3. Electronic Theses and Dissertations (ETDs) - All submissions
Permanent URI for this communityhttps://wiredspace.wits.ac.za/handle/10539/45
Browse
3 results
Search Results
Item Characterization of graphene epoxy nanocomposite interface region by multiscale modelling(2018) Qhobosheane, Relebohile GeorgeThe aim of this study was to characterize graphene epoxy nanocomposite interfacial region using multiscale modelling. Molecular dynamics was used to study the nanocomposite at nano scale and finite element analysis at macroscale to complete the multiscale modeling. Coupling of these two scales was done by the use of a property averaging method known as Irving Kirkwood method. One to three sheets (1.8 %, 3.7 % and 5.4 % graphene weight fraction) of graphene were respectively reinforced with epoxy polymer to form a graphene epoxy nanocomposite. The normal and shear forces at the interfacial region of graphene epoxy nanocomposite were investigated by displacing graphene from epoxy to analyze the mechanical properties including the Youngs Modulus, shear modulus and traction forces. Molecular dynamics simulations were further studied through radial distribution function and molecular energy. The effects of graphene on the density distribution of epoxy in the nanocomposites were also analyzed. The results showed that the density when graphene is added sheet by sheet relatively increases until saturation, and then progressively decreases to a bulk value in regions further away from the interface. Improvements in Youngs Modulus and shear modulus of graphene epoxy model compared to normal epoxy resin were noticed. The dispersed graphene sheet improved the Elastic Modulus more than the agglomerated graphene sheets. The normal and shear forces versus displacement were plotted in order to characterize the interfacial region properties. The elastic constants determined by molecular dynamics were higher than those predicted at macroscale analysis due to the difference in scales. The nanocomposite with 3.7 % weight fraction of graphene gave the best properties of the interfacial region. The results from this model also showed close agreement with the available numerical experiments results from the literature data.Item Synthesis, characterization and application of 2d semiconducting layered inorganic nanostructures of In2S3 and WS2 in gas sensing(2018) Gqoba, Siziwe Sylvia2D semiconductor nanostructure based chemical sensors hold the promise of portable, fast, low power, simple and low cost technologies for the detection of volatile organic compounds (VOCs) and gases. Detection, monitoring and quantification of these analytes are important for the improvement of the quality of human life, safety and the surrounding environment. Their electrical conductance is extremely sensitive to changes in the local chemical environment and can be chemically modified to increase selectivity. Nanostructures have tunable band gaps and exhibit new and improved properties at the quantum confinement limit. The band gap is affected by the size, shape, dimensionality and chemical composition of the nanostructures. So, precise control of these factors is achieved by manipulating reaction parameters like time, temperature, choice of precursors and choice of capping agent as well as concentration. In this study, we unravel the effect of reaction parameters on the structural, optical and morphological properties of In2S3 and WS2 nanostructures during colloidal synthesis. The reaction parameters under investigation were time, concentration, solvent or capping agent and precursor. For instance, different shapes of ß-In2S3 were obtained for mono- and bi-ligand systems, reaction time, concentration of the precursor and solvent. Capping agents influence the growth kinetics and the shape of nanostructures. Well defined hexagonal ß-In2S3 nanostructures of the tetragonal phase were obtained as a function of time with elemental S and indium chloride reacted at 1:1 mole ratio in oleylamine (OLA) medium. OLA, an alkylamine played the triple role of the reducing agent, solvent and capping agent. An increased amount of S to In3+ proved to be an unfavourable condition for the formation of the hexagonal shape as seen with the 1:2 mole ratio. Hexadecylamaine (HDA) and octadecylamine (ODA), alkylamines like OLA were also used in separate experiments. The hexagonal shape like with OLA evolved with time for ODA while it never materialized for HDA. Dodecanethiol (1-DDT), a thiol ligand produced microspheres as a function of time. These alkylamines were then each used in bi-ligand systems with a controlled amount of 1DDT at a 1:1 mole ratio of In3+ and S. For OLA/1-DDT, the hexagonal morphology was favoured and retained regardless of the duration of the reaction time. However, the hexagonal shape transformed into nanorods with prolonged reaction time for the ODA/1-DDT combination. The morphology was rather elusive for HDA/1-DDT system even at extended reaction time. HDA and ODA yielded the cubic phase of ß-In2S3 in both the mono- and biligand systems. An increased amount of 1-DDT to OLA resulted in mixed morphologies regardless of the reaction time, once again proving the importance of concentration. It is interesting to note that hexagonal nanostructures were retained when 1-DDT was used as a source of S (1:1) with OLA serving a triple role. In the case of WS2, only OLA was used as a capping agent and the variation of reaction of time yielded nanoflowers, nanoflake-like and nanorod-like structures. The nanostructures of these semiconductors were used as components in chemi-resistive sensors for the detection and identification of NH3 gas and selected VOCs. Unlike their oxide counterparts, their gas sensing potential has been largely overlooked despite their capability of operating at room temperature. Preliminary studies on ß-In2S3 sensors based on the 330 min nanostructures showed gas sensing potential towards formaldehyde vapour. In the case of WS2 nanostructures, all the sensors regardless of the reaction time exhibited gas sensing potential. However, the percentage of that response was based on the morphology which was associated with the reaction time. For instance, the microflower morphology obtained at 15 and 45 min displayed the best response compared to 60, 180 and 240 min. However, 45 min had a higher response than 15 min because the ‘petals’which make up the microflowers had opened up. This meant that the reaction not only took place on the surface of the microflower but also in between the ‘petals’. It is well known that humidity is an interferant and can either reduce or improve a sensor’s performance. The sensor’s performances towards NH3 varied depending on the relative humidity they were operating under. Annealing of the sensors showed improved performance at lower temperatures while higher temperatures led to reduced performance. OLA, a long chain ligand renders the semiconductor an insulator thereby reducing its performance. Effect of replacement of OLA with shorter chain ligands on the gas response was also investigated. Mercaptoethanol (ME) and ethanedithiol (1, 2EDT) were used as the short chain ligands and showed improved response towards a lower concentration of NH3. Application of the OLA/WS2 sensors in a tristimulus analysis proved that they can be used in chemical sensor arrays despite the fact that they are made of the same chemical composition. The various morphologies obtained at different time intervals provided the distinguishing factor between the nanostructures.Item Developing electrical tree resistant epoxy nanodielectrics with improved thermal properties(2017) Hank, Andrew MarvinTwo of the main contributors to high voltage insulation failure are thermal and electrical stresses. The failures may be mitigated by using nanodielectrics. The enhanced effect of nanoparticles in nanodielectrics is attributed to an interaction zone/interphase around each individual nanoparticle between the nanoparticle and host polymer. However, particle clumping or agglomerates are a major challenge in nanodielectric technology. In this work mitigation of the clumping challenges was explored through Rheology in determining optimal particle loading levels. The nanodielectrics studies were Boron Nitride and Carbon Nanospheres in Araldite Epoxy. The rheology results indicated an optimal loading level of 1.09 vol % to 1.35 vol% for Boron Nitride in Epoxy and 0.33 vol% for Carbon Nanospheres in Epoxy. Microscopy, dielectric spectroscopy, electrical tree characterisation, thermal expansion and laser flash analysis were used to validate the efficacy of the rheology results. The results indicated improved properties of the resultant dielectric such as; increased mechanical stiffness, increased electrical resistance and the percolation threshold, partial discharge suppression and increased thermal conductivity at the glass transition temperature. This study has established a rheology-based technique incorporated in the manufacturing process to determine the optimal filler loading of C/Epoxy and BN/Epoxy nanodielectrics. Future work is recommended as investigating either new particle types such as Sulphur hexafluoride in Carbon Nanospheres or mixtures of Carbon Nanospheres and Boron Nitiride.