Browsing by Subject "Crystal growth"
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Item Coupled compositional and three-dimensional textural analysis of garnet porphyroblasts(1995) Chernoff, Carlotta Bridget; Carlson, William, 1952-Integration of garnet compositional data with quantitative textural data for garnet positions acquired from high-resolution computed X-ray tomography (X-ray CT) allows the growth zoning in a crystal to be assessed with a full knowledge of the locations and sizes of all other garnets in the specimen. This study of a specimen of garnetiferous quartzite from the Picuris Range of north-central New Mexico is the first to evaluate garnet compositional profiles for hundreds of crystals with a clear knowledge of the degree of isolation of each garnet. Using the X-ray CT imagery as a guide, a 47.9 cm³ volume of rock containing 1248 garnet crystals was serially sectioned by grinding to expose the central section of each garnet. Electron microprobe analyses through the centers of 470 garnets include profiles of Mn, Mg, Fe, Ca, Al, and Si at 10-20 μm spacing across 372 of the crystals and analyses of the cores of the remaining 98. Part I of this thesis presents results from several textural and compositional analysis techniques applied to the data set in order to assess the mechanisms that governed crystallization of garnet porphyroblasts. Textural data enable determination of statistical measures of the extent of ordering, clustering, and impingement of adjacent porphyroblasts and characterization of the crystal size distribution and degree of isolation of each porphyroblast. These data may be compared with predictions of quantitative textural features for porphyroblastic rocks resulting from different crystallization mechanisms. Likewise, normalized radius-rate relations computed from compositional zoning patterns in the garnets are compared with predicted relations for several growth-rate laws. Interpretation of these results indicates the dominance of diffusionally-influenced nucleation and diffusion-controlled growth mechanisms. In Part II, the effects of competition among adjacent porphyroblasts for garnet-forming nutrients during a diffusion-controlled nucleation-and-growth process are investigated. Measures of crystal isolation when compared with crystal sizes fail to provide a measure of diffusional competition, although detailed analysis of sizes of closely adjacent crystals reveals clear evidence for the effects of competition during growth. These data lead to the conclusion that the evidence for competitive growth is obscured by the effects of inhomogeneities in the precursor. Part III presents evidence for Ca disequilibrium during garnet growth. Compositional data for Mn, Mg and Fe indicate that their compositions evolved systematically during crystallization and achieved equilibrium in the intergranular medium at hand-sample scale, whereas Ca did not. Instead, Ca concentrations appear to reflect the local extent of reaction around each individual garnet crystal, yielding equilibrium for Ca at scales no larger than that of an individual porphyroblast. These observations are consistent with the speculation that the relative rate of diffusion of Mn, Mg and Fe through the intergranular medium is more rapid than that of Ca, permitting rock-wide equilibration for these species. Slower relative diffusion of Ca prevents equilibration at scales much larger than that of a single garnet crystal. These unexpected results challenge the current thinking about garnet crystallization and call into question the accuracy of thermobarometers based on cation-exchange and phase equilibria involving garnetItem Crystal growth of the cuprate superconductor La1.6-xNd0.4SrxCuO4 by the traveling solvent floating zone method(2018-10-04) Shi, Ao; Li, ElaineIn this thesis, my efforts of the single crystal growth of a cuprate series La₁.₆₋[subscript x]Nd₀.₄Sr[subscript x]CuO₄ by the travelling solvent floating zone method will be described. The availability of high quality single crystals is essential to facilitate experimental studies of high temperature superconductivity in the cuprates. Repeated attempts gradually lead to crystals with the improved qualities. A single pattern well indexed via the X-ray powder diffraction suggests that the crystals were grown with the correct chemistry and single phase. Laue back reflection, on the other hand, indicates that domains with different orientations in the same crystals still exist, and crystal growth processes can be further optimized. Discussions, and possible routes of further optimization of the growth process are suggested at the end of the thesis based on the principles of the travelling solvent growth. A continuous supply of large and better quality crystals would help offer more insight into this challenging strongly correlated system.Item Embedded dielectric microstructures in molecular beam epitaxy : high-quality planar coalescence toward enhanced optoelectronic materials(2018-10-29) Ironside, Daniel Joseph; Bank, Seth Robert; Wasserman, Daniel; Li, Xiaoqin (Elaine); Yu, Edward T; Wang, ZhengSeamless integration of embedded dielectric microstructures in III-V crystal growth is a continued area of research due to its numerous high-impact applications. Historically, investigations into embedded dielectric microstructures within existing crystal growth techniques were focused on blocking dislocations at the III-V/dielectric interface in the production of low defect relaxed high mismatched heteroepitaxy. However, recent efforts have broadened the use of embedded dielectric microstructures for enhancement of optoelectronic device functionality and development of monolithic growth schemes toward integrated photonic circuits. The central challenge of embedding dielectric microstructures in III-V materials is achieving single-crystal high-quality planar coalescence within existing conventional III-V crystal growth techniques without defect. While prevalent in the field of III-V crystal growth, solid-source Molecular Beam Epitaxy (MBE) has a well-known "coalescence problem," historically lacking approaches that achieve planar coalescence over dielectric microstructures. Limited coalescence is in large part due to low diffusion of III-adatoms on dielectric surfaces, typically below 300nm, readily forming polycrystalline deposition on dielectric surfaces exceeding this diffusion length. Several solid-source MBE highly-selective growth and lateral epitaxial overgrowth (LEO) growth approaches have been reported; however, none demonstrating complete planar coalescence over dielectric microstructures. In this dissertation, to overcome the "coalescence problem," we demonstrate for the first time a general methodology for an all-MBE growth of high-quality planar coalescence over a variety of embedded dielectric microstructures. Underpinning the approach, we developed a two-stage all-MBE growth approach for GaAs and InAs on (001) substrates, producing highly selective LEO and planarization, returning the growth front to the (001) surface. Characterization of the growth approach demonstrates for the first time an all-MBE approach to planar coalescence. In application of the two-stage all-MBE growth approach towards photonics, we demonstrate enhancement of quantum emitters using buried silica gratings arrays and develop several methodologies for embedded high-contrast photonic materials through self-formed air voids and molded air channel processes. Lastly, in application to high-quality relaxed high mismatch heteroepitaxy, we demonstrate for the first time an all-MBE approach to III-V metamorphic heteroepitaxy, demonstrating threading dislocation reduction in InAs/GaAs metamorphics with high fill factor embedded silica gratings. Thus, from the material presented here, we provide several significant advances to the long-standing challenge of marrying high-quality semiconductor crystal growth with dielectric microstructures, unlocking several high-impact applications, including high-quality material pathways for enhanced quantum emitters and embedded metasurfaces as well as an all-MBE approach toward heterogeneous III-V integration on silicon.Item 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 Growth optimization of WSe₂ and its sulfurization to WS₂(2019-07-09) Liu, Chison Qishan; Banerjee, SanjayTungsten diselenide has gained much interest within recent years after it was reported to have both p-type and ambipolar transport properties. And because most other transition metal dichalcogenides exhibit n-type transport properties, tungsten diselenide would help to further realize CMOS technology if one could find a more reliable way to synthesize it in large areas with high quality crystallinity. In this report I will be detailing my work on successfully synthesizing WSe₂, its sulfurization into WS₂, and discussing what I’ve observed in both the crystal quality and growth mechanisms. My goal is to provide a better understanding of the growth process in hopes of moving forward with improving future growth recipesItem 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.