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Impact of homogeneous strain on uranium vacancy diffusion in uranium dioxide

机译:均匀应变对二氧化铀中铀空位扩散的影响

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We present a detailed mechanism of, and the effect of homogeneous strains on, the migration of uranium vacancies in UO_2. Vacancy migration pathways and barriers are identified using density functional theory and the effect of uniform strain fields are accounted for using the dipole tensor approach. We report complex migration pathways and noncubic symmetry associated with the uranium vacancy in UO_2 and show that these complexities need to be carefully accounted for to predict the correct diffusion behavior of uranium vacancies. We show that under homogeneous strain fields, only the dipole tensor of the saddle with respect to the minimum is required to correctly predict the change in the energy barrier between the strained and the unstrained case. Diffusivities are computed using kinetic Monte Carlo simulations for both neutral and fully charged state of uranium single and divacancies. We calculate the effect of strain on migration barriers in the temperature range 800-1800 K for both vacancy types. Homogeneous strains as small as 2% have a considerable effect on diffusivity of both single and divacancies of uranium, with the effect of strain being more pronounced for single vacancies than divacancies. In contrast, the response of a given defect to strain is less sensitive to changes in the charge state of the defect. Further, strain leads to anisotropies in the mobility of the vacancy and the degree of anisotropy is very sensitive to the nature of the applied strain field for strain of equal magnitude. Our results suggest that the influence of strain on vacancy diffusivity will be significantly greater when single vacancies dominate the defect structure, such as sintering, while the effects will be much less substantial under irradiation conditions where divacancies dominate.
机译:我们提出了UO_2中铀空位迁移的详细机理和均质菌株的影响。使用密度泛函理论确定空位迁移途径和障碍,并使用偶极张量法解释均匀应变场的影响。我们报告了复杂的迁移途径和与UO_2中铀空位有关的非立方对称性,并表明需要仔细考虑这些复杂性,以预测铀空位的正确扩散行为。我们表明,在均匀应变场下,仅需要相对于最小值的鞍形偶极张量即可正确预测应变和非应变情况下的能垒变化。扩散率是使用动力学蒙特卡罗模拟法计算的,用于铀单空位和中空位的中性和完全充电状态。我们计算了两种空位类型在800-1800 K温度范围内应变对迁移壁垒的影响。低至2%的同质应变对铀的单空位和双空位的扩散率都有相当大的影响,单空位的应变效果比双空位更明显。相反,给定缺陷对应变的响应对缺陷的电荷状态变化不太敏感。此外,应变导致空位迁移率的各向异性,并且各向异性程度对于等量应变的应变对所施加应变场的性质非常敏感。我们的结果表明,当单个空位主导缺陷结构(例如烧结)时,应变对空位扩散率的影响将明显更大,而在空位主导的辐照条件下,该影响将不那么明显。

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