Synthesis, assembling and validation of solid oxide fuel cell units

The main objectives of this thesis have been: the control of the structure and microstructure of the SOFC components (electrodes and electrolyte); and the electrochemical characterization of the manufactured materials. For this purpose, several techniques have been used: tape casting, replication with molds or 3D printing. Among the large number of material¿s characterization techniques applied in this work; special interest has been devoted to those related with the rheology for the slurries characterization and to those related with Electrochemistry, specifically on Electrochemical Impedance Spectroscopy (EIS), for the electrical characterization. La0.6Sr0.4Co0.2Fe0.8O3-¿ (LSCF) with perovskite structure is very interesting because they exhibit high oxygen permeability at elevated temperatures. Manufacture of commercial LSCF by aqueous colloidal processing has been carried out. The surface behavior of LSCF as a function of pH and the effect of a polyelectrolyte (Duramax D3005) on the stability are studied using zeta potential technique. Concentrated suspensions were prepared with a solid content as high as 35 vol.%. The optimum dispersing conditions were determined by means of rheological measurements for obtaining stable and fluid slurry for tape casting technique. Yttria (8 mol%) stabilized zirconia (YSZ) has widely been used as electrolyte in solid oxide fuel cells (SOFC). In order to obtain fluid slips, rheological studies of aqueous suspensions of three different commercial YSZ powders dispersed with a polyacrylic acid-based dispersant agent have been performed. Their viscosity was optimized by controlling the dispersant concentration, pH and homogenization time using an ultrasound probe. An electrical study of the sintered tapes prepared under strict control of the rheology was done by electrochemical impedance spectroscopy. An innovative design, alternative to the conventional metal supported fuel cells (MSC) is proposed. This new design permits the reduction of ~65% of the metallic supporting material, hence a decrease of the cost of any MSC assembled in this configuration and it offers the opportunity of produce at mass-scale in a cost-effective way. Furthermore, the way of preparing the microstructured MSC with a metal layer of 150-200 µm, allows us to prepare any type of patterning and thickness. This new design of SOFC comprises a 200 µm layer of a honeycomb-metallic framework with hexagonal cells which supports a layer of electrolyte and can be used as current collector. Each hexagonal cavity is further functionalized with a thin 5-10 µm of Ni-YSZ anode, in direct contact with YSZ electrolyte. Finally, a cold sealing through an electrical resistance welding process is possible because it is used interconnect material on one side of each SOFCs. 3D printing technique as a new tool for controlling the microstructure of the materials was studied. Microstructured organic-based molds have been designed for the deposition of YSZ and crofer slurries. The design of SOFC 3D prototypes for being fully 3D printed with ceramic powders and photopolymers is proposed and some successfully proofs have been performed

Modelling and Simulation of the Primary Power Distribution of a Lunar Habitat

A MATLAB/Simulink model of the Primary Power Distribution System of a lunar habitat is presented. The model can be adapted to multiple scenarios, and is able to interface with computer models of other habitat subsystems. A constant supply of power is considered regardless of the source and the time of the day, regulating the bus voltage when required. The electrical system of the International Space Station is used for reference and validation. The model has been tested in two scenarios representing two locations on the surface of the Moon.Peer ReviewedPostprint (published version

A new survey of cool supergiants in the Magellanic Clouds

In this study, we conduct a pilot program aimed at the red supergiant population of the Magellanic Clouds. We intend to extend the current known sample to the unexplored low end of the brightness distribution of these stars, building a more representative dataset with which to extrapolate their behaviour to other Galactic and extra-galactic environments. We select candidates using only near infrared photometry, and with medium resolution multi-object spectroscopy, we perform spectral classification and derive their line-of-sight velocities, confirming the nature of the candidates and their membership to the clouds. Around two hundred new RSGs have been detected, hinting at a yet to be observed large population. Using near and mid infrared photometry we study the brightness distribution of these stars, the onset of mass-loss and the effect of dust in their atmospheres. Based on this sample, new a priori classification criteria are investigated, combining mid and near infrared photometry to improve the observational efficiency of similar programs as this.Comment: 39 pages, 10 figures. Accepted for publication in A&

A VLSI-oriented and power-efficient approach for dynamic texture recognition applied to smoke detection

The recognition of dynamic textures is fundamental in processing image sequences as they are very common in natural scenes. The computation of the optic flow is the most popular method to detect, segment and analyse dynamic textures. For weak dynamic textures, this method is specially adequate. However, for strong dynamic textures, it implies heavy computational load and therefore an important energy consumption. In this paper, we propose a novel approach intented to be implemented by very low-power integrated vision devices. It is based on a simple and flexible computation at the focal plane implemented by power-efficient hardware. The first stages of the processing are dedicated to remove redundant spatial information in order to obtain a simplified representation of the original scene. This simplified representation can be used by subsequent digital processing stages to finally decide about the presence and evolution of a certain dynamic texture in the scene. As an application of the proposed approach, we present the preliminary results of smoke detection for the development of a forest fire detection system based on a wireless vision sensor network.Junta de Andalucía (CICE) 2006-TIC-235

Spectral type, temperature and evolutionary stage in cool supergiants

In recent years, temperature scales in cool supergiants (CSGs) have been disputed, and the possibility that spectral types (SpTs) do not depend primarily on temperature has been raised. We explore the relations between different observed parameters and the capability of deriving accurate intrinsic stellar parameters from them through the analysis of the largest spectroscopic sample of CSGs to date from SMC and LMC. We explore possible correlations between different observational parameters, also making use of near- and mid-infrared colours and literature on photometric variability. Direct comparison between the behaviour of atomic lines (Fe I, Ti I, and Ca II) in the observed spectra and synthetic atmospheric models provides compelling evidence that effective temperature is the prime underlying variable driving the SpT sequence in CSGs. However, there is a clear correlation between SpT and luminosity, with later ones tending to correspond to more luminous stars with heavier mass loss. The population of CSGs in the SMC is characterised by a higher degree of spectral variability, early spectral types (centred on type K1) and low mass-loss rates (at least as measured by dust-sensitive mid-infrared colours). The population in the LMC displays less spectroscopic variability and later spectral types. The distribution of spectral types is not single-peaked. Instead, the brightest CSGs have a significantly different distribution from less luminous objects, presenting mostly M subtypes (centred on M2), and increasing mass-loss rates for later types. In conclusion, the observed properties of CSGs in the SMC and the LMC cannot be described correctly by standard evolutionary models. The very strong correlation between spectral type and bolometric luminosity, supported by all data from the Milky Way, cannot be reproduced at all by current evolutionary tracks.Comment: 25 pages, 24 figure

Additivity of reinforcing mechanisms during creep of metal matrix composites: Role of the microstructure and the processing route

During decades, great efforts have been devoted to understand and predict the creep behavior of discontinuously reinforced metal matrix composites, particularly aluminum alloy matrix materials. As a result of all these investigations, however, a confuse panorama of the precise role of the reinforcing particles on the enhanced creep response of these materials rules the present understanding. Here, an analysis of the reasons that have driven to this situation is made. Also, a simple and generalized framework of the relevant mechanisms required to understand this behavior is proposed. This view is based on previous work by these authors on 6061Al alloy, prepared by ingot and powder metallurgy, and 6061Al–15 vol.% SiCw composite, as well as a data analysis of published investigations aimed at clarifying the above situation. The additivity of the proposed contributions and the potential damage mechanism are discussed in the context of the processing route employed.Project MAT05-00527 from MEC, Spain.Peer reviewe

The population of M-type supergiants in the starburst cluster Stephenson 2

The open cluster Stephenson 2 contains the largest collection of red supergiants known in the Galaxy, and at present is the second most massive young cluster known in the Milky Way. We have obtained multi-epoch, intermediate-resolution spectra around the CaII triplet for more than 30 red supergiants in Stephenson~2 and its surroundings. We find a clear separation between a majority of RSGs having spectral types M0-M2 and the brightest members in the NIR, which have very late spectral types and show strong evidence for heavy mass loss. The distribution of spectral types is similar to that of RSGs in other clusters, such as NGC 7419, or associations, like Per OB1. The cluster data strongly support the idea that heavy mass loss and maser emission is preferentially associated with late-M spectral types, suggesting that they represent an evolutionary phase.Comment: Contribution to the Betelgeuse workshop, November 2012, Paris. To be published in the European Astronomical Society Publications Series, editors: Pierre Kervella, Thibaut Le Bertre & Guy Perri

Lunar ISRU Energy Storage and Electricity Generation

The survival of the astronauts and their equipment is the priority for any long-term exploration mission to the Moon. The provision of energy during the long lunar nights is a critical part of these missions. Several approaches have recently been considered to store and provide energy on the Moon by means of ISRU (In-Situ Resource Utilisation). We present a review of the energy requirements for a long mission scenario, and a trade-off analysis of the potentially suitable technologies for an ISRU-based system able to store heat and generate electricity. The most promising combinations of technologies are presented.Peer ReviewedPostprint (published version