Effect of sintering conditions on the microstructure and mechanical properties of ZrN as a surrogate for actinide nitride fuels

Pellets of sintered ZrN were studied to optimize the mechanical properties and microstructures needed in nitride fuel pellets, using ZrN as a surrogate for actinide nitrides and as potential component in low fertile and inert matrix fuels. Samples were prepared via sintering in either Ar or N₂ (with and without 6% H₂) and at 1300 °C or 1600 °C. A significant difference in the hardness was measured ranging from 1000 (Kg/mm²) in samples sintered at 1600 °C in argon to 100 (Kg/mm²) in samples sintered at 1300 °C in nitrogen. Samples with 6% hydrogen added to the sintering environment experienced a decrease in hardness, as well as an increase in intergranular cracking as compared to samples sintered without hydrogen, suggesting hydrogen embrittlement. Grain size was more uniform in samples sintered in pure Ar as compared to Ar-H₂, while the latter had a larger fraction of high angle grain boundaries than the former. Cracking around indents had a clear tendency to follow high angle boundaries, which were found to be intrinsically weak in ZrN.     

Structural study of the oxidation process and stability of NbOxNy coatings

In the present work, we study the oxidation behaviour of NbON multilayer films. The films were deposited by DC magnetron sputtering with a reactive gas pulsing process. The nitrogen flow was kept constant and the oxygen flow was pulsed. Pulse durations of 10 s produced multilayered coatings with a period of λ = 10 nm. Three different films with increasing duty cycles have been deposited.Rutherford backscattering spectroscopy (RBS) was used to study the chemical composition variations at different annealing temperatures (as-deposited, 400 °C, 500 °C and 600 °C) combined with X-ray diffraction (XRD) to identify the crystalline phases formed. At 400 °C, for all films a very thin layer starts to form at the surface with enhanced O concentration. The composition of the deeper part of the samples remains unchanged. At 500 °C, the oxide scale grows, encompassing about half the film thickness. At 600 °C, the process is finished and a single layer is formed with reduced Nb and increased O concentration. Fourier-transformation infrared spectroscopy (FTIR) results confirmed the increase of this surface oxidation, while XRD revealed that crystallization of Nb₂O₅ occurs at 600 °C.     

Electronic and atomic structure modifications of copper nitride films by ion impact and phase separation

We have studied electronic and atomic structure modifications of Cu₃N films under 100 keV Ne and 100 MeV Xe ion impact. Cu₃N films were prepared on R(11-2 surface)-cut-Al₂O₃ substrates at 250 °C by using a RF-magnetron sputter deposition method. X-ray diffraction (XRD) shows that unirradiated films are polycrystalline with (1 0 0) orientation of cubic structure. We find that the electrical resistivity (∼10 Ω cm before ion impact) decreases by more than two orders of magnitude after the Ne impact at a fluence of ∼10¹³ cm⁻², where no Cu phase separation is observed. For further ion impact (larger than ∼10¹⁵ cm⁻²), XRD shows Cu diffraction peak (Cu phase separation), and the resistivity decreases further (three orders of magnitude). Decomposition and phase separation are discussed based on these results, as well as temperature dependence of the resistivity and optical absorption. The results of 100 MeV Xe ion impact are compared with those of Ne ion impact.     

Characterization of ThO2-UO2 pellets made by co-precipitation process

The co-precipitation technique renders an excellent route to obtain a homogeneous mixture of ThO₂ and UO₂ powders. In this process, after the nitrate solutions of Th and U are mixed in the intended ratio, oxalic acid is added for co-precipitation. The precipitate is then dried and calcined to get a solid solution of ThO₂ and UO₂. In this study, ThO₂-30%UO₂ and ThO₂-50%UO₂ (% in weight) powders were characterized in terms of particle size, particle shape, surface area, phase content, O/M ratio etc. The pellets obtained by sintering these powders were characterized with the help of optical microscopy, scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). The XRD data for ThO₂-30%UO₂ and ThO₂-50%UO₂ pellets showed the presence of a small amount of U₃O₈ phase besides fluorite phase. The grain size of ThO₂-30%UO₂ and ThO₂-50%UO₂ was found to be 5.7 and 4.5 μm, respectively.     

Ion beam characterization of rf-sputter deposited AlN films on Si(1 1 1)

Aluminum nitride (AlN) thin films have been deposited on Si(1 1 1) substrates by using reactive-rf-magnetron-sputtering at 250 °C. The crystalline quality and orientation of the films have been studied by X-ray diffraction (XRD). We have observed that the films grow with c- or a-axis orientation. The composition, film thickness, impurities and stress are considered to be factors affecting the orientation and have been analyzed by Rutherford backscattering spectroscopy (RBS), nuclear reaction analysis (NRA) and XRD. Their effects on the film growth will be discussed. Surface morphology of the films will be also presented.     

Kinetics of precipitation of U4O9 from hyperstoichiometric UO2+x

For safe and reliable operation of fission reactors in space, the phase diagrams and reaction kinetics of systems used as nuclear fuels, such as U-O, U-N, U-C, are required. Diffraction allows identification of phases and their weight fractions as a function of temperature in situ, with a time resolution of the order of minutes. In this paper, we will provide results from a neutron diffraction experiment studying the U-O system. Using the neutron diffractometer HIPPO, the decomposition of UO_2+x into UO₂ and U₄O₉ as a function of temperature was investigated in situ. From the diffraction data, the participating phases could be identified as UO_2+x, UO₂ and U₄O_8.94 and no stoichiometric U₄O₉ was found. Results of the experiment were used to improve existing thermodynamic models. The presented techniques (i.e., neutron diffraction and thermodynamic modeling) are also applicable to the other systems mentioned above.     

Fabrication method for UO2 pellets with large grains or a single grain by sintering in air

The effects of a powder treatment, the sintering temperature and the sintering time on the grain growth of UO₂ pellets were investigated in air to obtain UO₂ pellets with large grains. Air could be used for sintering because an oxidation path above 1803 K does not pass through a two-phase (UO_2+x + U₃O_8−z) region. The UO₂ pellets sintered by the CO₂-air-CO₂-H₂ process consisted of a single grain or some large grains in the order of several millimeters.     

Synthesis of nanodimensional TiO2 thin films using energetic ion beam

Nanophases of TiO₂ are achieved by irradiating polycrystalline thin films of TiO₂ by 100 MeV Au ion beam at varying fluence. The surface morphology of pristine and irradiated films is studied by atomic force microscopy (AFM). Phase of the film before and after irradiation is identified by glancing angle X-ray diffraction (GAXRD). The blue shift observed in UV-vis absorption edge of the irradiated films indicates nanostructure formation. Electron spin resonance (ESR) studies are carried out to identify defects created by the irradiation. The nanocrystallisation induced by SHI irradiation in polycrystalline thin films is studied.     

Synthesis of dysprosium and cerium nitrides by a mechanically induced gas-solid reaction

A high energy ball milling process was used to produce dysprosium nitride and cerium nitride powders at room temperature. Dysprosium and cerium metal flakes were milled in a 275 kPa nitrogen atmosphere for 24 h at ambient temperatures. X-ray diffraction confirmed the formation of phase pure dysprosium nitride and cerium nitride powders. The median particle size of the resultant dysprosium nitride was measured as 4 μm using a laser scattering technique. The particle size of the cerium nitride was not measured due to its reactive nature.     

Characterization of (Th,U)O2 pellets made by advanced CAP process

Coated Agglomerate Pelletization (CAP) process is being developed by Bhabha Atomic Research Centre (BARC) for the fabrication of ThO₂-UO₂ mixed oxide fuel pellets. In order to improve the microstructures with better microhomogeneity, a study was made to modify the CAP process. The advanced CAP (A-CAP) process is similar to the CAP process except that the co-precipitated powder of mixed oxide, ThO₂-30%UO₂ or ThO₂-50%UO₂, is used for coating instead of U₃O₈ powder. The choice of ThO₂-UO₂ powders as the coating material is advantageous compared to U₃O₈, since the presence of large quantities of ThO₂ in UO₂ powder gives better self-shielding effect. In this paper, ThO₂ containing 4%UO₂ (% in weight) was prepared by the A-CAP process. Property measurements including microstructure and microhomogeneity were made by optical microscopy, scanning electron microscopy (SEM), electron probe microanalysis (EPMA), etc. It was found that the pellets sintered in air at 1400 °C showed a duplex grain structure and those sintered in Ar-8%H₂ at 1650 °C showed a very uniform grain structure with excellent microhomogeneity.     

Fabrication of dense (Th,U)O2 pellets through microspheres impregnation technique

ThO₂ microspheres were prepared by internal gelation process using a pre-boiled hexamethylenetetramine (HMTA), urea solution. The microspheres were characterized with respect to tap density, specific surface area and pore size distribution. An indigenously designed and fabricated apparatus was used for the impregnation of uranium in thoria microspheres. The loading of uranium was found to vary with the concentration of uranyl nitrate solution, operational vacuum and the time of impregnation. These process conditions were optimized to obtain soft (Th,U)O₂ microspheres containing 3-4 mol% of uranium, which are readily amenable for pelletization. The green pellets could be sintered to ∼96% of T.D. by heating in air up to 1350 °C for a period of 2-4 h. The polished surface of the fractured pellets showed a smooth surface without any berry structure. The shrinkage behaviour of the pellets was also studied in air using a dilatometer. The SEM studies of the pellets indicated a uniform microstructure with average grain size of 1 μm. The elemental scanning by the EDX method showed the uniform distribution of uranium in the microspheres and pellets.     

Micro-PIXE-RBS methods highlighting the influence of phosphorus on the in vitro bioactivity of sol-gel derived glass particles in the SiO2-CaO-P2O5 system

Ion beam analysis methods were used to characterize the interface of bioactive glasses with surrounding biological fluids. Glass particles in the SiO₂-CaO and SiO₂-CaO-P₂O₅ compositions were made by sol-gel processing and soaked in biological fluids for periods up to 4 days. The surface changes were characterized using PIXE-RBS, which are efficient methods for multielemental analysis and accurate trace elements quantification. Elemental maps of major and trace elements were obtained at a micrometer scale and revealed the bone bonding ability of the materials. Glass particles are quickly coated with a thin calcium phosphate-rich layer containing traces of magnesium. After a few days, SiO₂-CaO-P₂O₅ glass particles are entirely changed into calcium phosphates, whereas SiO₂-CaO particles exhibit a different behavior: the previously Ca-P enriched periphery has been dissolved and glass particles consist of a silicate network. Calculation of the Ca-P atomic ratios at the glass/biological fluids interface provides us with an explanation for this: an enduring apatitic phase seems to be formed at the periphery of SiO₂-CaO-P₂O₅ glass particles. Presence of phosphorus in the glass matrix thus has an influence on the amplitude and the kinetics of reaction of the bioactivity process. It might result in an improved chemical bond with living tissues.     

Effect of 200 MeV Ag ion irradiation on structural and electrical transport properties of Fe3O4 thin films

Thin films of Fe₃O₄ have been deposited on single crystal MgO(1 0 0) and Si(1 0 0) substrates using pulsed laser deposition. Films grown on MgO substrate are epitaxial with c-axis orientation whereas, films on Si substrate are highly 〈1 1 1〉 oriented. Film thicknesses are 150 nm. These films have been irradiated with 200 MeV Ag ions. We study the effect of the irradiation on structural and electrical transport properties of these films. The fluence value of irradiation has been varied in the range of 5 × 10¹⁰ ions/cm² to 1 × 10¹² ions/cm². We compare the irradiation induced modifications on various physical properties between the c-axis oriented epitaxial film and non epitaxial but 〈1 1 1〉 oriented film. The pristine film on Si substrate shows Verwey transition (T_V) close to 125 K, which is higher than generally observed in single crystals (121 K). After the irradiation with the 5 × 10¹⁰ ions/cm² fluence value, T_V shifts to 122 K, closer to the single crystal value. However, with the higher fluence (1 × 10¹² ions/cm²) irradiation, T_V again shifts to 125 K.     

Interface strain study of thin Lu2O3/Si using HRBS

The interface of thin Lu₂O₃ on silicon has been studied using high-resolution RBS (HRBS) for samples annealed at different temperatures. Thin rare earth metal oxides are of interest as candidates for next generation transistor gate dielectrics, due to their high-k values allowing for equivalent oxide thickness (EOT) of less than 1 nm. Among them, Lu₂O₃ has been found to have the highest lattice energy and largest band gap, making it a good candidate for an alternative high-k gate dielectric. HRBS depth profiling results have shown the existence of a thin (∼2 nm) transitional silicate layer beneath the Lu₂O₃ films. The thicknesses of the Lu₂O₃ films were found to be ∼8 nm and the films were determined to be non-crystalline. Angular scans were performed across the [1 1 0] and [1 1 1] axis along planar channels, and clear shifts in the channeling minimum indicate the presence of Si lattice strain at the silicate/Si interface.     

An energy dispersive micro X-ray diffractometer based on a combined system of polycapillary optics

An energy dispersive micro X-ray diffractometer based on a combined system of two polycapillary X-ray lenses is designed. The polycapillary X-ray lens in the excitation channel is either a polycapillary parallel X-ray lens (PPXRL) or a slightly focusing polycapillary X-ray lens (SFPXRL). The polycapillary X-ray lens in the detection channel is a PPXRL. At 6.4 keV and 2θ = 141.5°, the total resolution of the diffractometer based on a SFPXRL in the excitation channel and a PPXRL in the detection channel in Δd/d is 4.8%.     

Temperature accelerated dynamics study of migration process of oxygen defects in UO2

We studied the migration dynamics of oxygen point defects in UO₂ which is the primary ceramic fuel for light-water reactors. Temperature accelerated dynamics simulations are performed for several initial conditions. Though the migration of the single interstitial is much slower than that of the vacancy, clustered interstitial shows faster migration than those. This observation gives us important insight on the formation mechanism of high-burnup restructuring, including planar defects and grain sub-division (the rim structure), found in UO₂.     

Nuclear resonant reflectivity with standing waves for the investigation of a thin Fe layer buried inside a superconducting Si/[Mo/Si]45/Fe/Nb multilayer

A special multilayer sample Si/[Mo/Si]₄₅/⁵⁷Fe/Nb has been prepared for the depth selective investigations of the hyperfine fields in thin iron layer at low temperatures above and below the superconducting transition in the top Nb layer (T_c ∼ 8 K) by means of the nuclear resonant reflectivity with standing waves. The periodic multilayer [Mo/Si]₄₅ below the iron layer in our sample was used as “a standing wave generator”. A weak magnetic hyperfine splitting in the ⁵⁷Fe layer was detected just at low temperature. A slight variation of the nuclear resonant reflectivity time spectra measured above and below T_c was observed. At first it was supposed that this change of the spectrum shape is caused by the spatial modulation of ferromagnetic domains in the ⁵⁷Fe layer caused by a proximity effect. A closer analysis, however, reveals that the spectrum variations are due to just the changes of the relative weights of the magnetic and paramagnetic phases in ⁵⁷Fe layer.     

Combined grazing incidence RBS and TEM analysis of luminescent nano-SiGe/SiO2 multilayers

Multilayer structures with five periods of amorphous SiGe nanoparticles/SiO₂ layers with different thickness were deposited by Low Pressure Chemical Vapor Deposition and annealed to crystallize the SiGe nanoparticles. The use of grazing incidence RBS was necessary to obtain sufficient depth resolution to separate the signals arising from the individual layers only a few nm thick. The average size and areal density of the embedded SiGe nanoparticles as well as the oxide interlayer thickness were determined from the RBS spectra. Details of eventual composition changes and diffusion processes caused by the annealing processes were also studied. Transmission Electron Microscopy was used to obtain complementary information on the structural parameters of the samples in order to check the information yielded by RBS. The study revealed that annealing at 900 °C for 60 s, enough to crystallize the SiGe nanoparticles, leaves the structure unaltered if the interlayer thickness is around 15 nm or higher.     

Irradiation effect on dielectric properties and electrical conductivity of Au/SiO2/n-Si (MOS) structures

The Au/SiO₂/n-Si (MOS) structures were exposed to beta-ray irradiation to a total dose of 30 kGy at room temperature. Irradiation effect on dielectric properties of MOS structures were investigated using capacitance−voltage (C−V) and conductance−voltage (G/ω−V) characteristics. The C−V and G/ω−V measurements carried out in the frequency range from 1 kHz to 10 MHz and at various radiation doses, while the dc voltage was swept from positive bias to negative bias for MOS structures. The dielectric constant (ε′), dielectric loss (ε″), loss factor (tan δ) and ac electrical conductivity (σ_ac) were calculated from the C−V and G/ω−V measurements and plotted as a function of frequency at various radiation doses. A decrease in the ε′ and ε″ were observed when the irradiation dose increased. The decrease in the ε′ and ε″ of irradiated MOS structures in magnitude is explained on the basis of Maxwell−Wagner interfacial polarization. Also, the σ_ac is found to decrease with increasing radiation dose. In addition, the values of the tan δ decrease with increasing radiation dose and give a peak. From the experimental results, it is confirmed that the peak of loss tangent is due to the interaction between majority carriers and interface states which induced by radiation.     

Study of the electronic structures of oxygen doped in LiBaF3 crystal

The most likely substituting positions of impurity oxygen ions in LiBaF₃ crystals are studied using the general utility lattice program (GULP). The calculated results indicate that the main defect model is [] in the O:LiBaF₃ crystal. The electronic structures of the LiBaF₃ crystal with the defect [] are calculated using the DV-Xα method. It can be concluded from the electronic structures that the LiBaF₃ crystal with the defect [] will exhibit a 217-280 nm absorption band and the impurity oxygen will decrease core-valence luminescence yield.