Microstructural evolution of Y2O3 and MgAl2O4 ODS EUROFER steels during their elaboration by mechanical milling and hot isostatic pressing

Different ODS EUROFER steels reinforced with Y₂O₃ and MgAl₂O₄ were elaborated by mechanical milling and hot isostatic pressing. Good compromise between strength and ductility could be obtained but the impact properties remain low (especially for the Y₂O₃ ODS steel). The materials were structurally characterized at each step of the elaboration. During milling, the martensite laths of the steel are transformed into nano-metric ferritic grains and the Y₂O₃ oxides dissolve (but not the MgAl₂O₄ spinels). After the HIP, all the ODS steels remain ferritic with micrometric grains, surrounded by nano-metric grains for the Y₂O₃ ODS steels. The mechanisms in the Y₂O₃ ODS steels are complex: the Y₂O₃ oxides re-precipitate as nano-Y₂O₃ particles that impede a complete austenitization during the HIP. The quenchability of the ODS steels is modified by the milling process, the oxide nature and the oxide content. Eventually, the advantages and drawbacks of each oxide type are discussed.     

The effect of Y2O3 on the dynamics of oxidative dissolution of UO2

In this work, we have studied the impact of Y₂O₃ on the kinetics of oxidative dissolution of UO₂ and the consumption of H₂O₂. The second order kinetics of catalytic consumption of H₂O₂ on Y₂O₃ was investigated in aqueous Y₂O₃ powder suspensions by varying the solid surface area to solution volume ratio. The resulting second order rate constant is 10⁻⁸ m s⁻¹, which is of the same magnitude as for the reaction between H₂O₂ and UO₂. Powder experiments with mixtures of UO₂ and Y₂O₃ show that Y₂O₃ has no effect on the oxidative dissolution of UO₂, whereas the consumption of H₂O₂ seems to be slightly slower in the presence of Y₂O₃ and H₂ respectively. UO₂ pellets with solid inclusions of Y₂O₃ show a decrease in oxidative dissolution by a factor of 3.3 and 5.3 under inert and hydrogen atmosphere, respectively. The rate of H₂O₂ consumption is similar for all cases and is well in line with kinetic data from powder experiments. The effects of H₂ and Y₂O₃ on the oxidative dissolution of UO₂ under gamma irradiation are similar to those found in experiments with H₂O₂. No significant difference in dissolution between inert and reducing atmosphere can be observed for pure UO₂.     

Photoluminescence and XEL in Y2O3:Eu phosphor

Eu-activated Y₂O₃ phosphors were prepared by combustion synthesis and also by precipitation techniques. Photoluminescence and X-ray excited luminescence of prepared Y₂O₃:Eu phosphor, under two different techniques were compared and reported in this paper. Y₂O₃:Eu³⁺ phosphor were prepared by precipitation technique followed by annealing at 900 °C. It gives cubic nature of the particle that may be more favourable for high lumen output. X-ray excited luminescence of Y₂O₃:Eu³⁺ phosphors also reported in this paper.     

Defect volumes of BO2 doped Y2O3 (B = Ti, Zr, Hf and Ce)

Atomistic simulations have been employed to study the effect of BO₂ (fluorite) incorporation into the bixbyite oxide Y₂O₃. The energetically preferred defect mechanism and the associated lattice parameter changes that occur from BO₂ doping have been predicted. The addition of Group IV elements into Y₂O₃ can follow three different mechanisms. The energetically favourable method is through a mediated reaction for ZrO₂ and HfO₂ while for TiO₂ and CeO₂, reducing B⁴⁺ to B³⁺ provides the lowest energy reaction. ZrO₂ and HfO₂ doping results in the lowest volume changes.     

Laser-joined Al2O3 and ZrO2 ceramics for high-temperature applications

A laser process is presented that has been specially developed for joining oxide ceramics such as zirconium oxide (ZrO₂) and aluminium oxide (Al₂O₃). It details, by way of example, the design of the laser process applied for to producing both Al₂O₃-Al₂O₃ and ZrO₂-ZrO₂ joints using siliceous glasses as fillers.The heat source used was a continuous wave diode laser with a wavelength range of 808-1010 nm. Glasses of the SiO₂-Al₂O₃-B₂O₃-MeO system were developed as high-temperature resistant brazing fillers whose expansion coefficients, in particular, were optimally adapted to those of the ceramics to be joined. Specially designed measuring devices help to determine both the temperature-dependent emission coefficients and the synchronously determined proportions of reflection and transmission.The glass-ceramic joints produced are free from gas inclusions and macroscopic defects and exhibit a homogenous structure. The average strength values achieved were 158 MPa for the Al₂O₃ system and 190 MPa for the ZrO₂ system, respectively.     

Diffuse reflectance and X-ray photoelectron spectroscopy of uranium in ZrO2 and Y2Ti2O7

Diffuse reflectance measurements were made over the wavenumber range of 4000-20,000 cm⁻¹ at room temperature on monoclinic and stabilised ZrO₂, together with Y₂Ti₂O₇ having the pyrochlore structure, all of which were doped with U and sintered in various atmospheres. X-ray photoelectron spectroscopy measurements were also carried out on selected samples. In monoclinic and stabilised zirconia, U exhibited valence states of +4 and/or +5, depending on the sintering atmosphere and the presence of appropriate charge compensators. Using both diffuse reflectance and X-ray photoelectron spectroscopy, U was also observed as mainly U⁴⁺ and/or U⁵⁺ in U-doped Y₂Ti₂O₇ sintered at 1400 °C in air or Ar, although a small amount of U⁶⁺ also appeared to be present in some U-doped Y₂Ti₂O₇ samples heated in air.     

Formation of Y2O3 nanoclusters in nano-structured ferritic alloys: Modeling of precipitation kinetics and yield strength

The solubility product of Y₂O₃ in ferrite and the diffusion coefficient of yttrium in ferrite have been obtained by fitting a model based on the classical nucleation-growth-coarsening theory of precipitation, as adapted to an anisothermal heat treatment, to experimental small angle neutron scattering results of Y₂O₃ precipitate size distributions in a mechanically alloyed and consolidated Fe-15 at.%Cr-0.13 at.%Y-0.18 at.%O ferritic alloy. This precipitation model is coupled to a dispersed barrier model of structural hardening to predict the yield strength of the alloys as a function of heat treatment. The resulting model and thermodynamic/kinetic properties are then applied to better understand how the precipitation kinetics impact the yield stress in various anisothermal heat treatments, as compared to an isothermal heat treatment. The modeling results clearly indicate that the anisothermal heat treatments can be tailored to establish a higher density and a smaller size distribution of Y₂O₃ precipitates, which also increase the yield stress.     

Mechanical properties of Y2O3-doped W-Ti alloys

W and W alloys are currently considered promising candidates for plasma facing components in future fusion reactors but most of the information on their mechanical properties at elevated temperature was obtained in the 1960s and 1970s. In this investigation, the strength and toughness of novel Y₂O₃-doped W-Ti alloys manufactured by powder metallurgy were measured from 25 °C up to 1000 °C in laboratory air and the corresponding deformation and failure micromechanisms were ascertained from analyses of the fracture surfaces. Although the materials were fairly brittle at ambient temperature, the strength and toughness increased with temperature and Ti content up to 600 °C. Beyond this temperature, oxidation impaired the mechanical properties but the presence of Y₂O₃ enhanced the strength and toughness retention up to 800 °C.     

Radiation chemical synthesis and characterization of UO2 nanoparticles

In a deep repository for spent nuclear fuel, U(VI)(aq) released upon dissolution of the fuel matrix could, in reducing parts of the system, be converted to U(IV) species which might coalesce and form nanometer-sized UO₂ particles. This type of particles is expected to have different properties compared to bulk UO₂(s). Hence, their properties, in particular the capacity for oxidant consumption, must be investigated in order to assess the effects of formation of such particles in a deep repository. In this work, methods for radiation chemical synthesis of nanometer-sized UO₂ particles, by electron- and γ-irradiation of U(VI) solutions, are presented. Electron-irradiation proved to be the most efficient method, showing high conversions of U(VI) and yielding small particles with a narrow size distribution (22-35 nm). Stable colloidal suspensions were obtained at low pH and ionic strength (pH 3, I = 0.03). Furthermore, the reactivity of the produced UO₂ particles towards H₂O₂ is investigated. The U(IV) fraction in the produced particles was found to be ∼20% of the total uranium content, and the results show that the UO₂ nanoparticles are significantly more reactive than micrometer-sized UO₂ when it comes to H₂O₂ consumption, the major part of the H₂O₂ being catalytically decomposed on the particle surface.     

Chlorination of UO2, PuO2 and rare earth oxides using ZrCl4 in LiCl-KCl eutectic melt

A new chlorination method using ZrCl₄ in a molten salt bath has been investigated for the pyrometallurgical reprocessing of nuclear fuels. ZrCl₄ has a high reactivity with oxygen but is not corrosive to refractory metals such as steel. Rare earth oxides (La₂O₃, CeO₂, Nd₂O₃ and Y₂O₃) and actinide oxides (UO₂ and PuO₂) were allowed to react with ZrCl₄ in a LiCl-KCl eutectic salt at 773 K to give a metal chloride solution and a precipitate of ZrO₂. An addition of zirconium metal as a reductant was effective in chlorinating the dioxides. When the oxides were in powder form, the reaction was observed to progress rapidly. Cyclic voltammetry provided a convenient way of establishing when the reaction was completed. It was demonstrated that the ZrCl₄ chlorination method, free from corrosive gas, was very simple and useful.     

Electronic sputtering of Gd3Ga5O12 and Y3Fe5O12 garnets: Yield, stoichiometry and comparison to track formation

The results of present paper have shown that sputtering of yttrium iron garnet (Y₃Fe₅O₁₂) under swift heavy ions in the electronic energy loss regime is non-stoichiometric. Here we are presenting additional experimental results for gadolinium gallium garnet (Gd₃Ga₅O₁₂) as target. The irradiations were performed with different ions (⁵⁰Cr (589 MeV), ⁸⁶Kr (195 MeV) and ¹⁸¹Ta (400 MeV)) impinging perpendicularly to the surface. As earlier, the sputtering yield was determined by collecting the emitted gadolinium and gallium atoms on a thin aluminium foil, placed upstream above the target and analyzing the Al catcher by Rutherford backscattering. Also for Gd₃Ga₅O_12, the emission of Gd and Ga is non-stoichiometric. Sputtering appears above a critical electronic stopping power of S_th = 11.6 ± 1.5 keV/nm, which is larger than the threshold for track formation, in agreement with other amorphisable materials. In addition, the angular distribution of the sputtered species was measured for Y₃Fe₅O₁₂ and Gd₃Ga₅O₁₂ using 200 MeV Au ions impinging the surface at 20° relatively to the surface. For the two garnets the ratio of Y/Fe (and Gd/Ga) varies with the angle of emitted species and the stoichiometry seems to be preserved only for an emission perpendicular to the surface.     

Characterisation of an oxygen sensor based on In/In2O3 reference electrode

A potentiometric sensor for measuring oxygen activity in LBE has been developed since 2000 until today at ‘Institut Quimic de Sarria’ electrochemistry laboratories. This sensor is based on In/In₂O₃ reference electrode. The last experiments performed with this sensor have been directed to characterise the sensor. For this purpose, the following experiments in stagnant conditions have been performed: effect of the operating temperature from 300 to 500 °C, different covering gases (N₂ + 5% H₂, Ar 99.999%, and N₂ + 10 mg/L O₂) and comparison of different solid electrolytes (ZrO₂/Y₂O₃ and ZrO₂/MgO). Long-term experiments have also been performed to the see the stability of the signal with time.     

Phasengleichgewichte in den systemen UO2-UO2,67-ThO2 und UO2 + x-NPO2

Solid-state chemical investigations have established that in the compositional range UO₂-UO_2.67-ThO₃ of the U-Th-O ternary system, the following single-phase domains exist: U₃O₈, which does not dissolve any ThO₂ in the solid state; an ordered M₄O₉ phase on the section between U₄O₉ and U₂Th₂O₉, below ≈ 1150 °C; and a phase with fluorite structure which occupies a large part of the system and which at 1250 °C is bounded by the compositions UO₂-UO_2.25 (U_0.43, ThO_0.57)O_0.25-ThO₃. The maximum O/M ratio of the “fluorite” phase is O:(U + Th) = 2.25. The highest oxidation valency of uranium is 5.30; this value falls as more thorium oxide is incorporated in the (U.Th)O_{2 + x} “fluorite” phase.     

Pu2O3 and the plutonium hydriding process

The role of cubic Pu₂O₃ in the corrosion of PuO₂-coated Pu by H₂ was investigated. Experiments were conducted to demonstrate that nucleation of hydriding is promoted by formation of Pu₂O₃ sites in the oxide layer. The nucleation mechanism based on diffusion of hydrogen through the PuO₂ layer was evaluated and an alternative mechanism based on formation of catalytic Pu₂O₃ sites via the Pu-PuO₂ reaction is proposed. The possibility of active participation of other impurities and inclusions in the dioxide is also discussed.     

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.     

Kinetics of initial stage of sintering of UO2 and UO2 with Nd2O3 addition

The kinetics of initial stage sintering of UO₂ powder were reinvestigated, using Ar-10% H₂ atmosphere. The effect of the addition of neodynium oxide was studied. The results revealed that surface and grain boundary diffusion mechanisms act simultaneously. The values of activation energies were found to be $\text{48.48 ± 3.51 kcal/mole}$ in the temperature range 870–942°C and $\text{89.88 ± 9.87 kcal/mole}$ in the temperature range of 942–1030°C for UO₂, and $\text{115.61 ± 7.77 kcal/mole}$ in the temperature range 1030–1150°C for UO₂ + Nd₂O₃. An important decrease in the calculated diffusion coefficient occurs by the addition of Nd₂O₃.     

Reactivity of hydrogen peroxide towards Fe3O4, Fe2CoO4 and Fe2NiO4

The kinetics of CRUD oxidation by H₂O₂ has been studied using aqueous suspensions of metal oxide powder. Fe₃O₄, Fe₂CoO₄ and Fe₂NiO₄ were used as model compounds for CRUD. In addition, the activation energies for the reaction between H₂O₂ and the three CRUD models were determined. The rate constants at room temperature were determined to 6.6 (±0.4) × 10⁻⁹, 3.4 (±0.4) × 10⁻⁸ and 1.6 × 10⁻¹⁰ m min⁻¹ for Fe₃O₄, Fe₂CoO₄ and Fe₂NiO₄, respectively. The corresponding activation energies are 52 ± 4, 44 ± 5 and 57 ± 7 kJ mol⁻¹, respectively. The mechanism of the reaction is briefly discussed indicating that the final solid product in all three cases is Fe₂O₃. In addition to the experimental studies, the theoretical grounds for kinetics of reactions in particle suspensions are discussed. The theoretical discussion is also used to explain the somewhat unexpected trends in reactivity observed experimentally.     

Structure and elastic property of nanosized complex oxide particles in ferritic/martensitic alloy: An electron energy-loss spectroscopy study

The structure and elastic property of nanosized complex oxide particles in a ferritic/martensitic alloy containing titanium and silicon were studied by transmission electron microscopy (TEM) and electron energy-loss spectroscopy (EELS). The nanosized complex Y-Si-O particles were found in the matrix of the alloy in addition to Y-Ti-O, and the size of Y-Si-O is smaller than that of Y-Ti-O particles. The formation of Y_2.16Si_1.76O₇ and Y_2.15Ti_1.95O₇ were further confirmed by O K, Si L_2,3 and Ti L_2,3 edges, respectively. The bulk modulus of Y_2.16Si_1.76O₇ was shown to be lower than that of Y_2.15Ti_1.95O₇, which implies that the nanosized Y_2.16Si_1.76O₇ particles would provide more effective dislocation pinning at elevated temperatures.     

Standard molar Gibbs energy of formation of Pb5Bi8O17 and PbBi12O19 and phase diagram of the Pb-Bi-O system

Equilibrium phase fields of the ternary Pb-Bi-O system were established by long-term equilibration in the temperature range 660-840 K. Using these results, the partial phase diagram of the ternary system has been constructed. The standard molar Gibbs energy of formation of the ternary oxides, 〈Pb₅Bi₈O₁₇〉 and 〈PbBi₁₂O₁₉〉, was determined by measuring the equilibrium oxygen partial pressures over the relevant phase fields by static manometry in conjunction with a solid oxide electrolyte-based emf cell. Further, the standard molar Gibbs energy of formation of 〈Pb₃O₄〉 was also measured by the manometric method. The oxide that coexists with the Pb-Bi eutectic alloy (LBE) has been experimentally confirmed as [β-PbO]_ss. Using the thermochemical data measured, the compositions of Pb-Bi alloys that coexist with [β-PbO]_ss and ternary oxides were derived and reported.     

Ab initio simulation of yttrium oxide nanocluster formation on fcc Fe lattice

Using results of density functional theory (DFT) calculations the first attempt towards the understanding of Y₂O₃ particles formation in oxide dispersed strengthened (ODS) ferritic-martensitic steels was performed. The present work includes modeling of single defects (O impurity atom, Fe vacancy and Y substitute atom), interaction between substituted Y atoms, Y-Fe vacancy pairs and oxygen impurity atoms in the iron matrix. The calculations have showed the repulsive interaction between the two Y substitute atoms at any separation distances that might mean that the oxygen atoms or O atoms with vacancies are required to form binding between atoms in the yttrium oxide nanoclusters.