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The manufacturers of future aero engines would need new ultra-high temperature alloys for blade applications with capabilities beyond those of Ni based superalloys to meet performance and environmental targets. The new alloys should meet property goals set by industry and would be expected to operate under conditions where turbine entry temperatures could be as high as 1800 °C and alloy surface temperatures  1300 °C. Ni based alloys cannot be used because of the melting temperature of Ni. Nb silicide based alloys are strong candidates for blade applications owing to their lower density and potential to offer a balance of properties that can satisfy property goals. The oxidation of the new alloys in service must be comparable with that of single crystal Ni based superalloys, such as CMSX4, at 1050°C. The oxidation of the new alloys is significantly improved by additions of Sn. However, to date it is not understood how Sn affects the oxidation of these alloys in the pest regime and high temperatures. The research described in this thesis attempts to understand how Sn on its own and in synergy with Al and Cr individually and simultaneously contributes towards improving the oxidation behaviour, how the stability of Nb3Sn, Nbss, Nb5Si3 and Nb5Si3 and Laves phase in Nb silicide based alloys is affected by the addition of Sn in the alloys and what the role played by Sn is in the isothermal oxidation behaviour of alloys at 800 °C (in the pest regime) and 1200 °C. Model Nb-Si ternary alloys with low and high concentrations of Sn and higher order model Nb-24Ti-18Si based alloys with 5 at.% of each Al and Cr added individually and simultaneously were studied in the cast and heat treated conditions (100 h at 1200 °C, 1500 °C or 1450 °C depending on alloy). The nominal compositions (at.%) of the alloys were Nb-14Si-3Sn (ZX1), Nb-12.5Si-7.5Sn (ZX2), Nb-24Ti-18Si-5Cr-2Sn (ZX3), Nb-24Ti-18Si-5Cr-5Sn (ZX4), Nb-24Ti-18Si-5Al-2Sn (ZX5), Nb-24Ti-18Si-5Al-5Sn (ZX6), Nb-24Ti-18Si-5Cr-5Al-2Sn (ZX7) and Nb-24Ti-18Si-5Cr-5Al-5Sn (ZX8). The thesis presents a brief review of relevant literature on Nb silicide based alloys and outlines a method for making Sn containing alloys with control of Sn losses during clean melting using arc melting. The microstructures of the alloys were studied using XRD, SEM, SEM-EDS, EPMA-WDS and TGA. Tin suppressed the Nb3Si. The Nbss + Nb5Si3 metastable eutectic formed in all alloys except the alloy ZX8. Depending on alloy composition and solidification conditions transitions from anomalous to anomalous + normal to normal eutectics were observed from the bottom towards the bulk of ingots. Tin on its own increased the stability of the lamellar microstructure of the eutectic up to 1500 °C but the additions of Al and/or Cr had the opposite effect. Increasing the Sn content in the ternary alloys accelerated the Nb5Si3  Nb5Si3 transformation during heat treatment. The latter was also enhanced during the solidification of the alloys with only Cr (ZX3, ZX4). The Nb5Si3 was stable in all the heat treated higher order alloys. Increasing the Sn content led to formation of Nb3Sn in the cast microstructure. With the exception of alloy ZX3 (low Sn, only Cr) the A15 phase was stable at 1500 °C in all alloys. Aluminium on its own destabilised the Nbss in the heat treated alloy ZX6. Only in the alloy ZX3 (low Sn, only Cr) the Laves phase was not stable at 1500 °C. Tin with Al and/or Cr suppressed pest oxidation. The scales formed on all alloys at 1200 °C spalled off. The “best” alloys at 800 °C and 1200 °C respectively were the alloys ZX4 and ZX6. All alloys at 1200 °C followed linear oxidation kinetics. Cr and the presence of Laves phase and Al and the absence of Laves phase were important for oxidation at 800 °C and 1200 °C, respectively. The oxide scales formed at 800 °C consisted of Nb rich and Nb and Si rich oxides. The same oxides plus Ti rich oxide were in the scales formed at 1200 °C in all alloys with the exception of the alloy ZX5. Ti rich oxide was also observed in the scale of the alloy ZX4 at 800 °C. The improved oxidation behaviour of Sn containing alloys was attributed to Sn rich area(s) that formed below the scale/substrate interface in all alloys at 800 °C and 1200 °C, with the exception of the alloy ZX7 at 800 °C. The thickness of the Sn rich area increased with Sn concentration in the alloy and with temperature. In the Sn rich areas the Sn rich intermetallics Nb3Sn, Nb5Sn2Si, NbSn2 were formed. The latter was absent in the alloys ZX5 (“best” low Sn alloy at 1200 °C) and ZX6 (“best” high Sn alloy at 1200 °C) and in the alloy ZX8. In all the alloys all phases in the diffusion zone and bulk were contaminated by oxygen. In the diffusion zone the effect was very severe for the Nbss. In general the contamination of the Nbss by oxygen was worse than Nb5Si3. The oxygen concentration in the A-15 phase in the bulk did not change ( 6 at.%) with increasing Sn in the alloy, but its Sn content increased. At 800 °C the contamination of the bulk of ZX4 and ZX8 by oxygen shifted the phase equilibria and in the former alloy made the Laves phase unstable and in the latter caused the formation of Nbss and Nbss + Nb5Si3 lamellar microstructure (the Nbss was absent in the cast alloy). The contamination of the bulk of the alloy ZX8 by oxygen at 1200 °C caused a shift in phase equilibria and led to the destabilisation of the Nbss in the microstructure.

Staphylococci are opportunistic pathogens responsible for a range of infections. Many staphylococcal species are frequently found to be resistant to antibiotics. The environment is considered a potential reservoir of genes conferring antibiotic resistance, which known as the 'resistomes'. Monitoring the dissemination of antibiotic resistant staphylococci is instrumental to mitigating this global health risk. The overall aim of this study was to generate informative data regarding dissemination of antibiotic resistance in environmental and public settings. This included looking into the distribution, epidemiology characteristic and transfer of oxacillin resistant determinant mecA; gaining an insight into genomic features that contribute to multiple antibiotic resistance and pathogenicity of one S. epidermidis isolate; and understanding the stress responses in mediating oxacillin resistance in S. aureus. The use of MALDI-TOF MS allowed identification of staphylococci to species level. MALDI-TOF MS data were used for taxonomic analysis of staphylococci, and taxonomic data were then combined with isolation sites and antimicrobial susceptibility profiles to aid the understanding of dissemination of environmental resistant staphylococci. The widespread dissemination of antibiotic resistant staphylococci in the environment was demonstrated. 12% of staphylococci harboured mecA gene. Community associated SCCmec types IV and V were more prevalent than nosocomial associated SCCmec types I, II, and III in the environment. 52% of SCCmec were non-typable. In addition, 14 new environmental S. epidermidis MLST types were reported. 9 antibiotic resistant determinants that were responsible for the resistant to 7 antimicrobial classes have been identified in environmental S. epidermidis 118 (G6_2). Proteomic analysis revealed that stress responses, including SOS response, stringent response and heat shock response, mediate oxacillin resistance in S. aureus. These results demonstrate widespread multiple drug resistance in different staphylococcal species isolated from non-healthcare environments. This uncontrolled dissemination of multidrug resistant bacteria poses a potential public health threats.

Nowadays, a significant part of the research interest in optoelectronic devices is in making nanostructures such as heterojunctions, nanowires, quantum wells and quantum dots that dramatically change the electron behavior of the material compared to their bulk counterpart. In this dissertation, we applied ultrafast spectroscopy to study specifically the hot carrier separation and recombination dynamics in the CVD-grown monolayer molybdenum disulfide (MoS2) and graphene heterostructure, III-nitride quantum well LED structure and self-organized InGaN nanowires for photocatalysis. By using ultrafast pump-probe spectroscopy, we observed efficient electron and hole separation within a few hundred femtoseconds by interlayer charge transfer from graphene to MoS2 monolayer following initial excitation and thermalization. The transfer-back process to graphene happens in multiple timescales, the dynamics of which after the thermalization can only be modeled by a Porter-Thomas distribution for the charge transfer rate because the transfer back process after the thermalization is dominated by the material disorder. The III-nitride quantum well has been the most common LED structure since the 1990s. III-V compounds are also the only known material that can have tunable valence and conduction band levels that cover the chemical potential of a few important photochemical reactions such as water splitting and methane oxidation thus making them ideal for photocatalysis in converting solar energy into electrochemical energy. We studied the time-resolved photoluminescense (TRPL) and the time-resolved differential reflectance (TRDR) of the self-organized InGaN nanowires observed a fast decay component in the PL decay dynamics that were not observed in the DR measurements only when the InGaN is p-doped. We argue that the fast decay component in the PL decay dynamics indicates the charge separation due to the built-in polarization field near the surface of the nanowires. We also studied the temperature-dependent TRPL experiments and room temperature TRDR of the quantum disk-in-wire micro-LED structure that contains tilted quantum wells grown along the semi-polar facets. The TRPL decay curves always show faster dynamics than the TRDR. This is modeled with thermal activated non-radiative process that can be explained with Arrhenius’s Law applied to the lateral charge transport over the local indium composition fluctuation. Throughout this thesis, we study the ultrafast charge separation and relaxation by directly probing and by combining the PL and DR dynamics in different nanostructures. We have demonstrated a novel approach for detecting carrier spatial separation and the results revealed the important microscopic physics such as material disorder, band bending and local fluctuation that are crucial for future research and development.

Abstract: With the rapid development of internet technology, web page has evolved from a traditional rich-text information source to a multi-functional tool, which can serve images, audios and videos, act as the GUI (Graphical User Interface) components of distributed applications, and so on. Similarity evaluation of the modern web pages becomes more essential yet difficult. On one hand, while many search engine rely on keyword search, texts play less important roles in web pages. On the other hand, there exists a variety of browsers and platforms that support HMTL/CSS/JavaScript in different levels, causing a web page is displayed differently among browsers. To address these issues, we propose four research topics. The first topic is to identify semantic blocks on web pages. We propose a model for merging web page content into semantic blocks based on human perception. To achieve this goal, we construct a layer tree to remove hierarchical inconsistencies between visual layout and DOM tree of web pages; we translate the Gestalt Laws of grouping to computer compatible rules can train a classifier to combine the laws to a unified rule to detect semantic blocks. The second topic is to estimate visual similarity of web pages. Existing approaches use DOM (Document Object Model) trees or images, but they either only focus on the structure of web pages or ignore inner connections among web page features. Therefore, we provide the block tree to combine both structural and visual information of web pages. Using this block tree structure, we propose a visual similarity measurement. The purpose of the third topic is to improve the visual similarity measurement and use it to detect visual differences in web pages when they are rendered in different browsers. The extended subtree model that maps sub trees instead of each single node is introduced for the precision improvement. The forth topic utilize the improved visual similarity measurement to create an automated testing framework for cross-browser visual incompatibility detection. An automated testing tool is also designed. Major contribution of this thesis is two-folds. On the one hand, it enriches theoretical analysis in the detection of semantic content, visual similarity, and cross-browser differences for web pages. On the other hand, it also provides an insight for testing cross-browser incompatibilities in practice.

High intensity ultrasound has the ability to focus energy deep within the human body and generate clinically significant bioeffects at the target without damaging overlying tissue. We have developed a new technique to achieve tissue erosion (physical removal of tissue) at a tissue-fluid interface using high intensity pulsed ultrasound. This technique can be considered as a form of soft tissue lithotripsy, which we call histotripsy. The overall goal of this dissertation is to develop a noninvasive ultrasound procedure to perforate the atrial septum (i.e. the membrane separating the two atria) as the first step to treat neonates suffering Hypoplastic Left Heart Syndrome (HLHS). The feasibility of histotripsy to create the atrial septal perforation is demonstrated both in vitro and in vivo. Our results show that short (less than 10 acoustic cycles) high intensity pulses delivered at certain pulse repetition frequencies can achieve localized, sharp boundary perforations. The erosion process is highly dependent on acoustic parameters. Furthermore, real-time monitoring of erosion is developed using temporally changing acoustic backscatter. The initiation of variable acoustic backscatter is required to produce erosion and its appearance is an excellent indicator that the erosion process is progressing normally. The primary mechanism for the erosion process is believed to be acoustic cavitation in which microbubbles within the focused region are activated by the acoustic field. Vigorous microbubble activities at the tissue-fluid interface mechanically and systemically remove small tissue fragments. To understand the physical basis for the erosion process, and to form a rational basis for further optimization, direct optical observations combined with acoustic feedback are employed to monitor dynamics of the ultrasound generated bubble cloud. In summary, we show that high intensity ultrasound pulses have the ability to physically remove soft tissue in a highly localized manner. The histotripsy technique will be developed as a potentially effective non-invasive tool in the treatment of intact atrial septum in neonatal patients with HLHS, as well as many other clinical procedures where mechanical removal of soft tissue would be indicated.

The purpose of radar signal processing is to extract desired data from radar signals. Testing of the radar signal processor requires that one produce a test consisting of a sequence of digitized radar sensor data. If the signal processor is modeled in VHDL then the sequence of sensor data will be applied to the model in a VHDL test bench. Generation of the sensor data and sensor models for VHDL testbenches is a labor intensive task which cannot be performed manually. This thesis concentrates on the transformation of the mathematical representations of Synthetic Aperture Radar signals and sensors into VHDL models, and provides the mathematical underpinning for the other testbench work. Cadence/Comdisco SPW is used to describe the behavioral model of the test bench, which is hierarchically constructed using the primitives in the SPW library. A parameterized VHDL model is constructed from the block diagrams in SPW to generate the test for the VHDL signal processing model under test.