Browsing by Subject "Interfaces"
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Item Conditioning variables at interfaces for Spanish pronominal subject expression : heritage language learners across proficiency levels(2012-05) Witte, Jennifer Lauren; Koike, Dale April; Toribio, Almeida JacquelineContributing to a more detailed understanding of heritage language (HL) learner levels of Spanish competence, this study examines the first-person singular Spanish pronominal subject expression by 17 HL learners across three levels of proficiency in oral interviews – as rated by modified ACTFL Proficiency Guidelines: intermediate (n = 3), intermediate-advanced (n = 4), and advanced (n = 10). Using a variationist approach, I analyze five conditioning variables (including discourse function and verb class) that have been found to affect pronominal expression in native speaker Spanish (Bentivoglio, 1980; Cameron, 1992, 1994; Cacoullos & Travis, 2010; Otheguy et al, 2007; Silva-Corvalán, 1989, 2001; Travis, 2005, 2007), as well as two additional variables (correct verbal inflection and presence of hesitation phenomena) seen in second language acquisition studies. Following theories of interface vulnerability (cf. Montrul, 2011b; Sorace, 2004, 2005; Sorace & Serratrice 1995), I hypothesized that the incremental development of sensitivity to conditioning variables would be reflective of the complexity of the interface at which the variable is conditioned. Based on previous studies of HL leaners (Montrul, 2004; Montrul & Rodríguez-Louro, 2006) and second language learners (Geeslin & Gudmestad, 2010a, Rothman, 2007a, 2007b, 2009), I predicted that the variables related to the more vulnerable and complex interface of syntax-discourse/pragmatics would be acquired only by the advanced proficiency group. My analysis indicated that HL pronominal expression is in free variation and unconstrained by any of the interface-related variables at the intermediate-advanced proficiency level. Neither the intermediate-advanced nor the advanced proficiency level HL leaners showed native-like sensitivity to variables conditioned at the syntax-discourse/pragmatic interface. Importantly, the only variable that was significant in first-person singular subject expression by HL leaners of all proficiency levels was the presence of hesitation phenomena. These results support the claim (Evans, 1985; Fehringer & Fry, 2007; Kormos, 1999; Riazantseva, 2001; van Hest 1996a, 1996b; Verhoeven, 1989) of a relationship between hesitation phenomena in oral speech and anxiety and increased processing demands while speaking the heritage language. Comparing the results across three proficiency levels, I analyze when and to what degree HL learners show sensitivity to these variables. The findings illustrate the benefits of extending the application of sociolinguistic methodology (specifically the comparative multivariable analysis and the stepwise logistic regression procedure) to HL and second language acquisition studies because it facilitates a fine-grained examination and comparison of leaners across proficiency levels.Item The effect of epitaxial strain and R³+ magnetism on the interfaces between polar perovskites and SrTiO₃(2011-05) Monti, Mark Charles; Markert, John T.; Markert, John T.; de Lozanne, Alex; Tsoi, Maxim; Yao, Zhen; Campion, AlanWe have embarked on a systematic study of novel charge states at oxide interfaces. We have performed pulsed laser deposition (PLD) growth of epitaxial oxide thin films on single crystal oxide substrates. We studied the effects of epitaxial strain and rare-earth composition of the metal oxide thin films. We have successfully created TiO₂ terminated SrTiO₃ (STO) substrates and have grown epitaxial thin films of LaAlO₃ (LAO), LaGaO₃ (LGO), and RAlO₃ on STO using a KrF pulsed excimer laser. Current work emphasizes the importance of understanding the effect of both epitaxial strain and R³+ magnetism on the interface between RAlO₃ and STO. We have demonstrated that the interfaces between LAO/STO and LGO/STO are metallic with carrier concentrations of 1.1 x 10¹⁴ cm[superscript -2] and 4.5 x 10¹⁴ cm[superscript −2], respectively. Rare-earth aluminate films, RAlO₃, with R = Ce, Pr, Nd, Sm, Eu, Gd, and Tb, were also grown on STO. Conducting interfaces were found for R = Pr, Nd and Gd, and the results indicate that for R [does not equal] La the magnetic nature of the R³+ ion causes increased scattering with decreasing temperature that is modeled by the Kondo effect. Epitaxial strain between the polar RAlO₃ films and STO appears to play a crucial role in the transport properties of the metallic interface, where a decrease in the R³+ ion size causes an increase in sheet resistance and an increase in the onset temperatures for increased scattering.Item Experimental studies on magnetic field and pressure dependence of high power microwaves at a dielectric surface(Texas Tech University, 2002-05) Hemmert, David J.The number of anticipated military applications of high power microwaves has greatiy increased in the past few years. Several limitations in the operation of high power microwave devices have been identified including breakdown at dielectric interfaces. These limitations prevent modem devices from being used to their full capabilities. Current research has focused on different materials, geometries, and coatings of dielectric windows. To truly understand how to maximize a dielectric window's capabilities, the basic physics of breakdown at a dielectric interface must be understood. In the work reported here, studies are made using a simple interface geometry for testing at power density levels up to 10 MW/cm^. An alumina sample with a planar geometry is placed in a rectangular waveguide such that the sample surface is parallel to the microwave electric field and perpendicular to the microwave propagation. A 4 MW, S-band magnetron is coupled to a traveling wave resonant ring to produce power levels for breakdown up to 100 MW. High speed (nanosecond) and high spatial resolution diagnostics are used to study the breakdown phenomena at the sample surface. The studies identified evidence of secondary electron emission avalanche occurring on the sample surface initiating breakdown as well as evidence of the microwave magnetic field contributing significantly to the avalanche effect for high power breakdown. This was corroborated by a significant difference in breakdovm levels on the upstream interface side of the sample compared to breakdown on the downstream side. Further studies compared gas breakdown to gas/interface breakdown and identified pressure regions in which breakdown is avalanche dominated or collision dominated.Item On Coating Durability of Polymer Coated Sheet Metal under Plastic Deformation(2011-08-08) Huang, Yu-HsuanPolymer coated sheet metal components find diverse applications in many industries. The manufacturing of the components generally involves forming of sheet metal into the desired shape and coating of the formed part with organic coating. An alternative manufacturing route is to coat the sheet metal first before forming. The change in the manufacturing sequence can potentially improve cost and reduce environmental impact. This approach, however, requires the coating to survive the deformation process. Thus, the effect of plastic deformation on coating adhesion is of primary interest to many engineers and researchers. This research aims at developing a methodology to predict the adhesion of coating after metal forming processes. A pull-off apparatus that measures the coating pull-off stress was used to indicate the coating adhesion strength. Several types of specimen were designed to obtain uniaxial tension, biaxial tension, and tension-compression deformation modes on pre-coated sheet by using a uniaxial tensile tester. Experimental results from two selected polymer coated sheet metals show that coating adhesion was affected by plastic deformation. An analytical model based on a virtual interface crack concept was developed to indicate the adhesion potential of the coating-substrate interface. From interfacial fracture mechanics, the initial adhesion potential is defined as the energy release rate characterized by the virtual interface crack and the initial pull-off stress. The analytical model was used to predict coating adhesion loss after deformation in uniaxial tension mode. The analytical model predictions agreed well with experimental results. Finite element analysis tool was applied to simulate more complex deformation modes in stamping of coated sheet meals. The stress field near the interface crack tip was used to calculate the energy release rate and predict the adhesion loss under different deformation modes. The predictions obtained from numerical method are also in good agreements with the experimental results in biaxial tension and tension-compression modes. The research has led to a better understanding of the effects of plastic deformation on coating adhesion. The developed adhesion test methods can be used to generate useful information on coating durability for diverse practical use. It is also expected that the results of the research will facilitate the development of better polymer coated sheet metal to be used in sheet metal forming processes.Item Particle dynamics and microrheology at liquid-liquid interfaces(Texas Tech University, 2009-05) Wu, Chih-yuan; Dai, Lenore L.Solid-stabilized emulsions, often refer to as Pickering Emulsions, have the ability to provide a simple and convenient experimental template to meet various requirements, such as changing property of solid particles, oil phase viscosity and interfacial curvature. The confocal laser scanning microscope (CLSM) is a useful and convenient tool to investigate the dynamics of particles at emulsions interfaces. Here we have employed Pickering emulsions as an experimental template and confocal laser scanning microscopy as a tool to study the dynamics of solid particles at liquid-liquid interfaces. In Chapter 4.1 and 4.2, the diffusion behavior of colloidal particles at oil-water interfaces is studied using Pickering emulsions as templates. The solid particles are sulfate modified polystyrene microparticles with diameters of 1.1 10-6 m and the oil phase is polydimethylsiloxane oil (PDMS) or octamethyltrisiloxane with different viscosities. The confocal laser scanning microscopy is used as a tool to observe the dynamics of microparticles at emulsions interfaces. The main discussion focuses on the effects of interfacial curvature, cluster size, and oil phase viscosity on the diffusive behavior of solid particles at oil-water interfaces. In Chapter 4.3 and 4.4, both of one-particle microrheology and two-particle microrheology are used to investigate the microrheologiccal data at the poly (dimethylsiloxane) oil-water interfaces. The dynamics of charged microparticles are observed using confocal laser scanning microscopy (CLSM) at the liquid-liquid interfaces. The experimental template is Pickering emulsions. For one-particle microrheology, the different charged sub-microparticles are used as tracers and mobility of the sub-microparticles depend largely on the viscoelastic properties of the oil phase and the wettability of the solid particles. However, those phenomena can be ignored in two-particle microrheology. In addition, the potential of microrheology at liquid-liquid interfaces is also developed in both two methodologies. The apparent loss modulus, storage modulus, and relaxation time of the oil-water interfaces are gathered at the liquid-liquid interfaces. Finally, the difference between the results of one- and two-particle microrheology is compared.Item Systems engineering processes for a student-based design laboratory(2009-12) Garner, Michael Dax; Bishop, Robert H., 1957-; Guerra, Lisa A.A student-based university environment for engineering design and development is much different from a product development environment within the aerospace industry. Therefore, a different approach to systems engineering should be considered. By its very nature, a university product development laboratory thrives on creativity and rejects bureaucracy. Experience shows that continuity and discipline within a project is crucial for success. The practice of systems engineering enables technical project discipline. Systems engineering is the art and science of developing an operable system that meets requirements within imposed constraints. The purpose of this thesis is to describe the systems engineering processes and techniques necessary for a student-based project, and explicitly show how to implement these processes. Although attempts have been made to utilize a few systems engineering techniques in past projects, many students did not properly and consistently apply those techniques to the technical design work. The goal of the thesis is to tailor the NASA systems engineering processes to a student-based design laboratory environment and to apply the methodologies to the mission design of Paradox. The Picosatellite for Autonomous Rendezvous and Docking on-Orbit eXperiment, or Paradox, is the second of four missions to demonstrate autonomous rendezvous and docking with a picosatellite-class satellite. A strong technical contribution highlighted within the thesis involves developing an open architecture rendezvous targeting algorithm for the Paradox mission in the face of large mission architecture uncertainties. The robust targeting algorithm builds from previous work utilizing an optimizer based on the Clohessey-Wiltshire equations and an iterative Lambert targeter. The contribution extends the rendezvous transfer times by including a multi-revolution Lambert targeter. The rendezvous algorithm will perform successfully given any launch vehicle and target spacecraft vehicle supporting the notion of an open architecture to satisfy the mission. The development of the algorithm is embedded within the context of the systems engineering processes to clearly showcase the intimate connection between systems engineering processes and the technical engineering design of a mission.