Structure Of Ion Tracks Research Articles

Nanoscale core-shell structure and recrystallization of swift heavy ion tracks in SrTiO3.

It is widely accepted that the interaction of swift heavy ions with many complex oxides is predominantly governed by the electronic energy loss that gives rise to nanoscale amorphous ion tracks along the penetration direction. The question of how electronic excitation and electron-phonon coupling affect the atomic system through defect production, recrystallization, and strain effects has not yet been fully clarified. To advance the knowledge of the atomic structure of ion tracks, we irradiated single crystalline SrTiO3 with 629 MeV Xe ions and performed comprehensive electron microscopy investigations complemented by molecular dynamics simulations. This study shows discontinuous ion-track formation along the ion penetration path, comprising an amorphous core and a surrounding few monolayer thick shell of strained/defective crystalline SrTiO3. Using machine-learning-aided analysis of atomic-scale images, we demonstrate the presence of 4-8% strain in the disordered region interfacing with the amorphous core in the initially formed ion tracks. Under constant exposure of the electron beam during imaging, the amorphous part of the ion tracks readily recrystallizes radially inwards from the crystalline-amorphous interface under the constant electron-beam irradiation during the imaging. Cation strain in the amorphous region is observed to be significantly recovered, while the oxygen sublattice remains strained even under the electron irradiation due to the present oxygen vacancies. The molecular dynamics simulations support this observation and suggest that local transient heating and annealing facilitate recrystallization process of the amorphous phase and drive Sr and Ti sublattices to rearrange. In contrast, the annealing of O atoms is difficult, thus leaving a remnant of oxygen vacancies and strain even after recrystallization. This work provides insights for creating and transforming novel interfaces and nanostructures for future functional applications.

Illustrating the atomic structure and formation mechanism of ion tracks in polyethylene terephthalate with molecular dynamics simulations

This work reports molecular dynamics (MD) simulations of the formation of detailed ion track structures at the atomic scale in polymers. To simulate the initial chemical reactions and the subsequent thermal movement of polymer molecules, an adapted thermal spike model was implemented using a charge-implicit reactive force field potential. The MD simulation reproduces the physical–chemical process of ion track formation observed in the experiment, e.g., the bond breakage and creation, gas production and release, carbonization effect, and the relative radial density distributions, which are consistent with small angle X-ray scattering results of ion tracks in polyethylene terephthalate generated with 15.9 keV nm−1 Au and 10.9 keV nm−1 Xe ions, respectively. Based on the MD simulation, the ion tracks in polymers show a heavily-damaged core region with an inhomogeneous nanoporous structure, and a surrounding transition region with the mass density increasing gradually back to the same value as the pristine sample.

Influence of the Structure of Ion Tracks in YBCO on the Superconducting Properties of Composite Wires

Influence of the Structure of Ion Tracks in YBCO on the Superconducting Properties of Composite Wires

Observation of latent ion tracks in semicrystalline polymers by scanning electron microscopy

AbstractAdvances in nanotechnology and materials science require further improvement of metrology of nanostructured polymers, in particular, polymers modified by high energy ion beams. The observation of latent ion tracks using various microscopy methods is an important part of studies on heavy ion effects in solids. However, scanning electron microscopy (SEM) has not been utilized for polymers. In the present study, it is shown how SEM can be used to observe latent tracks in semicrystalline polymers. The procedure includes the embrittlement of a polymer specimen by controlled photooxidation and its subsequent fracture. Latent tracks are clearly visible on fractured surfaces as structureless stripes surrounded by an inhomogeneous semicrystalline matrix. Using this method, the latent tracks of Kr, Xe, Au, and Bi ions with energies of 1–11 MeV/u in polyethylene terephthalate and polypropylene films are observed and their diameters are estimated. In contrast to transmission electron microscopy, the suggested novel technique detects the outer track shell consisting of an amorphized polymer. Therefore, SEM observations can complement other commonly used techniques to comprehensively characterize the structure of ion tracks in polymers.

Evidence for a critical dose above which damage to carbonate ester bonds in PADC appear after gamma ray and ultra soft X-ray exposures

It has been confirmed that the density of carbonate ester in poly(allyl diglycol carbonate) (PADC), which is a well-known polymeric nuclear track detector CR-39, begins decreasing above a critical dose of about 60 kGy during exposures to Co-60 gamma rays and Ultra Soft X-rays (USX: Kα (Al) with energy 1.49 keV). After exposure to Co-60 gamma rays at 77 K and room temperature (RT), a significant difference in Electron Spin Resonance (ESR) spectra has been observed at both sides of a critical dose of 60 kGy, where is a threshold for the degradation of carbonate ester in PADC. Namely, the types of free radicals in PADC are different from each other below and above this critical dose. Changes in FT-IR spectra of carbonate ester in PADC above critical doses for gamma ray and USX exposures have also been observed, while that of ether decreases linearly with increasing absorbed dose. These results are well concordant with previous results obtained with 28 MeV electron beams. They also indicate that at least two hits of electrons are needed to destroy the carbonate ester in PADC. The layered structure of ion tracks in PADC can be explained thanks to these experimental results. Also, some clues for the identification of created radicals have been obtained from the results using both ESR and IR.

Energy dependent structure of Xe ion tracks in YBCO and the effect on the superconductive properties in magnetic fields

The morphology and structure of damaged regions (tracks) produced by swift heavy 167, 77, and 46 MeV 132Xe23+ ions in YBCO-based second generation industrial high temperature superconductors wires (2G HTS) were studied using transmission electron microscopy. It was shown that ions produce tracks aligned along the ion trajectory but of morphology depending on their energy: continuous, nearly continuous, or discontinuous tracks like prolate ellipsoids of 10–15 nm in length and spheroids of 5 nm in diameter. The damaged regions of about 5 nm in diameter contain an amorphous material with a lower density compared to the pristine YBCO. The material density drops from 6300 kg m−3 for YBCO matrix to 3600 kg m−3 inside the tracks. Barium enrichment was found in the vicinity of the track at a distance of about 10 nm from the center of the defect. Ion damage tracks with different morphologies showed different effectivenesses as pinning centers. Tracks composed of discontinuous pinning centers—spheroids of about 5 nm in diameter—bring the best enhancement of the critical current density. It occurs for the lowest ion energy (46 MeV) in the range of energy loss of 8.9 keV/nm–4.7 keV/nm for Xe ions. The samples showed highest critical current densities of 56 MA/cm2 (4.2 K) and 3 MA/cm2 (77 K) in self-field, while in magnetic fields of 8 T, the values of 17 MA/cm2 (4.2 K) and 1.6 MA/cm2 (77 K) were achieved. The reduction of the superconducting volume corresponding to the amorphous radiation defects did not exceed 4% from the total sample volume.

Structure, morphology and annealing behavior of ion tracks in polycarbonate

Ion tracks created in polycarbonate foils by irradiation with 2.2 GeV Au ions were characterized using a combination of small-angle x-ray and neutron scattering (SAXS/SANS) and Fourier transform infrared spectroscopy (FTIR). The ion tracks were found to consist of a cylindrical damage core with a radius of ∼2.5 ± 0.2 nm and a relative density approximately 5% below that of the pristine polycarbonate. Upon exposure to thermal annealing between 100 and 200 °C, the tracks were observed to double in size. Simultaneously, this led to a recovery in the density of the ion track, reaching a value just below that of the pristine polymer. A mechanism is proposed that explains this behavior by diffusion of radiolysis products/material flow into the under-dense track core from the surrounding region. Treatment of the tracks with UV radiation has shown no significant change in the track structure and size.

Damage threshold and structure of swift heavy ion tracks in Al2O3

Structure changes and their formation threshold in swift heavy ion (SHI) tracks in Al2O3 are studied using a combined start-to-end numerical model. The hybrid approach consists of the Monte-Carlo code TREKIS, describing kinetics of the electronic subsystem, and classical Molecular Dynamics for lattice atoms. The developed approach is free from a posteriori fitting parameters. Simulations of Xe 167 MeV ion impacts show that relaxation of an excess lattice energy results in formation of a cylindrical discontinuous disordered region of about 2 nm in diameter. Recent transmission electron microscopy observations agree with these results. The threshold of an SHI track formation is estimated to be ~6.1 keV nm−1. Calculated x-ray diffraction patterns of irradiated material demonstrate more pronounced damage of the Al atoms sublattice near SHI trajectories. Modeling of Xe ion tracks overlapping demonstrates that the damaged area can be restored to a near virgin state. Estimations give 6.5 nm as the minimal distance between the Xe ion trajectories resulting in recovery of the transformed structure produced by the previous ion.

New insights on ion track morphology in pyrochlores by aberration corrected scanning transmission electron microscopy

Abstract

Structure of ion tracks in ceria irradiated with high energy xenon ions

Structure and accumulation behavior of ion tracks in CeO2 irradiated with 200 MeV Xe ions were examined by transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) to obtain fundamental knowledge on the microstructure evolution induced by fission fragments in nuclear fuels and transmutation targets, which is of importance for the development of advanced fuel/target materials at high burn-up conditions. Bright-field (BF) TEM images of ion tracks from an inclined direction showed Fresnel contrast along penetrating path of incident ions. The signal intensity of high-angle annular dark-field (HAADF) STEM images was decreased at the core damage region of ion tracks along the path of ions, revealing the reduction of atomic density inside the ion track. Preferential formation of smaller and larger ion tracks was observed at a high ion fluence of 1 × 1014 cm−2 compared to a low ion fluence of 1 × 1011 cm−2. Results were discussed due to the coalescences and incomplete recovery of the core damage regions during the overlap of high density electronic excitation damage, which is induced during the repetition of the formation and recovery of ion tracks within an influence region.

Structural response of titanate pyrochlores to swift heavy ion irradiation

The structure, size, and morphology of ion tracks resulting from irradiation of five different pyrochlore compositions (A2Ti2O7, A = Yb, Er, Y, Gd, Sm) with 2.2 GeV 197Au ions were investigated by means of synchrotron X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). Radiation-induced amorphization occurred in all five materials analyzed following an exponential rate as a function of ion fluence. XRD patterns showed a general trend of increasing susceptibility of amorphization with increasing ratio of A- to B-site cation ionic radii (rA/rB) with the exception of Y2Ti2O7 and Sm2Ti2O7. This indicates that the track size does not necessarily increase with rA/rB, in contrast with results from previous swift heavy ion studies on Gd2Zr2-xTixO7 pyrochlore materials. For Y2Ti2O7, this effect is attributed to the significantly lower electron density of this material relative to the lanthanide-bearing pyrochlores, thus lowering the electronic energy loss (dE/dx) of the high-energy ions in this composition. An energy loss normalization procedure was performed which reveals an initial increase of amorphous track size with rA/rB that saturates above a cation radius ratio larger than Gd2Ti2O7. This is in agreement with previous low-energy ion irradiation experiments and first principles calculations of the disordering energy of titanate pyrochlores indicating that the same trends in disordering energy apply to radiation damage induced in both the nuclear and electronic energy loss regimes. HRTEM images indicate that single ion tracks in Yb2Ti2O7 and Er2Ti2O7, which have small A-site cations and low rA/rB, exhibit a core-shell structure with a small amorphous core surrounded by a larger disordered shell. In contrast, single tracks in Gd2Ti2O7 and Sm2Ti2O7, have a larger amorphous core with minimal disordered shells.

Structure of Defects and Microstructure Evolution in Oxide Ceramics – Role of Electronic Excitation and Selective Displacement Damage

Fluoriteand spinel-type oxide ceramics, such as yttria-stabilized zirconia, ceria, magnesium aliminate spinel, are known to be exceptionally resistant to irradiation damage with energetic particles. Those oxide compounds, thereofore, have potential applications to advanced nuclear fuels and innert matrices of transmutaion targets. Fundamental understanding of radiation damage is apparently essential to assess the microstructure stability of fuel/target materials under the hostile environment. In this presentation, structure and stability of radiation-induced defects in the oxide ceramics is reported. Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) is utilized to examine the following topics focucing on role of electronic excitation and selective displacement damage: (1) effects of selective displacement damage of oxygen sublattice in fluorite-type oxides, such as stabilided cubic ZrO2, CeO2 [1-3], (2) atomic structure of ion tracks in CeO2 and MgAl2O4 induced by swift heavy ions, and microstructure evolution at high fluences under overlapping irradiation of high density electronic excitation damage [4-6], and (3) production and stability of defects in MgAl2O4 and alumina (-Al2O3) under synergistic irradiation with dispacement damage and electronic excitation [7].

Orientation dependent annealing kinetics of ion tracks in c-SiO2

The structure and thermal response of amorphous ion tracks formed along the [112¯0], [101¯0], and [0001]-directions in crystalline quartz have been investigated using small angle x-ray scattering. The radii of the ion tracks vary by about 5% (0.3 nm) for tracks along different crystallographic directions. Molecular dynamics simulations reproduce this anisotropy along the [101¯0] and [0001] directions and suggest that differences in thermal conductivity along these directions are partly responsible for this observation. Using in situ annealing, tracks along the [101¯0] and [0001] directions were shown to recrystallize during thermal annealing around 960–1020 °C with activations energies around 6 eV, while those along the [112¯0]-direction already disappeared at 640 °C with a significantly lower activation energy around 3–4 eV.

Atomic structure of ion tracks in Ceria

We have investigated atomic structure of ion tracks in CeO2 irradiated with 200MeV Xe ions by transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). TEM observations under inclined conditions showed continuous ion tracks with diffraction and structure factor contrast, and the decrease in the atomic density of the ion tracks was evaluated to be about 10%. High resolution STEM with high-angle annular dark-field (HAADF) technique showed that the crystal structure of the Ce cation column is retained at the core region of ion tracks, although the signal intensity of the Ce cation lattice is reduced over a region 4–5nm in size. Annular bright field (ABF) STEM observation has detected that the O anion column is preferentially distorted at the core region of ion tracks within a diameter of 4nm. The core region of ion track in CeO2 is determined to contain a high concentration of vacancies or small vacancy clusters and to generate interstitials in surrounding regions.

Effects of electronic energy loss on the behavior of Nd2Zr2O7 pyrochlore irradiated with swift heavy ions

Nd2Zr2O7 pyrochlore was irradiated with swift heavy ions in order to investigate the effects of electronic energy loss on the structural changes induced in this material. XRD and TEM results show that both pyrochlore–fluorite phase transformation and amorphization occur in Nd2Zr2O7. Ion tracks are thus composed of a mixture of anion-deficient fluorite phase and amorphous zones. A new phenomenological model (heterogeneous track overlap model – HTOM), which assumes a direct impact mechanism coupled with a single track overlap process to fit the transformation build-ups, is proposed to estimate the areas of the various track regions. The variation of electronic energy loss mainly influences the structure of ion tracks (and weakly their diameter), and essentially concerns the amount of amorphous phase versus the amount of fluorite counterpart.

Phenomenological model for the formation of heterogeneous tracks in pyrochlores irradiated with swift heavy ions

Zirconate and titanate pyrochlores were irradiated with swift heavy ions in order to investigate the effects of the chemical composition on the structural changes induced by high electronic excitation. Both transmission electron microscopy and X-ray diffraction results show that the structural modifications induced by irradiation with 120-MeV U ions are strongly dependent on the sample composition: Gd 2Ti 2O 7 is readily amorphized, whereas Gd 2Zr 2O 7 is transformed into a radiation-resistant anion-deficient fluorite structure. For Sm 2Zr 2O 7, Eu 2Zr 2O 7 and Nd 2Zr 2O 7, more complex behavior is observed, since both pyrochlore–fluorite phase transformation and amorphization occur. A new phenomenological model (the heterogeneous track overlap model, HTOM), which assumes a direct impact mechanism coupled with a single track overlap process, is proposed to describe the formation of heterogeneous track structures consisting of mixed anion-deficient fluorite and amorphous domains. The pyrochlore composition mainly influences the structure of ion tracks (and weakly their diameter), and essentially concerns the amount of amorphous phase vs the amount of fluorite counterpart.

I–V behavior of transition metal oxides′ nanoparticles confined in ion tracks

Behavior of transition metal oxide (TMO) nanoparticles (Fe3O4, NiO, Co3O4, and Mn3O4) inserted in ion tracks has been studied in the presence of magnetic field. The special structure of ion tracks in dielectric layer on semiconductors is known as TEMPOS—‘Tunable Electronic Materials with Pores in Oxide on Silicon’. TEMPOS structure offers a high surface to volume ratio resulting in fast response time and high sensitivity of sensors fabricated by inserting suitable materials in the ion tracks. We have already reported the behavior of ferrofluids (aqueous and non-aqueous) inserted in the TEMPOS structures and its feasibility as earth’s magnetic field sensor. In continuation to this study, a comparative study between different transition metal oxides inserted in the ion tracks is being presented here with an aim to understand their response in confined geometry. This study shows that Fe3O4 (ferrofluid) is the best choice for ion track-based magnetic field sensor as compared to NiO, Co3O4, and Mn3O4. Its response to magnetic field can be tailored by the dilution of the ferrofluid and annealing.

Single-ion tracks inGd2Zr2−xTixO7pyrochlores irradiated with swift heavy ions

Swift xenon ions (1.43 GeV) were used to systematically investigate the radiation response of pyrochlores in the ${\text{Gd}}_{2}{\text{Zr}}_{2\ensuremath{-}x}{\text{Ti}}_{x}{\text{O}}_{7}$ binary in the electronic energy loss regime. Ion-induced structural modifications were characterized by synchrotron x-ray diffraction, Raman spectroscopy, and transmission electron microscopy. The structure of ion tracks depends on the pyrochlore composition, and the damage cross section increases with the Ti content. In general, single ion tracks consist of an amorphous track core, surrounded by a crystalline, but disordered, defect-fluorite-structured shell. That is in turn surrounded by a defect-rich pyrochlore region. The decrease in the size of these different track zones, with increasing Zr content, is a result of the enhanced radiation stability of Zr-rich pyrochlore within individual swift heavy ion tracks.

The atomic structure of disordered ion tracks in magnesium aluminate spinel

Atomic resolution transmission-electron microscopy (TEM) observations and analysis have been undertaken on magnesium aluminate spinel to understand the structure of ion tracks induced by swift heavy ions. A combination of TEM techniques, which includes high-resolution and bright-field (BF) imaging, and high angular resolution electron channeling spectroscopy (HARECXS) analysis, disclosed the atomic structure of ion tracks. Swift heavy ions induce cation disordering along the latent tracks for a widespread region of 10 nm in diameter, which is much larger than a strained region detected by BF diffraction contrast. A preferential migration of cations is induced from the tetrahedral to octahedral interstitial site at the core regions of ion tracks under a condition of higher electronic excitation of (dE/dx)e = 35 keV/nm.

Atomic structure and disordering induced by 350 MeV Au ions in MgAl2O4

Microstructure change and atomic disordering in MgO·nAl2O3 (n=1.1) irradiated with 350MeV Au ions (Se=35keV/nm) were investigated through transmission electron microscopy (TEM) and high angular resolution electron channeling X-ray spectroscopy (HARECXS) techniques. High resolution TEM revealed that each ion track maintains crystalline structure. The core region of ion track is found to reveal a lattice fringe with a half period of spinel matrix, suggesting the phase transformation from spinel to rock-salt structure. HARECXS analysis clearly showed progress of cation disorder at a significantly large region of ∼10nm in diameter. These results are compared with the previous results of 200MeV Xe ion irradiated spinel (Se=25keV/nm). The structure of ion tracks is found to consist of three concentric circle structures: the defective core region (∼2nm in diameter), strained region (∼5nm) and cation disordered region (10–12nm).