Papers by Keyword: Irradiation Effects

Abstract: Post-irradiation data on the neutron-induced swelling behaviour and resistivity changes in silicon carbide often does not show a clear trend. This makes a quantitative comparison between different studies difficult. To address the diverging results after irradiation in different studies, a thorough reference study is performed on high quality β-silicon carbide. The results show the response to neutron irradiation may be significantly influenced by structural defects present before irradiation. These findings open a way to improve the accuracy of silicon carbide irradiation temperature monitors.

Abstract: The phase-field method is a very powerful tool to model the phase transformation and microstructural evolution of solids at mesoscopic scale. However, several important phenomena, like defect formation, grain boundary motion, or reconstructive phase transitions require an atomic scale study. Recently an approach called the quasi-particle approach, based on the Atomic Density Function theory was developed to incorporate the atomic-level crystalline structures into standard continuum theory for pure and multicomponent systems. This review focuses on the description of different computational methods used to model microstructural evolution and self-assembly phenomena at mesoscopic and atomistic scales. Various application examples of these methods are also presented.

Abstract: We present here an overview of native point defects calculations in silicon carbide using Density Functional Theory, focusing on defects energetics needed to understand self-diffusion. The goal is to assess the availability of data that are necessary in order to perform kinetic calculations to predict not only diffusion properties but also the evolution of defect populations under or after irradiation. We will discuss the spread of available data, comment on the main defect reactions that should be taken into account, and mention some of the most recent promising developments.

Abstract: Both bulk and thin film amorphous carbon were irradiated using a nitrogen ion beam and changes in surface roughness and composition after ion beam irradiation have been studied. Amorphous carbon thin films were prepared from toluene vapor using plasma enhanced chemical vapor deposition. Ion irradiation was performed at room temperature using a nitrogen ion beam and the ion beam energy was varied from 0.2 to 1.5 keV under the constant ion current density. Surface morphology was observed with atomic force microscopy (AFM). Depth profiles of nitrogen in the irradiated specimens were analyzed by X-ray photoelectron spectroscopy (XPS). AFM observations reveal that after the ion beam irradiation the surface of the bulk amorphous carbon becomes rough, while the surface of the amorphous carbon films becomes smooth. However, the notable difference in the surface roughness is hardly observed between low- and high-energy ion irradiation. From XPS studies, it is found that the nitrogen concentration near the surface increases after the ion irradiation for both bulk and thin films and irradiated nitrogen ions are combined with carbon, resulting in formation of carbon nitride layers. Depth profiles of nitrogen show that for the bulk specimen low-energy ion irradiation is more effective for the carbon nitride formation than high-energy ion irradiation, while for the thin films high-energy ions are implanted more deeply than low-energy ions.

Abstract: We studied the defects at the origins of the permanent radiation-induced attenuation in four g-rays irradiated single-mode germanosilicate optical fibers (~1 MeV; 1.2 kGy; 0.3 Gy/s) in the spectral range 400 - 1700 nm. We determined the wavelength dependence of the following cladding codopant influences: germanium (0.3 %), phosphorus (0.3 %), fluorine (0.3 %) on the germanosilicate (13 %) fiber radiation responses. We identified some of the different color centers produced by g-rays and we evaluated their localization in the fiber cross-section through the determination of the radial distribution of the radiation-induced absorption at 633 nm. We also evidenced the strong interactions between these three codopants. In particular, our results showed that the properties of the phosphorus-related color centers, which mainly determine the fiber infrared radiation sensitivity, are strongly influenced by the germanium- and fluorine-codoping.

Abstract: Radiation-induced losses and paramagnetic centers were investigated in phosphorusdoped and P-free multimode germanosilicate optical fibers after g-rays (~1 MeV) and ultraviolet (5 eV) exposures. After both types of irradiation, the same defects seem to be responsible of the fiber absorption in the spectral range 400 to 1650 nm. In particular, the P1 centers and the Phosphorus Oxygen Hole centers are created in both cases in the phosphorus-doped fibers and explain the high permanent radiation-induced attenuation levels observed in this fiber type. Luminescence and electron spin resonance measurements (77 K, ~9.38 GHz) on irradiated samples confirm that the GeE’, SiE’ and NBOHC defects are also generated in the different irradiated samples. From this study, it seems that the pertinence of a multimode fiber for nuclear space or civil applications could be estimated through low-cost ultraviolet measurements.