Browsing by Subject "Crystal growth"
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Item A liquid-phase epitaxy system(Texas Tech University, 1978-08) Burrows, Ian Lyndsay CunliffeNot availableItem Deep traps in liquid phase epitaxially grown AlGaAs layers at AlGaAs heterojunctions(Texas Tech University, 1983-05) Rys, AndrzejNot availableItem Evidence from high-temporal-resolution strain rates for strain softening due to episodic fluid influx at Passo del Sole, Central Swiss Alps(2012-12) Stacy, Sarah Jean; Carlson, William, 1952-; Cloos, Mark; Ketcham, Richard ABerg (2007) determined hand-sample-scale high-temporal-resolution strain rates from rotated garnet for two samples of quartzose pelitic gneiss at Passo del Sole, Central Swiss Alps, documenting a correlation between dramatic increases in strain rate and compositionally anomalous garnet growth zones. Considering additional evidence that these anomalous zones resulted from externally derived ephemeral fluid flow, he concluded that increased strain rates at Passo del Sole are the result of strain softening caused by fluid influx. This study tests Berg's interpretation by calculating new hand-sample-scale high-temporal-resolution strain rates for two additional samples of the same gneiss: a control sample (Sample PDS 03-30) that shows no unusual zoning patterns, implying that it was unaffected by fluids; and another (Sample PDS 03-2) that features a prominent Ca spike, suggesting that it has been affected by fluid influx. Unique features of garnet from this locality--contemporaneity of chemical zones, near-simultaneous nucleation, size-proportional growth, and rock-wide chemical equilibrium--were exploited to calculate strain rates of unprecedentedly high temporal resolution. Thermodynamically modeled temperatures for several growth-zone boundaries in each garnet crystal were combined with a regional heating rate of 11.5 ± 3.5 °C/Myr (presumed constant) and measured deflections of inclusion trails in each zone to calculate strain rates for several discrete time increments during garnet growth. Sample PDS 03-2 displays a 2- to 16-fold increase in strain rate that correlates with growth of the high-Ca zone; strain rates are 0.4 x 10⁻¹⁴ s⁻¹ to 4.1 x 10⁻¹⁴ s⁻¹ for zones with normal Ca concentration and 9.1 x 10⁻¹⁴ s⁻¹ to 17.9 x 10⁻¹⁴ s⁻¹ for the high-Ca zone. Distinct amongst all analyzed samples from Passo del Sole, Sample PDS 03-30--which has not been affected by fluid influx--shows no fluctuations in strain rates, which remain low and similar (0.2 x 10⁻¹⁴ s⁻¹ to 2.6 x 10⁻¹⁴ s⁻¹) across all zones. Results from this study therefore further substantiate the correlation of high strain rates with compositionally anomalous zones, strengthening the interpretation that elevated strain rates at Passo del Sole result from strain softening caused by episodic, externally controlled flow of fluids through the system during synkinematic garnet growth.Item Quantitative studies of porphyroblastic textures(2000) Hirsch, David Marshall, 1969-; Carlson, William, 1952-Spatial correlation functions, which quantify spatial relationships among porphyroblasts over a range of length scales, can be used in combination with other techniques of quantitative textural analysis to constrain crystallization mechanisms in metamorphic rocks. The utility, reliability, and robustness of these functions, however, depend critically upon correct methods of calculation and application to geological samples. Application of the L' -function, Pair Correlation Function, and Mark Correlation Function (Stoyan and Stoyan, 1994) to artificial arrangements of crystals yields results consistent with their predetermined ordering and clustering qualities. These results serve as a foundation for the interpretation of more complex simulated and natural crystal arrays. Analysis of artificial and simulated crystal arrays in which ordering signals are obscured in various ways (displacing crystals in an ordered array by increasing amounts, reducing the number of crystals, and increasing the sample's aspect ratio) demonstrates that these scale-dependent functions are robust indicators of effects diagnostic of certain crystallization mechanisms, even in complex circumstances. The effects of clustering of nucleation sites, however, can strongly obscure any underlying signal that might reveal crystallization mechanisms. The L' -function and the Pair Correlation Function are sensitive to short-range ordering of crystals, which may reflect suppression of nucleation in the vicinity of growing porphyroblasts. The Mark Correlation Function is sensitive to size-isolation correlations, which may reflect retardation of growth among crystals competing for nutrients. Interpretation of these functions, however, requires careful attention to proper calculation of Monte Carlo simulations, which are used to identify values of the functions that constitute a null-hypothesis region for comparison to samples with unknown ordering and clustering characteristics. To yield functional values commensurate with those calculated for a particular natural rock specimen, each simulation must match as closely as possible several critical features of the natural rock, including the set of crystal radii, limitations on the observability of crystals, and the shape and size of the bounding surface of the sample. Crystallization mechanisms in seven previously studied garnetiferous rocks from three localities (Carlson et al., 1995; Denison and Carlson, 1997) have been re-assessed using both scale-dependent correlation functions and single-valued spatial statistics, both evaluated by comparison to rigorously computed null-hypothesis regions. The results confirm previous inferences that the nucleation and growth rates of the garnet porphyroblasts in these specimens were governed by rates of diffusion through the intergranular medium.Item Recrystallization of guaifenesin from hot-melt extrudates containing Acryl-EZE® or Eudragit® L100-55(2008-05) Bruce, Caroline Dietzsch, 1976-; McGinity, James W.The physical stability of guaifenesin in melt-extruded acrylic matrix tablets was investigated. The initial study found that recrystallization was caused by guaifenesin supersaturation in Eudragit[Trademark] L100-55, and that the instability was confined to tablet surfaces. Drug release was not affected by crystal growth as guaifenesin is very water soluble. The addition of a polymer in which guaifenesin showed a higher solubility to the matrix blend decreased recrystallization on storage as supersaturation levels dropped. The second investigation identified heterogeneous nucleation as an additional factor in guaifenesin recrystallization. A quantitative assay showed that talc in matrix tablets accelerated the onset and extent of the recrystallization due to a nucleating effect on guaifenesin. Storage under elevated humidity conditions promoted recrystallization as well, but crystal growth was not correlated with water uptake, which implied a nucleating effect of moisture on guaifenesin. The third study investigated the effect of aqueous film-coating of the matrix tablets to stabilize amorphous guaifenesin using either hypromellose or ethylcellulose as coating polymers. The selection of the coating polymer influenced crystal morphology, and was a major factor in delaying the onset of crystallization, ranging from 1-3 weeks (ethylcellulose film-coatings) to 3-6 months (hypromellose film-coatings). Higher weight gains retarded recrystallization. Factors promoting drug and polymer diffusion, such as long curing times and elevated temperatures during both curing and storage, incomplete film coalescence and high core drug concentrations all resulted in an earlier onset of crystallization. The effects of single-screw extrusion (SSE) and twin-screw extrusion (TSE) of diltiazem hydrochloride and guaifenesin-containing blends in Eudragit[Trademark] L100-55 on drug morphology and dispersion were studied in the fourth project. Guaifenesin solubilized diltiazem hydrochloride, and plasticized Eudragit[Trademark] L100-55. Extrusion temperature influenced the drug morphology in single-screw extrudates, while TSE rendered all formulations amorphous due to higher dispersive mixing capabilities. Drug distribution improved with extrusion temperature and by TSE over SSE. Homogeneous matrices showed the slowest drug release at pH 1.0. Recrystallization was inversely correlated to drug distribution. In conclusion, the physical stability of guaifenesin in hot melt-extruded acrylic matrix tablets was shown to be affected by formulation, processing and post-processing factors.Item Sulfate Induced Heave: Addressing Ettringite Behavior in Lime Treated Soils and in Cementitious Materials(2012-02-14) Kochyil Sasidharan Nair, Syam KumarCivil engineers are at times required to stabilize sulfate bearing clay soils with calcium based stabilizers. Deleterious heaving in these stabilized soils may result over time. This dissertation addresses critical questions regarding the consequences of treating sulfate laden soils with calcium-based stabilizers. The use of a differential scanning calorimeter was introduced in this research as a tool to quantify the amount of ettringite formed in stabilized soils. The first part of this dissertation provides a case history analysis of the expansion history compared to the ettringite growth history of three controlled low strength mixtures containing fly ash with relatively high sulfate contents. Ettringite growth and measurable volume changes were monitored simultaneously for mixtures subjected to different environmental conditions. The observations verified the role of water in causing expansion when ettringite mineral is present. Sorption of water by the ettringite molecule was found to be a part of the reason for expansion. The second part of this dissertation evaluates the existence of threshold sulfate levels in soils as well as the role of soil mineralogy in defining the sensitivity of soils to sulfate-induced damage. A differential scanning calorimeter and thermodynamics based phase diagram approach are used to evaluate the role of soil minerals. The observations substantiated the difference in sensitivity of soils to ettringite formation, and also verified the existence of a threshold level of soluble sulfates in soils that can trigger substantial ettringite growth. The third part of this dissertation identifies alternative, probable mechanisms of swelling when sulfate laden soils are stabilized with lime. The swelling distress observed in stabilized soils is found to be due to one or a combination of three separate mechanisms: (1) volumetric expansion during ettringite formation, (2) water movement triggered by a high osmotic suction caused by sulfate salts, and (3) the ability of the ettringite mineral to absorb water and contribute to the swelling process.Item The fabrication and characterization of a GaAs-AlGaAs diode(Texas Tech University, 1981-05) Burrows, Ian Lyndsay CunliffeNot available