Diffusion of implanted europium in glassy carbon

Abstract

In my PhD study glassy carbon samples were implanted with 250 keV europium ions at room temperature and at 100 oC. The main purpose of my study was to study the diffusion properties of the europium in the glassy carbon. After implantation the samples were investigated by several techniques to determine the damage created by the ion bombardment processes, the distribution of the europium atoms and possible reactions between the implanted europium and the glassy carbon substrate. The samples were also isochronally annealed in vacuum for a wide ranges of temperatures and again investigated after each heat treatment. The following heat treatments were done. Room temperature europium implanted samples were annealed at 200-1000 oC for 1 h. Another set of room temperature implanted samples were annealed at 300-500 oC for 2 h. 100 oC implanted samples were annealed at 200-1000 oC for 1 h. These samples then were analysed to investigate the annealing of the initial damage, the diffusion of the implanted europium and possible reactions between the europium and glassy carbon induced by the ion bombardment and annealing. The analytical techniques used in this study were RBS (Rutherford backscattering spectrometry), Raman spectroscopy and SEM (scanning electron microscopy). The diffusion and phase formation behavior of the europium ions were investigated with Rutherford backscattering spectrometry (RBS), i.e. to see under the various high temperatures used, how much these ions diffused. Raman spectroscopy was used to study the structural changes in the glassy carbon due to ion bombardment and annealing. The diffusion coefficient (D) values for room temperature implanted sample which was annealed for 1 h at 200 and 300 oC were 2.3 x 10 -19 and 1.2 x 10 -19 m2/s, respectively. Annealing at 400 oC led to a loss of the implanted europium of about 30%. However increasing the temperature did not change this value and the retained europium in the glassy carbon samples was about 70% compared to the as-implanted samples for this range of temperature (400-1000 oC). The diffusion coefficient for room temperature implanted europium and then annealed for 2 h is the same that for 1 h. There was no europium lost at 300-400 oC, while for the annealed sample from 425-500 oC for 2h, the europium loss was about 25-45 %. The diffusion coefficients for the 100 oC implanted sample and then annealed at 200 and 300 oC were 1.24 x 10 -19 and 1.9 x 10 -19 m2/s, respectively. Good fitting of the experimental depth profiles to theory was obtained for the samples annealed from 300-700 oC indicating that the diffusion was Fickian. The depth profiles for the samples annealed from 800-1000 oC could no longer be fitted to the solution to the Fick diffusion differential equation. However, there was still europium migration to the surface resulting in europium loss reaching about 50% at 1000 oC which is higher than the europium melting point (Tmelting = 826 oC). SEM images showed the topography of the surfaces due to ion bombardment and annealing. Furthermore, it also helped to understand the changes in morphology of samples (i.e. to see how much the bonding of materials is changed). There was also evidence of either europium oxide or carbide formation. The appearance of europium on the surface was due to diffusion of europium to the glassy carbon surface. Raman results showed that ion implantation caused the two distinct D and G peaks of pristine glassy carbon to merge together into a broad peak. This is an indication that ion implantation caused amorphisation of implanted layer of the glassy carbon. By annealing the implanted sample to 1000 °C resulted in only partial recovery of the glassy carbon structure and it did not reach the pristine glassy carbon structure. This is due to retained radiation damage even after annealing at temperatures higher than the melting point of europium i.e. 826 oC. Finally, all the results were analyzed to make a recommendation about the suitability of glassy carbon as a storage materials for one of the important fission products, i.e. europium. The diffusion/segregation of europium to the surface to surface, amorphisation of glassy carbon after ion implantation are the positive aspect of the research. These aspects were resulted in that glassy carbon might be taken into a good nuclear storage material.