Void Alignment Research Articles

Irradiation-induced swelling of pure chromium with 5 MeV Fe ions in the temperature range 450–650 °C

A surface coating using pure chromium has been proposed to increase the accident tolerance of Zircaloy cladding in pressurized water reactors. However, there is not much irradiation experience with Cr alloys and especially pure Cr. In the present study, pure chromium was irradiated with 5 MeV Fe ions to 50 peak dpa (displacements per atom) at temperatures of 450, 500, 550, 600 and 650 °C. Then irradiation at the peak swelling temperature of 550 °C was conducted to 50, 100, 150 peak dpa. Swelling at 50 dpa was observed over the entire temperature range studied, 450–650 °C, but appeared to be decreasing strongly at the temperature boundaries of the experiment. After an initial transient of rapid swelling, chromium was observed to swell at a rate of ~0.03–0.04%/dpa (up to 120 local dpa), which is much lower than pure Fe at 0.2%/dpa. This low swelling rate was found to be relatively insensitive to dpa rate, which varied by a factor of ~2 over the depth of data collection. Swelling was observed to begin quickly with an incubation period less than 10 dpa. Self-organization in the form of void ordering was observed to be developing at 50 dpa, becoming better defined with increasing dose. The void alignment direction is determined to be the 〈111〉 axial direction.

Stress concentrations around voids in three dimensions: The roots of failure

We present stress concentration factors at the edges of 3D voids in uniaxial compression. Variations in these stress concentration factors due to Poisson's ratio of the host material, void shape, and void-void proximity are explicitly quantified. Voids loaded by a vertical compressive stress are hypothesised to fail in one or two ways: (1) tension crack development due to tensile stress concentrations at the void poles; or (2) compressive stress concentrations at the void sides causing the void wall to fail in shear. The stress concentration factors in this study are found using the numerical displacement discontinuity method. Equations are provided to assess the proximity of a void to the two hypothesised modes of failure. For 3D voids thinned in an axis that lies parallel to the compressive stress these have increasingly high stress concentrations at their sides. Changes in stress concentrations due to the Poisson's ratio of the matrix are minor, apart from for the intermediate stress at the void sides. Void shape, void separation, and void alignment are critical factors in the concentration of stresses. This suggests a significant departure from strength predictions for porous material that are based solely on scalar values of porosity.

Void alignment and density profile applied to measuring cosmological parameters

We study the orientation and density profiles of the cosmological voids with SDSS10 data. Using voids to test Alcock-Paczynski effect has been proposed and tested in both simulations and actual SDSS data. Previous observations imply that there exist an empirical stretching factor which plays an important role in the voids' orientation. Simulations indicate that this empirical stretching factor is caused by the void galaxies' peculiar velocities. Recently Hamaus et al. found that voids' density profiles are universal and their average velocities satisfy linear theory very well. In this article we first confirm that the stretching effect exists using independent analysis. We then apply the universal density profile to measure the cosmological parameters. We find that the void density profile can be a tool to measure the cosmological parameters.

Cellular Automata and X-Ray Microcomputed Tomography Images for Generating Artificial Porous Media

AbstractProblems involving fluid flow in porous media are of great interest in many scientific and technological areas. The development of numerical methods at the pore level allows simulating such phenomena considering characteristics and heterogeneities of the medium normally ignored in the macroscopic approach. To achieve the computational implementation of those numerical methods, it is necessary to initially define the domain (porous media) in which the simulations will be held. In this study, a simple, yet powerful methodology of representing a porous medium by means of cellular automata (CA) and microcomputed tomography is presented. Two parameters are proposed to link the tomographic images with those generated by CA. The first one considers the porosity, whereas the second one takes into account void alignment inside the image. The methodology computes the parameters in every automata image generated in a time range and compares them with the parameters of the tomographic image until certain user...

Analysis of the anisotropy of point defect diffusion in hcp Zr

A combination of density functional theory (DFT), kinetic Monte Carlo and mean-field rate theory is applied to analyze point defect migration and its effect on the observed growth of hexagonal close-packed (hcp) Zr under 1MeV electron irradiation. DFT is used to study stability of various configurations of vacancies and self-interstitial atoms (SIAs) and migration barriers. The data are used in kinetic Monte Carlo modeling of defect diffusion at different temperatures. It is found that both defects exhibit anisotropic diffusion, predominantly parallel to the basal planes. The ratio of diffusion coefficients parallel and perpendicular to the basal planes is found to be higher for vacancies as compared to SIAs at temperatures below ∼600K. This raises doubts that the observed radiation growth in Zr irradiated with 1MeV electrons, namely positive strains in prismatic and negative strains in basal directions, and void alignment along basal planes, can be accounted for by the anisotropy of point defect diffusion, which predicts opposite strain signs. It is speculated that formation of small SIA clusters with higher diffusion anisotropy may be responsible for the experimental observations.

Improving the Quality of Epitaxial Foils Produced Using a Porous Silicon-based Layer Transfer Process for High-Efficiency Thin-Film Crystalline Silicon Solar Cells

A porous silicon-based layer transfer process to produce thin (30-50 μm) kerfless epitaxial foils (epifoils) is a promising approach toward high-efficiency solar cells. For high efficiencies, the epifoil must have high minority carrier lifetimes. The epifoil quality depends on the properties of the porous layer since it is the template for epitaxy. It is shown that by reducing the thickness of this layer and/or its porosity in the near-surface region, the near-surface void size is reduced to <;65 nm and in certain cases achieve a 100 nm-thick void-free zone below the surface. Together with better void alignment, this allows for a smoother growth surface with a roughness of <;35 Å and reduced stress in the porous silicon. These improvements translate into significantly diminished epifoil crystal defect densities as low as ~420 defects/cm 2. Although epifoils on very thin porous silicon were not detachable, a significant improvement in the lifetime (diffusion length) of safely detachable n-type epifoils from ~85 (~300 μm) to ~195 μs (~470 μm) at the injection level of 10 15/cm 3 is achieved by tuning the porous silicon template. Lifetimes exceeding ~350 μs have been achieved in the reference lithography-based epifoils, showing the potential for improvement in porous silicon-based epifoils.

On the onset of void ordering in metals under neutron or heavy-ion irradiation

Formation of void lattices is observed in a number of metals and alloys under high-energy particle bombardment. The conditions were derived for destabilisation of homogeneous void arrangement using an approach developed for the description of lane formation in pedestrian crowds. The model is based on the Foreman's mechanism of void alignment due to one-dimensionally migrating clusters of self-interstitial atoms. The results show that spatial correlations between voids should exist above some very small size, unless correlations with other defects prevail. It is shown that spatial correlations of voids with dislocations and second-phase precipitates should also evolve and provide a powerful driving force for further swelling.

Simulations of the effects of 1-d interstitial diffusion on void lattice formation during irradiation

This paper describes the use of simulation techniques to examine some of the processes involved in the alignment of voids under the influence of one-dimensional self interstitial atom (1-d SIA) transport. The work follows the paper of Heinisch and Singh [1] on this topic but a different and simpler methodology is used. Besides repeating the scenarios studied by Heinisch and Singh, the effects of re-nucleation and the influence of vacancies have been introduced. One of the important processes that emerged from the results was the barrier to precise void alignment caused by the SIA-induced coalescence of aligned voids. This appears to prevent the formation of stable void lattices by any 1-d SIA transport mechanism, a point supported by the initial void alignment in the mechanism requiring swelling values well above those found experimentally. A full consideration of the void lattice phenomenon shows that the one-dimensional diffusion of self-interstitials central to the production bias model of irradiation damage cannot be the only mode of anisotropic diffusion available under irradiation.

A two-dimensional molecular dynamics simulation of thin film growth by oblique deposition

Atomistic, molecular dynamics simulations are employed to investigate the relationship between film microstructure and deposition conditions (substrate temperature, deposition kinetic energy, and deposition angle). Increasing substrate temperature and deposition kinetic energy leads to fewer voids, smaller voids, smoother surfaces, and higher film density. As the deposition angle increases, the film microstructure changes from a dense film, with few voids, to a microstructure in which nearly colinear tracks of elongated voids form and, finally, to a highly porous structure of well-formed columns. The angle along which the voids are elongated and the orientation of the void tracks are the same and increase monotonically with the deposition angle (the column angles follow the same trend as the deposition angle). Void formation, void alignment into tracks, and the columnar structure are all attributable to shadowing effects, which become more pronounced with increasing deposition angle. The variation of the column/void track angle β with deposition angle α fits well with the classical tangent law at low angles, but is overpredicted by the tangent law at α≳60°, consistent with experiment. The column angle β decreases slowly with increasing deposition kinetic energy due to increased surface mobility.

Glide of interstitial loops produced under cascade damage conditions: Possible effects on void formation

Abstract In the present communication we first summarize experimental and theoretical evidence for and then discuss the consequences of the production of small glissile self-interstitial loops under cascade damage conditions. We propose the production (“production bias”) and the incorporation of such loops into extended sinks such as dislocations and grain boundaries as an efficient driving force for void swelling, in particular in regions adjacent to grain boundaries. We also suggest that void lattice formation may be the result of void alignment caused by the one-dimensional glide of self-interstitial loops.

The void lattice in α-Al2O3

Alignment of irradiation-induced voids along specific crystallographic directions has been observed in many monatomic bcc, fee and cph metals. Observations of void alignment in diatomic materials, however, have been limited to just three cases: electron-irradiated CaF2 and SrF2 and neutron or heavy-ion irradiated α-Al2O3. We describe here a study of the evolution of the void lattice with increasing dose in heavy-ion irradiated α-Al2O3.Thin (∽50μm) polished, single crystals of α-Al2O3 were irradiated with 4MeV Ne+ and Ar+ ions to doses up to 30dpa over a range of temperatures from 500-900°C. Specimens were progressively backthinned through the damage profile by ion-beam milling and were examined at various stages in a Philips EM400 STEM. Voids were imaged using phase (defocused) contrast techniques.