Initial Vacancy Concentration Research Articles

Understanding the dose-rate effect on loop characteristics in Kr+-irradiated CeO2 by in-situ TEM study

Irradiation of CeO2 with heavy ions is commonly used to accelerate the evaluation of the irradiation performance of UO2 nuclear fuel. However, the differences in damage rates between ions and neutrons have led to doubts about this method, making it especially important to study the dose rate effect. Here, two kinds of dose rates were used to in-situ irradiate CeO2 using 400 keV Kr+ at 550 °C and 700 °C to study the dose rate effect on loop characteristics. High dose rate (HR) led to low-density and small-sized dislocation loops compared to low dose rate (LR), which was attributed to the initial high concentration of oxygen vacancies and enhanced recombination of point defects. HR was found to promote the direct nucleation of perfect dislocation loops (PDLs), while under LR irradiation, the PDLs tended to be formed by the transformation of Frank dislocation loops (FDLs) through a typical unfaulting. At a same dose, the proportion of PDL under the HR condition was much higher than that under the LR condition. Temperature greatly affected the loop characteristics. High temperature caused large-sized and small dense loops, as well as high PDL proportion. Moreover, the newly nucleated PDL tended to be close to the pre-existing PDLs and had the same Burgers vector as the adjacent PDL under HR irradiation at 700 °C. Current results provide underlying insights for assessing the effect of dose rate on irradiation-induced loop evolution and a reference for understanding the dose rate effect.

Initial Vacancy-Dependent High-Temperature Creep Behavior of Nanocrystalline Ni by Molecular Dynamics Simulation

Nanocrystalline metals possessing excellent mechanical strength have great potential to replace traditional metal materials as structural materials, but their poor resistance to creep deformation seriously restricts their engineering applications at high temperatures. The high-temperature creep behavior of nanocrystalline Ni with different volume fractions of initial vacancies ranging from 0% to 10% was studied systematically by molecular dynamics simulation in this study. The results showed that the steady-state creep displacement first increased and then decreased with increasing initial vacancy concentration, reaching the maximum when the initial vacancy concentration was 6%. The microstructural characteristics, such as quantity increment and distribution of the vacancies, the number and types of dislocations, and shear strain distribution during creeping, were analyzed in detail. The deformation-induced vacancies formed at the grain boundary (GB) in the initial creep stage, and their variation trend with the initial vacancy concentration was consistent with that of the creep displacement, indicating that the initial vacancy-dependent high-temperature creep behavior of nanocrystalline Ni was mainly determined by the rapidly increasing number of vacancies at the GB in the initial creep stage. Afterwards, the deformation-induced, vacancy-assisted 1/6{112} Shockley partial dislocation activities dominated the creep deformation of nanocrystalline Ni in the steady-state creep stage. The results can provide theoretical support for expanding the application of nanocrystalline metals from the perspective of crystal defect engineering.

Enhanced Proton Conduction with Low Oxygen Vacancy Concentration and Favorable Hydration for Protonic Ceramic Fuel Cells Cathode

Protonic ceramic fuel cells (PCFCs), as an efficient energy storage and conversion device, have great potential to solve the serious problems of energy shortage and environmental pollution. Improving the proton conductivity of the promising cathode materials is an effective solution to promote the widespread application of PCFCs at low temperatures (450-650 °C). Herein, considering the high oxygen reduction reaction (ORR) activity of BaCoO3-based perovskite oxide and beneficial proton uptake capacity of Zn-doping, we construct BaCo0.4Fe0.4Zn0.1Y0.1O3-δ (BCFZnY) as the PCFCs cathode, and compare it with the classic triple-conducting cathode BaCo0.4Fe0.4Zr0.1Y0.1O3-δ (BCFZrY). Different from the general strategy of increasing the initial oxygen vacancy concentration of cathode materials, this work unveils that enhancing the hydration of perovskite oxide with low oxygen vacancy concentration is a more effective strategy to accelerate the proton diffusion in the electrode. Therefore, the BCFZnY cathode achieved excellent proton conductivities of 8.05 × 10-3 and 6.38 × 10-3 S cm-1 as obtained by hydrogen permeation measurements and peak power densities of 982 and 320 mW cm-2 in a BaZr0.1Ce0.7Y0.1Yb0.1O3-δ-based anode-supported fuel cell at 600 and 450 °C, respectively.

Portevin-Le Chatelier Characterization of Quenched Al-Mg Alloy Sheet with Different Mg Concentrations.

In the present study, the PLC characteristic parameters and DSA mechanism of Al-(2.86~9.41) Mg alloy sheets were investigated during tensile testing at room temperature with a tensile rate of 1 × 10−3 s−1. On the basis of the solution Mg concentrations in the α-Al matrix, the initial vacancy concentration, the second-phase particle configuration and the recrystallized grain configuration are almost the same by quenching treatment. The results show that the type of room-temperature tensile stress–strain curves of quenched Al-(2.86~9.41) Mg alloy sheets varied according to the Mg content. The type of stress–strain curve of the Al-2.86 Mg alloy sheet was B + C, while the type of stress–strain curve of the Al-(4.23~9.41) Mg alloy sheets was C. When the quenched Al-(2.86~9.41) Mg alloy sheets were stretched at room temperature, the strain cycle of the rectangular waves corresponding to the high stress flow and stress drop amplitude on the zigzag stress–strain curve of alloy sheets increased with increasing the Mg content. Moreover, the strain cycle of and on the stress–strain curve of alloy sheets increased gradually with increasing tensile deformation. The yield stress of quenched Al-(2.86~9.41) Mg alloy sheets increased gradually with increasing the Mg content. Moreover, the critical strain corresponding to yield stress and the critical strain corresponding to the occurrence of the PLC shearing band of alloy sheets both increased with increasing the Mg content. However, the difference in flow strain value between and of alloy sheets decreased gradually with increasing the Mg content.

Effect of Vacancies on Dynamic Response and Spallation in Single-Crystal Magnesium by Molecular Dynamic Simulation

The effect of vacancies on dynamic response and spallation in single-crystal magnesium (Mg) is investigated by nonequilibrium molecular dynamics simulations. The initial vacancy concentration (Cv) ranges from 0% to 2.0%, and the shock loading is applied along [0001] and [10–10] directions. The simulation results show that the effects of vacancy defects are strongly dependent on the shock directions. For shock along the [0001] direction, vacancy defects have a negligible effect on compression-induced plasticity, but play a role in increasing spall damage. In contrast, for shock along the [10–10] orientation, vacancy defects not only provide the nucleation sites for compression-induced plasticity, which mainly involves crystallographic reorientation, phase transition, and stacking faults, but also significantly reduce spall damage. The degree of spall damage is probably determined by a competitive mechanism between energy absorption and stress attenuation induced by plastic deformation. Void evolution during spallation is mainly based on the emission mechanism of dislocations. The {11–22} <11–23> pyramidal dislocation facilitates the nucleation of void in the [0001] shock, as well as the {1–100} <11–20> prismatic dislocation in the [10–10] shock. We also investigated the variation of spall strength between perfect and defective Mg at different shock velocities. The relevant results can provide a reference for future investigations on spall damage.

Leakage current characteristics and DC resistance degradation mechanisms in Nb doped PZT films

The correlation between defect chemistry, leakage currents, and time-dependent dielectric breakdown was studied for PbZr0.52Ti0.48O3 (PZT) films doped with 0.5, 1, 2, or 4 mol. % Nb. As the samples are nearly intrinsic (that is, close to n- to p-type transition), signatures for both hole hopping between Pb2+ and Pb3+ and electron trapping by Ti4+ were observed. For all doping levels, the dominant conduction mechanism transitioned from Poole–Frenkel emission at lower electric fields to Schottky emission at higher electric fields. The electric field for this transition diminishes from 172 to 82 kV/cm with decreasing Nb concentration. The concomitant modification of the Schottky barrier height from 1.24 to 0.95 eV with decreasing Nb concentration is attributed to Fermi level pinning via oxygen vacancies. The DC resistance degradation was controlled by Schottky emission from 250 to 400 kV/cm. It was found that the lifetime of the films increases with increasing the Nb level. The effective Schottky barrier height for 2 mol. % Nb-doped PZT films decreased from 1.12 to 0.85 eV during degradation. This is related to the movement of oxygen vacancies toward the cathode and the observation of Ti3+ near the cathode, which are proved via thermally stimulated depolarization current and electron energy loss spectroscopy, respectively. Furthermore, Schottky emission starts to control the conduction at lower electric fields after degradation as a result of oxygen vacancy accumulation near the cathode. This, in turn, decreases the potential barrier height for electron injection from the Pt electrode into the PZT films. The mechanisms for time-dependent dielectric breakdown in PZT films will thus be a strong function of the initial oxygen vacancy concentration and its distribution within the PZT films.

Production, annihilation and migration of point defects in cyclic straining

Interactions of mobile dislocations in persistent slip bands with ladder-like structure are analyzed and production rates of point defects, preferably vacancies, are derived. Previous resistivity measurements in copper single crystals and polycrystals are used to derive the growth of vacancy concentration during cyclic loading at low temperatures. The parameters of the growth law are derived from the initial growth rate and saturated vacancy concentration. The consequences of vacancy production in localized cyclic straining on the surface relief formation are analyzed. At very low temperature vacancies are produced and annihilated only athermally and vacancy concentration saturates. Localized deformation in persistent slip bands produces high vacancy concentration which outcomes in formation of tiny extrusions. At ambient and slightly elevated temperatures vacancies are steadily produced and migrate from the persistent slip bands to the matrix. This results in formation of persistent slip markings consisting of extrusions and intrusions. The shape of extrusions and intrusions depends on the on the dislocation arrangement in the PSB and in the neighbor matrix and their history during cyclic straining. Fatigue cracks start from sharp-edged intrusions.

Production, Annihilation and Migration of Point Defects in Cyclic Straining

Interactions of mobile dislocations in persistent slip bands with ladder-like structure are analyzed and production rates of point defects, preferably vacancies, are derived. Previous resistivity measurements in copper single crystals and polycrystals are used to derive the growth of vacancy concentration during cyclic loading at low temperatures. The parameters of the growth law are derived from the initial growth rate and saturated vacancy concentration. The consequences of vacancy production in localized cyclic straining on the surface relief formation are analyzed. At very low temperature vacancies are produced and annihilated only athermally and vacancy concentration saturates. Localized deformation in persistent slip bands produces high vacancy concentration which outcomes in formation of tiny extrusions. At ambient and slightly elevated temperatures vacancies are steadily produced and migrate from the persistent slip bands to the matrix. This results in formation of persistent slip markings consisting of extrusions and intrusions. The shape of extrusions and intrusions depends on the on the dislocation arrangement in the PSB and in the neighbor matrix and their history during cyclic straining. Fatigue cracks start from sharp-edged intrusions.

Continuum-scale modeling of helium bubble bursting under plasma-exposed tungsten surfaces

We present a comparison between a continuum-scale drift-diffusion-reaction cluster dynamics prediction of helium retention in low-energy helium plasma exposed tungsten and experimental measurements, in a temperature regime that did not produce tungsten fuzz. Our cluster dynamics model, Xolotl, has been successfully benchmarked to high helium implantation flux MD simulations at relatively low implanted fluence. In this article, we also describe the extension of the Xolotl DDR model to incorporate the effect of bubble bursting, which is observed in very high rate MD simulations, as well as MD simulations at longer times than simulated in our prior benchmarking comparison. The bursting model parameters have been tuned by comparing to MD simulations at a flux of 5.0 × 1027 m−2 s−1, and also compared to lower implanted fluence simulations performed at ~4.0 × 1025 m−2 s−1. This article then reports on the consistency of the Xolotl predictions with respect to the size of the simulated cluster phase space (i.e. the maximum cluster size), initial vacancy concentration, and bubble growth trajectory (maximum number of helium atoms per vacancy). Finally, our simulation results are compared to helium plasma experiments that did not produce fuzz. While the Xolotl predictions including bubble bursting are in quantitative agreement with high-flux MD simulations, the initial comparison to plasma exposure experiments at a flux on the order of 1021 m−2 s−1 disagree by more than an order of magnitude, and in fact cannot reproduce the trends in helium retention with varying exposure temperature. Modifying the initial vacancy concentrations and helium cluster diffusion behavior in Xolotl leads to a reasonable agreement with the experimental observations, although the underlying physical explanation for these modifications remains unclear. The predicted helium content at experimentally relevant fluxes has been shown to be relatively insensitive to the parameters used in the bubble bursting model implemented in Xolotl, although these parameters have a larger influence at higher flux. More systematic comparisons between the modeling predictions with both experiments and MD simulation results is expected to improve the bubble bursting model in Xolotl in the future.

Phase field modelling on the growth dynamics of double voids of different sizes during czochralski silicon crystal growth

To investigate their dynamics and interaction mechanisms, the growth process of the two voids with different sizes during Czochralski silicon crystal growth were simulated by use of an established phase field model and its corresponding program code. On the basis of the several phase field numerical simulation cases, the evolution laws of the double voids were acquired as follows: the phase field model is capable to simulate the growth process of double voids with different sizes; there are two modes of their growth, that is, either mutual integration or competitive growth; the exact moment of their fusion can be also captured, and it is τ of 7.078 (simulation time step of 14156) for the initial vacancy concentration of 0.02 and the initial space between two void centers of 44Δx.

Dissociative diffusion mechanism in vacancy-rich materials according to mass action kinetics

Two sets of diffusion-reaction numerical simulations using a finite difference method (FDM) were conducted to investigate fast impurity diffusion via interstitial sites in vacancy-rich materials such as Cu(In,Ga)Se2 (CIGS) and Cu2ZnSn(S, Se)4 (CZTSSe or CZTS) via the dissociative diffusion mechanism where the interstitial diffuser ultimately reacts with a vacancy to produce a substitutional. The first set of simulations extends the standard interstitial-limited dissociative diffusion theory to vacancy-rich material conditions where vacancies are annihilated in large amounts, introducing non-equilibrium vacancy concentration profiles. The second simulation set explores the vacancy-limited dissociative diffusion where impurity incorporation increases the equilibrium vacancy concentration. In addition to diffusion profiles of varying concentrations and shapes that were obtained in all simulations, some of the profiles can be fitted with the constant- and limited-source solutions of Fick’s second law despite the non-equilibrium condition induced by the interstitial-vacancy reaction. The first set of simulations reveals that the dissociative diffusion coefficient in vacancy-rich materials is inversely proportional to the initial vacancy concentration. In the second set of numerical simulations, impurity-induced changes in the vacancy concentration lead to distinctive diffusion profile shapes. The simulation results are also compared with published data of impurity diffusion in CIGS. According to the characteristic properties of diffusion profiles from the two set of simulations, experimental detection of the dissociative diffusion mechanism in vacancy-rich materials may be possible.

Comparative molecular dynamics study of fcc-Al hydrogen embrittlement

Hydrogen embrittlement studies using reactive molecular dynamics have been conducted for Al metal nanoslabs with oxidized surfaces. These have provided evidence for possible grain boundary decohesion and void formation related range of hydrogen embrittlement mechanisms at different loading conditions with different initial defect configurations. Particularly, the effect of initial vacancy concentration on hydrogen diffusivity and localization, as well as on the amount of plasticity and grain formation is estimated. The results indicate significant reduction in diffusivity due to vacancy concentration, as well as hydrogen embrittlement effect through increase in dislocation emission and increase in dislocation slip propagation rate.

Characterisation and modelling of vacancy dynamics in Ni–Mn–Ga ferromagnetic shape memory alloys

The dynamics of vacancies in Ni–Mn–Ga shape memory alloys has been studied by positron annihilation lifetime spectroscopy. The temperature evolution of the vacancy concentration for three different Ni–Mn–Ga samples, two polycrystalline and one monocrystalline, have been determined. The formation and migration energies and the entropic factors are quite similar in all cases, but vary slightly according to composition. However, the number of jumps a vacancy has to overtake to reach a sink is five times higher in the single crystal. This is an expected result, due to the role that surfaces and grain boundaries should play in balancing the vacancy concentration. In all cases, the initial vacancy concentration for the samples quenched from 1173K lies between 1000ppm and 2000ppm. A phenomenological model able to explain the dynamics of vacancies has been developed in terms of the previous parameters. The model can reproduce the vacancy dynamics for any different kind of thermal history and can be easily extended to other alloys.

Growth Dynamics of Single Void during Czochralski Silicon Crystal Growth Using Phase-Field Modeling

To investigate the growth dynamics of the single void during Czochralski silicon growth as well as capture the basic features of the diffusion-controlled dynamic mechanisms, a phase field method has been developed. The free energy of the system involving the chemical free energy and the gradient energy is presented. Numerical tests were performed to examine the capability of this model, and the results show that: the void grows due to the absorption of vacancies in the matrix, which essentially reduces the free energy of the system; with the growth of the void, there forms vacancy concentration gradient towards the void in the matrix; the increase of initial vacancy concentration contributes to a larger void size and growth rate.

Effects of orientation and vacancy defects on the shock Hugoniot behavior and spallation of single-crystal copper

The effect of crystal orientation on the dynamic response of single-crystal copper is explored by using a molecular dynamics method with a realistic embedded atom potential. Three crystallographic orientations ([1 0 0], [1 1 0] and [1 1 1]) along which the shock waves propagate are investigated. For each orientation, an initial vacancy concentration (Cv) ranging from 0% to 2% is introduced to also take the effect of vacancy defects into consideration. The simulations characterize the defective Cu system in terms of shock Hugoniot behavior, local microstructure evolution and spall strength during the shock wave propagation. The shock wave is applied by pulling a layer of piston atoms at a velocity of 0.25–2 km s−1 with the Hugoniot pressure ranging from 15 to 120 GPa. The simulation results show that shock responses and the role of vacancy defects are strongly anisotropic for the three orientations investigated. For shock along the [1 0 0] orientation, vacancy defects have a negligible effect on the shock Hugoniot curve us–up relationship, but play a role in increasing spall damage. In contrast, for shock along the [1 1 0] and [1 1 1] orientations, vacancy defects not only significantly reduce the anisotropic level of us–up relationships, but also significantly decrease spall damage. Based on this, the orientation-dependent effects of vacancy defects on spallation strength and spall damage are analyzed by examining the defective evolution of single-crystal copper microstructure.

Molecular simulation of structural relaxation in ultrathin polymer films

Structural relaxation and physical aging in glassy polymer films have attracted much attention in the past two decades due to their strong correlation with the lifetime of polymer-based nano-devices. Currently, the observed physical aging in polymer films was explained on the basis of a free volume diffusion model (FVDM) that has not yet been validated by simulations at the molecular level. Here we performed a Monte Carlo simulation by introducing the vacancy diffusion mechanism (similar to FVDM) to investigate the structural relaxation in ultrathin polymer films. The results show that the local average density of segments increases linearly with the logarithm of time, similar to the fluorescence intensity and dielectric strength measurements in experiments. The responses of relaxation rates in ultrathin films to temperatures are consistent with that for glassy polymer thin films in experiments. The emergence of a peak of relaxation rates with decreasing temperature can be attributed to the competition of two mechanisms (segment mobility and accurate initial vacancy concentration) from the molecular levels. Our results demonstrate that the simulations with a vacancy diffusion model could indeed provide new insights from the molecular levels to understand the structural relaxation and physical aging in ultrathin polymer films.

Kr and Xe irradiations in lanthanum (La) doped ceria: Study at the high dose regime

In order to understand cavity and bubble formation and growth in oxide nuclear fuel materials, ion beam irradiation experiments were conducted with two common fission gas species: Kr and Xe. Ceria (CeO 2) was selected as a surrogate material for uranium dioxide (UO 2) due to its many similar properties to UO 2. Ion beam energies were chosen such that both cavities and gas bubbles structures were induced by ion irradiations. The ion irradiation experiments were carried out at 600 °C, at which temperature, cavity/gas bubble structures are believed to be immobile in this material. Lanthanum (La) was chosen as a dopant in CeO 2 to investigate the effect of impurities. The presence of La in the CeO 2 lattice also introduces a predictable initial concentration of oxygen vacancies, similar to the introduction of oxygen vacancies by the existence of Pu 3+ in MOX fuel [1]. The influence of two La concentrations, 5% and 25%, were examined. The study focused on the high dose regime where cavity/gas bubble structures were clearly identifiable with their sizes and number densities readily measurable. Cavity/gas bubble coarsening by coalescence was identified with TEM (Transmission Electron Microscopy) characterizations of as-irradiated La doped CeO 2 specimens. The results revealed that lanthanum trapping has significant influence on the cavity/bubble growth in the material lattice by comparing the cavity/gas bubble size distributions between 5% La doped ceria and 25% La doped ceria. Lattice and kinetic Monte Carlo calculations described in a previous work have provided insights to the interpretations of the experimental results [2]. Solid state Xe precipitates were observed in low energy Xe implantation in 5% La doped ceria to a very high fluence of 1 × 10 17 ions/cm 2 at 600 °C. The solid state Xe precipitate structures are represented by faceted morphology. Very similar observations of solid state/near solid state Xe bubbles were made by Nogita et al. in the outer region of UO 2 pellet irradiated to a pellet average burnup of 49 GWd/t [3].

Fission gas transport and its interactions with irradiation-induced defects in lanthanum doped ceria

To help understand the mechanisms of irradiation-induced defect formation and evolution in nuclear fuel, systematic experimental efforts have been carried out. Ceria (CeO 2) was selected as a surrogate material for Uranium Dioxide (UO 2) due to many similar properties. Lanthanum (La) was chosen as a dopant in CeO 2 to investigate the effect of impurities. The presence of La in the CeO 2 lattice introduces a predictable initial concentration of oxygen vacancies, making it possible to characterize hypostoichiometric effects in CeO 2. The influence of two La concentrations, 5% and 25%, were examined. In situ Transmission Electron Microscopy (TEM) experiments were used to study the evolution of defect clusters and the influence of irradiation with two common fission products: Xe and Kr. The irradiations were performed on thin film, single crystal materials. The irradiation damage caused formation of dislocation loopsat 600 °C and defect clusters at room temperature. Dislocation networks form as the result of interactions of defect clusters. The dislocation loops were determined to be mainly of 1/9[1 1 1] interstitial type loops. Quantitative results were obtained to characterize the fluence and temperature effects of irradiation. Slow defect kinetics were found with irradiation on 25% La doped CeO 2 at 600 °C and it is attributed to the higher concentration of oxygen vacancies due to high La dopant level.

Properties of doped ceria solid electrolytes in reducing atmospheres

In this work we present investigations of stability of rare earth-doped ceria electrolytes in reducing atmosphere. The effect of dopant type and dopant concentration on reducibility was studied on the basis of thermogravimetric and impedance spectroscopy measurements on materials grouped into two series: Ce 1 − x Gd x O 2 − x/2 (0 ≤ x ≤ 0.4) and Ce 0.85R 0.15O 1.925 (R = Y, Nd, Sm, Gd, and Dy). Relationship between an initial vacancy concentration introduced by the amount of dopant and the characteristic temperature of reduction was found. Much less pronounced dependence was observed for different dopants with the same concentration, which indicates that it is the dopant and vacancy concentration and not the dopant type, which is responsible for reducibility of ceria electrolytes. Impedance spectroscopy measurements allowed for calculation of changes of oxygen ions transport number during the reduction process.

The effect of vacancies on dynamic response of single crystal Cu to shock waves

Using molecular dynamics (MD) simulations, we investigate the effect of vacancies on the dynamic response of single crystal Cu to [100] shock loading, including plasticity and spallation, for an initial vacancy concentration (cv) ranging from 0% to 2%. A fixed impact velocity is adopted, for which plasticity and spall do not occur in the defect-free Cu during compression or tension. We show that shear flow strength (compressional or tensile) and spall strength decrease with increasing cv. At the MD scales, the vacancy effect becomes pronounced for cv>0.25%, where heterogeneous nucleation of plasticity prevails. Tensile plasticity may play a key role in inducing local heating and the power-law reduction in spall strength. Void nucleation occurs preferentially at highly sheared (plastically deformed) sites.