N. Shamir

Initial adsorption kinetics of oxygen on polycrystalline copper

The initial adsorption kinetics of oxygen on a sputter-cleaned surface of polycrystalline copper was studied at room-temperature utilizing combined measurements with Auger electron spectroscopy (AES) and time-of-flight spectrometry of direct recoils (TOF-DR). These combined measurements enable one to distinguish between adsorption processes occurring simultaneously on different structural arrays present on the polycrystalline surface. Modeling the surface as composed of two types of arrays, i.e. “rough” (e.g. ridged planes, defects and irradiation damaged areas) and “smooth”, it is possible to obtain quantitative kinetic data regarding adsorption on these local arrays. The “rough” arrays on polycrystalline copper display kinetic behavior similar to the ridged (110) plane of copper single-crystal. The “smooth” arrays on the other hand display initial sticking probabilities higher than obtained for single-crystals. This difference has been accounted for by an “equilibrium spillover” model assuming surface migration of adsorbed oxygen from the “rough” to the “smooth” arrays.

The effect of N2+ and C+ implantation on uranium hydride nucleation and growth kinetics

Abstract Hydrogen attack on uranium and uranium alloys may cause embrittlement and hydride formation that are undesirable in nuclear fuel technology. Implantation of the uranium surface by a high dose of energetic ions modifies the surface in a way that delays the hydrogen attack and slows the growth rate of the hydride. The implanted surfaces also exhibited better passivation to air oxidation. In the present study, 45 keV N 2 + and C + ions with a dose of 6·10 17 ions/cm 2 were implanted (separately) in pure uranium. The incipient hydriding nucleation and growth kinetics of the implanted uranium samples were measured in a hot-stage microscopy system. The surface was continuously monitored, during the hydrogenation process, by a TV camera and recorded on videotape. The reaction was stopped, for various experiments, at different reaction steps by pumping the hydrogen out. SEM micrographs revealed, especially for the C + implanted samples, a morphology in which the hydride appears as blisters, seemingly under the implanted layer. The hot-stage micrographs were analyzed by image-analysis procedures yielding the nucleation and growth rates for the implanted vs. unimplanted specimens. Possible explanations are suggested for the passivation effects imparted by ion implantation.

Heat-induced redistribution of surface oxide in uranium

Abstract The redistribution of oxygen and uranium metal at the vicinity of the metal-oxide interface of native and grown oxides due to vacuum thermal annealing was studied for uranium and uranium-chromium alloy using Auger depth profiling and metallographic techniques. It was found that uranium metal is segregating out through the uranium oxide layer for annealing temperatures above 450°C. At the same time the oxide is redistributed in the metal below the oxide-metal interface in a diffusion like process. By applying a diffusion equation of a finite source, the diffusion coefficients for the process were obtained from the oxygen depth profiles measured for different annealing times. An Arrhenius like behavior was found for the diffusion coefficient between 400 and 800°C. The activation energy obtained was E a = 15.4 ± 1.9 kcal/mole and the pre-exponential factor, D 0 = 1.1 × 10 −8 cm 2 / s . An internal oxidation mechanism is proposed to explain the results.

Surface spectroscopy studies of the oxidation behavior of uranium

Abstract Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) techniques were utilized to study the oxidation behavior of clean uranium surfaces, at very low pressures of various atmospheres (UHV, H 2 , O 2 , and CO 2 ), at room temperature. Both for O 2 and CO 2 , a precursor chemisorbed oxygen species has been identified at the very initial stage of the oxidation reaction. This chemisorbed oxygen transforms to the oxide form at a rate which depends on the pressure of the oxidizing atmosphere. Residual gaseous carbon compounds which are present even under UHV conditions result in the simultaneous formation of surface carbide which accompanies the initial stage of oxidation. This carbide however decomposes later as oxidation proceeds. Adventitious hydrocarbon adsorption occurs on the formed oxide layer.

The kinetics and mechanism of cerium hydride formation

Abstract The kinetics of the reaction between cerium metal and gaseous hydrogen were studied within the temperature range 0–100 °C and the pressure range 0.1–32 atm. Combined kinetic rate measurements and metallographic observations enabled the evaluation of the intrinsic kinetic parameters of the reaction, i.e. the velocities of the hydride front progression and their temperature-pressure dependence. Arrhenius temperature plots of the velocities yielded a pressure-independent apparent activation barrier of about 0.2 ± 0.05 eV per H atom. For each temperature an apparent power pressure dependence of about 1 3 was displayed. Different possible mechanisms are discussed. It has been concluded that the rate-limiting step for this reaction is the penetration of hydrogen from the surface (of the hydride layer) into the subsurface region. The activation energies for this penetration process and for the desorption of chemisorbed hydrogen (from the hydride surface) were estimated to be 0.24 and 0.09 eV respectively.

Hydrogen-oxygen accumulation on polycrystalline uranium surfaces

Abstract The kinetics of accumulation of hydrogen on clean and on oxidized uranium surfaces was studied by combined direct recoil spectrometry (DRS), Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) techniques. For the sputter-cleaned surface an island growth progression characterizes the accumulation kinetics of hydrogen, whereas on the oxidized surface a random two-site Langmuir model seems to fit the initial chemisorption process. Similar combined DRS and AES measurements were applied to study the kinetics of accumulation of oxygen on H 2 predosed uranium as compared to the clean (H-free) case. The pre-chemisorption of hydrogen modifies the mechanism of oxygen incorporation and the structure of the product overlayer.

The leaching-activation process of Urushibara catalysts

Abstract The acid-leaching process resulting in the activation of Urushibara-Ni catalysts was studied by means of X-ray photoelectron spectroscopy (XPS), X-ray diffraction, scanning electron microscopy (SEM) combined with X-ray energy dispersion (EDX), and wet chemical analysis. The leaching-induced activation has been shown to involve the dissolution of a coating surface layer dominated by zinc compounds and the exposure of metallic nickel. Metallic zinc coexisting with the nickel on the activated surface prevents (at low oxidation doses) the oxidation of the inherently active nickel, by gettering the oxidizing molecules. Exposing the active surface to high oxidation doses results in the oxidation of both metallic constituents and the loss of catalytic activity.