Interstitial Trapping Research Articles

Trapping and thermal migration of the first- and second-row atoms in Ar, Kr and Xe crystals.

Trapping and temperature-induced migration (TIM) of the first- and second-row atoms A from H to Ne in the face-centered cubic rare gas RG = Ar, Kr and Xe crystals are investigated within the classical crystal model parameterized by the empirically modified pairwise potentials. New ab initio coupled cluster A-RG potentials computed in a uniform way for all the atoms A are used to represent the atom-crystal interactions. Absolute and relative stabilities of the substitutional and interstitial trapping sites, their structures, interstitial migration pathways, related activation energies and rough estimates of the TIM rates are obtained. The isotropic model, which neglects non-zero atomic electronic orbital momentum, reveals that migration of interstitial atoms along the network of conjugated fcc octahedral voids is the generic case for atomic mobility. Anisotropic interactions with a crystal inherent to P-state atoms B, C, O and F are accounted for using the non-relativistic diatomics-in-molecule method. Depending on its sign, interaction anisotropy can alter the structures of interstitial trapping sites and transition states remarkably. This, in turn, can dramatically affect the TIM rates. Comparison with reliable experimental data available for oxygen and hydrogen indicates a systematic overestimation of the measured activation energies, by 30% at worst. A comprehensive literature review accomplished for other atoms reveals a lack of information on the TIM processes and rates, though makes it possible to verify a part of the present results on the trapping site energies and structures.

First-principles investigation of hydrogen interaction with Cu/WC interface

The WC-Cu composite used as a thermal barrier interlayer between tungsten and CuCrZr heat sink can effectively alleviate the thermal strain mismatch. In this work, the interaction of hydrogen (H) and Cu/WC interface are investigated by performing the first-principles density functional theory calculations. Considering the H solubility in the WC bulk, we found that the most energetically favorable site is not the traditional octahedral interstitial site but is the projection site of octahedral interstitial on the W basal plane. The H diffusion barrier for octahedral interstitial sites and the diffusion path refers to the channel for H penetration and migration into the bulk WC is as high as 0.773 eV. According to the interface calculations, it indicated that the C-terminated interface with the atoms in WC followed the stacking sequence of the Cu (111) plane can yield the strong adhesion for (111)Cu/(0001)WC interface. Furthermore, the higher dissolution energy for H in this Cu/WC interface indicates no interstitial segregation or trapping site for H in the region due to the localization of interface electrons. Note that the H atom in WC also exhibits a high diffusion barrier. Both of these factors combine to determine that the interface can effectively block the penetration behavior of H into the WC, and the WC-Cu composite materials can use as the H permeation barrier. Besides, our ab initio molecular dynamics (AIMD) simulation also confirmed that H positioned at the interface can spontaneously migrate from the interface to the Cu matrix, and the interface can block the H penetration.

Thermodynamic trapping and diffusion model for multiple species in systems with multiple sorts of traps

This work presents a trapping and diffusion model for multiple species of solute atoms in a system with multiple sorts of traps based on irreversible thermodynamics. We use two established approaches describing the interaction between lattice and traps, i.e. transient trapping and theory based local thermodynamic equilibrium, and extend them to systems with multiple species. Consequently, the presented theory and its numerical implementations cover effectively any kinetics of exchange between the lattice and traps as well as site competition effects within traps for any system size. The presented theory can be applied for several phenomena in material science. For example, (co-) segregation of solutes at grain boundaries, trapping of interstitials during precipitation processes and hydrogen interaction with material defects and other interstitials. Simulations of charging and discharging are presented for several plate-like samples containing multiple sorts of traps occupied by multiple species. The role of trapping parameters is demonstrated and discussed for charging and discharging behavior, site competition effects and the interaction of trapping kinetics with diffusion kinetics for multiple species.

Trapping and migration of P-state atoms in rare gas solids: effect of angular momentum anisotropy for model O(3P) and C(3P) atoms

Classical simulations of model O(P) atom isolated in Ar, Kr and Xe crystals with the variable magnitude and sign of the angular momentum anisotropy are performed. The guest–host interaction anisotropy remarkably affects the structure of the atomic trapping sites and the barriers to the temperature-induced atomic mobility. For interstitial trapping, inversion of anisotropy induces a kind of critical phenomenon, which transforms the migration path saddle point to a minimum and another saddle. As a result, a new peculiar axially symmetric interstitial site is formed with the migration barrier reduced. Preliminary ab initio-based study of the C(P) atom, whose interactions with Ar, Kr and Xe have the anisotropy opposite to oxygen, fully confirms the model analysis. Our results emphasise profound effect of the angular momentum anisotropy on the structure and dynamics of the open-shell atoms at matrix isolation.

Flux effects on point defect behavior by tracking loop evolution using combined irradiation method

Irradiation flux effects are directly related to the behavior of point defects, and it is challenging to observe them experimentally. During combined irradiation, the interstitial loops produced after the first irradiation proved to be good indictors of point defect behaviors during the second irradiation with different fluxes (∼50 times of difference). For specimens without Si, the growth of pre-existing interstitial loops in the second irradiation was more distinct at lower flux, whereas the number density of nucleated defects was significantly larger at higher flux. This was attributed to the long-range diffusion of interstitials toward pre-existing sinks in the low-flux irradiation. When the flux was high, such long-range diffusion was interrupted by the rapid production of cascades and point defects. For specimens with Si, the long-range diffusion was hindered by the interstitial trapping effect of Si, which was assumed to reduce the difference between the low-flux and high-flux irradiation.

Spatial distribution of defects in a plastically deformed natural brown diamond

Photoluminescence, Raman mapping, cathodoluminescence and transmission electron microscopy (TEM) have been carried out on a “zebra” diamond, containing both brown and colourless bands. The stone was cut into two and one part was given high-pressure high temperature (HPHT) treatment, removing the brown colouration. The parts were then cut into (110) sections. In the untreated stone the morphology of brown stripes is consistent with that of slip bands formed during plastic deformation and Raman mapping shows they are under strong compressive stress. Photoluminescence from N3 and H3 centres, as well as lines at 406.3 nm, 491.3 nm and 535.9 nm, are correlated with brown bands in the untreated sample, while cathodoluminescence shows that band-A luminescence is anticorrelated. HPHT treatment reduces internal stress, and eliminates or reduces correlated luminescence. TEM reveals long straight dislocations and dislocation dipoles in the brown bands, consistent with deformation by slip and concurrent intrinsic point defect production, while clear bands have curved and tangled dislocation networks. We postulate that vacancies produced by plastic deformation aggregate into clusters responsible both for the brown colouration and an increase in volume that results in compressive stress. The 535.9 nm line has characteristics of an interstitial-type defect and may be formed by the trapping of interstitials generated during plastic deformation.

Microstructure–Property Correlation in a Laser Powder Bed Fusion Processed High‐Strength AF‐9628 Steel

Laser powder bed fusion additive manufacturing (LPBF‐AM) of a low‐alloy, high‐performance AF‐9628 steel results in exceptionally high strength and good ductility. The reasons for such mechanical properties are investigated through detailed microscopy performed at several length scales. Thus, the characterization of melt pool, porosity, grain morphology, phases, and dislocations is performed in the as‐printed material. The as‐printed material consists of only 0.004 vol% of uniformly distributed porosity, single‐phase martensitic laths with an average lath size of ≈2.5 μm, the absence of carbides indicating interstitial trapping of C atom, and high dislocation density in the martensitic laths. Experimental data through microscopy are then fed in analytical models for calculating strengthening contributions from various strengthening mechanisms. Calculated yield strength agrees well with experimentally determined value, and therefore, activation of various strengthening mechanisms is established in as‐printed AF‐9628.

Comparative studies of helium behavior in copper and tungsten using molecular dynamics simulations

Helium (He) behavior in copper (Cu) and tungsten (W) bulk has been investigated and compared by molecular dynamics simulations. In contrast to the diffusion of He in W, interstitial He and He2 in Cu have higher diffusion coefficients and lower migration energies, which causes He atoms to rapidly coalesce into helium clusters. Once a Hen cluster (n ≥ 3) forms, trap mutation occurs in a short time. W atoms are less likely to be projected by Hen clusters. The binding energies are calculated to prove the results. For He2 in Cu, the dissociation of He2 and trap mutation both occur above 700 K. The total phenomenon exhibits Arrhenius behavior. The effective capture radii (ECRs) of an interstitial He trapping another one in Cu are obtained, which differ slightly from that in W. They decrease as the temperature increases, which deserves more attention, as the ECRs are required in MC and RT models.

Energetics, migration and trapping of Zn interstitials in ZnO

Zn interstitial (Zni) is one of the fundamental intrinsic defects in ZnO and prominently affects the physical properties of the material. Here, the energetics and migration properties of Zni have been studied in ion implanted ZnO using a new approach based on the Li marker diffusion. Specifically, ZnO single crystals were implanted with 3.2 keV/amu B and BF2 ions and the release of Zni from the implanted regions during annealing was correlated with the advance of the characteristic Li depleted region into the bulk of the samples. Using this methodology, we calculate the activation energy of 1.45 eV to govern the process for B implants. Assuming that the migration energy of Zni to be of ~0.6 eV, as discussed previously in literature, a barrier for releasing Zni from the implanted region may be estimated as ~0.8–0.9 eV. Meanwhile, in the BF2 implanted samples, the migration of Zni is less efficient, as interpreted from the Li redistribution features in these samples; in particular, it is argued that Zni may be trapped by defect complexes related to the presence of F.

Electrospun 3D composite nano-flowers for high performance triple-cation perovskite solar cells

Three dimensional (3-D) flower-shaped SnO2-TiO2 nano-structure has been synthesized by electro-spinning and incorporated on top of sol-gel ZnO ETL to fabricate highly efficient (highest efficiency: 17.25%) triple-cation (methyl ammonium, formamidinium and rubidium cations) based perovskite solar cell (PSC). The flower-based PSCs demonstrate superior photovoltaic performance compared to control ZnO or one-dimensional (1-D) fiber-shaped nano-structure ETL based devices. Nano-structured ETLs passivate the interstitial trap sites in pristine ZnO by intercalation of metal atoms in host ZnO lattice matrix and increase the n-type conductivity of the (nano-structured) ETL films by reducing the functional groups on ZnO surface. The accumulated ions at the perovskite/ETL interface are also well-distributed and hence the accumulation capacitance is significantly reduced in nano-structured ETL based PSCs, due to the branch-structured ETL network. Moreover, the nano-flower based PSC demonstrates superior charge transfer property, compared to nano-fiber based PSC owing to enhanced material crystallinity and higher effective surface area of 3-D nano-flower network, with respect to 1-D nano-fiber structure. The photo-current hysteretic phenomena are also most suppressed in nano-flower based PSC, due to mitigated electrode polarization mechanism in it. Adding to the merits, PSCs incorporating nano-flower ETL demonstrate enhanced device stability compared to the control devices, retaining about 92% of its initial efficiency even after a month. The enhanced device stability with nano-flower based PSC is contributed by the lower hydrophilicity, lower extent of functional surface hydroxyl group and lower content of vacant interstitial trap sites of the respective ETL film.

Production and annealing of the paramagnetic defects in as-grown and oxygen doped floating zone silicon irradiated with high fluence 3.5 MeV and 27 MeV electrons

The production and thermal stability of the irradiation paramagnetic point defects (IPPDs) in the as-grown, standard float-zone silicon (STFZ) and oxygen doped float-zone silicon (DOFZ) irradiated at room temperature with high fluence 3.5 MeV (1 × 1017 cm−2) low energy and 27 MeV (2 × 1016 cm−2) high energy electrons were investigated by Electron Spin Resonance (ESR). The nature and concentration of the IPPDs identified in the as-irradiated and isochronally annealed up to 300 °C samples by ESR measurements under intense above the gap 1.06 µm in-situ laser illumination, were found to depend on oxygen concentration and electrons energy. While irradiation of STFZ with electrons produced as main IPPDs, besides divacancies, larger tetravacancy and pentavacancy cluster defects, irradiation of DOFZ resulted mainly in divacancies, vacancy-oxygen and interstitial oxygen-carbon impurity pairs. The observed variation in the nature of the main resulting IPPDs with the concentration of incorporated oxygen is explained by differences in the dominant defects production mechanisms. Thus in STFZ with lower oxygen concentration (1 × 1016 cm−3) the dominant production mechanisms are the direct defects formation from a chain of neighboring vacancies by a cascade of secondary recoils across the path of the high energy irradiating particle and the divacancies diffusion and trapping/aggregation. Meanwhile, in DOFZ with larger oxygen content (1.2 × 1017 cm−3) the primary vacancy and interstitial trapping by the oxygen and carbon impurities is the dominant defect production mechanism. Variations in the concentration and nature of the IPPDs observed during isochronal annealing are discussed in terms of defects thermal activated diffusion and recombination processes.

Punch-Through Stop Doping Profile Control via Interstitial Trapping by Oxygen-Insertion Silicon Channel

Interstitial trapping by oxygen-inserted silicon channel results in blocking of boron and phosphorus transient enhanced diffusion as well as retention of channel boron profiles during the gate oxidation process. The enhanced doping profile control capability is applicable to punch-through stop of advanced CMOS devices and its benefits to 28nm planar CMOS and 20nm bulk FinFET devices projected by TCAD have been discussed.

Strong trapping and slow diffusion of helium in a tungsten grain boundary

We have investigated the segregation, trapping and diffusion of He in a ∑3<110>{111} W grain boundary (GB) using combined techniques of ab initio and classical atomistic simulations. We show that, with an average segregation energy of −3.20 eV, the strong He trapping can be attributed to a GB interstitial trapping or a vacancy trapping mechanism, while an average energy barrier of 1.97 eV leads to a slow diffusion of He in the GB plane. We further reveal by molecular dynamics simulations that the He diffusion will be dictated by GB migration through the motion of GB disconnections. Interestingly, we also observe a He-induced GB structural transition in classical simulations. The present work suggests that the GB does not provide fast transport channel for He, providing useful reference for the possible application of polycrystalline W under He irradiation in advanced nuclear fusion reactors.

Local viscosity and solvent relaxation experienced by rod-like fluorophores in AOT/4-chlorophenol/m-xylene organogels

Organogels prepared from AOT/4-chlorophenol/m-xylene are immobile in the macroscopic sense, with a well-characterized internal structure. However, the molecular level dynamics inside the gels is not too clear, although a very slow structural relaxation has been reported previously. Using a set of rod-like fluorophores, we find that the rotational mobility of a small guest molecule inside the gel can be extremely fast, indicating presence of sufficiently low-microviscosity domains. These domains consist of m-xylene solvent molecules trapped in the interstices of fiber bundles comprising columnar stacks of 4-chlorophenol surrounded by AOT molecules. However, interstitial trapping of m-xylene does retard its own dynamics, which explains the slow solvent relaxation inside the gels. Hence, the state of m-xylene in the organogel may be characterized as “bound”, in contrast to the “free” state in neat m-xylene.

Influence of trap connectivity on H diffusion: Vacancy trapping

A model is given for the effective diffusion of interstitial solutes in the presence of traps. It goes beyond Oriani's by taking into account, in a simple way, the connectivity between interstitial trap sites. It shows, in particular, that the typical dimension of a network of trap sites, connected by low barriers, appears squared in the diffusion coefficient. Therefore, a large precipitate can be inefficient if it offers a fast diffusion path, even if each individual trap site is deep. The model is illustrated in the case of hydrogen trapping at vacancies in Ni, using ab initio calculations for migration barriers and Kinetic Monte Carlo for validation. Trapping/detrapping kinetic parameters for “Thermal Desorption Spectra” analysis are also given.

Atomic‐scale simulation of space weathering in olivine and orthopyroxene

AbstractClassical molecular dynamics was used to study the annealing of anion and cation Frenkel defects in olivine and orthopyroxene minerals. While it was found that for both minerals, reorganization of the Si–O bonds, often accompanied by large Si displacements, occurs to maintain the fourfold coordination of the SiO4 tetrahedra, important differences are observed in their annealing behavior. Specifically, cation defects are substantially more mobile in olivine than in orthopyroxene leading to rapid annihilation of cation Frenkel defects and formation of extended defects in olivine. By contrast, the diffusion rate of anion defects in orthopyroxene is much higher than that in olivine and also exhibits large anisotropy. Consequently, it was found that diffusion in orthopyroxene occurs without significant annihilation of anion Frenkel defects or trapping of anion interstitials or vacancies into clusters. The results obtained here are discussed in the context of space weathering of olivine and orthopyroxene. Specifically, two important observations are made which may explain previous experimental results. First, ion irradiation experiments that show reduced tolerance for radiation damage in orthopyroxene may be explained by the rapid, one‐dimensional anion mobility which prevents healing of the lattice. Second, laser heating experiments which show that orthopyroxene has enhanced tolerance to reduction and the evolution of nanophase Fe inclusions could be due to the observed rapid anion diffusion in orthopyroxene, which might allow the bulk to act as a reservoir for the surface.

Determination of depths of multiple traps for interstitials and their influence on diffusion kinetics

Trapping of interstitial atoms by foreign substitutional atoms is a phenomenon that significantly influences the chemistry of the system as well as the diffusivity of the interstitial components. In their previous paper (Svoboda et al 2013 Modelling Simul. Mater. Sci. Eng. 21 065012), the present authors determined the trapping energy and the chemical diffusion coefficient of an interstitial component for a dilute system by assuming only one type of trap. This model is now generalized for non-dilute systems, and it accounts for the fact that each interstitial trap position can be influenced by several foreign atoms.The model is applied to a non-dilute Fe–Cr–rather dilute C system neglecting interstitial–interstitial interactions. Fitting of the chemical potential of the interstitial component given by the model to that given by thermodynamic databases provides parameters allowing calculation of the depth of the trap depending on the number of foreign atoms surrounding the trap position. The results of fitting indicate that the model is physically sound. Based on an established concept, subsequently, the chemical diffusion coefficient of the interstitial is calculated for different chemical compositions.

Atomic Scale Simulation of Dislocation Loops Formation in Thin Foil under High Energy Electron Irradiation

Using a personnel computer, we have simulated the diffusion and agglomeration of point defects in thin foil under high energy electron irradiation. The physical model has been developed by using the Monte Carlo technique. Four types of reactions are assumed to take place: di-interstitial creation by agglomeration of two free interstitials, vacancy-interstitial annihilation, interstitial trapping by dislocation loops and interstitial annihilation on the sample surfaces. In the simulation only interstitials are mobile and extended defects are assumed to be interstitial type. We have calculated the concentration of point defects, extended defects and the size of the latter. We compared them to the results of the Chemical Reaction Rate Theory (CRRT). It has been found that the dislocation loops are distributed in the center of material leaving areas denuded close to the surface and the loops radius is also strongly dependent on the location of the defect in thin foil with respect to the results of experimental and CRRT. To explain the origin of these phenomena we have exploited the spatial distribution of vacancies close to free surfaces and around dislocation loops. These types of informations are totally missing in the CRRT and experimental.

Molecular dynamics simulations of the diffusion and coalescence of helium in tungsten

Molecular dynamics (MD) simulations are performed on the diffusion and coalescence of helium in tungsten. A new method for determining the effective capture radii (ECRs) and the dissociation energies of helium-related defects is proposed in this work. It is observed that the ECR of an interstitial helium atom trapping helium interstitials (denoted as He–Hen, n=1–3) decreases with increasing temperature, except for He–He2 at T<400K. The traditional view that the ECR is approximately equal to the lattice constant, which has been widely used in kinetic Monte Carlo (KMC) and rate theory (RT) models, is only valid in some cases. However, the ECR between an interstitial helium atom and a substitutional helium atom (denoted as He–HeV) always approximates the third nearest-neighbor tetrahedral positions of the HeV. The diffusion coefficients Dn for helium clusters are also investigated. He2 migrates more quickly than a single He atom does at T<400K, whereas the diffusion path of He2 changes at higher temperatures. Another counterintuitive observation is that D5>D3>D4 at T<500K, which can be attributed to the disordered structure of He5. The Arrhenius relation describes the diffusion of Hen well in the temperature range from 300K to 550K, whereas the diffusion is not a standard thermally activated process at higher temperatures. Taken together, these results help elucidate the initial stage of helium bubble formation in tungsten as well as the requirements of long-term evolution methods such as KMC or RT models.

A computational model of hemorrhage and dehydration suggests a pathophysiological mechanism: Starling-mediated protein trapping

We sought to understand the degree to which a single computational cardiovascular model could replicate the typical responses of healthy subjects through a breadth of blood loss patterns and whether such a model could illuminate the cause-effect relationships that underlie the observed responses. The model consisted of compartments for the upper body, lower body, viscera, and kidneys as well as a four-chambered heart and a pulmonary compartment. Transcapillary fluid flux was governed by Starling forces, whereas lymphatic flow was driven by hydrostatic tissue pressure and scaled by a lymphatic activation term. We adjusted parameters based on results from one protocol involving moderate continual blood loss in a canine model. Next, we simulated six additional protocols spanning euvolemic and dehydrated subjects and compared in silico behavior with in vivo hemodynamic responses and fluid shifts. The model was able to replicate group-averaged behavior (i.e., within 1 or 2 SEs) of the rate and quantity of vascular refill and the associated cardiac output during slow, moderate, and rapid ongoing blood losses, the restitution after the cessation of blood loss, and the absence of restitution in dehydrated subjects. The model suggested that the earlier phase of restitution, i.e., transcapillary fluid shifts, was antagonistic to the later phase of restitution, i.e., protein return via lymphatics. This phenomenon was termed "interstitial protein trapping." In conclusion, the model appears valid for a range of blood loss patterns and prehydration states. Further investigation into the in vivo relevance of interstitial protein trapping is justified.