Browsing by Subject "Amorphous substances"
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Item Crystallization of amorphous solid films(2003-05) Safarik, Douglas Joseph; Mullins, C. B.Below ~130 K, H2O can exist for prolonged periods in a thermodynamically unstable, non-crystalline solid form known as amorphous solid water (ASW). When warmed to above 135 K, ASW crystallizes to the thermodynamically favored state, cubic ice I, on a laboratory time scale. Despite the relevance of ASW crystallization to a variety of scientific problems ranging from astrophysical phenomena to cryopreservation, the kinetics of this transformation are largely uncharacterized, and its mechanism is not fully understood. In the present work, the crystallization kinetics of vapor-deposited, nonporous ASW films less than one micron thick are investigated experimentally near 140 K. The amorphous to crystalline transition is characterized using a probe molecule, chlorodifluoromethane (CHF2Cl), whose adsorbed states and hence desorption kinetics are sensitive to the crystallinity of solid water surfaces. The transformation kinetics of very thick ASW films are found to be both independent of specimen size and consistent with simultaneous homogeneous nucleation and isotropic growth of crystalline ice grains. As the ASW film thickness is reduced from 385 nm to 55 nm, however, the rate of surface crystallization decelerates, in apparent conflict with a homogeneous nucleation and growth mechanism. In an attempt to explain this behavior, a geometrical model of phase transition kinetics at the surface of solids, with special consideration of finite specimen size in one dimension, is constructed. For materials in which nucleation occurs spatially randomly, phase change is predicted to decelerate when film thickness is reduced below the mean crystal grain size. This phenomenon originates from a reduction in the number of crystallites available to transform the surface as the sample becomes thinner. Good quantitative agreement between this simple model and the experimental data is attained using a minimum of kinetic parameters, suggesting it captures the essential physics of ASW crystallization. These model fits also yield preliminary estimates of crystalline ice growth and nucleation rates. Finally, an experimental protocol and corresponding model of phase change that together permit accurate quantification of nucleation and growth kinetics (when both processes occur simultaneously) is developed. Using the outlined methodology, crystalline ice growth and nucleation rates near 140 K are found to be ~1 Å/s and ~1010 cm-3s -1, respectively, and to exhibit Arrhenius temperature dependencies with activation energies of ~50 and ~170 kJ/mol.Item Growth and characterization of CVD Ru and amorphous Ru-P alloy films for liner application in Cu interconnect(2007-12) Shin, Jinhong, 1972-; Ekerdt, John G.Copper interconnect requires liner materials that function as a diffusion barrier, a seed layer for electroplating, and an adhesion promoting layer. Ruthenium has been considered as a promising liner material, however it has been reported that Ru itself is not an effective Cu diffusion barrier due to its microstructure, which is polycrystalline with columnar grains. The screening study of Ru precursors revealed that all Ru films were polycrystalline with columnar structure, and, due to its strong 3D growth mode, a conformal and ultrathin Ru film was difficult to form, especially on high aspect ratio features. The microstructure of Ru films can be modified by incorporating P. Amorphous Ru(P) films are formed by chemical vapor deposition at 575 K using a single source precursor, cis-RuH₂(P(CH₃)₃)₄, or dual sources, Ru₃(CO)₁₂ and P(CH₃)₃ or P(C6H5)₃ The films contain Ru and P, which are in zero-valent states, and C as an impurity. Phosphorus dominantly affects the film microstructure, and incorporating > 13% P resulted in amorphous Ru(P) films. Metastable Ru(P) remains amorphous after annealing at 675 K for 3 hr, and starts recrystallization at ~775 K. The density of states analysis of the amorphous Ru(P) alloy illustrates metallic character of the films, and hybridization between Ru 4d and P 3p orbitals, which contributes to stabilizing the amorphous structure. Co-dosing P(CH)₃ with Ru₃(CO)₁₂ improves film step coverage, and the most conformal Ru(P) film is obtained with cis-RuH2(P(CH₃)₃)₄; a fully continuous 5 nm Ru(P) film is formed within 1 µm deep, 8:1 aspect ratio trenches. First principles density functional theory calculations illustrate degraded Cu/Ru adhesion by the presence of P at the interface, however, due to the strong Ru-Cu bonds, amorphous Ru(P) forms a stronger interface with Cu than Ta and TaN do. Cu diffusion studies at 575 K suggests improved barrier property of amorphous Ru(P) films over polycrystalline PVD Ru.Item Highly supersaturated aqueous solutions by design of amorphous pharmaceutical nanoparticles(2007-12) Matteucci, Michal Elizabeth, 1977-; Johnston, Keith P., 1955-; Williams, Robert O., 1956-For 40% of currently discovered drugs which are poorly water soluble, engineering amorphous nanoparticles with rapid dissolution and enhanced solubility can improve their absorption. Antisolvent precipitation by mixing organic drug solutions with aqueous solutions produced sub-300 nm amorphous nanoparticle dispersions. Polymeric stabilizers increased the nucleation rate by lowering the interfacial tension and adsorbed to particle surfaces to inhibit growth by condensation and coagulation. An increase in the stabilizer concentration decreased the average particle size until reaching a threshold where the particles were < 300 nm for the poorly water soluble drug, itraconazole. The amorphous itraconazole nanoparticle dispersions dissolved at pH 1.2 to produce high supersaturation levels up to 90-times the equilibrium solubility. The supersaturation increased with particle curvature, as described qualitatively by the Kelvin equation. A thermodynamic analysis indicated the stabilizer maintained amorphous ITZ in the solid phase with a fugacity 90-times the crystalline value, while it did not influence the activity coefficient of ITZ in the aqueous phase. Recovery of the amorphous nanoparticles from water was achieved by adding salt to desolvate the polymeric stabilizers and flocculate the particles, which could then be rapidly filtered. The flocculation under constant particle volume fraction produced open flocs which were redispersible in water to their original ~300 nm size, after filtration and drying. Amorphous particles were preserved, as flocs were formed below the drug's glass transition temperature. After flocculation/filtration, medium surface area (2-5 m²/g) particles dissolved rapidly in pH 6.8 buffer with 0.17% surfactant to an unusually large supersaturation up to 17, comparable to that for high surface area (13-36 m²/g) particles. However, the decay in supersaturation was much slower for the medium surface area particles, as the smaller excess surface area of undissolved particles produced slower nucleation and growth from solution. In contrast, the maximum supersaturation was far lower for more conventional low surface area solid dispersions of drug in polymers, because of crystallization of undissolved solid during slow dissolution. The ability to design the particle morphology to manipulate the level in supersaturation in pH 6.8 media, offers new opportunities in raising bioavailability in gastrointestinal delivery.Item Low-energy electron driven reactions in layered methanol/amorphous solid water films(2008-05) Akin, Minta Carol, 1980-; Mullins, C. B.Understanding the radiolysis of impure water and resulting reactions is crucial to many fields. Reactions driven by low energy electrons (LEE) are of special interest, as high-energy radiation generates large quantities of these electrons, which then provide the energy for most subsequent reactions. Interfacially located reactions are also of particular interest, both as models for heterogeneously distributed reactions occurring during radiolysis, and in their own right, as radiation-driven reactions at interfaces are responsible for key processes such as corrosion and DNA damage. To study LEE-driven reactions at interfaces, thin-layered films of amorphous solid water (ASW) and methanol were grown under ultra-high vacuum conditions using molecular beam techniques. The films were exposed to a beam of low-energy (100eV or less) electrons, and studied using electron-stimulated desorption (ESD) and temperature programmed desorption (TPD). ESD studies indicated that methanol moves through a water film during deposition at 80 K but not at 50 K. This transport was not seen during thermal annealing, but radiation-induced mixing was observed at all temperatures. Major and minor LEE radiation products of pure methanol films were identified and found to be consistent with previous results. Products of LEE irradiated layered methanol/water films were determined for the first time using ESD and TPD spectra, and found to be limited to H₂, O, O₂, CH₂O, C₂H₆, CO, CO₂, CH₃OCH₃, and CH₃CH₂OH. The effect of adding methanol to an ASW film on the production in ASW of H₂ and O₂ was also examined. The interface created by the addition of CH₃OH to ASW was found to generate H₂ in previously non-reactive regions of the water film by increasing water-water and water-methanol reactions. Radiative mixing of CH₃OH and ASW enhanced this effect, presumably by increasing the region of disrupted H-bonding in the ASW. In contrast, the addition of CH₃OH at low coverages suppressed O₂ production in both unprocessed and preprocessed ASW layers. Modeling indicates that methanol scavenging of the O₂ precursor OH and of the reaction-driving electrons is responsible for this reduction in O₂ signal.Item A sorption and dilation investigation of amorphous glassy polymers and physical aging(2001-08) Punsalan, David Troy; Koros, William J., 1947-The goal of this work was to investigate the effect of physical aging on penetrant sorption and dilation in glassy polymers. At the present time, this topic is fundamental in nature but may be relevant to previously observed declines in the productivity of polymeric gas separation membranes. Though physical aging is well known to occur in glassy polymers, it is often neglected in most contexts. However, since gas sorption and diffusion occurs on a molecular scale, reduction of unrelaxed volume due of physical aging may have a large impact on the macroscopically observed manifestations of these phenomena. In addition to experimental investigations of the effect of physical aging on the polymerpenetrant environment, various models of sorption and dilation are studied. Of the numerous models available in the literature, the theory of dual mode sorption, Sanchez-Lacombe lattice fluid equation of state and sitedistribution model have previously demonstrated notable success and are applied to three polymers of varying chain flexibility: Matrimid® , Ultem® and Lexan® (Tg = 313, 215 and 150°C respectively). Since the lattice fluid equation of state is intend for use on equilibrium media, only partial descriptions of solubility are expected. A variation of the Sanchez-Lacombe equation of state which takes into account the non-equilibrium nature of the glassy state, suitably called the NonEquilibrium Lattice Fluid model, is also considered. In this work, the sorption and dilation data were used to study the presence of unrelaxed volume in glassy polymer materials and how it is affected by physical aging. A variety of other characterizational techniques were explored as well. Substantial changes in the sorption, dilation and CO2 partial molar volume due to physical aging were observed for bulk films of Matrimid® and Lexan® , but not for Ultem® . Gas solubility was found to be lower in thin ( l =0.1mm) Matrimid® films than in thick films (l =25.4mm) giving rise to lower values of the so-called “Langmuir sorptive capacity” for the thinner sample. This particular result along with other experimental observations made in this work support claims based on the proposed “diffusion of free volume” mechanism of aging that explains thickness dependent phenomena in glassy polymers.Item Transport mechanisms in nanoscale amorphous solid water films(2006) McClure, Sean Michael; Mullins, C. B.Amorphous solid water (ASW) is a disordered, glassy form of water, which upon heating above T~135-140 K, crystallizes to cubic ice. Despite much interest and research, many properties of this glassy water phase remain contentious. Particular controversy surrounds the location of its glass transition temperature (Tg) and details regarding relaxation dynamics (viscosity, diffusivity) as water is heated and/or cooled through Tg. In this dissertation, transport mechanisms in layered, isotopically-labeled (H2 18O, H2 16O) nanoscale ASW films (~ 10-100 nm) are investigated via temperature programmed desorption (TPD) techniques. ASW films are known to fracture during crystallization, presumably due to stresses generated within the film during the nucleation and growth process. This fracturing produces pathways for vapor-phase transport of desorbing molecules within the film. Translational motion of H2O is, in all cases, observed coincident with crystallization (and hence film fracture) during heating and desorption of layered ASW films. Comparison with a bulk diffusion model illustrates that the observed intermixing is inconsistent with a bulk diffusion mechanism. Through the use of hydrophobic, diffusion "barrier" layers (CCl4, CHCl3) I have been able to demonstrate that bulk diffusion in ASW is likely very small prior to crack/fracture formation. Transport properties of dilute, glassy nitric acid films (0 - 2.2 mol % HNO3) are also investigated. Results demonstrate that the presence of dilute amounts of HNO3 dramatically reduce crystallization-induced film fracture. Intermixing experiments using structured films of dilute HNO3/H2 16O and labeled water (H2 18O) demonstrate that water intermixing during crystallization is substantially reduced. Combined, the experimental results suggest that the intermixing observed in thin ASW films during crystallization is due to a porosity-mediated transport mechanism. This implies that the ASW bulk self-diffusivity near crystallization is not 'fragile' in nature, in contrast with previous results. Instead, these results suggest that ASW is likely either (i) a glass or (ii) a strong liquid prior to T~160 K. The latter case (ii) would require a change in water dynamics from its known 'fragile' behavior at higher temperatures (T~231-273 K) to 'strong' behavior at low temperatures (T<160 K).