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摘要:
钒(V)是核聚变反应堆结构材料的重要候选材料。实验表明杂质氧(O)会对V的结构和力学性能产生极大的影响。采用基于密度泛函理论的第一性原理方法研究了O在V中热力学稳定性、扩散特性以及与缺陷空位的相互作用。O在V中易于占据八面体间隙位，其溶解能为-4.942 eV。O在间隙位的最佳扩散路径为八面体间隙位→四面体间隙位→八面体间隙位，扩散激活能为1.728 eV，在此基础上对不同温度下的扩散系数在文中给出了详细分析。O在V中与空位存在很强的吸引相互作用，1个O原子和2个O原子被空位捕获时的捕获能分别为-0.484 eV和-0.510 eV。当O原子的数量超过3，其捕获能变为正值0.382 eV，因此单空位最多能够结合2个O原子，这意味着“O1-vacancy”和“O2-vacancy”团簇在V中很容易形成。这些研究结果将对V基合金在核聚变反应堆中的最终应用具有一定的参考价值。
Abstract:Vanadium (V) is identified as a promising candidate of the structural materials in fusion reactors. Experimental results have demonstrated that the impurity oxygen (O) has great influence on the structure and mechanical properties of V. Employing a first-principles method based on the density functional theory, we study the stability and diffusion property of impurity O as well as its interaction with defect vacancy in V. O atom is energetically favorable to occupy the octahedral insterstitial site with the solution energy of -4.942 eV. The intrinsic optimal diffusion route of O in the interstitial site is octahedral insterstitial site→tetrahedral insterstitial site→octahedral insterstitial site, and the diffusion activation energy is calculated to be 1.728 eV. The diffusion coefficients of O at the different temperature are systematically analyzed. We demonstrate that there is the strong attractive interaction between O and vacancy in V. The trapping energies of one and two O atoms are-0.484 eV and -0.510 eV, respectively. With the increase of the number of O atoms, the trapping energy of the third O becomes the positive value of 0.382 eV, meaning that vacancy cannot bind the additional O atom again. Thus, one vacancy can accommodate as many as two O atoms. It is revealed that the "O1-vacancy" and "O2-vacancy" clusters are easily formed in V. The current results can provide a very useful reference for V as a candidate structural material in a fusion reactor.
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Key words:
- vanadium /
- impurity oxygen /
- diffusion property /
- defect vacancy /
- first-principles
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图 2 O在V中的原子结构和价电荷密度分布
Figure 2. Atomic structure and valence charge density distribution of O in V
图 5 O与空位结合的最佳电子密度等值面
“+”代表O结合在等值面的6个能量最低位置。
Figure 5. Isosurface of optimal electron density for O binding at one vacancy
图 6 “O1-vacancy”团簇在(001) 平面上的电荷密度分布
V原子和O原子分别处在不同的(001) 平面。
Figure 6. Charge density distribution of "O1-vacancy" cluster on (001) plane
图 7 O在V中以“sequential”方式进入单空位时捕获能随O原子数目增加的变化曲线
Figure 7. Changing curve of trapping energy of O with increase of number of atoms at one vacancy in V via "sequential" way
表 1 O在128原子V超晶胞中八面体间隙位和四面体间隙位中溶解能的测试结果
Table 1. Tested solution energy results of O at OIS and TIS in 128-atom V supercell
eV 测试对象 截止能 k点 250 300 350 400 450 1×1×1 2×2×2 3×3×3 4×4×4 EOs(OIS) -4.928 -4.935 -4.942 -4.943 -4.943 -4.902 -4.935 -4.942 -4.942 EOs(TIS) -3.693 -3.761 -3.779 -3.778 -3.778 -3.689 -3.762 -3.779 -3.778 注：先选取一个较大的8×8×8 k点对截止能进行测试，测试结果取为350 eV，并采用350 eV的截止能对k点进行测试，测试结果取为3×3×3 k点。 
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表 2 O在V中300 ~1000K温度范围内的扩散系数
Table 2. Diffusion coefficient of O in V at different temperatures from 300 to 1000K
T/K D/(m2·s-1) 300 2.16×10-36 400 3.91×10-29 500 8.84×10-25 600 7.05×10-22 700 8.35×10-20 800 3.00×10-18 900 4.86×10-17 1000 4.51×10-16
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