Electrical conductivity and non-stoichiometry in the (U,Gd)O2±x system

The isothermal electrical conductivity and oxygen potential of the (U,Gd)O_2±x solid solution were measured in various oxygen partial pressure regions at 1200 °C and 1300 °C. The electrical conductivity gradually decreased with decreasing oxygen partial pressure even in the hypo-stoichiometric region. These findings were in contrast to the implication of a hypo-stoichiometry where the electrical conductivity is increased through the formation of oxygen vacancies. The (U_1−yGd_y)O_2−y/2 was defined as a new stoichiometric composition to determine the relationship between the deviation of the oxygen composition from stoichiometry and oxygen partial pressure. The dependence of the new oxygen deviation, z in (U_1−yGd_y)O_2−y/2+z, on the oxygen partial pressure corresponds to the dependence of the electrical conductivity, and thus a consistent defect structure model can be deduced from both the dependence curves. It suggests that the defect type is oxygen interstitial even below the oxygen composition of 2.     

Phase transitions in U3O8−z: I, heat capacity measurements

The heat capacity of U₃O_8−z with various O/U ratios was measured in the range from 250 to 750 K, and λ-type heat capacity anomalies were found in each sample. The transition temperatures were 487 and 573 K for UO_2.663, 490 and 576 K for UO_2.656 and 508, 562 and 618 K for UO_2.640. The entropy changes of the transitions were 0.44 and 0.39 J K⁻¹mol⁻¹ for UO_2.663, 0.58 and 0.47 J K⁻¹mol⁻¹ for UO_2.656 and 0.62, 0.51 and 0.25 J K⁻¹mol⁻¹ for UO_2.640, increasing as O/U decreases. The enthalpy change due to the transition varied linearly with the transition temperature except for UO_2.640, showing the presence of the same mechanism of phase transition among the samples with various O/U ratios. The mechanism of the phase transition was discussed on the assumption that the transition is originated from the order-disorder rearrangement of U⁵⁺ and U⁶⁺ with a consequent displacement of atoms, similarly to the case of U₄O_9−y.     

Sputtering yield of Ti1−xBx films by 2 keV deuterium bombardment

We deposited titanium borides (Ti_1−xB_x; 0.40 < x < 0.77) by the co-sputter coating method and measured their sputtering yield by 2 keV deuterium ion bombardment as a function of their chemical composition at room temperature. The total sputtering yield is found to increase with increase of the boron content in Ti_1−xB_x. The total sputtering yield of stoichiometric TiB₂ is estimated to be 2.8 × 10⁻², about the same as those reported previously. Concerning the partial sputtering yield, that of the titanium does not depend on the chemical composition, but that of the boron increases with increase of the boron content. These experimental results could be explained by assuming that the partial sputtering yield is proportional to the spatial concentration of each atom in the Ti_1−xB_x matrix.     

2 MeV Si ion implantation damage in relaxed Si1−xGex

The damage produced by implanting, at room temperature, 3 μm thick relaxed Si_1−xGe_x layers with 2 MeV Si⁺ ions has been measured as a function of Ge content (x = 0.04, 0.13, 0.24 or 0.36) and Si dose in the dose range 10¹⁰–10¹⁵ cm⁻². The accumulation of damage with increasing dose has been studied as a function of Ge content by Rutherford Backscattering Spectrometry, Optical Reflectivity Depth Profiling and Transmission Electron Microscopy and an increased damage efficiency in Si_1−xGe_x with increasing x is observed. The characteristics of implantation-induced defects have been investigated by Electron Paramagnetic Resonance. The results are discussed in the context of a model of the damage process in SiGe.     

Density measurement of amorphous SixGe1 − x alloys

The atomic density of amorphous Si_xGe_{1 − x} alloys (x = 1, 0.85, 0.67, 0.50, 0.20 and 0) has been measured. Mono-crystalline Si_xGe_1−x layers Were implanted with 1.50–2.75 MeV Si²⁺ and Ge²⁺ ions to produce the amorphous material. Using surface profilometry and RBS/channeling, it was found that amorphous alloys are less dense than the crystalline alloys, and that Vegard's law underestimates the a-Si_xGe_1−x density.     

Role de l''iodure de cesium sur le developpement des reactions combustible-gaine

TEM of thin foils taken from irradiated fuel element ((UPu)O_2−x fuel in type 316 stainless steel cladding) shows precipitates rich in iron and chromium in the fuel, together with a reduction of chromium content in the cladding. We have undertaken simulation experiments out-of-pile to study the compatibility, both isothermally and in thermal gradients, of the steel/CsI and steel/(CsI + UO ₂) systems, making use of metallography, X-ray diffraction and microprobe analysis, with a view to gaining a better understanding of the reactions at the fuel cladding interface in reactor service. The results of the simulation and reactor-irradiation experiments have enabled us to prepare a transport mechanism in the gas phase, of van Arkel type, involving stainsless steel (at low temperature), (UPu)O_2±x (at high temperature) caesium iodide serving as vector. This transport mechanism which essentially involves displacement of manganese, chromium and iron from cold zones to hot zones, is possible only in the presence of an oxygen source (and the mixed oxide, (UPu)O_2±x, in particular).     

Mass spectrometric investigation of the vaporization of li2tio3(s)

The vaporization of Li₂TiO₃(s) has been investigated by the mass spectrometric Knudsen effusion method. Partial pressures of Li(g), LiO(g), Li₂O(g), Li₃O(g) and O₂(g) over Li₂TiO₃(s) have been obtained in the temperature range 1180–1628 K. When the vaporization of Li₂TiO₃(s) proceeds, the content of Li₂O in the Li₂TiO₃(s) sample decreases. The phase of the sample is a disordered Li₂TiO₃ solid solution above 1486 K. The enthalpies of formation and the atomization energies for LiO(g) and Li₃O(g) have been evaluated from the partial pressures to be ΔH^o_f0(LiO, g) = 65.4 ± 17.4 kJ/mol, ΔH^o_f0(Li₃O, g) = − 207.5 ± 56.6 kJ/mol, D^o₀(LiO) = 340.5 ± 17.4 kJ/mol and D^o₀(Li₃O) = 931.6 ± 56.6 kJ/mol, respectively.     

Chemical thermodynamic representation of AmO2−x

The AmO_2−x solid solution data set for the dependence of the oxygen potential on the composition, x, and temperature was retrieved from the literature and represented by a thermodynamic model. The data set was analysed by least-squares using equations derived from the classical thermodynamic theory for the solid solution of a solute in a solvent. Two representations of the AmO_2−x data were used, namely the Am_5/4O₂–AmO₂ and AmO_3/2–AmO₂ solid solution. No significant difference was found between the two, and the Am_5/4O₂–AmO₂ solution was preferred on the basis of the phase diagram. From the results the Gibbs energy of formation of Am_5/4O₂ has been derived.     

A comparative EPR study of ion implantation induced damage in Si, Si1−xGex (x ≠ 0) and SiC

Electron Paramagnetic Resonance (EPR) measurements have been made to investigate the build up of damage in silicon in relaxed crystalline Si_1−xGe_x (x = 0.04, 0.13, 0.24, 0.36) and in 6H-SiC as a result of increasing the ion dose from low levels (10¹² cm⁻²) up to values (10¹⁵ cm⁻²) sufficient to produce an amorphous layer. Si, Si_1−xGe_x (x ≠ 0) and SiC were implanted at room temperature with 1.5 MeV Si, 2 MeV Si and 0.2 MeV Ge ions respectively. A comparison is made between the ways in which the type and population of paramagnetic defects depend on ion dose for each material.     

Contribution to the study of the U---Cs---Mo---I---O system

The reaction in the U---Cs---Mo---I---O system at 1073 K have been studied as a function of oxygen potential. The chemical constitution and the morphology of the phases formed were examined by electron-probe microanalyzer and X-ray diffractometer. The existence of a two-phase field UO_2+x + Cs₂UO₄ was confirmed in the U---Cs---O system. The threshold oxygen potential was determined for the decomposition of CsI by UO_2+x to form Cs₂U₄O₁₂ and gaseous iodine. The effect of molybdenum on the UO_2+x−CsI reaction was also investigated. Predominance of Cs₂MoO₄ over Cs₂UO₄ was verified in a certain range of oxygen potentials. Some assessments and interpertations of the experimental results were made with the aid of thermodynamic calculations.     

Formation of uranium mononitride by the reaction of uranium dioxide with carbon in ammonia and a mixture of hydrogen and nitrogen: II. Reaction rates

Kinetics of the carbothermic synthesis of UN from UO₂ in an NH₃ stream and a mixed 75% H₂ + 25% N₂ stream were studied in the temperature range of 1400–1600°C by X-ray analysis and weight change measurement of the sample. The weight change was divided into two parts; i.e. weight loss due to carbothermic reduction of UO₂ and weight loss due to removal of carbon by hydrogen. The former followed the first-order rate equation −1n(1 − ₀) = k₀t, and the latter the rate equation of phase boundary reaction 1 − (1 − _c)^1/3 = k_ct. The apparent activation energy of the former was in the range of 320–380 kJ/mol. The value of the latter in an NH₃ stream was 175–185 kJ/mol, which was smaller than that in a mixed 75% H₂ + 25% N₂stream (285 kJ/mol). In this method, the rate of the removal of carbon by hydrogen determines that of the formation of high purity UN.     

Rapid thermal annealing of arsenic implanted relaxed Si1−xGex

The electrical activity and redistribution during rapid thermal annealing (RTA) of high concentrations of As implanted into epitaxially grown, relaxed Si_1−xGe_x for x≤0.5 have been studied as a function of composition x and RTA parameters. At a given RTA temperature the maximum carrier concentration decreases and the redistribution increases with increasing x. Maximum carrier concentrations and junction depths as a function of composition and RTA parameters are given.     

The behaviour of single atoms of molybdenum in urania

Computer simulation techniques are used to investigate the behaviour of single atoms of Mo in UO_2±x. In UO_2−x, Mo is calculated to be present as neutral atoms in bound Schottky trio sites. In UO_2+x, most of the Mo is calculated to be in isolated uranium vacancy sites with Mo ionisation increasing with the O/U ratio. The behaviour near stoichiometric composition is more complex and is found to be very sensitive to changes in O/U ratio. An approximation to the free energy change associated with Mo incorporation in urania is plotted as a function of O/U ratio and Mo concentration. Although this plot is found to be in agreement with the observed insoluble character of Mo in urania, at high O/U ratios and very low Mo concentrations, Mo in solution may be preferred over Mo in the gaseous state.     

Thermal stability and brazing characteristics of Zr0.7−xMxBe0.3 (M=Ti or Nb) ternary amorphous filler metals

The effects of Ti or Nb substitution on the thermal stability and brazing characteristics of Zr_0.7−xM_xBe_0.3 (M=Ti or Nb) ternary amorphous alloys were investigated in order to improve properties of Zr–Be binary amorphous alloy as a new filler metal for joining zirconium alloy. The Zr_0.7−xM_xBe_0.3 (M=Ti or Nb; 0x0.1) ternary amorphous alloys were produced by melt-spinning method. In the selected compositional range, the thermal stability of Zr_0.7−xTi_xBe_0.3 and Zr_0.7−xNb_xBe_0.3 amorphous alloys are improved by the substitution of titanium or niobium for zirconium. As the Ti and Nb content increases, the crystallization temperatures increase from 610°C to 717°C and 610°C to 678°C, respectively. These amorphous alloys were put into practical use in joining bearing pads on zircaloy cladding sheath. Using Zr–Ti–Be amorphous alloys as filler metals, smooth interface and spherical primary particles (proeutectic phase) appear in the brazed layer, which is the similar microstructure of using Zr_0.7Be_0.3 binary amorphous alloys. In the case of Zr–Nb–Be amorphous alloys, Ni-precipitated Zr phase that may cause some degradation in ductility and corrosion-resistance is formed at both sides of the brazed layer.     

Characterization of Si/GexSi1 − x heterojunction bipolar transistors formed by Ge ion implantation in Si

Epitaxial Si/Ge_xSi_{1 − x} heterojunctions were formed by high dose Ge ion implantation in Si followed by rapid thermal annealing at 1000°C for 10 s. This technique was adopted to fabricate Si/Ge_xSi_{1 − x} heterojunction n-p-n bipolar transistors (HBT) using a self-aligned, double polycrystalline silicon process commonly used for fast Si bipolar transistors. The devices are characterized by a 60 nm wide neutral base with a Ge concentration peak of ≈ 7 at.% at the base-collector junction. Good static and dynamic electrical characteristics are demonstrated and discussed.     

RBS investigations of buffer layers between YBa2Cu3O7−x and silicon

The deposition of high-quality high-T_c superconducting films on silicon wafers for future hybrid electronic devices is strongly hampered by the interdiffusion between films and substrate. This effect degrades the superconducting properties seriously and is a strong function of temperature. Since high processing temperatures are inevitable for good films, suitable buffer layers are needed to reduce the interdiffusion. We have investigated the combinations ZrO₂/Si(100), BaF₂/Si(100), and noble-metal/TiN/Si(100) at temperatures up to 780°C in oxidizing ambient. YBa₂Cu₃O_7−x films have been deposited onto the buffer layers by laser ablation. Thereafter the interfaces have been analyzed by Rutherford backscattering. So far only ZrO₂ has demonstrated sufficient stability to serve as a buffer layer for the laser-ablated YBa₂Cu₃O_7−x films. All other combinations suffer from interdiffusion or oxidation.     

Micro-RBS characterisation of the chemical composition and particulate deposition on pulsed laser deposited Si1−xGex thin films

The formation and deposition of particulates by pulsed laser deposition of Si_1−xGe_x semiconductor alloy thin films are discussed. Using Rutherford backscattering spectrometry with micrometer lateral resolution (micro-RBS) the film composition was measured with high accuracy, even in the presence of particulates with a high areal density of 20,000–30,000 particulates per mm². We show that on impact of a particulate, the part of the thin film which is already deposited probably melts and its Ge content segregates to the surface.     

Thermodynamic studies on Cs4U5O17(s) and Cs2U2O7(s) by emf and calorimetric measurements

The oxygen potentials over the phase field: Cs₄U₅O₁₇(s)+Cs₂U₂O₇(s)+Cs₂U₄O₁₂(s) was determined by measuring the emf values between 1048 and 1206 K using a solid oxide electrolyte galvanic cell. The oxygen potential existing over the phase field for a given temperature can be represented by: Δμ(O₂) (kJ/mol) (±0.5)=−272.0+0.207T (K). The differential thermal analysis showed that Cs₄U₅O₁₇(s) is stable in air up to 1273 K. The molar Gibbs energy formation of Cs₄U₅O₁₇(s) was calculated from the above oxygen potentials and can be given by, Δ_fG⁰ (kJ/mol)±6=−7729+1.681T (K). The enthalpy measurements on Cs₄U₅O₁₇(s) and Cs₂U₂O₇(s) were carried out from 368.3 to 905 K and 430 to 852 K respectively, using a high temperature Calvet calorimeter. The enthalpy increments, (H⁰_T−H⁰₂₉₈), in J/mol for Cs₄U₅O₁₇(s) and Cs₂U₂O₇(s) can be represented by, H⁰_T−H⁰_298.15 (Cs₄U₅O₁₇) kJ/mol±0.9=−188.221+0.518T (K)+0.433×10⁻³T² (K)−2.052×10⁻⁵T³ (K) (368 to 905 K) and H⁰_T−H⁰_298.15 (Cs₂U₂O₇) kJ/mol±0.5=−164.210+0.390T (K)+0.104×10⁻⁴T² (K)+0.140×10⁵(1/T (K)) (411 to 860 K). The thermal properties of Cs₄U₅O₁₇(s) and Cs₂U₂O₇(s) were derived from the experimental values. The enthalpy of formation of (Cs₄U₅O₁₇, s) at 298.15 K was calculated by the second law method and is: Δ_fH⁰_298.15=−7645.0±4.2 kJ/mol.     

Melting behaviour of oxide systems for heterogeneous transmutation of actinides. III. The system Am–Mg–O

Unknown thermodynamic data of a number of americium oxides were estimated. Phase diagrams of related binary and ternary systems in the Am–Mg–O system were calculated by transposing solution model parameters of the Pu–Mg–O system to the Am–Mg–O system. According to the calculated results, the liquid phase in the MgO–AmO_2−x system starts to form at 1930 K in the oxygen pressure range 4.3×10⁻⁷ to 4.5 bar, and at the temperature varying from 2221 K to 2356 K at an oxygen pressure higher than 4.5 bar.     

Investigation of in-reactor creep and irradiation growth of zirconium-2.5 wt% niobium channel tubes

We summarize the diametral creep results obtained in the MR reactor of the Kurchatov Institute of Atomic Energy on zirconium-2.5 wt% niobium pressure tubes of the type used in RBMK-1000 power reactors. The experiments that lasted up to 30 000 h cover a temperature range of 270 to 350°C, neutron fluxes between 0.6 and 4.0 ×10¹³ n/cm² · s (E > 1 MeV) and stresses of up to 16 kgf/mm². Diametral strains of up to 4.8% have been measured. In-reactor creep results have been analyzed in terms of thermal and irradiation creep components assuming them to be additive. The thermal creep rate is given by a relationship of the type ε_th = A₁ exp [(A₂ + A _3σt) T] and the irradiation component by ε_rad = A_4σtø(T − A₅), where T = temperature, σ_t = hoop stress, ø = neutron flux and a₁ to A₅ are constants. Irradiation growth experiments carried out at 280° C on specimens machined from pressure tubes showed a non-linear dependence of growth strain on neutron fluence up to neutron fluences of 5 × 10²⁰ n/cm². The significance of these results to the elongation of RBMK reactor pressure tubes is discussed.