published_or_final_version / Chemistry / Master / Master of Philosophy

A novel way of producing nanostructures in the past decade has been through the use of an anodized alumina template. This template has dense, self-ordered nanometric pores that grow in the oxide as the aluminum is being anodized. This technique is a fairly new method of processing nanostructures, and much study and research is presently being done to understand the formation mechanisms of the highly ordered pores. Ultra-pure aluminum foil and pure aluminium single crystal plates were anodized to create porous anodized alumina, and using it as a template to electro-deposit Nickel nanostructures. The effects different anodizing parameters have on oxide creation were studied, and the results obtained from studying the effects of substrate purity and texture of the anodized aluminum substrate on the morphology of the alumina template, through the use of X-ray diffraction and scanning electron microscopy. / Nickel nanowires were prepared by DC electrodeposition inside the porous alumina template with a gold-palladium coating serving a conductive base. The nanowires have a diameter of 65 nm, and their length depends on the deposition time. The nanowires can uphold a position perpendicular to the substrate by partially dissolving the alumina template. They also have a tendency to gather together once the template is partially removed.

Thesis (M. S.)--Mechanical Engineering, Georgia Institute of Technology, 2009. / Committee Chair: Bert Bras; Committee Member: David Rosen; Committee Member: Jeannette Yen

Contents: a. The oxidation of metals by atomic and molecular oxygen -- b. Calculation of the heats of formation of point defects in some transition metal oxides.

Anodic oxidation of a commercial dye, methylene blue (MB), from aqueous solutions using an electrochemical cell is reported. Data are provided on the effects of eight different types of supporting electrolytes, concentration of electrolytes, initial dye concentration, current and electrolytic time on the percentage removal of methylene blue. Anodic oxidation was found to be effective in achieving the removal of methylene blue from aqueous solutions. The optimised electrolytic conditions, for the removal of methylene blue (MB), were applied to the removal of azure A (AA), azure B (AB), azure C (AC), toluidine blue 0 (TBO), new methylene blue (NMB), dimethyl methylene blue (DMMB), thionine (TH), methylene green (MG), methyl violet (MV), Nile blue (NB), neutral red (NR), acridine orange (AO) and resorufin (RS) from aqueous solutions containing sodium chloride. Results indicated that between 84 to 100% of each dye of phenothiazine was removed during 60 minutes of electrolysis. The percentage removals for the phenothiazine dyes followed the following decreasing order: (MG ≈ MV) > (DMMB ≈ AA) > (AB ≈ AC ≈ NMB) > TBO > TH. However, the azine, acridine and oxazine dyes showed between 98 to 99% colour removal and the following decreasing order: NB ≈ NR > AO ≈ RS. Strongly electron withdrawing substituents such as nitro group or carbonyl group increases the degradation of the phenothiazine chromophore, whereas the electron donating groups such as amino and alkyl amino groups decrease the degradation. Anodic oxidation studies were extended to the destruction of eight permitted food colours, with azo and triarylmethane chromophore, from aqueous solution containing either sodium chloride or sodium sulphate as a supporting electrolyte. Again, sodium chloride was found to be the best supporting electrolyte and between 97 to 100% colour removal was achieved after 60 minutes of electrolysis. The percentage removal for the single azo based colourants followed the following increasing order: carmoisine > sunset yellow FCF > amaranth > ponceau 411 > tartrazine. However, the binary and ternary mixtures of food colour showed the following increasing order: blue > green > yellow food colours. The extractions and applications of 54 different types of natural dyes (53 from plants and one from animal origin) are evaluated using simple techniques. The extracted natural dyes were applied in dyeing three types of textile fabrics viz: a) paj silk, b) brushed cotton twill and c) crystallized shimmering satin. The effects of two eco-friendly mordants (alum and iron) on the dyeing process were compared with the dyeing process without mordants. The colour fastness to wash and light (both natural and artificial sun light) of these natural dyes were also assessed. The results showed that out of the 54 dyestuffs studied, 32 plants are potentially able to produce marketable natural dyes. These dyes produced good colour and met minimal performance standards for colour fastness to light and washing. The addition of mordants generally increased the fastness properties. Silk gave the best performance of dyeing uptake and stability. Cotton gave the poorest fastness properties. The overall results showed that, considering molecular associations, the fastness properties were of the order: anthraquinones and tannins> indigoid > flavones> flavonols > flavanols> carotenoids> anthocyanins. Anodic oxidation studies were extended to the destruction of ten natural dyes from aqueous solutions containing either sodium chloride or sodium sulphate as a supporting electrolyte. Anodic oxidation was effective in achieving the removal of green tea (35%), spinach (69%), Langdale yellow and turmeric (95%), carmine, saffron, henna (97%), beetroot, karkade and sumac (98%). However, TOC measurements and the UV analyses indicated that some organic intermediate compounds were formed in the presence of sodium chloride.

Under suitable conditions anodic metal oxidation leads to growth of complex porous structures. The initiation and growth of these structures is an interesting and challenging task for electrochemical modelling. One must identify chemical reactions in a multi-phase framework, derive a proper partial differential equations and solve them in time dependent domains. In this work, electrochemical model for the oxide growth in nano scales is presented. Physically motivated equations are formulated with precise mathematical meaning and existence of solutions is studied. Electrostatic potential fulfilling high-field conduction law and interfacial jump conditions is sought for. Numerical discretization is performed with the use of finite element method and free boundaries are tracked with characteristic level-set functions. Basic mechanism governing the growth of porous structures is given and numerical experiments are explained on it's basis. This thesis presents novel contributions to the electrochemical and mathematical picture of nanopores growth.

No description available.

An equifield strength model has been established to elucidate the formation mechanism for the highly ordered alumina pore arrays and titanium oxide nanotubular arrays prepared via a common electrochemical methodology, anodisation. The fundamentals of the equifield strength model was the equilibrium between the electric field driven oxidation rate of the metal and electric field enhanced dissolution rate of oxide. During the anodic oxidation of metal, pore initiation was believed to generate based on dissolution rate difference caused by inhomogeneity near the metal/oxide interface. The ionic nanoconvection driven by the electric force exerted on the space charge layer in the vicinity of electrolyte/oxide interface is established to be the main driving force of the pore ordering at the early stage of the anodisation. While the equifield strength requirement governs the following formation of the single pore and the self-ordering of random distributed pore arrays during the anodisation process. Hexagonal patterned Al2O3 nanopore arrays and TiO2 nanotubular arrays have been achieved by anodisation of corresponding metal substrates in proper electrolytes. The two characteristic microstructural features of anodic aluminium oxide (AAO) and anodic titanium oxide (ATO) were investigated using scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM). The observations of the hemispherical electrolyte/oxide and oxide/metal interfaces, uniform thickness of the oxide layer, as well as self-adjustment of the pore size and pore ordering can be well explained by the equifield strength model. Field enhanced dissociation of water is extremely important in determination of the porosity of anodic metal oxide. The porosity of AAO and ATO films was found to be governed by the relative dissociation rate of water which is dependent on anodisation conditions, such as electrolyte, applied voltage, current density and electric field strength. Using an empirical method, the relations between the porosity of the AAO (ATO) films and the anodisation parameters, such as electric field strength, current density and applied voltage, have been established. Besides, the extent that an external electric field can facilitate the heterolytic dissociation of water molecule has been estimated using quantum-chemical model computations combined with the experimental aspect. With these achievements, the fabrication of anodic metal oxide films can be understood and controlled more precisely. Additionally, the impacts of other factors such as the electrolyte type and the temperature effect on the morphology of the anodic products were also investigated. Some important experimental evidences on the pore diameters variation with applied voltage in the anodisation of aluminium and the titanium were obtained for future investigation of the anodic metal oxide formation processes.

The topic of this thesis is the design of an automatic preanodizer to be used to preadjust precision tantalum resistors by anodizing their surfaces. The purpose of the preanodizer is to generate a controllable voltage ramp to be applied to a set of tantalum resistors which are immersed in a suitable electrolyte. The slope and maximum value of the voltage ramp are set to produce the current required for proper anodization of the resistors. The rate of anodization determines the rate of change of the resistor value. This thesis covers the design of an electronic device to meet a set of requirements specified by the Western Electric Company. The primary tasks the device has to perform are the generation of a linear voltage ramp of adjustable slope and maximum voltage and the supply of a given maximum current. Metering circuits are provided to measure, store and display the current and the maximum value reached by the ramp voltage.

<p>An electrolytic cell was designed and constructed for the preparation of TiO2 nanotubes. Conditions of anodic oxidation were established to reproducibly prepare TiO2 nanotubes of average length 35-50 &mu / m vertically orientated relative to the plain of a pure titanium metal sheet. A non-aqueous solution of ethylene glycol containing small percentage of ammonium fluoride was used as the electrolyte with an applied voltage of 60 V. The morphology and dimensions of the nanotube arrays were studied by scanning (SEM) and transmission (TEM) electron microscopy. The effect of calcination under different conditions of temperature and atmosphere (nitrogen, argon and air) were assessed by both X-ray diffraction (XRD) and cyclic voltammetry (CV). Cyclic voltammetry studies were made possible by construction of a specially designed titanium electrode upon which the nanotubes were prepared. CV studies established a positive correlation between crystallinity and conductivity of the nanotubes. Doping of the nanotubes with nitrogen and carbon was established by elemental analysis, X-ray photoelectron spectroscopy (XPS) and Rutherford back scattering (RBS). The effect of nonmetal doping on the band gap of the TiO2 nanotubes was investigated by diffuse reflectance spectroscopy (DRS).</p>