Thesis (M. Phil.)--Hong Kong University of Science and Technology, 2003. / Includes bibliographical references. Also available in electronic version. Access restricted to campus users.

Thesis (Ph. D.)--University of Hong Kong, 1987.

published_or_final_version / Physics / Doctoral / Doctor of Philosophy

published_or_final_version / Dental Materials Science / Master / Master of Science in Dental Materials Science

published_or_final_version / Dental Materials Science / Master / Master of Science in Dental Materials Science

As a wide band gap semiconductor, Zinc Oxide (ZnO) has recently attracted considerable attention due to its wide applicability. Transition metal (TM) doped ZnO is one of the most intensive research field in the last decade because of its possible application in spintronics devices. The Co-implanted ZnO has been considered as one of the most promising candidate in the field of diluted magnetic semiconductor (DMS). In this study, the magnetic and structural properties of Co-implanted ZnO single crystal were investigated. ZnO single crystals were implanted with 100 keV-Co ions at 300K with a fluence of 1014cm-2and subsequently annealed at 750oC and 900oC respectively. The samples were studied by secondary ion mass spectrometry, X-ray diffraction, photoluminescence, X-ray photoemission spectroscopy and vibrating sample magnetometer. The as-implanted sample seemed to be phase pure while the spinel secondary phase ZnCo2O4 was present in 750oC and 900oC samples showed trace ofCo3O4,Zn0.52Co2.48O4 and metallic Co cluster. All the samples exhibited ferromagnetism at room temperature in low field region, and the magnetic moment was found to decrease and increase after the heat treatment. In high field region, typical paramagnetism was the dominating magnetic property. More than one ferromagnetic mechanism is involved to explain the experimental results. / published_or_final_version / Physics / Master / Master of Philosophy

Zirconia has now been increasingly used in modern dental prosthetic practice due to its high mechanical strength, good esthetics, and excellent biocompatibility. However, the application of zirconia-based dental restorations is still constrained by its chemical inertness and the resultant relatively weak bonding properties, including porcelain to zirconia bonding, and resin to zirconia bonding. Therefore, many investigations have been carried out on the development of an effective method of surface modification on zirconia for improving its bonding ability. The aim of this laboratory study was to evaluate the effects of some new modified surface treatments on the adhesion durability of dental zirconia prosthetic system. In Part I and Part II, the application of laser surface treatment was examined. Laser energy was utilized and applied on zirconia surface before porcelain veneering procedure. Its influence on porcelain zirconia integration interface was mechanically tested and compared with sandblasting treatment. The changes in porcelain zirconia shear bond strength and mechanical strength values of zirconia with the modulation of output energy were recorded. In Part III and Part IV, several types of coating treatment, such as tribochemical approach, silica powder coating, and zirconium silicate coating, were compared with their effects on resin zirconia bonding. The changes in resin zirconia shear bond strength under different aging conditions were observed. The elemental analysis was also performed for clarifying the chemical composition of zirconia surface. The application of laser produced a flake-like micro-retentive structure on zirconia surface. Laser irradiation with the output energy higher than 11.3 W/cm2 was effective in increasing porcelain to zirconia bond strength values. The biaxial flexural strength of zirconia was not significantly affected by laser irradiation with the settings in this study. However, most of the laser treatments still demonstrated slightly lower flexural strength values compared with the control group. No changes in crystalline structure were detected after laser treatment with X-ray Diffraction (XRD) technique. Resin to zirconia shear bond strength was significantly increased after surface coatings. Without surface treatment the bonding between resin and zirconia was susceptible to artificial aging effects. Zirconia and silica-coating groups demonstrated the highest resistance to hydrolytic influence. On the other hand, the chemical changes of zirconia surface were still in need of clarification. It was concluded that porcelain zirconia bonding could be effectively enhanced by applying both sandblasting and a new laser irradiation approach. Laser irradiation might be a potential approach as a surface treatment for improving the quality of porcelain zirconia bonding interface. The strong and reliable resin zirconia integration could not be achieved without appropriate surface pre-treatment. This coating treatment is a promising approach for strengthening resin to zirconia adhesion. / published_or_final_version / Dentistry / Doctoral / Doctor of Philosophy

Nanoparticles/nanocrystals have been recognized by their remarkable and technologically attractive properties which are different from those of bulk materials due to their ultra large surface area and extremely fine nanostructure. Highly sophisticated properties such as optical, magnetic, electronic, catalytic, mechanical, chemical and tribological properties can be obtained by advanced nanostructured coatings, making them desirable for industrial applications. This thesis encompasses the fabrication of nano/micro-structures of cuprous oxide (Cu2O) including nanocrystals, nanowires, nanocrystalline coatings and co-deposition of Cu2O/Cu by electrodeposition. The investigation in this thesis involved a systematic study by using a simple two-electrode system with copper sulphate as the electrolyte at pH 4.0 at room temperature and without the aids of any additive. The substrates under investigation included silicon wafer, stainless steel plate, highly oriented pyrolytic graphite (HOPG) and silver wire. By changing the kinds of the substrates, deposition modes, and deposition potentials, different types of nano/micro-structures of Cu2O were yielded. Nanometer-to-micron sized Cu2O single crystals were fabricated on an Au/Pd sputter-coated silicon wafer and stainless steel cathode. Different morphologies of the crystals were studied and their structural characterization was performed. The Cu2O crystals were generally of an octahedral shape. A growth mechanism was proposed to explain the morphologies of the observed nano/micro-structures. The effect of the electrodeposition parameters such as deposition voltage and deposition time, on the size of the crystals and their coverage on the substrates was examined. Highly aligned Cu2O nano/microwires were fabricated on the step edges of the HOPG substrate. With the same deposition potentials, longer deposition time would increase the diameter of the wires. Various crystal morphologies including flower-like and butterfly-like structures, and dendrites and truncated octahedra were observed on the working electrode of HOPG. Some of the morphologies were revealed for the first time by the one-step electrodeposition. To investigate the differences between yield by DC and pulsed electrodeposition respectively, the sizes and coverages of the deposited crystals on the substrate by DC and pulsed electrodeposition were compared. Two completely different forms of Cu2O, namely uniform nanocrystalline coatings and isolated single crystals, were fabricated on Ag microwires by pulsed and DC electrodeposition respectively. The results illustrated a very different effect on crystal nucleation between DC and pulsed electrodeposition, and suggested the possibility of using different voltage waveforms for electrodeposition in order to fabricate coatings with different nano/micro-structures on substrates. Gradual transition in the deposition products from pure Cu2O to pure Cu during electrodeposition with various DC potential was investigated. The percentage of Cu content was studied in depth for a full picture of the relationship between the depositing potentials and the compositions of the deposits. This thesis provides a method to fabricate nanocrystalline Cu2O, Cu and Cu/Cu2O on substrates in a single step without the use of additives. / published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

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published_or_final_version / Medical Sciences / Master / Master of Medical Sciences