3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item Investigation of the effects of zinc oxide nanoparticles and synthesized cellulose nanocrystals (CNCs) on emulsion-based drilling fluids(2019) Aka, Tiemele Wilfried AndersonDrilling Mud holds an important role in the drilling process in such a way that it is a determinant key to the success of the operation as well as the money spent throughout the process. Indeed the success and the cost of the operation can be severely impacted by some challenges experienced while drilling such as temperature and pressure conditions which leads to fluid loss, fluid deterioration...As a result there is a need to formulate a fluid with desirable rheological properties to withstand such undesirable parameters. Therefore this work was aimed to improve emulsion drilling fluids (EDFs) based nanoparticles with enhanced properties. Many investigations were performed to find a proper emulsion stability as well as a good drilling fluid performance. The stability of the prepared emulsion drilling fluids was done using surfactant with different concentrations for several days. After several days of preparation, the EDFs containing DTAB as surfactant have showed a better emulsion stabilizer compared to the Triton X-100 ones. In addition an investigation combining both NPs and surfactants confirmed the used of NPs to improve DF and revealed the effective use of ZnO NPs for drilling fluids application and preferentially with DTAB as surfactant. Following that result, the 2nd part of the work was based on the synthesis and characterization of CNCs as NPs to formulate EDF with DTAB as surfactant. The CNCs NPS were successfully obtained via the method of oxidation of microfibrillated cellulose through TEMPO-mediate and after characterization using TEM, spherical NPs with small size varying from 10-50nm were observed. The FANN® Model 35 viscometer served to display the behavior of the shear stress and viscosity of the prepared fluids against variable shear rate at variable NPs and temperature concentration. The rheological and filtration properties were increase with increase in CNCs content from 0.8 to 1.2% of fluid in room temperature and with an increase in temperature.Item Iinvestigation of ZnO, and AZnO and rare earth doped ZnO thin films for spectral conversion and application to solar cells(2018) Otieno, Francis OtienoRecently Zinc oxide has drawn a resurgent attention in semiconductor industry due to its interesting properties with diverse application potential. These properties include high exciton binding energy, high resistance against radiation, high breakdown voltage, insensitivity to visible light, and easy wet chemical etching. The high quantum efficiency for emission by ZnO has seen it being considered a strong candidate for solid-state white lighting applications as well as transparent conductor electrode in solar cells. In order to realize efficient utilization of the multi-functional properties of ZnO for electronic and opto-electric applications, ZnO is usually doped with different elements. Such doping is aimed at enhancing and controlling its electrical, optical and multi-functional properties. Typical dopants widely used are trivalent atoms categorized as group III in the periodic table (Al, In, Ga) through substitution of cations. The as-grown ZnO thin film is usually n-type semiconductor with structural, electrical and optical properties that can be varied depending on the growth conditions as well as post deposition treatment such as thermal annealing. The use of RF sputtering for ZnO deposition has been explored in this work through varying deposition time, RF power and the partial pressure of oxygen. The films were then subjected to ex-situ thermal annealing in Argon filled furnace leading to a significant increase in grain size. Rare earth (RE) doping of materials has been widely investigated owing to the prominent and desirable optical and magnetic properties. Typically trivalent rare earths elements such as Sm+, Tb3+ and Eu3+ are investigated in this research project. ZnO doped with RE has exhibited electroluminescence, thus highlighting its potential for photovoltaic applications as a bi-functional layer. A doped ZnO layer is thus simultaneously utilized as transparent conducting electrode and as a spectral conversion layer. The RE doped luminescent materials provide an opportunity to effectively use the high energy and sub-band gap energy photons from the solar spectrum that would have otherwise been lost in direct band gap absorbers. In solar cells, they have been applied with an intention to reduce the fundamental thermalization losses arising as a result of the intrinsic properties of the semiconductor material namely: (a) sub-bandgap photon loss (b) thermalization of charge carriers resulting from absorption of high energy photons. From the X-ray diffraction (XRD) patterns both pristine and doped ZnO thin films showed growth along the c-axis of the wurtzite structure. The peaks were found to match the reflection planes of (100), (002) and (102) with all the diffraction peaks being well indexed to the wurtzite structure of ZnO of the space group P63mc, which is consistent with the standard values reported in JCPDS, card no. 03-0888. The structural properties of the material were investigated using a -scanning electron microscope (SEM) and Atomic force microscopy (AFM) where the particle size, roughness, skewness and kurtosis were found to change with growth condition and annealing temperature. Most importantly, the results indicated that the photoluminescence (PL) properties reflect the quality of the pristine and doped ZnO. The films were then used in the fabrication of the solar cells as a bi-functional layer and thus as a proof of concept of good transparent conducting oxides (TCOs) and for spectral conversion. RBS measurements indicated the depth profile distribution of Zn, O and various rare earths which showed homogeneity in depth distribution without any external impurity.Item An assessment of Sn-W-Nb-Ta oxide minerals from the Bugesera district of Rwanda and geological context(2018) Buurman, InnesThe Sn‐W‐Nb‐Ta oxide minerals of Rwanda are rarely found in the same area and predominantly occur in different deposits. Gatumba is known for its pegmatite related Nb‐Ta, and Sn mineralisation, Nyakabingo for its hydrothermal vein type W mineralisation, and Rutongo for its hydrothermal vein type Sn mineralisation. Very little geological research has been performed on the Bugesera district in Rwanda. Therefore, the aim of this study is to enhance the knowledge base on the Bugesera district by studying four mining areas (Nyagasagara, Gakurazo, Kageyo and Kageyo Extension) on the Hard Metal Rwanda (HMR) license, their locations relative to regional structures and G4 granites, and the mineral grade at each of the deposits. The mineralogy and mineral chemistry were only studied on the concentrate material from Nyagasagara, for comparison with literature and to determine whether the mineralisation was primary or secondary. The HMR license is located in between the Bugesera granite and the Bugesera syncline fold. Nyagasagara and Kageyo occur adjacent (< 4km) to known large scale structures, granites and/or pegmatites, Gakurazo is located in between Nyagasagara and Kageyo, and the Kageyo Extension is located on the rim of the Bugesera granite. No outcrops of granite and/or pegmatite were observed at any of the sites. The highest mineral grade was recorded at Nyagasagara Tunnel 1 (5.56% cassiterite) with the lowest grade recorded at Kageyo and Kageyo Extension (0.01% coltan). Around 60% of the cassiterite grains from Nyagasagara, reports to the coarser size fraction of +4 mm and +2 mm, with the shape of the grains ranging from angular to very angular. Cassiterite is the only oxide mineral of interest present at Nyagasagara, as confirmed by the whole rock chemistry and optical microscopy on the concentrate samples from Nyagasagara. The mineral chemistry of the cassiterite plots in the rare element granite and pegmatite field on the binary Nb + Ta and Fe + Mn diagram. Most of the cassiterite contains more Nb compared to Ta (Nb>Ta) and minor Fe2O3. The cassiterite also contained numerous coltan inclusions and exsolution products. This study concludes that the HMR license area is situated in a suitable regional geological setting for Sn‐ W‐Ta‐Nb oxide minerals to be found. However, the Nyagasagara deposit is an eluvial deposit consisting of only cassiterite minerals, which have a magmatic origin, believed to be related to rare element granites.Item The synthesis, characterization and photocatalytic activity of various morphologies and sizes of ZnO photocatalysts(2018) Nkabinde, Siyabonga SiphoThe increasing abundance of industries, together with the high human population density has prompted the pollution of the hydrosphere with organic and inorganic matter at a very high rate. To triumph over the problems caused by water pollution, and to comply with strict environmental regulations, researchers have been focusing on the development of new or improvement of existing water purification processes. One such process is known as Advanced Oxidation Processes (AOPs), and is based on using light quanta in conjunction with a photocatalyst (TiO2, ZnO, CdS, etc.) in order to degrade organic pollutants in an aqueous medium. An important factor in AOPs is the type of the photocatalyst being used as it controls the rate at which dyes are degraded when exposed to light quanta with energy higher or equals to its band gap energy. The photocatalytic activity of a photocatalyst is controlled by its properties such as surface area, crystallinity and morphology. These properties can be controlled by varying reaction parameters such as reaction time, type of precursor, and pH used when it is being synthesized. Furthermore, the photocatalytic activity also depends on operational parameters under which the photocatalyst is used. The operational parameters include factors such as the photocatalyst concentration, concentration of organic compounds in solution, pH at which the dye exists in nature, and light intensity. Zinc oxide nanoparticles were synthesized was synthesized using microwave assisted heating method. Microwave assisted heating method was chosen over conventional colloidal method due to its ability to heat reaction mixtures homogeneously and very short reaction times. The synthesis of ZnO nanoparticles using a co-precipitation method between Zn(CH3COO)•2H2O and NH3/NaOH via microwave assisted heating and their photocatalytic activity investigated. The crystalline structure, morphology and optical properties of as-synthesized ZnO were characterized by Powder X-ray diffraction (PXRD), Transmission Electron Microscopy (TEM) Scanning Electron Microscopy (SEM) and Ultra-violet Visible (UV-Vis) and Photoluminescence Spectrophotometer. ZnO was first synthesized using cetyltrimethylammonium bromide (CTAB) as capping agent and ammonia (NH3) as precipitating agent in order to assess its photocatalytic activity in the degradation of Rhodamine B. Rhodamine B was used as a model dye as it is frequently used in industry. Rod like ZnO nanoparticles were produced with an average length of 608 nm and a width of 205 nm. The operational parameters to be used during the course of the research were determined by performing photocatalyst concentration, dye concentration, light intensity, and pH studies. The extent of direct hydrolysis of the Rhodamine B dye under UV light without the photocatalyst was first measured to eliminate the possible contribution from the undesired variables to the overall efficiency. It was observed that using only light without a photocatalyst or vice versa could not degrade the dye, a combination of the two was needed for dye degradation. The ZnO photocatalyst was found to efficiently degrade the Rhodamine B dye at photocatalyst concentration of 160 ppm, light intensity of 210 W and at pH 8. Morphological diversity was achieved by varying reaction parameters such as pH of the precursor solution and changing the type of zinc metal salts. Varying the pH changed the growth nature of ZnO forming pseudo-spherical nanoparticles at pH 7, bullet-like nanoparticles at pH 10 and rod-like nanoparticles at high pH of 14. The photocatalytic degradation experiments revealed that ZnO nanoparticles with different morphologies degraded Rhodamine B at different rates. The pseudo-spherical, bullet-, and rod-like nanoparticles degraded the dye at 150, 180, and 210 min, respectively. The difference in the rate of degradation was attributed to surface area differences and proportion of exposed polar facets (i.e. [0001], [000-1]) on the surface of the different morphologies. Different sizes of the ZnO nanoparticles were prepared by varying the reaction time length, with short reaction time producing smaller particles than the longer reaction time. The photocatalytic activity of the nanoparticles was examined for the photocatalytic degradation of Rhodamine B as the test dye in aqueous solution under solar irradiation. The size of the nanoparticles was found to be highly dependent on the reaction time. Particle size influenced the photocatalytic activity with the smaller sized nanoparticles being more efficient in degrading Rhodamine B than the larger nanoparticles. The reason for the high catalytic activity was attributed to higher surface area. The as-synthesized ZnO photocatalyst showed good photocatalytic stability and can be reused four times with only gradual loss of activity. Thus, it is an efficient photocatalytic material for degrading contaminated coloured wastewater for reuse in textile industries under mild conditions. The syntheses of ZnO nanoparticles using different precursors Zn(NO3)2•6H2O, Zn(CH3COO)2•2H2O, ZnCl2, and ZnSO4.H2O, resulted in different structural, optical and photocatalytic activity. The difference in the properties of the nanoparticles synthesized in this study was attributed to the fact that the counter anions (i.e. NO3-, Cl-, SO42-, and CH3COO-) in the zinc metal salts are capable of coordinating to the crystal planes of ZnO differently. The different coordinating abilities of the counter anions resulted in dissimilar rates of growth and different morphologies. The BET surface area measurements were found to be 13.25, 12.34 12.02, and 1.24 m2g-1 for the NO3-, CH3COO-, Cl-, and SO42- counter anions, respectively. The difference in surface area indicated that the binding abilities of the counter anions to the decreased in the order NO3- > CH3COO- > SO42- > Cl-, with the NO3- anion binding more strongly to the crystal planes (i.e. [0001] and [000-1]) of ZnO and hence hindering rapid growth of the nanoparticles. Photocatalytic degradation studies indicated that the nanoparticles with the highest surface area were more active than those with lower surface area.