Functioning satellite communication is a key part in everyday life and this thesis aims to contribute to the research area by exploring the characterisation of electrostatic discharges on antennas. Currently there is a gap in the research field when it comes to discharges on antennas and this thesis aims to try to fill that gap. Currently most research involves expensive electron beam and vacuum chamber testing. Simpler and more cost effective test methods are not as explored. This thesis explores current spacecraft charging and Electrostatic Discharge (ESD) standards, and how they can be used to analyze the charging and discharging characteristics of an antenna. The thesis involves how such a analysis is done and what limitations it may have. Two practical direct injection tests were also performed as to see how the antenna reacts to discharges of different energies and voltages. Finally a proposal of a test setup for an electron beam test was done. The main findings of the thesis is that there may be a risk of sustaining damage from the level of discharges measured despite the antenna fulfilling current standards and recommendations. Another take-away is that more research is required to establish if simpler analytic or practical tests are applicable for characterising discharges on the antenna.

No / We investigated five different methods for evaluating the electrostatic interaction between atoms in force field calculations on organic solids. Atomic charges and multipoles were obtained by fitting them to the molecular electrostatic potential, calculated in vacuum with an ab initio quantum mechanical method. Multipole moments were derived using three schemes, differing in the order in which the monopoles, dipoles and quadrupoles were fitted. For comparison, Gasteiger charges were also calculated. Using these electrostatic models, the lattice parameters and the molecular geometry of 48 organic crystals were optimised with the DREIDING force field. During the optimisation, the atomic multipoles were rotated with their local environment to account for molecular flexibility. For comparative reasons, rigid-body optimisations were performed on a subset of structures. The results were analysed in terms of structural parameters of the lattice and the molecules, and, for the ten polymorphic systems present in the test set, in terms of relative stability. On average, the multipole methods were not superior to the point charge methods for the full optimisation. For rigid molecules, however, the multipole models gave a substantial improvement in lattice parameters. No evidence was found that parameters for van der Waals energies need to be refitted for a specific electrostatic model. Energy differences between polymorphs were less than 5 kcal mol¿1 in eight out of ten cases, independent of the electrostatic model used. The results show that our use of distributed multipoles to describe the intra-molecular as well as inter-molecular electrostatic interactions does lead to an improvement in accuracy for rigid molecules, but not for flexible molecules. The investigation shows that accurate descriptions of the intra-molecular as well as the inter-molecular energies are crucial for the successful optimisation of crystal structures of organic solids.

Zellmembranen sind hochspezialisierte Mehrkomponentenlegierungen, welche sowohl die Zelle selbst als auch ihre Organellen umgeben. Sie spielen eine entscheidende Rolle bei vielen biologisch relevanten Prozessen wie die Signaltransduktion und die Zellbewegung. Aus diesem Grund ist eine genaue Charakterisierung ihrer Eigenschaften der Schlüssel zum Verständnis der Bausteine des Lebens sowie ihrer Erkrankungen. Besonders Krebs steht im engen Zusammenhang mit Veränderungen der biomechanischen Eigenschaften vom Gewebe, Zellen und ihren Organellen. Während Veränderungen des Zytoskeletts von Krebszellen im Fokus vieler Biophysiker stehen, ist die Bedeutung der Biomechanik von Zellmembran weitgehend unklar. Zellmembranen faszinieren Wissenschaftler jedoch nicht nur wegen ihrer biomechanischen Eigenschaften. Sie sind auch Beispiele für eine selbstorganisierte und heterogene Landschaft, in der Prozesse fernab des Gleichgewichtes, wie z.B. räumliche und zeitliche Musterbildungen, auftreten. Die vorgelegte Dissertation untersucht erstmals umfassend die zentrale Rolle der Zellmembran und ihrer molekularen Architektur für die Signalübertragung, die Biomechanik und die Zellmigration. Hierfür werden einfache Modellmembranen aber auch komplexere Vesikel und ganze Zellen mittels etablierter physikalischer Methoden analysiert. Diese reichen von Fourier- Analysen zur Charakterisierung von thermisch angeregten Membranundulationen über Massenspektrometrie und ‘Optical Stretcher’ Messungen von ganzen Zellen bis hin zur Filmwaagentechnik. Des Weiteren wird ein Modellsystem vorgestellt, welches sowohl einen experimentellen als auch einen mathematischen Zugang zum ‘ME-switch’ ermöglicht. Die vorgelegte Dissertation bietet neue Einblicke in wichtige Funktionen von Zellmembranen und zeigt neue therapeutische Perspektiven in der Membran- und Krebsforschung auf.

<p>Theoretical studies of dusty plasmas have been performed by focusing attention principally on collective phenomena and on grain motion. This thesis consists of a collection of seven published papers that explore both the collective behavior of a complex plasma system as well as the dynamics of grains in plasmas. In paper 1, a mechanism that explains the energy gain which leads to the self excited grain oscillations is theoretically formulated. The newly developed mechanism explains the observed self excited oscillations through the coupling of plasma sheath fluctuations with the electrostatic force, which holds the dust grain. In paper 2, theoretical and simulation studies have been conducted to study the vertical oscillations of dust grains that are levitated in plasma sheaths, under low pressure conditions. The oscillations were driven either by an external force or by a plasma number density modulation. The proposed model gives a full picture of the dust grains dynamics and is capable of successfully explaining the experimental observations. Paper 3 explores both theoretically and numerically the origin of the nonlinearities that lead to the observed oscillation resonances. The feature of the confining potential well which traps the grain, the influence of an electrode voltage modulation on the trapping well, and hence on the grain dynamics, and the resulting nonlinear resonances are analyzed in detail. The numerical simulations presented successfully reproduce a broad range of dynamical phenomena, including the self excited oscillations, for a range of different parameters. Paper 4 is dedicated to the analysis of the propagation of Dust Acoustic Waves (DAW) in a medium with an equilibrium dust density distribution. It has been theoretically shown that only some harmonics of the dust density distribution will influence the propagation of the DAW, thus modifying its frequency. Paper 5 presents a theoretical and numerical analysis of the excitation of higher harmonics of electrostatic dust cyclotron waves. The instability is driven by the ion and electron currents flowing along the magnetic field. The dispersion relation and the wave instability conditions have been derived, and a detailed numerical analysis has been performed. In Paper 6, we explore theoretically some cross field instabilities of low frequency, long wavelength electrostatic modes in fully and weakly ionized plasmas. It is shown that in a magnetoplasma with a transverse equilibrium dc electric field, the energy associated with the cross field motion of the plasma particles can be coupled to low frequency electrostatic waves. Paper 7 explores the properties and instabilities of low frequency electrostatic waves propagating in a current carrying magnetoplasma with equilibrium density and field aligned ion flow with a transverse gradient. The paper contains previous results as limiting cases, together with additional instabilities related to the equilibrium plasma density distribution. </p>

Theoretical studies of dusty plasmas have been performed by focusing attention principally on collective phenomena and on grain motion. This thesis consists of a collection of seven published papers that explore both the collective behavior of a complex plasma system as well as the dynamics of grains in plasmas. In paper 1, a mechanism that explains the energy gain which leads to the self excited grain oscillations is theoretically formulated. The newly developed mechanism explains the observed self excited oscillations through the coupling of plasma sheath fluctuations with the electrostatic force, which holds the dust grain. In paper 2, theoretical and simulation studies have been conducted to study the vertical oscillations of dust grains that are levitated in plasma sheaths, under low pressure conditions. The oscillations were driven either by an external force or by a plasma number density modulation. The proposed model gives a full picture of the dust grains dynamics and is capable of successfully explaining the experimental observations. Paper 3 explores both theoretically and numerically the origin of the nonlinearities that lead to the observed oscillation resonances. The feature of the confining potential well which traps the grain, the influence of an electrode voltage modulation on the trapping well, and hence on the grain dynamics, and the resulting nonlinear resonances are analyzed in detail. The numerical simulations presented successfully reproduce a broad range of dynamical phenomena, including the self excited oscillations, for a range of different parameters. Paper 4 is dedicated to the analysis of the propagation of Dust Acoustic Waves (DAW) in a medium with an equilibrium dust density distribution. It has been theoretically shown that only some harmonics of the dust density distribution will influence the propagation of the DAW, thus modifying its frequency. Paper 5 presents a theoretical and numerical analysis of the excitation of higher harmonics of electrostatic dust cyclotron waves. The instability is driven by the ion and electron currents flowing along the magnetic field. The dispersion relation and the wave instability conditions have been derived, and a detailed numerical analysis has been performed. In Paper 6, we explore theoretically some cross field instabilities of low frequency, long wavelength electrostatic modes in fully and weakly ionized plasmas. It is shown that in a magnetoplasma with a transverse equilibrium dc electric field, the energy associated with the cross field motion of the plasma particles can be coupled to low frequency electrostatic waves. Paper 7 explores the properties and instabilities of low frequency electrostatic waves propagating in a current carrying magnetoplasma with equilibrium density and field aligned ion flow with a transverse gradient. The paper contains previous results as limiting cases, together with additional instabilities related to the equilibrium plasma density distribution.

La diffraction des rayons X permet d’obtenir des informations sur la structure atomique et même sur la distribution de charges de composés sous forme cristalline, ce qui est d’une importance fondamentale pour la compréhension de leurs propriétés. Accéder expérimentalement à une description de la distribution de charges de systèmes macromoléculaires reste rarement possible malgré les améliorations techniques. Pour pallier cette limite, la transférabilité des paramètres de distributions de charges est un moyen fiable d’obtenir pour ces systèmes un modèle estimé et d’en déduire leurs propriétés électrostatiques. Les résultats présentés dans ce travail de thèse s’intègrent dans une dynamique visant à étendre les méthodes initialement réservées pour l’étude des petites molécules aux systèmes macromoléculaires. Il s’articule autour du développement de la suite de logiciels MoPro et de la banque de données ELMAM (Experimental Library of Multipolar Atom Model) pour l’étude des énergies des interactions électrostatiques au sein du site actif de complexes enzyme-inhibiteur. L’étude du fidarestat, un inhibiteur de l’holoenzyme aldose réductase, réalisée à partir de données obtenues à très haute résolution, est exposée dans ce manuscrit et a servi notamment à l’amélioration de la banque ELMAM en vue de l’étude des complexes holoenzymes aldo-keto réductase. A cette occasion, la légitimité du recours aux modèles transférés de distribution de charges a été discutée pour la première fois par une estimation statistique des incertitudes sur les énergies d’interaction électrostatique entre enzyme et inhibiteur / X-ray diffraction allows to obtain information about atomic structure and charge density distribution of crystal-state compounds, which is of main interest for the understanding of their properties. Reaching experimentally charge density distribution description of macromolecular systems is rarely possible despite technical improvements. To get around this limit, the transferability of charge density distribution parameters is a reliable way to obtain for these systems estimated model and to deduce their electrostatic properties. Works introduced in this PhD thesis manuscript take part in the will of extending methods initially for study of small molecules to macromolecular systems. It is centered on the development of the MoPro software suite and of ELMAM database (Experimental Library of Multipolar Atom Model) for the study of electrostatic interaction energies in enzyme-inhibitor complexes’ active site. The study of fidarestat, an inhibitor of aldose reductase holoenzyme, performed using ultra-high resolution data, is exposed in this manuscript and allowed to improve ELMAM database for the study of electrostatic interaction in aldo-keto reductase holoenzyme complexes. Moreover, the legitimacy of using transferred charge density distribution models was discussed for the first time, thanks to statistical estimation of uncertainties on electrostatic interaction energies between enzyme and inhibitor

Zellmembranen sind hochspezialisierte Mehrkomponentenlegierungen, welche sowohl die Zelle selbst als auch ihre Organellen umgeben. Sie spielen eine entscheidende Rolle bei vielen biologisch relevanten Prozessen wie die Signaltransduktion und die Zellbewegung. Aus diesem Grund ist eine genaue Charakterisierung ihrer Eigenschaften der Schlüssel zum Verständnis der Bausteine des Lebens sowie ihrer Erkrankungen. Besonders Krebs steht im engen Zusammenhang mit Veränderungen der biomechanischen Eigenschaften vom Gewebe, Zellen und ihren Organellen. Während Veränderungen des Zytoskeletts von Krebszellen im Fokus vieler Biophysiker stehen, ist die Bedeutung der Biomechanik von Zellmembran weitgehend unklar. Zellmembranen faszinieren Wissenschaftler jedoch nicht nur wegen ihrer biomechanischen Eigenschaften. Sie sind auch Beispiele für eine selbstorganisierte und heterogene Landschaft, in der Prozesse fernab des Gleichgewichtes, wie z.B. räumliche und zeitliche Musterbildungen, auftreten. Die vorgelegte Dissertation untersucht erstmals umfassend die zentrale Rolle der Zellmembran und ihrer molekularen Architektur für die Signalübertragung, die Biomechanik und die Zellmigration. Hierfür werden einfache Modellmembranen aber auch komplexere Vesikel und ganze Zellen mittels etablierter physikalischer Methoden analysiert. Diese reichen von Fourier- Analysen zur Charakterisierung von thermisch angeregten Membranundulationen über Massenspektrometrie und ‘Optical Stretcher’ Messungen von ganzen Zellen bis hin zur Filmwaagentechnik. Des Weiteren wird ein Modellsystem vorgestellt, welches sowohl einen experimentellen als auch einen mathematischen Zugang zum ‘ME-switch’ ermöglicht. Die vorgelegte Dissertation bietet neue Einblicke in wichtige Funktionen von Zellmembranen und zeigt neue therapeutische Perspektiven in der Membran- und Krebsforschung auf.:1 Introduction 2 Background 2.1 The Cell Membrane 2.1.1 Lipids in Cell Membranes 2.1.2 Membrane Proteins 2.1.3 An Overview on Membrane Models 2.1.4 Lipid Rafts 2.2 Model Membranes – An Experimental Access to Cell Membranes 2.2.1 Surface Tension and Thermodynamic Equilibrium 2.2.2 Langmuir Monolayer 2.2.3 The Polymorphism of Langmuir Monolayers 2.2.4 Membrane Vesicles 2.3 Biological Membranes as Semiflexible Shells 2.3.1 Elasticity of Soft Shells 2.3.2 Helfrichs Theory About Bending Deformations 2.3.3 Membrane Undulation 2.4 Membranes in Cell Signaling 2.4.1 Signal Transduction Fundamentals 2.4.2 Phosphoinositides 2.4.3 Phosphatidylinositol Signaling Pathway 2.4.4 The Myristoyl-Electrostatic Switch 2.5 Reaction-Diffusion Systems 2.5.1 Diffusion 2.5.2 Michaelis-Menten Kinetics 2.5.3 Reaction-Diffusion Systems 3 Methods, Materials and Theory 3.1 Optical Microscopy 3.1.1 Fluorescence Microscopy 3.1.2 Phase Contrast Microscopy 3.2 Cell Culture and GPMV Formation 3.2.1 Tumor Dissociation and Cell Culturing of Primary Cells 3.2.2 Cell Lines and Cell Culturing 3.2.3 Preparation of Giant Plasma Membrane Vesicles 3.3 Optical Stretcher 3.4 Fourier Analysis of Thermally Excited Membrane Fluctuations 3.4.1 The Quasi-Spherical Model – Membrane Fluctuations 3.4.2 Determination of the Bending Rigidity 3.5 Mass Spectrometry 3.5.1 MALDI-TOF Mass Spectrometry 3.5.2 ESI Mass Spectrometry 3.6 Migration, Invasion and Cell Death Assays 3.7 Langmuir-Blodgett Technique 3.7.1 Langmuir Troughs and Film Balances 3.7.2 Experimental Setup and Monolayer Preperation 3.7.3 Phospholipids, Dyes and Buffer Solutions 4 Fluctuations in Cell Membranes 4.1 Cell Membrane Softening in Human Breast and Cervical Cancer Cells 4.1.1 Bending Rigidity of Human Beast and Cervical Cell Membranes 4.1.2 MALDI-TOF Analysis of Lipid Composition 4.1.3 Summary and Discussion 4.2 Targeting of Membrane Rigidity – Implications on Migration 4.2.1 ESI Tandem Analysis of Lipid Composition 4.2.2 Biomechanical Behavior of Whole Cells and Membranes 4.2.3 Migration and Invasion Behavior 4.2.4 Summary and Discussion 5 Oscillations in Cell Membranes 5.1 Mimicking the ME-switch 5.1.1 DPPC/PIP2 monolayers at the presence of MARCKS 5.1.2 Lateral organization of PIP2 in DPPC/PIP2 monolayers 5.1.3 Translocation of MARCKS 5.1.4 Phosphorylation of MARCKS by PKC 5.1.5 Summary and Discussion 5.2 Dynamic Membrane Structure Induces Temporal Pattern Formation 5.2.1 Mechanism of the Oscillation 5.2.2 Modeling the ME-switch 5.2.3 Time Evolution 5.2.4 Phase Diagrams and Open Systems 5.2.5 Summary and Discussion 6 Conclusion and Outlook Appendix Bibliography List of Figures List of Abbreviations Acknowledgement

Thesis (MScEng (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 2009. / ENGLISH ABSTRACT: In the primary circuit components of high temperature reactors, various unwanted particles have been found. These particles include, for example, graphite and silver- 110 (110mAg). The silver-110 (110mAg) particles are radioactive, with a half-life of 253 days. The presence of radioactive particles in the primary circuit components constitutes an unwanted maintenance problem from a radiation hazard point of view. The development of a method to remove these particles from the helium stream is therefore needed. This thesis consideres two possible methods of removing silver from the helium stream, namely laser ablation of microparticles and electrostatic precipitation. These methods require the generation of silver ions or charged particles, the deflection of these particles in a helium gas stream passing through an electric field and the subsequent plate-out of these particles onto deflection electrodes. To find a suitable method of generating ions, various methods to generate the silver ions were investigated and evaluated. These methods include existing ion sources, thermionic, field and photoelectric emission and laser ablation of microparticles. No existing ion sources could be found which could be utilised in helium at high pressure. From calculations it was concluded that thermionic, field and photoelectric emission could also not be used to raise the energy of the emitted electrons sufficiently to ionize silver in a helium flight path. These methods were found not to be feasible ion sources in helium at high pressures. However, laser ablation of microparticles was found to constitute a feasible technology. Laser ablation was successfully utilised by Nichols et al. (2000) to deflect silver nanoparticles in an electric field across a two bar helium stream. An apparatus, similar to the one developed by Nichols et al. (2000), was designed and built. The apparatus included a silver insertion mechanism and tests with this apparatus were called the microparticle tests. To determine the efficacy of the silver insertion mechanism, the microparticle tests were done without the use of a laser. It was found that a laser was not necessary as microparticles collected on both the deflection electrodes. Dielectrophoresis was proposed as a possible explanation for the deflection and the plate-out of the mcroparticles. To theoretically model the deflection of the silver particles, two models were proposed, namely the deterministic and the stochastic deflection models. The latter describes the deflection of atoms, ions and polarized particles by using probability theory. From this model it was found that the Brownian motion force is far larger that the force created by the polarizibility of the atom due to an electric field. The deterministic deflection model describes the deflection of larger particles in a continuum. From this model it was found that a silver microparticle with a radius of 3 mm in a helium stream with bulk velocity of 0.0198 m/s would deflect 4.6 mm per helium flight path length of 140 mm. From these calculation it was found that the apparatus which had been built was not long enough to deflect and plate-out all the silver microparticles. The dielectrophoresis force on nanoparticles cannot be calculated, as the theory of dielectrophoresis is only valid for particles with diameter larger than 1 mm. Changes were therefore made to the apparatus to generate nanoparticles as their mobility is larger than that of microparticles. The nanoparticles were created by means of an arc discharge in helium; therefore tests with this modified apparatus were called the arc discharge test. The nanoparticles so created, deflected and deposited on both deflection electrodes. With the use of an atomic force microscope some of the particles could be classified as microparticles. According to the deterministic deflection model they should not have deflected. Combined with the fact that oxygen was in the plasma, due to the oxidation of the electrodes, a hypothesis of bipolar charging was thus proposed. The deterministic deflection model was used and supplemented with field and diffusion charging calculations, to support this hypothesis. A reasonable correlation between the theoretical model and this experimental results was obtained. Based on the arc discharge test, electrostatic precipitation was proposed as the indicated means of scrubbing silver and other particles such as graphite from a helium stream. It is recommended that a new apparatus be built and that the deterministic deflection model be used to predict the deflection of the particles. With this apparatus the uncertainties of breakdown voltage, the effect of thermionic emission and the size of the particles, all of which have been identified as being important, can then also be determined. / AFRIKAANSE OPSOMMING: In die primêre komponente van die kringloop van hoë temperatuurreaktors, was verskillende ongewenste partikels soos grafiet en silwer-110 (110mAg) teenwoordg. Silwer- 110 (110mAg) is radioaktief met ’n halfleeftyd van 253 dae. Vanuit ’n radiasie-risiko oogpunt word daar onderhoudsimplikasies geskep deur die teenwoordigheid van radioaktiewe partikels in die primêre komponente. Die ontwikkeling van ’n metode om hierdie partikels uit die heliumstroom the verwyder was dus nodig. Hierdie tesis ondersoek twee moontlike metodes van verwydering van silwer uit die heliumstroom, naamlik laser-ablasie van mikropartikels en elektrostatiese presipitasie. Hierdie metodes benodig die generasie van silwer ione of gelaaide partikels, die defleksie daarvan in ’n heliumstroom wat deur ’n elektriese veld vloei en die platering van die partikels op defleksie elektrodes. Om ’n geskikte metode the vind wat ione genereer was, verskillende metodes om die silwer ione te verkry, ondersoek en geëvalueer. Hierdie metodes sluit in bestaande ioonbronne, termioniese, veld en fotoëlektriese emissie en laser-ablasie van mikropartikels. Geen ioonbronne was gevind wat gebruik kan word in helium by hoë druk nie. Die gevolgtrekking is gemaak vanaf berekeninge dat termioniese, veld en fotoëlektriese emissie ook nie gebruik kan word om die energie van die voortgebronge elektrone genoeg te verhoog om silwer in ’n heliumstroom te ioniseer nie. Daar was gevind dat hierdie metodes nie geskik is as ioonbronne in helium by hoë druk nie. Daarenteen was laser-ablasie van mikropartikels gevind om ’n geskikte tegnologie voor te stel. Laser-ablasie van mikropartikels was suksesvol deur Nichols et al. (2000) gebruik om silwer nanopartikels te deflekteer in ’n elektriese veld oor helium van twee bar. ’n Apparaat soortgelyk aan Nichols et al. (2000) se eksperiment, was dus ontwerp en gebou. Die apparaat het ’n silwer insitmeganisme bevat en toetse met hierdie apparaat was die mikropartikel toetse genoem. Om die effektiwiteit van die insitmeganisme te bepaal, was toetse gedoen sonder opstelling van die laser. Daar was eksperimenteel gevind dat die laser nie nodig was nie, omdat mikropartikels op beide defleksie elektrodes geplateer het. Dielektroforese was voorgestel as ’n moontlike verduideliking vir deflektering en platering vir die silwer mikropartikels. Om die defleksie van silwer partikels teoreties te moduleer was twee modelle voorgestel, naamlik deterministiese en stogastiese defleksiemodelle. Laasgenoemde beskryf die defleksie van atome, ione en gepolariseerde partikels deur gebruik te maak van waarskynlikheidsteorie. Die stogastiese defleksiemodel dui aan dat die Brownian bewegingskrag veel groter is as die krag wat geskep word deur die polarisasie van ’n atoom as gevolg van ’n elektriese veld. Die deterministiese defleksiemodel beskryf die defleksie van groter partikels in ’n kontiuum. Met hierdie model was gevind dat silwer mikropartikels met ’n radius van 3 mmin ’n heliumstroom van snelheid van 0.0198 m/s, 4.6 mm sal deflekteer per 140 mm van heliumstroom lengte. Dit bewys dat die apparaat wat gebou was, se lengte onvoldoende was om al die silwer mikropartikels te deflekteer en te laat neerslaan. Die dielektroforese krag van nanopartikels kan nie uitgewerk word nie, omdat die dielektroforese model slegs geldig is vir partikels groter as 1 mm. Veranderings was dus aan die apparaat gemaak om nanopartikels te genereer omdat hul mobiliteit hoër is as die van mikropartikels. Die nanopartikels was geskep deur gebruik van ’n boogontlading in helium; daarom was toetse met hierdie gemodifiseerde apparaat die boogontladingstoets genoem. Die nanopartikels wat so geskep was, het gedeflekteer en het op beide elektrodes neergeslaan. Met die gebruik van ’n atomiese krag mikroskoop was dit gevind dat sommige van hierdie partikels mikropartikels was. Volgens die deterministiese defleksiemodel moes hul nie gedeflekteer het nie. Gekombineerd met die feit dat daar, weens oksidasie van die elektrodes, suurstof in die plasma was, was ’n hipotese van bipolêre lading voorgestel. Die deterministiese defleksiemodel is saam met die veld- en diffusielading gebruik om hierdie hipotese te staaf. ’n Redelike korrelasie tussen die teoretiese en eksperimentele data was gevind. Gebaseer op die boogontladingstoets, was elektrostatiese presipitasie voorgestel as ’n metode om silwer en ander partikels soos grafiet uit ’n heliumstroom te verwyder. Daar word voorgestel dat ’n nuwe apparaat gebou word en dat die deterministiese defleksiemodel gebruik word vir die bepaling van defleksie van die partikels. Deur die nuwe apparaat te gebruik kan die onsekerhede van deurslagspanning, effek van termioniese emissie en grootte van die partikels wat geidentifiseer is as belangrik, ook bepaal word.

Thesis (MScEng (Process Engineering))--University of Stellenbosch, 2005. / 199 leaves single sided printed, preliminary pages i-vii and numbered pages 1-191. Includes bibliography, list of figures and tables. Digitized using a Hp Scanjet 8250 Scanner to pdf format (OCR). / ENGLISH ABSTRACT: This study provided scientific proof of the precipitation of iron and other metal species in the HAL circuit at the Mineral Separation Plant of Namakwa Sands. Iron and aluminium hydroxides were the most abundant precipitate complexes that formed in the HAL circuit. Sulphates formed bridges with the iron complexes. It was found that precipitation of iron in the HAL circuit was a function of the pH, ferric ion concentration and temperature of the process water. Experimental work provided abundant evidence that the precipitated iron hydroxide and other species adsorb to the surfaces of zircon and rutile mineral particles due to opposite zeta potentials of the mineral particles and iron hydroxide. These adsorbed species altered the electrostatic potential of the mineral surfaces, which reduced the electrostatic separation efficiency of these minerals. It was determined that an improvement of 7% could be expected if the precipitation and attachment of iron could be minimised. Based on the experimental results, conceptual processes were developed and iterative simulations were set up to determine the optimum solution that would maximise the removal of dissolved iron. This solution would ensure clean mineral surfaces free of any adsorbed precipitates. This process circuit was constructed in May to August 2004 at a total capital cost of R4.8m and removed 99.97% of the total dissolved iron prior to caustic addition. Prime zircon recoveries were increased by 3-7%. / AFRIKAANSE OPSOMMING: Hierdie studie het wetenskaplike bewyse gelewer dat yster en ander spesies neerslae vorm in die HAL proses van die Mineraal Skeidings Aanleg van Namakwa Sands. Yster- en aluminium hidroksiede was die vernaamste neerslag komplekse wat gevorm het in die HAL proses. Sulfaat spesies het verbindings gevorm met die yster komplekse. Die neerslag van yster was ‘n funksie van pH, ferric ioon konsentrasie en temperatuur van die proses water. Eksperimentele werk het genoegsame bewyse gelewer dat die yster hidroksied neerslag en ander spesies adsorbeer op die oppervlaktes van sirkoon en rutiel minerale weens teenoorgestelde zeta potensiale van die mineraal partikels en yster hidroksiede. Hierdie geadsorbeerde spesies het die elektrostatiese potentiaal van die mineraal oppervlaktes verander wat sodoende die elektrostatiese skeidingsvermoë van hierdie minerale verminder het. ‘n Verbetering van 7% in elektrostatiese skeiding van sirkoon en rutiel minerale kan verwag word indien die neerslag en adsorbsie van yster minimiseer word. Konseptuele prosesse, gebasseer op die eksperimentele resultate, is ontwikkel. Itteratiewe simulasies is opgestel om die optimum proses te bepaal wat sodoende die maksimum verwydering van die opgeloste yster sou verseker. Hierdie oplossing sou skoon mineraal oppervlaktes, vry van enige geadsorbeerde neerslae, verseker. Hierdie prosesaanleg was opgerig in Mei tot Augustus 2004 teen ‘n kapitaalkoste van R4.8m en dit het 99.97% van die totale opgeloste yster verwyder voordat natrium hidroksied bygevoeg is. Primêre sirkoon opbrengste het sodoende toegeneem met 3-7%.

Thesis (MSc (Chemistry and Polymer Science))--Stellenbosch University, 2007. / In this study, ten selected boron-nitrogen compounds and three borane carbonyl complexes were investigated by a number of computational methods. It is well known that the B-N dative bond is shorter in the solid state than in the gas phase. The B-CO distance, on the other hand, displays the opposite effect. Quantum mechanical techniques at the Hartree-Fock, Møller-Plesset second-order and Density Functional Theory level were used to calculate the geometries of the isolated molecules and to compare them with those found in molecular clusters built to model the solid state. It was found that calculated geometries were very sensitive to the choice of the basis set. The effects of dipole-dipole interactions were further investigated by applying an external electric field with varying strength to isolated molecules, and by replacing the central molecule in a cluster with a different compound. The B-N bond was found to respond much more to the applied field than the B-CO bond. An effort was made to correlate the lengthening or shortening of the dative bond to the strength of the crystal field, the latter being calculated classically from point charges. Unfortunately, large differences were noted between the charges calculated with common methods like Mulliken or Merz-Kollman-Singh. Furthermore, an analysis of 67 crystal structures taken from the Cambridge Structural Database did not reveal a correlation between the length of the B-N bond and the crystal field calculated with Charge Equilibration charges. Finally, a valence force field was developed for H3N-BH3. It was shown that a much better fit of the vibrational spectrum can be obtained if the B-N stretching mode is assigned to the 603 cm-1 band rather than the peak observed at 968 cm-1.