A Rutherford backscattering study on radiation damage and the diffusion of krypton implanted into 6H-SiC

Abstract

A pebble bed modular reactor (PBMR) is a modern type high-temperature gas-cooled nuclear reactor (HTGR). The fuels of the PBMR are in the form of small multi-layered spheres called triple-coated isotropic (TRISO) particles. A key feature of this PBMR technology is the entrapment of the fission products (FPs) within the TRISO particles. Silicon carbide (SiC) is used as the main layer in the TRISO particles. Given the sophistication of the TRISO design the release of silver (Ag) has motivated a thorough investigation concerning the ability of SiC to entrap other fission products. In this project volume diffusion of Kr in 6H-SiC was investigated under the influence of radiation damage. Kr (360 keV) ions were implanted into 6H-SiC at three different temperatures, i.e. room temperature, 350 oC and 600 oC, up to a fluence of 2×1016 ions/cm2. The radiation damage retained after implantation was assessed with the Rutherford backscattering technique in the channelling mode (RBS-C). Annealing of radiation damage and diffusion of the implanted Kr were investigated during isochronal annealing in the temperature range 1000 – 1500 oC in steps of 100 oC for 5 hours using RBS-C and Rutherford backscattering spectroscopy (RBS), respectively. The room temperature implantation amorphised the 6H-SiC to a depth of approximately 280 nm from the surface. This occurred because the thermal energy of the atoms at this temperature was not high enough to allow the displaced atoms to recombine with their designated lattice positions. The high temperature implantations did not amorphise the 6H-SiC. The implantation at these temperatures did, however, cause a distortion of the 6H-SiC because of the defects and/or defect clusters that were retained. The 350 oC implantation retained a high damage density as compared to the 600 oC implantation. The reason for the decrease in damage density with increasing temperature can be explained in terms of the thermal energy available for the atoms to move around in the SiC. A high temperature implies a higher mobility of the atoms thus increasing the probability of the displaced atoms to recombine with their designated lattice positions. Consequently, a slight diffusion of the Kr was also observed at the high temperature implantations relative to the room temperature implantation. The Kr depth profiles were broader for the high temperature implantation. Implantation at different temperatures caused different degrees of retained radiation damage in the SiC, consequently, isochronal annealing was done to assess the recovery of the SiC and also the diffusion of the implanted Kr inside the SiC under the different conditions. Epitaxial re-crystallisation from the amorphous-crystalline interface was observed after annealing the room temperature implanted sample at 1000 oC. However, no change in the Kr depth profile was observed after annealing at 1000 and 1100 oC for 5 hours. Annealing the same sample from 1100 to 1300 oC in steps of 100 oC for 5 hours did not result in any further epitaxial re-crystallisation. There, however, was a slight change of the SiC from the surface region at 1200 oC. The Kr depth profile started to broaden slightly at 1200 oC. An increased broadening was further observed at 1300 oC. In both instances the Kr depth profiles maintained an approximately Gaussian shape. This change in the Kr depth profile implies that there was Fickian type diffusion of the Kr at these temperatures. Annealing at 1400 oC resulted in a loss of about 30% of the Kr accompanied by a shift of the Kr depth profile towards the surface. These changes occurred simultaneously with the major epitaxial re-crystallisation of the SiC from the amorphous-crystalline interface. Further annealing at 1500 oC caused an additional loss of about 20% of the Kr accompanied by a pronounced shift towards the surface. This also occurred concurrently with the remarkable re-crystallization of the SiC. The Kr depth profile changes that occurred at 1400 and 1500 oC resulted in an asymmetric Kr profile and thus cannot be explained in terms of the Fickian diffusion process. The observed abrupt changes at 1400 and 1500 oC are consistent with the influence of thermal etching. This is because the thermal etching effect could have influenced the RBS spectrum and resulted in asymmetric depth profiles due the surface inhomogeneity. Unlike in the room temperature implantation case where the thermal energy had to be enough to allow (1) excess defects to escape the disordered region; (2) provide sufficient mobility to allow atomic re-ordering, and finally (3) allow for the formation of appropriate bonds, in the high temperature case there was a consistent decrease in the retained damage with each annealing cycle. Through-out the annealing cycles the 350 oC implantation retained more damage than the 600 oC implantation. In all the annealing instances there was no observable change in the Kr depth profiles implying that no diffusion took place despite the re-ordering of the displaced host atoms. The stability of the Kr atoms in their implanted positions is a possible contributor to the resistance of the SiC from returning to its virgin crystalline structure as observed through the RBS-C spectrum. This is because the Kr atoms exist as point defects in the SiC lattice thus causing the de-channelling of the He ions as they penetrate the SiC. This, in addition to the de-channelling from the extended defects, caused an increased backscattering spectrum from the host atoms. Thorough-out the entire isochronal annealing experiments in the temperature range 1000 – 1500 oC the 6H-SiC retained all of the implanted Kr.