A. Debelle

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.

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.

Comprehensive study of the effect of the irradiation temperature on the behavior of cubic zirconia

Cubic zirconia single-crystals (yttria-stabilized zirconia (YSZ)) have been irradiated with 4 MeV Au2+ ions in a broad fluence range (namely from 5 × 1012 to 2 × 1016 cm−2) and at five temperatures: 80, 300, 573, 773, and 1073 K. Irradiated samples have been characterized by Rutherford backscattering spectroscopy in channeling mode, X-ray diffraction and transmission electron microscopy techniques in order to determine the disordering kinetics. All experimental results show that, whatever is the irradiation temperature, the damage build-up follows a multi-step process. In addition, the disorder level at high fluence is very similar for all temperatures. Thus, no enhanced dynamic annealing process is observed. On the other hand, transitions in the damage accumulation process occur earlier in fluence with increasing temperature. It is shown that temperature as low as 573 K is sufficient to accelerate the disordering process in ion-irradiated YSZ. ACKNOWLEDGMENTS

Statistical Nature of Atomic Disorder in Irradiated Crystals.

Atomic disorder in irradiated materials is investigated by means of x-ray diffraction, using cubic SiC single crystals as a model material. It is shown that, besides the determination of depth-resolved strain and damage profiles, x-ray diffraction can be efficiently used to determine the probability density function (PDF) of the atomic displacements within the crystal. This task is achieved by analyzing the diffraction-order dependence of the damage profiles. We thereby demonstrate that atomic displacements undergo Lévy flights, with a displacement PDF exhibiting heavy tails [with a tail index in the γ=0.73-0.37 range, i.e., far from the commonly assumed Gaussian case (γ=2)]. It is further demonstrated that these heavy tails are crucial to account for the amorphization kinetics in SiC. From the retrieved displacement PDFs we introduce a dimensionless parameter f_{D}^{XRD} to quantify the disordering. f_{D}^{XRD} is found to be consistent with both independent measurements using ion channeling and with molecular dynamics calculations.

RaDMaX: a graphical program for the determination of strain and damage profiles in irradiated crystals

RaDMaX (radiation damage in materials analysed with X-ray diffraction) is a user-friendly graphical program that allows the determination of strain and damage depth profiles in ion-irradiated crystals. This task is achieved by fitting experimental X-ray diffraction data, recorded in symmetrical θ–2θ geometry, with a dynamical diffraction model parametrized with variable strain and damage profiles based on B-spline functions. The strain and damage profiles can be graphically manipulated so as to fit the calculated curve to the experimental data. Automatic fitting procedures (generalized simulated annealing and conventional least squares) are also implemented. RaDMaX is free and open source (CeCILL licence) and can be downloaded from http://aboulle.github.io/RaDMaX.

Recovery effects due to the interaction between nuclear and electronic energy losses in SiC irradiated with a dual-ion beam

Single and dual-beam ion irradiations of silicon carbide (SiC) were performed to study possible Synergetic effects between Nuclear (Sn) and Electronic (Se) Energy Losses. Results obtained combining Rutherford backscattering in channeling conditions, Raman spectroscopy, and transmission electron microscopy techniques show that dual-beam irradiation of SiC induces a dramatic change in the final sample microstructure with a substantial decrease of radiation damage as compared to single-beam irradiation. Actually, a defective layer containing dislocations is formed upon dual-beam irradiation (Sn&Se), whereas single low-energy irradiation (Sn alone) or even sequential (Sn + Se) irradiations lead to full amorphization. The healing process is ascribed to the electronic excitation arising from the electronic energy loss of swift ions. These results shed new light on the long-standing puzzling problem of the existence of a possible synergy between Sn and Se in ion-irradiation experiments. This work is interesting ...

Diffuse X-ray scattering from ion-irradiated materials: a parallel-computing approach

A computational method for the evaluation of the two-dimensional diffuse X-ray scattering distribution from irradiated single crystals is presented. A Monte Carlo approach is used to generate the displacement field in the damaged crystal. This step makes use of vector programming and multiprocessing to accelerate the computation. Reciprocal space maps are then computed using GPU-accelerated fast Fourier transforms. It is shown that this procedure speeds up the calculation by a factor of ∼190 for a crystal containing 109 unit cells. The potential of the method is illustrated with two examples: the diffuse scattering from a single crystal containing (i) a non-uniform defect depth distribution (with a potentially bimodal defect size distribution) and (ii) spatially correlated defects exhibiting either long-range or short-range ordering with varying positional disorder.

Depth-dependent phase change in Gd2O3 epitaxial layers under ion irradiation

Epitaxial Gd2O3 thin layers with the cubic structure were irradiated with 4-MeV Au2+ ions in the 1013–1015 cm−2 fluence range. X-ray diffraction indicates that ion irradiation induces a cubic to monoclinic phase change. Strikingly, although the energy-deposition profile of the Au2+ ions is constant over the layer thickness, this phase transformation is depth-dependent, as revealed by a combined X-ray diffraction and ion channeling analysis. In fact, the transition initiates very close to the surface and propagates inwards, which can be explained by an assisted migration process of irradiation-induced defects. This result is promising for developing a method to control the thickness of the rare-earth oxide crystalline phases.

Ion beam synthesis of ZrCxOy nanoparticles in cubic zirconia

{110}-oriented yttria-stabilized zirconia single crystals have been implanted with low-energy C ions in an axial direction, at room temperature and at 550 °C. Room temperature ion implantation generated a damage layer that contains the expected dislocation loop clusters. Strikingly, the high temperature implantation produced zirconium oxycarbide nanoparticles (ZrCxOy) at a shallow depth in the yttria-stabilized cubic zirconia crystal, with a diameter in the range of 4–10 nm. Moreover, in the high concentration region of implanted C ions, between 100 and 150 nm below the surface, a number of large precipitates, up to 20 nm, were observed.