3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item Supercapacitive performance of nickel(II) tetrapyrazinoporphyrazine modified titanium carbide nanosheets(2021) Tsoari, ThokozaniElectrochemical capacitors are known to be energy storage devices with outstanding power delivery and uptake devices. Their prominent drawback is extremely low, and at best, modest energy density.. This research presents a new pathway into the exploration of two-dimensional materials as high energy electrode materials in symmetrical supercapacitor systems. We, for the first time in the reported history of MXenes and metallotetrapyrazinoporphyrazine complexes, report the combination of the lightest MXene, Ti2CTx, with nickel(II) tetrapyrazinoporphyrazine as active electrode material for supercapacitors in aqueous electrolyte to evaluate the resulting electrochemical performance. This novel composite was obtained by a simple intercalation of MXene via delamination by dissolved nickel(II) tetrapyrazinoporphyrazine. The rationale of this combination is to achieve synergized electrochemical performance through the intermix of unique respective properties from these materials: the metallic conductivity, capacitive, pseudocapacitive properties emanating from the surface physiochemistry of MXene nanosheets, and the redox catalytic activity of N4 metallo-macrocyclic complex. The electrochemical performance of alkaline, neutral, and acidic electrolyte systems is assessed by employing cyclic voltammetry, galvanostatic charge-discharge and electrochemical impedance spectroscopy. The highest obtained specific gravimetric capacitance is 109.5 F/g the nickel(II) tetrapyrazinoporphyrazine modified MXene. The broadest full cell operating window was 0.8 V for near-neutral based electrolyte. Full cells in basic media exhibited the most inferior performance in terms of stability as drastic capacitance fall off is observed after 2000 cyclesItem Hard, wear resistant Fe-B-C composites produced using spark plasma sintering(2017) Rokebrand, Patrick PierceFe-B-C composites were produced, from boron carbide and iron powders, using spark plasma sintering. This provided information on the effects of rapid sintering on densification, composition and the microstructure of the materials produced. The composition range included a selection high Fe contents (69.3, 78 and 80.9 vol. % Fe-B4C) and high B4C concentrations (1, 3, 5 vol. % Fe-B4C). The properties of the materials were investigated to determine the potential for using relatively cheap Fe and B4C powders to produce hard, wear resistant materials. High Fe-B4C composites were sintered at 900, 1000 and 1100°C at 60 MPa. Densification increased with increasing temperature and at 1100° each composition achieved ≥ 97 % densification. The materials reacted during sintering with the main phases observed being Fe2B and Fe3(B,C) whilst additional phases formed were FeB, C and Fe23(B,C)6.Comparing the phases that were produced to Fe-B-C phase diagrams showed deviations from expected compositions, indicating the non-equilibrium nature of producing the composites using SPS. Although the composites were not at equilibrium, all the B4C reacted and could not be maintained, even with fast heating and cooling rates. The properties of the materials were dependent on both densification and the phases that were present after sintering. Materials containing higher amounts of the Fe2B phase showed higher hardness and fracture toughness results, up to 13.7 GPa and 3.5 MPa.m0.5 respectively for the 69.3 vol. % Fe-B4C. The materials were sensitive to grain and pore growth which negatively affected properties at 1100°C. The transverse rupture strength of 388.3 MPa for 80.9 vol. % Fe-B4C composite was the greatest, and showed evidence of both intergranular and transgranular fracture. The strength was affected by a fine dispersion of porosity at the grain boundaries, throughout the material, and free carbon in the structure was detrimental to the strength of the 69.3 % Fe-B4C. The wear rates were lower using Si3N4 wear balls compared to stainless steel balls, where 69.3 vol. % Fe-B4C showed the best wear rates, 8.9×10-6 mm3/Nm (stainless steel ball) and 1.77×10-6 mm3/Nm (Si3N4 ball), due to the higher Fe2B composition and free carbon acting as a lubricant during sliding. 1, 3 and 5 vol. % Fe-B4C composites were sintered to densities above 97 % of theoretical at 2000°C and 30 MPa. The formation of a transient FeB liquid phase assisted densification. 1 % Fe-B4C attained hardness and fracture toughness up to 33.1 GPa and 5.3 MPa.m0.5 with a strength of 370.5 MPa. Thermal mismatch between the FeB phase and B4C caused high residual stresses at the interface which led to cracking and pull-out of the FeB phase. Residual carbon at the grain boundary interface exacerbated the pull-out effect. Increasing Fe and the subsequent FeB phase had an embrittling effect. The materials suffered severe wear of up to 36.92×10-6 mm3/Nm as a result of the pull-out with the remaining porosity acting as a stress raiser. 20 vol. % of the Fe in each system was substituted with Ti to reduce the presence of residual carbon. Although in some case the properties of the respective compositions improved, residual carbon was still present in the composites.Item Development of diffusion carbide coatings(1993) Fazluddin, Shahed BhaggaThe formation of VC, NbC, and Cr-carbide coatings on steels immersed in molten borax baths containing carbide-forming constituents has been known for some time. A study was made of the formation of carbide coating on steel specimens treated in molten borax cont.ad.Lnr.q ferro-vanadium and V20S as bath additives. The prevalence of oxidizing or reducing conditions in the bath was found to playa decisive role in the formation of the VC layer. The influence of treatment factors such as time, temperature, and bath composition on the thickness of the coating was investigated. A detailed investigation into the behaviour of baths containing V205 and Al was carried out in this regard. Microstructural examination of coated specimens was performed using optical and electron microscopy. Microhardness tests, X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS) analyses were carried out in order to characterise the layers produced. The corrosion behaviour of the vanadf.um carbide coating was evaluated by accelerated electrochemical means. Samples of mild steel and EN9(O.5% C) steel were treated for various lengths of time in order to obtain coatings of various thicknesses. The corrosion resLst.ance of the carbide coating was found to be superior to that of the untreated base alloys. Five diffet"ent tool steel materials were a.lso treated. Microscopy examination and hardness testing of the VC coated tool steels was performed in order to assess the effects of treatment by this process on the bas~ material. The austenitizing temperature and the nature of the carbides in the matrix of the tool steels was found to play an important role in this respect.Item A spectroscopic study of Fe phases in cemented carbides(2016) Mosse, Ibwanga SavTungsten carbide (WC) is characterized by its high strength, toughness, hardness, its high resistance to wear and can also be employed at high temperatures. It is used mainly in the form of cemented tungsten carbides which are produced by combining grains of tungsten carbide into a binder matrix element, for example cobalt (Co). Tungsten carbide is commonly used in industrial machinery as cutting tools and abrasives. The primary aim of this project is to investigate the effects of iron (Fe) as an alternative/additional binder in a tungsten carbide system. Therefore, two samples WC-10wt%Co-6wt%TiC and WC-10wt%Co-6wt%TiC-20wt%Fe alloys were prepared by milling and followed by sintering. Several studies have been undertaken in this project to ascertain the effect of Fe on the structural, electronic, magnetic and physical properties of the as-milled and as-sintered samples. A number of different experimental methods were applied to give such information. Transmission Mössbauer spectroscopy and conversion electron Mössbauer spectroscopy were employed as the main techniques to determine the charge states of Fe, Fe phases and other complex phases in the WC-10wt%Co-6wt%TiC-20wt%Fe alloy from the hyperfine interaction parameters. In addition, applied Vickers hardness test, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD) and strain analysis were utilized as complementary characterization techniques. The Vickers hardness value of 1358±70 HV was measured for the WC-Co-TiC sample whilst a lower value of 820±41 HV was found for the WC-Co-TiC-Fe sample. The microstructure analysis (SEM/EDS) confirmed the presence of WC in the matrix, and Co, Ti, and Fe as the binder elements. XRD results show the formation of the FeCo alloy in the as-milled powder and as-sintered samples. The strain analysis was performed on the as-milled powder, and the as-sintered samples by adopting the method used in accordance with the Topas description, defined in the Topas manual. The WC-Co-TiC sample showed that the tungsten carbide phase appears to have little strain whilst the titanium carbide phase appeared to have no strain. In the WC-Co-TiC-Fe sample, all phases show no strain. The Mössbauer spectrum at room temperature acquired from transmission Mössbauer spectroscopy was fitted with one sextet S1 attributed to -Fe. Best fits to the data obtained from conversion electron Mössbauer spectroscopy required four spectral components: two sextets S1 and S2 assigned to FeCo, one doublet D1 assigned to FeWC and one single SL1 assigned to FeTi alloy.Item Optimizing the surface integrity of machined Ti-6A-4V using advanced carbide inserts and minimum quantity lubrication(2016) Ofem, Nweoyi LekamThis research is focused on optimizing the surface integrity of Ti-6Al-4V using advanced carbide inserts and minimum quantity lubrication (MQL). Experiments were designed to machine twenty Ti-6Al-4V blocks under dry and MQL lubricating conditions using innovative cemented carbide inserts produced by Liquid Phase Sintering (LPS) and Spark Plasma Sintering (SPS). The cutting speed, feed rate, and depth of cut, were considered as machining parameters, while the cutting force, temperature, tool wear, surface roughness and residual stress were considered as performance characteristics. The results obtained for surface roughness and residual stress measurements were used to analyze the surface integrity of the machined Ti-6Al-4V samples. The influence of sintering techniques and milling parameters on performance characteristics and surface integrity were investigated from experimental results obtained. Discrepancy in behavioral trends with respect to lubricating conditions was also investigated. The analyses of milling results showed that cutting speed played a major role in the optimization of surface integrity of the Ti-6Al-4V work pieces followed by the depth of cut. The results also showed that the LPS inserts performed better than the SPS inserts due to their better mechanical properties (higher fracture toughness (KIC) and transverse rupture strength). The application of MQL was also observed to significantly reduce milling temperatures resulting in better surface integrity. However, an optimized surface integrity of the Ti-6Al-4V samples was achieved during MQL milling with the 10Co-L insert at a low finishing speed of 75m/minute and a reduced depth of cut of 0.5mm.Item The alpha-eta transformation in cobalt with particular reference to the use of cobalt in cemented carbides(2015) Rees, G JA study of the reduction of black cobalt oxide, using hydrogen as the reductant, showed that the temperature and time used for reduction exerted a profound influence on the proportions of face centred cubic (a) and hexagonal close packed (e) cobalt allotropes obtained in the reduced metal powder. Reduction under certain conditions yielded a faulted hexagonal cobalt powder product which contained stacking faults. The influence of oxygen and titanium on cobalt materials showed that these impurities did not stabilise the proportions of either the cubic or hexagonal phases under the conditions, which were used. The grinding of cobalt powders from different sources showed different tendencies towards agglomeration, when acetome was used as the milling fluid. It was found that powders which contained an initially higher proportion of the hexagonal phase -had a strong tendency towards agglomeration during the initial stages of grinding. The importance of the different phases of cobalt present in the metal powders, and their different agglomeration tendencies on grinding, on the milling required for satisfactory densification of cobalt - tungsten carbide compacts has been shown; it was (iv) found that hard metal alloys prepared using powders with an initially higher proportion of the hexagonal phase required less milling to achieve a given sintered density, than did mixtures prepared with other cobalt powder*;. A correlation analysis between the properties of the milled hard metal powders and the properties of the sintered compacts was carried out; it was found that the most important correlation was between the specific surface area of the milled hard metal powder and the coercivity of the sintered product.Item Synthesis of Tungsten Oxide Nanostructures by Laser Pyrolysis(2012-02-01) Govender, MalcolmThis dissertation discusses the synthesis method known as laser pyrolysis. The theory on laser pyrolysis has been inferred since 1975, but it is insufficient in predicting the products that can be formed. This is due to the use of a laser, which leads to indecisive reaction pathways from precursor to product. In this work, the laser wavelength and power are varied to initiate a starting point in understanding the complex nature of the laser–precursor interaction, in addition to studying the resulting nanomaterial that is formed by the corresponding laser pyrolysis parameters. The results are justified based on linear and nonlinear optical processes, as well as photophysical and photochemical processes. Experiments to produce tungsten trioxide nanowires were conducted, but similar products could not be achieved, due to the difficulty in emulating ‘sensitive’ variables such as gas pressure and flow rates. However, it was discovered for the first time using this method that six-sided tungsten oxide “stars” can be grown.