Defect Diffusion Coefficient Research Articles

Improved passivation performance of selective laser melted Inconel 718 alloy via tempering treatment

The passivation performance of the selective laser melted Inconel 718 alloy after tempering treatments was investigated, and the underlying mechanism for the improved passivation film protectiveness was explored. The dislocation reversion and the mitigated segregation of Nb, Mo and Ti at Laves and δ phases decreased the storage energy of lattice distortion were critical factors for the formation of more protective passivation film in chloride-containing solutions. The film formation rate increased after tempering, presenting the increased electric field strength and the decreased dissolution rate accordingly, as well as for the point defect diffusion coefficient and the oxygen vacancy flux therein.

Influence of Grain Size and Film Formation Potential on the Diffusivity of Point Defects in the Passive Film of Pure Aluminum in NaCl Solution

The influence of grain size on the corrosion behavior of pure aluminum and the defect density and diffusion coefficient of surface passive films were investigated using electron backscatter diffraction (EBSD) and electrochemical testing techniques, based on the point defect model (PDM). Samples with three different grain sizes (23 ± 11, 134 ± 52, and 462 ± 203 μm) were obtained by annealing at different temperatures and times. The polarization curves and electrochemical impedance spectroscopy results for the pure aluminum in the 3.5% NaCl solution showed that with decreasing grain size, the corrosion current (icorr) decreased monotonously, giving rise to a noble corrosion potential and a large polarization resistance. The Motte–Schottky results showed that the passive films that formed on pure aluminum with fine grains of 23 ± 11 μm had a low density (3.82 × 1020 cm−3) of point defects, such as oxygen vacancies and/or metal interstitials, and a small diffusion coefficient (1.94 × 10−17 cm2/s). The influence of grain size on corrosion resistance was discussed. This work demonstrated that grain refinement could be an effective approach to achieving high corrosion resistance of passive metals.

Comparing Electrochemical Passivation and Surface Film Chemistry of 654SMO Stainless Steel and C276 Alloy in Simulated Flue Gas Desulfurization Condensates.

This research examines the behavior of electrochemical passivation and the chemistry of surface films on 654SMO super austenitic stainless steel and C276 nickel-based alloy in simulated condensates from flue gas desulfurization in power plant chimneys. The findings indicate that the resistance to polarization of the protective film on both materials initially rises and then falls with either time spent in the solution or the potential of anodic polarization. Comparatively, 654SMO exhibits greater polarization resistance than C276, indicating its potential suitability as a chimney lining material. Mott-Schottky analysis demonstrates that the density of donors in the passive film formed on 654SMO exceeds that on C276, potentially due to the abundance of Fe oxide in the passive film, which exhibits the characteristics of an n-type semiconductor. The primary components of the passive films on both materials are Fe oxides and Cr oxides. The formation of a thin passive film on C276 in the simulated condensates is a result of the low Gibbs free energy of nickel oxide and low Cr content. The slower diffusion coefficient of point defects leads to the development of a thicker and more compact passive film on the surface of 654SMO.

A solid-state electrochemical device for studying thermodynamic properties of non-stoichiometric oxides: Application to UO2+x

We report attempts at determining the evolution of the oxygen activity in hyper-stoichiometric uranium dioxide for very low values of deviation from stoichiometry, using a solid-state electrochemical device. To this end, solid-state oxidation cycles coupled with oxygen activity measurements are carried out on a dense polycrystalline uranium oxide sample. We thus follow the evolution of the oxygen activity in the material as it oxidises or reduces, for temperatures ranging from 1093 to 1273 K. The data are subsequently analysed using an oxygen point defect model and equilibrium constants, corresponding to the various supposed point defect equilibria, are determined. We show that the model faithfully reproduces the trends observed in the experimental data. A more detailed analysis of the equilibrium constants obtained enables us to identify more precisely the limitations of the experimental method. In particular, we highlight the fact that the presence of residual oxygen in the device prevents us from determining the desired physical quantities for stoichiometry deviations less than 10−3. Ways of circumventing these difficulties which would further allow us to determine point defect diffusion coefficients are outlined.

Data-driven methods for diffusivity prediction in nuclear fuels

The growth rate of structural defects in nuclear fuels under irradiation is intrinsically related to the diffusion rates of the defects in the fuel lattice. The generation and growth of atomistic structural defects can significantly alter the performance characteristics of the fuel. This alteration of functionality must be accurately captured to qualify a nuclear fuel for use in reactors. Predicting the diffusion coefficients of defects and how they impact macroscale properties such as swelling, gas release, and creep is therefore of significant importance in both the design of new nuclear fuels and the assessment of current fuel types. In this article, we apply data-driven methods focusing on machine learning (ML) to determine various diffusion properties of two nuclear fuels—uranium oxide and uranium nitride. We show that using ML can increase, often significantly, the accuracy of predicting diffusivity in nuclear fuels in comparison to current analytical models. We also illustrate how ML can be used to quickly develop fuel models with parameter dependencies that are more complex and robust than what is currently available in the literature. These results suggest there is potential for ML to accelerate the design, qualification, and implementation of nuclear fuels.

The Sluggish Diffusion of Cations in CeO2 Probed through Molecular Dynamics and Metadynamics Simulations

Cation diffusion in fluorite‐structured CeO2, though far slower than anion diffusion, is an important, high‐temperature process because it governs diverse fabrication and degradation phenomena. Herein, cation diffusion is studied by means of classical molecular dynamics and metadynamics simulations. Three different mechanisms are examined: migration involving an isolated cerium vacancy, migration involving a cerium vacancy in a defect associate with an oxygen vacancy, and migration involving a cation divacancy. For each mechanism, defect diffusion coefficients are calculated as a function of temperature, from which the respective activation enthalpy of defect migration is obtained. Through comparisons with experimental cation diffusion data (specifically, of the absolute magnitude of the cation diffusivity as well as its activation enthalpy), it is concluded that cation diffusion takes place predominantly neither by isolated vacancies nor by cation vacancy–oxygen vacancy associates but by cation divacancies.

Concentrations and diffusion coefficients of thermal equilibrium point defects in silicon crystals

The dependencies of concentrations of thermal equilibrium vacancies and interstitials on temperatures in Si crystals are determined directly, which has been a long-standing issue since the 1950s. They are evaluated by combining the formation energies and self-diffusion entropies deduced from the analyses of self-diffusion coefficients, and migration entropies deduced from diffusion coefficients of point defects. The concentrations as the number density of thermal equilibrium vacancies and interstitials at temperature T (K) are determined to be 5 × 1022exp(6.5)exp(–3.85 eV/k B T) and 5 × 1022exp(10.6)exp(–4.3 eV/k B T) cm−3, respectively. The diffusion coefficients of vacancies and interstitials are determined to be 2.7 × 10−3exp(–0.45 eV/k B T) and 2.5 × 10−2 exp(–0.49 eV/k B T) cm2 s−1, respectively. The results are discussed in comparison with those reported experimentally.

Point defect model for passivity breakdown on hyper-duplex stainless steel 2707 in solutions containing bromide at different temperatures

Passivity breakdown on HDSS 2707 has been studied and the electrochemical data are interpreted in terms of the Point Defect Model. Pitting parameters of HDSS 2707 are determined, including the polarizability at bl/s interface, the defect annihilation rate, and the defect diffusion coefficient. The breakdown potential is demonstrated to be linearly related to log [Br-], pH, and the square root of potential scan rate (v1/2), and follows a near-normal distribution. The critical vacancy concentration calculated from the PDM is consistent with that estimated crystallographically from the chromic barrier layer, and the dominant point defect is further confirmed by Mott-Schottky measurements.

Passivation behavior and surface chemistry of 316 SS in the environment containing Cl− and NH4+

The passivation behavior and surface chemistry of 316 stainless steel (316 SS) in the environment containing NH4+ and Cl− are studied systematically in this paper. The influence of NH4+ on the passive film formed on 316 SS in the presence of Cl− was studied by electrochemical testing, point defect model (PDM), and X-ray photoelectron spectroscopy (XPS). The results show that NH4+ is favorable for the passive film formed on 316 SS. According to the PDM, it is concluded that the film performance is affected by the defect density and the defect diffusion coefficient, while the addition of low concentration NH4+ has little influence on the defect density but reduces the defect diffusion coefficient to delay the dissolution of the passive film, increasing the corrosion resistance of the passive film. In addition, the XPS results show that the addition of NH4+ does not have much influence on the double-layer structure, but increases the contents of Fe(III) and Cr(OH)3 in the surface layer of the passive film, making the surface layer of the passive film denser and resistant to erosive ions.

Ethylenediamine-assisted growth of multi-dimensional ZnS nanostructures and study of its charge transfer mechanism on supercapacitor electrode and photocatalytic performance

In recent years, much interest has been raised by materials with multi-purpose characteristics as the performance of electrochemical energy devices such as supercapacitors and photocatalytic activities depend strongly on the properties of materials. This study delineates various parameters like morphology, energy band gap, charge transfer resistance, different defect states, diffusion coefficient and functional groups adsorbed on the surface of material to assess the performance of supercapacitor electrodes and photocatalytic degradation efficiency of synthesised multi-dimensional ZnS nanostructures. Ethylenediamine (EN)-mediated multi-dimensional ZnS nanostructures were grown by the solvothermal route. One-dimensional (1D), 2D and 3D morphologies were obtained by varying the ratio of de-ionised water and EN, taken as 1:3, 1:2 and 1:1, respectively. The EN molecules effectively capped most of the surfaces of the ZnS nanoparticles formed, preventing agglomeration of nanoparticles due to the decrement in surface energy. The oriented attachment of these clusters resulted in the formation of 1D, 2D and 3D morphologies. The plausible chemistry in the formation of 1D, 2D and 3D nanostructures has been elaborated. Charge transfer properties of prepared electrodes have been examined using the electrochemical impedance spectroscopy (EIS) technique because better charge transfer causes diminishing electron/hole recombination and hence better photodegradation efficiency. Among the synthesised materials, the 2D nanostructure degraded the eosin Y dye to maximum 90.71% efficiency with rate constant 34 × 10−3 min−1. 2D nanostructures possess better charge transfer and hence better photodegradation efficiency. Various studies using methods of UV-vis, Fourier-transform infrared, Brunauer–Emmett–Teller, x-ray photoelectron spectroscopy and photoluminescence spectra are in good agreement with the obtained photodegradation results. After analysing cyclic voltammetry curves and EIS, a higher diffusion coefficient is obtained for 1D nanostructure material, hence a higher specific capacitance and higher energy density of 159.12 F g−1 and 22.75 KWh kg−1 are found in this case. Only 9% loss of specific capacitance is found after 1000 cycles, showing a relatively high cycling stability in 3D nanostructures. The excellent supercapacitive property can be attributed to the porous structure and high specific surface area. Thus, the synthesised multi-dimensional ZnS nanostructures are proved to be a potential candidate for both photocatalytic and supercapacitor electrode performance.

Фактор предпочтения базисной краевой дислокационной петли в цирконии. Численный анализ

Recent numerical calculations of the diffusion coefficients of radiation point defects in hexagonal crystals have made it clear that the main assumption of the radiation growth theory of zirconium (DAD - diffusional anisotropy difference) does not allow one to describe the radiation growth correctly. Thus, the elastic ideology (EID - elastic interaction difference), based on the concept of the flow bias factor, remains relevant. Therefore, the bias factor for the basic edge loop of zirconium in a toroidal reservoir was calculated numerically (using the finite difference method), taking into account the elastic anisotropy of the hexagonal crystal. The toroidal geometry of the reservoir makes it possible to calculate the flows for a loop of any size and without any correction of the elastic field in its area of influence. The dependences of the loop bias factor on its radius and nature are obtained for various sink densities. The essential role of the form of the boundary condition on the outer surface of the reservoir is shown. The prospects for further research in the construction of the theory of the radiation growth of zirconium based on the elastic ideology are discussed.

Correlated Brownian motion and diffusion of defects in spatially extended chaotic systems.

One of the spatiotemporal patterns exhibited by coupled map lattices with nearest-neighbor coupling is the appearance of chaotic defects, which are spatially localized regions of chaotic dynamics with a particlelike behavior. Chaotic defects display random behavior and diffuse along the lattice with a Gaussian signature. In this note, we investigate some dynamical properties of chaotic defects in a lattice of coupled chaotic quadratic maps. Using a recurrence-based diagnostic, we found that the motion of chaotic defects is well-represented by a stochastic time series with a power-law spectrum 1/fσ with 2.3≤σ≤2.4, i.e., a correlated Brownian motion. The correlation exponent corresponds to a memory effect in the Brownian motion and increases with a system parameter as the diffusion coefficient of chaotic defects.

Modelling of water transport through mixed‐ion conducting dense ceramics

AbstractThis study develops and demonstrates a model that characterizes defect transports, responsible for water transport within dense ceramics, and calculates the diffusion coefficients for those defects. The multi‐species mass transfer processes within yttrium doped barium cerates are modelled by applying the Nernst‐Planck equation to the system. The Nernst‐Planck equation with suitable boundary conditions is adopted to compute defect diffusion coefficients in COMSOL Multiphysics. All related equations, based on charge and defect conservation, are solved numerically and validated experimentally. The model also predicts the concentration distribution of the defects and potential profiles throughout the membranes. The results provided convenient insights about the water transport and charge distribution as a function of membrane thickness.

Defect Chemistry and Transport within Dense BaCe0.7Zr0.1Y0.1Yb0.1O3 − δ (BCZYYb) Proton-Conducting Membranes

This paper uses previously published conductivity measurements as the basis for establishing thermodynamic and transport properties for BaCe0.7Zr0.1Y0.1Yb0.1O3 − δ (BCZYYb) proton-conducting membranes. BCZYYb is mixed ionic-electronic conducting (MIEC) doped perovskite that is dominantly a proton-conducting ceramic. In addition to protons, BCZYYb permits transport of oxygen vacancies and O-site polarons. The present analysis considers the effects of immobile polaron traps. The paper reports enthalpies ΔH° and entropies ΔS° for the defect reactions, and defect diffusion coefficients in Arrhenius form. The paper incorporates the thermodynamic and transport properties into a Nernst–Planck model to characterize BCZYYb performance as a protonic-ceramic membrane for electrolyzer and fuel-cell applications. Especially at temperatures below 600°C, BCZYYb can achieve high faradaic efficiencies (i.e., low electronic leakage) as an electrolyzer membrane.

Interplay of defect doping and Bernal-Fowler rules: A simulation study of the dynamics on ice lattices

Protonic defects on ice lattices induced by doping with acids such as HCl and HF or bases such as KOH can facilitate order-disorder transitions. In laboratory experiments KOH doping is efficient in promoting the ordering transition from hexagonal ice I to ice XI, but it is ineffective for other known ice phases, for which HCl can trigger hydrogen ordering. Aiming at understanding these differences, random-walk simulations of the defect diffusion are performed on two- and three-dimensional ice lattices under the constraints imposed by the Bernal-Fowler ice rules. Effective defect diffusion coefficients are calculated for a range of dopants, concentrations, and ice phases. The interaction of different defects, incorporated by different dopants, is investigated to clarify the particular motion-enhancing role played by complementary defect pairs.

Multi-scale model for point defects behaviour in uranium mononitride

A multiscale approach was used to study the properties of point defects in uranium mononitride (UN). In this work we used combination of several methods: ab initio calculations; molecular dynamics simulations with a new interatomic potential; thermodynamic model. Density functional theory (DFT) calculations are used for fitting of the parameters of the angular-dependent interatomic potential, as well as for evaluation of the defects formation and migration energies. Molecular dynamics (MD) simulations are applied to analyze what migration/formation mechanisms are activated at finite temperatures and to calculate diffusion coefficients of point defects. The thermodynamic model for description of concentrations and diffusivities for point defects in non-stoichiometric UN1+x is proposed.

The diffusion of point defects in uranium mononitride: Combination of DFT and atomistic simulation with novel potential

The properties of point defects in uranium mononitride (UN) are studied by ab initio calculations and molecular dynamics simulations with a new interatomic potential. Density functional theory (DFT) calculations are used for fitting of the parameters of the angular-dependent interatomic potential, as well as for evaluation of the defects formation and migration energies. Molecular dynamics (MD) simulations are applied to analyse what migration mechanisms are activated at finite temperatures and to calculate diffusion coefficients of point defects. It is shown that the U antisite defects play an important role in the U-rich UN1−x. During migration the interstitial uranium is able to knock-out nitrogen atom, and this act leads to formation of U in antisite and N interstitial. This effect results in dependence of the diffusivity of U-interstitials on the concentration of defects in the N sublattice. Another peculiarity of UN is the large athermal concentration of U-vacancies in the N-rich UN1+x. This is due to close formation energies of nitrogen Frenkel pairs and Schottky defects. In addition, the applicability of the new potential for description of various phase transitions in UN is discussed.

Modeling defect transport during Cu oxidation

A defect transport model describing Cu oxidation kinetics to Cu2O and CuO is presented. The model supports findings that the dominant defects in Cu2O are singly charged and neutral copper vacancies, and in CuO doubly charged copper vacancies. In this work, CuO is treated as a charge transfer insulator and from the model a reaction enthalpy for Cu vacancies in CuO was estimated to be 250±10kJ/mole. Using the model, defect-diffusion coefficients are extracted from copper oxidation experiments and activation energies for vacancy-diffusion in Cu2O and CuO were found to be 54kJ/mole and 32kJ/mole, respectively.

Calculation of diffusion coefficients of defects and ions in UO2

This paper has presented molecular dynamics calculations of the diffusion coefficients of interstitials, vacancies, and vacancy complexes of oxygen and uranium in UO2, as well as the coefficients of ion diffusion provided by these defects. The interatomic potentials have been chosen by comparing the defect formation energies with data of the DFT + U calculations. The results of the calculations have been compared with experimental data on the annealing of defects and the measurements of self-diffusion coefficients of ions. The limitations of the model of point defects for the description of the self-diffusion in nominally stoichiometric UO2 have been discussed.

Effect of Cl− on the Properties of the Passive Films Formed on 316L Stainless Steel in Acidic Solution

The corrosion behavior of 316L stainless steel (316L SS) has been investigated in solutions containing various concentrations of chloride ions by using potentiodynamic polarization, capacitance measurement and Mott–Schottky relationship analysis (M–S). The result indicates that passive currents change slightly with the addition of chloride ions. The pitting potential (Epit) decreases linearly with log[Cl−]. Correspondingly, the point defect diffusion coefficient (Do) of the passive film increases linearly with increasing log[Cl−]. The results also indicate that the pitting corrosion of 316L SS follows the adsorption mechanism in NaCl solution.