Diffusion Of Point Defects Research Articles

Quantitative analysis of local fine structure on diffusion of point defects in passive film on Ti

The diffusion of point defects in passivation determines all physico-electrochemical processes in materials corrosion and protection. In this study, a multiscale quantitative methodology was developed to reveal the diffusivity of oxygen vacancies on titanium surface (～5.0 nm thickness) in 1.0 M H2SO4 solution based on X-ray absorption fine structure (XAFS), combining with electrochemical impedance spectroscopy (EIS) and Mott-Schottky. EIS equivalent circuit exhibits that the high electronic field across passive film is 1.06 × 106 V cm–1. Mott-Schottky space charge capacitance indicates n-type semiconducting film with the donor density of ～1021 cm–3. Three important structure parameters have been firstly proposed from XAFS theoretical calculation at Ti K-edge to quantitatively describe the point defects transport between metal/film/solution interfaces. The half-jump distance equals half Ti–Ti distance subtly changing with oxidation potentials. The coordination coefficient is defined as the probability of activation jump from Ti–O coordination. And the Debye-Waller factor of structure disorder is related to the donor density of Mott-Schottky. Overall, the diffusivity of oxygen vacancies is in the range of (1.84–4.71) × 10–17 cm2 s–1 under the electric field, which determines the interfacial equilibrium of passivation and dissolution during potentiodynamic polarization. And the activation energy for motion has a minimum of 0.89 eV at 1.5 V. This multiscale quantitative methodology predicts well the potential dependent steady-state in the corrosive system.

Effect of Nickel Concentration on Radiation-Induced Diffusion of Point Defects in High-Nickel Fe–Cr–Ni Model Alloys during Neutron and Electron Irradiation

Effect of Nickel Concentration on Radiation-Induced Diffusion of Point Defects in High-Nickel Fe–Cr–Ni Model Alloys during Neutron and Electron Irradiation

Alkali Atoms Diffusion Mechanism in CuInSe2 Explained by Kinetic Monte Carlo Simulations

AbstractAdaptive kinetic Monte Carlo simulation (aKMC) is employed to study the dynamics and the diffusion of point defects in the CuInSe2 lattice. The aKMC results show that lighter alkali atoms can diffuse into the CuInSe2 grains, whereas the diffusion of heavier alkali atoms is limited to the Cu‐poor region of the absorber. The key difference between the diffusion of lighter and heavier alkali elements is the energy barrier of the ion exchange between alkali interstitial atoms and Cu. For lighter alkali atoms like Na, the interstitial diffusion and the ion‐exchange mechanism have comparable energy barriers. Therefore, Na interstitial atoms can diffuse into the grains and replace Cu atoms in the CuInSe2 lattice. In contrast to Na, the ion‐exchange mechanism occurs spontaneously for heavier alkali atoms like Rb and the further diffusion of these atoms depends on the availability of Cu vacancies. The outdiffusion of alkali substitutional atoms from the grains results in the formation of Cu vacancies which in turn increases the hole concentration in the absorber. In this respect, Na is more efficient than Rb due to the higher concentration of Na substitutional defects in the CuInSe2 grains.

A molecular dynamics simulation of energetics and diffusion of point defects in a Au–Ag alloy

For revealing an aging mechanism for self-irradiation in a Pu–Ga alloy, we carried out a molecular dynamics (MD) simulation on a substitutional material, i.e., Au–Ag alloy. In this work, we estimate physical and microscopic properties of the Au–Ag alloy containing various point defects using a MD method, in particular, formation energy for point defects, migration energy for point defects diffusion into interstitial sites, and diffusion coefficient for the Au–Ag alloy containing point defects, such as vacancy, He atom and He-vacancy (He-V) cluster. The results indicate that volumetric heat capacity and linear expansion coefficient would decrease due to the various point defects, and He atom has the most remarkable influence on the physical properties of the Au–Ag alloy for point defects considered in this work. The formation energy of Au and Ag self-interstitial atom indicates that Octa1 is the most stable site, and structural stability of octahedral (Octa) interstitial sites for the He atom obeys \(\hbox {Octa1}> \hbox {Octa2}> \hbox {Octa4} > \hbox {Octa3}\). For the \(\hbox {He}_{n}\hbox {V}_{m}\) cluster, the formation energy of the defect structure is most stable at \(n = m\). The diffusion coefficient of the He-V cluster is relatively smaller, showing that vacancy defects would further decrease atomic diffusion. An influence of various point defects on the diffusion velocity in the Au–Ag alloy obeys the He-V \(\hbox {cluster}> \hbox {He}> \hbox {vacancy}> \hbox {Ag} > \hbox {Au}\).

Point defect sink strength of low-angle tilt grain boundaries: A phase field dislocation climb model

Evaluating the point defect sink strength of grain boundaries is crucial for understanding the metal behavior of plasticity and damage under irradiation. In this paper, the point defect sink strength of low-angle symmetrical tilt grain boundaries is investigated by the phase field dislocation climb model under irradiation. The results indicate that sink strength of grain boundary is not only determined by the long-range point defect diffusion but also the short-range point defect absorption by the dynamic climbing of grain boundary dislocations. All of the study findings prove that the irradiation induced creep deformation of grain boundaries is essential for evaluating the radiation tolerance of materials.

Passivation behavior and surface chemistry of 2507 super duplex stainless steel in artificial seawater: Influence of dissolved oxygen and pH

Passivation and film chemistry of 2507 super duplex stainless steel in modified artificial seawater (ASW) are investigated. Removal of dissolved oxygen decreases the electric field strength and point defect diffusivity of the passive film. Passive current density in acidified ASW is higher than that in aerated ASW due to increased potential drop across film/solution interface, donor density and point defect diffusivity. The content of oxidized Cr, Fe(II) and hydroxides increases and film thickness reduces with acidification. In acidified ASW, passive film become denser from pre-passivation to passivation region and pores form accompanied with the increase of oxidized Fe after transpassivation.

Modeling the Influence of Strain Fields Around Precipitates on Defect Equilibria and Kinetics Under Irradiation in ODS Steels: A Multi Scale Approach

Resistance to radiation damage is a crucial property for structural materials in fission or fusion reactors. The particle-matrix interface in oxide dispersion strengthened (ODS) steels is considered a sink for point defects created during irradiation. In this work we address thequestion, if elastic strain fields around oxide particles cause a long-range interaction between the precipitates and point defects. We use kinetic Monte Carlo simulations to simulate the diffusion of point defects under the influence of elastic strain fields caused by Y2O3 and Y2Ti2O7 precipitates. We show that there is essentially no interaction between vacancies and the strain fields, while the sink strength of precipitates for interstitials increases with misfit strain between precipitate and matrix. Consequently, the interstitial concentration decreases with misfit strain, while due to reduced interstitial-vacancy recombination the vacancy concentration increases with misfit strain. The total change of point defect concentration with misfit strain is, however, rather limited.

Modification of GaN thin film on sapphire substrate optical properties under weak magnetic fields

We present a study of the modifications to the optical transmittance of GaN thin film on sapphire due to weak magnetic field (WMF) treatment. Measurements were performed in the 320–1100 nm wavelength range at 300 K, evidencing a change in transmission spectra for wavelengths above 700–800 nm after the WMF treatment. We use a model with three layers to simulate transmittance spectra. We have found that changes of the optical thickness induced by WMF are not sufficient to explain discrepancies between experimental and simulated spectra. To obtain a complete agreement it is necessary to consider a change in the refractive index of GaN and/or in the reflection coefficients for the interface air-GaN. We interpret such WMF-related effects as due to the diffusion of point defects after dissolution of metastable complexes from inner boundaries to surface of the structure. These defects modify optical properties of the GaN film.

Long-time point defect diffusion in ordered nickel-based binary alloys: How small kinetic differences can lead to completely long-time structural evolution

In this paper, we characterize the effect of defect kinetics on the stability of ordered nickel-based binary alloys, NiFe, NiCo and NiCu, using the kinetic Activation-Relaxation Technique (k-ART), an unbiased off-lattice kinetic Monte Carlo method with on-the-fly catalog building, that can provide kinetic pathways over second scales taking full account of chemical and elastic effects. We generate the full energy landscape surrounding vacancy and self-interstitial diffusion for L10 NiFe, an alloy with promising magnetic properties, with those of model L10 NiCo and NiCu, and combine this information with unbiased long-time kinetic simulations to characterize the link between specific microscopic diffusion mechanisms and overall phase stability. Our simulations demonstrate an unexpected richness and diversity: even though these alloys display similar proprieties like atomic radius, single vacancy and interstitial diffuse along totally different pathways that explain the relative stability of ordered structure.

A novel approach to quantifying the kinetics of point defect absorption at dislocations

Microstructure evolution under creep loading and/or irradiation is largely affected by the two-way coupling between dislocation induced plasticity and point defect diffusion as mediated by the internal elastic strain field within the medium. The magnitude of this effect has been quantified only in idealized, simplified environments such that estimates of absorption rates of point defects to dislocations ignore the collective effects on capture rates arising from particular distribution of dislocations in a given network, the interaction of point defects with the internal strain, or in many cases both. To quantify these effects, we advance a novel methodology to determine point defect capture rates locally within a spatially resolved finite difference solution to the diffusion problem for arbitrarily directed dislocations and the corresponding strain fields. In addition to examining the effects of these couplings on local and effective absorption rates of dislocation networks, the potential consequences of these interactions on other microstructure evolution phenomena are examined using void swelling under irradiation as an example. Our study indicates that local rates of defect mediated microstructural changes such as cavity growth can vary strongly as a consequence of the arrangement of the dislocation network and the corresponding strain state. The heterogeneous aspects of the resultant defect populations are shown to provide mechanisms for microstructural evolution under irradiation that are not available in the conventional homogeneous theory.

Formation energy and diffusion barrier of point defects in three uranium compounds as nuclear fuels

During fission neutron irradiation, point defects are generated in uranium fuel, and the point defects inevitably have some effects on the properties of the uranium fuel. In particular, the point defects accumulate through diffusion, resulting in formation of body defects that degrade the properties of the fuel. Using the first principles calculation method, based on the density functional theory, we calculated the elastic properties of three kinds of uranium compounds (UO2, UN, and UC). The results are in agreement with calculated and experimental results of others. On the basis of verifying the accuracy of the calculation model, we investigated formation energy and diffusion barrier of three kinds of point defects (vacancy, self-interstitial atom, and hydrogen/helium (H/He) atoms) in the three kinds of uranium compounds. The formation energy of the U vacancy is lower than that of the O, N, and C vacancy in the three kinds of uranium compounds, whereas the interstitial U atom has a higher formation energy than the interstitial O, N, and C atoms. The formation energy of the interstitial H atom is much lower than that of the interstitial He atom in the three kinds of uranium compounds, and the H atom prefers to occupy the O, N, and C vacancy while the interstitial He atom prefers to occupy the U vacancy. The diffusion of the interstitial H atom in UO2 is easier than that of the interstitial He atom, whereas the diffusion of the interstitial He atom in UN and UC is easier than that of the interstitial H atom. The diffusion of the interstitial self-atom in UO2 is more difficult than that of the self-atom in UN and UC. The work provides a fundamental understanding of the point defects formation and diffusion to develop uranium fuel by controlling defect formation.

Modelling of dislocation-solute interaction in ODS steels: Analytic bond-order potential for the iron-yttrium system

An interatomic potential for iron-yttrium is presented that allows to investigate the diffusion of yttrium in iron. The potential provides a complete description of the elastic and thermal properties of yttrium. The iron-yttrium interaction is fitted with focus on yttrium point defects in an iron matrix, as the interaction and diffusion of point defects is the crucial part of the formation of precipitates in ODS steels. The formation energy of substitutional yttrium atoms, the binding energy to vacancies and the migration barriers are very well reproduced. The potential is used to investigate the interaction between substitutional yttrium atoms and edge dislocations. It shows a significant attraction between yttrium atoms and the stress field of the dislocation which leads to yttrium segregation and pinning of dislocation motion. Pipe diffusion of yttrium atoms within the edge dislocation core is investigated by calculating migration barriers. The potential predicts a significant reduction of the activation barriers in the tensile part of the stress field around an edge dislocation. We conclude that yttrium pipe diffusion is relevant for the precipitate formation in ODS steels.

Atomistic modeling of α’ precipitation in Fe-Cr alloys under charged particles and neutron irradiations: Effects of ballistic mixing and sink densities

The α′ precipitation in Fe-Cr alloys under electron, neutron and ion irradiations is modelled by cluster dynamics and atomistic kinetic Monte Carlo simulations. The former method provides the evolution of the density of point defect sinks. This density is then introduced in the Monte Carlo which models the diffusion of point defects and atoms. By comparison with previous studies, this provides a better description of the evolution of point defect concentrations and therefore of the balance between two competing irradiation effects: the acceleration of diffusion and the ballistic mixing. The prediction of the model is in agreement with existing atomic-scale experiments already reported after electron, ion and neutron irradiations or obtained for the present study. Irradiation is found to accelerate the precipitation by orders of magnitude, except for the case of ion irradiation at high doses rates, where the simulations predict a ballistic dissolution of α′ precipitates. According to our model, this dissolution is mainly due to a high sink density, which reduces the concentrations of point defects and limits the acceleration of diffusion. This effect may be reinforced by carbon contamination, which reduces the mobility of point defects.

Damage accumulation and structural modification in c-plane and a-plane GaN implanted with 400 keV Kr and Gd ions

GaN is the most actively studied wide-bandgap material, applicable e.g. in short-wavelength optoelectronic devices, high-electron-mobility transistors, and semiconductor lasers. The crystallographic orientation of an implanted crystal can significantly influence the optical properties of the implanted layer, reflecting the rearrangement of the crystal matrix after annealing. The annealing procedure, influencing dynamic recovery, point defect diffusion and large defect stabilisation, depending on the GaN crystal orientation and the used ion implantation parameters, is still an important issue to be studied. We have studied the structural and compositional changes of the GaN-epitaxial-layers of c-plane and a-plane orientations grown by MOVPE and implanted with Gd and Kr ions using the ion energy of 400 keV and ion fluences of 5 × 1014 cm−2, 1 × 1015 cm−2 and 5 × 1015 cm−2 with subsequent annealing at 800 °C in ammonia. Dopant depth profiling was accomplished by Rutherford backscattering spectrometry (RBS). Induced structure disorder and its recovery during subsequent annealing were characterised by RBS channelling and Raman spectroscopy. Ion-implanted c-plane and a-plane GaN exhibit significant differences in damage accumulation simultaneously with post-implantation annealing, inducing a different structural reorganization of the GaN structure in the buried layer depending on the introduced disorder level, i.e. depending on the ion-implantation fluence and ion mass.

Electronic excitations induced climb of dislocations in swift heavy ion irradiated AlN and AlxGa1−xN

Recent work has shown that electronic excitations play a role in the creation of optical point defects in AlN although the material had been considered during a long time as insensitive to electronic energy loss and radiolysis. We address here an indirect observation of the effect of electronic excitations on point defect in AlN. Unprecedented climb of screw dislocations under swift heavy irradiation in irradiated AlN is studied. This is a sign to point defect diffusion towards the dislocation, which allows the increase of the length of the dislocation line. It is demonstrated that the climb, thus the point defect creation and/or their mobility is mainly induced by electronic excitations. This happens above an electronic stopping power threshold much lower than the one for ion track formation in AlN. This climb of dislocation was also observed for the same irradiation conditions in AlxGa1−xN (x = 0.5; 0.7; 0.8).

Mechanics of point defect diffusion near dislocations and grain boundaries: A chemomechanical framework

Diffusion of point defects during irradiation is simulated via a two-way coupling between mechanical stress and defect diffusion in iron. This diffusion is based on a modified chemical potential that includes not only the local concentration of radiation-induced defects, but also the influence of the residual stress field from both the microstructure (i.e. dislocations or grain boundaries) and the eigenstrain caused by the defects themselves. Defect flux and concentration rates are derived from this chemical potential using Fick’s first and second laws. Mean field rate theory is incorporated to model the annihilation of Frenkel pairs, and increased annihilation near grain boundaries is included based on the elastic energy of each grain boundary. Mechanical equilibrium is coupled with diffusion by computing eigenstrain from point defects and adding this to the total strain. Intrinsic stresses associated with the dislocations and grain boundaries are calculated using dislocation and disclination mechanics. Through this two-way-coupled model, regions of low concentration are seen near grain boundaries, and sink efficiency is calculated for different types of microstructure. The results show that the two-way mechanical coupling has a strong influence on sink efficiency for dislocation loops. The results also suggest that misorientation is a poor metric for determining sink efficiency, with sink density and elastic energy being much more informative.

Study of point defects diffusion in nickel using kinetic activation-relaxation technique

Point defects play a central role in materials properties. Yet, details regarding their diffusion and aggregation are still largely lacking beyond the monomer and dimer. Using the kinetic Activation Relaxation Technique (k-ART), a recently proposed off-lattice kinetic Monte Carlo method, the energy landscape, kinetics and diffusion mechanisms of point defect in fcc nickel are characterized, providing an exhaustive picture of the motion of one to five vacancies and self-interstitials in this system. Starting with a comparison of the prediction of four empirical potentials — the embedded atom method (EAM), the original modified embedded atom method (MEAM1NN), the second nearest neighbor modified embedded atom method (MEAM2NN) and the Reactive Force Field (ReaxFF) —, it is shown that while both EAM and ReaxFF capture the right physics, EAM provides the overall best agreement with ab initio and molecular dynamics simulations and available experiments both for vacancies and interstitial defect energetics and kinetics. Extensive k-ART simulations using this potential provide complete details of the energy landscape associated with these defects, demonstrated a complex set of mechanisms available to both vacancies and self-interstitials even in a simple environment such as crystalline Ni. We find, in particular, that the diffusion barriers of both vacancies and interstitials do not change monotonically with the cluster size and that some clusters of vacancies diffuse more easily than single ones. As self-interstitial clusters grow, moreover, we show that the fast diffusion takes place from excited states but ground states can act as pinning centers, contrary to what could be expected.

First principles calculations for iodine atom diffusion in SiC with point defects

The formation energies, diffusion barriers, vibration frequencies of complex iodine defects in 3C-SiC are calculated. The effective diffusion rates of these complex defects are evaluated in the temperature range from 300 K to 2000 K. The iodine interstitial diffusion is the dominant diffusion mechanism in bulk SiC since its diffusion energy barriers is the lowest. On the contrary, if the iodine atom substitutes a Si or a C atom, the diffusion rate is the lowest. The diffusion rates of other iodine point defects coupled with vacancies are between the rates of these two cases. The vacancies near the iodine atoms are traps for the iodine and they can slow down the diffusion rate of iodine atoms. The experimental diffusion rate of iodine in SiC is between that of calculated interstitial and substitutional iodine. It may result that the experimental data is from the combination of all the point defect diffusion mechanisms.

Oxygen migration enthalpy likely limits oxide precipitate dissolution during tabula rasa

In industrial silicon solar cells, oxygen-related defects lower device efficiencies by up to 20% (rel.). In order to mitigate these defects, a high-temperature homogenization anneal called tabula rasa (TR) that has been used in the electronics industry is now proposed for use in solar-grade wafers. This work addresses the kinetics of tabula rasa by elucidating the activation energy governing oxide precipitate dissolution, which is found to be 2.6 ± 0.5 eV. This value is consistent within uncertainty to the migration enthalpy of oxygen interstitials in silicon, implying TR to be kinetically limited by oxygen point-defect diffusion. This large activation energy is observed to limit oxygen precipitate dissolution during standard TR conditions, suggesting that more aggressive annealing conditions than conventionally used are required for complete bulk microdefect mitigation.

Hybrid systems with Ag nanocrystals and Si nanostructures synthesized by ultra-low-energy ion beam synthesis

Hybrid systems based on silicon and silver nanocrystals (Si-NCs and Ag-NCs) are of considerable interest in photon conversion solar cells. Due to their plasmonic properties, Ag-NCs strongly increase the photoluminescence emission intensity of Si-NCs located in their vicinity, allowing, in principle, to solve the problem of their low emission yield. In this work, we have elaborated 2D networks of Ag-NCs and amorphous Si nanoparticles in a controlled way by using Ultra-Low-Energy Ion-Beam-Synthesis. In the proposed synthesis scheme, a 2D layer of Si-NCs is first obtained by implanting Si+ ions at ultra low energy (from 1 to 3 keV) in a SiO2 layer with subsequent high temperature thermal annealing. Then, Ag+ ions are implanted in the same matrix at energies between 3 and 10 keV and crystalline Ag-NCs are formed during the implantation step. Several configurations with either 2D arrays or a large band of Ag-NCs have been obtained following the Ag+ implantation energy. Enhancement of the PL emission from Si nanostructures, which is related to the presence of Ag-NCs, has been observed under specific arrangement of the two embedded subsystems. In this type of synthesis, a combination of physical phenomena including ion mixing, implantation damage, point defect, and thermal diffusion has been taken into account in order to explain and thus control the structural and the optical characteristics of the system.