Interstitial Migration Energy Research Articles

Properties of radiation-induced point defects in austenitic steels: a molecular dynamics study

Austenitic steels are recognized as excellent structural materials for pressurized water reactors due to their outstanding mechanical properties and radiation resistance. However, compared to the widely studied FeCrNi series of steels, little is known about the radiation resistance of FeCrNiMn steel. In this study, the generation and evolution of radiation-induced defects in FeCrNiMn steel were investigated by molecular dynamics simulations. The results showed that more defect atoms were produced in the thermal spike stage, but fewer defects survived at the end of the cascades in FeCrNiMn compared to pure Fe. Point defect properties were analyzed by molecular statics, and the formation energies of defects in FeCrNiMn were lower than those of pure Fe, while the migration energies were higher. Compared to FeCrNi, FeCrNiMn had smaller migration energies and a larger overlap of vacancy and interstitial migration energies. The low vacancy formation energies and widely overlapping migration energies suggested that the number of point defects in the thermal spike stage was higher, but the possibility of recombination was greater. Additionally, Mn exhibited the smallest interstitial formation energies and migration energies. The difference in defect migration energies revealed that vacancy and interstitial defects migrate through different alloy constituent elements. This study revealed the underlying mechanism for the excellent irradiation resistance of FeCrNiMn.

New Family of Interstitial Oxygen Ion Conductor Discovered By High-Throughput Computational Screening

Discovery and design of highly efficient oxygen-active materials which react with, absorb, and transport oxygen is essential for fuel cells and related applications. Currently, the predominant families of oxygen-active materials used in commercial devices are perovskites and fluorites, which diffuse oxygen via a vacancy mechanism and only have adequate oxygen diffusivity and surface exchange at high temperatures (e.g., ~800 ˚C), impeding their use at lower temperatures desirable for more cost-effective and long-lasting device operation. The development of alternative electrolyte/electrode materials with high oxygen ionic conductivity at low temperatures (e.g., room temperature to 400 °C) is of great interest. Interstitial oxygen ion conductors typically have significantly lower migration barriers than even state-of-the-art vacancy-mediated oxygen conductors, which can provide orders of magnitude kinetic enhancement at low temperatures. In addition, the interstitial oxygen defects typically become more thermodynamically favorable as the temperature is lowered, potentially providing an additional boost to the oxygen conductivity at low temperatures from a higher mediating defect concentration. The synergistic combination of low migration barriers and thermodynamically favorable interstitial oxygen content at low temperatures provides the tantalizing possibility that interstitial oxygen conductors may lead to transformative performance improvements in oxygen-active materials.In this work, we developed an approach to discover new interstitial-mediated oxygen conductors by performing high-throughput computational screening of the 34k oxide materials from the Materials Project database and discovered a new structural family of interstitial oxygen diffusers, perrierite-type oxide La4Mn5Si4O22+ 𝜹 (LMS). We used a hierarchy of screening criteria including the geometric free space, thermodynamic stability, synthesizability, redox-active elements, and presence of short diffusion pathways. This approach, based largely on simple, physically intuitive properties, quickly winnowed the field of 34k oxides down to just 345 oxides, an approximate 99% reduction in the search space considered, with essentially no significant computational cost. From this reduced set of promising materials, ab initio simulations were performed to calculate the interstitial oxygen formation energy and migration energy. Our screening has yielded several material families to date, among which the A4B5C4O22+ 𝜹, with La4Mn5Si4O22+ 𝜹 (LMS) as a representative material, was selected for investigation with higher-fidelity DFT hybrid functional calculations and experimental validations. The calculated formation energy of interstitial oxygen is -0.11 eV at a concentration of 2.3% under air condition, indicating that LMS is predicted to be oxygen hyperstoichiometric with an expected composition of La4Mn5Si4O22+0.5 in air. Two separate interstitial and interstitialcy (“kick-out”) diffusion mechanisms are revealed by ab initio molecular dynamics (AIMD) simulation. For the interstitial diffusion pathway, the moves through the channel in between the zigzag-arranged sorosilicate Si2O7 groups, with a barrier of 0.69 eV. For the interstitialcy diffusion pathway, the moves along the corner-sharing Si2O7-MnO2-Si2O7 framework by kicking out the lattice oxygen, with a barrier of 0.74 eV. These results align well with the experimental findings of stable interstitial oxygen content and high oxygen mobility in LMS (Please see a separate poster from Md Sariful Sheikhfor experimental results). This work provides a systematic approach to screening a large number of prospective materials and opens a new avenue of finding promising interstitial oxide ion conducting materials for potential applications in a range of fast oxygen ion conduction-based technologies.

Infrared emission from calcium fluoride anion defects observed in synthetic material and natural fluorites

Fluorine-based defect structures in calcium fluoride have previously been shown to emit visible-spectrum fluorescence when excited by UV light. This work shows the behaviour of a new fluorescence centre emitting in the near infrared at 1100–1200 nm when excited by visible light. This emission was most readily observed in natural fluorite samples, which have structural defects due to a long term exposure to environmental radiation, but was also induced in a synthetic calcium fluoride sample using intense UV light. Excitation of the near infrared emission reaches a maximum near the previously reported F-centre aggregate absorption peak. Emission is also “recharged” when excited by photons matching the fluorine interstitial migration energy. The measurement of this fluorescence could be used to monitor defects in optical grade fluorite caused by high power lasers and could also have applications in fluorite mineral detection during mining operations. Potential resetting of these optical defects is additionally discussed.

Ab initio study of effects of Al on the defect behaviors of AlxCoCrFeNi high entropy alloys

The influence of Al on the defect behaviors of AlxCoCrFeNi high-entropy alloys (HEAs) is studied by ab initio calculations. The lattice distortion, defect formation energies, and defect migration energies were calculated for different Al concentrations ranging from 0 to 6.25 at. %. First, the increase in the lattice distortion of AlxCoCrFeNi by the addition of Al is demonstrated, but the increment is smaller than that predicted by the atomic size difference of constituent elements. Second, although formation energies of Al vacancies and dumbbells containing Al are higher, the addition of Al decreases the average formation energies of vacancies and interstitials, which is probably due to the Al-induced local strain field and lattice distortion. The migration energies of interstitials and vacancies exhibit opposite variations, the migration energies of interstitials are increased while that of vacancies are decreased with the Al content. Moreover, the distributions of formation and migration energies of both defects are significantly broadening with increasing Al content, which could further enhance the characteristic effects of HEAs and play a more important role in the irradiation resistances of HEAs. Based on the calculation results, the effects of Al on the irradiation responses of AlxCoCrFeNi are discussed.

Computational Screening of La2NiO4+δ Cathodes with Ni Site Doping for Solid Oxide Fuel Cells.

Doping on the crystal structure is a common strategy to modify electronic conductivity, ion conductivity, and thermal stability. In this work, a series of transition metal elements (Fe, Co, Cu, Ru, Rh, Pd, Os, Ir, and Pt) doped at the Ni site of La2NiO4+δ compounds as cathode materials of solid oxide fuel cells (SOFCs) are explored based on first-principles calculations, through which the determinant factors for interstitial oxygen formations and migrations are discussed at an atomistic level. The interstitial oxygen formation and migration energies for doped La2NiO4 are largely reduced in contrast to the pristine La2NiO4+δ, which is explained by charge density distributions, charge density gradients, and Bader charge differences. In addition, based on a negative correlation between formation energy and migration barrier, the promising cathode materials for SOFCs were screened out between the doped systems. The Fe-doped structures of x = 0.25, Ru-doped structures of x = 0.25 and x = 0.375, Rh-doped structures of x = 0.50, and Pd-doped structures of x = 0.375 and x = 0.50 are screened out with interstitial oxygen formation energy less than -3 eV and migration barrier less than 1.1 eV. In addition, DOS analysis indicates that doping to La2NiO4+δ also facilitates the electron conductions. Our work provides a theoretical guideline for the optimization and design of La2NiO4+δ-based cathode materials by doping.

Migration energy of a self-interstitial atom in α-iron estimated by in situ observation of interstitial clusters at low temperatures using high-voltage electron microscopy

ABSTRACT Modeling cluster dynamics or rate theory to describe the microstructural evolution of irradiated materials requires a precise knowledge of the migration energy of a self-interstitial atom (SIA), a product of energetic particle radiation. We measured the evolution of the number density of SIA clusters in electron-irradiated α-iron at low temperatures (110–320 K) by in situ observation using high-voltage electron microscopy. We identified temperature-dependent physical quantities, including (1) the peak density of SIA clusters and (2) the critical defect-free zone thickness in a thin foil specimen, associated with interstitial mobility. By fitting these quantities to the Arrhenius relations derived by rate theory analysis, we obtained estimated interstitial migration energy values of and eV for (1) and (2), respectively.

Mobility of point defects in CoCrFeNi-base high entropy alloys

The in-situ observation of microstructural evolution under electron irradiation was performed in order to experimentally estimate the mobility of vacancy and interstitial in 316 stainless steel and CoCrFeNi-base high entropy alloys. The vacancy migration energy in CoCrFeNiMn and CoCrFeNiAl0.3 was estimated to be lower and higher compared with that in 316 stainless steel, respectively. While, the interstitial migration energy in those alloys seems to be almost the same. The increase in vacancy migration energy in CoCrFeNiAl0.3 is probably due to a strong interaction between vacancy and Al. The positron annihilation analysis revealed that vacancies might exist close to Co and Ni atoms in CoCrFeNi-base high entropy alloys. In addition, vacancies could also be captured by Mn and Al atoms in CoCrFeNiMn and CoCrFeNiAl0.3, respectively. The electron- and ion-irradiation at 400 and 300 °C up to 1.5 dpa resulted in the precipitation of the intermetallic compounds, such as CoFe, NiFe in CoCrFeNiMn, and Ni3Al in CoCrFeNiAl0.3. From these results, it is suggested that Co, Ni, Mn, and Al atoms could affect the vacancy mobility in HEAs, leading to the delay of microstructural evolution and the enhancement of precipitation under irradiation.

Radiation-induced defects in sapphire single crystals irradiated by a pulsed ion beam

The luminescence and thermal stability of defects formed in α-Al2O3 single crystals after powerful (300 keV) pulsed irradiation with C+/H+ ion beam were investigated. It was found by measuring of optical density, photoluminescence, and pulsed cathodoluminescence that ion irradiation induces both single F-, F+-centers and F2-type aggregate centers. An intense thermoluminescence band with a complex shape was observed in the broad temperature range of 350–700 K, its intensity decreases with increasing of the energy density of the ion beam. The thermal stability of the F-type defects produced in α-Al2O3 after irradiation with a pulsed ion beam is comparable to that in neutron-irradiated samples. The appropriate kinetics of annealing of radiation-induced defects has been analyzed in terms of the diffusion-controlled bimolecular reactions between F-type centers and complementary interstitial oxygen ions. Thus, two important kinetic parameters – the migration energy of mobile interstitials and pre-exponential – have been evaluated and discussed.

Rate theory study of the proton-irradiation induced dislocation loops in modified 310S steels

The evolution of dislocation loops in two types of modified 310S steel under proton irradiation are investigated with rate theory simulations. The growth of dislocation loops is promoted with the increase of irradiation dose and growth rate decreases at high dose, while the number density of the dislocation loops increases first with dose and then tends to be saturated gradually. At the initial stage of irradiation, large number of dislocation loops nucleate and the density increases rapidly. With the development of irradiation, the growth of dislocation loops gradually takes over. Simulation results are in good agreement with the experimental observations of transmission electron microscope. Present study shows that The irradiation resistance of Zr-added steel is slightly better than that of (Nb, Ta, W)-added steel, which may due to the smaller migration energy of interstitials and vacancies in Zr-added steel than that in the (Nb, Ta, W)-added one.

Моделирование радиационно-индуцированной сегрегации в сплавах Fe-Cr-Ni

In the model of radiation-induced segregation, based on the first and second Fick’s laws and with inverse Kirkendall effect considered, the concentration profiles of the components of the concentrated Fe-Cr-Ni alloy and radiation point defects are obtained at different temperatures, dislocation densities, dose rates. The concentration profiles were calculated from the start of irradiation to the steady state. The sensitivity of concentrations of alloy components near the surface is analyzed as function of input parameters (vacancies migration energy for Cr, Ni, Fe, interstitial migration energy, etc.).

Helium irradiated cavity formation and defect energetics in Ni-based binary single-phase concentrated solid solution alloys

Binary single-phase concentrated solid solution alloys (SPCSAs), including Ni80Co20, Ni80Fe20, Ni80Cr20, Ni80Pd20, and Ni80Mn20 (in atomic percentage), were irradiated with 200 keV He+ ions at 500 °C. He cavity size and density distribution were systematically investigated using transmission electron microscope. Here we show that alloying elements have a clear impact on He cavity formation. Cavity size is the smallest in Ni80Mn20 but the largest in Ni80Co20. Alloying elements could also substantially affect cavity density profile. In-depth examination of cavities at peak damage region (∼500 nm) and at low damage region (∼300 nm) demonstrates that cavity size is depth (damage) dependent. Competition between consumption and production of vacancies and He atoms could lead to varied cavity size. Density functional theory (DFT) calculations were performed to obtain the formation and migration energies of interstitials and vacancies. Combined experimental and simulation results show that smaller energy gap between interstitial and vacancy migration energies may lead to smaller cavity size and narrower size distribution observed in Ni80Mn20, comparing with Ni80Co20. The results of this study call attention to alloying effects of specific element on cavity formation and defect energetics in SPCSAs, and could provide fundamental understanding to predict radiation effects in more complexed SPCSAs, such as high entropy alloys.

How relative defect migration energies drive contrasting temperature-dependent microstructural evolution in irradiated ceramics

Ceramic materials have become widely used in various fields of material science, and ceramic oxides such as cubic zirconia (c-${\mathrm{ZrO}}_{2})$ and magnesia (MgO) are candidate materials for nuclear energy applications. For the corresponding in-service conditions of these materials, there is a crucial need in studies at moderate or high temperatures of the physical phenomena underlying the damage buildup under irradiation. In the present work, we show, using x-ray diffraction, that these two materials exhibit a similar damage accumulation process under ion irradiation at fixed temperature. However, they display an unexpected opposite damaging rate to changes in the irradiation temperature. In fact, as the temperature is increased, the final damage state is reached earlier in c-${\mathrm{ZrO}}_{2}$ while it is delayed in MgO. Rate equation cluster dynamics simulations show that defect clustering is favored over defect recombination in c-${\mathrm{ZrO}}_{2}$, but the situation is reversed for MgO, explaining the observed opposite response to temperature of the two materials. This contrasting behavior can be rationalized in terms of nonequivalent interstitial versus vacancy defect migration energies in MgO. We finally demonstrate that these results allow for a qualitative prediction of the evolution of the experimental irradiation-induced disorder with temperature, henceforth potentially reducing the cost in selecting and developing ad hoc materials.

First-principles study on the helium migration energies in B12X2 (X=O, Si, P, As) crystals for neutron absorber use

ABSTRACTBoron-carbide-based materials (B12X2) with two-atom instead of three-atom chains have better ductility, which indicates that they may be better alternatives to nuclear absorber materials than B4C. In this study, we investigated the migration energy of neutron-induced helium interstitials using density functional theory calculations. As a result, we discovered that the migration energy of helium in B12Si2 and B12O2 is lower than that in B4C, which suggests that these materials might be better in inhibiting the introduction of helium gas and subsequent volume expansion during the neutron irradiation. Moreover, we found that B12P2 and B12As2 have isotropic helium migration barriers, while B4C, B12Si2, and B12O2 exhibited a strong anisotropy in the helium migrations.

On the abnormal fast diffusion of solute atoms in α-Ti: A first-principles investigation

Solute atoms such as Fe, Co, and Ni diffuse abnormally fast in α-Ti, which influences significantly the mechanical properties of the titanium alloys. Various mechanisms (e.g., the interstitial diffusion mechanism and solute-vacancy complex mechanism) have been proposed to account for the fast diffusion of these solutes in α-Ti. To elucidate such diffusion mechanism, a first-principles method is employed to calculate the formation energies, migration energy barriers, and solute-vacancy binding energies of the substitutional and interstitial solute atoms including Al, Si, Sn, V, Mn, Fe, Co, Ni, and Cu in α-Ti. Based on the calculated parameters, the diffusion mechanisms are discussed. Comparing the formation energies of the substitutional and interstitial solutes, we find that all the solute atoms prefer the substitutional configuration to the interstitial one. The interstitial migration energy barriers are quite low for all the solutes. Al and Sn diffuse mainly through normal vacancy mediated mechanism due to the high substitutional to interstitial preferential energy that leads to very low fraction of interstitial solutes (about 10−15∼10−16 at 1000 K) at thermal equilibrium state and high interstitial diffusion activation energy. The 3d metal solute atoms, especially Mn, Fe, and Co, are fast diffusers and the diffusion coefficients are dominated by the interstitial mechanism because of their sizable thermal equilibrium interstitial fractions (several percent at 1000 K). The solute-vacancy complex mechanism is not likely to account for the fast diffusions in α-Ti. We show that the direct chemical interaction between the solute and matrix atoms determines the site-occupancy of the solute atoms in α-Ti besides the atomic size effect that was commonly believed to be responsible for the fast diffusions.

Unveiling the effects of A-site substitutions on the oxygen ion migration in A2−xA′xNiO4+δ by first principles calculations

The effects of A-site substitutions on the interstitial oxygen formation energy and the migration energy in layered A2-xA'xNiO4+δ (A = selected lanthanides, A' = Ba, Sr, Ca) are investigated by first principles calculations. The interstitial oxygen formation energy is negative, in the range of -4.81 eV to -3.45 eV, strongly supporting easiness of formation of the interstitial oxygen defects in the (A,A')O rock salt plane. The Pr2NiO4+δ compound shows the lowest formation energy, indicating the highest amount of interstitial oxygen. Doping with alkaline earth cations (A') increases the formation energy of the interstitial oxygen, which prefers to be located far away from the dopants. Nevertheless, Ca seems to be the best choice, due to relatively low formation energy. Calculations for the four kinds of diffusion paths allow it to be predicted that the oxygen transport in A2-xA'xNiO4+δ is governed by the interstitialcy mechanism in the ab plane, because of the significantly lower energy barriers for this mechanism. An interesting finding is achieved for A2NiO4+δ (A = Pr, Nd, Sm), for which the energy barriers for the interstitialcy transport are negative (-0.47 eV, -0.33 eV and -0.02 eV, respectively), implying that the transition state is more stable than the assumed initial state. A new structural configuration is proposed in this work, with the adjacent apical oxygen located at the adjacent interstitial site, which shows ca. 0.5 eV lower free energy than that of the initial model. This result provides a new understanding for the location of the interstitial and the adjacent apical oxygens from an energetic point of view and supports previously published experimental data. It is found that alkaline earth doping at the A-site deteriorates the interstitial oxygen diffusion in La2-xA'xNiO4.25 materials, but concerning overall transport properties, Ca seems to be a good dopant from an energetic point of view, when compared with Ba and Sr.

Anomalous Kinetics of Diffusion-Controlled Defect Annealing in Irradiated Ionic Solids.

The annealing kinetics of the primary electronic F-type color centers (oxygen vacancies with trapped one or two electrons) is analyzed for three ionic materials (Al2O3, MgO, and MgF2) exposed to intensive irradiation by electrons, neutrons, and heavy swift ions. Phenomenological theory of diffusion-controlled recombination of the F-type centers with much more mobile interstitial ions (complementary hole centers) allows us to extract from experimental data the migration energy of interstitials and pre-exponential factor of diffusion. The obtained migration energies are compared with available first-principles calculations. It is demonstrated that with the increase of radiation fluence both the migration energy and pre-exponent are decreasing in all three materials, irrespective of the type of irradiation. Their correlation satisfies the Meyer-Neldel rule observed earlier in glasses, liquids, and disordered materials.The origin of this effect is discussed. This study demonstrates that in the quantitative analysis of the radiation damage of real materials the dependence of the defect migration parameters on the radiation fluence plays an important role and cannot be neglected.

Lattice sites of ion-implanted Mn, Fe and Ni in 6H-SiC

Using radioactive isotopes produced at the CERN-ISOLDE facility, the lattice location of the implanted transition metal (TM) ions 56Mn, 59Fe and 65Ni in n-type single-crystalline hexagonal 6H-SiC was studied by means of the emission channeling technique. TM probes on carbon coordinated tetrahedral interstitial sites (TC) and on substitutional silicon sites (SSi,h+k) were identified. We tested for but found no indication that the TM distribution on SSi sites deviates from the statistical mixture of 1/3 hexagonal and 2/3 cubic sites present in the 6H crystal. The TM atoms partially disappear from TC positions during annealing at temperatures between 500 °C and 700 °C which is accompanied by an increase on SSi and random sites. From the temperature associated with these site changes, interstitial migration energies of 1.7–2.7 eV for Mn and Ni, and 2.3–3.2 eV for Fe were estimated. TM lattice locations are compared to previous results obtained in 3C-SiC using the same technique.

Anisotropic Migration of Defects under Strain Effect in BCC Iron**Supported by the National Natural Science Foundation of China under Grant Nos 11375242, 11675230 and 91426301.

The basic properties of defects (self-interstitial and vacancy) in BCC iron under uniaxial tensile strain are investigated with atomic simulation methods. The formation and migration energies of them show different dependences on the directions of uniaxial tensile strain in two different computation boxes. In box-1, the uniaxial tensile strain along the direction influences the formation and migration energies of the dumbbell but slightly affects the migration energy of a single vacancy. In box-2, the uniaxial tensile strain along the direction influences the formation and migration energies of both vacancy and interstitials. Especially, a dumbbell has a lower migration energy when its migration direction is the same or close to the strain direction, while along these directions, a vacancy has a higher migration energy. All these results indicate that the uniaxial tensile strain can result in the anisotropic formation and migration energies of simple defects in materials.

Formation, migration, and clustering energies of interstitial He in α-quartz and β-cristobalite

Precipitation of implanted helium (He) is detrimental to many nuclear materials. A solid in which implanted He does not precipitate, but rather remains in solution and diffuses readily is potentially of interest for applications requiring resistance to He-induced damage. We use density functional theory (DFT) calculations to examine He interstitial formation, migration, and clustering energies in two SiO2 polymorphs: α-quartz and β-cristobalite. Our findings show greater He solubility and mobility in the latter than in the former. This difference appears to be due primarily to the unlike atomic-level structures of α-quartz and β-cristobalite, rather than their differing densities. Our findings also suggest that He is unlikely to cluster in either material. The behavior of He in α-quartz and β-cristobalite, and similar forms of silica make them promising materials for further investigation for potential use in applications requiring resistance to He-induced damage.

Defect energetics of concentrated solid-solution alloys from ab initio calculations: Ni0.5Co0.5, Ni0.5Fe0.5, Ni0.8Fe0.2 and Ni0.8Cr0.2.

It has been shown that concentrated solid solution alloys possess unusual electronic, magnetic, transport, mechanical and radiation-resistant properties that are directly related to underlying chemical complexity. Because every atom experiences a different local atomic environment, the formation and migration energies of vacancies and interstitials in these alloys exhibit a distribution, rather than a single value as in a pure metal or dilute alloy. Using ab initio calculations based on density functional theory and special quasirandom structures, we have characterized the distribution of defect formation energy and migration barrier in four Ni-based solid-solution alloys: Ni0.5Co0.5, Ni0.5Fe0.5, Ni0.8Fe0.2, and Ni0.8Cr0.2. As defect formation energies in finite-size models depend sensitively on the elemental chemical potential, we have developed a computationally efficient method for determining it which takes into account the global composition and the local short-range order. In addition we have compared the results of our ab initio calculations to those obtained from available embedded atom method (EAM) potentials. Our results indicate that the defect formation and migration energies are closely related to the specific atoms in the structure, which further determines the elemental diffusion properties. Different EAM potentials yield different features of defect energetics in concentrated alloys, pointing to the need for additional potential development efforts in order to allow spatial and temporal scale-up of defect and simulations, beyond those accessible to ab initio methods.