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ZrO 2陶瓷耐高温、耐腐蚀、抗辐照性能强, 是极具前景的反应堆惰性基质燃料和锕系元素固化材料. 本文联合使用热峰模型和分子动力学方法, 模拟了核辐射环境下ZrO 2的相变过程: 基于热峰模型, 从快速重离子注入后能量沉积和传导的多物理过程出发, 建立热扩散方程, 求得ZrO 2晶格温度时空演变特性; 然后运用分子动力学方法模拟了该热峰作用下, 单斜ZrO 2相变的微观物理过程. 研究发现, 电子能损为30 keV·nm –1的单一快速重离子注入后, ZrO 2中心产生一个半径为7 nm的柱形径迹, 径迹中心晶格迅速熔融, Zr原子配位数由7降至4—6, 2 ps时开始结晶并形成空洞, 空洞周围为非晶区, 非晶区外Zr原子配位数变为8, 同时X射线衍射(X-ray diffraction, XRD)计算和分析结果确认发生了单斜相向四方相的转变. 随着热峰能量向周围传递, 相变区逐渐扩大. 经热峰计算和分子动力学模拟, 辐照诱导ZrO 2由单斜相转为四方相的快速重离子的电子能损阈值为21 keV·nm –1.Owing to its excellent corrosion, radiation and high temperature resistance, ZrO 2has 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 ZrO 2in 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 ZrO 2lattice temperature are obtained by solving the coupled diffusion equations numerically. Then the phase transformation of ZrO 2form 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 ZrO 2after 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 ZrO 2, 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 ZrO 2with 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.
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Keywords:
- ZrO2/
- phase transition/
- thermal spike/
- molecular dynamics
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