Recent ucei_basic items

Toward ZnO Light Emitting Diode

Mon, 20 Oct 2008 00:00:00 +0000

It is estimated that about 25% of all electrical energy is used for lighting purposes. Light-emitting diode (LED)-based lighting technologies can be a factor of nearly 10 times more efficient than incandescent lighting and a factor of about 2-3 times more efficient than fluorescent lighting technologies. It is predicted by Department of Energy (DOE) that by 2025 the use of solid-state lighting will reduce national energy consumption for lighting by 29%. The cumulative energy savings from 2006 to 2025 would result in more than 125 billion dollars of savings to consumer electricity bills. LEDs based on Gallium Nitride (GaN) materials are currently widely developed to conserve electrical energy in numerous lighting applications, such as headlights of vehicles, traffic lights, outdoor displays, and some indoor lighting. The trend towards higher brightness, lower operating current, and longer lifetime has increased the needs for developing alternative next-generation LEDs. Zinc...

Fundamental Studies of Diffusion and Reactions in Hydrogen Storage Materials

Mon, 20 Oct 2008 00:00:00 +0000

Hydrogen can serve as an energy carrier in a carbon-neutral system of energy production and use [1,2], but adequate hydrogen storage materials are still lacking in spite of many decades of investigations. In addition to being reversible and meeting stringent weight % and volume criteria, candidate materials must exhibit favorable kinetics for hydrogen uptake and release. The fundamental mechanisms of the (de)hydrogenation process have remained elusive to date. We have initiated a study of the relevant reactions, resulting in an identification of the dominant defect species involved in hydrogen transport in non-metallic hosts. While the concepts discussed here are general, we illustrate them with detailed first-principles results for sodium alanate. We identify hydrogen-related point defects as the essential mediators of hydrogen transport. A novel finding of this work is that the defects are positively or negatively charged, and hence their formation energies are Fermi-level...

Development of a Thermoacoustic Model for Evaluating Passive Damping Strategies

Wed, 14 Feb 2007 00:00:00 +0000

Lean premixed prevaporized (LPP) gas turbine generators have become popular in energy conversion applications to meet strict emission requirements. Because the combustion process is very lean, combustion instabilities due to acoustic perturbations are more likely to occur than in a less lean fuel combustion process. Current design of damping strategies for mitigating these instabilities is often based on empirical trial and error, which precludes the possibility of determining an optimal configuration. A combustion system whose elements consist of flames, passive dampers, and ducts must be optimized to reduce or completely eliminate combustion instabilities. Hence, a modular simulation tool is developed to examine the interaction of plane acoustic waves with typical combustion system elements. The simulation tool represents these interactions in the form of transfer matrices, which can be modularly arranged for exploring a variety of configurations. In this work, a heuristic...

Nanoscale Heat Conduction across Metal-Dielectric Interfaces

Fri, 14 Oct 2005 00:00:00 +0000

We report a theoretical study of nanoscale heat conduction across nanolaminates consisting of alternating layers of metal and dielectric materials. Nanolaminates are promising as thermal barrier coatings for energy generation and conversion applications because they offer unique opportunities to achieve superior thermal performance without compromising mechanical strength or chemical protection characteristics. A continuum two-fluid energy transport equation is solved to predict the thermal resistance of a metallic film bounded by dielectric materials. Analysis of existing experimental data is consistent with the present model, suggesting that electron-phonon spatial nonequilibrium plays an important role in heat conduction across metal-dielectric interfaces.

Counterflow Extinction of Premixed and Nonpremixed Methanol and Ethanol Flames

Fri, 14 Oct 2005 00:00:00 +0000

Experimental and numerical studies are conducted on extinction of methanol and ethanol flames. Two flame types are considered: premixed and nonpremixed. The studies are performed in the counterflow configuration. The burner used in the experiments is made up of two opposing ducts. In the premixed configuration the reactive stream, made up of fuel, oxygen, and nitrogen, is injected from one duct, and a nitrogen stream is injected from the other duct. In the nonpremixed configuration the fuel stream is made up of fuel and nitrogen, and it is opposed by an oxidizer stream made up of air. The fuels are prevaporized by flowing nitrogen through a heated bath of liquid fuel. The velocities of the reactant streams at the injection planes are calculated from measured flowrates. These velocities are used to calculate the strain rate. The temperature of the fuel stream and that of the nitrogen stream at the injection plane are measured using thermocouples. Critical conditions of extinction...

Thermoelectric Coolers and Power Generators Using Self-assembled Ge Quantum Dot Superlattices

Tue, 29 Mar 2005 00:00:00 +0000

The proposed goal of this proposal is to explore the Ge quantum dot superlattices for thermoelectric device applications. Over 1-year period, we have worked on this project extensively and achieved the following research results.

Improvement in Oxidation Behavior of Nanostructured CoNiCrA1Y Bond Coat Dispersed with Nano-size Alumina Particles

Thu, 19 Dec 2002 00:00:00 +0000

The oxidation behavior of a nanostructured CoNiCrA1Y bond coat dispersed nano-sized alumina particles was compared to that of a conventional coarse-grained CoNiCrA1Y bond coat. It was found that after a 1000ºC/24 h oxidation a protective alumina layer was developed on the nanostructured coating, whereas a mixed and less protective oxide layer was observed on the conventional coating. Correspondingly, spallation of the oxide layer was not observed with the nanostructured coating but did occur with the conventional coating. The improvement in oxidation behavior observed in the nanostructured coating was suggested to be due to its fine-grained structure, which provides rapid pathways (grain boundaries) for A1 diffusion to form a protective scale of pure alumina. Furthermore, the dispersion of the nano-sized alumina particles was effective in suppressing grain growth and in promoting the rapid formation of a protective alumina scale and thus significantly improving the oxidation...

Synthesis and Oxidation Behavior of Nanocrystalline MCrA1Y Bond Coats

Thu, 19 Dec 2002 00:00:00 +0000

Thermal barrier coating (TBC) systems protect turbine blades against high-temperature corrosion and oxidation. They consist of a metal bond coat (MCrAIY, M = Ni, Co) and a ceramic top layer (ZrO2/Y2O3). In this work the oxidation behavior of conventional and nanostructured HVOF NiCrAIY coatings has been compared. Commercially available NiCrAIY powder was mechanically cryomilled and HVOF sprayed on a nickel alloy foil to form a nanocrystalline coating. Free-standing bodies of conventional and nanostructured HVOF NiCrAIY coatings were oxidized at a 1000ºC for different time periods in order to form the thermally grown oxide (TGO) layer. The experimants show an improvement in oxidation resistance in the nanostructured coating when compared to that of the conventional one. This behavior is a result of the formation of a continuous AI2O3 layer on the top surface of the nanostructured HVOF NiCrAIY coating. This layer protects the coating from further oxide protrusions present in...

Carrier and Phonon Spectrum Modification in Quantum Dot Superlattices Designed for Thermoelectric Applications

Tue, 5 Nov 2002 00:00:00 +0000

Quantum dot superlattices have recently been proposed for thermoelectric applications. The predicted improvement of the thermoelectric figure of merit in such structures should come from the decreased lattice thermal conductivity due to additional boundary scattering and acoustic phonon spectrum modification, as well as change in the carrier transport and density of states. Here we outline a theoretical model to calculate carrier and phonon dispersion in such structures and present results for Ge/Si quantum dot superlattices. We argue that one can tune the mini-band carrier transport and phonon dispersion in such a way that electron-phonon coupling is suppressed. The latter may open up a novel way for the enhancement of the thermoelectric figure of merit.

Measurement of Hydrocarbon Fluxes due to Natural Seepage in the Northern Santa Barbara Channel

Tue, 5 Nov 2002 00:00:00 +0000

We describe the design, calibration, and deployment of a buoy and gas capture assembly for measuring bubbling gas flux in oceans and lakes. The assembly collects gas in a chamber while continuously measuring the position of the gas-water interface that forms as gas accumulates. Interface position is determined from the differential pressure between the chamber and ambient seawater. A spar buoy provides flotation and stability to reduce vertical motions due to surface waves. The gas collection assembly and spar, referred to as a flux buoy, is suitable for deployment from small boats under conditions of light wind and small waves. We are using the flux buoy to determine the spatial distribution of natural hydrocarbon seepage off the south-central California coast. Hydrocarbon seepage from continental shelves may be an important source of atmospheric methane.