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The behaviour of iodine, before and after annealing, in pyrolytic carbon (PyC) has been studied using the Rutherford backscattering spectrometry (RBS), X-ray diffraction (XRD) and scanning electron microscopy (SEM). PyC is used as a coating material in the design of the nuclear fuels such as the TRISO particles. TRISO particles are used to produce nuclear energy in nuclear reactors such as the PBMR. Iodine is one of the radioactive fission products produced during the production of nuclear energy by the nuclear fuels. The PyC layers in TRISO particles acts as a barrier for fission products. The main aim of this study was to investigate the effectiveness of PyC as a barrier of iodine diffusion. 360 keV iodine ions were implanted into the PyC to a fluence of 1×1015 iodine ions per cm2, at room temperature. After implantation the PyC samples were annealed (in vacuum) isochronally at 900 °C, 1000 °C, 1100 °C and 1200 °C; all for 9 hours. XRD measurements were performed using a cobalt (Co) XRD spectrometer on θ-2θ configuration; before and after the annealing temperatures. In-lens SEM images were also obtained for the PyC samples before and after heat treatment. RBS measurements were performed using a Van de Graaff accelerator and a 1.6 MeV He+ beam. XRD and SEM were used to study the structure of PyC before and after the annealing. Ion implantation and high temperature treatment affected the structure of PyC. Literature has shown that ion implantation caused structural damages in the implanted region of the PyC, reducing the degree of preferred orientation of the graphitic layers of the PyC in that region. The XRD results showed that high temperature annealing caused an increase in the degree of preferred orientation of the graphitic layers in the PyC used which is in agreement with other studies. Comparison of our SEM results to other reports has shown that the PyC used has a structure similar or close to the laminar structures of PyCs, with medium to high degree of preferred orientation. RBS was used to produce the iodine depth profiles, which provided insight on implantation depth of iodine in PyC as well as iodine behaviour after annealing. The results showed that iodine concentration in the implanted profile decreased with increasing annealing temperature. The full width at half maximum (FWHM) of the iodine profiles, obtained using the computer program Genplot, showed that after heat treatments of 900 °C, 1000 °C and 1100 °C more iodine initially diffused deeper into the PyC bulk than towards the PyC surface. At 1200 °C, the diffusion of iodine towards the PyC surface increased. It was proposed that this iodine behaviour was associated with the changes in the PyC structure due to ion implantation and high temperature treatment of the PyC. The obtained iodine profiles and the corresponding FWHM did not show evidence that the diffusion of iodine in both directions could be attributed to Fickian diffusion mechanism; hence no activation energy for the iodine diffusion in laminar PyC was determined. Copyright / Dissertation (MSc)--University of Pretoria, 2012. / Physics / unrestricted

This thesis concerns method development of new synthetic routes by applying electrophilic hypervalent iodine reagents, such as trifluoromethyl-benziodoxole (Togni reagent) and fluoro-benziodoxole. The first project involved the addition of an oxygen moiety and trifluoromethyl group across double and triple bonds (both groups derived from the hypervalent iodine reagent). We observed that electron donating substituents on the aromatic ring of the substrate accelerated the oxytrifluoromethylation reaction. This transformation was further expanded to halo-trifluoromethylation reaction of a vinyl silane substrate. We also developed a copper mediated cyanotrifluoromethylation reaction, which was accelerated by PCy3 additive. This transformation allowed for the creation of two new C-C bonds in a single addition reaction. The direct C-H trifluoromethylation reaction of quinones was achived using the Togni-reagent in the presence of B2pin2 additive. The intriguing additive effects of both B2pin2 and PCy3 inspired us to examine the mechanism of these transformations. Fluoro-benziodoxole is the fluoroiodane analogue of the trifluoromethylating Togni reagent. We developed a AgBF4 mediated geminal difluorination of styrenes using this fluoroiodine reagent. In this process one fluorine atom came from the fluoroiodane, while the other fluorine was derived from the tetrafluoroborate ion. A similar approach was applied for the 1,3-oxyfluorination and difluorination of cyclopropanes. Similarly, this fluorinative ring opening of unactivated cyclopropanes involved the introduction of an electrophilic fluorine atom from the fluoroiodane reagent and a nucleophilic one from the tetrafluoroborate ion. This reaction was extended to synthesis of 1,3-oxyfluorinated products. When alkenes reacted with the fluoro-benziodoxole reagent in the presence of palladium catalyst the iodofluorination reaction occurred.  Both the iodine and fluorine atoms were derived from the fluoroiodane reagent. The iodofluorination reaction with disubstituted and cyclic alkenes proceeded with high regio- and stereoselectivity.

The chemistry of fluorolkyl hypervalent iodine reagents has witnessed a great boast in recent years. These compounds are highly attractive as drug candidates, advanced materials and agrochemicals as described in detail in the Introduction. Despite this fact, applications of these reagents in biological studies are rather rare and under developed. The goal of this thesis is therefore the development of mild and metal-free methods in order to fill this gap. Two ways of application of fluoroalkyl hypervalent iodine reagents in labeling of biologically relevant compounds was explored. First, the applicability of previously reported parent Togni CF3 and their analogous tetrafluoroethyl reagents in radical fluoroalkylation of electron-rich substrates such as indole and pyrrole derivatives using sodium ascorbate as reductant was described. This afforded trifluoromethyl or 1,1,2,2-tetrafluoroethyl containing products in moderate to high yields. Next, same reagents were applied for labeling of several peptides and proteins bearing aromatic amino acids in their structure. This way, peptides and proteins containing electron-rich aromatics such as Trp, Phe, Tyr and His were reacted with fluoroalkyl groups with high selectivity toward Trp. In the second part of the work, a different approach of radical...

Over the past 100 years, there has been significant progress in limiting the development of iodine deficiency disorders globally. The recognition that congenital cretinism can be eradicated through early iodine supplementation along with the implementation of salt iodination in many countries has substantially decreased the prevalence of severe iodine deficiency. Yet, there are still populations that are mildly iodine deficient, likely due to lack of mandated iodine supplementation programs. While some countries are qualified as iodine sufficient based upon the general population’s median urinary iodine concentration, the sub-population of pregnant women may be mildly to moderately iodine deficient. Thyroid hormone demand increases during gestation to support fetal thyroid function and neurodevelopment. Pregnant women require increased iodine intake during pregnancy to account for this increased demand and to make up for increased renal iodide losses. The American and European Thyroid Associations both recommend that pregnant women in mild to moderately deficient populations take a daily 150mcg iodine supplement while planning a pregnancy, pregnant, and lactating. Studies into the impact of mild to moderate iodine deficiency in pregnancy on long-term neurocognitive outcomes of offspring have shown varied results. Some have found that iodine sufficiency or supplementation in pregnancy is associated with higher intelligence quotient scores in offspring. Others have found no significant effect of iodine sufficiency and iodine supplementation on developmental outcomes. This proposal is a double-blind, placebo-controlled, randomized study evaluating the effect of iodine supplementation in pregnancy on long-term neurocognitive outcomes of offspring. The study will be implemented in the mildly iodine deficient population of Southern England. Results from this study will contribute to and likely expand upon the current body of literature. They will also bolster current recommendations for iodine supplementation in pregnancy. In addition, this study may have important public health consequences including discussions around mandatory salt iodization in mild to moderately deficient populations.

Molybdenum compounds present a series of baffling chemical puzzles. By no means are all of these yet even clearly defined and many may never be. This complexity results from the ability of molybdenum to form six different oxidation states, which can possess three different coordination numbers. Furthermore, molybdenum compounds readily disproportionate to yield mixed valence compounds; and finally, molybdate ions readily aggregate in solution to form a variety of polyions by polymerization-condensation reactions which are controlled by pH and the concentration of the system. Molybdenum' s complexity can best be summed up in the words of the late Edgar Smith who described molybdenum to his classes as “ambidextrous, bi-sexual and polygamous.”

No description available.