Formation Of Interstitial Clusters Research Articles

Ab initio study of helium behavior near stacking faults in 3C-SiC

First-principles calculations are used to investigate the effects of stacking faults (SFs) on helium trapping and diffusion in cubic silicon carbon (3C-SiC). Both extrinsic and intrinsic SFs in 3C-SiC create a hexagonal stacking sequence. The hexagonal structure is found to be a strong sink of a helium interstitial. Compared to perfect 3C-SiC, the energy barriers for helium migration near the SFs increase significantly, leading to predominant helium diffusion between the SFs in two dimensions. This facilitates the migration of helium towards interface traps, as confirmed by previous experimental reports on the nanocrystalline 3C-SiC containing a high density of SFs. This study also reveals that the formation of helium interstitial clusters near the SFs is not energetically favored. The findings from this study enhance our comprehension of helium behavior in faulted 3C-SiC, offering valuable insights for the design of helium-tolerant SiC materials intended for reactor applications.

Molecular dynamics simulation of primary irradiation damage in Ti-6Al-4V alloys

Displacement cascade behaviors of Ti-6Al-4V alloys are investigated using molecular dynamics (MD) simulation. The embedded atom method (EAM) potential including Ti, Al and V elements is modified by adding Ziegler-Biersack-Littmark (ZBL) potential to describe the short-range interaction among different atoms. The time evolution of displacement cascades at the atomic scale is quantitatively evaluated with the energy of primary knock-on atom (PKA) ranging from 0.5 keV to 15 keV, and that for pure Ti is also computed as a comparison. The effects of temperature and incident direction of PKA are studied in detail. The results show that the temperature reduces the number of surviving Frenkel pairs (FPs), and the incident direction of PKA shows little correlation with them. Furthermore, the increasing temperature promotes the point defects to form clusters but reduces the number of defects due to the accelerated recombination of vacancies and interstitial atoms at relatively high temperature. The cluster fractions of interstitials and vacancies both increase with the PKA energy, whereas the increase of interstitial cluster is slightly larger due to their higher mobility. Compared to pure Ti, the presence of Al and V is beneficial to the formation of interstitial clusters and indirectly hinders the production of vacancy clusters.

Scaling laws and stability of nano-sized defect clusters in niobium via atomistic simulations and statistical analysis

The predictions of scaling laws for the structure and properties of defect clusters are generally limited to small defect clusters in their ground-state configurations. We investigated the size and geometrical configuration dependence of nano-sized defect clusters in niobium (Nb) using molecular dynamics. We studied the structure and stability of large clusters of size up to fifty defects for vacancies and one hundred defects for interstitials, as well as the role of helium and metastable configurations on the stability of these clusters. We compared three different interatomic potentials in order to determine the relative stability of these clusters as a function of their size and geometrical configurations. Additionally, we conducted a statistical analysis to predict the formation and binding energies of interstitial clusters as a function of both their size and configuration. We find that the size dependence of vacancy and interstitial clusters can be approximated by functional forms that account for bulk and surface effects as well as some considerations of elastic interactions. We also find that helium and metastable configurations can make vacancy and interstitial clusters thermally stable depending on the configuration. Our parameterized functional forms for the formation and binding energies are valid for a very broad range of defect sizes and configurations making it possible to be used directly in a coarse-grained modeling strategy such as Monte Carlo, cluster dynamics or dislocation dynamics which look at defect accumulation and evolution in microstructures.

On the fabrication of micro- and nano-sized objects: the role of interstitial clusters

Ion-induced bending phenomena were studied in free-standing nano-sized Al cantilevers with thicknesses in the range of 89–200 nm. The objective is to present a predictive and useful model for the fabrication of micro- and nano-sized specimens. Samples were irradiated in a Tescan Lyra dual beam system with 30 kV Ga+ ions normal to the sample surface up to a maximum fluence of ~ 2 × 1021 m−2. Irrespective of thickness, all samples bent initially away from the Ga+ beam; as irradiation proceeded, the bending direction was reversed. The Al cantilever bending behavior is discussed in terms of depth-dependent volume change due to implanted Ga atoms, radiation-induced point defects and interstitial clusters. A kinetic model is designed which is based on a set of rate equations for concentrations of vacancies, interstitial atoms, Ga atoms and clusters of interstitial atoms. The bending crossover is explained by the formation of sessile interstitial clusters in a zone beyond the Ga+ penetration range. Model predictions agree with our experimental findings.

Visualization of T Cell-Regulated Monocyte Clusters Mediating Keratinocyte Death in Acquired Cutaneous Immunity

It remains unclear how monocytes are mobilized to amplify inflammatory reactions in T cell-mediated adaptive immunity. Here, we investigate dynamic cellular events in the cascade of inflammatory responses through intravital imaging of a multicolor-labeled murine contact hypersensitivity model. We found that monocytes formed clusters around hair follicles in the contact hypersensitivity model. In this process, effector T cells encountered dendritic cells under regions of monocyte clusters and secreted IFN-γ, which mobilizes CCR2-dependent monocyte interstitial migration and CXCR2-dependent monocyte cluster formation. We showed that hair follicles shaped the inflammatory microenvironment for communication among the monocytes, keratinocytes, and effector T cells. After disrupting the T cell-mobilized monocyte clusters through CXCR2 antagonization, monocyte activation and keratinocyte apoptosis were significantly inhibited. Our study provides a new perspective on effector T cell-regulated monocyte behavior, which amplifies the inflammatory reaction in acquired cutaneous immunity.

Off-stoichiometric defect clustering in irradiated oxides

A cluster dynamics model describing the formation of vacancy and interstitial clusters in irradiated oxides has been developed. The model, which tracks the composition of the oxide matrix and the defect clusters, was applied to the early stage formation of voids and dislocation loops in UO2, and the effects of irradiation temperature and dose rate on the evolution of their densities and composition was investigated. The results show that Frenkel defects dominate the nucleation process in irradiated UO2. The results also show that oxygen vacancies drive vacancy clustering while the migration energy of uranium vacancies is a rate-limiting factor for the nucleation and growth of voids. In a stoichiometric UO2 under irradiation, off-stoichiometric vacancy clusters exist with a higher concentration of hyper-stoichiometric clusters. Similarly, off-stoichiometric interstitial clusters form with a higher concentration of hyper-stoichiometric clusters. The UO2 matrix was found to be hyper-stoichiometric due to the accumulation of uranium vacancies.

Theoretical study of deposition-induced point defects in ZnO

Using density functional theory based molecular dynamics simulations, we have investigated plasma – ZnO(0001) interactions for four different surface sites. The influence of 30eV Ar, Zn, and O bombardment on atomic and electronic modifications of ZnO(0001) has been explored. We observe exchange diffusion, channeling, O2 desorption, formation of Frenkel pairs and vacancies, Ar incorporation, and O vacancy – Zn interstitial cluster formation, which is significant for growth and properties. Zn and O form covalent and ionic bonds in ZnO, which are severed and rearranged in the vicinity of defects. These electronic structure changes may in turn alter the transport properties.

Electrical resistivity measurement of Fe-0.6%Cu alloy irradiated by neutrons at 14–19 K

Electrical resistivity measurement is a useful experimental method for investigating the recovery of defects that are induced by irradiation in metals and alloys. In this study, an Fe-0.6%Cu alloy, used to model steel from old commercial reactor pressure vessels, was irradiated by neutrons at a low temperature range of 14–19 K with a dose of about 1.3 × 1020 neutrons/m2 (E > 0.1 MeV) in the Kyoto University Reactor (KUR); electrical resistivity measurement was performed during irradiation and after annealing of the irradiated sample from 20 K to 300 K to investigate the migration of point defects in the Fe-0.6%Cu alloy. The electrical resistivity was measured at 14–19 K. With the increase in the irradiation dose, the electrical resistivity increased linearly. Four peaks appeared at 70 K, 100 K, 150 K, and 260 K, in the change of electrical resistivity during annealing of the irradiated sample up to 300 K. The former two peaks were caused by the recombination of interstitials and vacancies, and the latter two peaks were caused by the formation of interstitial clusters and the migration of vacancies. Compared with previous electron irradiation results, the former two peaks represent new data, as does the ratio of recombination caused by close-pair and correlation to that caused by migrations of mixed-interstitials Fe–Cu and vacancies decreased in neutron irradiation.

Influence of temperature histories during reactor startup periods on microstructural evolution and mechanical properties of austenitic stainless steel irradiated with neutrons

This paper addresses influence of two different temperature profiles during startup periods in the Japan Materials Testing Reactor and a boiling water reactor upon microstructural evolution and mechanical properties of austenitic stainless steel irradiated with neutrons to about 1 dpa and 3 dpa. One of the temperature profiles was that the specimens experienced neutron irradiation in both reactors, under which the irradiation temperature transiently increased to 290 °C from room temperature with increasing reactor power during reactor startup periods. Another was that the specimens were pre-heated to about 150 °C prior to the irradiation to suppress the transient temperature increase. Tensile tests at 290 °C and Vickers hardness tests at room temperature were carried out, and their microstructures were observed by FEG-TEM. Difference of the temperature profiles was observed obviously in interstitial cluster formation, in particular, growth of Frank loops. Although influence of neutron irradiation involving transient temperature increase to 290 °C from room temperature on the yield strength and the Vickers hardness is buried in the trend curves of existing data, the influence was also found certainly in increment of in yield strength, existence of modest yield drop, and loss of strain hardening capacity and ductility. As a result, Frank loops, which were observed in austenitic stainless steel irradiated at doses of 1 dpa or more, seemed to have important implications regarding the interpretation of not irradiation hardening, but deformation of the austenitic stainless steel.

Molecular dynamics simulation of the early stages of self-interstitial clustering in silicon

We have studied the early stages of self-interstitial clustering in silicon using molecular dynamics simulation techniques. We have generated silicon samples of over 200,000 atoms where we introduced a 0.5% extra concentration of self-interstitials. Then samples were annealed at several temperatures. During the simulations we observed the formation of interstitial clusters with different atomic structures, ranging from spherical and amorphous-like clusters, to highly ordered extended configurations such as (110) chains, {111} rod-like defects and dislocation loops, and {100} planar defects. This last type of defects, while common in germanium, have not been observed in silicon until very recently, in ultra-fast laser annealing experiments. The particular morphology of formed interstitial clusters is found to be related to the annealing temperature, as it is observed in the experiments. From the molecular dynamics simulations we have analyzed the atomic mechanisms leading to the formation and growth of interstitial clusters, with special attention to the newly found {100} planar defects.

Annealing in tellurium-nitrogen co-doped ZnO films: The roles of intrinsic zinc defects

In this article, the authors have conducted an extensive investigation on the roles of intrinsic zinc defects by annealing of a batch of Te-N co-doped ZnO films. The formation and annihilation of Zn interstitial (Zni) clusters have been found in samples with different annealing temperatures. Electrical and Raman measurements have shown that the Zni clusters are a significant compensation source to holes, and the Te co-doping has a notable effect on suppressing the Zni clusters. Meanwhile, shallow acceptors have been identified in photoluminescence spectra. The NO-Zn-Te complex, zinc vacancy (VZn)-NO complex, and VZn clusters are thought to be the candidates as the shallow acceptors. The evolution of shallow acceptors upon annealing temperature have been also studied. The clustering of VZn at high annealing temperature is proposed to be a possible candidate as a stable acceptor in ZnO.

Comparative study of defect evolution in carbon implanted strained SiGe and SiSn layers

AbstractBy combining secondary ion‐mass spectrometry, transmission‐electron microscopy (TEM) and Rutherford‐backscattering spectrometry we show that the redistribution of implanted carbon atoms around epitaxially strained Si/SiGe layers results in their accumulation on the Si side and depletion on the SiGe side. On the contrary, uphill diffusion of carbon into SiSn layers takes place in the case of Si/SiSn structures. The TEM study demonstrates formation of dislocation loops, stacking faults and interstitial clusters in the Si/SiGe layers, but elimination of interstitial dislocation loops and suppression of tin precipitates in the Si/SiSn layers. We deduced different evolution of dislocation loops and a precipitate is due to dopant‐defect complexes. The complex formation is enhanced by separation of implanted point defects in strain‐fields of Si/SiSn and Si/SiGe layers. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Radiation damage in cubic ZrO2 and yttria-stabilized zirconia from molecular dynamics simulations

We perform molecular dynamics simulation on cubic ZrO2 and yttria-stabilized zirconia (YSZ) to elucidate defect cluster formation resulting from radiation damage and evaluate the impact of Y dopants. Interstitial clusters composed of split-interstitial building blocks, i.e. Zr–Zr or Y–Zr were formed. Oxygen vacancies control cation defect migration; in their presence, Zr interstitials aggregate to form split interstitials, whereas in their absence Zr interstitials remain immobile, as isolated single-interstitials. Y-doping prevents interstitial cluster formation due to sequestration of oxygen vacancies.

Nucleation of interstitial dislocation loops in metals under electron irradiation

A hybrid model of the dislocation loop nucleation in metals during electron irradiation is pro� posed. The model is based on the kinetic diffusion mechanism of formation and growth of threedimensional interstitial atomic clusters and the thermodynamic mechanism of intermittent transformation of these clus� ters in dislocation loops. The existence of critical input rate GC of freely migrating point defects is established. When G G C , the number of interstitial atoms increases (n 0 ≥ 4). By analyzing experimental data for the electronirradiated aluminum, it is shown that, using the formulas of the proposed model, one can cal� culate all parameters of nucleation of interstitial dislocation loops at a given magnitude of the input rate of freely migrating defects without performing experimental studies.

Proton irradiation-induced creep of ultra-fine grain graphite

The objective of this work was to systematically study the effects of applied tensile stress, dose rate, temperature, and accumulated dose on the in situ proton irradiation-induced creep behavior of graphite. Experiments were performed on ultra-fine grain POCO Graphite Inc. ZXF-5Q. The irradiation creep rate exhibited a linear dependence on applied tensile stress, dose rate, and temperature and no dependence on dose to a total accumulated value of 1.0dpa. These dependencies showed reasonably good agreement with in-reactor creep data. Analysis of the crystal structure and crystallite size parameters following irradiation under load revealed that the most-probable source of structural change is formation of interstitial clusters between basal planes and uncollapsed vacancy lines, in agreement with changes observed for graphite irradiated in-reactor. The parametric dependencies and the change of the lattice parameters, and crystallite dimensions, were compared with dependencies from several potential mechanisms of irradiation-induced creep and revealed, via process of elimination, that Stress-Induced Preferential Absorption (SIPA) is the most likely mechanism to explain irradiation-induced creep in this material.

Interstitial zinc clusters in zinc oxide

Doped zinc oxide (ZnO) exhibits anomalous Raman modes in the range of 270 to 870 cm${}^{\ensuremath{-}1}$. Commonly, the resonance at 275 cm${}^{\ensuremath{-}1}$ is attributed to the local vibration of Zn atoms in the vicinity of extrinsic dopants. We revisit this assignment by investigating the influence of isotopically purified zinc oxide thin films on the frequency of the vibrational mode around 275 cm${}^{\ensuremath{-}1}$. For this purpose, undoped and nitrogen-doped ZnO thin-films with Zn isotope compositions of natural Zn, ${}^{64}$Zn, ${}^{68}$Zn, and a 1:1 mixture of ${}^{64}$Zn and ${}^{68}$Zn were grown by pulsed laser deposition. The isotopic shift and the line shape of the Raman resonance around 275 cm${}^{\ensuremath{-}1}$ are analyzed in terms of three different microscopic models, which involve the vibration of (i) interstitial zinc atoms bound to extrinsic defects, (ii) interstitial diatomic Zn molecules, and (iii) interstitial zinc clusters. The energy diagram of interstitial Zn-Zn bonds in a ZnO matrix is derived from density functional theory calculations. The interstitial Zn-Zn bond is stabilized by transferring electrons from the antibonding orbital into the ZnO conduction band. This mechanism facilitates the formation of interstitial Zn clusters and fosters the common $n$-type doping asymmetry of ZnO.

The influence of ∑3 twin boundaries on the formation of radiation-induced defect clusters in nanotwinned Cu

Abstract

Leakage Reduction by Thermal Annealing of NiPtSi Silicided Junctions and Anomalous Grain-Incompatible Pt Network

Using highly reliable damage-free junctions, the effectiveness and limitation of Pt addition for the stabilization of thin NiSi films are accurately specified and practically formulated in terms of the thermally induced leakage. In addition to the thermal leakage, the unexpected emergence of initial leakage is also witnessed and attributed to the emission of Si interstitials during silicidation and the subsequent formation of boron interstitial clusters. Rapid evanescence of the initial leakage by post-annealing is also successfully demonstrated owing to the Pt-induced thermal stabilization. Moreover, unlike other Pt distributions considered so far, Pt atoms are revealed to concentrate in a distinctive manner, forming an anomalous in-layer web-like structure which even extends within single NiSi grains. This grain-incompatible Pt network is thought to be a remnant of Pt-aggregation around grain boundaries of an earlier metal-rich silicide phase (e.g., Ni2Si), incorporated and left intact in the final phase (i.e., NiSi). Such intermediate-phase Pt-rearrangement may have interfered with the phase transition sequence and reoriented the final NiSi grains to constitute a crystallographically stable and thermally robust interface structure, resulting in the effective stabilization by Pt addition.

Effect of boron on interstitial-related luminescence centers in silicon

Photoluminescence measurements have been used to investigate the optically active defect centers formed by silicon implantation and a subsequent anneal at 275, 400, or 525 °C. The presence of boron in p-type silicon is found to produce deleterious effects on the luminescence of the interstitial-related W- and X-centers as well as a lower energy broad luminescence band. This effect has not been previously reported but it is consistent with the suppression of interstitial-related {311} extended defect formation in the presence of high boron concentrations at higher annealing temperatures. The results presented in this letter provide insight into the role of boron in the initial stages of interstitial cluster formation.

Mutual recombination and clusterization effect of the vacancy and interstitial barriers on radiation hardening materials

There is proposed the nonlinear model of dose dependence saturation of the yield strength on the base of the vacancy and interstitial barrier interaction in this work. Processes of mutual recombination of vacancy and interstitial barriers and formation of vacancy and interstitial clusters are taken into consideration. In the framework of the model, the analytical equations corresponding to the evolution of the barrier densities and yield strength are obtained. It is shown that the yield strength of irradiated materials decreases with the increasing intensity of barrier recombination processes, the dependence being nonlinear. Also it is shown that the model is valid both for low doses and large doses on the stage of radiation hardening.