金属钯及其氚化物中氦行为的理论研究
摘要
核能是解决能源问题的一种理想的替代能源。在核能的利用中,材料中的氦效应是一个至关重要的问题。材料中引入氦(He)的途径主要有：贮氚材料中的氚通过p衰变而产生氦(3He)；材料内部不断通过α衰变产生高能α粒子(4He)；聚变反应堆第一壁材料表面不断受到氘、氚等离子体中聚变产物的轰击而产生He原子；裂变反应堆和聚变反应堆中产生的中子与结构材料发生(n,α)核反应而产生氦(4He)等。材料中产生的氦,由于其特殊的满壳层电子结构而不溶于基体材料,氦的聚集会对材料本身的宏观物理性能和力学系能产生影响,即核脆。因而,弄清楚He原子在材料中与各种缺陷的微观作用机理,对于核能工业和材料工业都是很有必要的。金属钯(Pd)是一种固氦能力很强的单质,其氚化物也是一种重要的贮氚材料,因而我们选取钯及其氚化物作为研究对象。
本文应用第一性原理密度泛函理论和平面波赝势方法,利用VASP程序软件包系统地研究了金属Pd及其氚化物中的氦行为。计算结果如下：(1)计算了Pd中间隙位置He原子的稳定性、氦原子的迁移情况以及氦-空位小团簇(HemVn)的稳定性。结果表明：Pd中空位有很强的捕获He原子的能力；He在Pd中间隙位置的稳定情况依次是空位、八面体间隙、四面体间隙；间隙氦原子不能直接在邻近的八面体间隙之间迁移,只能先通过四面体间隙位置,再达到八面体间隙位置,即迁移路径为O-T-O；HemVn团簇形成能随着He原子浓度的增加而增加,而且没有空位的He团簇比有空位的He团簇形成能高,空位使得He更容易形成团；HemVn团簇的结合能很大程度上依赖于团簇中的He对空位的比例(He／V)而不是团簇本身的大小,当He／V的比例约等于1的时候,是团簇中最稳定的构型；HemVn团会通过发射出He原子或者空位来改变自身的氦与空位的比例从而到达稳定,稳定后的离解能大约为2.5 eV,当He／V比例小于1时,会发射空位,从而使He／V比例增加,当He／V比例大于1时,会发射出现氦原子和自间隙原子,从而使团中He／V比例降低。(2)计算了Pd中氢(H)、H-He对的稳定性情况,及Pd氚化物中氦行为,并对比分析了Pd及其氚化物中不同的氦行为。结果表明：H在Pd中间隙位置的稳定情况和He很不一样,其稳定性位置依次是八面体间隙、四面体间隙、空位；当H和He原子同时存在于Pd中相邻的八面体间隙位置时,形成一种稳定的Hocta-Heocta对；H和He原子同时位于Pd中单空位的最近邻处时,He原子易被空位捕获,H原子依然位于原来的八面体间隙为形成稳定的Hocta-Hesub对；钯氚化物中氦原子的聚集容易导致局部的晶格扭曲,相比Pd中He而言,其氚化物中更有利于He小团簇聚集。
The nuclear energy is the hope of sustainable development of energy source, and the effect of helium (He) in nuclear materials is very important in the use of nuclear energy. He in materials can be introduced by various ways, such as the beta decay of tritium in tritium-storage materials which produce 3He, the alpha decay in materials which produce high energy 4He, and so on. Because of the low solubility in the host materials, the accumulation of helium is known to result in the formation of He clusters or bubbles, which can induce large changes of micro-structure in the host lattice, leading to irradiation defect induced by swelling and high temperature embrittlement in structural materials with aging time. Thus the properties of helium atoms in materials are considered as the particular concern and it is very important to understand the micro-mechanism of helium interaction with different defects in materials. At the same time, palladium (Pd) has the strongest capacity to retain helium and its tritide is an excellent tritium-storage material, so we choose palladium and its tritide to perform our research.
First-principles calculations based on density functional theory (DFT) have been performed to study the properties of helium behavior in palladium and its tritide. The results are as follows:(1) we calculated the properties of the vacancy, substitutional, and small helium-vacancy clusters HemVn (m, n=0-4) in palladium. The result indicates that the vacancy has the strongest ability of capturing helium atoms and the octahedral interstitial configuration is more stable than the tetrahedral one. In the palladium crystal, helium atom migrates from one octahedral interstitial site to another through the O-T-O path. The formation energies of HemVn clusters are increasing with the increase of helium atoms, and they are larger without than with vacancies. In the HemVn clusters, He is most strongly bound to large vacancies, and the binding energies mainly depend on the ratio of helium to vacancy in the clusters rather than the cluster size and the configuration is the most stable as the ratio of helium to vacancy is one. The HemVn clusters change their ratio of helium to vacancy through emitting helium atoms or vacancies to stabilize the configuration, and the dissociation energy is about 2.5 eV. When the ratio of helium to vacancy is less than one, the clusters will increase the ratio through emitting vacancies, otherwise, emitting helium atoms and self-interstitial atoms to decrease the ratio. (2) The stability of H and H-He pairs in Pd and helium behavior in Pd tritide have also been calculated, meanwhile, we compared the difference between helium behavior in Pd and its tritide. The result indicates as follows:The stability of H in Pd is different from He in Pd, the vacancy has the weakest ability of capturing the H atoms, and the octahedral interstitial configuration is more stable than the tetrahedral one. When the H and He atoms exist in the Pd in the adjacent octahedral interstitial site, they relax and form a stable Hocta-Heocta pair. When H and He atoms located simultaneously in the vicinity of a single vacancy in Pd, the He atoms can easily captured by the vacancy and the H atom remains in the original octahedral sites, then the Hocta-Hesub pair is predicted to be the lowest energy configuration. Comparing with He in Pd, the He atoms in Pd tritide can easily lead to local lattice distortions and small He clusters.
本文应用第一性原理密度泛函理论和平面波赝势方法,利用VASP程序软件包系统地研究了金属Pd及其氚化物中的氦行为。计算结果如下：(1)计算了Pd中间隙位置He原子的稳定性、氦原子的迁移情况以及氦-空位小团簇(HemVn)的稳定性。结果表明：Pd中空位有很强的捕获He原子的能力；He在Pd中间隙位置的稳定情况依次是空位、八面体间隙、四面体间隙；间隙氦原子不能直接在邻近的八面体间隙之间迁移,只能先通过四面体间隙位置,再达到八面体间隙位置,即迁移路径为O-T-O；HemVn团簇形成能随着He原子浓度的增加而增加,而且没有空位的He团簇比有空位的He团簇形成能高,空位使得He更容易形成团；HemVn团簇的结合能很大程度上依赖于团簇中的He对空位的比例(He／V)而不是团簇本身的大小,当He／V的比例约等于1的时候,是团簇中最稳定的构型；HemVn团会通过发射出He原子或者空位来改变自身的氦与空位的比例从而到达稳定,稳定后的离解能大约为2.5 eV,当He／V比例小于1时,会发射空位,从而使He／V比例增加,当He／V比例大于1时,会发射出现氦原子和自间隙原子,从而使团中He／V比例降低。(2)计算了Pd中氢(H)、H-He对的稳定性情况,及Pd氚化物中氦行为,并对比分析了Pd及其氚化物中不同的氦行为。结果表明：H在Pd中间隙位置的稳定情况和He很不一样,其稳定性位置依次是八面体间隙、四面体间隙、空位；当H和He原子同时存在于Pd中相邻的八面体间隙位置时,形成一种稳定的Hocta-Heocta对；H和He原子同时位于Pd中单空位的最近邻处时,He原子易被空位捕获,H原子依然位于原来的八面体间隙为形成稳定的Hocta-Hesub对；钯氚化物中氦原子的聚集容易导致局部的晶格扭曲,相比Pd中He而言,其氚化物中更有利于He小团簇聚集。
The nuclear energy is the hope of sustainable development of energy source, and the effect of helium (He) in nuclear materials is very important in the use of nuclear energy. He in materials can be introduced by various ways, such as the beta decay of tritium in tritium-storage materials which produce 3He, the alpha decay in materials which produce high energy 4He, and so on. Because of the low solubility in the host materials, the accumulation of helium is known to result in the formation of He clusters or bubbles, which can induce large changes of micro-structure in the host lattice, leading to irradiation defect induced by swelling and high temperature embrittlement in structural materials with aging time. Thus the properties of helium atoms in materials are considered as the particular concern and it is very important to understand the micro-mechanism of helium interaction with different defects in materials. At the same time, palladium (Pd) has the strongest capacity to retain helium and its tritide is an excellent tritium-storage material, so we choose palladium and its tritide to perform our research.
First-principles calculations based on density functional theory (DFT) have been performed to study the properties of helium behavior in palladium and its tritide. The results are as follows:(1) we calculated the properties of the vacancy, substitutional, and small helium-vacancy clusters HemVn (m, n=0-4) in palladium. The result indicates that the vacancy has the strongest ability of capturing helium atoms and the octahedral interstitial configuration is more stable than the tetrahedral one. In the palladium crystal, helium atom migrates from one octahedral interstitial site to another through the O-T-O path. The formation energies of HemVn clusters are increasing with the increase of helium atoms, and they are larger without than with vacancies. In the HemVn clusters, He is most strongly bound to large vacancies, and the binding energies mainly depend on the ratio of helium to vacancy in the clusters rather than the cluster size and the configuration is the most stable as the ratio of helium to vacancy is one. The HemVn clusters change their ratio of helium to vacancy through emitting helium atoms or vacancies to stabilize the configuration, and the dissociation energy is about 2.5 eV. When the ratio of helium to vacancy is less than one, the clusters will increase the ratio through emitting vacancies, otherwise, emitting helium atoms and self-interstitial atoms to decrease the ratio. (2) The stability of H and H-He pairs in Pd and helium behavior in Pd tritide have also been calculated, meanwhile, we compared the difference between helium behavior in Pd and its tritide. The result indicates as follows:The stability of H in Pd is different from He in Pd, the vacancy has the weakest ability of capturing the H atoms, and the octahedral interstitial configuration is more stable than the tetrahedral one. When the H and He atoms exist in the Pd in the adjacent octahedral interstitial site, they relax and form a stable Hocta-Heocta pair. When H and He atoms located simultaneously in the vicinity of a single vacancy in Pd, the He atoms can easily captured by the vacancy and the H atom remains in the original octahedral sites, then the Hocta-Hesub pair is predicted to be the lowest energy configuration. Comparing with He in Pd, the He atoms in Pd tritide can easily lead to local lattice distortions and small He clusters.
引文
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