Thermal stability of radiation-induced optical centers in non-stoichiometric spinel crystals

The successive anneals of the neutron-irradiated non-stoichiometric spinel crystals MgO · 2.2Al₂O₃ leads to incremental change of optical spectra that demonstrates two main bands whose intensity and spectral position depends on annealing temperature. While temperature increases from 450 to 750 K, one of the bands shifts from photon energy of 4.2–5.1 eV. Another one shifts in the opposite direction from 6.4–5.6 eV. This effect can be attributed to coagulation of radiation induced defects near cation vacancies and change of the energy levels and transitions in F⁺- and F-centers in neutron-irradiated crystals. The final position of these two bands 5.1 and 5.6 eV corresponds to transitions in F⁺- and F-centers in non-stoichiometric spinel, respectively. The investigation of optical centers induced at subsequent UV-illumination of neutron-irradiated crystals annealed to temperature of 750 K, and comparison with as-grown crystals, shows the existence of residual concentration mainly of the antisite defects and partially of the anionic vacancies in the neutron irradiated and annealed crystals. The existence of two temperature stages where optical centers at antisite defects are effectively destroyed may indicate the presence of spatially correlated and isolated antisite defects in irradiated spinel.     

Calculations of the trapping and migration of vacancies and nickel self-interstitials in the presence of rare gases and dislocations

Recent models of swelling, void growth, and solute segregation under irradiation all require knowledge of the trapping and migration of vacancies and self-interstitials in the presence of lattice defects. The present calculations include trapping of both vacancies and nickel self-interstitials to substitutional and interstitial rare gas atoms. The results show a systematic dependence on rare gas atom size. It is found for example, that a vacancy is bound to a small fixed rare gas interstitial (He) by ～0.5 eV and to a large fixed interstitial (Xe) by ≥3 eV. In addition, a fixed substitutional rare gas or rare gas interstitial is found to be a strong trap for a self-interstitial. It is found that a single vacancy can significantly affect the migration energy of another vacancy. For example, a 0.4 eV decrease in migration energy is found at a distance of three half-lattice constants. However, this interaction is of limited range; at distances greater than five half-lattice constants vacancy migration is unaffected. The migration of vacancies near the core of a partial dislocation was also investigated. This partial is found to provide a 1 eV (compared to 1.4 eV in the bulk) path for the pipe diffusion of vacancies. In addition, the activation energy for vacancy migration along the slip plane is reduced by as much as 0.2 eV.     

Thermal effects in 10 keV Si PKA cascades in 3C-SiC

We present a molecular dynamics study of the influence of temperature on defect generation and evolution in irradiated cubic silicon carbide. We simulated 10 keV displacement cascades, with an emphasis on the quantification of the spatial distribution of defects, at six different temperatures from 0 K to 2000 K under identical primary knock-on atom conditions. By post-processing the simulation results we analyzed the temporal evolution of vacancies, interstitials, and antisite defects, the spatial distribution of vacancies, and the distribution of vacancy cluster sizes. The majority of vacancies were found to be isolated at all temperatures. We found evidence of temperature dependence in C and Si replacements and C_Si antisite formation, as well as reduced damage generation behavior due to enhanced defect relaxation at 2000 K.     

An atomistic approach to self-diffusion in uranium dioxide

The formation and mobility of point defects in UO₂ have been studied within the framework of the Density Functional Theory. The ab initio Projector Augmented Wave method is used to determine the formation and migration energies of defects. The results relative to intrinsic point defect formation energies using the Generalized Gradient Approximation (GGA) and GGA+U approximations for the exchange-correlation interactions are reported and compared to experimental data. The GGA and GGA+U approximations yield different formation energies for both Frenkel pairs and Schottky trios, showing that the 5f electron correlations have a strong influence on the defect formation energies. Using GGA, various migration mechanisms were investigated for oxygen and uranium defects. For oxygen defects, the calculations show that both a vacancy and an indirect interstitial mechanism have the lowest associated migration energies, 1.2 and 1.1 eV respectively. As regards uranium defects, a vacancy mechanism appears energetically more favourable with a migration energy of 4.4 eV, confirming that oxygen atoms are much more mobile in UO₂ than uranium atoms. Those results are discussed in the light of experimentally determined activation energies for diffusion.     

Stability and migration property of helium and self defects in vanadium and V-4Cr-4Ti alloy by first-principles

The stability and migration behavior of helium and self defects in vanadium and V-4Cr-4Ti alloy are studied by first-principles calculations. The tetrahedral site is found as the most stable configuration for interstitial He, followed by the octahedral and substitutional sites. Among the self defects, the monovacancy has lower formation energy (1.71 eV for V and 2.14 eV for V-4Cr-4Ti alloy) than the self interstitial ones. The migration energies for He hopping between the tetrahedral sites are 0.06 and 0.09 eV for vanadium and V-4Cr-4Ti alloy, respectively. Our calculations reveal strong repulsion between two interstitial He atoms and strong attraction between He and vacancy, suggesting that vacancy acts as a trapping site for He impurity and a seed for further bubble formation.     

Native defect properties in β-SiC: Ab initio and empirical potential calculations

There is considerable ambiguity regarding the formation of native defects and their clusters in silicon carbide (SiC), since different empirical potentials give different results, particular for the stability of interstitial configurations. Density functional theory (DFT) is used to study the formation and properties of native defects in β-SiC. The DFT results are compared with those calculated by molecular dynamics (MD) simulations using the Tersoff potentials, with modified cut-off distances and parameters obtained from the literature. The formation energy of vacancies and antisite defects obtained by DFT calculations are in good agreement with those given by the Tersoff potential, regardless of the cut-off distances, but for interstitials there is a disparity between the two methods, depending on the cut-off distances used in the Tersoff potential. The present results provide guidelines for evaluating the quality and fit of empirical potentials for large-scale simulations of irradiation damage (displacement cascades) and point defect migration (recombination or annealing) in SiC.     

Molecular dynamics modelling of radiation damage in normal, partly inverse and inverse spinels

The radiation response of perfect crystals of MgAl₂O₄, partially inverted MgGa₂O₄ and fully inverse MgIn₂O₄ were investigated using molecular dynamics. Dynamical cascades were initiated in these spinels over a range of trajectories with energies of 400 eV and 2 keV for the primary knock-on event. Collision cascades were set up on each of the cation and anion sublattices and were monitored up to 10 ps. Simulations in the normal MgAl₂O₄ spinel for the 2 keV energy regime resulted in similar defect structures as obtained at the post-threshold 400 eV energies, with little clustering occurring. The predominant defect configurations were split interstitials and cation antisites. For the inverse spinels, a much wider variety of lattice imperfections was observed. More defects were also produced due to the formation of interstitial–vacancy cation chains and oxygen crowdions.     

Hydrogen release in SiO2: Source sites and release mechanisms

We investigate molecular scale mechanisms for radiation-induced release of hydrogen from precursor sites using density functional theory applied to a fully periodic model of SiO₂. We focus on proton release from H-decorated oxygen vacancies in the bulk oxide. After hole-capture at the vacancy, a proton can hop to an energetically favorable bound state at a neighboring oxygen atom. In -quartz, this release mechanism has an activation energy of about 1.2 eV. In amorphous silica, this hop has a range of low barriers, from 0.1 to 0.5 eV. Furthermore, another proton release mechanism involves cracking of H₂ molecules by a reaction with an isolated, positively charged Si-dangling bond.     

Theoretical study of helium insertion and diffusion in 3C-SiC

Insertion and diffusion of helium in cubic silicon carbide have been investigated by means of density functional theory. The method was assessed by calculating relevant properties for the perfect crystal along with point defect formation energies. Results are consistent with available theoretical and experimental data. Helium insertion energies were calculated to be lower for divacancy and silicon vacancy defects compared to the other mono-vacancies and interstitial sites considered. Migration barriers for helium were determined by using the nudged elastic band method. Calculated activation energies for migration in and around vacancies (silicon vacancy, carbon vacancy or divacancy) range from 0.6 to 1.0 eV. Activation energy for interstitial migration is calculated to be 2.5 eV. Those values are discussed and related to recent experimental activation energies for migration that range from 1.1 [P. Jung, J. Nucl. Mater. 191–194 (1992) 377] to 3.2 eV [E. Oliviero, A. van Veen, A.V. Fedorov, M.F. Beaufort, J.F. Bardot, Nucl. Instrum. Methods Phys. Res. B 186 (2002) 223; E. Oliviero, M.F. Beaufort, J.F. Bardot, A. van Veen, A.V. Fedorov, J. Appl. Phys. 93 (2003) 231], depending on the SiC samples used and on helium implantation conditions.     

金红石辐照损伤的分子动力学模拟

为了研究辐照条件下金红石的耐辐照损伤能力，采用GULP软件包拟合出了与实验值吻合的势函数，并采用LAMMPS软件包计算出了金红石的离位阈能和高能粒子反冲条件下的位移级联。通过统计球坐标系下266个出射方向的离位能，利用缺陷形成概率的定义得出Ti和O原子的离位阈能分别为(78.3±1.0) eV和(42.6±2.0) eV。采用VORONOI缺陷统计方法，计算了300 K、10 keV出射能量条件下缺陷数量随辐照时间演化信息，结果表明：Ti原子作为初始出射原子产生的缺陷数量整体高于O原子产生缺陷的数量，在最大无序阶段产生的空位、填隙和不同类型反位缺陷通过空位-填隙复合作用和kick-out机制逐渐减少，有效地降低了晶体的无序度，提高了基材耐辐照损伤性能。     

Phase equilibrium of PuO2−x − Pu2O3 based on first-principles calculations and configurational entropy change

Combination of an oxygen vacancy formation energy calculated using first-principles approach and the configurational entropy change treated within the framework of statistical mechanics gives an expression of the Gibbs free energy at large deviation from stoichiometry of plutonium oxide PuO₂. An oxygen vacancy formation energy 4.20 eV derived from our previously first-principles calculation was used to evaluate the Gibbs free energy change due to oxygen vacancies in the crystal. The oxygen partial pressures then can be evaluated from the change of the free energy with two fitting parameters (a vacancy-vacancy interaction energy and vibration entropy change due to induced vacancies). Derived thermodynamic expression for the free energy based on the SGTE thermodynamic data for the stoichiometric PuO₂ and the Pu₂O₃ compounds was further incorporated into the CALPHAD modeling, then phase equilibrium between the stoichiometric Pu₂O₃ and non-stoichiometric PuO_2−x were reproduced.     

Atomic diffusion mechanism of Xe in UO2

We have investigated vacancy-assisted diffusion of Xe in uranium dioxide (UO₂) calculating incorporation, binding, and migration energies. All the energy values have been obtained using the density functional theory (DFT) within the generalized gradient approximation (GGA) and the projector-augmented-wave (PAW) method. Considering spin-polarization effect, we find that the computed migration energy is reduced by and agrees well with experimental data compared to those obtained from non-magnetic calculations. We also find that an oxygen vacancy lowers the migration energy of a uranium vacancy by about 1 eV, enhancing an effective movement of vacancy clusters consisting of both uranium and oxygen vacancies. Furthermore, the strain energy of Xe is large enough to contribute to the clustering of vacancies making it the driving force for the vacancy-assisted diffusion of Xe in UO₂. In summary all the calculated results suggest that the trivacancy is a major diffusion pathway of Xe in UO₂.     

Formation and properties of defects and small vacancy clusters in SiC: Ab initio calculations

Large-scale ab initio simulation methods have been employed to investigate the configurations and properties of defects in SiC. Atomic structures, formation energies and binding energies of small vacancy clusters have also been studied as a function of cluster size, and their relative stabilities are determined. The calculated formation energies of point defects are in good agreement with previously theoretical calculations. The results show that the di-vacancy cluster consists of two C vacancies located at the second nearest neighbor sites is stable up to 1300 K, while a di-vacancy with two Si vacancies is not stable and may dissociate at room temperature. In general, the formation energies of small vacancy clusters increase with size, but the formation energies for clusters with a Si vacancy and nC vacancies (V_Si-nV_C) are much smaller than those with a C vacancy and nSi vacancies (V_C-nV_Si). These results demonstrate that the V_Si-nV_C clusters are more stable than the V_C-nV_Si clusters in SiC, and provide possible nucleation sites for larger vacancy clusters or voids to grow. For these small vacancy clusters, the binding energy decreases with increasing cluster size, and ranges from 2.5 to 4.6 eV. These results indicate that the small vacancy clusters in SiC are stable at temperatures up to 1900 K, which is consistent with experimental observations.     

Vacancy formation and migration energies in strained crystals

The formation and migration energies of vacancies in crystals under hydrostatic strain are considered in model fee and bcc crystals using the technique of computer simulation. Equilibrium and non-equilibrium interatomic potentials have been employed for iron, molybdenum, copper and nickel. In the case of Ni, a simple density-dependence of the potential has also been incorporated. Significant differences between the potential forms and crystal structures are reported. For the purpose of comparison, Mukherjee's empirical relation between the formation energy and Debye temperature is exploited to obtain the variation of this energy with lattice parameter. By way of contrast, the effect of the vacancy migration energy of a neighbouring vacancy is also considered in the model crystals.     

Interaction of deuterium with lattice defects in nickel

The trapping of ion-implanted deuterium (D) in fee Ni is investigated by ion-beam-analysis techniques. Two lattice-defect traps have been observed with trap-binding enthalpies 0.24 eV and 0.43 eV referred to an untrapped solution site. The lattice location of D when associated with the defect traps is obtained by the channeling technique following anneals at various temperatures. The detailed analysis of these channeling data is based on a comparison with multirow continuum-model calculations of the angular yields for different D positions. These channeling calculations are extended by introducing a parameter δψ which encompasses the spreading in transverse energy caused by effects such as, for example, electron and nuclear multiple scattering. Also new and improved theoretical calculations based on the effective medium scheme of the equilibrium positions of H isotopes at defects, especially vacancies, are presented. The calculations show that D is delocalized over the entire vacancy, with a maximum density in the region between the vacancy and the nearest octahedral site. This picture is supported by the finding that the channeling data for D trapped to vacancies cannot be interpreted in terms of a single lattice site, and preference in site occupancy is found for D displaced from the vacancy towards the octahedral and (smaller) tetrahedral sites, respectively.     

The structural and dynamical properties of Al clusters adsorbed on Ni surface

The impact-induced deposition of Al₁₃ clusters with icosahedral structure on Ni(0 0 1) surface was studied by molecular dynamics (MD) simulation using Finnis–Sinclair potentials. The incident kinetic energy (E_in) ranged from 0.01 to 30 eV per atom. The structural and dynamical properties of Al clusters on Ni surfaces were found to be strongly dependent on the impact energy. At much lower energy, the Al cluster deposited on the surface as a bulk molecule. However, the original icosahedral structure was transformed to the fcc-like one due to the interaction and the structure mismatch between the Al cluster and Ni surface. With increasing the impinging energy, the cluster was deformed severely when it contacted the substrate, and then broken up due to dense collision cascade. The cluster atoms spread on the surface at last. When the impact energy was higher than 11 eV, the defects, such as Al substitutions and Ni ejections, were observed. The simulation indicated that there exists an optimum energy range, which is suitable for Al epitaxial growth in layer by layer. In addition, at higher impinging energy, the atomic exchange between Al and Ni atoms will be favourable to surface alloying.     

钨中空位及其团簇的能量学和动力学性质参数

在总结前人钨中空位及其团簇的能量学和动力学行为的研究成果基础上，采用第一性原理方法系统计算了钨中空位及其团簇的结合能和扩散能垒。研究发现，交换关联泛函PW91和PBE较PBEsol、AM05和LDA更适合用于计算钨空位的能量学性质。基于第一性原理计算结果对文献中单空位形成能、双空位作用性质等争议性问题进行了讨论，并对钨经验势进行了评估。研究结果表明，钨中孤立单空位间总是相互排斥，而空位团簇（V_n>3）对单空位具有很强的吸引作用，其结合能随着所含空位个数增多呈现波动性增大的趋势。空位团簇稳定结构可通过最小化Wigner-Seitz表面积来确定，其结合能与V_n与V_n-1之间的Wigner-Seitz面积之差呈正比。     

反应堆压力容器钢中溶质元素空位型扩散机理研究

辐照或热老化导致元素偏析和沉淀析出是反应堆压力容器(reactor pressure vessel,RPV)钢性能退化的主要影响因素,点缺陷与合金/杂质元素结合与扩散是引起元素偏析和沉淀析出的主要原因。本文利用分子动力学方法研究了反应堆压力容器钢中几种主要合金/杂质元素(Cu、Ni、Mn、P)的空位型扩散机理。研究了空位与合金/杂质元素的结合性能;基于多频模型计算了合金/杂质元素的空位风参数和扩散系数。通过计算发现,Cu、P与第1近邻、第2近邻空位均具有较大的结合能,Ni与第2近邻空位具有较大的结合能;溶质元素的空位风均随着温度的升高而增大,表明在高温下合金/杂质元素均倾向通过与空位互换位置而扩散。     

Quantum mechanical calculations of uranium phases and niobium defects in γ-uranium

Depleted uranium (U) from fuel enrichment processes has a variety of applications due to its high density. With the addition of a small concentration of niobium (Nb), U becomes stainless. Nb is fully miscible with the high-temperature γ phase of U and tends to segregate upon cooling below 1050 K. The starting point of segregation is the configuration of Nb substitutional or interstitial defects. Using quantum mechanical calculations, the authors find that the formation energy of a single vacancy is 1.08 eV, that of Nb substitution 0.59 eV, that of Nb interstitial at octahedral site 1.58 eV, and that of Nb interstitial at tetrahedral site 2.35 eV in the dilute limit of isolated defects; all with reference to a reservoir of the pure γ phase U and pure Nb. The analysis of electronic structures reveals the correlation of formation energies of Nb defects with the local perturbations of electron distribution. Higher formation energy of Nb defects correlates with larger perturbation. Based on this study, Nb atoms thermodynamically prefer to occupy substitutional sites in the γ phase U.     

Ab initio-based diffusion theory and tracer diffusion in Ni-Cr and Ni-Fe alloys

In this paper, ab initio modeling is used to predict diffusion relevant thermodynamic and kinetic information for dilute Ni-Cr and Ni-Fe alloys. The modeling results are then used to determine the phenomenological coefficient matrices and the tracer diffusion coefficients for both vacancy and interstitial mediated diffusion. In addition to predicting diffusion coefficients, this ab initio-based approach provides information typically inaccessible to experiments, including the different contributions to diffusion (e.g., electronic excitation effects), the species dependence of interstitial diffusion, and the deviations from Arrhenius-type relations, which are often used to describe and extrapolate experimental diffusion data. It is found that: (1) Cr is the fastest diffusing species in Ni by both vacancy and interstitial diffusion, followed by Fe and then Ni. The enhanced diffusivity of Cr is primarily due to differences in migration barriers and binding energies, not pre-exponential factors. (2) Fe and Cr solutes in Ni have weak interactions with vacancies but Cr solutes bind strongly to interstitial defects. (3) Cr exhibits non-Arrhenius behavior in both vacancy and interstitial mediated diffusion. (4) Temperature dependent electronic contributions have a significant impact on the diffusion in some cases. (5) The vacancy diffusion mechanism in Ni-Cr changes as a function of temperature resulting in vacancy-solute drag below 460 K.