published_or_final_version / Electrical and Electronic Engineering / Master / Master of Philosophy

published_or_final_version / Physics / Master / Master of Philosophy

published_or_final_version / Electrical and Electronic Engineering / Master / Master of Philosophy

published_or_final_version / Physics / Doctoral / Doctor of Philosophy

No description available.

<p> Although the glass transition temperature (<i>T_g</i>) and dynamics of polymers confined to the nanoscale have been studied for twenty years, a physical understanding is still lacking. The reason for a polymer species dependent <i>T_g</i>-confinement effect and the role of neighboring polymer domains in perturbing the <i>T_g</i> of a confined species are areas with a need for greater study as they will inform many of the decisions regarding the use of polymers in nanomaterials. </p><p> In this work, fluorescence spectroscopy is used as the primary tool to characterize <i>T_g</i> in a number of systems. First, micelle core <i>T_g</i> and critical micelle temperatures can be determined via pyrenyl label fluorescence for block copolymers in organic solvent at polymer contents which cannot be reliably characterized by other standard methods. Next, measurements were extended to miscible polymer-polymer blend systems where two component <i>T_gs</i> can be determined via a single pyrene-labeled component. Fluorescence can characterize systems with small component <i>T_g</i> differences and near-infinitely dilute blend components unlike scanning calorimetry. </p><p> Studies of the near-infinitely dilute blend components reveal that a 0.1 wt% polystyrene component can have its <i>T_g</i> tuned over a 150 &deg;C range depending on the blend partner. Analogous tunability of <i>T_g</i> is also reported in multilayer film systems with an ultrathin PS layer surrounding by bulk neighboring domains. The same limiting <i>T_g</i> is reported by PS for a given neighbor indicating a common physical origin of perturbations in both systems. The perturbations are correlated with fragility which also tracks with the magnitude of <i>T_g</i>-confinement effects in single layer polymer films. Thus, fragility provides a unifying explanation of confinement effects in multilayer films, blends, and single layer films (in the absence of attractive interactions). </p><p> Surface wave dynamics are also examined in ultrathin polystyrene layers on various substrates. It is demonstrated that surface dynamics become much slower than anticipated by capillary wave theory as the film thickness decreases. Additionally, surface wave dynamics become orders of magnitude faster as the modulus of the supporting substrate decrease.</p>

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Current melt texturing processes can only produce high-quality High Temperature Superconducting (HTSC) domains of a few centimetres in size. Increasing the size of the domain by a joining technique is investigated. The parent HTSC melt textured domains of YBa2CU307-li (Y 123), were used in this research to investigate the joining technique. The solder powders used to form the seam were HTSC YbBa2Cu307-li (YbI23), which has a peritectic temperature of approximately 925 cC, Yb2BatCu\07_o (Yb211), Silver Oxide (Ag20) and Y123. A total of 8 different superconducting 'solders' were manufactured using these powders. Microstructural analyses using a scanning electron microscope (SEM) and an optical microscope were conducted on each sample. It was found that mechanisms occurring during the growth of the seam affect the performance of the join. During the growth, three interfaces are formed. The interface between the parent and the growth front produced an excellent join whereas most of the microstructural defects were present at the intersection of the two growth fronts also known as the impingement boundary. The effect of the addition of Yb211 and silver particles to the join were investigated. It was found that although Yb211 particles improve the flux pinning and viscosity of the solder and the silver improves the intergranular critical current density (Jc) [1], they also reduce the effective cross-sectional area of the joined sample. Thus, the amount of non-superconducting particles added needs to be optimised. Transport measurements were taken to evaluate the current carrying capacity of each sample. It was found that Sample 2-A (Yb123 + 5%Ag20) has the highest Jc of approximately 142 A/cm 2 at 80 K while Sample 3-A (Y123 + 5%Ag20) has the poorest Jc of about 37 A/cm2 at 80 K. Resistivity vs. Temperature graphs show that all joined samples were superconducting implying that the joining process was successful in establishing a superconducting join. Magnetic field maps of the parent and joined samples were captured using a Hall Probe. It was found that the joined sample could trap almost 95 % of the field that could be trapped by the parent. From these results, it was concluded that joining HTSC bulk pieces is possible using an external 'soldering' agent. / Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2005.

A stainless steel, high temperature, phase change micro actuator has been demonstrated using the solid-liquid phase transition of mannitol at 168°C and In-Ag transient liquid phase diffusion bonding. Joints created with this bonding technique can sustain temperatures up to 695°C, while being bonded at only 180°C, and have thicknesses between 1.4 to 6.0 μm. Physical vapour deposition, inkjet printing and electroplating have been evaluated as deposition methods for bond layers. For actuation, cavities were filled with mannitol and when heated, the expansion was used to deflect a 10 μm thick stainless steel membrane. Bond strengths of the joints are found to be in the region of 0.51 to 2.53 MPa and pressurised cavities sustained pressures of up to 30 bar. Bond strength is limited by the bond contact area and the surface roughness of the bonding layers.

No description available.