Browsing by Subject "Amorphous"

Now showing 1 - 4 of 4
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    Effects of nanocrystalline silicon inclusions in doped and undoped thin films of hydrogenated amorphous silicon.
    (2009-12) Blackwell, Charlie Pearman
    Hydrogenated amorphous silicon has attracted considerable interest as a low-cost material for various large-area electronic devices, such as scanners, thin film transistors employed in flat panel displays, and photovoltaic devices. A major limitation of amorphous silicon is a light-induced degradation of the photoconductivity and dark conductivity, associated with the creation of metastable dangling bond defects. Recent reports that mixed phase thin films, consisting of silicon nanocrystallites embedded within a hydrogenated amorphous silicon matrix, display a resistance to this light-induced degradation have motivated the development of a novel deposition system to synthesize such materials. Conventional techniques to generate such amorphous/nanocrystalline mixed phase films involve running a Plasma Enhanced Chemical Vapor Deposition system very far from those conditions that yield high quality amorphous silicon. A dual-plasma co-deposition system has thus been constructed, whereby the silicon nanoparticles can be fabricated in one chamber, and then injected into a second plasma reactor, in which the surrounding amorphous silicon is deposited. The deposition process, as well as structural, optical, and electronic characterization of these films, including the dark conductivity, photoconductivity, infra-red absorption spectra, micro-RAMAN spectra, and the optical absorption spectra, will be discussed for these films.
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    Electronic transport in mixed-phase hydrogenated amorphous/nanocrystalline silicon thin films.
    (2010-08) Adjallah, Yves Gbemonde
    The opto-electronic properties of amorphous/nanocrystalline hydrogenated silicon (a/nc-Si:H) mixed-phase thin films are investigated. Small crystalline silicon particles (3-5 nm diameter) synthesized in a flow-through reactor are injected into a separate capacitively-coupled plasma (CCP) chamber where mixed-phase hydrogenated amorphous silicon is grown by Plasma Enhanced Chemical Vapor Deposition (PECVD) deposition techniques. This dual-chamber co-deposition system enables the variation of crystallite concentration incorporated into a series of a-Si:H films deposited simultaneously. The structural, optical and electronic properties of these mixed-phase materials are studied as a function of the silicon nanocrystal concentration. That is, we compare a sequence of films deposited in a single run, where the location of the substrate in the CCP chamber determines the density of embedded nanocrystals. Raman spectroscopy is used to determine the volume fraction of nanocrystals in the mixed phase thin films. At a moderate concentration of silicon crystallites, the dark conductivity and photoconductivity are consistently found to be up to several orders of magnitude higher than in mixed phase films with either low or heavy nanocrystalline inclusions. These results are interpreted in terms of a model whereby for low nanocrystal concentrations conduction is influenced by the disorder introduced into the a-Si:H film by the inclusions, while at high nanocrystal densities electronic transport is described by a heterojunction quantum dot model. The thermopower of the undoped a/nc-Si:H has a lower Seebeck coefficient, and similar temperature dependence, to that observed for undoped a-Si:H. In contrast, the addition of nanoparticles in doped a/nc-Si:H thin films leads to a negative Seebeck coefficient (consistent with n-type doping) with a positive temperature dependence, that is, the Seebeck coefficient becomes larger at higher temperatures. The temperature dependence of the thermopower of the doped a/nc-Si:H is similar to that observed in unhydrogenated a-Si grown by sputtering or following high-temperature annealing of a-Si:H, suggesting that charge transport may occur via hopping in these materials.
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    Enabling Direct Compression Tablet Development Of Celecoxib Through Solid State Engineering
    (2020-08) Wang, Kunlin
    Tablets are the most desirable solid oral dosage form for patients. Direct compression (DC) tablet formulation is the most economical, robust and efficient way of tablet manufacture. Being sensitive to properties of the Active Pharmaceutical Ingredient (API), direct compression tablet formulation is not available for the high dose non-steroidal anti-inflammatory drug, celecoxib (CEL) due to the undesirable properties of the commercial solid form of CEL, including low bulk density, poor flowability and tablet lamination issues. The solid form used in commercially available CEL capsules is a polymorph of CEL, Form III. Form III CEL is a needle shaped crystal, which is exceptionally elastic. This high elasticity, verified by nanoindentation and three-point bending tests, is unfavorable for good tablet quality and performance during high speed tableting. Through understanding the molecular interactions by analyzing the CEL crystal structure, a structural model for high elasticity is built and validated by Raman spectroscopy. Interlocked molecular packing without slip plane and the presence of isotropic hydrogen bond network are major structural features responsible for both the exceptional elastic flexibility and high stiffness of the CEL crystal. CEL Form III exhibits unsatisfactory flowability and tablet lamination issues for DC tablet manufacturing. Pharmaceutically acceptable solvates of CEL offer better flow, compaction and dissolution properties than CEL Form III. Two stoichiometric solvates of CEL and N-methyl-2-pyrrolidone (NMP) are extensively characterized and examined, which establishes a clear crystal structure-property relationship essential for crystal engineering of CEL. Through crystal engineering, a DC tablet formulation of CEL is successfully developed using the dimethyl sulfoxide (DMSO) solvate of CEL. This pharmaceutically acceptable solvate is highly stable and also exhibited much improved manufacturability compared to CEL Form III, including better flowability, lower elasticity and bulk density (superior tablet quality) as well as better dissolution performance. As a Class II drug in the biopharmaceutics classification system with low solubility and high permeability, the high dose of CEL is partially attributed to its limited solubility. Amorphous CEL, although providing solubility advantages as the thermodynamically high energy state, is unstable and prone to crystallization. The study of crystal growth of amorphous CEL reveals a fast glass-to-crystal growth mode at room temperature with a surface-enhanced mechanism. This paves the way for future development of a stable amorphous solid dispersion tablet product of CEL with improved dissolution performance and tablet manufacturability. In summary, by understanding the structural origin of undesired properties of CEL, successful development of the most patient-compliant tablet dosage form by direct compression can be achieved. This sets an excellent example of utilizing a solid state engineering approach to effectively overcome challenges encountered in direct compression tablet development.
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    Structure-property relationships of solids in pharmaceutical processing
    (2012-11) Chattoraj, Sayantan
    Pharmaceutical development and manufacturing of solid dosage forms is witnessing a seismic shift in the recent years. In contrast to the earlier days when drug development was empirical, now there is a significant emphasis on a more scientific and structured development process, primarily driven by the Quality-by-Design (QbD) initiatives of US Food and Drug Administration (US-FDA). Central to such an approach is the enhanced understanding of solid materials using the concept of Materials Science Tetrahedron (MST) that probes the interplay between four elements, viz., the structure, properties, processing, and performance of materials. In this thesis work, we have investigated the relationships between the structure and those properties of pharmaceutical solids that influence their processing behavior. In all cases, we have used material-sparing approaches to facilitate property assessment using very small sample size of materials, which is a pre-requisite in the early stages of drug development when the availability of materials, drugs in particular, is limited. The influence of solid structure, either at the molecular or bulk powder levels, on crystal plasticity and powder compaction, powder flow, and solid-state amorphization during milling, has been investigated in this study. Through such a systematic evaluation, we have captured the involvement of structure-property correlations within a wide spectrum of relevant processing behaviors of pharmaceutical solids. Such a holistic analysis will be beneficial for addressing both regulatory and scientific issues in drug development.