1.State Key Laboratory of Electrical Insulation and Power Equipment, School of Electrical Engineering, Xi’an Jiaotong University, Xi’an 710049, China 2.Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. 11690040, 11690041), and the State Key Laboratory of Intense Pulsed Radiation Simulation and Effect, Northwest Institute of Nuclear Technology, China (Grant No. SKLIPR1709)
Received Date:06 November 2020
Accepted Date:23 February 2021
Available Online:25 June 2021
Published Online:05 July 2021
Abstract:Owing to its excellent corrosion, radiation and high temperature resistance, ZrO2 has been considered as a strong candidate material for inert fuel for the incineration of actinides. In this paper, a combination of thermal spike model and molecular dynamics is used to simulate the phase transition process of ZrO2 in the nuclear radiation environment. Based on the thermal spike model, two coupled diffusion equations are established with considering the multiple physical process of energy deposition and transmission after the implantation of swift heavy ions into target material. The space-time evolution characteristics of ZrO2 lattice temperature are obtained by solving the coupled diffusion equations numerically. Then the phase transformation of ZrO2 form monoclinic phase to tetragonal phase under the thermal spike is investigated on an atomic scale by means of molecular dynamics. It is found that a cylindrical track with a radius of 7 nm is generated in the center of ZrO2 after the implantation of swift heavy ion with an electronic energy loss of 30 keV·nm–1. The lattice melts immediately in the center of track, accompanied with the coordination number of Zr decreasing from 7 to 4–6. Then at about 2 ps, the melting zone gradually turns cool and recrystallized. And in the center of the melting zone, voids begin to form and are surrounded by a highly disordered amorphous region. Meanwhile, tetragonal phase of ZrO2, whose coordination number of Zr is 8, is formed at the periphery of the amorphous region, which is also confirmed by the XRD calculation results. As energy transfers from track center to the surround, the tetragonal region gradually develops into the whole system, accompanied with the increase of voids size. The simulation results indicate that the irradiation of ZrO2 with swift heavy ions can lead to a transformation from the monoclinic to the tetragonal phase when the deposited electronic energy loss exceeds an effective threshold ~21 keV·nm–1, greater than the experimental value (12 keV·nm–1), which was mainly due to the large difference between the simulated and measured incident ion fluences and the accuracy of the force field used in the molecular dynamics. Keywords:ZrO2/ phase transition/ thermal spike/ molecular dynamics
图3展示了不同位置和不同时刻的晶格温度变化趋势, 入射离子的电子能损为30 keV·nm–1, 为实验值(10—50 keV·nm–1)的中位数. 图 3 晶格温度分布曲线 (a) 不同位置晶格温度随时间演化; (b) 不同时刻晶格温度的径向分布 Figure3. Distributions of lattice temperature: (a) Evolution of lattice temperature at different radial distances; (b) radial distribution of lattice temperature at different times.
由热峰模型得出的晶格温度径向分布, 反映了快速重离子辐照后, 沉积在不同位置处的原子动能. 根据计算结果, 60 fs时径迹内各处温度基本已达最大值, 故取此时刻晶格温度, 标定不同位置处原子动能. 经分子动力学模拟, 得出不同时刻ZrO2微观结构变化如图4所示. 图 4 不同时刻ZrO2原子结构变化. 所有原子沿热峰注入方向投影在(010)平面上 Figure4. Atomic structure of ZrO2 at different times. All the atoms are projected on (010) plane along the injection direction of thermal spike.
由图5可知, 辐照前XRD图谱在28.2°, 31.5°, 34.2°, 35.3°, 49.3°和50.1°出现衍射峰, 分别对应m-ZrO2的($ {\overline{1}} 11$), (111), (002), (200), (022), (220)晶面衍射(JCPDS. No 83-0936), 表明辐照前ZrO2为单斜相. 经快速重离子辐照后, 单斜相特有的特征峰消失, 并且在30.3°, 34.6°, 50.3°, 50.8°, 59.4°, 60.3°出现新的特征峰, 与t-ZrO2的(101), (002), (112), (200), (103), (211)衍射晶面良好对应(JCPDS. No 88-1007), 证明在快速重离子辐照作用下, ZrO2由单斜相转为了四方相. ZrO2晶体中, 单斜相的Zr原子配位数为7, 四方相的Zr原子配位数为8, 因此可根据Zr原子配位数的变化情况研究辐照条件下ZrO2相变过程. 图6展示了不同配位数的Zr原子空间分布随时间演化过程. 图 6 不同时刻不同配位数Zr原子空间分布. 所有原子沿离子入射方向投影在(010)平面上, 颜色不同代表配位数不同 Figure6. Spatial distribution of Zr atoms with different coordination number at various times. All the atoms are projected on (010) plane along the injection direction of thermal spike. Different coordination numbers are distinguished by the atomic color.
为获得辐照诱导ZrO2由单斜相转为四方相的快速重离子的电子能损阈值, 在电子能损为15—30 keV·nm–1范围内进行了多次热峰计算和分子动力学模拟. 图7展示了Zr原子配位数随电子能损值变化趋势. 图 7 配位数随电子能损值变化 Figure7. The change of coordination number with electronic energy loss.
如图7所示, 电子能损值为15—20 keV·nm–1时, 配位数为7的Zr原子个数随电子能损值增加而减少, 未出现配位数为8的Zr原子, 表明ZrO2仍保持单斜相. 当电子能损值为21 keV·nm–1时, 72.4% Zr原子配位数由7变为8, 表明ZrO2已经由单斜相转为四方相. 由此得出辐照诱导单斜ZrO2相变的快速重离子的电子能损阈值为21 keV·nm–1. 电子能损值为21—30 keV·nm-1时, 随着电子能损值增加, 配位数为8的Zr原子个数先增加后减小, 占Zr原子总数比例在71.5%—76%之间. 当入射离子电子能损分别为20和21 keV·nm–1时, 辐照后不同配位数的Zr原子空间分布如图8所示. 图 8 不同配位数Zr原子空间分布 (a) 电子能损值为20 keV·nm–1; (b)电子能损值为21 keV·nm–1. 所有原子沿离子入射方向投影在(010)平面上, 颜色不同代表配位数不同 Figure8. Spatial distribution of Zr atoms with different coordination number: (a) Se = 20 keV nm–1; (b) Se = 21 keV nm–1. All the atoms are projected on (010) plane along the injection direction of thermal spike. Different coordination numbers are distinguished by the atomic color.