Size Of Defect Clusters Research Articles

Defect-induced resonances and magnetic patterns in graphene

We investigate the effects of point and line defects in monolayer graphene within the framework of the Hubbard model, using a self-consistent mean field theory. These defects are found to induce characteristic patterns into the electronic density of states and cause non-uniform distributions of magnetic moments in the vicinity of the impurity sites. Specifically, defect induced resonance bound states in the local density of states are observed at energies close to the Dirac points. The magnitudes of the frequencies of these resonance states are shown to decrease with the strength of the scattering potential, whereas their amplitudes decay algebraically with increasing distance from the defect. For the case of defect clusters, we observe that with increasing defect cluster size the local magnetic moments in the vicinity of the cluster center are strongly enhanced. Furthermore, non-trivial impurity induced magnetic patterns are observed in the presence of line defects: zigzag line defects are found to introduce stronger-amplitude magnetic patterns than armchair line defects. When the scattering strength of these topological defects is increased, the induced patterns of magnetic moments become more strongly localized.

Computer simulations of large-scale defect clustering and nanodomain structure in gadolinia-doped ceria

The aggregation of defects in gadolinia-doped ceria (GDC) into clusters and larger domains has been studied by atomistic computer simulation. It is found that sub-nanoscale defect clusters prefer a pyrochlore-type structure in which the dopants and vacancies are at next-nearest-neighbor sites, and have a tendency to grow into larger clusters. It was determined that, as defect clusters grow into nanoscaled domains, the C-type rare earth structure, in which the dopants and vacancies are at nearest-neighbor sites, becomes more stable. Simulations suggest that nanodomains serve as the precursor of phase separation and can be easily formed during synthesis. It is believed that doping concentration limits the size of the nanodomains, and this causes GDC to favor small pyrochlore-type clusters at lower concentrations, but C-type nanodomains at higher concentration. Because of this transition, GDC is expected to show initially an increase in conductivity and then a decrease with increasing doping concentration. The lattice parameter of GDC should also show the same trend and could be used as an indicator of the predominant defect structure. The cation mobility is believed to be another important factor limiting the size of defect clusters, and can be used to control the formation of nanodomains during synthesis and thereby improve the electrolyte performance.

Effect of vacancy defect clusters on the optical property of the aluminium filter used for the space solar telescope

We investigate the microstructures of the pure aluminium foil and filter used on the space solar telescope, irradiated by photons with different doses. The vacancy defect clusters induced by proton irradiation in both samples are characterized by transmission electron microscopy, and the density and the size distribution of vacancy defect clusters are determined. Their transmittances are measured before and after irradiating the samples by protons with energy E = 100 keV and dose φ = 6 × 1011/mm2. Our experimental results show that the density and the size of vacancy defect clusters increase with the increase of irradiation doses in the irradiated pure aluminium foils. As irradiation dose increases, vacancies incline to form larger defect clusters. In the irradiated filter, a large number of banded void defects are observed at the agglomerate boundary, which results in the degradation of the optical and mechanical performances of the filter after proton irradiation.

A Molecular Dynamics Study of Helium-Vacancy Clusters Production due to Cascades in α-Iron

Molecular dynamics (MD) methods are utilized to study the displacement cascades in α-Fe containing different concentrations of substitutional He atoms. Primary knock-on atom (PKA) energies, Ep, from o.5 keV to 20 keV are considered at a temperature of 100 K and 600 K, and the results are compared with those performed in pure α-Fe. There are distinct differences in the number and size of defect clusters within displacement cascades with and without substitutional helium atoms. Particularly, the number and size of helium-vacancy clusters generally increase with increasing helium concentration and PKA energy. However, the number of He-vacancy (He-V) clusters increases with increasing temperature, the mean size of He-V clusters is independent on temperature for the same He concentration and energy recoils.

Mechanical properties and microstructures in F82H steel irradiated under alternating temperature

Reduced-activation martensitic steel F82H was irradiated at alternating temperatures between 230 and 350 °C to the accumulated damage level of 1.5 dpa using an irradiation capsule with temperature control independent of reactor power. Tensile tests were conducted in order to investigate the effects of the irradiation temperature variations on mechanical properties of F82H. Electron microscope observations were performed for the irradiated F82H to evaluate microstructural evolution of the specimens following varying temperature irradiation. Yield stress of the F82H irradiated at 50% alternating temperature between 230 and 350 °C was relatively large compared with the other temperature variations in this study. Size and number densities of dislocation loops were observed to be affected by changing irradiation temperature. The distinctive hardening behavior could be interpreted by the difference in the size and density of the defect clusters in terms of the effect of varying temperature.

Defect cluster formation and radiation hardening in molybdenum neutron-irradiated at 80 °C

Molybdenum was neutron-irradiated near 80 °C to doses of 7.2 × 10 −5, 7.2 × 10 −4, 7.2 × 10 −3, 0.072 and 0.28 dpa. Post-irradiation examination included electrical resistivity and tensile properties measured at room temperature. Microstructure of irradiated specimens was examined by TEM and the defect cluster density and cluster mean size were characterized. Measurements of electrical resistivity and cluster density showed sublinear defect accumulation behavior. The mean size of visible defect clusters increased with increasing dose. Yield stress decreased at 7.2 × 10 −5 and 7.2 × 10 −4 dpa, then increased significantly with increasing dose up to 0.072 dpa and saturated. It appeared that there was a transition in hardening from weak obstacles to strong obstacles. It is suggested that the formation of sessile defect clusters in neutron-irradiated Mo is mainly associated with diffusive nucleation and growth rather than in-cascade clustering.

Investigations of nano size defects in InP induced by swift iron ions

Indium phosphide (InP) samples were irradiated with swift (100MeV) 56Fe7+ ions with different fluences varying from 5×1012 to 2×1014cm−2 at room temperature. Atomic force microscopy (AFM) and high resolution X-ray diffraction (HRXRD) have been used to investigate the irradiation effects. AFM observations revealed the presence of nanosized defect clusters in all irradiated InP samples. Size (diameter) and density of defect clusters was found as a function of ion fluence. Root mean square (r.m.s) surface roughnesses measured using the Nanoscope software supplied with the AFM instrument were found to change from 0.33nm to 7.49nm. HRXRD studies revealed the presence of radiation-damaged layer (strained peak) in high fluence (2×1014cm−2) Fe ion irradiated InP. The screw dislocations, out of plane strain and lattice mismatch of irradiated samples have been studied.

Molecular dynamics study of the interactions between dislocation and imperfect stacking fault tetrahedron in Cu

The microstructure of irradiated face centered cubic alloys with low stacking fault energy is distinguished by the formation of a high number density of nanometer size stacking fault tetrahedra (SFT). A recent transmission electron microscopy investigation of high-energy proton irradiated copper [16] has shown that nearly 50% of the visible SFT population are not perfect SFTs, but rather consist of truncated SFT and/or groups of overlapping SFT. This paper presents the results of atomistic molecular dynamics simulations of the interaction between gliding dislocations, of either edge or screw character, and truncated SFT or overlapping SFT. The most common result of the edge dislocation interaction with a truncated SFT is defect shearing, ultimately leading to complete separation into two smaller defect clusters. Partial absorption of the truncated SFT is the most common result of the interaction with a screw dislocation, resulting in the formation of super-jog (or helical) segments as the defect is absorbed into the dislocation core. The resulting non-planar screw dislocation is self-pinned with reduced mobility and is re-emitted as a similar truncated SFT as the applied shear stress is increased. The re-emitted truncated SFT is often rotated and translated relative to the original position. These observations are consistent with the hypothesis that shearing (decreased defect cluster size) and dislocation dragging of the defect clusters by partial absorption into the dislocation core contributes to the formation of defect-free channels.

Microstructure of the F82H martensitic steel irradiated in STIP-II up to 20 dpa

The F82H martensitic steel was irradiated in the STIP-II experiment in SINQ up to 20 dpa in a temperature range of 110–400 °C. Transmission electron microscope observations have been performed to study the effects of radiation on the microstructure. The results show that at ⩽350 °C, the size of defect clusters or dislocation loops increases with both dose and temperature. The density of defect clusters or dislocation loops increases with increasing dose but decreases with temperature. A high-density of small He bubbles are visible in samples irradiated at ⩾165 °C with ⩾750 appm He. The bubble size increases while the density decreases with increasing irradiation temperature. These results agree with the previous observations of STIP-I specimens. In a specimen irradiated to 20.3 dpa with 1800 appm He at 400(±50) °C, large voids up to 50 nm in size were detected. The bubbles showed a bimodal size distribution. The martensite lath structure was almost completely removed by radiation. Meanwhile, the M 23C 6 precipitates along prior martensite lath and austenite boundaries disappeared and new M 23C 6 precipitates were formed in the matrix of new grains. In all cases, no obvious segregation of bubbles on grain boundaries was observed. However, it was detected that helium bubbles locate preferentially at pre-existing dislocations.

Molecular dynamics characterization of as-implanted damage in silicon

We have analyzed the as-implanted damage produced in silicon by B, Si and Ge ions using molecular dynamics (MD) simulations. Implantations were carried out at 50 K to avoid damage migration and annealing. In order to make a statistical study of the damage features, we have simulated hundreds of independent cascades for each ion for the same nuclear deposited energy. We have obtained that the average number of displaced atoms (DA) from perfect lattice positions and the size of defect clusters formed increases with ion mass. This dependence has not been obtained from equivalent binary collisions simulations. This indicates that multiple interactions play an important role in the generation of damage. Amorphous regions are directly formed during the collisional phase of the cascade of Ge and Si ions.

Raman studies in nanocrystalline lead (II) fluoride

Nanocrystalline lead fluoride was prepared by an inert gas condensation technique underultra-high vacuum conditions. Structural studies were carried out with x-ray diffractionanalysis. As-prepared and vacuum annealed samples were found to contain both orthorhombic(α) andcubic (β)phases of PbF2. The annealed samples contain dominantly the cubic phase. Grain sizes were found to be inthe range from 21 to 43 nm. Raman scattering measurements have been performed on thesesamples in different frequency regions. Characteristic phonon vibrational modes such asT2g for β-PbF2 have been observed in addition to some modes corresponding toα-PbF2. The Raman lineswere assigned to β and α-PbF2 on comparison with the already reported, both calculated and experimental, values. Somenew modes have also been observed at the higher frequency region. The modes at higherfrequencies were correlated to vibrations of electronic centres whose density varied withannealing temperature. The presence of electronic centres was investigated byphotoluminescence studies. The widths of the Raman spectral lines were found to decreasewith the increase in grain size. Low-frequency Raman studies revealed the presence ofstructural defect clusters. The defect cluster size was found to increase with annealingtemperature.

Observation and analysis of defect cluster production and interactions with dislocations

The current understanding of defect production fundamentals in neutron-irradiated face centered cubic (FCC) and body centered cubic (BCC) metals is briefly reviewed, based primarily on transmission electron microscope observations. Experimental procedures developed by Michio Kiritani and colleagues have been applied to quantify defect cluster size, density, and nature. Differences in defect accumulation behavior of irradiated BCC and FCC metals are discussed. Depending on the defect cluster obstacle strength, either the dispersed barrier hardening model or the Friedel–Kroupa–Hirsch weak barrier model can be used to describe major aspects of radiation hardening. Irradiation at low temperature can cause a change in deformation mode from dislocation cell formation at low doses to twinning or dislocation channeling at higher doses. The detailed interaction between dislocations and defect clusters helps determine the dominant deformation mode. Recent observations of the microstructure created by plastic deformation of quenched and irradiated metals are summarized, including in situ deformation results. Examples of annihilation of stacking fault tetrahedra by gliding dislocations and subsequent formation of mobile superjogs are shown.

Microstructure in martensitic steels T91 and F82H after irradiation in SINQ Target-3

T91 and F82H martensitic steels were irradiated in the Swiss spallation neutron source Target-3 to a maximum dose of 11.8 dpa in a temperature range of 90–360 °C. Transmission electron microscope observations were performed to study radiation effects on the microstructure at different irradiation doses and temperatures. The results for the unirradiated and irradiated conditions indicate that there is no significant difference between the microstructures of the two steels. High-density helium bubbles of size about 1 nm are observed in samples irradiated at temperatures above about 200 °C to about 10 dpa with 550 appm He. The helium bubbles observed in T91 steel have slightly higher density but smaller size as compared to those in F82H steel at the same irradiation conditions. The density and size of defect clusters in a T91 sample are almost the same as those in a F82H sample at the same dose. In both steels, the size increases while the density drops rapidly with irradiation temperature above about 250 °C. The amorphization of M 23C 6 precipitates was observed again in T91 and F82 samples irradiated at 220 °C and below.

The primary damage in Fe revisited by Molecular Dynamics and its binary collision approximation

ABSTRACTMolecular Dynamics (MD) is a very powerful tool for studying displacement cascades initiated by the neutrons when they interact with matter and thus evaluate the primary damage. The mean number of point defects created can be obtained with a fair standard error with a reasonable number of cascade simulations (10 to 20 [1]), however other cascades characteristics (spatial distribution, size and amount of defect clusters …) display a huge variability. Therefore, they may need to be studied using faster methods such as the Binary Collision Approximation (BCA) which is several order of magnitude less time consuming. We have investigated the point defect distributions subsequent to atomic collision cascades by both MD (using EAM potentials for Fe) and its BCA. MD and its BCA lead to comparable point defect predictions. The significant similarities and differences are discussed.

Clustering theory of atomic defects

Clustering of atomic defects leads to changes in the microstructure of materials, and hence induces drastic variations in their properties. In many technical fields, the role of defect clustering is very significant, and is sometimes limiting to further progress. We present here a comprehensive review of the theory of atomic defect clustering under non-equilibrium conditions, particularly encountered during irradiation of materials with energetic particles, as well as during material processing by energetic sources. These conditions are met in a wide range of technical applications, ranging from nuclear and fusion energy to microelectronics and surface engineering. We first present a general stochastic framework for the evolution of atomic clusters, and show how this can be described within the context of death-and-birth processes. This leads to the well-known master equation for microscopic atomic clusters. In the limiting case of a Poissonian process for the transition probabilities between cluster sizes, the master equation tends, in the macroscopic limit, to the mean-field approximation embodied by the theory of rate processes. When atomic clusters grow or shrink by the absorption of single atomic defects, a continuum Fokker-Planck approximation can be derived. Within this approximation, the evolution of interstitial loops, voids, bubbles, and general clusters of complex phases is presented, and in some cases, good agreement with experiments is obtained. It is shown that because of coalescence reactions, the evolution of surface atomic clusters during atom deposition processes is best described by kinetic moment equations, directly derived from rate equations. It is shown that breaking the symmetry of space or time leads to drastic variations in the size and space distributions of defect clusters. Examples are given for pulsed irradiation conditions, where it is shown that non-linear rate processes enhance cluster formation during on-time, and could lead to their dissolution during the off-time at high temperature. On the other hand, fluctuations are shown to result in instabilities and spatial self-organization of defect clusters. Description of pattern formation during irradiation, such as void and interstitial loop lattices, is very well described by a Ginzburg-Landau type equation, reminiscent of phase transitions under thermodynamic equilibrium conditions.

Simulation of the kinetics of defect accumulation in copper under neutron irradiation

Stochastic annealing simulations were originally developed for and successfully used to describe the short-term annealing stage of defect evolution within individual displacement cascades. Applied to a sufficiently large volume with periodic boundaries, stochastic annealing simulations can also provide a description of the evolution of defects produced during continuous irradiation. The irradiation is simulated by successive introduction of collections of defects representing the primary damage state of individual cascades placed in the simulation volume randomly in time and space. Cascade energies and the rate of their occurrence are chosen to approximate the damage due to the neutron flux of the 14 MeV neutron source Rotating Target Neutron Source-II (RTNS-II). The cascades are chosen from a library of cascades generated in molecular dynamics (MD) simulations for recoil energies from 5 to 25 keV. The numbers of each type of defect, defect cluster size distributions, as well as the positions of the defects within the crystal, are monitored as a function of time. Simulated defect cluster densities as a function of dose up to 0.1 dpa at room temperature are compared to experimental results. The simulated cluster densities are within about a factor of two of the experimental results over several orders of magnitude of dose. The effects on damage accumulation due to dose, dose rate, cascade overlap and interstitial cluster mobilities are demonstrated with examples.

Defect distribution and the diffuse X-ray diffraction pattern of wüstite, Fe 1−x O

We evolve a physical picture of the defect structure of wustite, presenting a step by step description of how the complex diffuse diffraction patterns arise and are influenced by various possible real-space variables such as defect distribution, defect cluster size, number of interstitials and lattice strain. A motif of diffuse incommensurate superlattice peaks around each main Bragg peak position is indicative of the presence of a paracrystal-like distribution of defects. The most significant result of the present work is that in order to explain the presence of the asymmetric central peak within this diffraction motif it is necessary that the lattice is inhomogeneous. That is, there exist regions containing the paracrystal array of defect clusters interspersed with regions containing no defects. Of all the possible single cluster types the V13T4 (Koch-Cohen) clusters appear to us to give diffraction patterns most similar in detail to the observed patterns, but there is also evidence for the presence of a proportion of the larger V16T5 clusters.

Fully Atomistic Analysis of Diffuse X-Ray Scattering Spectra of Silicon Defects

ABSTRACTDiffuse X-ray scattering is a useful method for studying defects in silicon and metals. Although the traditional approaches of analyzing experimental diffuse X-ray scattering data have given much information about the size of defects and defect clusters, they are not very well suited for determining the atomic configuration. We present a fully atomistic computational method to calculate the diffuse X-ray scattering line profile of an arbitrary atomic configuration, and compare line profiles of point defects and Frenkel pair configurations with experiment.

Identification of the nature of neutron-irradiation-induced small point defect clusters in nickel by means of electron irradiation and defect structure development from collision cascade

In order to identify the nature of small point defect clusters produced from cascade collisions, either made of interstitials or vacancies, an effective method is introduced in which the identification is made from the behavior of defects under electron irradiation. Material investigated is nickel neutron-irradiated with several facilities. Under electron irradiation, neutron-irradiation-induced defects having stacking fault tetrahedron (SFT) contrast shrank and disappeared, others having dislocation loop contrast grew. Majority of extremely small defects (≤ 1 nm), over 3/4 of all the defect clusters (bulk, 300 K) and about 95% of all the defect clusters (thin foil, 573 K), disappeared and are clarified to be of vacancy type. The size and spatial distributions of defect clusters in foil specimens and comparison of defect structures in thin foils with those in bulk disclosed the role of freely migrating interstitials.

Dependence of observed cascade defects on neutron spectrum and dose in Au and Ag irradiated with fission and fusion neutrons at low temperature

Cryotransfer transmission electron microscopy (TEM) experiments have been performed at about 110 K for Ag and Au irradiated with fission neutrons at 6 K and fusion neutrons at 20 K. More than 50% of the observed defect clusters were of interstitial type. The average size of defect clusters increased as neutron dose became higher. This result was explained by the effect of freely migrating interstitial atoms produced by newly developed displacement cascades. The effect of the difference in neutron spectrum was manifested in the number of defect clusters in a group of closely existing clusters (‘subcascade’ structure). There was a possibility that interstitial-type defect clusters were nucleated in the close vicinity of subcascades.