L. Thomé

Raman spectroscopy study of heavy-ion-irradiated α-SiC

Raman spectroscopy was used to investigate the structure of ion-irradiated α-SiC single crystals at room temperature and 400 °C. Irradiations induce a decrease of the Raman line intensities related to crystalline SiC, the appearance of several new Si–C vibration bands attributed to the breakdown of the Raman selection rules, and the formation of homonuclear bonds Si–Si and C–C within the SiC network. For low doses, the overall sp3 bond structure and the chemical order may be almost completely conserved. By contrast, the amorphous state shows a strong randomization of the Si–Si, Si–C and C–C bonds. The relative Raman intensity decreases exponentially versus increasing dose due to the absorption of the irradiated layer. The total disorder follows a sigmoidal curve, which is well fitted by the direct impact/defect stimulated model. The chemical disorder expressed as the ratio of C–C bonds to Si–C bonds increases exponentially versus the dose. A clear correlation is established between the total disorder and the chemical disorder. The increase of temperature allows the stabilization of a disordered/distorted state and a limitation of damage accumulation owing to the enhancement of the dynamic annealing.

High-Energy Heavy-Ion Irradiations of Fe85B15 Amorphous Alloy: Evidence for Electronic Energy Loss Effect

Amorphous metallic Fe85B15 alloy has been irradiated at low temperature with Ar, Kr and Xe ions of initial energies of 1.8, 2.7 and 3.0 GeV, respectively. Electrical resistance was measured in situ on samples piled up along the beam direction. It is shown that above a given electronic stopping power threshold the electronic losses play a crucial role in radiation-induced damage.

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.

Phase transformations induced by high electronic excitation in ion-irradiated Gd2(ZrxTi1−x)2O7 pyrochlores

The pyrochlore oxides (A2B2O7) exhibit a remarkable range of structural, physical, and magnetic properties related to their various chemical compositions. This article reports the phase transformations induced by high electronic excitation in pyrochlores of the Gd2(ZrxTi1−x)2O7 family irradiated with swift ions. The structural changes, investigated by using several analytical techniques (x-ray diffraction, Raman spectroscopy, and transmission electron microscopy), strongly depend on the chemical composition. The high electronic excitation along the ion trajectory results in the amorphization of ion tracks for Gd2Ti2O7 and Gd2TiZrO7, whereas a defective fluorite structure is formed in Gd2Zr2O7. Moreover, the results underline the existence of an electronic stopping power threshold of 6 keV/nm for amorphizable compounds and 10 keV/nm for Gd2Zr2O7, below which phase transformations do not occur. Finally, the study of the thermal recovery of irradiated pyrochlores provides the recrystallization temperature fo...

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...

Radiation Damage Induced by Electronic-Energy Loss in Amorphous Metallic Alloys

The electrical resistance of amorphous metallic Fe85B15 ribbons irradiated with 3 GeV Xe ions at different tilting angles with respect to the incident ion beam has been measured in situ at 77 K. The results show that irradiation induces large sample growth due to electronic-energy loss effects. The existence of a new mechanism leading to damage creation in metallic alloys by electronic excitation alone is also demonstrated.

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.

Amorphization and dynamic annealing of hexagonal SiC upon heavy-ion irradiation: Effects on swelling and mechanical properties

Structural, mechanical, and dimensional evolutions of silicon carbide (SiC) induced by heavy-ion irradiations are studied by means of Rutherford backscattering spectrometry and channeling (RBS/C), nanoindentation, and surface profilometry measurements. 4H- and 6H-SiC single crystals were irradiated with 4 MeV Au2+ and 4 MeV Xe+ ions at room temperature (RT) or 400 °C. Using a Monte Carlo program to simulate the RBS/C spectra (MCCHASY code), we find that Au ion irradiation at RT induces a total silicon sublattice disorder related to full amorphization at a dose of about 0.4 displacement per atom (dpa). A two-step damage process is found on the basis of the disordered fractions deduced from RBS/C data. Complete amorphization cannot be reached upon both Au and Xe ion irradiations at 400 °C up to about 26 dpa because of the dynamic annealing of defects. When complete amorphization is reached at RT, the Young’s modulus and Berkovich hardness of irradiated 6H-SiC samples are lower by, respectively, 40% and 45% ...

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.