by Wong Ho Man, Bruce. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1999. / Includes bibliographical references (leaves [60-61]). / Text in English; abstracts in English and Chinese. / by Wong Ho Man, Bruce. / Acknowledgements --- p.i / Abstract --- p.ii / 摘要 --- p.iii / Table of Contents --- p.iv / List of figures --- p.vi / Chapter Chapter1 --- Introduction --- p.1 / Chapter Chapter 2 --- Fabrication of erbium-doped sol-gel derived silica --- p.4 / Chapter 2.1 --- Introduction to the sol-gel process --- p.4 / Chapter 2.1.1 --- The procedure for silica formation - ´ؤ --- p.7 / Chapter 2.1.2 --- Changes during the sol-gel process --- p.11 / Chapter 2.2 --- Factors affecting the fluorescence --- p.15 / Chapter 2.2.1 --- Clustering of Er3+ ions in the silica samples --- p.15 / Chapter 2.2.2 --- Retention of hydroxyl group OH/absorbed water in silica - --- p.16 / Chapter 2.3 --- The role of Aluminum in dissolving Er3+ ions in Si02 network --- p.18 / Chapter 2.4 --- Thermal dehydroxylation --- p.20 / Chapter 2.5 --- Characterization of the Erbium-doped silica samples --- p.24 / Chapter 2.5.1 --- Absorption spectrum --- p.24 / Chapter 2.5.2 --- "Refractive index, density and microhardness" --- p.29 / Chapter Chapter 3 --- Fluorescence spectroscopy --- p.35 / Chapter 3.1 --- Introduction --- p.35 / Chapter 3.2 --- Experimental set-up --- p.36 / Chapter 3.3 --- "Fluorescence spectrum with variation in Er3+ concentration, Al/Er molar ratio and annealing temperature" --- p.38 / Chapter Chapter 4 --- Decay lifetime measurements of the 4S3/2 energy level --- p.47 / Chapter 4.1 --- Introduction --- p.47 / Chapter 4.2 --- Experimental set-up --- p.48 / Chapter 4.3 --- Increase in decay lifetimes with Aluminum-codoping --- p.49 / Chapter Chapter 5 --- Conclusions and further works --- p.58 / References

No description available.

Thesis (PhD)--University of South Australia, 1997

The system kaolinite - carbon black consisting of cheap precursors has been investigated, in terms of its potential to form A1₂ O₃ - SiC composites. The carbothermal reduction process of mullite and silica was studied, in detail, in the range 1275° to 1810° C and over different periods, both under sintering as well as hot-pressing conditions. It was established that the reduction of mullite and silica starts around 1450° C, where the rate of reaction is very slow. Until about 1800° C during the reduction of mullite, SiO₂ gets preferentially reduced, thus forming a composite ceramic consisting of SiC and A1₂ O₃ phases. The kinetics of the formation of SiC + A1₂ O₃ were followed in the range 1590° - 1660° C and it was noted that under hot-pressing conditions they follow a contracting cylinder model. The rate of reaction increased with the increase in temperature and followed a parabolic path with time because of the geometry of the hot-pressed specimens at each temperature. This indicated that the gas diffusion in and out of the system along the edges of the cylindrical specimens is the rate controlling step. The activation energy of the reduction process was calculated to be 922 KJ/mole. The application of pressure prior to the carbothermal reduction process seemed to be not favourable for the formation of SiC and A1₂ O₃, however, when applied after the beginning of soaking period, this greatly improved the densities and formation of SiC and A1₂ O₃. The microstructure of the samples was analysed using SEM and TEM. It was found that the grain size of the composite ceramic was of the order of 0.2μm. SiC was present mainly in the form of fine platelets. Finally, the isothermal compaction behaviour of the system was studied under a constant pressure in the temperature range 1200° C - 1800° C, during which the formation and carbothermal reduction of mullite and silica took place. A mathematical model based on the least squares fitting was used to fit the compaction curves. Due to the complex nature of the compaction data an empirical approach was used to interpret the data and a viscoelastic model was developed. It was found that the interactive-double-Kelvin unit having two elastic and two viscous components explained the type of compaction behaviour observed in the kaolinite + C system. One of the viscous components (η₁) and one of the elastic components (M₁) were found to be temperature sensitive. It is concluded that starting from the cheap precursors (kaolinite and carbon black) a particulate composite of A1₂ O₃,-SiC can be produced by hot-pressing technique. SiC-whisker formation is not encountered in this system. The very fine grain size of the paniculate composite, resulting in a small flaw size, should provide the composite ceramic with good mechanical properties. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

Thesis (Ph.D.) - University of Queensland, 2005. / Includes bibliography.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Earth and Planetary Sciences, 1981. / Microfiche copy available in Archives and Science. / Bibliography: leaves 52-55. / by Guyton Hampton Watkins, Jr. / M.S.

The mean-square displacement amplitudes (MSDA) of the tetrahedral cations Si and Al are compared to the MSDA of their coordinated O atoms in framework silica polymorphs and aluminosilicate structures. Criteria are established which indicate order, structural disorder or substitutional disorder in a framework crystal. / Master of Science

The last decade has seen many new developments in the foundry industry, among them the CO₂ process for curing molds and cores. The CO₂ process consists essentially of mixing dry, clay-free, silica sand with an organic liquid sodium silicate binder, then ramming this mix into molds or core boxes and injecting CO₂ gas. The CO₂ gas reacts with the sodium silicate binder forming a silica gel which hardens rapidly in atmospheric conditions. The CO₂ molds are more resistant to metallostatic pressure and erosion than either green or dry sand molds but more expensive also. The CO₂ cores are hardened rapidly without the benefit of a baking cycle characteristic of the production of conventional organically-bonded cores. The CO₂ cores are more economically compared with other core-making processes than the CO₂ molds are compared with other sand-molding processes. The casting property, fluidity, is a qualitative measure of the ability of a metal to completely fill a mold cavity and is normally expressed as inches of flow in a small channel. Mold material variables, gating and flow-channel variables, and metallurgical variables, such as metal composition and the number of degrees superheat, all affect the fluidity values. This paper presents a spiral fluidity pattern for determining the fluidity of aluminum-silicon alloys in CO₂ molds made by a standard procedure. A standardized molding, melting, and pouring procedure is suggested to control certain fluidity variables while investigating the influence of other fluidity variables. Finally, a statistical method is presented to ascertain the significance of the effect certain variables may have on fluidity. Preliminary investigations pertinent to the major objective of this paper indicate aluminum-silicon alloys are less fluid in CO₂ molds than in green sand. Investigations also indicate that certain variabilities in testing procedure which an operating foundry might encounter have no statistically significant effect on fluidity. / Master of Science

In the present work a new synthesis route, a hybrid between solid state reaction and proteic sol-gel method, is applied to a scintillator material based on Ca2Al2SiO7 (CAS). The luminescence mechanisms for the nanostructured scintillator is studied a mechanism that describes the luminescence process, when excited with X-rays, is proposed. The same system was also prepared via other 2 different methodologies, solid state traditional route, proteic sol-gel route, and 3 different solvents were used, for comparison in the hybrid method. In all cases the CAS phase was found as indicated by X-ray powder diffraction. During the thermal evolution of CAS precursors prepared via hybrid route assisted by coconut water, intermediate oxide phases were formed and they reacted with SiO2 to form the final CAS phase. SEM images indicated that the organic molecules present in the coconut water play an important role in the nanoparticle formation defining the average size and morphology, Samples prepared via the hybrid route showed particle with spherical shape with average size of (36 ± 15) nm. Analysis carried out via photoemission spectroscopy indicated that Si ions are the most abundant cation on surface of the particles produces via hybrid methods and solid state reaction. X-ray absorption (XAS) revealed that the Si coordination environment did not change during the synthesis. The XAS technique also indicated that the main valence for the dopants in CAS structure and the emission spectra obtained via pholuminescence and X-ray excited optical luminescence (XEOL) confirmed that the main emissions are related to the dopants on the CAS matrix. Extended X-ray absorption fine structure revealed the location of the dopants in the CAS matrix, and the most probable defect generated for the dopant presence. XEOL excitation spectra showed different behaviors for the luminescence around K edge of the matrix elements. The XEOL decay time indicated that fast scintillators were produced when CAS were doped with Ce3+ with characteristic constants lower than 40 ns. The results time dependent XEOL emission also showed that the luminescence decay curves are influenced by the presence of electrons and holes shallow traps in the CAS electronic structure. / No presente trabalho uma nova rota de síntese, híbrida entre as sínteses de estado sólido e sol-gel proteico, é aplicada a um material cintilador baseado na matriz cristalina Ca2Al2SiO7 (CAS). Adicionalmente, o mecanismo de luminescência destes cintiladores nanoestruturados é estudado e um modelo é proposto. Para efeito de comparação com a rota de síntese híbrida, amostras foram também preparadas por sol-gel proteico e síntese de estado sólido, e com a variação do solvente na síntese híbrida. Os resultados de difração de raios X mostraram que as amostras apresentaram a fase cristalina de Ca2Al2SiO7 após calcinação acima de 1300°C. Durante a evolução térmica dos precursores preparados pelo método híbrido assistido por água de coco, fases cristalinas intermediárias são formadas e estas reagem com o SiO2, até a formação do CAS. As imagens obtidas por microscopia eletrônica de varredura indicaram que as moléculas orgânicas presentes na água de coco são decisivas no controle do tamanho e morfologia das nanopartículas obtidas pelo método híbrido, e as partículas apresentam formato esférico e tamanho médios de (36 ± 15) nm. A espectroscopia de fotoemissão revelou que o Si é o elemento mais abundante na superfície das amostras produzidas pelos métodos híbridos e síntese de estado sólido. Enquanto a absorção de raios X (XAS) mostrou que a coordenação do Si não muda durante a síntese. A XAS também indicou as valências mais abundantes dos íons dopantes do CAS e os espectros de emissão fotoluminescente e de luminescência óptica estimulada por raios X (XEOL) confirmaram que estes são os canais luminescentes dos materiais. Com a análise da estrutura fina de absorção de raios X foi possível localizar os sítios ocupados pelos dopantes no CAS, assim como estimar o tipo de defeito mais provável, gerado pela presença destes íons. Os espectros de excitação XEOL mostraram comportamentos diferentes para a luminescência na região das bordas K de absorção dos elementos da matriz. As medidas de tempo de decaimento da luminescência indicaram a obtenção de cintiladores muito rápidos, com constantes características menores que 40 ns em todas as amostras que contém Ce3+ e que o processo de decaimento também está associado à presença de armadilhas de portadores de cargas na estrutura eletrônica do material.

Indiana University-Purdue University Indianapolis (IUPUI) / In adhesive dentistry, obtaining a good bond is a fundamental goal. It has been suggested that filler addition to the adhesives would increase the bonding strength of the adhesive layer. Halloysite aluminosilicate nanotubes (HNTs) are biocompatible, hydrophilic, durable, and have high mechanical strength. These advantages make them good candidates to be used as reinforcing agents for improving the properties of dental adhesives. The objective of this study was to evaluate the effect of incorporating HNTs into a commercial two-step etch-and-rinse adhesive system or one-step self-etch adhesive system on dentin shear bond strength. HNTs were incorporated into the two commercial adhesive systems in 0 wt%, 5 wt%, 10 wt%, and 20 wt%. The commercial control adhesives and the experimental adhesives were used to bond occlusal dentin of 120 extracted human molar teeth and then tested for shear bond strength by a universal testing machine with a semi-circular edge at a crosshead speed of 1.0 mm/min. Debonded specimens were examined under light microscopy to evaluate the fracture pattern. Resin-dentin interface were evaluated under scanning electron microscopy (SEM) after bonding dentin slabs using commercial control adhesives and experimental adhesive that showed numerically highest shear bond strength from each adhesive system. Two-way ANOVA was used to evaluate the effects of adhesive system and nanofiller content on shear bond strength. Pair-wise comparisons between groups were made using Fisher's (LSD) (p < 0.05). For the self-etch adhesive system, only incorporation of 5 wt% showed a significant increase in shear bond strength to dentin compared with the commercial control group. For the etch-and-rinse adhesive system, there was no significant difference in shear bond strength between HNTs filled adhesives groups and the commercial control group. Resin-dentin interface SEM evaluation showed nanotubes infiltrated into dentinal tubules. In conclusion, incorporating the self-etch adhesive system with 5 wt% HNTs increased the bond strength to dentin. Incorporation of up to 10 wt% filler concentration into both the self-etch and the etch-and-rinse adhesive systems did not adversely affect the bond strength to dentin or the handling properties. HNTs can penetrate along with resin tags into dentinal tubules, which could expand the use of their unique properties.