3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item Thermodynamic and Kinetic Study of the Production of Carbon Nanotube Yarn from Chemical Vapour Deposition Reactor(2019) Mahangani, NdanganeniOvercoming the low production rate of carbon nanotubes (CNTs) could be instrumental to reducing their cost of production and enhancing its wide application. Understanding the kinetics of the production of yarn from CNTs, through kinetic model development could assist in the optimisation of the production and this makes the application of yarn as a replacement to filament in incandescent bulb promising. In this study, kinetic of the production of CNTs, an intermediate in the production of yarn, is presented. Several experiments were conducted using Ferrocene as catalyst in a CVD reactor. The reaction of CH4 on Ferrocene catalyst is via heterogeneous catalysis because methane is in gaseous phase and Ferrocene in solid phase. Langmuir Hinshelwood was used to develop a kinetic model based on these two- phase phenomena. The effect of CH4 concentration was investigated as well. The derived kinetic model fits the experimentally measured data with 95% confidence interval. Good quality CNTs were obtained at methane flow rate of 125 ml/min. The CNTs produced at this flow rate has high purity, low tube diameter and spinnable long array as compared to the one produced at 100 ml/min and 150 ml/min. Yarn was produced at this flow rate (125 ml/min) at a reactor temperature of 900, 950 and 1000oC. Characterisation techniques such as Scanning Electron Microscopy (SEM), Transition Electron Microscopy (TEM), Energy-Dispersive X-ray (EDX), Gas Chromatography (GC) and Raman Spectroscopy were used to analyse the product from experiments. At all studied reactor temperatures CNT yarns were synthesised as observed in SEM images. Four Probe method was used to measure the electrical conductivity of as-produced CNTs and yarn. By using the proposed CH4 flow rate (125 ml/min) the produced CNT yarn was found to be metallic and electrically conductive. The studied electrical conductivity of as-produced CNTs were found to be approximately 2 times higher than their yarn. Yarn produced at reactor temperature of 950oC proved to have high quality and more electrically conductive than those produced at 900 and 1000oC. The thermodynamic properties of CNTs yarn were studied using TGA/DSC equipment. Polynomial models for predicting Specific heat capacities of yarns produced in this study were developed. The results showed that the temperature at which yarn is produced has an effect on a thermodynamic property such as heat capacities, enthalpy and entropy.Item Improving polysulfone membrane resistance to carbon dioxide induced plasticization during natural gas separation using functionalized carbon nanotubes(2019) Aberefa, Oluseyi AdebisiCommercial natural gas, generally methane (CH4), is described as the most efficient energy source with a calorific value per mass of approximate 50.1 MJ/kg. However, raw natural gas is made up of mostly methane and a considerable quantity of other components like C2+, CO2, water and hydrogen sulphide (H2S) in varying composition from well to well depending on the geographical location and condition. Membrane technology for gas separation technology has become an acceptable method for CO2 removal from natural gas. Majority of commercial natural gas separation membrane systems are polymeric due to processing feasibility and low cost. However, conventional polymeric membranes have reached a trade-off threshold between permeability and selectivity, and of great concern is CO2 induced plasticization due to harsh industrial conditions such as high feed pressure. The addition of carbon nanotubes (CNTs) into polymers appears to offer a unique solution to the deficiencies of conventional polymeric membrane systems. In this study, carbon nanotubes (CNTs) were produced using a catalyst-assisted chemical vapour deposition method. Ferrocene was used as the catalyst. Effects of carbon sources (acetylene and methane) and production conditions (temperature and type of carrier gas) on the physicochemical property of as-produced CNTs were investigated using field emission scanning electron microscopy (FESEM), energydispersive X-ray spectroscopy (EDS), electronic precision balance (EPB) and Raman spectroscopy. The best of as-produced CNTs was purified and functionalized using carboxylation protocol. The surface chemistry, thermal stability, textural property and crystallinity of the functionalized CNTs (FCNTs) were obtained using Fourier transform infrared spectroscopy (FTIR), thermo-gravimetric analysis (TGA), N2 physisorption at 77 K and X-ray diffraction (XRD), respectively. CNT produced from methane and argon at 900 oC displayed the best quality with ID/IG ratio of 0.17 while the CNT produced from acetylene and mixture of argon and hydrogen at 1000 oC has the highest yield of 1.78 mg/s. FTIR confirms successful functionalization of CNTs. The degree of functionalization obtained from TGA is consistent with that of EPB. N2 physisorption at 77 K indicates an increase in pore volume and average pore size of the FCNTs indicating more adsorption sites for the adsorbate. Thereby suggesting that FCNT could be a good filler in membrane synthesis for gas separation. Dense mixed matrix membranes (MMMs) were synthesized by incorporating functionalized CNTs (FCNTs) at different wt. % (0.2, 0.5, 1.0, and 1.5) into polysulfone (PSF) using evaporation method. The MMMs were characterized using field emission scanning electron microscopy (FESEM), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), atomic force microscopy (AFM) and texture analysis (TA). Single gas permeation experiment was conducted using a custom-built permeation set-up via constant volume variable pressure technique while mixed gas permeability was measured on a constant pressure variable volume gas permeation apparatus. Surface and cross-section morphology of the MMMs as depicted by FESEM show that the membranes are dense, indicating that the membranes consist of dense structure whose porosity is not observable under an electron microscope. Depth-at-max analysis of AFM images of the synthesized membranes further supports that the pores of the MMMs possess dense structure. TGA indicates improved thermal stability of the MMMs with increasing wt. % loading of FCNTs. DSC results show that the glass transition temperature (Tg) of the MMMs was also improved with the addition of FCNTs. However, the increase in wt. % loading of FCNTs does not significantly increase the Tg of the MMMs. Tensile strength and Young’s modulus of the MMM obtained from TA show an increase in these mechanical properties with the addition of FCNTs. Single gas permeation results indicate an increase in CO2 permeability as the wt. % loading of FCNTs increases up to 1.0 wt. % loading. Mixed gas separation shows similar trend without sacrificing CO2/CH4 selectivity. CO2 induced plasticization of the membranes was investigated experimentally and by phenomenological modelling. The sorption isotherms showed a typical dual-mode sorption model behaviour for all the types of synthesized membranes with the CO2 concentration increasing rapidly at low pressures, which indicated a hole-filling of the Langmuir sites. At higher pressure, the rate of increase in the concentration declines due to the saturation of the Langmuir sites, the further increase in the concentration at higher pressure is because of the penetrant sorption in Henry’s law sites. The permeation isotherm of pure PSF indicated plasticization onset at CO2 pressure of 12 bar. The MMMs showed no sign of plasticization over the pressure range tested (0-20 bar). This indicated an improvement in the CO2 plasticization resistance of PSF by the addition of FCNTs. The developed model assumed three possible interface interactions in the MMMs; polymer matrix – polymer matrix, CNT – polymer matrix, and aligned CNT – CNT interface interactions. The model also accounted for the plasticization of the membrane. The model equation was solved by using the Levenberg-Marquardt algorithm in the Matlab environment to obtain the plasticization potential and the fraction of the gas held in Langmuir sites that have mobility within the membrane. The model CO2 permeability is in good agreement with CO2 permeability obtained experimentally. The PSF plasticization potential obtained is similar to ones reported in the literature. The fraction of the gas held in Langmuir sites that have mobility within the membrane increases with an increase in FCNT wt. % loading which shows high diffusivity through the microvoids with respect to diffusivity through the polymer matrix. This is an indication of an increase in Langmuir sorption sites for the penetrant thereby improving gas permeabilitItem Characterization and leaching studies of cobalt and nickel-polysulfone nanocomposite membranes for potential water treatment(2018) Enemuo, Ngozi DorisIn spite of the undeniable importance of water for survival, a considerable number of people have no access to clean usable water as a result of high contamination of the water sources available to them. The consumption of the unclean water leads to epidemic diseases such as cholera and some other diseases whose adverse effect on the health of the people can range from mild to fatal. Although the conventional methods of water treatment are available, the challenges associated with their applications prompts researches towards finding more efficient alternatives. Application of nanotechnology in water treatment membranes is one of such unconditional means that is gaining ground in the field of membrane technology. This work aimed at preparing nanocomposite membranes with a potential application in water treatment. This was done by modifying polysulfone using nickel and cobalt nanoparticles aimed at improving the polysulfone for effective usage in water treatment. The properties of the modified membranes were obtained and the stability of the embedded nanoparticles assessed. The synthesized nickel and cobalt nanoparticles were characterized using X-ray diffraction, and Transmission electron miscroscopy. The properties of the membranes were determined using Scanning electron miscroscopy, Atomic force miscroscopy, Thermogravimetric analysis, Contact angle measurements. The leaching of the nanoparticles from the nanocomposite polysulfone membranes was assessed using Inductively coupled plasma mass spectroscopy. For the membranes modified with Ni, the hydrophilicity improved from 89.05o to 75.49o. Co nanoparticles also improved the hydrophicity from 89.05o to 80.52o, however the PSF-10 membrane exhibited contact angle of 89.45o as a result of agglomeration at such high amount of Co. The pore sizes were increased from 0.307 µm to 0.833 µm and 0.307 µm to 1.79µm for the Co and Ni modified membranes respectively. In addition, the embedded nanoparticles were stable in the PSF matrix.Item Group III-V nanostructures application in gas sensing(2018) Nyembe, Sanele G.The advent of nanoscience and nanotechnology has made it possible to control several properties such as material shape, size and stability. However, different production approaches are often required. Engineered surfaces with tailor-made properties such as large surface area or specific reactivity are used routinely in a range of applications such as in fuel cells, catalysis, etc. Nanomaterials with unique morphologies have been developed and used in fields such as electronic device manufacture, chemistry and engineering. Synthesis of gold icosahedral (Ih) and decahedral (Dh) nanostructures was successfully achieved through a two-step heterogeneous nucleation process. Citrate stabilised seeds were used to grow these nanostructures. Cetyltrimethylammonium bromide (CTAB) was used to promote fast growth of Au nanostructure along the [111] crystal lattice plane. Dh and Ih nanostructures are known to be thermodynamically unfavourable above size of 5 nm. Successful growth of such nanostructures above this critical size limit was explained in terms growth kinetics. Gold nanostructures were found to have an average particle size of 50 nm and a narrow size distribution range of 45 nm to 55 nm. The seeds were fast-handled during growth to enhance the formation of these nanostructures. In-depth characterisation of these nanostructures confirmed that they formed via crystal twinning mechanism. Synthesis of gold nanostructures with different sizes was achieved through a three-step heterogeneous nucleation process. Different types of seeds were prepared using different stabilizers (Citrate and CTAB). Citrate and CTAB stabilized seeds were used to grow Au nanostructures separately. The CTAB stabilized seeds showed polydispersity, suggesting the presence of various shapes. These seeds produced agglomerated particles of various shapes with a wide particle diameter distribution ranging from 50 nm to 200 nm. The triangular Au nanostructures present in the mixture had a 3 dimensional morphology (i.e. pyramid shape) as confirmed by atomic force microscopy (AFM). The citrate stabilized seeds were monodisperse and they yielded well dispersed Au nanostructures with uniform morphologies. These Au nanostructures had an average particle size of 50 nm and a narrow size distribution range of 45 nm to 55 nm. iii Laser assisted synthesis of silicon nanowires (SiNWs) was achieved through the use of gold and nickel nanoparticles as catalysts. The diameter of the resulting SiNWs was found to be dependent on that of the catalyst. The gold catalysed silicon nanowires were unevenly curved and branched owing to the high kinetic energy possessed by gold nanoparticles (AuNPs) at relatively high processing temperature. The use of nickel as catalyst resulted in the formation of several SiNWs on a single nickel catalyst due to interconnection of the nickel metal particles at processing temperature. The morphology of SiNWs catalysed by both nickel and gold was controlled by optimising the laser energy during ablation. Indium phosphide nanowires (InPNWs) with an average diameter of 87 nm were successfully synthesized through thermal chemical vapour deposition (CVD) method. The smooth surface nanowires showed a relatively narrow size distribution of 70 nm to 105 nm. Temperature programmed deposition (TPD) was used to study the thermodynamic behaviour of gas desorption. The study revealed that gaseous CO and CH4 molecules bind to InPNW surface through chemical and physical adsorption. Redhead method was used to estimate the enthalpy energy of desorption for CO and CH4 to be 142 kJ/mol and 38 kJ/mol. The sorption temperature ranges were found to be 220 ̊C to 260 ̊C for CO and -50 ̊C to -20 ̊C for CH4. InPNWs were used to fabricate a gas sensor electronic device and were tested for performance. The device showed a quick response time of 29.19 s for CO at 250 ̊C. Indium arsenide nanowires (InAsNWs) with an average diameter of 45 nm were successfully synthesized using homogeneous catalysis approach and chemical vapour deposition method. Succesful synthesis of InAsNWs was achieved at a temperature of 700°C suggesting that solution-liquid-solid growth mechanism was involved. Synthesis conditions were optimised to minimise InAsNWs polytypism and stacking faults. The results showed that InAsNWs have significant adsorption sites and affinity for CO and H2S gases due to the formation of electron accumulating surface. The calculation of enthalpy energy of desorption revealed that interaction of InAsNWs and CO was through physisorption. InAsNWs showed a response time of 72s for CO at 250 ̊C. Characterization of the nanostructures was carried out using high resolution transmission electron microscope (HRTEM), Raman spectroscope, high resolution scanning electron microscope (HRSEM), UV-Vis spectrometer, X-ray diffractometer (XRD), temperature programmed desorption (TPD) and diffuse reflectance infrared Fourier transform spectroscope (DRIFTS).Item Synthesis of carbon nanotubes - polyphenylene sulfone composite membranes for waste water treatment from petroleum sources(2017) Phasha, Motshamonyane JacobOil and gas operations produce high volumes of wastewater in the form of finely dispersed oil/ water (o/w) emulsions, which have detrimental effects on the environment. The current most feasible method used to mitigate the environmental impacts caused by the emulsion (produced water) from oil and gas operations is polymer membrane technology. However, polymer membranes are susceptible to fouling and concentration polarization, which leads to the necessity for frequent membrane replacement, thus loss of operating time and high operation cost. This motivates the need to investigate ways of modifying the polymer membrane in order to make it more resistant to fouling and concentration polarization. This study is concerned with circumventing the challenges experienced by polymer membrane during crude oil/ water mixture ultra-filtration by infusing the polymer membrane with nano particles. The aim of the research was to investigate the influence of addition of CNTs on the modified membranes in treatment of waste water from petroleum source. The Wet Impregnation method was used for the preparation of the bimetallic catalyst (Fe-Co catalyst supported on Zeolite), Chemical vapor deposition (CVD) method was used to prepare the carbon nanotubes (CNTs) and Phase inversion (PI) method was used for the preparation of the polymer nanocomposite membrane. The bimetallic catalyst was characterized using scanning electron microscope (SEM) and X-ray diffraction (XRD). The CNTs were characterized using Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR) and Transmission electron microscopy (TEM). The prepared polymer nanocomposite membranes were characterized using SEM, FTIR, goniometer (for contact angles) and TAXT plus texture analyzer (for tensile strength test). Functionalized carbon nanotubes were used as membrane fillers or modifiers to improve the filtration properties of the polymeric membrane, ultimately forming nanocomposite polymer membranes. This increased hydrophilicity, chemical, mechanical and physical properties of the polymer membrane, made them to perform better during filtration than pristine polymer membranes. The performance of the nanocomposite membranes were evaluated and it was determined that the nanocomposite polymer membrane with a loading 0.4 wt.% functionalized carbon nanotubes performed better than pristine membrane and other CNTs loaded nanocomposite polymer membranes. The pristine membrane (0 wt% CNTs) showed a higher contact angle (79o) which crosses ponds to the inability to soak up water. The 0.4 wt% nanocomposite polymer membrane showed the lowest contact angle of 72 o, this validated an improvement in the properties of the membrane, in particular hydrophilicity. The 0.4 wt% nanocomposite polymer membrane showed a superior mechanical strength, with a breaking force at 4 N relative to the other membranes of the same thickness. 0.4 wt% nanocomposite polymer membrane showed the highest permeate flux of 120 L/m2.h compared to the pristine membrane, which showed a permeate flux of 63 L/m2.h. The permeate flux of 0.4 wt% nano-composite polymer membrane increased with the operating pressure.Item A CO2 capture technology using carbon nanotubes with polyaspartamide surfactant(2016-07-13) Ngoy, Jacob MasialaTechnologies for the separation of CO2 from flue gas require a feat of engineering for efficient achievement. Various CO2 capture technologies, including absorption, adsorption, cryogenics and membranes, have been investigated globally. The absorption technology uses mainly alkanolamine aqueous solutions, the most common being monoethanolamine (MEA); however, further investigation is required to circumvent its weakness due to degradation through oxidation, material corrosion and high energy costs required for regeneration. Attractive advantages in adsorption technology, including the ability to separate the more diluted component in the mixture with a low energy penalty, have been a motivation for many researchers to contribute to the advancement of adsorption technology in CO2 capture. The challenge in CO2 adsorption technology is to design a hydrophobic and biodegradable adsorbent with large CO2 uptake, high selectivity for CO2, adequate adsorption kinetics, water tolerance, and to require low levels of energy for regeneration processes. The existing adsorbent such as activated carbon, silica gel, zeolites, metal organic frameworks and others, have been ineffective where moisture occurs in flue gas. This work provides an advanced adsorption technology through a novel adsorbent, MWNT-PAA, designed from the noncovalent functionalization of multi-walled carbon nanotubes (MWNTs) by polyaspartamide (PAA) as product of amine grafted to polysuccinimide (PSI). Three types of PAA were prepared using ethylenediamine (EDA), 1, 3 propanediamine (PDA) and monoethanolamine (MEA) drafted to PSI to give PSI-EDA, PSI-PDA and PSI-MEA respectively. The CO2 adsorption capacity was 13.5 mg-CO2/g for PSI-PDA and 9.0 mg-CO2/g for PSI-MEA, which decreased significantly from PSI where the CO2 adsorption capacity was 25 mg-CO2/g. PSIEDA was selected as PAA, because the CO2 adsorption capacity was 52 mg-CO2/g which doubled from PSI. The polymer polyethylenimine (PEI), the most commonly polymer used in CO2 capture, was found to be non-biodegradable, while the polymer PAA showed the presence of CONH as a biodegradable bond functionality, occurring in the MWNT-PAA, as confirmed through Fourier Transform Infrared (FTIR) analysis. The adsorbent MWNT-PAA was demonstrated to have a water tolerance in the temperature range 25-55 ℃, where CO2 adsorption capacity increased with the increase of water in the adsorbent. The highest CO2 adsorption capacity recorded was 71 mg-CO2/g for the moist MWNT-PAA using 100% CO2 and 65 mg-CO2/g for the mixture of 14% CO2 with air. Under the same conditions, the dry MWNT-PAA adsorbed 70 and 46 mg-CO2/g respectively (100%, 14% CO2). The 2 regenerability efficiency of the MWNT-PAA absorbent was demonstrated at 100 ᵒC; after 10 cycles of adsorption-desorption 99% of adsorbed gas was recovered in the desorption process. The heat flow for the thermal swing adsorption system resulted in the net release of heat over the complete cycle; a cycle includes adsorption (heat release) and desorption (heat absorbance). Thus, this MWNT-PAA adsorbent demonstrates an advantage in terms of overall energy efficiency, and could be a competitive adsorbent in CO2 capture technology.Item CVD synthesis of nitrogen doped carbon nanotubes using iron pentacarbonyl as catalyst(2012-02-24) Ghadimi, NafiseIn this dissertation, the synthesis of nitrogen doped carbon nanotubes (N-CNTs) was performed successfully, using a floating catalyst chemical vapour deposition (CVD) method. Fe(CO)5 was utilized as the catalyst and acetonitrile and toluene as nitrogen and carbon sources respectively. Two different procedures were used to add reagents to the reactor: an injection method and a bubbling method. The effect of nitrogen concentration and physical parameters such as reaction temperature, gas flow rate on the morphology, crystallinity and thermal stability of the tubes was studied. The synthesized materials were characterized by means of Raman spectroscopy, TGA and TEM analyses. The presence of nitrogen was confirmed by the presence of the bamboo formations in the tubes by TEM. A comparison of the data from the numerous reactions revealed that N-CNTs can be made from Fe(CO)5 and acetonitrile. Further the main conclusions achieved using the injection method were: i) the maximum number of tubes with bamboo structure were made using on acetonitrile concentration of 15%, ii) The best growth temperature to make N-CNTs was 850 oC, iii) An increase in acetonitrile concentration decreased the yield of NCNTs and iv) Tubes with the narrowest outer diameters were made using an acetonitrile concentration of 15%.