Assuming a simple force field involving only short range Non-Coulombic molecular energy terms along with P1 symmetry, a variation of the SQLOO model (Boisen and Gibbs, 1993) successfully generates the structure types of both α and β1 quartz along with at least five alternative structure types of silica not yet observed to our knowledge. These structure types are identified by the existence of symmetry elements represented in the optimized atomic coordinates and cell parameters that define a minimizer in the model. A family of minimizers is discovered through the combined use of Monte Carlo simulated annealing followed by quasi-Newton minimization techniques. The results are in contrast with the assertion made by Tse et al. (1992) that new structure types of SiO₂ can only be arrived at by Molecular Dynamic methods. By varying the parameters used in the minimization process, different families of structure types are discovered. Several structure types were found to have high symmetries. These results are in contrast with the findings by Kramer et aL (1991) that the stability of high symmetry structures of silica are stabilized in part by ionicity. The results reported here are for calculations involving Z = 3 and 6 formula units. This strategy may be useful in the prediction of possible high silica zeolite structure types. An examination of the atomic displacement parameters (ADPs) obtained for TO₄ tetrahedra (T = Si, Al) suggest rigid TO bonds are more common in non-framework than in framework silicates. Correlated motion is found among the ADPs that is consistent with TLS rigid body motion. For these data, the translational motion is represented by the ADPs of the central T atom while both the librational and translational motion is contained in those of the surrounding O atoms. The libration angle for rigid tetrahedra is linearly dependent on the difference between the isotropic equivalent displacement parameter of the T and O atoms, B(T) and B(O), respectively. The value of B(O) is on average twice that of B(T) with a maximum value of ~ 2.0Ų. Variations in the SiO bond lengths of rigid tetrahedra in the silica polymorphs is related only to f_s(O). Rigid TO and OO bonds are a necessary but not sufficient condition for rigid body motion. Nonrigid tetrahedra may represent crystals containing disorder or problems with the refinement. The computer program EXCALIBR (Bloss and Riess, 1973; Bloss, 1981, p. 202) has been rewritten and markedly improved. Like EXCALIBR, EXCALIBR II solves optical extinction data, as determined with a spindle stage, and determines the optic axial angle 2V and the orientation of the crystal’s optical indicatrix. EXCALIBR II uses a modification to Joel’s equation as a means of obtaining the optic axes of a crystal. Furthermore, EXCALIBR II successfully solves extinction data where one optic axis of a biaxial crystal is 90° to the spindle axis, an orientation that had thwarted its predecessor. EXCALIBR II also accurately determines the optical indicatrix orientation for uniaxial crystals. After solving extinction data for several different wavelengths and/or temperatures, EXCALIBR II calculates the angular change of each optic direction with wavelength and/or temperature along with the error on the angle. Using a simple t-test, it then computes a p-value to aid in the decision as to whether the optical direction truly exhibits dispersion. This is a more valid and sensitive procedure than the χ² test used by EXCALIBR, particularly because the covariance in each optic vector’s coefficients are taken into consideration and the results are invariant to the vector’s orientation. / Ph. D.

No description available.

Thesis (Ph.D (Physics)) --University of Limpopo, 2005 / We have derived a potential model for °uorapatite Ca10(PO4)6F2, ¯tted to structure, elastic constants and vibrational frequencies of the phosphate groups, which is compatible with existing calcite and °uorite potential mod- els. We then modelled the structure and stabilities of the dry and hydrated f0 0 0 1g, f1 0 1 0g, f1 0 1 1g, f1 1 2 0g, f1 0 1 3g and f1 1 2 1g surfaces, which calculations con¯rmed the experimental dominance of the f0 0 0 1g surface, which is prominently expressed in the calculated thermodynamic morphologies. The dehydrated morphology further shows the experimental f1 1 2 1g twinning plane, while the f1 0 1 0g cleavage plane is expressed in the hydrated morphology. Molecular adsorption of water has a stabilising e®ect on all six surfaces, where the surfaces generally show Langmuir be- haviour and the calculated hydration energies indicate physisorption (73 - 88 kJ mol¡1). The chains of °uoride ions surrounded by hexagonal calcium channels can become distorted in two major ways during relaxation: either by a shortening/lengthening of the FF distances, when the channel is perpendicular to the surface, or by distortion of the CaF bonds when the channel is parallel to the surface. Both distortions occur when the channel runs at an angle to the surface. Other relaxations include compression of the calcium sub-lattice and rotation of surface phosphate groups. We have modelled adsorption of a range of organic molecules onto dif- ferent °uorapatite surfaces, due to the importance of organic/ inorganic in- teractions in biological situations. We have selected organic molecules that represent a model for the carboxylic acids, alkyl hydroxamates and those 3 that contain both the aldehyde and hydroxyl functional groups. Adhesion of these organic molecules on the surfaces has shown strong interaction between the surface's Ca ions and the molecule's oxygens, more especially the car- bonyl oxygens than any other interactions. It was found that the number of interactions between the ions of adsorbate molecule and the mineral surfaces thus contribute signi¯cantly to the exothemicity of adsorption. Further more, simulations of apatite thin ¯lms at a range of ®-quartz surfaces have shown how the strength of adhesion between thin ¯lms of ap- atite material and ceramic silica surfaces is crucially dependent upon both the orientation of the ¯lm relative to the substrate and the nature of the silica surfaces, a ¯nding that is important in a wide number of applications, from basic geological research on intergrowth of phosphate and silicate rock minerals to the search for more e®ective surgical implant materials. It was shown that although the unrelaxed quartz surface is more reactive toward the apatite ¯lm, the more regular thin ¯lm structures grown at the pre-relaxed quartz surfaces lead to more stable interfaces. Film growth at the unrelaxed quartz surface is energetically increasingly unfavorable, whereas growth at the pre-relaxed surface is calculated to continue beyond the ¯rst layer, where the adhesion energy is convergent with the layer growth of the thin ¯lm. Ad- hesion of apatite thin ¯lm on hydroxylated surfaces of ®-quartz has shown to be energetically less favourable than at dry surfaces. This was because the thin ¯lm interact mainly with the hydroxyl ions on the surface of quartz. However, the adhesion energy is still convergent with layer growth of the thin ¯lm on the hydroxylated surfaces. / National Research Foundation of South Africa (NRF), Council for Scientific and Industrial research (CSIR), and the University of the North

Ce travail de thèse présente la modélisation des phénomènes interfaciaux des Silicates de Calcium Hydratés (C-S-H) en présence de diverses solutions salines. Les C-S-H sont synthétisés en laboratoire sous la forme de suspensions en solution. La caractérisation du solide par diffraction des rayons X et par résonnance magnétique nucléaire permet de valider le processus de synthèse. L'étude de particules colloïdales en solution, tels les C-S-H, requiert la caractérisation globale de l'interface, c'est-à-dire la surface du solide, la région interfaciale et la solution. Le modèle numérique développé dans le cadre de travail de recherche prend en compte les spécificités physiques de ces trois régions. La modélisation des intéractions ioniques s'articule expérimentalement sur les mesures de potentiel zêta et de rétention ionique. Le modèle considère les phénomènes de double couche et de complexation de surface pour interpréter les données de potentiel zêta. Une telle interprétation permet également de déterminer quantitativement la fixation des ions par les C-S-H, due à l'adsorption spécifique et à l'attraction électrostatique. La modélisation des interactions de surface est couplée chimiquement à un modèle de solubilité. Ce couplage physico-chimique fait intervenir la notion d'activité ionique. Le coefficient d'activité des ions en solution est calculé par la méthode MSA (Mean Spherical Approximation) qui est applicable à de fortes concentrations ioniques. Les C-S-H sont étudiés en présence de NaCl, Na2SO4, NaOH et CaCl2. Les résultats du modèle mettent en évidence la dépendance de la charge de surface en fonction de la composition de la solution, ainsi que la corrélation implicite entre les interactions interfaciales et la rétention ionique. Cette corrélation est vérifiée par l'application du modèle à des pâtes de C3S hydraté. L'un des objectifs majeurs de la thèse est l'étude du comportement des ions chlore. L'étude démontre leur très faible interactivité envers les C-S-H, ce qui constitue une information importante, car, dans le cas d'une pâte de ciment hydraté, les C-S-H sont généralement considérés comme étant les principaux responsables de la rétention du chlore. Par le développement d'un module d'interactions physiques, ce travail de recherche contribue significativement à l'étude du transport ionique dans les matériaux cimentaires.

Equilibrium properties of the MgO-Al₂O₃-SiO₂ (MAS) system are modeled using techniques from statistical and quantum mechanics. The fundamental structural units in this model are the closed shell ions: Mg²⁺, Al³⁺, Si⁴⁺, and O²⁻. The equilibrium properties of the MAS system are determined by the interactions among these ions and by the environment (i.e. temperature and pressure). The interactions are modeled using coulombic, dispersion, and repulsive forces. Two parameters appearing in the repulsive terms for each cation-oxygen interaction are fitted from properties of quartz, corundum, and periclase crystals. The effects of the environment on the liquid and solid compositions found in this system are calculated using a Monte Carlo technique involving the generation of a Markov chain of configurations; each configuration being a "snapshot" of the particles in the liquid or solid material being studied. The properties of the material are derived from averaging appropriate quantities over all the configurations. Enthalpies of formation, heat capacities, and volumes of seven compositions in the MAS system have been calculated using this method. All are within three percent of the corresponding experimental values. Radial distribution functions for these runs show the competition among the cations for the common anion, oxygen, under charge and mass balance constraints. The electronic structure of several molecular clusters in the MAS system are examined using ab initio linear combinations of atomic orbitals (LCAO) techniques. The assumptions used in LCAO calculations are examined and a small, balanced basis set for the MAS system is presented. The Mg-, Al-, and Si-O interactions are all found to be highly ionic using this basic set. Using a first principles technique, the two body effective pair potentials assumed for the Monte Carlo calculations were shown to be physically reasonable.

Hafnium and Zirconium based gate dielectrics are considered potential candidates to replace SiO2 or SiON as the gate dielectric in CMOS processing. Furthermore, the addition of nitrogen into this pseudo-binary alloy has been shown to improve their thermal stability, electrical properties, and reduce dopant penetration. Because CMOS processing requires high temperature anneals (up to 1050 °C), it is important to understand the diffusion properties of any metal associated with the gate dielectric in silicon at these temperatures. In addition, dopant penetration from the doped polysilicon gate into the Si channel at these temperatures must also be studied. Impurity outdiffusion (Hf, Zr) from the dielectric, or dopant (B, As, P) penetration through the dielectric into the channel region would likely result in deleterious effects upon the carrier mobility. In this dissertation extensive thermal stability studies of alternate gate dielectric candidates ZrSixOy and HfSixOy are presented. Dopant penetration studies from doped-polysilicon through HfSixOy and HfSixOyNz are also presented. Rutherford backscattering spectroscopy (RBS), heavy ion RBS (HI-RBS), x-ray photoelectron spectroscopy (XPS), high resolution transmission electron microscopy (HR-TEM), and time of flight and dynamic secondary ion mass spectroscopy (ToF-SIMS, D-SIMS) methods were used to characterize these materials. The dopant diffusivity is calculated by modeling of the dopant profiles in the Si substrate. In this disseration is reported that Hf silicate films are more stable than Zr silicate films, from the metal interdiffusion point of view. On the other hand, dopant (B, As, and P) penetration is observed for HfSixOy films. However, the addition of nitrogen to the Hf - Si - O systems improves the dopant penetration properties of the resulting HfSixOyNz films.

Orientador: Renata Marques de Melo Marinho / Banca: Tarcísio José Arruda Paes Júnior / Banca: Laís Regiane da Silva Concílio / Resumo: No presente trabalho, avaliou-se o efeito de diferentes concentrações de ácido fluorídrico (5% e 10%), na resistência de união entre uma cerâmica de silicato de lítio reforçado por zircônia (sem cristalização adicional, com queima de glaze e com queima adicional de cristalização) e um cimento resinoso, com e sem envelhecimento. Avaliou-se também o comportamento em fadiga de uma cerâmica à base de silicato de lítio reforçado por zircônia (sem cristalização adicional, com de queima de glaze e com queima adicional de cristalização) cimentada adesivamente a um material análogo a dentina (NEMA G10), na ausência ou presença de envelhecimento. Para o teste de microtração os blocos cerâmicos de silicato de lítio reforçado por zircônia foram cortados em blocos menores e divididos aleatoriamente em 12 grupos (N = 72; n = 6). No teste de fadiga os espécimes foram em formato de discos (diâmetro de 12 mm e espessura de 1,2 mm) cimentados a discos de resina epoxi Nema G10 (diâmetro de 12mm e espessura de 2,3mm), divididos em 6 grupos (N = 120; n = 20) e submetidos a ensaio de fadiga pelo método da escada (100.000 ciclos, 20 Hertz de frequência). Os dados resistência de união (MPa) e fadiga (N) foram submetidos à ANOVA 2-fatores, e respectivamente submetidos aos testes de Tukey e Bonferroni (p<0,05). Os resultados mostraram diferença estatisticamente significante para o fator "termociclagem" (ANOVA 2 fatores, p<0,05) sendo os grupos sem termociclagem superiores aos grupos com envelhecimento... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract : This study evaluated the effect of different hydrofluoric acid concentrations (5% and 10%) on the bond strength between a zirconium-reinforced lithium silicate ceramic (without additional crystallization, glaze firing and additional crystallization firing) and a resinous cement, with and without aging. The fatigue behavior of a zirconium-reinforced lithium silicate-based ceramic (without additional crystallization, glaze firing and additional crystallization firing) adhesively cemented to a material like dentin (NEMA G10) was also evaluated with or without thermal aging. For the microtensile test, the zirconia reinforced lithium silicate ceramic blocks were cut into smaller blocks and randomly divided into 12 groups (N=72; n=6). In the fatigue test, discs (diameter 12 mm and thickness 1.2 mm) were cut, cemented to NEMA G10 epoxy resin discs (diameter 12 mm and thickness 2.3 mm), assigned into 6 groups (N=120, n=20) and subjected to the fatigue test by the ladder method (100,000 cycles, 20 Hertz frequency). The bond strength (MPa) and fatigue (N) data were submitted to 2-Way ANOVA, and respectively submitted to Tukey and Bonferroni tests (p <0.05). The results showed a statistically significant difference for the "thermocycling" factor (2-Way ANOVA, p <0.05), with the groups without thermocycling been superior to the aged groups (Tukey). The Bonferroni test indicated that within the same condition (with or without thermocycling), the groups with additional crystallization or glaze firing showed higher results than the groups without additional firing. Thus, the bond between ZLS and the resin cement was negatively affected by thermal aging in water. On the other hand, the fatigue behavior of ZLS discs cemented on NEMA G10 was higher in the groups that received glaze or crystallization firing and lower in the aged groups...(Complete abstract click electronic access below) / Mestre

Thesis. 1977. Ph.D.--Massachusetts Institute of Technology. Dept. of Materials Science and Engineering. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE. / Vita. / Includes bibliographical references. / Ph.D.

Thesis. 1976. Sc.D.--Massachusetts Institute of Technology. Dept. of Materials Science and Engineering. / Microfiche copy available in Archives and Science. / Vita. / Includes bibliographical references. / Sc.D.

Acid leaching of different natural chrysocolla samples under ambient conditions demonstrated that the chemical and chrystallographic inhomogenity significantly affects the rate and that it is virtually impossible to use the mineral as a model for the kinetics of hydrometallurgical processing of copper ores.Dissolution experiments, stability diagrams,and X-ray powder diffraction studies are undertaken and studied in this research and results are given.It is found that a complete solid solution series exists in Nature with most tungstenian wulfenites and molybdenum stolzites containing a range of compositions. The first occurrence of a molybdenum stolzite from Broken Hill, Australia, which also contains small amounts of chromium is noted.Raman microprobe spectroscopy has proven to be a very quick and effective non-destructive method for semi-quantitative analysis of the wulfenite-stolzite and powellite-scheelite solid-solution series and in detecting very small amounts of substituted chromate in wulfenite, stolzite and anglesite / Doctor of Philosophy (PhD)