In this work, the effect of composition and crystal orientation relationship on theinterface fracture toughness of niobium/sapphire system was studied. Interfaces were synthesized by either physical vapor deposition o...
In this work, the effect of composition and crystal orientation relationship on theinterface fracture toughness of niobium/sapphire system was studied. Interfaces were synthesized by either physical vapor deposition or ion beam assisted deposition. Silver was deposited to weaken the interface and crystal orientation was used to strengthen it. Several techniques were used to assess the interface fracture toughness, including microscratch, nanoindentation, microwedge scratch, and delamination of patterned lines. Results showed a general trend in which the interface fracture toughness decreased with the amount of silver. Ion bombardment during film deposition significantly increased the interface fracture toughness through a combination of interface mixing and a controlled orientation relationship.
A major limitation of the current technology for GaN epitaxy is the availability of suitable substrates matched in both lattice constant and thermal expansion coefficient. One alternative for the development of GaN su...
A major limitation of the current technology for GaN epitaxy is the availability of suitable substrates matched in both lattice constant and thermal expansion coefficient. One alternative for the development of GaN substrates rests in the application of halide vapor phase epitaxy (HVPE) to produce GaN films at high growth rates. In this paper, we describe the growth of thick GaN films via the HVPE technique on (0001) sapphire and (111) Si substrates. At a temperature of 1030°C, films are grown at rates between 70 and 90 μm/hr, yielding total thicknesses exceeding 200 μm on sapphire. DCXRD measurements of GaN/sapphire indicate FWHM values less than 220 arcsec on 180 μm thick films. Room temperature PL measurements of GaN/sapphire indicate strong emission at 3.41 eV, with a FWHM value of 65 meV. Moreover, no detectable deep level emission was found in room temperature PL measurement. Under optimized conditions, films are morphologically smooth and optically clear. The GaN morphology appears to be a strong function of the initial nucleation conditions, which in turn are strongly affected by the partial pressure of GaCl. HVPE growth on (111) Si substrates is accomplished using an AlN MOVPE buffer layer.
This paper describes the development of the Unmanned Port Security Vessel (UPSV), a small autonomous surface vehicle designed to support maritime domain awareness in port and harbor environments. The UPSV is capable o...
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This paper describes the development of the Unmanned Port Security Vessel (UPSV), a small autonomous surface vehicle designed to support maritime domain awareness in port and harbor environments. The UPSV is capable of rapidly producing fine resolution, shallow-water bathymetry maps using a multibeam sonar, detecting chemical threats using an on board mass spectrometer and monitoring oceanographic parameters using off-the-shelf instruments. In this paper we discuss the significance and scientific advantages of sensor fusion on a small autonomous robotic platform, as well as detailing the control methods chosen for such a vehicle. Command and control of the robotic platform is implemented using the lightweight communications and marshalling (LCM) library, a general purpose software infrastructure for sensor integration and feedback control. Based on this foundation we have built a flexible software architecture that includes mission planning, real-time navigation and control, payload management and telemetry to support a variety of maritime domain awareness missions. Using the developed software and control methods, the UPSV is capable of autonomously generating high fidelity bathymetric maps while simultaneously conducting chemical surveys with a mass spectrometer. We demonstrate the utility of the software architecture and hardware integration through results from both simulation and experiments. We present the results of a set of field trials to evaluate the performance of the UPSV in operational settings. These experiments include system identification for estimation and control, feedback control performance evaluation, demonstration of autonomous high-resolution bathymetric mapping and demonstration of in-situ chemical sensing. The performance of the UPSV's control system is characterized with the presented navigation data, and the results of the bathymetric surveys and in-situ chemical sensing are detailed. The implementation of this functionality on a small, d
This article presents the development of L 1 adaptive-control theory and its application to safety critical flight control system (FCS) development. Several architectures of the theory and benchmark examples are anal...
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This article presents the development of L 1 adaptive-control theory and its application to safety critical flight control system (FCS) development. Several architectures of the theory and benchmark examples are analyzed. The key feature of L 1 adaptive-control architectures is the decoupling of estimation and control, which enables the use of arbitrarily fast estimation rates without sacrificing robustness. Rohrs's example and the two-cart system are used as benchmark problems for illustration. NASA's flight tests on subscale commercial jet verify the theoretical claims in a set of safety-critical test flights.
For online monitoring and prediction of surface or near-surface changes such as corrosion, cracks and crack-like defects, propagation of surface acoustic waves (SAW) and pseudo surface acoustic waves (PSAW) in a layer...
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We have developed a quantitative model, based on a two-dimensional finite difference enthalpy method, which accounts for the localized melting behavior of thin Si films on substrates. The model incorporates radiative ...
We have developed a quantitative model, based on a two-dimensional finite difference enthalpy method, which accounts for the localized melting behavior of thin Si films on substrates. The model incorporates radiative and conductive heat flow components and takes account of the phase changes that occur during zone-melting recrystallization. Emphasis is placed on the effects resulting from the differences in reflectivity and emissivity between solid and liquid Si. The model provides quantitative information concerning the temperature profile of the Si film and the configuration of the solid-liquid interface. Results of the analysis indicate that there exist two distinct types of transition behavior: i) reflectivity-change dominated and ii) emissivity-change dominated. Partial melting and a nonplanar solid-liquid interface are characteristics of the reflectivity-change dominated behavior. The emissivity-change dominated behavior, on the other hand, can be characterized by explosive-like melting and a planar solid-liquid interface. The conditions and physical factors which give rise to these behaviors are discussed.
A novel model reduction strategy for forced dissipative infinite-dimensional nonlinear dynamical systems is described. Unlike popular but empirical methods, this new approach does not require extensive data sets from ...
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We have investigated the nucleation and growth of gallium nitride (GaN) films on silicon and sapphire substrates using halide vapor phase epitaxy (HVPE). GaN growth was carried out on bare Si and sapphire surfaces, as...
We have investigated the nucleation and growth of gallium nitride (GaN) films on silicon and sapphire substrates using halide vapor phase epitaxy (HVPE). GaN growth was carried out on bare Si and sapphire surfaces, as well as on MOVPE-grown GaN buffer layers. HVPE growth on MOVPE GaN/AIN buffer layers results in lower defect densities as determined by x-ray than growth directly on sapphire. HVPE GaN films grown directly on sapphire exhibit strong near-edge photoluminescence, a pronounced lack of deep level-based luminescence, and x-ray FWHM values of 16 arcsec by an x-ray θ-2θ scan. The crystallinity of GaN films on sapphire is dominated by the presence of rotational misorientation domains, as measured by x-ray ω-scan diffractometry, which tend to decrease with increasing thickness or with the use of a homoepitaxial MOVPE buffer layer. The effect of increasing film thickness on the defect density of the epilayer was studied. In contrast, the HVPE growth of nitride films directly on silicon is complicated by mechanisms involving the formation of silicon nitrides and oxides at the initial growth front.
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