Cathodoluminescence (CL) Microanalysis (spectroscopy and microscopy) provides unique high sensitivity, high spatial resolution information about the defect structure and distribution of defects in wide band gap materials and therefore is an ideal technique with which to investigate the microstructural processes induced by irradiation. CL microanalytical techniques allow the in situ monitoring and post irradiation assessment of electron irradiation induced damage. Changes in the defect structure and surface topography of electron irradiated silicon dioxide polymorphs and related silicates including pure crystal quartz, pure silica glasses, pure amorphous fused quartz and alkali-borosilicate glasses, have been investigated and compared using CL microanalysis and Scanning Probe Microscopy (SPM) techniques. CL and SPM evidence shows all specimens are sensitive to electron irradiation. CL evidence is consistent with the production and micro-segregation of irradiation induced defects. The observed damage is highly correlated with the electron irradiation induced changes in the surface topography of the investigated specimens.

We show evidence from transmission electron microscopy of an unusual change in columnar defect morphology in YBCO upon annealing to 600°C. The disappearance of the nanotwinned, but not the larger scaled thermal twinned, structure is found. A removal of the large fluctuations in the diameter of the amorphous column, preserving a narrow continuous column, is surprisingly discovered. Correlations with magnetization measurements demonstrate the greater efficiency of vortex pinning at 77 K by the annealed defect structure.

Refinement of several topologically generated displacement cascades in silica has been conducted using molecular dynamics (MD) simulation. Several metastable amorphous silicas with substantially different medium-range order (as characterized by ring topologies) were obtained. However, their total correlation functions were found scarcely distinguishable. Major structural reconstruction was observed when the refinement took place above a glass transition temperature, below which the cascades largely retained their original topological ring structures. Attempts are made to correlate topological ring distributions with the first sharp diffraction peak, which may in turn provide some insight into the medium range structures of irradiated silicas.

Microstructural examination and in situ post-irradiation annealing studies were carried out on 20% cold-worked 316 stainless steel (SS) hexagonal duct material following irradiation in the reflector region of the EBR-II reactor. Stainless steel hexagonal ducts were used to house reactor subassemblies and provide a valuable source of information on irradiation behavior of reactor structural materials at lower dose-rates (on the order of 10-8 dpa/sec) than previously examined. The microstructural development of samples irradiated to doses of 1, 20 and 30 dpa is examined, while the post-irradiation annealing behavior of a sample irradiated to 20 dpa is described. Annealing studies were performed at 370 and 500°C to examine the kinetics of radiation damage recovery as a function of annealing temperature. The initial (pre-annealed) microstructures consists of a substantial density of irradiation induced chromium-rich M23C6 and M6C carbides which form both on the grain boundaries and within the grain interiors. Recovery of the cold- work is evident in the 1 dpa sample while samples irradiated to 20 and 30 dpa possess dense populations of voids and dislocation structures consisting of networks of line dislocations and faulted dislocation loops. Results indicate that post-irradiation annealing of the samples at 370°C for 1 hour has little effect on the microstructure, while further annealing at 500°C for 1 hour results in void shrinkage, the formation of small cavities, and a reduction in the dislocation loop and network density.

Ion implantation has become a versatile and powerful technique for synthesizing nanometer-scale clusters and crystals embedded in the near-surface region of a variety of hosts. The resulting nanocomposite materials often show unique optical, magnetic, and electronic properties. Here we review some of the principal features of this nanophase materials synthesis technique and discuss the outstanding experimental difficulties that currently hamper the development of devices based on the many unique properties of these nanocomposite materials. Possible solutions to these problems and future research directions are discussed.

Analysis of the effect of annealing on change in void and loop size distributions provided insights that complement microstructural characterization using transmission electron microscopy (TEM). The predictions of concurrent vacancy-type and interstitial-type damage annealing are applied to measurements in neutron-irradiated austenitic stainless steels. Irradiation at 330°C produces void and loop microstructures for which the measured annealing response is in accord with predictions. Irradiation at 275° produces only Frank loop microstructure for which the annealing response cannot be predicted from measured microstructures. The model predictions are based on an assumed vacancy source not detected using TEM. The measured loop microstructure is typically reported to be interstitial in character but this analysis suggests that a significant component of the loop population is vacancy-type damage based on defect inventory and kinetic arguments.

The correlation between the structural and optical properties of Si nanocrystals embedded in SiO2 is the key factor to understand their emission mechanism. However, there is a great difficulty in imaging Si nanocrystals in SiO2 and measuring their size distribution because of the lack of contrast in electron microscopy. We have used here a new method for imaging Si nanocrystals by using high resolution electron microscopy in conjunction with conventional electron microscopy in dark field conditions. Regarding the optical properties, the band-gap energies and photoluminescence have been measured by direct and independent methods. The results have allowed experimental determination, for the first time in this material, of the experimental Stokes shift between absorption and emission as a function of crystallite size. The experimental band-gap versus size correlates well with the most accurate theoretical predictions. Moreover, the photoluminescence energy emission versus crystallite size shows a parallel behaviour to that of band-gap energy. Consequently, the experimental Stokes shift is independent of nanocrystal size and is found to be 0.26±0.03 eV. This value is almost twice the energy of the Si-O vibration (0.134 eV). These results suggest that the dominant emission of Si nanocrystals passivated with SiO2 is a fundamental transition spatially located at the Si-SiO2 interface and with the assistance of a local Si-O vibration.

Cross-section transmission electron microscopy was used to investigate the microstructure of single crystal silicon carbide and polycrystalline silicon nitride and aluminum nitride following room temperature irradiation with either 245 MeV Kr or 710 MeV Bi ions. The fluences ranged from 1×1012/cm2 (single track regime) to 1×1013/cm2. Ion track formation was observed in the Bi ion-irradiated Si3N4 specimen in regions where the electronic stopping power exceeded a critical value of ∼15 keV/nm (depths <24 μm). Ion track formation was not observed at any depth in 245 MeV Kr ion-irradiated Si3N4, in which the maximum electronic stopping power was 14.5 keV/nm. There was no evidence for track formation in either SiC or AlN irradiated with 710 MeV Bi ions, which indicates that the threshold electronic stopping power for track formation in these two ceramics is >34 keV/nm. The high resistance of SiC and AlN to track formation may be due to their high thermal conductivity, but further study is needed to quantitatively evaluate the suitability of the various track formation models.

Dilute III-Nx-V1-x alloys were successfully synthesized by nitrogen implantation in GaAs and InP. The fundamental band gap energy for the ion beam synthesized III-Nx-V1-x alloys was found to decrease with increasing N implantation dose in a manner similar to that commonly observed in epitaxially grown GaNxAs1-x and InNxP1-x thin films. The fraction of N occupying anion sites (“active” N) in the GaNxAs1-x layers formed by N implantation was thermally unstable and decreased with increasing annealing temperature. In contrast, thermally stable InNxP1-x alloys with N mole fraction as high as 0.012 were synthesized by N implantation in InP. Moreover, the N activation efficiency in InP was at least a factor of two higher than in GaAs under similar processing conditions. The low N activation efficiency (<20%) in GaAs can be improved by co-implanting Ga and N in GaAs.

A reduced activation ferritic/martensitic steel, F82H (IEA heat), developed for fusion energy applications was irradiated at 300 and 500°C to 5 dpa in the High Flux Isotope Reactor (HFIR). Changes in yield strength, deformation mode, and strain-hardening capacity were seen, with the magnitude of the changes dependent on irradiation temperature. Irradiation at 300°C led to a significant loss of strain-hardening capacity with a large change in yield strength. There was a tendency for a reduction in strain rate to cause a decrease in yield strength and elongation. Irradiation at 500°C had little effect on strength, but a reduction in strain rate caused a decrease in ductility. In order to determine the contributions of different microstructural features to strength and to deformation mode, transmission electron microscopy (TEM) specimens were prepared from the gage sections of the tested (strained) flat tensile specimens and examined; fracture surfaces were examined by scanning electron microscopy (SEM). The fracture surfaces showed a martensitic mixed quasi-cleavage and ductile-dimple fracture in the center at both irradiation temperatures. The microstructure in the necked region irradiated at 300°C showed defect free bands, which may be dislocation channels. This suggests that dislocation channeling could be the dominant deformation mechanism in martensitic steels irradiated at 300°C, resulting in the loss of strain-hardening capacity.

The primary damage state in 3C-SiC has been comprehensively studied by molecular dynamics using a modified Tersoff potential. The simulations examined damage produced by Si and C primary knock-on atoms (PKA) with energies from 0.25 to 30 keV. The study also generated statistics of defect production by simulating a number of PKAs at each energy. The defect production efficiency decreases with increasing PKA energy, as observed previously in metals. However, the cascade lifetime is very short (less than 1 ps), localized melting does not occur, the defect arrangements are highly dispersed, and the tendency for defects to form clusters is much less compared to the case of metals. Frenkel pairs on the C sublattice are more numerous than Si Frenkel pairs, and 10-20% of the displacements are in the form of anti-site defects.

In this work we report the structural and optical properties of ion implanted GaN. Potential acceptors such as Ca and Er were used as dopants. Ion implantation was carried out with the substrate at room temperature and 550 °C, respectively. The lattice site location of the dopants was studied by Rutherford backscattering/channeling combined with particle induced X-ray emission. Angular scans along both [0001] and [1011] directions show that 50% of the Er ions implanted at 550 oC occupy substitutional or near substitutional Ga sites after annealing. For Ca we found only a fraction of 30% located in displaced Ga sites along the [0001] direction. The optical properties of the ion implanted GaN films have been studied by photoluminescence measurements. Er- related luminescence near 1.54 μm is observed under below band gap excitation at liquid helium temperature. The spectra of the annealed samples consist of multiline structures with the sharpest lines found in GaN until now. The green and red emissions were also observed in the Er doped samples after annealing.

Clusters of self-interstitial atoms (SIAs) formed in displacement cascades in metals irradiated with energetic particles play an important role in microstructure evolution under irradiation. They have been studied in the fcc and bcc metals by atomic scale computer simulation and in this paper we present the first results of a similar study of SIA clusters in an hcp crystal. Clusters of 4 to 30 SIAs were modelled over a wide temperature range using molecular dynamics and a many-body Finnis-Sinclair type interatomic potential for Zr. The results show a qualitative similarity of the dynamic properties of clusters to those for cubic metals. In particular, all clusters larger than 4 SIAs exhibit fast thermally-activated one-dimensional glide, which is in a <1120> direction in the hcp case. Due to the crystallographic features of the hcp lattice, this mechanism leads to two-dimensional mass transport in basal planes. Smaller clusters (≤ 4 SIAs) exhibit behaviour peculiar to the hcp structure, however, for they can migrate two-dimensionally in the basal plane. The jump frequency, activation energy and correlation factors of clusters have been estimated and comparison drawn between the behaviour of SIA clusters in different structures.

Physical properties of multiple-energy B-ion implanted C60 thin films were investigated for various doses. Fourier Transform Infra-red Spectroscopy (FTIR) results indicate the structural transformation of C60 to amorphous carbon phase during implantation. The conductivity type of the implanted films is found to be p-type and the conductivity measurements reveal a dramatic increase in the conductivity with ion implantation. Temperature dependent conductivity shows the semiconducting nature of the B-ion implanted films. The optical absorption coefficient and optical gap of the implanted films have been observed as a function of B-ion dose. Measurements on implanted films subjected to thermal annealing indicate the removal of the defects caused during the implantation. Ion implantation-induced defects are found to partially annihilate with the annealing temperature. Electrical conductivity and optical gap are determined in the post-implanted films. The observation of the systematic increase in the conductivity of the annealed films is due to the removal of the defects and the formation of defect free boron impurity acceptor.

Molecular dynamics (MD) simulations have been employed to study cascade overlap and defect accumulation processes during amorphization in 3C-SiC. A large number of 10 keV displacement cascades were randomly generated in a model crystal to achieve the amorphous state, and the corresponding dose is consistent with experimental observations. The results show that most defects are single interstitials and mono-vacancies at low dose, whereas the amorphous or disordered clusters, which consist of interstitials and antisite defects, appear at intermediate dose levels. These local disordered regions play an important role in the amorphization of SiC. At higher doses, a significant proportion of antisite defects is created during continued cascade overlap. The increase in interstitials and antisite defects with increasing dose suggests that the primary driving force for the crystalline-to-amorphous transition under these ion irradiation conditions in SiC is due to the accumulation of both Frenkel pairs and antisite defects.

As part of the shutdown of the EBR-II reactor, structural materials were retrieved to analyze the effects of long-term irradiation on mechanical properties and microstructure. In this work, the effect of low dose rate irradiation (10−7 to 10−8 dpa/s) on grain boundary composition in 316 and 304 stainless steels was analyzed. Samples were taken from surveillance specimens and subassemblies irradiated in the reflector region of EBR-II at temperatures from 371-390°C to maximum doses of 30 dpa. The effects of dose, dose rate, and bulk composition on radiation- induced segregation are analyzed. In 316 stainless steel, changes in grain boundary chromium and nickel concentrations occur faster than changes in iron and molybdenum concentrations. In 304 stainless steel, decreasing the dose rate increases the amount of grain boundary segregation. For a dose of 20 dpa, chromium depletion and nickel enrichment are greater in 304 stainless steel than in 316 stainless steel, the difference most likely due to dose rate. In both 304 and 316 stainless steels, the presence of a grain boundary precipitate significantly changes the composition of the adjacent grain boundary.

The mechanisms of dislocation-defect interactions are of practical importance for developing quantitative structure-property relationships, mechanistic understanding of plastic flow localization and predictive models of mechanical behavior in metals under irradiation. In copper and other face centered cubic metals, high-energy particle irradiation produces hardening and shear localization. Post-irradiation microstructural examination in Cu reveals that irradiation has produced a high number density of nanometer sized stacking fault tetrahedra. Thus, the resultant irradiation hardening and shear localization is commonly attributed to the interaction between stacking fault tetrahedra and mobile dislocations, although the mechanism of this interaction is unknown. In this work, we present a comprehensive molecular dynamics simulation study that characterizes the interaction and fate of moving dislocations with stacking fault tetrahedra in Cu using an EAM interatomic potential. This work is intended to produce atomistic input into dislocation dynamics simulations of plastic flow localization in irradiated materials.

An atom probe tomography study has been performed to estimate the matrix composition, and the size, composition and number density of the ultrafine copper-enriched precipitates that formed under neutron irradiation in a submerged arc weld (Weld 73W). This high copper (0.27 at. % Cu) weld was examined after a typical stress relief treatment of 40 h at 607°C, after neutron irradiation to a fluence of 1.8 × 1023 n m-2 (E > 1 MeV) at a temperature of 288°C, after irradiation and thermal annealing for 168 h at 454°C, and after irradiation, thermal annealing and re-irradiation to an additional fluence of 0.8 × 1023 n m-2 (E > 1 MeV). A high number density of ultrafine copper-enriched precipitates were found to form on irradiation. These precipitates were found to coarsen and decrease in number density after the annealing treatment at 454°C. These precipitates were present after re-irradiation and some additional subnanometer diameter copper- enriched precipitates were also observed. Segregation of nickel, manganese, silicon, copper and phosphorus atoms to dislocations was also observed.

A 14 MeV D-T(fusion) neutron irradiation was carried out at fusion neutron source facility (FNS) in Japan Atomic Energy Research Institute (JAERI). Specimen temperature was controlled to 288 K. Fluence was 6.1 × 1017to 1.1 × 1021 n/m2. Both TEM thin foil and bulk specimens were irradiated at the same position. At 1018 n/m2, defects observed were single isolated dot defects. With increasing fluence, dot defects changed to complicate structure and made groupings. In a dot group, interstitial clusters and vacancy clusters were observed together. The present result was explained by the modeling that point defects in a nascent damage cascade move in crystal at 288 K and form their defect groupings.

In the present study we correlate between the secondary electron emission (SEE) of variously treated Xe+ ion-damaged diamond films and their bonding structure in the near-surface region as identified by near edge X-ray absorption fine structure (NEXAFS) spectroscopy and X-ray photoelectron spectroscopy. The 50 keV Xe+ ion bombardment of hydrogenated polycrystalline diamond films to a dose of 2×1015 cm−2 results in the transformation of near-surface diamond to sp2-bonded amorphous carbon, increased oxygen adsorption, shift of the electron affinity from negative to positive, and strong degradation of its electron emission properties, although it does not induce a pronounced depletion of hydrogen. Exposure of the ion-bombarded films to microwave (MW) hydrogen plasma treatment for 30 min produces negative electron affinity diamond surfaces, but only partially regenerates SEE properties, retains some imperfection in the near-surface atomic layers, as determined by NEXAFS, and the concentration of oxygen remains relatively high. Subsequent annealing to 610 °C produces oxygen-free diamond films and somewhat increases their SEE. Annealing to 1000 °C results in desorption of the surface hydrogen, formation of a positive electron affinity surfaces and drastically degrades their electron emission properties. Prolonged, up to three hours MW hydrogen plasma treatment of as-implanted diamond films gradually improves the crystal quality and results in further increase of SEE intensity. This treatment does not, however, substantially reduce the concentration of oxygen in the previously damaged diamond, indicating its bulk diffusion during or after ion bombardment. To fully recover electron emission properties it is necessary to both remove the defects and hydrogenate the diamond surfaces.