<p> This study investigated localized corrosion behavior of austenitic stainless steels under stressed and unstressed conditions, as well as corrosion of metallic thin films. While austenitic stainless steels are widely used in corrosive environments, they are vulnerable to pitting and stress corrosion cracking (SCC), particularly in chloride-containing environments. The corrosion resistance of austenitic stainless steels is closely tied to the alloying elements chromium, nickel, and molybdenum. Polarization curves were measured for five commercially available austenitic stainless steels of varying chromium, nickel, and molybdenum content in 3.5 wt.% and 25 wt.% NaCl solutions. The alloys were also tested in tension at slow strain rates in air and in a chloride environment under different polarization conditions to explore the relationship between the extent of pitting corrosion and SCC over a range of alloy content and environment. The influence of alloy composition on corrosion resistance was found to be consistent with the pitting resistance equivalent number (PREN) under some conditions, but there were also conditions under which the model did not hold for certain commercial alloy compositions. Monotonic loading was used to generate SCC in in 300 series stainless steels, and it was possible to control the failure mode through adjusting environmental and polarization conditions. Metallic thin film systems of thickness 10-200 nm are being investigated for use as corrosion sensors and protective coatings, however the corrosion properties of ferrous thin films have not been widely studied. The effects of film thickness and substrate conductivity were examined using potentiodynamic polarization and scanning vibrating electrode technique (SVET) on iron thin films. Thicker films undergo more corrosion than thinner films in the same environment, though the corrosion mechanism is the same. Conductive substrates encourage general corrosion, similar to that of bulk iron, while insulating substrates supported only localized corrosion.</p><p>

<p> Abrupt changes in grain boundary energy (GBE) and character are associated with a change in GB complexion, GB mobility and exaggerated grain growth. In this thesis, complexion transitions and their effect of relative GBE and GB character distributions (GBCD) are studied for ceramics: europium doped spinel (Eu doped spinel), yttria doped (Y-doped), undoped alumina (Al₂O₃), and strontium titanate (SrTiO₃). </p><p> The population of GB planes (GBPD) of Eu doped spinel was investigated at temperatures before and after a previously identified a complexion transition. The microstructure changed from unimodal (1400 &deg;C) to bimodal (1600 &deg;C) and the relative area of {111} increased. This information was used to understand the GBs involved in the transition. </p><p> Atomic force microscopy was used to measure relative GBE from thermal grooves on the surfaces of Al₂O₃, 100 ppm Y-doped Al₂O₃, and 500 ppm Y-doped Al₂O₃ heated between 1350 &deg;C and 1650 &deg;C. The relative GBE of Al₂O₃ decreased slightly with increased temperature. When the doped samples were heated, there was an overall increase in the relative GBE, interrupted by abrupt reductions (increases) in relative GBE (mobility) between 1450 &deg;C and 1550 &deg;C. When the 100 ppm Y-doped sample was cooled, there was an increase in the relative GBE at the same complexion transition temperature, indicating that the transition is reversible. </p><p> Relative GBE was measured from thermal grooves on (SrTiO₃ in a region of non-Arrhenius grain growth. Between 1350 &deg;C and 1390 &deg;C, an abrupt decrease in relative GBE and exaggerated grain growth indicated that a complexion transition occurred. Solidified liquid eutectic wet the boundaries at 1550 &deg;C so these GB interfaces could not be compared to the lower temperatures. </p><p> 3D microstructures of samples heated below (1350 &deg;C), within (1390 &deg;C), and above (1425 &deg;C) the non-Arrhenius region in (SrTiO₃ were measured. Internal dihedral angles indicated that slowly growing grains have lower GBE than more rapidly growing grains. Low GBE grains increased in fraction with increased temperature until 1425 &deg;C. The GBE distribution and GBCD at 1425 &deg;C indicated a correlated high population and low energy for the {001}, the (111) sigma 3 and (2&macr;21) sigma 9 GBs. </p><p>

<p> Heusler based functional or smart materials are a deep well of solutions to future energy, heat transport and mechanization problems. The half-metallic ferromagnetic nature of these crystalline intermetallic compounds is the source of their extraordinary properties. The loss of this magnetic ordering places limits on the range of application temperatures making knowledge of the Curie point of these novel materials essential for understanding of their limitations. High throughput continuous wavelet transform spectrum analysis of magnetic balance data generated on a custom modified Setaram Setsys Evolution 16/18 Differential Scanning Calorimeter-Differential Thermal Analyzer with simultaneous Thermogravimetric Analyzer was performed on select Fe, Co and Mn based Heusler compounds. The phase stability of Co-Fe-Si compounds is explored in relation to the high-Curie Temperature <i>Co2FeSi</i> and <i>Fe2CoSi </i> compounds via generation of equilibrium ternary isothermal phase diagrams at 1160 &deg;C and 800 &deg;C to enable greater control of the microstructure for future thermomechanically processed bulk smart device fabrication.</p>

Magister Scientiae Dentium - MSc(Dent) / Several factors play a role in stability of impressions made from elastomeric impression materials. These include; polymerization shrinkage, loss of by-products during condensation, thermal contraction from oral temperature to room temperature, imbibition when exposed to water, disinfectant or high humidity and incomplete recovery from deformation due to viscoelastic behavior. An ideal impression material should be dimensionally stable over time to allow for pour at the convenience of the operator. Several studies evaluated the dimensional accuracy of elastomeric impression materials based on various factors including effects of repeat pour, temperature, humidity, disinfectants, impression techniques, and filler loading amongst others. Most of the previous studies did not use the standardized method described by the ADA specification for elastomeric impression materials. / South Africa

Despite advances in diagnostic procedures and treatments, the overall survival rate from cancer has not improved substantially over the past 30 years. One promising development is the encapsulation of toxic cancer chemotherapeutic reagents within biocompatible nanocomposite materials. The targeted stimuli triggered drug release restrict the toxic drugs to the tumour site, thereby reducing the effects of “free drug” on healthy tissues. One of the most versatile and safe materials used in medicine are iron oxide nanoparticles. This project describes the development of several formulations based on magnetite nanoparticles for drug delivery applications. Utilising magnetic nanoparticles in drug delivery systems allowed for the synergistic effects of hyperthermia and heat triggered drug released. The drug delivery systems developed in this project include magnetoliposomes, magnetic micelles, mesoporous silica-magnetite core-shell nanoparticles, liposome capped mesoporous silica-magnetite core-shell nanoparticles (protocells) and polymer capped mesoporous silica-magnetite core-shell nanoparticles. The drug loading and release profiles of the developed nanomaterials were assessed using two different anticancer drugs; Mitomycin C (MMC) and Doxorubicin (DOX). The drug loading content and drug loading efficiency for different nanocomposites ranged from 0.48 to 10.30% and 16.16 to 85.85%, respectively. Drug release profiles were studied in vitro at 37°C at pH 5.5 and pH 7.4 and at hyperthermia elevated temperature of 43°C to evaluate the effects of pH and temperature on the release profiles. An AC magnetic field with frequency of 406 kHz and variable field of up to 200 G was used to induce magnetic heating and keep the temperature within hyperthermia treatment range. Compared to uncapped mesoporous silica nanoparticles capping the mesopores of the silica nanoparticles with liposome or polymer reduced the drug release by 52.7% and 41.5%, respectively. The efficacy of doxorubicin-containing nanoparticles were evaluated in vitro against breast cancer and glioblastoma cell lines where different formulations demonstrated comparable or increased cytotoxicity compared to free drug. The cells treated with DOX loaded nanoparticles and hyperthermia demonstrated up to 89% lower viability compared to cells treated with free DOX. Silica coated magnetic nanoparticles were also used as enzymes (Pseudomonas Fluorescens Lipase (PFL) and Candida Rugosa Lipase (CRL)) supports in catalysis reactions. The enzymes were immobilised onto nanoparticles through physical adsorption and chemical bonding. The immobilised lipases were used in hydrolysis of pNPP and hydrolysis of cis-3,5-diacetoxy-1-cyclopentene to investigate the catalytic activity of the immobilized enzymes compared to free enzymes. The results indicated that free lipases provided slightly higher conversion than immobilised lipases in the first cycle however, the immobilised lipases were easily recycled and reused in sequential cycles which provides higher total yield per mg of lipase. The chemically immobilised lipase exhibited good reusability without loss of its activity in sequential cycles, however the physically adsorbed lipase showed reduced activity which could be explained by loss of enzyme during recycling between successive reactions. The CRL lipase activity were further assessed in the presence of an AC field where the results showed that exposure to the AC magnetic field resulted in increased lipase activity. The effect of reaction temperature on immobilised lipase activity were studied by performing the hydrolysis of cis-3,5-diacetoxy-1-cyclopentene at two temperatures of 25°C and 37°C where it was observed that both lipases exhibited higher activity at higher temperature which could be due to the fact that for PFL and CRL the optimum temperature is close to 37°C.

Dynamic wind tunnel tests were made on plunging rectangular cylinders with various depth to width ratios (b/h) and the results were compared with predictions based on the quasi-steady approach. To restrain the models to plunging oscillation they were attached, top and bottom, to transverse shafts guided by hydrostatic air bearings. An electromagnetic eddy current damping device was developed to obtain accurately calibrated viscous damping. Dynamic displacements were measured with a variable coupling electromagnetic transducer designed specifically for this apparatus. Direct force measurements were made for the square cylinder to obtain the lateral force variation with angle of attack in the Reynolds number range of the dynamic tests. The velocity-amplitude curves for those cylinders which oscillated are presented. The curves for b/h = 1.00 agreed with the predictions of the quasi-steady approach, and could be reduced onto one curve by using the ratios of applied damping. The longer rectangles did not prove to be amenable to analysis by the quasi-steady approach, and appeared to be strongly influenced by the instantaneous wake geometry. Rectangular cylinders of b/h over 4.00 did not plunge. Hot wire measurements showed that the Strouhal frequency was present in the wake of the plunging cylinder, occurring simultaneously with the plunging frequency. / Applied Science, Faculty of / Mechanical Engineering, Department of / Graduate

A simple method is presented, suitable for the repetitive testing necessary to investigate fully the bond characteristics in the anchorage zones of pretensioned pre-stressed members. Conditions in one anchorage zone of a member are simulated, the central zone of constant stress being replaced by a rigid steel frame. Loads in the wire on either side of the specimen are measured by load cells incorporating strain gauges. The experimental series comprised thirty specimens, of which five were prestressed with bright wire. Seventeen specimens were prestressed with rusted wire and the remainder were cast with rusted wire under zero prestress. The results show the superior anchorage characteristics of rusted wire over bright wire and the exponential nature of the load pickup of the wire in the anchorage zone. A relation is suggested between end pull in at the free end and ultimate load anchored. A short description of an investigation of the relaxation behaviour of prestress wires is appended to the thesis. / Applied Science, Faculty of / Civil Engineering, Department of / Graduate

Tension tests have been performed on LiF single crystals. The results of strain rate change tests are not wholly consistent with current theories of deformation processes. The number of mobile dislocations appears to depend sensitively on the stress prevailing during straining. A short investigation of the effect of removing surface layers during deformation was undertaken. The surfaces of chemically polished LiF single crystals are not sites of strong work-hardening. The effects of magnesium-rich surface layers on the mechanical properties of LiF have been studied. The yield stress, critical tensile stress, and work-hardening slope increase linearly with layer thickness, while the strain to fracture decreases rapidly with increasing layer thickness. The results of tension tests, supplemented by metallographic evidence, indicate that the yield stress and the critical tensile stress are not determined by surface source operation. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

The free-edge effect around holes in composite laminates has recieved less attention than the straight free-edge problem. Mathematical analysis of free-edge stresses around holes have mostly been numerical. The present work develops a simple approximate solution of the hole problem which allows for low cost computation. The method assumes that only the deviations of the ply stresses from the homogeneous plate solution of in-plane stresses around holes contribute to the interlaminar effects. It is then possible to use an equilibrium argument to calculate the interlaminar stresses at the hole boundary. The results obtained show good agreement with numerical results from the literature for a wide range of laminates, predicting the general shapes and signs of interlaminar stress' distributions reasonably well. Experimental observations of delamination found in the literature also agree with the present results. An experimental study of the damage development around holes under quasi-static loading for a number of different laminates is reported. The delamination observed at the hole boundaries are found to be in good qualitative agreement A simple semi-quantitative correlation between these results and a stress combination function of the three interlaminar stress components is also derived. The problems associated with the development of reliable methods of delamination prediction are also discussed. / Applied Science, Faculty of / Mining Engineering, Keevil Institute of / Graduate

The literature in Vacuum Induction Melting is briefly reviewed, with especial emphasis on refractory practices and refractory-metal interactions. Since it was determined that in both steel and superalloy vacuum melting in large furnaces lining failures are associated with attack on the cement joints, tests were performed to characterize this attack and determine the most suitable cements from this standpoint. The presence of low-stability oxides (SiO₂, P₂O₅,...) was shown to be the main reason for the cement's low resistance to metal attack. It is suggested that in the case of steels, diffusion of the less stable oxides may be the rate controlling step in the corrosion process. In the case of superalloys, the high interaction between oxygen and the elements present in the alloy (Ti, Al, Cr) causes an extreme depression of the oxygen activity in the melt, hence enhancing the dissolution of all refractory oxides. In order to produce cements with a very small content of the low-stability oxides, the use of fluorides as fluxes was attempted. The cements so produced performed well, as far as resistance to attack, adherence to bricks and technological properties were concerned. To verify the validity (on a large scale) of the mechanisms observed and proposed in the tests, samples from industrial Vacuum Furnaces were examined. It was concluded that the processes occurring in a large furnace can be rationalized based on the test observations. Also comments were made on the need for improved pouring facilities, if the products of the metal-refractory interaction are to be kept out of the final material produced. This is because in the present state of the refractory technology and practice, these interactions cannot be avoided. / Applied Science, Faculty of / Materials Engineering, Department of / Unknown