1,721,132 research outputs found
New approaches to solar tracking and concentration through numerical optimization of lens arrays
Full text linkSolar concentrators are essential for enabling several solar energy applications, including high-efficiency photovoltaic conversion and high-temperature solar thermal energy. These concentrators require accurate solar tracking, commonly performed by rotating them towards the sun, which adds to the bulk and complexity of the system.
In this thesis, we investigate optically tracking the sun using millimeter-scale translation instead of rotating the complete concentrator — a concept known as tracking integration. We show how the performance of these systems can be pushed beyond the current state of the art through a broad exploration of the design space using a custom sequential ray-tracer in combination with memetic multi-objective optimization algorithms. We explore two classes of tracking integration: beam-steering lens arrays that consist of an afocal stack of lens arrays, and microtracking concentrators that concentrate sunlight to an array of discrete focal spots where micro-PV cells can convert the solar energy to electricity. Further, we propose possible étendue-squeezing solar concentrator designs that may benefit from tracking integration.
First, we perform a broad exploration of beam-steering lens array configurations for full-day stationary solar tracking. We identify several promising configurations, including one capable of redirecting sunlight into a 2000x concentration ratio at a two-axis ±60◦tracking range. Finally, we demonstrate a line-focus concentrator with a simulated effective concentration ratio of 218x at a ±1◦ acceptance angle that employs étendue squeezing to go beyond the conventional two-dimensional concentration limit. To the best of our knowledge, this is the first demonstration of how a line-focus concentrator can be directly designed as a three-dimensional concentrator to operate beyond the 2D limit (which is 57x at the ±1◦ acceptance angle). This concentrator, combined with beam-steering lens arrays, may enable the development of a new class of highconcentration trough-like solar concentrators
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Relating stress corrosion cracking behavior to microstructural and surface properties of biocompatible AZ31 alloy
Full text linkMagnesium (Mg) and its alloys have attracted significant attention as temporary implant materials due to their excellent biocompatibility with human physiology. In fact, Mg is essential to the human metabolism as a cofactor for many enzymes and Mg ions are well-known to facilitate tissue-healing. In addition, the mechanical properties (density, elastic modulus, yield strength and ultimate tensile strength) of Mg and its alloys resemble those of natural bone reducing the risk of the stress-shielding-related problems observed with other metallic implant materials such as stainless steel, titanium and Co-Cr alloys. However, despite their high potential, Mg and its alloys are not yet utilized in biomedical applications. This is due to the (1) rapid corrosion and degradation in the human body that leads to a loss of mechanical integrity before tissues have sufficient time to heal, (2) the evolution of hydrogen as corrosion product accompanied by hydrogen pocket formation that hampers healing or even cause the death of patients through the blockage of the blood stream and (3) the sudden fracture of implants due to the simultaneous action of the corrosive human-body-fluid and mechanical loads through corrosion-assisted cracking phenomena (stress corrosion cracking (SCC) and corrosion fatigue (CF)).
In the past years, several approaches have been developed to improve the corrosion resistance of Mg and its alloys. These approaches can be divided into two main groups, one characterized by the modification of the bulk and the other by the modification of the surface. Among the former, Severe Plastic Deformation (SPD) techniques, such as Equal Channel Angular Pressing (ECAP), have attracted attention as possibility for inducing a very fine and homogeneous microstructure throughout all the samples. The latter group relies on surface modifications obtained by mechanical processing (e.g. cryogenic machining) or by the protection through coatings deposited by various techniques (e.g. sputter and Atomic Layer Deposition (ALD)). However, the assessment of the effectiveness of the different approaches in improving the resistance of Mg and its alloys to corrosion-assisted cracking phenomena is still underexplored.
In an attempt to understand the fundamental mechanisms linking the microstructural and surface properties to the SCC susceptibility, this thesis investigates how selected procedures initially intended for improving the corrosion resistance of Mg and its alloys impact the SCC susceptibility of AZ31 alloys in Simulated Body Fluid (SBF) at 37 °C. The procedures selected from an extensive literature review investigating the different procedures used to improve the corrosion behavior and the mechanisms regulating the SCC phenomenon were ECAP, cryogenic machining and coatings obtained by means of ALD.
1, 2 and 4 passes of ECAP were carried out on an AZ31 alloy and samples subjected to one pass of ECAP have been shown to be less susceptible to SCC compared to the material in the as-received condition (the elongation to failure was increased by 150%) due to the improved corrosion resistance as a consequence of a reduced grain size. The reduced SCC susceptibility after one pass of ECAP was also confirmed by the morphology of the fracture surfaces that reveals an increased ductility compared to the as-received material. However further ECAP processing (2 and 4 passes) are reported to worsen the SCC susceptibility due to an increased brittleness of the material as a consequence of an increased amount of hydrogen evolved. This is due to the unfavorable texture evolution, as confirmed by the mechanical characterization (tensile tests and hardness measurements).
AZ31 samples were machined under cryogenic cooling and afterwards subjected to Slow Strain Rate Tests (SSRTs) at a strain rate of 3.5·10-6 s-1 to evaluate the SCC susceptibility. Cryogenic machined samples were characterized by lower SCC susceptibility than dry cut samples (the elongation to failure was increased by 28%) as a consequence of their improved corrosion performances due to the presence of a wider nanocrystalline layer, resulting in a faster formation of passivating surface oxides, and to the presence of compressive residual stresses instead of tensile.
Being ALD a recently developed technique still underexplored in terms of corrosion and biological properties, it was compared to sputter technique in terms of corrosion protectiveness and the induced biocompatibility of three different coatings were evaluated. The ALD technique has been shown to provide the better corrosion protection (assessed by means of potentiodynamic polarization curves and hydrogen evolution experiments) both in case of smooth and rough surfaces due to an increased surface integrity (observed by SEM and XPS analyses). In addition, in the case of 3D porous structures, the improvements provided by the ALD technique were even higher as a consequence of the line-of-sight limitation of sputtering (confirmed by means of SEM analyses). In addition, the biocompatibility of TiO2, ZrO2 and HfO2 coatings obtained by means of ALD have been investigated by means of MTS assay on L929 cells and the HfO2 coatings were shown to provide the best biocompatibility due to the highest corrosion resistance. This can be reasoned by their lower wettability and their higher electrochemical stability and surface integrity (in terms of cracks and pores). TiO2, though generally considered a biocompatible coating, was found to provide the lowest improvements in terms of corrosion resistance and cell viability. Interestingly, TiO2 coatings are characterized by grade 3 cytotoxicity after 5 days of culture due to their high corrosion rate, which does not meet the demands for cellular applications. These results indicate the strong link between biocompatibility and corrosion protection and signify the need of considering the latter when choosing a biocompatible coating to protect temporary Mg based alloys before implantation.
Finally, the SCC susceptibility of TiO2 and ZrO2 ALDed coated AZ31 alloys have been evaluated and the ZrO2 coated samples were reported to have the lowest SCC susceptibility. In fact, the elongation to failure of the TiO2 coated samples were increased by 125% and that of ZrO2 coated samples by 220%. The different SCC susceptibility was attributed to the improved corrosion of the ZrO2 coated samples compared to the TiO2 coated samples as a consequence of four main aspects, i.e. different cohesive energies, different wettability, different defect densities and sizes and different mechanical properties
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
3D Printing and Carbonization of High Performing Polyimide Formulations
No full textStereolitografisk 3D-printing (SLA) er en moden form for additiv tilvirkning, og blir i økende grad brukt til produksjon av deler som inngår i et ferdig produkt. I denne oppgaven utforskes muligheten for å bruke SLA til å produsere gassdiffusjonslagene som inngår i proton-utvekslende brensel-celler. En studie av eksisterende litteratur presenteres,og denne studien indikerer at oppløsningen som er oppnåelig med SLA er kompatibel med designkravene for gassdiffusjonslag. Teknikker for effektiv generering av gitterkonstruksjoner med slik oppløsing har blitt utviklet. Dette er gjort ved å hoppe over 3D modellering som et steg i prosessen og isteden produsere 2D tverrsnitt med et python script. Sammenligninger som er utført viser at denne teknikken er størrelsesordener raskere enn vanlig 3D-modellering. Videre tillater teknikken nøyaktig innstilling av energidoseringen gjennom pikselbasert intensitetskontroll. Dette muliggjorde 3d-printing av gitterkonstruksjoner med bjelker bare en piksel brede (50uM), adskilt av mellomrom kun tre piksler (150uM) brede på en kommersielt tilgjengelig DLP-SLA 3D-printer. Denne masteroppgaven undersøker også to polyimidbaserte materialer som kandidatmaterialer for de 3D-printede gassdiffusjonslagene. Resultatene indikerer at det er mulig å karbonisere UV-herdbar polyimid, men også at mer arbeid er nødvendig for å avgjøre om materialet er egnetStereolithographic 3D printing (SLA) is a mature additive manufacturing technology and is gaining traction for the production of certain end-use parts. In this thesis, the possibility of using SLA to produce the gas diffusion layers found in proton exchange membrane fuel cells is explored. A study of the existing literature is presented, which indicates that the resolution of SLA is compatible with the design requirements for gas diffusion layers. A technique for efficiently designing lattice structures with sufficient resolution and a large number of features have been developed. This has been done by avoiding 3D modeling and instead directly producing 2D cross sections with a Python script. Comparisons show that this technique could be orders of magnitudes faster than conventional 3D modeling. Further, the technique allows for precise tuning of the energy dosage through pixel-based intensity control, this enables 3D printing of lattice structures with beams only one pixel wide 50 μm, separated by only three pixels 150 μm gaps on a commercially available a DLP-SLA printer. The thesis also investigates two polyimide-based materials as candidate materials for the 3D printed gas diffusion layers. The results indicate that it is possible to carbonize UV-curable polyimide, but also that more work is needed to determine if the material is suitable
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
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