Browsing by Subject "Nucleation"
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Item Characterization of low density oxide surface sites using fluorescent probes(2013-12) McCrate, Joseph Michael; Ekerdt, John G.Low density surface sites are believed to play an important role in processes occurring on oxide surfaces, including catalysis and particle and film nucleation. However, our understanding of the role and chemical nature of such sites play in these processes is limited by the inability to experimentally detect minority surface sites in many oxide systems. The research performed for this dissertation is focused on developing a surface science technique utilizing fluorescent molecules to titrate specific surface sites on planar fused silica surfaces in an ultra-high vacuum (UHV) environment. High sensitivity (low detection limit) is achieved by using derivatives of perylene, a high quantum yield fluorophore. High specificity is attained by employing perylene derivatives with functional groups designed to react chemically with and titrate various sites. In addition to titrating the well-studied hydroxyl sites with perylene-3-methanol (density ~ 10¹⁴ cm⁻²), which is used to establish the technique, the detection of strained siloxane sites (~ 10¹² cm⁻²), ) with perylene-3-methanamine and oxygen vacancy sites (~ 10¹¹ cm⁻²), ) with 3-vinyl perylene is demonstrated. Particle nucleation on oxides is suspected to involve defects that trap adatoms and form critical nuclei. Using this technique, the possible role strained siloxane and oxygen vacancy sites play in trapping adatoms during the nucleation of Ge nanoparticles on silica surfaces is examined.Item Controlling nucleation and growth of ultra-thin ruthenium films in chemical vapor deposition(2016-05) Liao, Wen; Ekerdt, John G.; Korgel, Brian A; Hwang, Gyeong S; Hildebrandt Ruiz, Lea; Ferreira, PauloAs feature sizes in microelectronic devices decrease, ultra-thin (< 3 nm) and smooth diffusion barriers are required to prevent copper from diffusing into the surrounding dielectric layers and to limit electron scattering at the copper-liner surface. Chemical vapor deposition (CVD) is one route to these barriers. The inhibitor gas adsorbs on metal nanoparticles, forces additional nucleation and enhances nucleation density. Island growth combined with a sparse nucleation density leads to film roughness and the deposition of more metal mass than is needed to form a film of sufficient thickness to function as a copper diffusion barrier when compared to a uniformly-thick metal film. In the first study, a higher nucleation density and smoother Ru film is achieved in CVD with CO addition during growth. CO competes with Ru3(CO)12 for free hydroxyl adsorption. The CO addition to Ru3(CO)12 deposition at proper timing and effective partial pressure reduces the film growth rate, surface roughness and nanocrystalline grain size by chemical vapor deposition. The second study reports the use of ammonia to inhibit the growth of previously-nucleated ruthenium islands and force the nucleation of additional islands such that thinner films form as the islands coalesce with continued growth using Ru3(CO)12. The ammonia addition reduces the film nanocrystallinity and the films appear X-ray amorphous with the highest ammonia partial pressure during film deposition. In the third study, films grown from Ru(tBu-Me-amd)2(CO)2 form a 2D wetting layer before 3D particle growth is observed. CO and ammonia addition to the gas phase during film growth from Ru(tBu-Me-amd)2(CO)2 leads to smoother films by inducing surface reconstructions during the film growth; these gases also lead to films with lower resistivity and lower crystalline character. Overall, this research is to understand how blocking adsorbed moieties effect the nucleation of metals on a silica substrate, and to discover the principles leading to ultra-thin and smooth metallic films in CVD.Item Crystallization of metamorphic garnet : nucleation mechanisms and yttrium and rare-earth-element uptake(2014-05) Moore, Stephanie Jean; Carlson, William, 1952-This dissertation focuses on two areas of garnet porphyroblast crystallization that have until now remained largely uninvestigated: epitaxial nucleation of garnet porphyroblasts and yttrium and rare earth (Y+REE) uptake in metamorphic garnet. The mechanism of epitaxial nucleation is explored as a step towards determining which aspects of interfaces are significant to interfacial energies and nucleation rates. Garnet from the aureole of the Vedrette di Ries tonalite, Eastern Alps, shows a clear case of epitaxial nucleation in which garnet nucleated on biotite with (110)grtItem Growth and characterization of Ru films deposited by chemical vapor deposition : towards enhanced nucleation and film properties(2009-12) Thom, Kelly Marriott; Ekerdt, John G.As device dimensions in integrated circuits scale down, there is an increasing need to deposit ultra-thin, smooth, continuous films for use in applications such as the liner in back end processing. The liner must have good adhesion to both Cu and the dielectric, act as a Cu diffusion barrier, and be conductive enough to allow the electroplating of Cu. Ruthenium (Ru) has been considered as a possible material to be implemented into the liner due to its low electrical resistivity, high thermal and chemical stability, and negligible solubility with copper. Chemical vapor deposition (CVD) is an attractive growth technique for Ru films because it allows conformal deposition in high-aspect ratio features. However, there are some limitations that must be overcome in the deposition of Ru films. CVD Ru films suffer from poor nucleation on oxide and nitride substrates. Poor nucleation leads to rough, large-grained polycrystalline columnar films, which may not coalesce into a continuous film until the thickness greatly exceeds the requirements for the liner. This dissertation presents surface chemistry and film growth studies involving Ru CVD and focuses on improving the nucleation and properties of Ru films. In situ surface analysis techniques including X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD) were used to study the fundamental adsorption behavior of the Ru precursor, (2,4- dimethylpentadienyl)(ethylcyclopentadienyl)Ru or DER, on polycrystalline Ta, both with and without iodine adsorbed on the Ta. Based upon these results, CVD films were grown using DER/O₂, and it was shown that nucleation and film properties can be improved by the addition of methyl iodide. Ru films grown using DER/O₂ show sparse nucleation, which leads to very rough surface topography and large polycrystalline columnar grains. The addition of methyl iodide during growth significantly improves nucleation and results in smoother, smaller-grained films. Iodine adsorbs on the initially-formed Ru islands and continuously segregates through the film to the surface during the entire deposition. In addition, CVD films grown with Ru₃(CO)₁₂ were studied. Use of the Ru₃(CO)₁₂ precursor results in thin, ultra-smooth films that show little to no columnar grain structure.Item High-silica zeolite nucleation from clear solutions(Texas A&M University, 2006-04-12) Cheng, Chil-HungUnderstanding the mechanism of zeolite nucleation and crystallization will enable the zeolite science community to tune zeolite properties during synthesis in order to accommodate the purposes of various applications. Thus there has been considerable research effort in "deciphering" the mechanism by studying the growth course of tetrapropylammonium (TPA)-mediated silicalite-1 using several techniques, such as dynamic light scattering (DLS), small-angle X-ray/neutron scattering (SAXS/SANS), and nuclear magnetic resonance (NMR). While these studies have generated a more comprehensive picture on the silicalite-1 growth mechanism, the general application of the mechanism and how it could be applied to other zeolite systems have not been addressed. This work initially tried to apply the insights developed from the TPAsilicalite- 1 clear solution synthesis by investigating the nanoparticles formation and zeolite growth in several tetraethyl orthosilicate (TEOS)-organocation-water solutions heated at 368 K using SAXS. The results are in contrast to TEOS-TPAOH-water mixtures that rapidly form silicalite-1 at 368 K. These results imply that the developed TPA-silicalite-1 nucleation and crystallization mechanism is not universally applicable to other zeolite systems and TPA-silicalite-1 itself could be a special case. With this in mind, the next goal of this work uses in situ SAXS to revisit silicalite-1 growth kinetics prepared by using several TPA-mimic organocations and some asymmetric geometry organocations. The results clearly show the TPA cation is an extraordinarily efficient structure-directing agent (SDA) due to its moderate hydrophobicity and perfect symmetric geometry. Any perturbation of the hydrophobicity and symmetry of SDA leads to a deterioration of zeolite growth. This work further investigates the influences of alcohol identity and content on silicalite-1 growth from clear solutions at 368 K using in situ SAXS. Several tetraalkyl orthosilicates (Si(OR)4, R = Me, Pr, and Bu) are used as the alternative silica sources to TEOS in synthesizing silicalite-1. Increasing the alcohol identity hydrophobicity or lowering the alcohol content enhances silicalite-1 growth kinetics. This implies that the alcohol identity and content do affect the strength of 1) hydrophobic hydration of the SDA and 2) the water-alcohol interaction, through changing the efficiency of the interchanges between clathrated water molecules and solvated silicate species.Item On the role of microstructure in ductile failure(2011-08) Ghahremaninezhad Mianji, Ali; Ravi-Chandar, K.; Mear, Mark E.; Liechti, Kenneth M.; Huang, Rui; Benzerga, AmineFailure in structural materials occurs initially by localization of deformation, and subsequently through a process of nucleation, growth and coalescence of voids. Predicting material failure requires a careful investigation of the different stages of damage evolution at the multiple scales. The main objective of this thesis is to explore the evolution of damage and to correlate this with the deformation of the material at the continuum and microstructural levels. This is accomplished through macroscopic measurements of strain evolution using digital image correlation and microscale measurements of strain and damage using optical and scanning electron microscopy. Three materials with different microstructure were examined. In oxygen-free, high-conductivity copper, a high-purity material without appreciable second phase particles, strain levels in the order of three were observed in the material without any trace of damage. Failure was observed to be triggered by plastic instability in the form of shear bands and the emergence of a prismatic cavity that grows in a self-similar fashion by an alternating slip mechanism. In Al 6061-T6, a material with a dispersion of second phase particles at a volume fraction of about 0.01, nucleation of damage does not appear until plastic strain levels of 0.5 to 1.0. Once damage in the form of particle fracture or decohesion at the interface initiates, subsequent failure follows by the void nucleation, growth and coalescence; but, dominated by the fluctuations in the distribution of second phase particles, final separation occurs in a highly localized layer of material on the order of the grain size, corresponding to a small increase in the overall strain. In nodular cast iron, a material with an initial porosity of about 0.10, growth of voids was observed initially, but this was terminated by a transition of the deformation into a localized region. Phenomenological models based on strain-to-failure and micromechanical models based on a mechanistic description of the microscale deformation are evaluated in light of the above examination of failure in these three classes of materials.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 Structural transformation rates in zinc-doped nickel fluorotitanate(Texas Tech University, 1983-12) Casey, Kelly GambleNot availableItem 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.