<p>  </p> <p>Aluminum (Al) possesses a plenitude of remarkable properties, such as strong corrosion resistance, high thermal and electrical conductivity, and high specific strength. However, Al and its alloys are still remarkably weaker than most high strength steels and susceptible to drastic softening at high temperatures, preventing many applications where its low density would be beneficial. Severe plastic deformation can yield ultra-fine grained Al alloys with similar strengths as steels, although they are highly unstable even at room temperature. Nanotwinned (NT) metals have demonstrated concomitant strength and ductility, enabled by twin boundaries which simultaneously act to inhibit dislocation motion and generate partial dislocations that aid in plasticity. In spite of having a high stacking fault energy, nanotwins have been introduced into Al alloys using transition metal solutes during magnetron sputtering. This thesis aims to explore the impact Zr has on the microstructure, deformation, and thermal stability of nanotwins in NT Al.</p> <p>Our studies identify how Zirconium (Zr) aids in the formation of a significant volume fraction of 9R phase and an abundance of finely spaced incoherent twin boundaries, leading to a maximum hardness of 4.2GPa. They further uncover through <em>in-situ</em> micropillar compression that NT Al-Zr alloys are highly deformable and reach a flow stress of ~1.1GPa. Constant strain rate nanoindentation tests demonstrate the enhanced strain rate sensitivity in NT Al-Zr alloys. Zr is also identified to be a remarkable thermal stabilizer when incorporated into NT Al-Co alloys, with no apparent softening up to 450 °C (0.78 T­m). The influence of substrate texture on nanotwinned Al-Zr alloys microstructure was also thoroughly explored.</p>

No description available.

Natural products have been a significant source of new drugs, especially in treating cancer, infectious diseases, hypertension, and neurological disorders. Although many natural metabolites have been screened and yielded pharmaceutically important drugs, many potential sources of natural product drug therapies still need to be investigated, including lichens. Lichens are symbiotic systems consisting of a filamentous fungus and a photosynthetic partner (an eukaryotic alga and/or cyanobacterium). Lichens produce an impressive variety of unique secondary compounds and have been used as ingredients in folk medicines for centuries. Demonstrated biological roles based on lichen chemistry include: antibiotics, anti-proliferative, antioxidants, anti-HIV, anti-cancer, immunomodulation, and anti-protozoans. Although North America is home to an impressive variety of lichen species, there is limited research to examine the biological potentials of these lichens. The core goal of this dissertation research has been to investigate some of the biological roles including, antibiotic, antioxidant, and anticancer potentials using lichen crude extracts and their metabolites collected from various locations in the United States. Antibiotic screening of crude extracts of 36 lichen species demonstrated inhibitory effects against Escherichia coli, Staphylococcus aureus, Pseudomonas aeruginosa, and Methicillin-resistant S. aureus (MRSA). Generally, acetone extractions were found to be more effective than methanol extractions. It has also been shown that L. vulpina extract was bacteriocidal against MRSA with a relatively slow kill rate that disrupts cell membrane integrity and cell division as possible modes of action. Antioxidant screening of extracts from 11 lichen species, using the Oxygen Radical Absorbance Capacity (ORAC) assay, showed that lichen extracts inhibited the oxidative degradation of the fluorescent molecule (fluorescein-sodium salt) by the oxygen free radical initiator AAPH (2,2'-azobis(2-aminidopropane) dihydrochloride Acetone extracts as well as pure compounds from lichen species showed cytotoxic effects against Burkitt's lymphoma (Raji) cells and a colon cancer cell line (HT29 and SW620). They decreased proliferation, arrested cell cycle at various stages and force the cell to undergo apoptosis. The tested extracts or pure compounds were not toxic to normal cells. In colon cancer apoptosis took place independent of casapase-3. The results of this dissertation showed that lichen compounds merits for further investigation.

Convolutional neural networks are equivariant to translations; equivariance to other symmetries, however, is not defined and the class output may vary depending on the input's orientation. To mitigate this, the training data can be augmented at the cost of increased redundancy in the model. Another solution is to build an equivariant neural network and thereby increasing the equivariance to a larger symmetry group. In this study, two convolutional neural networks and their respective equivariant counterparts are constructed and applied to the symmetry groups D4 and C8 to explore the impact on performance when removing and adding batch normalisation and data augmentation. The results suggest that data augmentation is irrelevant to an equivariant model and equivariance to more symmetries can slightly improve accuracy. The convolutional neural networks rely heavily on batch normalisation, whereas the equivariant models achieve high accuracy, although lower than with batch normalisation present.

Die Untersuchung von III-V-Heterostruktur-Grenzflächen spielt aufgrund des starken Einflusses der Grenzflächen auf die Eigenschaften von Halbleiterbauelementen eine elementare Rolle. Die Transmissionselektronenmikroskopie (TEM) hat sich als eines der geeignetsten Methoden für die Analyse von Grenzflächen erwiesen. Jedoch stellt die Interpretation von zweidimensionalen Projektionen, insbesondere bei dreidimensionalen (3D) Strukturen, eine Herausforderung dar, da die Information über die Tiefe in der Projektion verloren geht. In dieser Arbeit wird diskutiert, dass Grenzflächen als 3D Objekte gesehen werden können, insbesondere bei großen Rauheiten oder chemischen Durchmischungen. Zur Charakterisierung von vergrabenen III-V-Halbleitergrenzflächen wurde eine neue analytische Methode auf Basis der Elektronentomographie entwickelt. Diese Methode wird anhand eines (Al,Ga)As/GaAs-Mehrschichtsystems als Fallstudie vorgestellt. Es wird gezeigt, dass die tomographische Rekonstruktion von Grenzflächen ausgenutzt werden kann, um sogenannte Iso-Konzentrationsflächen zu erhalten, welche die Lage einer festgelegten Konzentration an der Grenzfläche beschreiben. Sie erlauben die Erstellung topographischer Höhenkarten, die die Rauheit von Grenzflächen visualisieren und ermöglichen die Bestimmung wesentlicher Grenzflächenparameter wie die quadratische Rauheit oder die lateralen und vertikalen Korrelationslängen. Zusätzlich wurden Isoflächen an unterschiedlichen Konzentrationsniveaus verwendet, um topographische Karten der Grenzflächenbreite zu erzeugen. Die Stärke der Methodik liegt in der Möglichkeit, alle chemisch-strukturellen Parameter von vergrabenen Grenzflächen mit einer einzelnen Tomographiemessung zu bestimmen. Die Applikation dieser neuen Grenzflächentomographie-Technik wird an unterschiedlichen Halbleitermaterialien für optische Anwendungen demonstriert: An einer (Al,Ga)As/GaAs-Mikrokavität, einem (Al,Ga)N/GaN-Bragg-Reflektor und einem 3D (In,Ga)N/GaN-Nanodraht. / The investigation of III-V heterostructure interfaces plays a key role in developing novel semiconductor devices due to the strong influence of interfaces on device characteristics. Transmission electron microscopy (TEM) has proven to be one of the most suitable tools for an interface analysis. The interpretation of two-dimensional projections can be challenging, especially for three-dimensional (3D) structures, since the depth information is lost in the projection. It is discussed in this thesis that interfaces can be seen as 3D objects, particularly when interfaces are subject to large roughnesses or chemical intermixing. A new analytical method for the comprehensive characterization of buried III-V semiconductor interfaces based on electron tomography was developed. This method is applied to an (Al,Ga)As/GaAs multilayer system as a case study. It is shown that the tomographic reconstruction of a tomography needle of this material can be exploited to obtain so-called iso-concentration surfaces. These surfaces describe the positions of a chosen concentration value at the interfaces. They enable the creation of topographic height maps revealing the roughness of several interfaces. The height maps allow the determination of essential interface parameters as the quadratic mean roughness or the lateral and vertical correlation lengths using the height-height correlation function. In addition, height-difference maps based on isosurfaces corresponding to different concentration levels were used to generate topographic maps of the interface width. The methodology’s main strength is the ability to determine all chemical-structural parameters of buried interfaces with a single measurement. This thesis presents the application of this new interfacial tomography technique on semiconducting materials used for optical application purposes: An (Al,Ga)As/GaAs microcavity, an (Al,Ga)N/GaN Bragg reflector and a complex, 3D (In,Ga)N/GaN multi-shell nanowire.

One conventional method for studying dislocations uses the Transmission Electron Microscope (TEM), a complex and expensive piece of equipment which requires extensive specimen preparation in order to thin the specimens to electron transparent thickness. Newer High Resolution Electron Backscatter Diffraction (HREBSD) methods of determining geometrically necessary dislocation content via cross-correlation promise to be able to produce estimates of the dislocation density of the sample over a larger area with considerably less preparation time and using a much more accessible instrument. However, the accuracy of the new EBSD technique needs more experimental verification, including consideration of possible changes in the specimen dislocation density due to the different preparation methods. By comparing EBSD and TEM dislocation measurements of Electron Transparent platinum specimens prepared using the Focused Ion Beam (FIB), along with EBSD dislocations measurements of specimens prepared by both FIB and mechanical polishing techniques, this paper seeks to verify the accuracy of the new method and identify any changes in the specimens’ apparent dislocation density caused by the different preparation processes.

No description available.

No description available.

No description available.

No description available.