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.

This thesis presents the design and fabrication of a re-configurable micromirror array which can be used as a component of an optical microspectrometer. In an optical microspectrometer, an array of mechanically positionable micromirrors can be implemented as a reconfigurable exit slit to selectively focus particular wavelengths of a diffracted spectrum onto the detector stage. The signal to noise ratio and response time of an optical microspectrometer can be vastly improved by this technique. In the approach presented here, a hybrid bulk- and surface- micromachining process is demonstrated for fabrication of a 1XN array of micromirrors. The reconfigurable micromirrors presented here comprise of two elements, a surfacemicromachined positioning mechanism, and a bulk-micromachined mirror. These elements are finally integrated using a flip-chip bonding technique. The integrated micromirror assembly can be positioned by means of a driving mechanism consisting of arrayed electrothermal actuators. Various techniques for fabricating the micromirror array components are discussed in detail in this thesis along with a review of techniques applicable for integrating the individual components. In order to enhance the efficiency of the positioning system, the classic electrothermal actuators were redesigned in this research. The modified design of thermal actuators is introduced in this thesis. An analysis of the modified thermal actuators is also presented to demonstrate the validity of the suggested modifications.

Development and studies of new electrochemical methods for the detection of various biologically and environmentally relevant substances are the focus of this dissertation. A dual amperometric sensor, capable of the simultaneous, real-time determination of NO and O,2, has been developed and optimized. Many tests were performed in order to reduce cross-talking between the two sensors, and an electro-deposited polymer, poly-5-amino-1-naphthol, was shown to reduce the cross-talking to insignificant levels. The use of bismuth-based electrodes in the detection of various metals has been explored. A bismuth bulk electrode has been developed, optimized, and used for the individual and simultaneous determination of Pb(II), Cd(II), and Zn(II). The fundamental electrochemistry of several bismuth-based electrodes in the system used for Cr(VI) analysis has also been explored, and many interactions among the electrode material, ligand, and analyte were observed, particularly the formation of a soluble bismuth-ligand complex. Electrochemical analysis of Cr(VI) was attempted at all of the bismuth-based electrodes, with success at the thin bismuth film electrode. A series of surface modifications were made to the glassy carbon substrate, in an attempt to remove any co-adsorbed contaminants and to understand the sensitivity of the chromium detection process. Inevitably, it was found that the contaminating source was contaminants in the nitrogen gas used for solution deaeration. Upon switching to argon, detection became highly reproducible and showed strong linearity with the Cr(VI) concentration.

<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>

The aim of this thesis is to manufacture Ti/APC-2 hybrid composite laminates and obtain its mechanical properties and fatigue characteristics at elevated temperatures. Ti/APC-2 laminates were composed of two layers of APC-2 and three layers of titanium sheets. For superior bonding ability between titanium and APC-2, chromic anodic method was adopted to treat titanium sheets in manufacturing process and APC-2 was stacked according to cross-ply [0/90]s and quasi-isotropic [0/45/90/-45] sequences. Then, the modified curing process was adopted to fabricate Ti/APC-2 hybrid composite laminates. Tension and fatigue tests carried out with MTS 810 and MTS 651 environmental control chamber to lift and maintain experimental temperatures, such as 25¢XC, 75¢XC, 100¢XC, 125¢XC and 150¢XC. From static tensile tests, the mechanical properties of cross-ply and quasi-isotropic composite laminates, such as ultimate strength, longitudinal stiffness were gained and the stress-strain diagrams of laminates were also plotted from testing data at elevated temperature. From fatigue tests we obtained laminate¡¦s fatigue resistance properties and the experimental data of applied stress vs. cycles were plotted as S-N diagrams at elevated temperature. From the tensile and fatigue tests, the important remarks were summarized as follows. First, no matter what the APC-2 stacking sequence was, the ultimate strength and longitudinal stiffness decreased while temperature rising, especially at 150¢XC; second, a turning point appeared at each stress-strain diagram that kink angle caused the decrease of stiffness while temperature rising; third, combining fatigue data and stress-strain diagrams we analogized a presumption that the region before turning point was in elastic behavior and after turning point in plastic deformation; fourth, quasi-isotropic laminates had better fatigue resistance than that of cross-ply laminates; sixth, the longitudinal stiffness before turning point was in good agreement with the prediction by using the modified ROM, however, after turning point the errors became large.

published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

<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>

An electrochemical investigation of the corrosion of iron and Fe18Cr based stainless steel alloys was undertaken with particular emphasis on the nucleation and growth of surface films. Chronoamperometry was shown to be a sensitive technique to investigate the initial stages of film formation and growth. In a variety of acidic (pH < 7), alkaline and alkaline cyanide electrolytes, providing dissolution of the substrate metal could occur rising current transients, similar to those reported in electrocrystallisation studies, were observed when the electrode was stepped to the appropriate potential. This indicated that at these potentials the surface film formed via the nucleation and growth of discrete nuclei. A significant aspect of this study was visual evidence of this nucleation and subsequent growth of the film provided by scanning electron microscopy which supported the electrochemical data and interpretation thereof. Existing electrocrystallisation models were used to evaluate the experimental rising current transients. While these models gave an indication as to the prevailing nucleation and growth mechanism, they were found to be inadequate in describing anodic oxide formation on an oxidising substrate. A qualitative model was proposed. In acidic electrolytes, rising chronoamperometric transients were observed for Fe, Cr and Fe18Cr at passive potentials and for FexCr (x = 16,18, 20,23% Cr) and alloys 444, 4732, 4733, 304L and 316L at transpassive potentials. The transients were shown to be sensitive to variations of potential, temperature, electrolyte and alloy composition. A systematic investigation of the influence of temperature (20 0 C - 1200 C) on the chronoamperometric, cyclic voltammetric and rotating ring - disc electrode behaviour of Fe in O.5M and 1.0M NaOH was also undertaken. In alkaline electrolytes, the formation of a duplex surface film was proposed, with x-ray photoelectron spectroscopy indicating that the protective base layer consisted of FeO while Fez03 and FeOOH constituted the upper layer. Base layer formation was favoured with increasing temperature and increasing hydroxide ion concentration of the electrolyte. Addition of OAM NaCN to O.5M and 1.0M NaOH had a marked effect on the electrochemistry of the system, with CN- inhibiting surface film formation, particularly of the upper layer. A mechanism for the oxidation of Fe in alkaline and alkaline cyanide electrolytes was proposed. / Thesis (Ph.D.)-University of Natal, 1994.

The purpose of this research was to investigate the factors that enhance conductive anodic filament (CAF) formation in printed wiring boards. The variables studied were (1) flux formulation, (2) conductor spacing, (3) operating voltage, and (4) temperature. A Weibull distribution of failure times due to CAF was observed. A novel test circuit was designed and implemented that allowed the mean time to failure to be determined for boards processed with three different fluxes, at 0.5 mm and 0.75 mm conductor spacings and at 150V and 200V. The boards were aged at 85%RH and a temperature of 75°C, 85°C or 95°C. It was found that the flux formulation affected the rate of CAF formation. A modified linear aliphatic polyether flux with a chloride activator had a significantly different activation energy than control printed wiring boards or those boards processed with a poly(ethylene/propylene) glycol flux or a poly(ethylene/propylene) glycol flux with a bromide activator. The addition of bromine to a poly(ethylene/propylene) glycol flux decreased the rate of CAF formation as compared to poly(ethylene/propylene) glycol without a halide activator. The inter-relation between voltage and conductor spacing was quantified as a L4/V2 relationship for the plated through hole test pattern used in this study. 325V/mm was found to be a critical voltage gradient for the formation of CAF. The maximum temperature of the reflow profile also greatly enhances CAF formation and decreases the mean time to failure. Microscopic analysis showed distinct differences in CAF morphology between the various processed boards. Control boards had small halo-like CAF formations around a separated fiber / epoxy interface. CAF that formed on boards processed with poly(ethylene/propylene) glycol or poly(ethylene/propylene) glycol with a bromine activator had a stratified appearance that penetrated well into the epoxy. Boards that were processed with the modified linear aliphatic polyether with chlorine activator had a striated morphology that also penetrated into the epoxy. All CAFs were consistently copper and chlorine containing despite the use of a bromine containing flux. Electron diffraction revealed that a CAF observed in this study was synthetic atacamite. Stainless steel (i.e., iron, nickel, and chromium) residues were also observed as a result of drill bit breakage during PTH formation.

This work focuses on a simple template assisted approach for fabricating I-III-VI semiconductor nanowire arrays. Vertically aligned nanowires of p-CIS of controllable diameter and thickness are electrodeposited, from an acidic electrolyte solution, inside porous aluminum templates using a three electrode set up with saturated calomel electrode as the reference. AAO template over ITO-glass was used as starting template for the device fabrication. The deposited CIS is annealed at different temperatures in a reducing environment (95% Ar+ 5% H2) for 30 minutes. X-ray diffraction of the nanowires showed nanocrystalline cubic phase structures with a strong orientation in the <112> direction. The effective bandgap of the deposited CIS nanowires determined using the Near Infrared (NIR) Spectrometer was found to be 1.07eV. The type of CIS electrodeposited inside the porous alumina template is determined to be p-type from the Schottky diode obtained with ITO-CIS-Au structure. Schottky diodes were characterized and analyzed at room temperature.