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Effetto della morfologia e della composizione chimica di polveri in YSZ sulle prestazioni di barriere termiche per applicazioni industriali.
No full textLe crescenti temperature d'ingresso dei gas nelle moderne turbine ad uso industriale (Industrial Gas Turbine, IGT) e aviation richiedono che i rivestimenti ceramici (Thermal Barrier Coatings, TBCs) abbiano un’elevata resistenza al ciclaggio termico e alla corrosione causata dai depositi di silicati fusi, comunemente noti come "CMAS", in quanto costituiti principalmente da ossidi come CaO, MgO, Al2O3 e SiO2. L'obiettivo di questo studio è stato quello d’investigare l'influenza combinata della morfologia della polvere e della composizione chimica sulla resistenza al CMAS ed al ciclaggio termico di rivestimenti ceramici monostrato e multistrato creati attraverso la tecnica di deposizione APS (Air Plasma Spraying). Innanzitutto, sono stati esaminati rivestimenti monostrato in ZrO2 + 7-8 wt.% Y2O3 (7-8YSZ) con microstrutture sia porose che dense (Dense-Vertically Cracked, DVC), ottenuti a partire da polveri con tre morfologie differenti: Agglomerated and Sintered (A&S), Hollow-Spherical (HOSP) e Fused & Crushed (F&C). Sono poi stati studiati due diversi tipi di rivestimenti multistrato: una prima serie di rivestimenti erano costituiti da uno strato inferiore di 7-8YSZ standard poroso e da uno strato superiore con un elevato contenuto di ittrio (ZrO2-55wt.% Y2O3), entrambi ottenuti utilizzando tutti e tre i tipi di polvere (A&S, HOSP o F&C). Lo scopo principale dello strato di ZrO2-55wt.% Y2O3 è stato quello di aumentare la resistenza chimica al CMAS e, poiché la maggiore fragilità della struttura cubica della composizione ZrO2-55wt.% Y2O3 rispetto alla composizione 7-8YSZ con struttura tetragonale potrebbe peggiorare la resistenza a ciclaggio termico del sistema, sono stati depositati sistemi multistrato con diverse proporzioni tra lo spessore dei singoli strati e/o diversi spessori totali per ottimizzarne l’architettura. Il secondo tipo di TBC multistrato è stato costituito da un primo strato interno fatto di 7-8YSZ standard, per mitigare lo stress termomeccanico grazie alla sua favorevole tenacità alla frattura, ed un secondo strato superiore composto da ZrO2-20wt.% Y2O3, con lo scopo principale di ridurre la conducibilità termica del sistema. In questo caso, sia gli strati inferiori, sia quelli superiori sono stati ottenuti utilizzando polveri agglomerate delle rispettive composizioni, ottenendo sistemi con microstrutture sia porose che DVC per entrambi gli strati. Inoltre, è stata esplorato, in alcuni di questi sistemi, anche l'uso di un terzo strato "flash" aggiuntivo costituito da zirconato di gadolinio per conferire resistenza a CMAS. Sono state condotte caratterizzazioni mediante FEG-SEM, EDX e micro-Raman per investigare microstruttura e composizione chimica. Test di nano indentazione high-speed mapping e pillar splitting sono stati eseguiti per valutare le prestazioni dei rivestimenti e il loro comportamento meccanico. La corrosione da attacco CMAS, la resistenza a fatica da ciclaggio termico (Thermal Cycling Fatigue, TCF) e l'adesione a trazione sono state analizzate su tutti i campioni.The increasing turbine inlet temperatures in modern gas turbines have raised concerns about the durability (thermal cycling) and the corrosion resistance of ceramic Thermal Barrier Coatings (TBCs) to molten silicate deposits, commonly known as "CMAS" due to their main constituents (CaO-MgO-Al2O3-SiO2). The objective of this study was to investigate the combined influence of powder morphology and chemical composition on the CMAS and thermal cycling resistance of ceramic monolayer and bi-layer coatings deposited through Atmospheric Plasma Spraying (APS). Three powder morphologies of Yttria-Stabilized Zirconia (YSZ) were examined: porous Agglomerated and Sintered (A&S) granules; Hollow Spherical (HOSP) powders; and dense, irregular Fused and Crushed (F&C) particles. Monolayer coatings were made of ZrO2 stabilized with 7-8 wt.% Y2O3 (7-8YSZ) with both porous and dense-vertically cracked (DVC) microstructures. Two types of bi-layer coatings were studied. The first type consisted of a bottom layer of porous standard 7-8YSZ and a top layer composed of a porous, high-yttrium ZrO2-55wt.% Y2O3 layer. Both layers were obtained using all three powder types (A&S, HOSP, or F&C). In this system the main purpose of the ZrO2-55wt.% Y2O3 top layer was to enhance the resistance to "CMAS" melts. Because of its cubic structure, ZrO2-55wt.% Y2O3 is more brittle than 7-8YSZ; therefore, in order to minimize the detrimental effects on thermal cycling resistance, the bi-layer systems were designed with different ratios between the individual layer thicknesses and/or different total thickness. The second type of bi-layer TBC consisted of an inner layer made of standard 7-8YSZ to mitigate thermomechanical stress due to its favorable fracture toughness, and a top layer composed of ZrO2-20wt.% Y2O3, the main purpose of which is, in this case, to reduce the thermal conductivity of the TBC. Agglomerated powders were employed both for bottom and top layer of their respective compositions, resulting in systems with either porous or Dense-Vertically Cracked (DVC) microstructures for both layers. Additionally, the use of an extra "flash" third layer made of gadolinium zirconate in dedicated systems was explored to confer "CMAS" resistance. FEG-SEM, EDX, and micro-Raman analyses, were conducted to investigate microstructure and chemical composition. Nanoindentation high-speed mapping and pillar splitting test were performed to assess the coatings' performance and evaluate the mechanical behavior. Corrosion by molten CMAS, Thermal Cycling Fatigue (TCF) resistance and tensile adhesion were characterized on all samples
CMAS corrosion and thermal cycling fatigue resistance of alternative thermal barrier coating materials and architectures: A comparative evaluation
No full textThe corrosion of ceramic thermal barrier coatings (TBCs) by molten silicate deposits, usually known as “CMAS”
from their main constituents (CaO-MgO-Al2O3-SiO2), is an issue of increasing concern in modern gas turbines as
the turbine inlet temperatures are increased to enhance thermodynamic efficiency. Because conventional ZrO2-
7wt%Y2O3 (7YSZ) dissolves quite readily in a CMAS melt, many alternative materials have been proposed, but
there are not many comparative studies among them. Multi-layer architectures featuring a tougher 7YSZ bottom
layer and a more brittle, but more CMAS corrosion-resistant top layer have also been proposed; therefore, a
comparison among these architectures is also in order. In this paper we studied comparatively the resistance to
CMAS corrosion and to thermal cycling fatigue (an essential pre-requisite for any TBC system) of Gd2Zr2O7,
ZrO2–55wt%Y2O3 and Gd/Yb/Y co-doped ZrO2, both in the form of single, dense-vertically cracked (DVC) layers
deposited by plasma spraying onto an MCrAlY bond coat, and as top layers with a bottom layer of either porous
or DVC 7YSZ. It was found that Gd2Zr2O7 resists CMAS corrosion, without any grain-boundary dissolution,
slightly better than does ZrO2–55wt%Y2O3. They both develop a solid Gd- or Y-apatite layer (respectively) at the
interface with the CMAS melt, driven by the rather large difference in optical basicity between these compounds
and CMAS itself, but the Y-apatite layer is less continuous and, therefore, a bit less protective. Gd/Yb/Y co-doped
ZrO2, instead, suffers as much grain-boundary dissolution in contact with molten CMAS as does 7YSZ. A
Gd2Zr2O7/porous 7YSZ system would therefore exhibit simultaneously high resistance to CMAS dissolution and
to thermal cycling fatigue, although there is a risk that the CMAS melt might infiltrate the segmentation macro-
cracks and the microcracks of the Gd2Zr2O7 layer and undermine the porous 7YSZ bottom layer
The effect of ceramic YSZ powder morphology on coating performance for industrial TBCs
No full textThe increasing turbine inlet temperatures in modern gas turbines have raised concerns about the corrosion of ceramic thermal barrier coatings (TBCs) caused by molten silicate deposits, commonly referred to as “CMAS” due to their main constituents (CaO-MgO-Al2O3-SiO2). The objective of this study was to investigate the combined influence of powder morphology and chemical composition on the CMAS resistance and thermal cycling resistance of ceramic monolayer and bi-layer coatings created through Atmospheric Plasma Spraying (APS). Three powder morphologies were examined: porous Agglomerated and Sintered (A&S) granules, Hollow Spherical (HOSP) powders, and dense, irregular Fused and Crushed (F&C) particles. Monolayer 7-8YSZ coatings with both porous and dense vertically cracked (DVC) microstructures, and bi-layer coatings consisting of a bottom layer of porous standard 7-8YSZ and a top layer composed of a porous high‐yttrium ZrO2–55 wt% Y2O3 were obtained using all three powder types (A&S, HOSP, or F&C). Furthermore, the bi-layer systems were deposited with different ratios between the individual layer thicknesses and/or different total thickness. FEG-SEM, EDX, and micro-Raman analyses, were conducted to assess the coatings' performance. Nanoindentation high-speed mapping and pillar splitting test were performed to evaluate the mechanical behaviour. The study on 8YSZ monolayers shows that coatings from a F&C feedstock exhibit higher density, reducing the CMAS penetration. However, these coatings demonstrate poorer thermal cycling performance due to increased stiffness and thermal stresses. Coatings from HOSP and A&S powders allow CMAS penetration but offer stress relief pathways, enhancing the coating's ability to withstand thermal stresses. Bi-layer coatings with a 55YSZ top coat show superior CMAS resistance compared to 7-8YSZ monolayer coatings, with limited penetration causing top coat peeling. The thickness ratio between the layers also affects thermal cycling resistance, where a thinner 55YSZ layer extends the TBC lifetime
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
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
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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