Browsing by Author "Abdelbagi, Hesham A. A."
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- Encapsulating nuclear fuel kernels (i.e., uranium) in thin films, such as silicon carbide (SiC) and carbon allotropes, prevents the release of most radioactive fission products. Since SiC fails to retain cesium (Cs) within the fuel structure, the present study investigates the benefits of combining chemically stable SiC and zirconium (Zr) layers to prevent the escape of Cs from nuclear fuels. Polycrystalline SiC samples were implanted with 300 keV Cs ions at room temperature (RT) to a fluence of 1 ×1016 cm2. Selected as-implanted SiC samples were then coated with a 150 nm thick Zr layer. The annealing of the as implanted and coated samples was performed in vacuum at temperatures from 900 to 1000 ◦C, which is similar to the normal reactor operation temperatures. Our investigations show that the annealing of the as-implanted, uncoated SiC samples, at 900 and 1000 ◦C indeed leads to the diffusion of Cs atoms through the SiC surface with retentions of 47 ±5 % and 26 ±5 %, respectively. On the other hand, annealing of the coated samples at 900 ◦C resulted in the migration of Zr atoms toward the SiC layer and formation of cesium zirconate (Cs2ZrO3). However, annealing at a higher temperature (i.e., 1000 ◦C) resulted in the sublimation of Cs2ZrO3 and the formation of zirconium carbide (ZrC) in Zr-SiC samples, resulting in different Cs loss mechanisms than in SiC samples. Therefore, a Zr-SiC double layer may not be beneficial for maintaining Cs within the fuel structure when reactor operation temperatures approach the melting point of Cs2ZrO3, which is 1010 ◦C.
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- 2025| Nature PortfolioHighly oriented pyrolytic graphite (HOPG) structural changes caused by gallium (Ga) implantation at room temperature were investigated. Ga ions were implanted into HOPG at different energies (10, 20, and 30 keV) and fluences (ranging from 2×1015 to 5×1016Ga+/cm2). To monitor structural changes in the samples post-implantation, Raman spectroscopy was employed. The Raman spectra of the pristine HOPG sample displayed low-intensity D peaks at 1359 cm⁻1 and high-intensity G peaks at 1582 cm⁻1. After implantation with 10 keV at a fluence of 5×1016Ga+/cm2, a decrease in G peak intensity was observed, accompanied by an increase in its full width at half maximum (FWHM), indicating defect formation in the HOPG structure. In contrast, implantation with 30 keV at the same fluence (5×1016Ga+/cm2) resulted in the merging of the D and G peaks into a broad peak, signifying the amorphization of HOPG. These results confirm that ion energy plays a significant role in the amorphization of HOPG. Furthermore, implantation with 20 keV Ga ions at fluences≤2×1016Ga+/ cm2 introduced some defects in the HOPG structure, while higher fluences (≥4×1016Ga+/cm2) led to complete amorphization. After comparing the Raman results with the threshold displacement per atom (dpa) values calculated using the SRIM (Stopping and Range of Ions in Matter) software, it is evident that the HOPG used in this study required a very high dpa (exceeding 35 dpa) for complete amorphization, significantly exceeding the previously suggested range of 0.2 dpa to 3 dpa. The findings of this study align with very few prior results, where no amorphization was observed above 3 dpa. However, further research and testing are necessary to quantify the dpa required for HOPG amorphization.
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- Polycrystalline SiC samples were first implanted with 200 keV selenium (Se) ions at room temperature (RT) to a fluence of 1 ×1016 cm2. Thereafter, some of the pre-implanted SiC samples were separately implanted with helium (He) ions of 17 keV to a fluence of 1 ×1017 cm2 at RT and 500 ◦C. The samples were then annealed in a vacuum at 1000 ◦C for 5 h. Raman spectroscopy and transmission electron microscopy (TEM) were used to study the influence of Se and He co-implantation and annealing on the microstructure of SiC. Rutherford backscattering spectrometry (RBS) was used to study the migration behavior of Se in SiC before and after co-implantation and annealing. Implantation of Se at RT amorphized the SiC near the surface implanted region. Co-implantation at RT created a high portion of free carbon atoms in the damaged region accompanied by the formation of He nano- bubbles, while co-implantation at 500 ◦C led to some recrystallization of initially amorphous SiC and formation of larger He bubbles. Annealing at 1000 ◦C caused some recrystallization in both as-implanted (SiC implanted with Se only) and co-implanted samples. However, the recrystallization was accompanied by the formation of graphite crystals (with an average size of 10 nm) in the samples co-implanted at RT. This indicates that co- implantation at RT induces more detrimental effects on the physical integrity of SiC. RBS results showed no change in Se distribution in SiC after subsequent implantation with He ions at RT and 500 ◦C. However, annealing at 1000 ◦C caused the Se atoms to migrate towards the bulk of SiC in the co-implanted samples, while no significant migration of Se atoms was observed in the as-implanted samples (SiC implanted with Se only) annealed under the same conditions. This suggests that He assisted Se migration in SiC.
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- The effect of helium (He), silver (Ag) and strontium (Sr) ions implantation and annealing on the surface and structural properties of SiC as well as the migration of Ag and Sr was investigated in this study. Ag and Sr were sequentially implanted at 360 keV and 280 keV, respectively, each to a fluence of 2 × 1016 cm−2 at 600 °C. Some dual implanted (Ag + Sr-SiC) samples were additionally implanted with He of 17 keV to a 1 × 1017 cm−2 at 350 °C, forming triple-implanted samples (Ag + Sr + He-SiC). Both dual and triple implanted samples underwent isochronal annealing at 1100, 1200, and 1300 °C for 5 h. Ag, Sr and He implantation introduced defects in both dual and triple implanted samples. However, triple-implanted samples developed surface blisters and holes due to the migration of He bubbles. At 1100 °C, partial recovery of structural damage was observed in both dual- and triple-implanted samples, but graphite formed in the latter, and holes persisted. At higher temperatures (i.e., 1200 and 1300 °C), dual-implanted samples showed significant structural recovery, whereas the graphite in triple-implanted samples impeded the healing of defects. Depth profiling revealed minimal changes in Ag and Sr distributions and concentrations in dual-implanted samples post-annealing up to 1300 °C. However, triple-implanted samples lost ∼20 % of Ag and Sr at 1100 °C due to sublimation via holes. At 1200 and 1300 °C, no further losses occurred, but Ag and Sr migrated toward the surface. These findings suggest that He implantation promotes the formation of holes in SiC, facilitating the loss of Ag and Sr at 1100 °C. Additionally, He-induced defects enhance the migration of Ag and Sr toward the surface during annealing at 1200 and 1300 °C.
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