Diffusion of silver in 6H-SiC

SiC is used as the main diffusion barrier in the fuel spheres of the pebble bed modular reactor (PBMR). The PBMR is a modern high temperature nuclear reactor. However, the release of silver from the fuel spheres has raised some doubts about the effectiveness of this barrier, which has led to many studies on the possible migration paths of silver. The reported results of these studies have shown largely differing results concerning the magnitude and temperature dependence of silver being transported through the fuel particle coatings. Results from earlier investigations could be interpreted as a diffusion process governed by an Arrhenius type temperature dependence. In this study, the silver diffusion in 6H-SiC was investigated using two methods. In the first method a thin silver layer was deposited on 6H-SiC by vapour deposition while in the second method silver was implanted in 6H-SiC at room temperature, 350°C and 600°C to a fluence of 2×1016 silver ions cm-2. Finally the effect of neutron irradiation on the diffusion of silver was investigated for the samples implanted at 350°C and 600°C. Silver depth profiles before and after annealing were determined by Rutherford backscattering (RBS). Both isothermal and isochronal annealing were used in this study. Diffusion coefficients as well as detection limits were extracted by comparing the silver depth profiles before and after annealing. The radiation damage after implantation and their recovery after isothermal and isochronal annealing were analysed by Rutherford backscattering spectroscopy combined with channelling. The results of in-diffusion of silver into 6H-SiC at temperatures below the melting point (960°C) using un-encapsulated 6H-SiC samples with 100 nm deposited silver indicated no in-diffusion of silver; however, disappearance of silver occurred at these temperatures. For the encapsulated samples, no in-diffusion of silver was observed at 800°C, 900°C and 1000°C but silver disappeared from the samples’ surface and was found on the walls of the quartz glass ampoule. This disappearance of silver was established to be due to the wetting problem that existed between silver and SiC. The room temperature implantation resulted in a completely amorphous surface layer of approximately 270 nm thick. Epitaxial re-growth from the bulk was already taking place during annealing at 700°C and the crystalline structure seemed to be fully recovered at 1600°C, for samples that were sequentially isochronally annealed from 700°C in steps of 100°C up to 1600°C. However, no silver signal was detected at this temperature, which left certain doubts regarding the crystalline structure of the samples at this temperature. This was speculated to be due to thermal etching of the top original amorphous layer while the deeper amorphous layer was epitaxial re-growth from the bulk. The decomposition of SiC, giving rise to a carbon peak in the RBS spectra due to evaporation of Si, was clearly observed on the same samples at 1600°C. Isothermal annealing at 1300°C for 10 h cycles up to 80h caused epixatial re-growth from the bulk during the first annealing cycle (10h). No further epitaxial re-growth from the bulk was observed up to 80h. This was believed to be due to the amorphous layer re-crystallising into crystals that were randomly oriented to the 6H-SiC substrate. No diffusion of silver was observed at temperatures below 1300°C but silver seemed to form precipitates at these temperatures. Diffusion of silver towards the surface accompanied by silver loss from the surface began at 1300°C and was very high at 1400°C, with silver profiles becoming asymmetric and closer to the surface. The loss of silver was already taking place at 1100°C. This loss was found to be due to the following: diffusion of silver towards the surface; the mass flow of silver via holes that were observed to be becoming larger with higher annealing temperatures on SiC surfaces and thermal etching of SiC. Isothermal annealing at 1300°C for 10h up to 80h caused diffusion of silver during the first annealing cycle, while no further diffusion was observed for any further annealing at the same temperature up to 80 h. The diffusion coefficient was not calculated due to the lack of information on the structural evolution of SiC during the first annealing cycle. Isothermal annealing at 1300°C and 1350°C for 30 minute cycles up to 120 minutes caused high diffusion during the first cycle and reduced diffusion during the second cycle, while no diffusion was observed for any further annealing longer than the second cycle. The higher diffusion during the first 30 minutes was due to ion induced amorphization. The diffusion of silver in amorphised SiC was measured at different temperatures in the range 1300°C to 1385°C and yielded to Do ~ 1.4 × 10-12 m2s-1 and Ea ~ 3.3 × 10-19 J. These values were found to be approximately the same as the values of silver diffusion in polycrystalline CVD-grown SiC found by our group which were due to grain boundary diffusion: Do ~ 4×10-12 m2 s-1 and Ea ~ 4×10-19 J. Implantation of silver at 600°C retained crystallinity although distortions occurred in the implanted region while implantation at 350°C also retained crystallinity but more distortions occurred as compared to silver implanted at 600°C. This was caused by the fact that at 600°C, the displaced atoms were more mobile because of their higher thermal energy than at 350°C. The higher thermal energy increased the probability of the displaced atoms combining with their original lattice sites. Annealing of these samples at 1300°C, 1350°C and 1500°C caused the annihilation of some defects but certain others were retained. No diffusion of silver was observed during annealing of the samples (implanted at 350°C and at 600°C) at 1300°C, 1350°C and 1500°C but silver moved towards the surface at 1500°C. The upper limit of the diffusion coefficient of D < 10-21 m2s-1 was obtained at 1300°C. The movement of silver towards the surface was found to be due to thermal etching at 1500°C. Neutron irradiation of these samples caused no silver diffusion but silver -110mAg, due to -109Ag capturing a neutron during neutron irradiation, was detected in the samples.Thesis (PhD)--University of Pretoria, 2010.Physicsunrestricte

Ion beam effects of 26.0 MeV Cu7+ ions in thin metallic and insulating films during heavy ion ERDA measurements

We report on an investigation carried out to determine effects of the probing beam on the structure of typical metallic and insulating thin films during Elastic Recoil Detection Analysis (ERDA) using a heavy ion beam. Metallic niobium nitride (NbN) and insulating calcium fluoride (CaF2) thin films (used as test samples) were irradiated by 26.0 MeV 63Cu7+ ions to fluences of 1.70 1014 ions/cm2 and 2.70 1014 ions/cm2, respectively. The effects of irradiation on the structural properties of the films were studied using Rutherford Backscattering Spectrometry (RBS), X-ray diffraction (XRD) and Atomic Force Microscopy (AFM). RBS results showed a significant (18%) reduction in the thickness of the CaF2 film due to electronic sputtering compared to only 1% reduction in the NbN film. XRD results showed no significant peak shifts in both films, but rather formation of unidentified peaks in the insulating film. AFM results indicated a substantial decrease in the average surface roughness of the insulating film and only a nominal increase in that of the metallic film. Results of electronic sputtering yield measurements carried out by ERDA are explained in terms of both the Coulomb explosion and the inelastic thermal spike models.NECSA, Tshwane University of Technology, University of Pretoria and iThemba LABS Gauteng.http://www.elsevier.com/locate/nimb2016-04-30hb201

The effect of annealing time on the structural and optical properties of ZnAl2O4:0.01% Cr3+ nanophosphor prepared via sol-gel method

Zinc aluminate (ZnAl2O4) host and 0.01% Cr3+ doped were successfully prepared using the sol–gel method. The annealing time (AT) was varied in the range of 0.5–19 h. The x-ray diffraction results showed that the AT does not affect the crystal structure of the prepared powders. Scanning electron microscopy (SEM) results showed that the morphology of the prepared nanophosphors was influenced by the AT. Energy dispersive x-ray spectroscopy (EDS) confirmed the homogeneous distribution of the constituent elements. Transmission electron microscopy (TEM) suggested that the average crystallites sizes of the ZnAl2O4 to be ~20 nm. Ultraviolet–visible (UV–Vis) spectroscopy results revealed that the bandgap (E g) of the prepared nanophosphor can be tuned by varying the AT. The emission peak at 390 nm is attributed to the intrinsic defects within the host material bandgap. The emission peak at 572 nm is attributed to both contribution from the host and Cr3+ (4T1 → 4A2) transition. The maximum PL intensity was observed from the samples annealed for 3 h. The International Commission on Illumination (CIE) chromaticity diagram showed a slight shift on the blue emission with an increase in AT.http://link.springer.com/journal/116642019-01-30hj2018Physic

The nature of surface defects in Xe ion-implanted glassy carbon annealed at high temperatures: Raman spectroscopy analysis

Please read abstract in the article.The National Research Foundation (NRF) of South Africa via iThemba LABS Materials Research Department (MRD) and University of Pretoria (South Africa).http://www.elsevier.com/locate/apsuschj2021Physic

Influence of radiation damage on diffusion of fission products in silicon carbide

The influence of irradiation induced damage on the transport of implanted species in poly and single crystalline silicon carbide is investigated. For this purpose published diffusion results of strontium, silver, iodine and cesium are compared with the associated evolution of defect profiles determined by α-particle channelling in a backscattering geometry. Strong diffusion takes place in the amorphized surface layer of room temperature implanted 6H-SiC during annealing at 1100 °C, which drops below the detection limit of 10-21 m2 s-1 as soon as re-crystallization is completed. Diffusion in samples implanted above the critical amorphization temperature is only observed when simultaneously a significant reduction of defect density occurs. No diffusion into the undamaged bulk is detected at temperatures up to 1500 °C. The observed diffusion behaviour is explained by a defect related trapping and release mechanism. Normal grain boundary diffusion of silver and iodine occurs in CVD-SiC.National Research Foundation and the Bundesministerium für Bildung und Forschunghttp://onlinelibrary.wiley.com/journal/10.1002/(ISSN)1610-1642ahb201

The effect of thermal annealing in a hydrogen atmosphere on tungsten deposited on 6H-SiC

Tungsten (W) film was deposited on a bulk single crystalline 6HeSiC substrate and annealed in H2 ambient at temperatures of 700 C, 800 C and 1000 C for 1 h. The resulting solid-state reactions, phase composition and surface morphology were investigated by Rutherford backscattering spectrometry (RBS), grazing incidence X-ray diffraction (GIXRD) and scanning electron microscopy (SEM) analysis techniques. These results are compared with the vacuum annealed results reported in our earlier work. As-deposited RBS results indicated the presence of W and O2 in the deposited thin film, the GIXRD showed the presence ofW,WO3, W5Si3 andWC. RBS results indicated the interaction betweenWand SiC was accompanied by the removal of oxygen at 700 C. The GIXRD analysis indicated the presence of W5Si3 and WC in the samples annealed at 700 C. At temperatures of 800 C and 1000 C, Wannealed in a H2 ambient further reacted with the SiC substrate and formed a mixed layer containing silicide phases and carbide phases, i.e.W5Si3, WSi2, WC and W2C. The SEM micrographs of the as-deposited samples indicated the W thin film had a uniform surface with small grains. Annealing at 800 C led to the agglomeration of W grains into clusters making the surface rough.National Research Foundation (NRF) (Grant number: 88661), South Africa.http://www.journals.elsevier.com/vacuum2017-07-30hb2016Physic

The new Heavy Ion ERDA set up at iThemba LABS Gauteng : multilayer thin film depth profiling using direct calculation and Monte Carlo simulation codes

We report here on the recently built Heavy Ion ERDA set up at iThemba LABS Gauteng; describing a typical application in the study of interfacial reactions in an Al2O3–Ti ceramic–metal multilayer structure annealed in vacuum at 800 C for 2 h. Depth profile extraction was found to be best obtained through combined use of direct calculation and Monte Carlo simulation codes as opposed to using just either of the methods. The obtained profile suggests a case of the Kirkendall effect, whereupon the inter-diffusion between the metal and the ceramic was largely due to the faster diffusion of the metal into the amorphous ceramic than diffusion of the ceramic elements into the metallic layer.National Research Foundation (iThemba LABS) and the International Atomic Energy Agency (IAEA).http://www.elsevier.com/locate/nimbhb201

Effects of annealing temperature on the structure and photoluminescence properties of the ZnO/ZnAl2O4/Ca5Al6O14/Ca3Al4ZnO10:0.1% Ce3+ mixed-phases nanophosphor prepared by citrate sol–gel process

Please read abstract in the article.The South African National Research Foundation (NRF) Thuthuka Programme and NRF incentive funding for rated researchers (IPRR).https://link.springer.com/journal/3392021-09-28hj2021Physic

Solid-state reactions between iridium thin films and silicon carbide in the 700 °C to 1000 °C temperature range

DATA AVAILABILITY : Data will be made available on request.Please read abstract in the article.The National Research Foundation (NRF) of South Africa.https://www.elsevier.com/locate/mtcommhj2024PhysicsNon

Influence of radiation damage on krypton diffusion in silicon carbide

Diffusion of krypton in poly and single crystalline silicon carbide is investigated and compared with the previously obtained results for xenon, which pointed to a different diffusion mechanism than observed for chemically active elements. For this purpose 360 keV krypton ions were implanted in commercial 6H-SiC and CVD-SiC wafers at room temperature, 350 °C and 600 °C. Width broadening of the implantation profiles and krypton retention during isochronal and isothermal annealing up to temperatures of 1400 °C was determined by RBS-analysis, whilst in the case of 6H-SiC damage profiles were simultaneously obtained by α- particle channelling. Little diffusion and no krypton loss was detected in the initially amorphized and eventually recrystallized surface layer of cold implanted 6H-SiC during annealing up to 1200 °C. Above that temperature thermal etching of the implanted surface became increasingly important. No diffusion or krypton loss is detected in the hot implanted 6H-SiC samples during annealing up to 1400 °C. Radiation damage dependent grain boundary diffusion is observed at 1300 C in CVD-SiC. The results seem to indicate, that the chemically inert noble gas atoms do not form defect-impurity complexes, which strongly influence the diffusion behaviour of other diffusors in silicon carbide.National Research Foundation of South Africa.http://www.elsevier.com/locate/nimb2016-07-31hb201