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Since high-mass stars form deeply embedded within dense molecular clumps, the evolution of young stars and of dense clumps is inextricably linked. Previous datasets, however, lack the information necessary to test the prevailing theories. Definitive tests require a sufficiently large sample of molecular clumps and maps of their gas temperatures, column densities, velocity dispersions, and velocities at a spatial resolution comparable to, or smaller than, the clump scale (~1 pc). The Radio Ammonia Mid-Plane Survey (RAMPS), a new molecular line survey of thermal NH3 and H2O masers, provides the necessary data. In this dissertation, I used RAMPS data and archival datasets to test several theories of high-mass star formation and to investigate the co-evolution of molecular clumps and high-mass stars. All theories of high-mass star formation make testable predictions regarding clump kinematics and gravitational stability. Analyses of RAMPS kinematic data revealed that the majority of molecular clumps, particularly those in early evolutionary stages, are unstable to gravitational collapse. Further, they display infall motions, a key prediction of the theory of competitive accretion. I also investigated the kinematics of molecular filaments by comparing their measured velocity gradients to those predicted by hydrodynamical simulations. The measured spatial distributions of velocity gradients are inconsistent with existing models. Feedback from protostars and stars is predicted to alter the properties of surrounding clumps. I investigated feedback size scales and found that high-mass protostellar and stellar feedback significantly changes the temperatures, chemical abundances, and velocity dispersions of clumps on scales of ~0.3 to 3 pc. Finally, I observed a massive molecular cloud filament undergoing an interaction with a supernova shock, which is accelerating, heating, and injecting turbulence into the filament's gas. Although the molecular cores hosted by the filament may remain gravitationally bound, the filament is gravitationally unbound and likely being dispersed. Given that the shock is removing a reservoir of gas that could have been accreted by the cores, these data suggest that the supernova is inhibiting star formation.

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High temperature, solvent resistant materials which also display good mechanical properties are desired for use as aerospace structural adhesives and polymer matrix/carbon fiber composites. High molecular weight amorphous poly(arylene ether sulfone) thermoplastic materials display many of these desirable characteristics but are deficient in solvent resistance. Previous attempts to prepare poly(arylene ether) based thermosets to improve solvent resistance have been largely unsuccessful due to processiblity issues from the low curing temperature and high glass transition temperature of the thermoset precursor. Incorporation of a high temperature curable (* 350°C) endgroup such as 3-phenylethynylphenol in the synthesis of controlled molecular weight poly(arylene ether sulfone) oligomers has allowed for a large processing window prior to the exothermic cure that forms the desired networks. Control of oligomer molecular weight and backbone structure has allowed for further control of the processing, thermal transitions and adhesive properties of the thermosets. A systematic series of phenylethynyl terminated oligomers derived from either bisphenol A, or wholly aromatic hydroquinone or biphenol has been synthesized and characterized to determine the influence of backbone structure, molecular weight, and endgroup structure on thermoset properties. The features most affected by backbone structure included thermal stability (weight loss behavior) as well as transition temperatures (Tg, Tm), and processing characteristics. Increasing molecular weight of the oligomer produced a decrease in the glass transition temperature of the network and an increase in the adhesive properties of the thermoset. Comparison of the curing behavior of the 3-phenylethynylphenol endcapped materials with other related phenylethynyl terminated compounds led to the synthesis and systematic investigation of the curing behavior of phenylethynyl endcappers in which the electronic environment in relation to the reactive ethynyl carbons was systematically varied. Electron withdrawing groups, eg. sulfone, ketone, imide on the aryl ring para to the acetylene bond enhanced the rate of cure and also appear to improve the lap shear adhesion to suface treated titanium adherands. Discussion of the background, synthesis and characterization are described in this dissertation. / Ph. D.

Over the past several years, energy efficiency and power density have become the top concerns for power conversion. Rising energy intensity leads to a higher cost of delivering power. Meanwhile, the demand for compact power supplies grows significantly. It requires power supplies with high efficiency, low profile and high power density. Dc-dc power conversion has been widely applied for industry, medial, military and airspace applications. Conventional PWM dc-dc converters have relatively low power transfer efficiency and low power density. In contrast, resonant dc-dc converters have numerous advantages for dc-dc power conversions. In this work, topologies and system optimization of resonant converters are investigated to meet challenges of high efficiency, high power density, low EMI, easy startup and over current protection. LLC resonant converters can achieve zero-voltage-switching (ZVS) for primary side devices and zero-current-switching (ZCS) for the secondary side rectifiers. The switching loss is minimized. LLC is very attractive to overcome the issues of conventional circuits. However, challenges still remain. First of all, for low-voltage high-current applications, the synchronous rectifier (SR) with lower conduction loss is a must for high efficiency. To solve the driving issues of SRs, a novel synchronous driving scheme is proposed. Experimental results demonstrate the considerable loss reduction with utilization of the proposed driving scheme. Secondly, dc-dc converters are required to meet EMI standard. This work proposes an EMI mode. Based on the proposed model, EMI analysis and noise attenuation techniques are proposed and verified by experiments. Thirdly, startup and over-load protection are another issues of LLC resonant converters. With proposed multi-element resonant converters, the current limit issues can be resolved. In addition, the proposed multi-element resonant converters can utilize higher-order harmonics to enhance power transfer. Fourthly, for high-current applications, the secondary side structure becomes very critical. An improved secondary side construction is proposed to alleviate ac termination losses and SR paralleling issues. Novel winding structures are proposed to reduce the winding loss. The magnetic integration technique is proposed and analyzed, and an optimal integrated transformer design is proposed, which has low loss and compact size. / Ph. D.

The volume reduction in power converters over the past several decades can chiefly be attributed to increases in switching frequency. It is to be expected that the trends towards miniaturization will maintain steady pressure to keep this pace of increasing switching frequencies of power converters. However certain fundamental limits in high frequency power conversion are being reached as frequencies are being pushed deeper into the megahertz range, inhibiting substantial further increases. The work reported in this dissertation is intended to systematically investigate the fundamental frequency limitations, identify some of the solutions for HF/VHF power conversion and to provide guidelines and tools to optimize the performance of power converters by maximizing frequency. A number of multi-megahertz power converters are examined to evaluate the present status and future trend of HF/VHF power conversion. An interesting trend between power level and frequency is observed. A general limitation about the power level and frequency, independent of design details, is derived from the physics of the semiconductor devices, which determines the upper bound of the power levels as frequency increases. A 250 MHz DC-DC power converter (derived from the Class E power amplifier) is analyzed and demonstrated with discrete components, which again verifies the trend between power level and frequency. The power losses in the semiconductor devices are discussed, and optimization criteria for minimizing the power losses of the devices, are discussed. By relating the power losses to the semiconductor materials' properties, a methodology for selecting proper materials is identified for high frequency and high efficiency power conversion. The frequency scaling effects of passive components, still dominating the volume of the modern power converter, is analyzed. A generic multi-disciplinary methodology is developed to analyze and maximize frequency and performance of passive components in terms of power density and efficiency. It is demonstrated how the optimum frequency can be identified, and how power conversion efficiency deteriorates beyond this optimum under a fixed maximum temperature. Power loss measurement is becoming more challenging as higher frequency and higher efficiency power conversion. To achieve an accurate power loss measurement in a high frequency, high efficiency power electronics system or component, limitations of electrical measurement are identified, and various calorimetric methods are surveyed. Calorimetric methods are more accurate due to the direct heat loss measurement. An advanced calorimetric system is proposed, analyzed, and tested, demonstrating about 5% error in total losses up to 25W. / Ph. D.