Diffusion of ion implanted iodine in 6H-SiC

The diffusion of iodine implanted 6H-SiC has been investigated using Rutherford backscattering Spectrometry (RBS). SiC is used as the main barrier in the modern high temperature gas cooled reactors. An understanding of the transport behaviour of iodine in 6H-SiC will shed some light into SiC’s effectiveness in the retention of fission products. Room temperature iodine implantations were performed to a fluence of 1 x 1016 iodine ions per cm2. Iodine depth profiles were determined by Rutherford backscattering spectroscopy. Previous studies indicate that iodine diffusion cannot be detected for temperatures below 1000oC. Isochronal annealing experiments for 5 hours cycles were then performed starting from 1000oC. 5 hour annealing cycles at 1200oC and 1300oC were also performed. These relatively short annealing cycles were used to study the evolution of the iodine depth profiles and to determine the temperature where the first noticeable broadening of the iodine implanted profile is observed. Broadening of the iodine profile was initially observed at temperatures around 1200oC. Isothermal annealing cycles were then performed at this temperature of 1200oC where the broadening was first observed. The annealing experiments were performed for a total annealing time of 60 hours. Isothermal annealing experiments were also performed at a lower temperature of 1100oC also for a total annealing time of 60 hours. The experimental data was analyzed by fitting the iodine depth profiles to a Gaussian function using the GENPLOT program. Diffusion coefficients were obtained by comparing the full widths at half maximum, (FWHM) of the iodine profiles before and after annealing. Results for 1100oC show that the diffusion coefficient is below 10-21 m2 s-1 . Solving the diffusion equation numerically for this diffusion coefficient shows that the iodine does not traverse more than 2 ìm through SiC after annealing for one year. Intact SiC coatings which have a thickness of 35 ìm in the fuel particles will therefore be able to prevent iodine release from the particle at this temperature. Results for 1200oC show that the diffusion coefficient is in the order of 10-20 m2 s-1. The results also reveal that iodine loss through the front surface is relatively low with only 15% of iodine loss observed after annealing for a total time of 60 hours. Further indications of SiC ability to retain iodine are observed when an iodine peak is still present after annealing for 30 hours at a temperature of 1500oC. Radiation damaged produced during ion implantation was investigated by Rutherford backscattering in conjunction with channeling. The results of the RBS/channeling reveal that the room temperature implantations produce an amorphous layer from the surface up to a depth of 260 nm. Some epitaxial regrowth is observed starting at approximately 1000oC.Dissertation (MSc)--University of Pretoria, 2011.Physicsunrestricte

Expenditure analysis and planning in a changed economy: a case study approach of Gweru City Council, Zimbabwe

The purpose of this study is to analyse Gweru City Council`s spending pattern and behaviour and to determine if this spending pattern is directed towards poverty reduction and economic development or not. Furthermore, to fit a log-differenced regression model to a historical financial dataset obtained from Gweru City Council Finance Department for the time period July 2009 to September 2012. Regression techniques were used to determine how Gweru City Council`s total income (dependent variable) is affected by its expenditure (independent variables). Econometric modeling techniques were employed for the evaluation of estimate tests, conducted to determine the reliability of the estimated model. The study concludes by providing some recommendations for possible financial plans which could be adopted by Gweru City Council and other local authorities in Zimbabwe for the well-being of Zimbabweans and economic development

South African Fast Food Outlets Contribution Towards Curbing Obesity: Managements’ Perspective

The aim of this study was to determine what the fast food restaurants in the Gauteng region are doing to combat obesity. Obesity is a global epidemic, with figures rising at an alarming rate every year. The food service industry has contributed significantly to the rise in obesity rates worldwide. Purposive sampling technique was utilised. From the target population selected, one manager per establishment was chosen to participate in the survey (one manager x 5 fast food outlets x 6 branches = 30 managers in total). The study used a quantitative research approach, utilising self-administered surveys which were distributed by the researchers. Summarily, the study observed that there was a statistically significant difference (p<0.05) among the managers regarding knowledge of healthy meals. Managers of two of the fast foods had better knowledge of healthy meals than managers of the other three fast foods. A lot still needs to be done across all sectors of the food industry. The South African government, specifically the Departments of Health and Tourism, should collaborate to develop regulations regarding inclusion of nutritional information on menus. Increasing fast food restaurant employees' awareness of the obesity epidemic can have a long-term impact because they will be able to easily promote healthier eating if they are better informed. The findings revealed that the managers agreed that more regulations were needed to help reduce obesity

Effect of Xe ion (167 MeV) irradiation on polycrystalline SiC implanted with Kr and Xe at room temperature

The effect of swift heavy ion (Xe 167 MeV) irradiation on polycrystalline SiC individually implanted with 360 keV Kr and Xe ions at room temperature to fluences of 2×1016 cm-2 and 1×1016 cm-2 respectively, was investigated using transmission electron microscopy (TEM), Raman spectroscopy and Rutherford backscattering spectrometry (RBS). Implanted specimens were each irradiated with 167 MeV Xe+26 ions to a fluence of 8.3×1014 cm-2 at room temperature. It was observed that implantation of 360 keV Kr and Xe ions individually at room temperature amorphized the SiC from the surface up to a depth of 186 and 219 nm respectively. Swift heavy ion (SHI) irradiation reduced the amorphous layer by about 27 nm and 30 nm for the Kr and Xe samples respectively. Interestingly, the reduction in the amorphous layer was accompanied by the appearance of randomly oriented nanocrystals in the former amorphous layers after SHI irradiation in both samples. Previously, no similar nanocrystals were observed after SHI irradiations at electron stopping powers of 33 keV/nm and 20 keV/nm to fluences below 1014 cm-2. Therefore, our results suggest a fluence threshold for the formation of nanocrystals in the initial amorphous SiC after SHI irradiation. Raman results also indicated some annealing of radiation damage after swift heavy ion irradiation and the subsequent formation of small SiC crystals in the amorphous layers. No diffusion of implanted Kr and Xe was observed after swift heavy ion irradiation.National Research Foundation (NRF)http://iopscience.iop.org0022-37272016-10-20hb201

Iodine assisted retainment of implanted silver in 6H-SiC at high temperatures

The effect of high temperature thermal annealing on the retainment and diffusion behaviour of iodine (I) and silver (Ag) both individually and co-implanted into 6H-SiC has been investigated using RBS, RBS-C and heavy ion ERDA (Elastic Recoil Detection Analysis). Iodine and silver ions at 360 keV were both individually and co-implanted into 6H-SiC at room temperature to fluences of the order of 1 1016 cm 2. RBS analyses of the as-implanted samples indicated that implantation of Ag and of I and co-implantation of 131I and 109Ag at room temperature resulted in complete amorphization of 6H-SiC from the surface to a depth of about 290 nm for the co-implanted samples. Annealing at 1500 C for 30 h (also with samples annealed at 1700 C for 5 h) caused diffusion accompanied by some loss of both species at the surface with some iodine remaining in the iodine implanted samples. In the Ag implanted samples, the RBS spectra showed that all the Ag disappeared. SEM images showed different recrystallization behaviour for all three sets of samples, with larger faceted crystals appearing in the SiC samples containing iodine. Heavy Ion ERDA analyses showed that both 109Ag and 131I remained in the co-implanted SiC samples after annealing at 1500 C for 30 h. Therefore, iodine assisted in the retainment of silver in SiC even at high temperature.National Research Foundation (NRF)http://www.elsevier.com/locate/nimbhb201

SEM analysis of ion implanted SiC

SiC is a material used in two future energy production technologies, firstly as a photovoltaic layer to harness the UV spectrum in high efficient power solar cells, and secondly as a diffusion barrier material for radioactive fission products in the fuel elements of the next generation of nuclear power plants. For both applications, there is an interest in the implantation of reactive and non-reactive ions into SiC and their effects on the properties of the SiC. In this study 360 keV Ag+, I+ and Xe+ ions were separately implanted into 6H–SiC and in polycrystalline SiC at various substrate temperatures. The implanted samples were also annealed in vacuum at temperatures ranging from 900 C to 1600 C for various times. In recent years, there had been significant advances in scanning electron microscopy (SEM) with the introduction of an in-lens detector combined with field emission electron guns. This allows defects in solids, such as radiation damage created by the implanted ions, to be detected with SEM. Cross-sectional SEM images of 6H–SiC wafers implanted with 360 keV Ag+ ions at room temperature and at 600 C and then vacuum annealed at different temperatures revealed the implanted layers and their thicknesses. A similar result is shown of 360 keV I+ ions implanted at 600 C into 6H–SiC and annealed at 1600 C. The 6H–SiC is not amorphized but remained crystalline when implanting at 600 C. There are differences in the microstructure of 6H–SiC implanted with silver at the two temperatures as well as with reactive iodine ions. Voids (bubbles) are created in the implanted layers into which the precipitation of silver and iodine can occur after annealing of the samples. The crystallinity of the substrate via implantation temperature caused differences in the distribution and size of the voids. Implantation of xenon ions in polycrystalline SiC at 350 C does not amorphize the substrate as is the case with room temperature heavy ion bombardment. Subsequent annealing of the implanted polycrystalline samples leads to increased thermal etching effects such as grain boundary grooving. Damage due to channelling (or non-channelling) in the different crystallites resulted also in differences in thermal etching in the crystallites.http://www.elsevier.com/locate/nimbhb201

Scanning electron microscopy of the surfaces of ion implanted SiC

This paper gives a brief review of radiation damage caused by particle (ions and neutrons) bombardment in SiC at different temperatures, and its annealing, with an expanded discussion on the effects occurring on the surface. The surface effects were observed using SEM (scanning electron microscopy) with an inlens detector and EBSD (electron backscatter diffraction). Two substrates were used, viz. single crystalline 6H-SiC wafers and polycrystalline SiC, where the majority of the crystallites were 3C-SiC. The surface modification of the SiC samples by 360 keV ion bombardment was studied at temperatures below (i.e. room temperature), just at (i.e. 350 C), or above (i.e. 600 C) the critical temperature for amorphization of SiC. For bombardment at a temperature at about the critical temperature an extra step, viz. postbombardment annealing, was needed to ascertain the microstructure of bombarded layer. Another aspect investigated was the effect of annealing of samples with an ion bombardment-induced amorphous layer on a 6H-SiC substrate. SEM could detect that this layer started to crystalize at 900 C. The resulting topography exhibited a dependence on the ion species. EBSD showed that the crystallites forming in the amorphized layer were 3C-SiC and not 6H-SiC as the substrate. The investigations also pointed out the behaviour of the epitaxial regrowth of the amorphous layer from the 6H-SiC interface.http://www.elsevier.com/locate/nimb2016-07-31hb201

Modelling of South African Hypertension: Application of Panel Quantile Regression

Hypertension is one of the crucial risk factors for morbidity and mortality around the world, and South Africa has a significant unmet need for hypertension care. This study aims to establish the potential risk factors of hypertension amongst adults in South Africa attributable to high systolic and diastolic blood pressure over time by fitting panel quantile regression models. Data obtained from the South African National Income Dynamics Study (NIDS) Household Surveys carried out from 2008 to 2018 (Wave 1 to Wave 5) was employed to develop both the fixed effects and random effects panel quantile regression models. Age, BMI, gender (males), race, exercises, cigarette consumption, and employment status were significantly associated with either one of the BP measures across all the upper quantiles or at the 75th quantile only. Suggesting that these risk factors have contributed to the exacerbation of uncontrolled hypertension prevalence over time in South Africa

Diffusion and surface effects of SiC implanted with fission product elements

The diffusion and surface effects of several fission product elements implanted in SiC have been investigated. SiC is used as the main barrier for fission products in modern high temperature gas cooled reactors. An understanding of the transport behaviour of the implanted ions and their interactions with SiC will shed some light into SiC‟s effectiveness in the retention of fission products. The diffusion behavior of cesium implanted into SiC was investigated by isochronal and isothermal annealing methods up to temperatures of 1500 ºC. Ion implantation was performed at three temperatures, which were room temperature, 350 and 600 ºC. The implantation fluences in all cases were in the order of 1016 ions per cm2. The implantation depth profiles before and after annealing were determined by Rutherford backscattering spectroscopy (RBS). The cesium diffusion results were compared with results from our previous studies on the diffusion behaviour of atoms of iodine, silver, strontium and xenon in SiC. Amorphisation of the SiC was observed for room temperature implantations but not for implantations at 350 ºC and at 600 ºC. A strong temperature dependence of irradiation induced diffusion was observed for the cesium implanted samples. For room temperature implanted samples, isochronal annealing cycles showed that almost 50% of the implanted cesium is lost after the first annealing cycle. This behavior was similar to previous results on the loss of strontium during the first annealing cycle and to some extent previous data on silver loss where also some loss was observed after the first annealing cycles. However this behavior was in sharp contrast to the diffusion behaviour of iodine and xenon where no losses were observed after similar annealing cycles. About 25 % of the cesium is lost in the case of the samples implanted at 350 ºC while no loss of implanted cesium is observed for samples annealed at 600 ºC. For new high temperature isochronal annealing studies of silver and iodine room temperature implanted samples, complete loss of the implanted silver was observed after annealing at 1500 ºC for 30 hours. In the case of the iodine implanted samples under the same conditions, a significant amount of iodine was retained. These results sho These results shoThese results shoThese results shoThese results shoThese results sho These results sho These results sho These results sho wed that silver wed that silverwed that silverwed that silver wed that silver wed that silver wed that silver diffuses faster diffuses faster diffuses faster diffuses faster diffuses faster diffuses faster diffuses faster diffuses faster through the initially amo through the initially amothrough the initially amothrough the initially amo through the initially amo through the initially amo through the initially amothrough the initially amo through the initially amothrough the initially amothrough the initially amothrough the initially amothrough the initially amorphised SiC than iodine. rphised SiC than iodine. rphised SiC than iodine. rphised SiC than iodine. rphised SiC than iodine. rphised SiC than iodine. rphised SiC than iodine. rphised SiC than iodine. rphised SiC than iodine. RBS results also showed that thermal decomposition of SiC at these temperatures due to the sublimation of silicon leaving a carbon layer on the surface appears to be more drastic in the case of the silver implanted samples as compared to the iodine implanted samples. Iodine appears to have some effect in slowing down the decomposition of SiC. This result was confirmed by Scanning Electron Microscopy (SEM) measurements. SEM also revealed that the surfaces of silver implanted samples contained more cavities than the iodine implanted samples after the annealing cycles. The thermal decomposition was also investigated by Raman spectroscopy. Raman spectroscopy was performed using two laser wavelength excitation regimes. These were the 514 nm laser (visible region) and the 244 nm laser (deep ultraviolet region, DUV). Raman spectra show that the surface region of SiC at 1600 ºC had completely decomposed and the top surface layer was now covered by a carbon layer. Raman measurements with laser in the visible region showed that the extent of decomposition at 1600 ºC was greater for the silver implanted samples than for the iodine implanted samples. When ions of both elements (iodine and silver) were co-implanted in the same wafer and annealed under the same conditions as the single element implanted samples, iodine was observed to assist in the retention and trapping of silver ions. This was observed via Secondary Ion Mass Spectrometry (SIMS), Heavy Ion Elastic Recoil Detection Analysis (HI-ERDA), and Atom Probe Tomography (APT).Thesis (PhD)--University of Pretoria, 2015.tm2015PhysicsPhDUnrestricte

Predictors of High Blood Pressure in South African Children: Quantile Regression Approach

Objective: To identify predictors of blood pressure (BP) in children and explore the predictors` effects on the conditional quantile functions of systolic blood pressure and diastolic blood pressure.Methods: A secondary data analysis was performed using data from the South African National Income Dynamics Study (2014-2015). From this particular secondary data, data for children aged between 10-17 years were extracted for analysis. The variables used in the study were systolic blood pressure (SBP), diastolic blood pressure (DBP), body mass index (BMI), age, smoking, alcohol consumption, exercises, gender and race. Two parameter estimation methods were used, ordinary least squares (OLS) and quantile regression (QR).Results: BMI had positive statistically significant estimated OLS and conditional quantile functions with both the BP measures except the 95th quantile for SBP. Age had also positive statistically significant estimated OLS and QR coefficients except for the 95th percentile, with both DBP and SBP respectively. Gender was found to be inversely related to both DBP and SBP except the 10th quantile for DBP. Race was partially significant to DBP. Smoking, alcohol consumption and exercises did not present any statistically significant relations with both DBP and SBP for all the estimated OLS and QR coefficients.Conclusion: BMI, age, gender and partially race were found to be predictors of BP in South African children using both OLS and QR techniques. Exercises, smoking and alcohol consumption did not present any statistically significant relations with both DBP and SBP probably because few participants exercise regularly, smoke and drink alcohol to bring out a significant change in both BP measurements