1,720,971 research outputs found
Resistenza a corrosione da CMAS di barriere termiche (TBCs): influenza della microstruttura, materiali e architettura del rivestimento
Full text linkLe barriere termiche (TBC) sono rivestimenti ceramici applicati su componenti metallici nelle sezioni calde delle turbine a gas industriali (IGT), dove le temperature dei gas sono superiori al punto di fusione dei substrati metallici. Le TBC forniscono isolamento termico alle parti metalliche sottostanti: agendo in sinergia con un sistema interno di raffreddamento ad aria, le TBC riducono le temperature superficiali del metallo. Lo “stato dell’arte” delle TBC è la zirconia stabilizzata con il 7-8% in peso di Y2O3 (YSZ) che ha un alto punto di fusione (2690°C), una stabilità di fase sopra i 1200°C, e bassa conducibilità termica. Un tipico sistema di rivestimento comprende un'architettura a doppio strato, in cui uno strato YSZ poroso e termoisolante è depositato mediante spruzzatura al plasma su un bondcoat metallico (leghe MCrAlY, M = Ni, Co o NiCo) che protegge il substrato di superlega dall'ossidazione e migliora l'adesione dello strato superiore. Durante l'esposizione ad alte temperature, tra questi due strati, si forma per ossidazione del bondcoat uno strato chiamato thermally grown oxide (TGO) costituito da α-Al2O3. Il cedimento delle TBC può essere dovuto sia a sollecitazioni termiche cicliche indotte dall’accensione e spegnimento delle turbine, che provocano la nucleazione e la crescita di cricche, sia alla degradazione chimica. Quando le turbine a gas operano in ambienti polverosi, possono ingerire polveri di silicati, che poi formano depositi fusi a base di miscele di CaO-MgO-Al2O3-SiO2 (CMAS) sulla superficie dei componenti delle parti calde.
Il presente lavoro è incentrato sullo studio di sistemi TBC con una maggiore resistenza chimica ai depositi di CMAS.
La prima parte di questa tesi studia l’infiltrazione e il meccanismo di reazione tra la CMAS e i rivestimenti 8YSZ con diverse microstrutture: porose ottenute tramite atmospheric plasma spraying (APS) utilizzando materie prime a standard e ad elevata purezza; dense-vertically cracked (DVC) APS; microstrutture colonnari ottenute tramite suspension plasma spraying (SPS). È stato riscontrato che la CMAS attacca la YSZ dissolvendo i suoi bordi grano e che l’utilizzo di un materiale a bassa purezza accelera la dissoluzione.
Nella seconda parte di questo lavoro, dopo aver identificato la microstruttura DVC come la più promettente in termini di resistenza a corrosione da CMAS, sono stati effettuati test su tre nuovi rivestimenti: ZrO2 drogata con Gd/Yb/Y, Gd2ZrO7 e ZrO2 con 55% in peso di Y2O3. Tutti sono stati prodotti con una microstruttura DVC e depositati sullo stesso tipo di bondcoat.
8YSZ porosi e DVC sono stati utilizzati come termine di confronto. Inoltre, sono stati testati sei sistemi a doppio strato, in cui la 8YSZ con microstruttura porosa o DVC è stata impiegata come strato inferiore ad uno strato DVC di Gd/Yb/Y, Gd2ZrO7 e ZrO2 con 55% in peso di Y2O3. Questi sistemi sono stati sottoposti a test di corrosione CMAS e test di ciclaggio termico (TCF).
I rivestimenti multistrato hanno mostrato una durata a TCF più lunga rispetto ai rivestimenti monostrato. D'altra parte, i test CMAS hanno mostrato che i nuovi materiali hanno una migliore resistenza alla corrosione. Il DVC Gd2ZrO7, ha mostrato un'eccellente resistenza alla corrosione CMAS dovuta alla formazione di uno strato solido di Gd-apatite all'interfaccia con CMAS fusa che blocca l'ulteriore reazione e rallentata la penetrazione di CMAS. La combinazione di uno strato superiore di Gd2ZrO7 con uno strato inferiore poroso 8YSZ mostra una maggiore resistenza a TCF. Sebbene il sistema a doppio strato non raggiunga la stessa resistenza TCF del puro YSZ, la combinazione tra una ragionevole durata a TCF e un'eccellente resistenza CMAS lo rende una buona scelta per le turbine che operano in condizioni ambientali ostili.Thermal barrier coatings (TBCs) are refractory-oxide ceramic coatings applied on metallic components in the hot section of industrial gas turbines (IGT), where gas temperatures are higher than the melting point of the metallic substrates. TBCs provide thermal insulation the underlying metal parts: acting in synergy with an internal air-cooling system, TBCs reduce the temperatures of the metal down to tolerable levels. The “state of the art” for TBCs is 7-8 wt.% (≈3.5 mol.%) Y2O3-stabilized zirconia (YSZ) with high melting point (2690 °C), phase stability up to 1200 °C, and low thermal conductivity. A typical coating system comprises a bi-layer architecture, where a porous, thermally insulating YSZ layer is deposited by plasma spraying onto a metallic bond coat (MCrAlY alloys, M = Ni, Co or NiCo) which protects the superalloy substrate against oxidation and improves the top layer adhesion. During exposure to high temperatures, between these two layers, a further layer called thermally grown oxide (TGO) made of α-Al2O3 is formed by oxidation of the MCrAlY bond coat. Failure of TBCs during service can be due either to the cyclic thermal stresses induced by starting and stopping the turbine, which cause the nucleation and growth of delamination cracks, or by chemical degradation. Specifically, when gas turbines operate in dusty environments, they can ingest silicate powders, which then form molten deposits based on CaO-MgO-Al2O3-SiO2 (CMAS) mixtures on the surface of hot-section components. The present work is especially focused on studying TBC systems with improved chemical resistance to molten CMAS deposits.
The first part of this Thesis studies the infiltration behaviour and reaction mechanism between the CMAS deposit and 8YSZ coatings with various kinds of microstructures: porous, layers from atmospheric plasma spraying (APS) of standard and high-purity YSZ feedstock; a dense-vertically cracked (DVC) APS layer; and a columnar YSZ coating obtained by suspension plasma spraying (SPS). It was found that CMAS attacks YSZ by dissolving its grain boundaries, and a low-purity material accelerates the dissolution by molten CMAS. It was also found that the DVC microstructure is effective for reducing the infiltration of molten CMAS.
In the second part of this work, having established the DVC microstructure as the most promising for improved resistance to CMAS corrosion, tests were carried out on three novel coating materials: Gd/Yb/Y co-doped ZrO2, Gd2ZrO7 and ZrO2-55 wt.%Y2O3. All were manufactured as DVC layers on the same type of MCrAlY bond coat. Porous and DVC 8YSZ were employed as terms of comparison. In addition, six ceramic bilayers systems were also tested, where 8YSZ with either porous or DVC microstructure was employed as a bottom layer under a DVC top layer of either Gd/Yb/Y co-doped ZrO2, Gd2ZrO7 or ZrO2-55 wt.%Y2O3. These systems were subjected to CMAS corrosion tests and thermal cycling fatigue (TCF) tests. Multilayered coatings showed longer thermal cycling fatigue life compared to monolayer coatings. On the other hand, CMAS tests showed that the novel materials do exhibit improved corrosion resistance. DVC Gd2ZrO7 layers, in particular, exhibited excellent CMAS corrosion resistance because the formation of a solid Gd-apatite layer at the interface with molten CMAS blocked further reaction and slowed down CMAS penetration. The combination of a Gd2ZrO7 top layer with a porous 8YSZ bottom layer shows enhanced resistance to thermal cycling fatigue. Although the bi-layer system does not attain the same TCF resistance of pure YSZ, the combination between reasonable TCF life and excellent CMAS resistance makes it a good choice for turbines operating in demanding environmental conditions
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
TiC–NiCr thermal spray coatings as an alternative to WC-CoCr and Cr3C2–NiCr
No full textTiC-based hardmetal coatings containing 25 or 40 vol% Ni-20 wt%Cr matrix (hereafter TiC–25NiCr and TiC–40NiCr) were obtained by High Velocity Oxygen-Fuel (HVOF) and High Velocity Air-Fuel (HVAF) spraying, starting from high-energy ball milled feedstock powders. These coatings are intended as critical raw materials-free solutions against wear and corrosion.
HVOF-sprayed coatings contain some more oxide inclusions than do HVAF ones, but, irrespective of the deposition conditions, TiC–40NiCr coatings are usually somewhat harder (800–900 HV0.3) than TiC–25NiCr ones. They also exhibit lower wear rates in ball-on-disc sliding tests against Al2O3 at room temperature. A hard asperity can indeed penetrate slightly deeper into TiC–25NiCr, as it deforms inelastically through microcracking. Bigger abrasive grooves are thus produced.
The wear resistance of TiC–40NiCr coatings compares favourably to that of a Cr3C2-25% (NiCr) reference, and even approaches that of WC-10 wt%Co-4wt.%Cr. TiC–40NiCr coatings are also more corrosion resistant than both reference materials when tested by electrochemical polarization in a 3.5% NaCl solution.
At 400 °C, to the contrary, TiC–25NiCr coatings exhibit better sliding wear resistance, whilst more severe abrasive grooving and adhesive tearing affect TiC–40NiCr samples. TiC–NiCr coatings are also unaffected by the transverse macro-cracking that was found to compromise the usefulness of WC-CoCr at 400 °C
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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