S. Zalkind

Oxidation of ion-bombarded vs. annealed beryllium

The mechanisms of O2 adsorption and oxidation of ion-bombarded and annealed highly oriented polycrystalline beryllium surfaces were comparatively studied. It was found out that the basic mechanism of oxygen adsorption involves O clusters for both surfaces and at a certain oxygen coverage nucleation and growth of oxide islands take place. Also, in both cases, lateral growth of the islands dictates a relatively fast oxidation rate, which is decreased following coalescence, turning to a slow inverse logarithmic growth. The introduction of surface and subsurface defects by the Ar þ ion-bombardment almost doubles the oxygen sticking coefficient, increases the oxide island thickness from � 2to � 3 monolayers (prior to coalescence) and dictates a higher oxidation rate in the high oxygen exposure regime. It was also found that annealing the surface, following sputtering, causes its smoothing, but in the initial heating process, in the range 300–500 K coarsening seems to occur. 2002 Published by Elsevier Science B.V.

The adsorption of H2O vs O2 on beryllium

The adsorption mechanism and initial oxidation of sputtered beryllium exposed to H2O and to O2 were studied using a combination of the DRS, AES and XPS techniques. For both cases, the “clustering” Langmuir type mechanism was found to fit the adsorption kinetics. The initial sticking coefficients, estimated from these fits, are S0(H2O) ≈ 1 and S0(O2) ≈ 0.07. Oxide islands, ∼ 3 monolayers thick, are formed in both cases, spreading laterally till a full layer is formed. For O2 the oxidation stops at this stage, while for H2O it continues at a lower rate, reaching a saturation level of about six monolayers. Observation of significant broadening of the AES O KVV peak in the stage of thickness growth for the H2O exposure, and DRS H depth profiling indicate that hydrogen is trapped in the oxide matrix. Possible hydrogen-enhanced-diffusion of ions through the oxide seems to enable the further growth.

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.

Adsorption of hydrogen on clean and oxidized beryllium studied by direct recoil spectrometry

Direct monitoring of hydrogen adsorption on clean and on pre-oxidized polycrystalline beryllium by means of direct recoil spectrometry (DRS) is reported. While no adsorption occurs for molecular hydrogen either on clean or on oxidized beryllium, atomic hydrogen adsorbs on the surface of clean Be, as well as at the sites available between oxygen clusters, formed by low oxygen pre-exposure. For higher oxygen pre-exposures, when oxide islands are formed, the apparent coverage of hydrogen is significantly lower than the amount needed for complementary full coverage on the Be surface. Possible origins for this observation are discussed.

Temperature dependent interactions of water vapor with a beryllium surface

Abstract The temperature dependence of the dissociation of H2O, adsorption of its products on a Be surface and the initial oxidation process were studied. In contrast to the room temperature one-step full dissociation, at 150 K the dissociation is into H and OH, the latter being adsorbed on the surface. Above a certain dose, adsorption of an ice layer occurs. Heating such an adsorbed ice layer to 200 K, causes some of it to desorb, leaving adsorbed hydroxyls on the surface. Further warming causes complete dissociation of the hydroxyls and oxidation of the surface. At 375 K the whole surface is transformed into an oxide. Exposing an O2 pre-oxidized Be surface to residual H2O for a long period, yields hydroxyl adsorption on the oxide. A time dependent mechanism of electron supply by tunneling through the oxide layer, causing the partial dissociation of the water molecules, or attachment to the surface of hydroxyl groups, is suggested.

Effects of preadsorbed hydrogen on the adsorption of O2, CO and H2O on beryllium

Abstract The adsorption of O 2 , CO and H 2 O on a hydrogen precovered beryllium surface was studied using direct recoils spectrometry and Auger electron spectroscopy. It was found that preadsorbed hydrogen, in addition to adsorption site blocking, changes the Be electronic structure in a way that hinders the adsorption of O 2 and CO on the surface. Apparently, repulsion between the preadsorbed hydrogen and adsorbed oxygen or CO creates depletion zones between them. For oxygen it also promotes agglomeration and penetration into the subsurface and leads to oxide nucleation at lower oxygen overages. For H 2 O, only a minor delay of adsorption and hydrogen replacement was observed.