Er site in Er + Au-implanted SiO2: Effect of annealing in reducing atmosphere

The Er site in Er + Au-implanted silica has been investigated by x-ray absorption spectroscopy, in particular after annealing in reducing atmosphere (H₂(4%):N₂(95%)) at temperature ranging from 100 to 800 °C. The EXAFS analysis shows that Er ions are surrounded by a first shell of O atoms, while the absence of signal from further coordination shell indicates a disordered site. The Er-O distance is lower than that of the Er₂O₃; it is suggested that the annealing in reducing atmosphere leads to a significant reduction of the first shell coordination number. Correspondingly, in the XANES region of the spectrum, it is observed a decrease in the white line intensity for annealing temperature higher than 400 °C; similar annealing treatments in inert atmosphere did not result in significant changes of the near-edge region of the X-ray absorption spectrum. These results enlighten that the annealing procedure, normally used to tailor the size distribution of the metal clusters present in the matrix and/or to modify the matrix structure, can also have an effect on the site of the Er ions, and possibly on the rare-earth optical properties.     

Oxygen defects created in CeO2 irradiated with 200 MeV Au ions

CeO₂ films were irradiated with 200 MeV Au ions in order to investigate the damages created by electronic energy deposition. In the Raman spectra of the ion-irradiated films, a broad band appears at the higher frequency side of the F_2g peak of CeO₂. The band intensity increases as ion fluence increases. Furthermore, the F_2g peak becomes asymmetric with a low-frequency tail. In order to understand the origin of these spectral changes, an unirradiated CeO₂ film was annealed in vacuum at 1000 °C. By comparing the results for the irradiation and for the annealing, it is concluded that the broad band obtained for irradiated samples contains the peak observed for the annealed sample. The F_2g peak becomes asymmetric with a low-frequency tail by the irradiation as well as the annealing. Therefore, the above-mentioned changes in the Raman spectra caused by 200 MeV Au irradiation is closely related to the creation of oxygen vacancies.     

Strain-driven (0 0 2) preferred orientation of ZnO nanoparticles in ion-implanted silica

We present an experimental and theoretical study on the structural properties of ZnO nanoparticles embedded in silica. The ZnO-SiO₂ nanocomposite was prepared by ion implanting a Zn⁺ beam in a silica slide and by annealing in oxidizing atmosphere at 800 °C. From an experimental point of view, the structural properties of the ZnO-SiO₂ nanocomposite were studied by using glancing incidence X-ray diffraction. According to the results, zinc crystalline nanoclusters with an average diameter of 13 nm are in the as-implanted sample. The annealing in oxidizing atmosphere promotes the total oxidation of the Zn nanoclusters and increases their size until to an average of 22 nm. Moreover, the formed ZnO nanocrystals have a preferential (0 0 2) crystallographic orientation. From a theoretical point of view, the preferential orientation of the ZnO nanoparticles can be explained satisfactory by the minimization of the strain energy of the nanoparticles placed in proximity of the surface of the matrix.     

Effects of MeV Si ions bombardment on the thermoelectric generator from SiO2/SiO2 + Cu and SiO2/SiO2 + Au nanolayered multilayer films

The defects and disorder in the thin films caused by MeV ions bombardment and the grain boundaries of these nanoscale clusters increase phonon scattering and increase the chance of an inelastic interaction and phonon annihilation. We prepared the thermoelectric generator devices from 100 alternating layers of SiO₂/SiO₂ + Cu multi-nano layered superlattice films at the total thickness of 382 nm and 50 alternating layers of SiO₂/SiO₂ + Au multi-nano layered superlattice films at the total thickness of 147 nm using the physical vapor deposition (PVD). Rutherford Backscattering Spectrometry (RBS) and RUMP simulation have been used to determine the stoichiometry of the elements of SiO₂, Cu and Au in the multilayer films and the thickness of the grown multi-layer films. The 5 MeV Si ions bombardments have been performed using the AAMU-Center for Irradiation of Materials (CIM) Pelletron ion beam accelerator to make quantum (nano) dots and/or quantum (quantum) clusters in the multilayered superlattice thin films to decrease the cross plane thermal conductivity, increase the cross plane Seebeck coefficient and cross plane electrical conductivity. To characterize the thermoelectric generator devices before and after Si ion bombardments we have measured Seebeck coefficient, cross-plane electrical conductivity, and thermal conductivity in the cross-plane geometry for different fluences.     

Structural studies of Ge nanocrystals embedded in SiO2 matrix

Germanium nanoparticles embedded in SiO₂ matrix were prepared by atom beam sputtering on a p-type Si substrate. The as-deposited films were annealed at temperatures of 973 and 1073 K under Ar + H₂ atmosphere. The as-deposited and annealed films were characterized by Raman, X-ray diffraction and Fourier transform infrared spectroscopy (FTIR). Rutherford backscattering spectrometry was used to quantify the concentration of Ge in the SiO₂ matrix of the composite thin films. The formation of Ge nanoparticles were observed from the enhanced intensity of the Ge mode in the Raman spectra as a function of annealing, the appearance of Ge(3 1 1) peaks in the X-ray diffraction data and the Ge vibrational mode in the FTIR spectra. We have irradiated the films using 100 MeV Au⁸⁺ ions with a fluence of 1 × 10¹³ ions/cm² and subsequently studied them by Raman and FTIR. The results are compared with the ones obtained by annealing.     

Synthesis and characterization of SnO2 nanoparticles embedded in silica by ion implantation

Tin dioxide nanoparticles embedded in silica matrix were fabricated by ion implantation combined with thermal oxidation. Silica substrate was implanted with a 150 keV Sn⁺ ions beam with a fluence of 1.0 × 10¹⁷ ions/cm². The sample was annealed for 1 h in a conventional furnace at a temperature of 800 °C under flowing O₂ gas. According to the structural characterization performed by X-ray diffraction and transmission electron microscopy techniques, metallic tetragonal tin nanoparticles with a volume average size of 12.8 nm were formed in the as-implanted sample. The annealing in oxidizing atmosphere promotes the total oxidation of the tin nanoparticles into tin dioxide nanoparticles with a preferential migration toward the surface of the matrix, where large and coalesced nanoparticles were observed, and a small diffusion toward the bulk, where smaller nanoparticles were found.     

Cluster formation, evolution and size distribution in Fe-Cu alloy: Analysis by XAFS, XRD and TEM

Fe-Cu alloys containing 1.3 at.% copper were studied as model systems for cluster formation in reactor pressure vessel steels. The samples were annealed at 775 K for different times and subsequently analyzed using X-ray absorption fine structure spectroscopy at the Cu-K-edge, X-ray diffraction and transmission electron microscopy. The results show that copper cluster formation might occur even with short annealing times. These clusters of about 1 nm size can switch easily from bcc iron-like structures to fcc copper, if the local copper concentration is high enough. While a short annealing time of 2.5 h at 775 K maintains a good dilution of copper in the bcc iron matrix, annealing for 312 h leads to large fcc copper precipitates. A linear combination analysis suggests that in the sample annealed 8 h, copper clusters are mostly formed with the same structure as the matrix. A co-existence of bcc and fcc clusters is obtained for 115 h of annealing. Transmission electron microscopy indicates the presence of precipitates as large as 60 nm size for an annealing time of 312 h, and X-ray diffraction provided complementary data about the clusters size distributions in all of the four samples.     

Nonlinear optical properties of MeV and keV ion beam synthesized Ag nanoclusters

Ag nanoclusters embedded in silica glass matrix have been synthesized by high fluence ion implantation using both keV and MeV ion beams. In keV implantation case, optical absorption shows an intense surface plasmon resonance (SPR) peak corresponding to the Ag clusters formed in the matrix. Transmission electron microscopy (TEM) measurements carried out on identically implanted SiO₂ thin films on a TEM catcher grid shows the presence of Ag nanoclusters of size around 4 nm in the matrix. However, for the MeV implantation case, the SPR peak appears in the optical absorption spectra only after air annealing the sample at 500 °C for one hour. For the annealed samples, TEM measurements show the presence of 6 nm sized Ag nanoclusters. On the other hand the as-implanted sample shows smaller nanoclusters with a lower particle density in the matrix. Interestingly, open aperture z-scan measurements carried out on keV implanted samples did not show any nonlinear absorption, while the MeV as-implanted as well as annealed samples showed nonlinear absorption. The nonlinear absorption coefficient of the MeV annealed sample is extracted from a fit to the z-scan data considering a three photon like absorption process.     

10 MeV Au ion irradiation effects in an MgO-HfO2 ceramic-ceramic (CERCER) composite

Room temperature ion irradiation damage studies were performed on a ceramic composite intended to emulate a dispersion nuclear fuel. The composite is composed of 90-mole% MgO and 10-mole% HfO₂. The as-synthesized composite was found to consist of Mg₂Hf₅O₁₂ (and some residual HfO₂) particles embedded in an MgO matrix. X-ray diffraction revealed that nearly all of the initial HfO₂ reacted with some MgO to form Mg₂Hf₅O₁₂. Ion irradiations were performed using 10 MeV Au³⁺ ions at room temperature over a fluence range of 5 × 10¹⁶-5 × 10²⁰ Au/m². Irradiated samples were characterized using both grazing incidence X-ray diffraction (GIXRD) and transmission electron microscopy (TEM), the latter using both selected-area electron diffraction (SAED) and micro-diffraction (μD) on samples prepared in cross-sectional geometry. Both GIXRD and TEM electron diffraction measurements on a specimen irradiated to a fluence of 5 × 10²⁰ Au/cm², revealed that the initial rhombohedral Mg₂Hf₅O₁₂ phase was transformed into a cubic-Mg₂Hf₅O₁₂ phase. Finally, it is important to note that at the highest ion fluence used in this investigation (5 × 10²⁰ Au/m²), both the MgO matrix and the Mg₂Hf₅O₁₂ second phase remained crystalline.     

H2 inhibition of radiation induced dissolution of spent nuclear fuel

In order to elucidate the effect of noble metal clusters in spent nuclear fuel on the kinetics of radiation induced spent fuel dissolution we have used Pd particle doped UO₂ pellets. The catalytic effect of Pd particles on the kinetics of radiation induced dissolution of UO₂ during γ-irradiation in containing solutions purged with N₂ and H₂ was studied in this work. Four pellets with Pd concentrations of 0%, 0.1%, 1% and 3% were produced to mimic spent nuclear fuel. The pellets were placed in 10 mM aqueous solutions and γ-irradiated, and the dissolution of was measured spectrophotometrically as a function of time. Under N₂ atmosphere, 3% Pd prevent the dissolution of uranium by reduction with the radiolytically produced H₂, while the other pellets show a rate of dissolution of around 1.6 × 10⁻⁹ mol m⁻² s⁻¹. Under H₂ atmosphere already 0.1% Pd effectively prevents the dissolution of uranium, while the rate of dissolution for the pellet without Pd is 1.4 × 10⁻⁹ mol m⁻² s⁻¹. It is also shown in experiments without radiation in aqueous solutions containing H₂O₂ and O₂ that ?-particles catalyze the oxidation of the UO₂ matrix by these molecular oxidants, and that the kinetics of the catalyzed reactions is close to diffusion controlled.     

Photoluminescence and photoluminescence excitation studies in 80 MeV Ni ion irradiated MOCVD grown GaN

We report damage creation and annihilation under energetic ion bombardment at a fixed fluence. MOCVD grown GaN thin films were irradiated with 80 MeV Ni ions at a fluence of 1 × 10¹³ ions/cm². Irradiated GaN thin films were subjected to rapid thermal annealing for 60 s in nitrogen atmosphere to anneal out the defects. The effects of defects on luminescence were explored with photoluminescence measurements. Room temperature photoluminescence spectra from pristine sample revealed presence of band to band transition besides unwanted yellow luminescence. Irradiated GaN does not show any band to band transition but there is a strong peak at 450 nm which is attributed to ion induced defect blue luminescence. However, irradiated and subsequently annealed samples show improved band to band transitions and a significant decrease in yellow luminescence intensity due to annihilation of defects which were created during irradiation. Irradiation induced effects on yellow and blue emissions are discussed.     

Synthesis of uranium nitride by a mechanically induced gas-solid reaction

A high energy, low-temperature, ball-milling route was used to directly produce uranium nitride. Pure uranium metal particles (∼100 μm) were ball milled under a 420 kPa nitrogen atmosphere for 24 h at ambient temperature to yield phase pure U₂N₃ powder as confirmed by X-ray diffraction and energy dispersive spectroscopy. The median particle size was measured to be approximately 6 μm.     

Structural and magnetic characterization of cobalt implanted GaN films

Cobalt ions were implanted into GaN films with multiple energies between 50 keV and 380 keV with two total fluences, 1.25 × 10¹⁶ and 1.25 × 10¹⁷ cm⁻², followed by annealing at temperatures between 600 and 850 °C. The crystal quality and surface morphology of as-implanted and subsequently annealed films were investigated by X-ray diffraction (XRD) 2θ scans, ω-rocking curve measurements and atomic force microscopy (AFM). The profiles of impurities and defects were analyzed by Rutherford backscattering spectrometry (RBS) in random and channeling configurations. The virgin GaN films have an excellent crystal quantity (χ_min = 1.4%) and in the implanted samples 60% disorder induced by ion implantation was recovered after annealing. The annealed sample become ferromagnetic, with a spontaneous magnetization of 0.1 emu/g and a coercive magnetic field of 100 Oe at 10 K, and the Curie point was found to be higher than room temperature.     

Growth of TiO2 surfaces following low energy (<40 eV) atom and small cluster bombardment

Low energy (10-40 eV) interaction of small Ti_xO_y clusters with a rutile (1 1 0) substrate was investigated using molecular dynamics with the aim of determining the influence of various parameters on surface growth and defect formation. Rutile growth was simulated through depositing randomly selected clusters with energies in the tens of eV range. Long time scale evolution was approximated through heating the substrate. A modified second-moment-Buckingham-QEq (SMB-Q) empirical potential was developed for the purpose. Crystal growth on amorphous and anatase TiO₂ substrates was also considered. Grown lattice layers were compared by visual inspection along with radial distribution function (RDF) plots. Bombardment at an energy of around 20 eV in an oxygen rich atmosphere with a high proportion of bombarding clusters, TiO, TiO₂, as opposed to single atoms, was found to produce rutile growth with the best crystallinity.     

X-ray absorption spectroscopy and X-ray photoelectron spectroscopy studies of CaSO4:Dy thermoluminescent phosphors

Extended X-ray absorption fine structure (EXAFS) measurements have been carried out on CaSO₄:Dy phosphor samples at the Dy L₃ edge with synchrotron radiation. Measurements were carried out on a set of samples which were subjected to post-preparation annealing at different temperatures and for different cycles. The EXAFS data have been analysed to find the Dy-S and Dy-O bond lengths in the neighbourhood of the Dy atoms in a CaSO₄ matrix. The observations from EXAFS measurements were verified with XANES and XPS techniques. On the basis of these measurements, efforts were made to explain the loss of thermoluminescence sensitivity of CaSO₄:Dy phosphors after repeated cycles of annealing at 400 °C in air for 1 h.     

Microwave sintering of 8 mol% yttria-zirconia (8YZ): An inert matrix material for nuclear fuel applications

This study focused on reducing overall processing time and temperature for fully stabilized zirconia, an inert matrix material candidate, to minimize the loss of actinides (that will be incorporated into the matrix material), while maintaining at least 90% theoretical density (TD). The effects of different processing routes on bulk density and microstructure were evaluated. The results obtained by adopting microwave sintering for 8 mol% Y₂O₃-ZrO₂ were compared to conventional sintering. A 20 min soak time at 1300 °C resulted in pellets with 90% TD for microwave-processed samples, compared to 77% TD for pellets processed conventionally. A similar density was obtained at lower temperature (1200 °C) by increasing the soak time to 100 min in microwave processing. This time and temperature resulted in 60% TD conventionally processed pellets. Compressive strength values obtained for a 1300 °C (20 min soak time) microwave-processed sample were higher (1600 MPa) as compared to a conventionally processed sample (1300 MPa).     

Characterization of the interaction layer grown by interdiffusion between U-7wt%Mo and Al A356 alloy at 550 and 340 °C

U(Mo) alloys are under study to get a low-enriched U fuel for research and test reactors. Qualification experiments of dispersion fuel elements have shown that the interaction layer between the U(Mo) particles and the Al matrix behaves unsatisfactorily. The addition of Si to Al seems to be a good solution. The goal of this work is to identify the phases constituting the interaction layer for out-of-pile interdiffusion couples U(Mo)/Al(Si). Samples γU-7wt%Mo/Al A356 alloy (7.1 wt%Si) made by Friction Stir Welding were annealed at 550 and 340 °C. Results from metallography, microanalysis and X-ray diffraction, indicate that the interaction layer at 550 °C is formed by the phases U(Al,Si)₃, U₃Si₅ and Al₂₀Mo₂ U, while at 340 °C it is formed by U(Al,Si)₃ and U₃Si₅. X-ray diffraction with synchrotron radiation showed that the Si-rich phase, previously reported in the interaction layer at 550 °C near U(Mo) alloy, is U₃Si₅.     

Electrical properties of annealed, fully metamict REE2FeBe2Si2O10

The electrical properties of annealed, fully metamict gadolinite REEFe²⁺Be₂Si₂O₁₀ are studied as a function of annealing temperature. Changes due to annealing are also probed by ⁵⁷Fe Mössbauer spectroscopy and X-ray diffraction. The electrical conductivity measured at f = 100 Hz between 110 and 750 K varies markedly, ranging from 10⁻¹⁰ to 10⁻⁶ S m⁻¹ for untreated samples and 10⁻⁹ to 10⁻³ S m⁻¹ for sample annealed in argon at 1373 K. Average measured activation energies for electrical conduction are 0.47 and 0.63 eV for ranges of 400-450 K and 500-600 K, respectively. The dielectric permittivity shows strong dispersion effects above 235 K. After high temperature annealing, the electrical conductivity shows a marked dispersion below 604 K. The combination of polaron hopping and hydroxyl anion migration is proposed for the electrical conduction mechanism.     

EXAFS and SR-XRD study on Mn occupations in Zn1−xMnxO diluted magnetic semiconductors

Mn-doped ZnO films were prepared by radio frequency (RF) magnetron sputtering on sapphire substrate. Mn content was determined by proton induced X-ray emission (PIXE). Only Mn, no other magnetic impurities such as Fe, Co and Ni were observed. Also, no precipitates such as MnO, Mn₃O₄ and other secondary phases or Mn clusters, were found by SR-XRD, even in Mn-doped content up to 11 at.%. EXAFS analyses showed that Mn atoms were incorporated into ZnO crystal lattice by occupying the sites of zinc atoms.     

Ferromagnetic modification of GaN film by Cu ions implantation

The structural and magnetic properties of Cu⁺ ions-implanted GaN films have been reported. Eighty kiloelectron-volt Cu⁺ ions were implanted into n-type GaN film at room temperature with fluences ranging from 1 × 10¹⁶ to 8 × 10¹⁶ cm⁻² and subsequently annealed at 800 °C for 1 h in N₂ ambient. PIXE was employed to determine the Cu-implanted content. The magnetic property was measured by the Quantum Design MPMS SQUID magnetometer. No secondary phases or clusters were detected within the sensitivity of XRD. Raman spectrum measurement showed that the Cu ions incorporated into the crystal lattice positions of GaN through substitution of Ga atoms. Apparent ferromagnetic hysteresis loops measured at 10 K were presented. The experimental result showed that the ferromagnetic signal strongly increased with Cu-implanted fluence from 1 × 10¹⁶ to 8 × 10¹⁶ cm⁻².