Defect Production Mechanisms Research Articles

Defect production in tungsten under sub-threshold energy irradiation: role of hydrogen and surface effects

Despite the low solubility of hydrogen isotopes (HIs) in tungsten (W), their concentration can reach up to ∼10 at.% after low-energy plasma irradiation. This is generally attributed to the vacancies that may accommodate excessive HIs. However, the kinetic energy of incident HIs transferred to W is far below the energy threshold to create a Frenkel pair, the underlying mechanism of defect production is still unclear. Here, we investigate the influence of H on the defect production in W using the molecular dynamic (MD) simulations. It is found that the threshold displacement energy (TDE) in bulk W slight decreases with the increasing of H concentration. This is due to the formation of H-vacancy complexes, which prevents the vacancy-interstitial recombination. More importantly, the H effects are significantly magnified in the surface region. On the one hand, the maximum kinetic energy transferred from 400 eV H to W can reach up to ∼21 eV due to the double-hit process, which is two times higher than that predicted by elastic collision model. On the other hand, the momentum transferred to W is completely random, including both the recoil direction upward and downward from the surface. Accordingly, the lowest TDE in W surface is only 15–21 eV at sub-surface layers with the depth of 6.7–11.1 Å, which is lower than the maximum kinetic energy transferred to W. Therefore, the low-energy HIs irradiation can create the defects in W surface directly. Our findings provide deep insight into defect production in W at sub-threshold energy and have wider implications for materials performance under low-energy ions irradiation.

Probing the temperature effects in the radiation stability of Nd2Zr2O7 pyrochlore under swift ion irradiation

The pyrochlore structure is of particular interest to the nuclear industry due to its potential for nuclear waste immobilization and as a host material for inert matrix fuels. Herein, the experimental results of the temperature and ion fluence dependence on the disordered ion tracks produced by the electronic excitation in the pyrochlore compound Nd2Zr2O7 are presented. The samples were exposed to extreme conditions of ion irradiation with 100 MeV 107Ag9+ ions at ion fluences ranging from 3 × 1011 ions/cm2 to 2 × 1013 ions/cm2 at temperatures of 300 K and 1000 K. The in-situ X-ray diffraction along with micro-Raman spectroscopic analysis on Nd2Zr2O7 samples irradiated at 300 K displays order-disorder phase transformation which is consistent with earlier reports. The direct-impact model is used to calculate the amorphous fraction which exists due to massive deposition of electronic-energy in a localized region, and the estimated value of track radius is 2.98 ± 0.56 nm. Furthermore, theoretical thermal spike simulations are performed to estimate the track radius (∼4.2 nm) which provides strong support for experimental results. To simulate the response of Nd2Zr2O7 under nuclear reactor environment, irradiation carried out at an elevated temperature that exhibits significant radiation tolerance. Its substantiality is revealed from comparative temperature dependent irradiation studies where a slow rate of amorphization is found at the elevated temperature because of the competition between defect production and thermal healing mechanism. Thus, obtained results strongly endorse the appositeness of Nd2Zr2O7 as inert matrix material for the fabrication of futuristic nuclear fuel.

Supported Two-Dimensional Materials under Ion Irradiation: The Substrate Governs Defect Production.

Focused ion beams perfectly suit for patterning two-dimensional (2D) materials, but the optimization of irradiation parameters requires full microscopic understanding of defect production mechanisms. In contrast to freestanding 2D systems, the details of damage creation in supported 2D materials are not fully understood, whereas the majority of experiments have been carried out for 2D targets deposited on substrates. Here, we suggest a universal and computationally efficient scheme to model the irradiation of supported 2D materials, which combines analytical potential molecular dynamics with Monte Carlo simulations and makes it possible to independently assess the contributions to the damage from backscattered ions and atoms sputtered from the substrate. Using the scheme, we study the defect production in graphene and MoS2 sheets, which are the two most important and wide-spread 2D materials, deposited on a SiO2 substrate. For helium and neon ions with a wide range of initial ion energies including those used in a commercial helium ion microscope (HIM), we demonstrate that depending on the ion energy and mass, the defect production in 2D systems can be dominated by backscattered ions and sputtered substrate atoms rather than by the direct ion impacts and that the amount of damage in 2D materials heavily depends on whether a substrate is present or not. We also study the factors which limit the spatial resolution of the patterning process. Our results, which agree well with the available experimental data, provide not only insights into defect production but also quantitative information, which can be used for the minimization of damage during imaging in HIM or optimization of the patterning process.

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.

Ab initio molecular dynamics simulations of AlN responding to low energy particle radiation

Ab initio molecular dynamics simulations of low energy recoil events in wurtzite AlN have been performed to determine threshold displacement energies, defect production and evolution mechanisms, role of partial charge transfer during the process, and the influence of irradiation-induced defects on the properties of AlN. The results show that the threshold displacement energies, Ed, along the direction parallel to the basal planes are smaller than those perpendicular to the basal planes. The minimum Ed values are determined to be 19 eV and 55 eV for N and Al atom, respectively, which occur along the [1¯1¯20] direction. In general, the threshold displacement energies for N are smaller than those for Al atom, indicating the N defects would be dominant under irradiation. The defect production mechanisms have been analyzed. It is found that charge transfer and redistribution for both the primary knock-on atom and the subsequent recoil atoms play a significant role in defect production and evolution. Similar to the trend in oxide materials, there is a nearly linear relationship between Ed and the total amount of charge transfer at the potential energy peak in AlN, which provides guidance on the development of charge-transfer interatomic potentials for classic molecular dynamics simulations. Finally, the response behavior of AlN to low energy irradiation is qualitatively investigated. The existence of irradiation-induced defects significantly modifies the electronic structure, and thus affects the magnetic, electronic and optical properties of AlN. These findings further enrich the understanding of defects in the wide bandgap semiconductor of AlN.

Heat-treatment induced defect formation in α-Al matrix in Sr-modified eutectic Al–Si alloy

An intensive formation of crystal defects in the α-Al matrix of an as-cast Sr-modified eutectic Al–Si alloy, induced by heat treatment with rapid heating, has been studied. This phenomenon arises from the fragmentation of the Si crystals and the diffusion of Al atoms into the cracks of the Si crystals. During the rapid heating process, a large temperature gradient is established within an as-cast Al–Si sample. It creates a tensile environment for the irregular-shaped and interconnected Si crystals, as the thermal expansion coefficient of the α-Al is ten times that of the Si. Under the thermal constraints, some Si crystals are broken, where the cracks produce the so-called “capillary force” that attracts the surrounding α-Al matrix to fill in them. As the migration of the Al atoms is substitutional, the Al diffusion creates fluxes of vacancies to the interior of the αAl matrix; thus, the crack volume is transferred to the αAl matrix. Due to the homogeneous distribution of the Si crystals and the random presence of the cracks, the αAl matrix is subjected to a varied migration of vacancies in site, in quantity and in direction. Such diffusion is further perturbed by local migration of Al atoms to accommodate the spheroidization of the Si induced by its shape instability. In this way, a large amount of crystal defects (vacancies and then dislocations) are produced in the α-Al matrix. The produced crystal defects then promote the recovery and even recrystallization of the α-Al matrix, resulting in its refinement. Such a mechanism of defect production is applicable to polycrystalline materials composed of phases with incompatible thermal and mechanical properties.

Surface effects and statistical laws of defects in primary radiation damage: Tungsten vs. iron

We have investigated the effect of surfaces on the statistics of primary radiation damage, comparing defect production in the bcc metals iron (Fe) and tungsten (W). Through molecular dynamics simulations of collision cascades we show that vacancy as well as interstitial cluster sizes follow scaling laws in both bulk and thin foils in these materials. The slope of the vacancy cluster size distribution in Fe is clearly affected by the surface in thin foil irradiation, while in W mainly the overall frequency is affected. Furthermore, the slopes of the power law distributions in bulk Fe are markedly different from those in W. The distinct behaviour of the statistical distributions uncovers different defect production mechanisms effective in the two materials, and provides insight into the underlying reasons for the differing behaviour observed in TEM experiments of low-dose ion irradiation in these metals.

Direct Observation of Defect Range and Evolution in Ion-Irradiated Single Crystalline Ni and Ni Binary Alloys.

Energetic ions have been widely used to evaluate the irradiation tolerance of structural materials for nuclear power applications and to modify material properties. It is important to understand the defect production, annihilation and migration mechanisms during and after collision cascades. In this study, single crystalline pure nickel metal and single-phase concentrated solid solution alloys of 50%Ni50%Co (NiCo) and 50%Ni50%Fe (NiFe) without apparent preexisting defect sinks were employed to study defect dynamics under ion irradiation. Both cross-sectional transmission electron microscopy characterization (TEM) and Rutherford backscattering spectrometry channeling (RBS-C) spectra show that the range of radiation-induced defect clusters far exceed the theoretically predicted depth in all materials after high-dose irradiation. Defects in nickel migrate faster than in NiCo and NiFe. Both vacancy-type stacking fault tetrahedra (SFT) and interstitial loops coexist in the same region, which is consistent with molecular dynamics simulations. Kinetic activation relaxation technique (k-ART) simulations for nickel showed that small vacancy clusters, such as di-vacancies and tri-vacancies, created by collision cascades are highly mobile, even at room temperature. The slower migration of defects in the alloy along with more localized energy dissipation of the displacement cascade may lead to enhanced radiation tolerance.

Interfacial carbon nanoplatelet formation by ion irradiation of graphene on iridium(111).

We expose epitaxial graphene (Gr) on Ir(111) to low-energy noble gas ion irradiation and investigate by scanning tunneling microscopy and atomistic simulations the behavior of C atoms detached from Gr due to ion impacts. Consistent with our density functional theory calculations, upon annealing Gr nanoplatelets nucleate at the Gr/Ir(111) interface from trapped C atoms initially displaced with momentum toward the substrate. Making use of the nanoplatelet formation phenomenon, we measure the trapping yield as a function of ion energy and species and compare the values to those obtained using molecular dynamics simulations. Thereby, complementary to the sputtering yield, the trapping yield is established as a quantity characterizing the response of supported 2D materials to ion exposure. Our findings shed light on the microscopic mechanisms of defect production in supported 2D materials under ion irradiation and pave the way toward precise control of such systems by ion beam engineering.

Mechanisms of primary long-lived defect production in pure quartz glasses under electron irradiation

Based on an analysis of the dependences of individual absorption band amplitudes in quartz glasses on the electron beam (EB) fluence, mechanisms of primary defect production in KS-4V, KU-1, and Corning 7980 glasses during their prolonged irradiation by EB pulses are determined.

Response of mechanically strained nanomaterials to irradiation: Insight from atomistic simulations

By combining analytical molecular-dynamics with density-functional theory simulations, we study the radiation hardness of mechanically strained low-dimensional nanosystems such as carbon nanotubes, graphene, and Si nanowires. We show that the radiation hardness of all these structures decreases with strain but the effect is most pronounced in nanowire due to the bulk structure of its core in contrast with the planar structure of nanotubes and graphene. Our results not only elucidate the microscopic mechanism of irradiation-induced defect production in strained nanomaterials but also provide quantitative information required for assessing the stability of nanocomponents in composite materials subjected to mechanical strain and irradiation, e.g., in space applications.

Basic properties of the F-type centers in halides, oxides and perovskites

We present a short survey of the optical properties of primary radiation-induced point defects in alkali halides, simple oxides and some ABO 3 perovskites. We discuss in details the optical properties of single electron F and F + centers in rock-salt (f.c.c.) alkali halides and oxides and show that the Mollwo–Ivey law well-known for the F-type centers in alkali halides may be extended for other rock-salt structure insulators. We also discuss the major differences in point defect production mechanisms in halides and oxides. We show that the Rabin–Klick diagram may be generalized for a whole family of alkali halides. The F-type center migration and aggregation into metal colloids in alkali halides and oxides is also discussed.

Molecular dynamics study of damage production in single-walled carbon nanotubesirradiated by various ion species

The irradiation-induced damage production in single-walled carbon nanotubes (CNTs) byseveral types of ions is investigated using the molecular dynamics method with analyticalpotentials. We found that, in the incident energy range 25–1000 eV, the bonding action orthe chemical effect of the ions could significantly enhance their damage capabilities toCNTs relative to that of non-bonding ions, and the dependence of damage yield on the ionmass is no longer monotonic. This is contrary to the previous viewpoint that thechemical aspect of the interaction is of no importance to the ion-induced defectproduction mechanism in CNTs. The bonding interaction of ions with CNTsalso increases their implantation probabilities into CNTs. The chemical erosioneffect of incident ions remarkably intensifies the sideward recoil from CNTs underirradiation while the downward recoil is still governed by the physical collision effect.

Atomistic simulations of irradiation effects in carbon nanotubes: an overview

In this article, we first review recent work on the basic science of the mechanisms of irradiation-induced defect production and migration in carbon nanotubes. We then discuss a few ways in which understanding of the irradiation effects can be used to modify the properties of single nanotubes as well as macroscopic materials based on them. The results illustrate that there indeed exist several ways in which defects can have potentially useful effects on nanotube-based materials.

Raman and optical absorption studies of proton bombarded CsI

CsI single crystals, which have the simple cubic structure, have been bombarded with 1MeV protons at a temperature close to 300K. Optical absorption and Raman studies have identified most of the defects created. These include F and F2 centres, and V centres having absorption bands at 2.7eV and 3.4eV, which grow together during irradiation. The Raman studies identify these latter centres as having the I3- structure. Isochronal and isothermal annealing experiments show a mutual decay of the F-type centres and these V centres in a second order reaction with an activation energy of 1.28eV. The results are discussed in relation to the excitonic mechanism of defect production and the formation of interstitial iodine aggregates of various types in alkali iodides.

Creation of defects with core condensation

Defects in superfluid $^{3}\mathrm{He}$, high-${T}_{c}$ superconductors, QCD color superfluids, and cosmic vortons can possess (anti)ferromagnetic cores, and theirgeneralizationss. In each case there is a second-order parameter whose value is zero in the bulk which does not vanish in the core. We examine the production of defects in the simplest $1+1$ dimensional scalar theory in which a second-order parameter can take nonzero values in a defect core. We study in detail the effects of core condensation on the defect production mechanism.

Multiscale Modelling of bcc-Fe Based Alloys for Nuclear Applications [PowerPoint Submission

AbstractUnderstanding the basic mechanisms that determine microstructure changes in neutron irradiated steels is vital for a safe lifetime management of existing nuclear reactors and a safe design of future nuclear options. Low-alloyed ferritic steels containing Cu, Ni, Mn and Si as principal solute atoms are used as structural materials for current reactor vessels. The microstructural evolution under irradiation in alloys is decided by the interplay between defect formation and thermodynamic driving forces, together determining the appearance of phase transformations (precipitation, segregation, …) and favouring or delaying the nucleation and growth of point-defect clusters, their diffusion and their mutual recombination or removal at sinks. A reliable description of the production, evolution and accumulation of radiation damage must therefore start from the atomic level and requires being able to describe multicomponent systems for timescales ranging from few picoseconds to years. This goal demands firstly the fabrication of interatomic potentials for alloys that must be both consistent with the thermodynamic properties of the system and capable of reproducing correctly the characteristic solute-point defect interactions, versus ab initio or experimental data. Secondly the performance of extensive molecular dynamics (MD) simulations, to grasp the main mechanisms of defect production, diffusion, mutual interaction, and interaction with solute atoms and impurities. Thirdly, the development of simulation tools capable of describing the microstructure evolution beyond the timeframe and lengthscale of MD, while reproducing as much as possible the atomic-level origin of the mechanisms governing the evolution of the system, including phase changes.In this presentation the results of recent efforts made in this direction in the case of Fe-Cu, Fe-Cr and Fe-Ni alloys, as basic model alloys for the description of steels of technological relevance, are highlighted. In particular, advanced techniques to fit interatomic potentials consistent with thermodynamics are proposed and the results of their application to the mentioned alloys are presented. Next, the development of advanced methods, based on the use of artificial intelligence, to improve both the physical reliability and the computational efficiency of kinetic Monte Carlo codes for the study of point-defect clustering and phase changes beyond the scale of MD, is reported. These recent progresses bear the promise of being able, in the near future, of producing reliable tools for the description of the microstructure evolution of realistic model alloys under irradiation.

The Simplest Quantum Model Supporting the Kibble-Zurek Mechanism of Topological Defect Production: Landau-Zener Transitions from a New Perspective

It can be shown that the dynamics of the Landau-Zener model can be accurately described in terms of the Kibble-Zurek theory of the topological defect production in nonequilibrium phase transitions. The simplest quantum model exhibiting the Kibble-Zurek mechanism is presented. A new intuitive description of Landau-Zener dynamics is found.

Tritium Induced Defects in Amorphous Silicon

ABSTRACTWe report the growth of tritium induced defects in tritium doped hydrogenated amorphous silicon (a-Si:H,T) as measured by electron spin resonance (ESR) and photothermal deflection spectroscopy (PDS). The measurements allow one to examine the accumulation of defects in a-Si:H,T where the defect production mechanism is known. Defects produced by tritium decay are found to be much less numerous than the number of decayed tritium atoms and they are metastable like Staebler-Wronski defects. These results provide new insight into the metastable defect creation and the role of hydrogen motion.

Study of the irradiation damage in SiC by ion channeling

The importance of silicon carbide as a wide band gap semiconductor is widely accepted and well documented. Its excellent physical properties (chemical inertness, high-temperature strength, low thermal expansion, extreme hardness) make it the most promising substitute for traditional semiconductors, especially when high-temperature, high-voltage power, and high-frequency devices are concerned. In the present work, the gradual amorphization of a SiC Lely (21R) crystal when irradiated with 8 MeV 7Li ions in a random direction up to a maximum dose of approximately 1×10 16 particles/cm 2, is being studied, using the progressive change of channeling parameters for different depths. The results refer to the energy region of ∼1 MeV/nucleon, and an attempt is made in order to explain the peculiarities of the experimental spectra and the mechanism of defect production in SiC. As in previous studies, a change in color was observed after irradiation in the random mode, indicating that the problem of irradiation damage in SiC caused by light ion beams needs further investigation.