Enhanced Direction of Arrival Estimation through Electromagnetic Modeling

Engineering is a high art that balances modeling the physical world and designing meaningful solutions based on those models. Array signal processing is no exception, and many innovative and creative solutions have come from the field of array processing. However, many of the innovative algorithms that permeate the field are based on a very simple signal model of an array. This simple, although powerful, model is at times a pale reflection of the complexities inherent in the physical world, and this model mismatch opens the door to the performance degradation of any solution for which the model underpins. This dissertation seeks to explore the impact of model mismatch upon common array processing algorithms. To that end, this dissertation brings together the disparate topics of electromagnetics and signal processing. Electromagnetics brings a singular focus on the physical interactions of electromagnetic waves and physical array structures, while signal processing brings modern computational power to solve difficult problems. We delve into model mismatch in two ways; first, by developing a blind array calibration routine that estimates model mismatch and incorporates that knowledge into the reiterative superresoluiton (RISR) direction of arrival estimation algorithm; second, by examining model mismatch between a transmitting and receiving array, and assessing the impact of this mismatch on prolific direction of arrival estimation algorithms. In both of these studies we show that engineers have traded algorithm performance for model simplicity, and that if we are willing to deal with the added complexity we can recapture that lost performance

Multiple cracking events in metal bi-layers on polymer substrates

Metal films on polymer substrates are used in a variety of applications such as flexible electronics, sensors, medical devices and aerospace including multilayer insulators and surface mirrors on satellites. A common way to assess the mechanical behavior of metal-polymer systems is with fragmentation testing, which strains the system under uniaxial tension. During straining cracks or localized deformation (necks) develop perpendicular to the loading direction and buckle delaminations occur parallel to the loading. From the crack spacing the fracture behavior can be determined and the interface adhesion energy can be measured from the buckles. Fragmentation testing has been used on single and multilayer films and has shown that brittle adhesion layers next to the substrate, can cause brittle cracking of normally ductile overlying films. A similar fracture behavior was observed here for the Inconel-Ag-Teflon system, but in this system, the top 30 nm Inconel film is the brittle layer inducing brittle cracking of the underlying 150 nm Ag film. Inconel acts as a corrosion protection for the Ag layer in surface mirrors on satellites in low earth orbit, where the material should not develop cracks upon mechanical loading. Observation of the Inconel surface during in-situ tensile straining revealed crack formation in the Inconel layer at less than 1% strain, which continues with increasing strain (primary cracks). At approximately 3% strain, the primary cracks in the Inconel overcoat act as stress concentrators and generate through thickness cracks in the Ag film (secondary cracks). The primary Inconel cracks had a saturation spacing of 1.5 µm, while the secondary Inconel-Ag saturation crack spacing was much larger at 12 µm. In-situ fragmentation experiments performed through the transparent Teflon substrate revealed only the secondary through thickness cracks and cross-sectional focused ion beam characterization provides further evidence for the two-stage cracking behavior. Using the shear lag model the interfacial shear stresses of the Inconel and Inconel-Ag layers were determined from the saturation crack spacings and observed fracture strains. These results further illustrate that brittle layers at any position are detrimental to the functionality of multi-layered metal-polymer systems and should be carefully considered for any application. Please click Additional Files below to see the full abstract

Reactive stream separation photography Final report

High speed photographic techniques to study impinging streams of propellants in experimental investigation of reactive stream separatio

Evolution of thickness dependent buckle geometries

Interfaces determine the overall reliability of multi-material components since they have to bear the distinct physical and chemical properties of the different adhering materials. In microelectronic applications, where several materials are implemented at small length scales, the main interest is on identifying the weakest interface, since it dictates the overall reliability of the implemented packages. The focus of the present study is set on a multi-layer stack composed of a rigid Si substrate with dielectric borophosphosilicate glass (BPSG) and a thin TiW film acting as adhesion promoter and diffusion barrier to the copper film, which are finally covered with 6 µm of polyimide (PI). Of main interest is a thorough characterization of the delamination of the various interfaces, which allow for a better understanding of the adhesion and the stress states present in the complex material stack. As a first step to study the interfacial behaviour, a peeling test was carried out to reveal the weakest interfaces resulting in three different delamination zones. Zone 1 delaminated at the BPSG-TiW interface and Zone 2 delaminated at the copper-PI interface (Fig 1a). An intermediate Zone 3 (Fig 1a) was identified, where straight buckles formed in the Cu-TiW layer parallel to the peeling direction at the TiW-BPSG interface (Fig 1b). Using these Zone 3 delaminations, the evolution of the buckle shape as a function of film thickness and layer stress was investigated using atomic force microscopy and X-ray diffraction. Of great interest is that with the Cu layer the buckles have a straight geometry (Fig 1b) indicating an isotropic stress. However, when the Cu layer is removed with chemical etching, the buckle morphology changes to a telephone cord geometry (Fig. 1c), maintaining the outer boundaries from the previous straight buckles shape. The change in geometry could be due to the change in film stress from isotropic to biaxial as well as the fact that the out of plane plasticity is constrained while the copper film is present. Both topics will be further discussed along with how the interfacial adhesion measurements may also be influenced by the change in buckle geometry. Please click Additional Files below to see the full abstract

Fatigue behavior of gold thin films at elevated temperatures studied by bulge testing

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