F. Garrido

Structure and composition of passive titanium oxide films

The thickness and composition of several kinds of titanium oxide films formed on a titanium substrate were determined by surface analysis techniques: X-ray photoelectron spectroscopy, Rutherford back scattering, X-ray diffraction and atomic force microscopy. Most titanium oxide samples were prepared by anodisation, using a galvanostatic procedure. The films were shown to be composed of an amorphous TiO2 outer layer (10–20 nm thick) and an intermediate TiOx layer, in contact with the TiO2 layer and the metallic substrate. The outer layer is sensitive to the environment: its thickness usually decreases with ageing in a corrosive solution. A stabilisation procedure was proposed in order to improve its ability to withstand corrosion.

Alpha-radiolysis effects on UO2 alteration in water

The behaviour of UO2/water interface under irradiation has been investigated as a function of alpha flux using an alpha beam provided by a cyclotron. The effects of alpha-radiolysis on UO2 alteration in aerated deionized water were studied by characterizing both the chemistry of irradiated aqueous solutions and the UO2 surface. Uranium (as uranyl ion UO22+) and hydrogen peroxide concentration (H2O2) increased whereas pH decreased in the irradiated solutions when alpha-beam flux increased. The formation of hydrated uranium peroxide (metastudtite UO4·2H2O) on UO2 leached surface was identified by X-ray diffractometry. The production of metastudtite can be considered as a direct effect of water radiolysis due to the production of radiolytic species H2O2, since its formation is known to occur out of irradiation via a precipitation reaction between hydrogen peroxide and uranyl ion. From both these observations and literature about hydrated uranium peroxide occurrence, the possibility of metastudtite formation on nuclear spent fuel in storage conditions is discussed.

Athermal crystallization induced by electronic excitations in ion-irradiated silicon carbide

Silicon carbide single crystals were irradiated at room temperature with low energy I ions and high energy Pb ions. It is found that the damaged layer formed by the elastic collisions generated during low energy I ion irradiation can readily be removed by the electronic excitations induced by swift Pb ions. This effect occurs at a temperature quite below that at which the conventional ion-beam induced crystallization process is generally achieved by nuclear energy loss. This finding is interesting both from a fundamental point of view for the understanding of the interaction of swift heavy ions with solids and for a large number of technological applications.

Multistep damage evolution process in cubic zirconia irradiated with MeV ions

This work reports the study, via the combination of Rutherford backscattering spectrometry and channeling, x-ray diffraction, and transmission electron microscopy experiments, of the damage formation in cubic yttria-stabilized zirconia single crystals irradiated with medium-energy (4 MeV) heavy (Au) ions. The damage buildup, which is accounted for in the framework of the multistep damage accumulation model, occurs in three steps. The first step at low fluences (up to 1015 cm−2), characterized by a regular increase in both the damage yield and the elastic strain, is related to the formation of small defect clusters. The second step in the intermediate fluence range (from 1015 to 5×1015 cm−2) leads to a sharp increase in the damage yield and to a large drop of the strain due to the formation of dislocation loops which collapse into a network of tangled dislocations. The third step at high fluences (above 5×1015 cm−2) exhibits a surprising decrease in the damage yield, which may be attributed to the reorgani...

Characterization and modelling of the ion-irradiation induced disorder in 6H-SiC and 3C-SiC single crystals

6H-SiC and 3C-SiC single crystals were simultaneously irradiated at room temperature with 100 keV Fe ions at fluences up to 4 × 1014 cm−2 (~0.7 dpa), i.e. up to amorphization. The disordering behaviour of both polytypes has been investigated by means of Rutherford backscattering spectrometry in the channelling mode and synchrotron x-ray diffraction. For the first time, it is experimentally demonstrated that the general damage build-up is similar in both polytypes. At low dose, irradiation induces the formation of small interstitial-type defects. With increasing dose, amorphous domains start to form at the expense of the defective crystalline regions. Full amorphization of the irradiated layer is achieved at the same dose (~0.45 dpa) for both polytypes. It is also shown that the interstitial-type defects formed during the first irradiation stage induce a tensile elastic strain (up to ~4.0%) with which is associated an elastic energy. It is conjectured that this stored energy destabilizes the current defective microstructure observed at low dose and stimulates the formation of the amorphous nanostructures at higher dose. Finally, the disorder accumulation has been successfully reproduced with two models (namely multi-step damage accumulation and direct-impact/defect-stimulated). Results obtained from this modelling are compared and discussed in the light of experimental data.

Damage induced by electronic excitation in ion-irradiated yttria-stabilized zirconia

This article presents a study of the damage production in yttria-stabilized cubic zirconia single crystals irradiated with swift heavy ions. The combination of techniques which probe the material at different spatial scales (Rutherford backscattering spectrometry in channeling geometry, x-ray diffraction, transmission electron microscopy, and atomic force microscopy) was used in order to gain information about the damage depth distribution, the disordering buildup, the nature of radiation defects, and the occurrence of microstructural modifications. The damage results from the formation of tracks, due to the huge electronic excitations induced in the wake of incident ions. The melting of the material in the core of tracks, via a thermal spike mechanism, leads to the creation of large hillocks at the surface of the crystals. The overlapping of ion tracks at high fluence (above similar to 10(12) cm(-2)) induces a severe transformation of the microstructure of the material. Nanodomains slightly disoriented from the main crystallographic direction are formed, with a size decreasing with increasing irradiation fluence. These results may be used to predict the damage evolution in other nonamorphizable ceramics irradiated with swift heavy ions.

Combined effects of nuclear and electronic energy losses in solids irradiated with a dual-ion beam

Single and dual-beam irradiations of oxide (c-ZrO2, MgO, Gd2Ti2O7) and carbide (SiC) single crystals were performed to study combined effects of nuclear (Sn) and electronic (Se) energy losses. Rutherford backscattering experiments in channeling conditions show that the Sn/Se cooperation induces a strong decrease of the irradiation-induced damage in SiC and MgO and almost no effects in c-ZrO2 and Gd2Ti2O7. The healing process is ascribed to electronic excitations arising from the electronic energy loss of swift ions. These results present a strong interest for both fundamental understanding of the ion-solid interactions and technological applications in the nuclear industry where expected cooperative Sn/Se effects may lead to the preservation of the integrity of nuclear devices.

Application of ion beams to nuclear waste issues:: evaluation of nuclear ceramics

Oxide ceramics are promising matrices for nuclear waste immobilization or transmutation. The most important issue concerning these materials is the investigation of their physico-chemical properties with respect to safety criteria. This paper shows the potentiality of ion beams for the evaluation of ceramics as nuclear waste matrices. Several items are concerned. The different sources of radiation damage occurring in the reactor and during the storage term may be simulated by irradiation of the matrix with a variety of ions in a wide energy range. The doping of the material with foreign species produced by radioactive desintegration may be performed by ion implantation of stable or radioactive isotopes. The modifications induced in the matrices may be characterized by nuclear microanalysis techniques using medium-energy light ion beams. Examples relative to these various items are provided.

Formation of nonlinear optical waveguides by using ion-exchange and implantation techniques

Abstract Composite materials consisting of metal nanoclusters embedded in glass matrices have been obtained by the combined use of ion-exchange and ion implantation processes, with possible application in the design of nonlinear all-optical switching devices. Optical waveguides containing either silver or copper clusters have been fabricated. Optical absorption and electron microscopy have been performed to detect the presence of metal clusters. Preliminary measurements have been also performed of the optical nonlinear response on both silver- and copper-containing glasses.

Stress field induced by swift heavy ion irradiation in cubic yttria stabilized zirconia

X-ray diffraction (XRD) was used to investigate the damage and the correlated stress induced by the slowing down of swift heavy ions in cubic zirconia polycrystals doped with 10 mol % Y2O3. Samples were irradiated at room temperature with 940 MeV Pb ions at fluences ranging from 5×1011 to 4×1013 cm−2. Changes of XRD profiles were examined at increasing fluences. Residual macroscopic stresses induced by irradiation were determined using XRD by the “sin2 ψ method.” The state of stress in the irradiated layer was described by a combination of: (i) a hydrostatic stress caused by the formation of damaged tracks leading to swelling and (ii) a biaxial stress imposed by the bulk undamaged material, which controls the lateral expansion of the surface damaged layer. The evolution of the stress as a function of irradiation fluence was also determined: the intensity of the hydrostatic stress increases from 80 to 460 MPa when the fluence is increased from 5×1011 to 4×1013 cm−2 and that of the biaxial stress increases ...