Optical surface quality and molecular dynamics modelling of ultra-high precision optical silicon machining
- Authors: Abdulkadir, Lukman Niyi
- Date: 2019-04
- Subjects: Lasers -- Industrial applications , Manufacturing processes , Materials science
- Language: English
- Type: Master's theses , text
- Identifier: http://hdl.handle.net/10948/66551 , vital:75600
- Description: Hard and brittle materials, such as silicon, silicon carbide etc., are widely used in aerospace, integrated circuit, and other fields due to their excellent physical and chemical properties. However, these materials display poor machinability owing to hardness, brittleness, non-linearity in machining process and complexities in selecting suitable machining parameters and tool geometry. These leads to low quality lenses due to subsurface damage and surface micro-crack. Additionally, it is experimentally very difficult to observe all nanoscale physical phenomena due to in-process measurement problems, inaccessible contact area of tool and workpiece, and the difficulty of surface analysis. With the use of molecular dynamics (MD) which is a comprehensive nanoscale modelling technique, proper selection of process parameters, tool geometry and online monitoring techniques, production of freeform optics is possible through Ultra-high precision diamond turning (UHPDT). Though, depending on view point, machinability in UHPDT may be in terms of tool wear rate, hardness, chip morphology, surface roughness, and other benchmarks. These situations have called for more insights, which on the long run will help to achieve high precision manufacturing with predictability, repeatability, productivity and high infrared (IR) optical quality. In this thesis, UHPDT of monocrystalline silicon at atomistic scale was conducted to investigate combined effects of edge radius, feed rate, cutting speed, depth of cut, rake and clearance angles hitherto not done so far. Using appropriate potential functions with the MD algorithm, comprehensive analysis of thermal effects, diamond tool wear, phase change, cutting forces and machining stresses (normal, shear, hydrostatic and von Mises) were carried out. Dislocation extraction algorithm (DXA) and radial distribution function (RDF) were used to evaluate dislocation nucleation, variations in bond lengths, microstructural transformation and represents structural changes in histogram form. Selected parameters for optical quality surface roughness were afterwards compared and optimized through response surface methodology (RSM) based on Box Behnken (BBD) and Taguchi L9 methods. The results indicated that, silicon atoms in the chip formation zone undergo high pressure phase transformation (HPPT) at high hydrostatic pressure and temperature Silicon microstructure transformed from four-coordinated diamond cubic structure (Si-I) to unstable six-coordinated body-centered tetragonal structure (β-silicon) which then transformed to amorphous silicon atoms (a-Si) through amorphization. These resulted in plastic deformation and defects in the machining zone causing subsurface damage. Stress analysis indicated that the compressive stress in the machining zone (i.e.amorphous region) suppressed crack formation contributing to continuous plastic flow which is responsible for silicon ductile-mode cutting. Furthermore, formation of silicon carbide which constituted diamond wear was observed to be by sp3 - sp2 diamond carbon atom disorder and tribochemistry. The tribochemistry occurred through both multiphase and solid-state single-phase reaction between diamond tool and silicon workpiece at cutting temperatures above and below 959 K. Both the experimental findings and the simulation results reveal that, at edge radius less than uncut chip thickness, tool wear was more of rake wear than flank wear. Tool wear and kinetic friction reduced as the edge radius approached the uncut chip thickness while forces, stresses and SCE increased. When machining silicon at different ratio, silicon stress state, SCE, SSD, forces (reduced with increase in clearance angle), shear plane, chip velocity and chip ratio increased as edge radius and rake angle increased, while, kinetic friction, chip length and thickness reduced. The crystal lattice of the machined surfaces and subsurface deformed layer depth increased with increase in edge radius, feed and rake angle. Amongst all tested and analysed parameters, feed rate had the highest influence on surface quality while depth of cut showed the least. Acoustic emission was also monitored during machining and its results statistically analysed. The trends of the monitored acoustic emissions showed its capability to adequately represent and predict surface roughness results. Based on the developed simulation model a novel method for quantitative assessment of tool wear was proposed. The proposed model can be used to compare tool wear using graphitization and tribochemistry to decide the path and mode of the diamond tool wear. Finally, based on the experiment results and predictive model, a novel combination and hierarchical arrangement of the considered factors capable of suppressing tool wear and improve attainable machined surface roughness when turning hard-to-machine materials was proposed. , Thesis (PhD) -- Faculty of Engineering, the Built Environment, and Technology, School of Engineering, 2019
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- Date Issued: 2019-04
Electrospinning of carbon nanofibers for investigating the behaviour of lead electrodeposits on the carbon surface
- Authors: Madikane, Zipporah Kayakazi
- Date: 2018
- Subjects: Materials science , Nanotechnology Nanostructured materials
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10948/31622 , vital:31629
- Description: Over the years, the lead acid battery’s performance in modern vehicles has improved considerably to adapt to the new demands of vehicle electronics and start-stop applications. Although vehicles are valuable for commuting, their high volume of usage has resulted in an increase of fuel prices and emitted carbon dioxide (CO2). Due to the drastic global warming concerns, vehicle manufactures have been pressured into developing a variety of hybrid electric vehicles (HEV) that would reduce their fuel consumption and thereby reduced CO2 emission. The HEVs construction requires the implementation of advanced battery systems that operate under high rate partial state of charge (HRPSoC) conditions. During battery’s discharge, the Pb contained on the negative plate is oxidised to insulating PbSO4 crystals. When the battery is charged, the PbSO4 is reduced back to Pb. With prolonged use at HRPSoC, the battery undergoes an irreversible process wherein small PbSO4 crystallites dissolve and precipitate into large passivating PbSO4 crystals that accumulate as a dense layer on the surface of the negative plate. This layer restricts the diffusion of the H2SO4 electrolyte into the reaction sites for further reactions, resulting in the battery to fail prematurely. Research has shown that the inclusion of appropriate carbon materials on the negative active material (NAM) improves the performance of the batteries in HRPSoC cycling. This study looked at the making of some of the carbon-based additives by using relatively cheap materials such as polyvinyl alcohol (PVA) polymer with a sucrose blend in ratios of 75:25, 50:50 and 25:75, respectively. The blended materials were electrospun into PVA/Sucrose nanofibers. These were further stabilised by iodine and carbonised at 500 °C in a tube furnace. The properties of the carbon fibres were investigated by a number of analytical techniques such as TGA, DSC, XRD, BET, FTIR and SEM. The obtained carbon fibre surfaces were further electroplated with Pb in order to investigate the chemical bonding, the relationship between the carbon-based material interface and the Pb was investigated by SEM and CV techniques. The results of the study demonstrated that the varied ratios of the PVA/Sucrose nanofibers influenced the differences in their respective chemical and physical properties. Thermal analysis demonstrated that at approximately 500 °C, the final residue of the PVA/Sucrose nanofibers decreased as the sucrose content decreased. iii Carbonisation of the nanofibers was confirmed by the structural changes corresponding to the disappearance of the major bonds of PVA and sucrose materials, as well as by the appearance of C=C stretches. Carbonisation was also confirmed by the XRD diffractogram that demonstrated two broad amorphous peaks that corresponded to the planes of the graphite structure. The broadness of the peaks confirmed the amorphous nature of the carbon fibres. The morphological studies demonstrated that upon blending PVA with increasing quantities of sucrose, the uniformity of the fibres was altered into beaded fibrous structures with large diameters due to the increased viscosity of the electrospinning solution. The behaviour of the Pb deposits on the carbon surfaces was influenced by the high quantity of PVA present on the blend and the fibrous structure of the carbonised material. The Pb had high affinity towards the fibrous carbon derived from 75:25 as it deposited widely across the surface. Whereas, the absence of Pb deposits on the surface of the carbon derived from the 25:75 ratio was an indication that the high sucrose content and the bulky non-fibrous structure hindered the electrodeposition. The electrochemical study findings demonstrated that the bare Pb electrode had two distinct current peaks corresponding to the anodic (discharge) and cathodic (charge) reactions of Pb/PbSO4. Upon the inclusion of the electrospun carbon materials on the Pb electrode, the intensities of the anodic and cathodic peaks were reduced. Implying that none of the carbon materials enhanced the electrochemical redox reactions of the Pb/PbSO4 couple in the H2SO4 electrolyte.
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- Date Issued: 2018