Loop formation by ion irradiation in yttria stabilized zirconia

Yttria stabilized zirconia (YSZ) is a candidate material focused as optical and insulating materials in nuclear reactors. Therefore, it is useful to investigate defect formation during irradiation, in order to assess YSZ resistance to radiation damage. In the present study, in situ transmission electron microscopy (TEM) observations were performed on YSZ during 30 keV Ne⁺ ion irradiation in the temperature range of 723–1123 K (using 100 K intervals). Results revealed that damage evolution morphology depends on irradiation temperature. For irradiations below 1023 K, defect clusters and bubbles were formed simultaneously. On the other hand, at 1123 K, only bubbles were formed in the initial stage of irradiation. Loops formed later following the bubble formation. It was also observed that, in the early stage of irradiation above 923 K, larger bubbles were formed along the loop planes compared with other areas.

TEM observations indicated that dislocation loops formed on three kinds of crystallographic planes: namely, {1 0 0}, {1 1 1} and {1 1 2} planes.     

Ion beam enhanced etching of LiNbO3

Single crystals of z- and x-cut LiNbO₃ were irradiated at room temperature and 15 K using He⁺- and Ar⁺-ions with energies of 40 and 350 keV and ion fluences between 5 × 10¹² and 5 × 10¹⁶ cm⁻². The damage formation investigated with Rutherford backscattering spectrometry (RBS) channeling analysis depends on the irradiation temperature as well as the ion species. For instance, He⁺-irradiation of z-cut material at 300 K provokes complete amorphization at 2.0 dpa (displacements per target atom). In contrast, 0.4 dpa is sufficient to amorphize the LiNbO₃ in the case of Ar⁺-irradiation. Irradiation at 15 K reduces the number of displacements per atom necessary for amorphization. To study the etching behavior, 400 nm thick amorphous layers were generated via multiple irradiation with He⁺- and Ar⁺-ions of different energies and fluences. Etching was performed in a 3.6% hydrofluoric (HF) solution at 40 °C. Although the etching rate of the perfect crystal is negligible, that of the amorphized regions amounts to 80 nm min⁻¹. The influence of the ion species, the fluence, the irradiation temperature and subsequent thermal treatment on damage and etching of LiNbO₃ are discussed.     

Effects of xenon implantation in spinel-zirconia/ceria composites

Two prethinned spinel specimens containing either Y_0.15Zr_0.85O₂ or Ce_0.5Zr_0.5O₂ particles were implanted with 200–400 keV Xe ions at 873 K using the IVEM-Tandem Facility at Argonne National Laboratory. In situ transmission electron microscopy (TEM) was conducted during the implantation in order to follow the evolution of the microstructure. At an ion fluence between 2.4x10²⁰ to 3x10²⁰ m⁻² (up to 50 dpa and 4.7 at %), large Xe bubbles of 50–100 nm developed at the boundaries of the small oxide particles, while a high density of dislocation loops (up to 8 nm in diameter) and much smaller bubbles (up to 4 nm in diameter) formed in the spinel matrix. No large bubbles were observed at the boundaries between the spinel grains. These results suggest that the boundaries between spinel and oxide particles are preferred sites for fission gas accumulation.     

Helium retention of vanadium alloy after energetic helium ion irradiation

Helium irradiation experiments of V–4Ti alloy were conducted in an ECR ion irradiation apparatus by using helium ions with energy of 5 keV. The ion fluence was in the range from 1 × 10¹⁷ He/cm² to 8 × 10¹⁷ He/cm². After the helium ion irradiation, the helium retention was examined by using a technique of thermal desorption spectroscopy (TDS). After the irradiation, the blisters with a size of about 0.1 μm were observed at the surface, and the blister density increased with the ion fluence. Two desorption peaks were observed at approximately 500 and 1200 K in the thermal desorption spectrum. When the ion fluence was low, the retained helium desorbed mainly at the higher temperature regime. As increase of the ion fluence, the desorption at the lower temperature peak increased and the retained amount of helium saturated. The saturated amount was approximately 2.5 × 10¹⁷ He/cm². This value was comparable with those of the other plasma facing materials such as graphite.     

Heavy-ion-induced luminescence of amorphous SiO2 during nanoparticle formation

Silica glass was implanted with negative 60 keV Cu ions at an ion flux from 5 to 75 μA/cm² up to a fluence of 1 × 10¹⁷ ions/cm² at initial sample temperatures of 300, 573 and 773 K. Spectra of ion-induced photon emission (IIPE) were collected in situ in the range from 250 to 850 nm. Optical absorption spectra of implanted specimens were ex situ measured in the range from 190 to 2500 nm.

IIPE spectra showed a broad band centered around 560 nm (2.2 eV) that was assigned to Cu⁺ solutes. The band appeared at the onset of irradiation, increased in intensity up to a fluence of about 5 × 10¹⁵ ions/cm² and then gradually decreased indicating three stage of the ion beam synthesis of nanoclusters: accumulation of implants, nucleation and growth nanoclusters. The IIPE intensity normalized on the ion flux is independent on the ion flux below 20 μA/cm²at higher fluences. The intensity of the band increased with increasing samples temperature, when optical absorption spectra reveal the increase of Cu nanoparticles size.     

Photoluminescence of Au formed in 12CaO · 7Al2O3 single crystal by Au-implantation

Au⁺ ion implantation with fluences from 1 × 10¹⁴ to 3 × 10¹⁶ cm⁻² into 12CaO · 7Al₂O₃ (C12A7) single crystals was carried out at a sample temperature of 600 °C. The implanted sample with the fluence of 1 × 10¹⁵ cm⁻² exhibited photoluminescence (PL) bands peaking at 3.1 and 2.3 eV at 150 K when excited by He–Cd laser (325 nm). This was the first observation of PL from C12A7. These two PL bands are possibly due to intra-ionic transitions of an Au⁻ ion having the electronic configuration of 6s², judged from their similarities to those reported on Au⁻ ions in alkali halides. However, when the concentration of the implanted Au ions exceeded the theoretical maximum value of anions encaged in C12A7 (2.3 × 10²¹ cm⁻³), surface plasmon absorption appeared in the optical absorption spectrum, suggesting Au colloids were formed at such high fluences. These observations indicate that negative gold ions are formed in the cages of C12A7 by the Au⁺ implantation if an appropriate fluence is chosen.     

Heavy ion irradiation effects in the rare-earth sesquioxide Dy2O3

Polycrystalline pellets of the rare-earth sesquioxide Dy₂O₃ with cubic C-type rare-earth structure were irradiated with 300 keV Kr²⁺ ions at fluences up to 5 × 10²⁰ Kr/m² at cryogenic temperature. Irradiation-induced microstructural evolution is characterized using grazing incidence X-ray diffraction (GIXRD) and transmission electron microscopy (TEM). In previous work, we found a phase transformation from a cubic, C-type to a monoclinic, B-type (C2/m) rare-earth structure in Dy₂O₃ during Kr²⁺ ion irradiation at a fluence of less than 1 × 10²⁰ Kr/m². In this study, we find that the crystal structure of the top and middle regions of the implanted layer transform to a hexagonal, H-type (P6₃/mmc) rare-earth structure when the irradiation fluence is increased to 5 × 10²⁰ Kr/m²; the bottom of the implanted layer, on the other hand, remains in a monoclinic phase. The irradiation dose dependence of the C-to-B-to-H phase transformation observed in Dy₂O₃ appears to be closely related to the temperature and pressure dependence of the phases observed in the phase diagram. These transformations are also accompanied by a decrease in molecular volume (or density increase) of approximately 9% and 8%, respectively, which is an unusual radiation damage behavior.     

XPS and optical studies of Xe-implanted and annealed YSZ single crystals

Xe⁺ ion implantation with 200 keV was completed at room temperature up to a fluence of 1 × 10¹⁷ ion/cm² in yttria-stabilized zirconia (YSZ) single crystals. Optical absorption and X-ray photoelectron spectroscopy (XPS) were used to characterize the changes of optical properties and charge state in the as-implanted and annealed crystals. A broad absorption band centered at 522 or 497 nm was observed in the optical absorption spectra of samples implanted with fluences of 1 × 10¹⁶ ion/cm² and 1 × 10¹⁷ ion/cm², respectively. These two absorption bands both disappeared due to recombination of color centers after annealing at 250 °C. XPS measurements showed two Gaussian components of O_1s spectrum assigned to Zr–O and Y–O, respectively, in YSZ single crystals. After ion implantation, these two peaks merged into a single peak with the increasing etching depth. However, this single peak split into two Gaussian components again after annealing at 250 °C. The concentration of Xe decreased drastically after annealing at 900 °C. And the XPS measurement barely detected the Xe. There was no change in the photoluminescence of YSZ single crystals with a fluence of 1 × 10¹⁷ ion/cm² after annealing up to 900 °C.     

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 350 MeV Au ions (S_e = 35 keV/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 10 nm in diameter. These results are compared with the previous results of 200 MeV Xe ion irradiated spinel (S_e = 25 keV/nm). The structure of ion tracks is found to consist of three concentric circle structures: the defective core region (2 nm in diameter), strained region (5 nm) and cation disordered region (10–12 nm).     

Analysis of charge-exchanged neutral particles during IBW heating in the HT-7 tokamak

The charge-exchange neutral particles fluxes and energy distribution in IBW heated plasma were investigated in the HT-7 tokamak. The RF frequency was 30 MHz and with an injecting power up to 200 kW. It is observed that the plasma performance is obviously enhanced by IBW heating. The electron temperature was increased by 0.5 keV and the central line averaged electron density was doubled. The neutral particle fluxes of high-energy increased and the bulk ions were heated during IBW heating. The ion temperature was increased by 0.3 keV and the ion heating efficiency of (2–3) eV kW⁻¹ × 10¹³ cm⁻³ was achieved. The velocity distribution of charge-exchanged neutral particles appears to be Maxwellian without high-energy tail ions up to the maximum RF power.     

Radiation damage induced by 5 keV Si ion implantation in strained-Si/Si0.8Ge0.2

The damage distributions induced by ultra low energy ion implantation (5 keV Si⁺) in both strained-Si/Si_0.8Ge_0.2 and normal Si are measured using high-resolution RBS/channeling with a depth resolution better than 1 nm. Ion implantation was performed at room temperature over the fluence range from 2 × 10¹³ to 1 × 10¹⁵ ions/cm². Our HRBS results show that the radiation damage induced in the strained Si is slightly larger than that in the normal Si at fluences from 1 × 10¹⁴ to 4 × 10¹⁴ ions/cm² while the amorphous width is almost the same in both strained and normal Si.     

Damage produced in magnesium aluminate spinel by high energy heavy ions including fission products of fission energy: microstructure modifications

The radiation stability of spinel MgAl₂O₄ against the impact of beams of a fission product at fission energy (70 MeV iodine) and at different fluences was investigated using Transmission Electron Microscopy (TEM). Specimens prethinned before irradiation were analysed by TEM and irradiated bulk specimens were investigated using cross-sectional TEM. Tracks were observed in pre-thinned specimens. Partial amorphisation was observed for the irradiation at the highest fluence. Recrystalization of the amorphous region, induced by the microscope electron beam was observed. A threshold value of 6 keV nm⁻¹ was determined for the amorphization of spinel under the above mentioned irradiation conditions. Moreover, profile measurements of the irradiated areas confirmed the large swelling values for this material when irradiated with fission products of fission energy. A thermal spike model was used to calculate the damage threshold for spinel using experimentally measured heavy-ion track radii, including results for other ions of up to very high energies. These accelerator-based fission product irradiations revealed an unexpected poor radiation stability, in contrast to the known good behaviour of the spinel against neutron or alpha particle damage.     

Enthalpy and heat capacity of LiAlO2 between 298 and 1700 K by drop calorimetry

The enthalpy of γ-LiAlO₂ was measured between 403 and 1673 K by isothermal drop calorimetry. The smoothed enthalpy curve between 298 and 1700 K results in H⁰(T) − H⁰(298 K)=−37 396 + 93.143 · T + 0.00557 · T² + 2 725 221 · T⁻¹ J/mol. The standard deviation is 2.2%. The heat capacity was derived by differentiation of the enthalpy curve. The value extrapolated to 298 K is C_p,298=(65.8 ± 2.0) J/K mol.     

Surface modification of MgAl2O4 and oxides with heavy ions of fission fragments energy

In this paper we report on results of surface modification in several candidate materials for inert matrix fuel hosts (MgAl₂O₄, MgO and Al₂O₃) induced by (0.5–5) MeV/amu Kr, Xe and Bi ion bombardment in the fluence range of 2 × 10¹⁰–10¹² ions/cm². The size and shape of nanoscale hillock-like defects, each of which was created by the impact of a single ion, have been studied by using atomic force microscopy (AFM) technique. It was found that mean hillock height on sapphire and spinel surfaces depends linearly on the incident electronic stopping power. The hillocks are highest in MgAl₂O₄, having a lower threshold for the lattice disorder in the bulk material via relaxation of electronic excitations. As a possible reason for the hillocks formation, the plastic deformation due to the defects created by the Coulomb explosion mechanism in the target subsurface layer is suggested.     

Irradiation effects of swift heavy ions in actinide oxides and actinide nitrides: Structure and optical properties

Actinide oxides have been used as nuclear fuels in the majority of power reactors working in the world and actinide nitrides are under investigation for the fuels of the future fast neutron fission reactors developed in Forum Generation IV. Radiation damage in actinide oxides UO₂, (U_0.92Ce_0.08)O₂, and actinide nitride UN has been characterized after irradiation with swift heavy ions. Fluences up to 3 × 10¹³ ions/cm² of heavy ions (Kr 740 Mev, Cd 1 GeV) available at the CIRIL/GANIL facility were used to simulate irradiation in reactors by fission products and by neutrons. The macroscopic effects of irradiation remains very weak compared with those seen in other ceramic oxides irradiated in the same conditions: practically no swelling can be measured and no change in colour can be observed on the irradiated part of a polished face of sintered disks. The point defects in irradiated actinide compounds have been characterized by optical absorption spectroscopy in the UV–Vis–NIR wavelength range. The absorption spectra before and after irradiation are compared, and unexpected stability of optical properties during irradiation is shown. This result confirms the low rate of formation of point defects in actinide oxides and actinide nitrides under irradiation. Actinide oxides and nitrides studied are >40% ionic, and oxidation state of the actinides seems to be stable during irradiation. The small amount of point defects produced by radiation (<10¹⁶ cm⁻²) has been identified from differences between the absorption spectrum before irradiation and the one after irradiation: point defects in oxygen or nitrogen lattices can be observed respectively in oxides and nitrides (F centres), and small amounts of U⁵⁺ would be present in all compounds.     

Structural evolution in Fe ion implanted Si upon thermal annealing

We have performed high-dose Fe ion implantation into Si and characterized ion-beam-induced microstructures as well as annealing-induced ones using transmission electron microscopy (TEM) and grazing-incidence X-ray diffraction (GIXRD). Single crystals of Si(1 0 0) substrate were irradiated at 623 K with 120 keV Fe⁺ ions to a fluence of 4 × 10¹⁷ cm⁻². The irradiated samples were then annealed in a vacuum furnace at temperatures ranging from 773 K to 1073 K. Cross-sectional TEM observations and GIXRD measurements revealed that a layered structure is formed in the as-implanted specimen with ε-FeSi, β-FeSi₂ and damaged Si, as component layers. A continuous β-FeSi₂ layer was formed on the topmost layer of the Si substrate after thermal annealing.     

Ion beam induced amorphization and recrystallization of Si/SiC/Si layer systems

Epitaxial, buried silicon carbide (SiC) layers have been fabricated in (100) and (111) silicon by ion beam synthesis (IBS). In order to study the ion beam induced epitaxial crystallization (IBIEC) of buried SiC layers, the resulting Si/SiC/Si layer systems were amorphized using 2 MeV Si²⁺ ion irradiation at 300 K. An unexpected high critical dose for the amorphization of the buried layers is observed. Buried, amorphous SiC layers were irradiated with 800 keV Si⁺ ions at 320 and 600°C, respectively, in order to achieve ion beam induced epitaxial crystallisation. It is demonstrated that IBIEC works well on buried layers and results in epitaxial recrystallization at considerably lower target temperatures than necessary for thermal annealing. The IBIEC process starts from both SiC/Si interfaces and may be accompanied by heterogenous nucleation of poly-SiC as well as interfacial layer-by-layer amorphization, depending on irradiation conditions. The structure of the recrystallized regions in dependence of dose, dose rate, temperature and crystal orientation is presented by means of TEM investigations.     

Structural stability of ZnAl2O4 spinel irradiated by low energy particles

It is now well known that irradiation of metals and alloys can drive materials into complex configurations. Several examples, like the occurrence of order–disorder phase transitions driven by irradiation, are discussed by many authors. To understand the behavior of ceramics under irradiation, several spinels were irradiated. In this paper, experimental results on the irradiation of ZnAl₂O₄ by low-energy particles (4 MeV Au²⁺ ions) are presented. An order–disorder phase transition is observed. The unusual behavior of this spinel under irradiation is discussed within the framework of Martin’s theory of driven alloys under irradiation.     

A generalized model for radiation-induced amorphization and crystallization of U3Si and U3Si2 and recrystallization of UO2

A rate-theory model of radiation-induced amorphization and crystallization of U₃Si during ion irradiation has been generalized to include U₃Si₂ and UO₂. The generalized model has been applied to ion-irradiation and in-reactor experiments on U₃Si and U₃Si₂ and provides an interpretation for the amorphization curve (dose required to amorphize the material as a function of temperature), for the ion-radiation-induced nanoscale polycrystallization of these materials at temperatures above the critical temperature for amorphization, as well as for the role of the small crystallites in retarding amorphization. An alternative mechanism for the evolution of recrystallization nuclei is described for a model of irradiation-induced recrystallization of UO₂ wherein the stored energy in the UO₂ is concentrated in a network of sinklike nuclei that diminish with dose due to interaction with radiation-produced defects. The sinklike nuclei are identified as cellular dislocation structures that evolve relatively early in the irradiation period. The complicated kinetics involved in the formation of a cellular dislocation network are approximated by the formation and growth of subgrains due to the interaction of shock waves produced by fission-induced damage to the UO₂.