Si bond.In the(100)[010]slip system,U_(3)Si_(2) has great deformation and the crystal breaks when the Si—Si bond length reaches a limit of 3.038 Å.For the(001)[100],(110)[1^(-)10]and(001)[110]slip systems of U_(3)Si_(2),the crystal is broken under small shear deformation,and the change of its bond length is not obvious,reflecting that the sudden decrease of the strain energy or stress in these several slip systems may be related to the straininduced structural phase transition of U_(3)Si_(2).