In this study, the Bragg reflectors deposited by DC and RF magnetron sputtering are composed of alternating layers of high and low acoustic impedance materials which have a thickness of one quarter wavelength at the desired resonance frequency, while the piezoelectric layer, aluminum nitride thin film, was deposited by RF reactive magnetron sputtering. The deposition parameters of Mo and SiO2 thin films are tuned according to the AFM measurement. With the optimal deposition parameters, the cross-sectional SEM images of multilayer Bragg reflector show smooth and clear interfaces which are important criterias for the SMR devices. The frequency responses of SMR show distinct resonant phenomenon near 1.4GHz and 2.5GHz with an excellent noise restraint. It indicates that the Bragg reflector made of Mo/SiO2 is suitable for the fabrication of SMR devices. Furthermore, the two resonant frequencies show that the bulk acoustic wave transmit in two different way, the longitudinal mode and the shear mode. The cause of the shear mode vibration is the tilt of the c-axis of AlN thin films. For c-axis inclined AlN thin films, the longitudinal and the shear mode vibration both exist, and the acoustic wave propagates with different acoustic velocities along the c-axis inclined AlN.

Thesis (Ph. D.)--Lehigh University, 2001. / Includes vita. Includes bibliographical references (leaves 123-127).

Thesis (Ph. D.)--University of Queensland, 2002. / Includes bibliographical references.

No description available.

No description available.

No description available.

Excessive release of neurotransmitter is a characteristic of epileptogenic cells. A number of lines of evidence implicate defects in the synaptic vesicle cycle as a cause of this excessive release. Synaptic vesicles are retrieved by more than one route in central nerve terminals. During mild stimulation the dominant synaptic vesicle retrieval pathway is classical clathrin mediated endocytosis. During elevated neuronal activity retrieval of synaptic vesicle membrane by bulk endocytosis is the predominant retrieval method. As it is triggered by strong stimulation, bulk endocytosis may be of importance in retrieval during epilepsy, however little is currently known about this pathway. In order to investigate the role of bulk endocytosis, we sought to establish a cell culture model of epilepsy, to develop an assay to distinguish retrieval by bulk endocytosis, and to use these tools to look at the molecular players controlling this form of endocytosis. Characterisation of bulk endocytosis through the development of tailored assay systems has revealed that bulk endocytosis is a fast event that is triggered during strong stimulation. Bulk endocytosis provides the nerve terminal with an appropriate mechanism to meet the demands of synaptic vesicle retrieval during periods of intense synaptic vesicle exocytosis. Inhibition of a dephosphorylation specific dynamin I-syndapin I interaction by competitive peptides inhibits activity dependent bulk endocytosis, implicating this interaction in a role in this method of synaptic vesicle retrieval. Having characterised the strength of stimulation needed to activate bulk endocytosis, and the speed at which it occurs, we also investigated the effects of known anti-epileptogenic drugs on bulk endocytosis in our central nerve terminal model system.

PhD, Faculty of Engineering, University of the Witwatersrand, 1990

Resin-composites have been used in dentistry as direct restorative materials for over fifty years ago, and meanwhile their properties have been substantially improved. Recently a new class of light-cured resin-composites known as 'bulk-fill' materials has been introduced, and has increased the practical application of resin-composites in comparison to conventional incrementally applied materials. The aim of the present research was to assess bulk-fill resin-composites in comparison to conventional materials with respect to their composition, polymeric structure properties and hygroscopic behaviour in oral and food simulating substances. A variety of bulk-fill and conventional resin-composites were tested in this study. Monomer composition of materials has been qualitatively and quantitatively determined using high performance liquid chromatography and mass spectroscopy, and their degree of conversion assessed using Fourier transform infrared spectroscopy. It was found that the degree of conversion of the bulk-fill resin-composites was generally comparable to that of conventional materials and appeared to be governed by the type and quantity of monomers comprising the resin matrix. To evaluate the cross-link density of the polymer matrix, the extent of post-irradiation hardness development, chemical softening, and thermal stability were assessed using microhardness and thermogravimetric analysis. Materials showed variable results and it was suggested that two networks were present in the polymerized resin matrix, a primary network forming immediately after curing followed by a secondary network. The secondary network seemed to have poor cross-link density with low resistance to the degradative effects of solvents. Monomer elution from cured materials was assessed using High performance liquid chromatography over three months' storage in three different media. Elution from bulk-fill resin-composites was found to be comparable to that of conventional materials, despite their increased incremental thickness, with remarkably minimal elution taking place from two bulk-fill materials examined. Monomer elution was shown to be highly dependent on the hydrophobicity of the base monomers and the network characteristics of the resin-matrix. UDMA-BisEMA based systems appeared to be more vulnerable in organic solutions than BisGMA and BisGMA-BisEMA based systems in terms of monomer elution. Sorption and solubility were assessed after one years' storage in water and artificial saliva. Water sorption and solubility of resin-composites were material-dependent and highly affected by the filler loading and initial degree of conversion of the polymeric matrix. BisEMA and UDMA-BisEMA based polymer networks appeared to be more hydrophobic and resistant to sorption and solubility than BisGMA based systems in water based media. The bulk-fill and conventional resin-composites tested were considered stable in the long-term water storage, with the exception of one conventional flowable material.

Thesis (Ph. D.)--Lehigh University, 2000. / Includes vita. Includes bibliographical references (leaves 291-301).