1,721,473 research outputs found
Luce Fabbri e o antifascismo
No full textExistem diversas conexões entre Luce Fabbri e Maria Lacerda de Moura, e uma delas é, definitivamente, o antifascismo. Luce Fabbri viveu com intensidade os acontecimentos do século XX: nasceu envolta às ideias anarquistas (Roma, 1908); com a ascensão do fascismo, passou por experiências de exílio (França e Uruguai); na América do Sul, enfrentou ameaças de governos antidemocráticos. Faleceu em Montevideo, capital do Uruguai, em 2000, e teve a efémera sorte de vislumbrar um mundo diferente d..
Gli obituari delle confraternite udinesi dei Fabbri e degli Alemanni
No full textIl volume racchiude le edizioni degli obituari di due confraternite udinesi bassomedievali: quella dei Fabbri e quella degli Alemanni. I manoscritti editi rappresentano ognuno a modo suo la storia e la compagine sociale dei sodalizi che li allestirono e vi annotarono i nomi dei propri defunti.
Fondata all'inizio del Trecento nel nome di san Nicolò e con intenti prettamente caritativi e assistenziali, la fraterna dei Fabbri si dotò di un obituario già alla metà del secolo, utilizzandolo in un arco di tempo esteso fino ai primi decenni del Cinquecento. Della seconda metà del Quattrocento, e con più rade annotazioni riferibili a meno di un cinquantennio, è invece l’obituario della fraterna, di carattere etnico, degli Alemanni.
Le edizioni dei due documenti sono precedute da altrettanti studi introduttivi che, in consonanza con gli interessi e le competenze delle due autrici (Laura Pani per i Fabbri e Vittoria Masutti per gli Alemanni) si soffermano rispettivamente sugli aspetti paleografici del primo manoscritto, e dunque sulla cultura grafica di coloro i quali in quasi due secoli vi registrarono obiti e lasciti, e su presenza e ruolo dei cittadini stranieri nel tessuto storico e sociale della Udine tre- e quattrocentesca
L’analisi di una foto e il metodo semiotico. A proposito del saggio di Paolo Fabbri e Tiziana Migliore sulla foto 14 maggio 1977
No full textLettura critica di un saggio di Paolo Fabbri e Tiziana Migliore sull'analisi di una foto-simbolo degli «anni di piombo».[...
Tailoring materials for intermediate temperature solid oxide fuel cells (IT-SOFCs) based on ceramic proton conducting electrolyte
Full text linkEsistono attualmente varie ragioni per cui ampio interesse scientifico e tecnologico è rivolto verso sistemi di generazione di energia alternativi rispetto ai metodi convenzionali (quali i sistemi a turbine o i motori a combustione interna). Dal punto di vista ecologico, cresce il bisogno di ridurre la produzione di sostanze inquinanti per far fronte a uno sviluppo sostenibile. Da un punto di vista socio-economico, invece, aumenta il bisogno di far fronte a un continuo aumento della richiesta di energia, mentre nello stesso tempo le principali fonti di energia, quali i combustibili fossili, si stanno esaurendo. E infine, da un punto di vista socio-politico, la scarsità delle attuali fonti di energia sta creando drammatiche tensioni tra le varie aree economiche del mondo. La diminuzione della dipendenza mondiale dai combustibili fossili e l’introduzione di forme di generazione di energia alternative potrebbero sanare tale situazione.
Il concetto di energia alternativa è stato introdotto già da vari anni. Ci sono diverse fonti d energia alternativa, come l’energia solare, l’energia eolica o la fusione nucleare. Un diverso approccio consiste nello sviluppo di sistemi di generazione di energia alternativi, che siano in grado di lavorare con alti rendimenti e di limitare al massimo la produzione di inquinanti. Allo stato attuale i motori a combustione interna presentano un’efficienza totale del 20-30%. Questo significa che solo il 20-30% dell’energia termica contenuta nel gasolio viene utilizzata come lavoro meccanico. Alti rendimenti si traducono, invece, in costi ridotti per unità di lavoro prodotto.
Le celle a combustibile sono sistemi di conversione di energia alternativi i quali permettono la conversione diretta dell’energia chimica dei reagenti in energia elettrica, producendo al contempo basse emissioni inquinanti.
Tra i diversi tipi di celle a combustibile, le celle a ossidi solidi (SOFCs) presentano vari vantaggi; tra i primi, lavorando ad alta temperatura (800-100°C) queste celle raggiungono valori di rendimento molto alti, permettono l’uso di diversi combustibili e l’unica emissione inquinante rilevante è quella di CO2, la quale rimane comunque un terzo di quella emessa da un motore a combustione interna a parità di kW/h prodotti. Tuttavia le alte temperature di lavoro comportano anche degli svantaggi: materiali costosi, elevati stress termici, difficoltà nel sigillare la cella, lunghi tempi di accensione e spegnimento del sistema. Per risolvere questi problemi la ricerca è orientata nell’abbassare la temperatura di funzionamento delle SOFCs nel cosiddetto range di temperature intermedie (400-700°C).
Abbassare la temperatura di funzionamento si traduce in un peggioramento delle performance dei vari componenti della cella, e per questo lo studio di nuovi materiali risulta essenziale nella prospettiva di rendere le SOFC commercializzabili.
Lo scopo di tale lavoro di tesi è appunto lo studio di materiali elettrolitici ed elettrodici che presentino buone proprietà conduttive a temperature di funzionamento intermedie e che allo stesso tempo siano chimicamente stabili.
Nel capitolo 1A della tesi si presentano i principi basilari di funzionamento di una SOFC e una breve illustrazione dei materiali più studiati in letteratura sia per le alte e intermedie temperature di funzionamento. In particolare, tra i materiali ceramici con buone proprietà conduttive a basse temperature si trovano i conduttori protonici. Nel Capitolo 2A vengono illustrate le principali proprietà chimico-fisiche ed elettrochimiche di tali materiali ceramici.
Molti ossidi perovskitici presentano conduzione protonica a temperature intermedie quando esposti ad atmosfera di idrogeno e/o vapore acqueo. Tuttavia nessuno di questi ossidi presenta contemporaneamente le due proprietà essenziali richieste ad un buon elettrolita: alta conducibilità ionica e buona stabilità chimica.
La seconda parte della tesi presenta i risultati del lavoro sperimentale svolto, il quale è stato rivolto alla preparazione e caratterizzazione di conduttori protonici ceramici elettrolitici con alta conducibilità e buona stabilità chimica e allo sviluppo di elettrodi a - hoc per tali elettroliti.
Il Capitolo 1B riporta l’ottimizzazione di una tecnica di sintesi sol gel per produrre i seguenti conduttori protonici: BaZr0.8Y0.2O3-δ (BZY) e BaCe0.8Y0.2O3-δ (BCY). Attraverso il metodo di sintesi ottimizzato si sono sintetizzate fasi singole dei suddetti composti. Le basse temperature di calcinazione richieste dal processo hanno portato a polveri di particelle nanometriche. I due composti sono stati sinterizzati in forma di pasticche circolari e caratterizzati elettricamente mediante spettroscopia di impedenza. Inoltre sono stati svolti test termici in flusso di anidride carbonica per valutare la stabilità chimica dei due composti, osservando una buona stabilità solo nel caso del BZY. Tuttavia le performance in cella di tale elettrolita si sono rilevate insufficienti rispetto ai target richiesti per la commercializzazione.
Nel Capitolo 2B si è cercato di implementare le prestazioni del BZY sostituendo nel sito B della struttura perovskitica diverse quantità di Ce. Gli elettroliti cosi prodotti sono stati analizzati ai raggi X, sotto il punto di vista della stabilità chimica e della conducibilità elettrica. Il miglior compromesso tra stabilità chimica e conducibilità elettrica è risultato il composto con stechiometria BaZr0.5Ce0.3Y0.2O3-δ.
Un ulteriore miglioramento della conduzione elettrica rispetto al BaZr0.5Ce0.3Y0.2O3-δ, pur mantenendo un’ottima stabilità chimica, è stato ottenuto realizzando un elettrolita “a doppio strato”, il quale è descritto nel Capitolo 3B. Una pasticca spessa 1 mm di BCY è stata protetta con uno strato sottile (circa un micron) di BZY cresciuto tramite la tecnica di deposizione a laser pulsato. Questo nuovo elettrolita ha presentato elevata conducibilità e buone prestazioni in cella in termini di stabilità chimica e densità potenza fornita.
Nel Capitolo 4B si sono invece investigati elettrodi funzionali per tali elettroliti a conduzione protonica. Un catodo composito e stato realizzato unendo un conduttore misto ionico/elettronico, La1-xSrxCo1-yFeyO3-δ (LSCF), e un conduttore misto protonico/elettronico BaCe0.9Yb0.1O3-δ (10YbBC). L’uso di catodi compositi aumenta i siti di reazione al catodo, diminuendo quindi le cadute di potenziale dovute alle reazioni catodiche.There are increasing reasons to explore alternatives to conventional energy generation methods (that is to say coal-fired steam turbine and gasoline internal combustion engine). From an ecological point of view, there is the need to reduce the polluting by-products of conventional energy generation. From a socio-economical standpoint, the worldwide demand for energy continues to rise as more and more nations join the group of the industrialized countries, while hydrocarbon fuels go to exhaustion. Finally, from a socio-political perspective, the situation described above has created several and often dramatic tensions between different world economic areas, as evidenced by frequent wars. Lowering the global dependence on oil might reduce such tensions. However, despite all of this, changes in the energy generation methods are extremely slow, as evidenced by the wide (if we cannot say total) use of the internal combustion engine.
The concept of alternative energy has been introduced a long time ago. Several different sources of energy are proposed, which can have the potential to replace conventional generation methods. Popular examples include solar radiation, wind motion, and nuclear fusion. Each of these technologies has its own set of problems that have slowed down its commercialization, but much research is being conducted to overcome these problems. In fact, the research towards the development of alternative, highly efficient, eco-friendly energy production technologies is expanding. There is a general push towards higher efficiencies. At present, automobiles based on internal combustion engines have an overall efficiency of about 20-30%. That is, only 20-30% of the thermal energy content of the gasoline is converted into useful mechanical work and the rest is wasted. Higher efficiencies translate into reduced energy costs per unit of work done.
Fuel cells, an alternative energy technology, have received growing interest in recent years since they represent one of the most promising energy production systems to reduce pollutant emissions. They are electrochemical devices that allow the direct conversion of chemical energy into electrical energy.
Among the different type of fuel cells, solid oxide fuel cells (SOFCs) offer great promise as a clean and efficient technology for energy generation and provide significant environmental benefits. They produce negligible hydrocarbons, CO or NOx emissions, and, as a result of their high efficiency, about one-third less CO2 per kW/h than internal combustion engines. Unfortunately, the current SOFC technology based on a stabilized zirconia electrolyte requires the cell to operate from 700 to 1000°C to avoid unacceptable ohmic losses. These high operating temperatures demand specialized (expensive) materials for fuel cell interconnectors, long start-up time, and large energy input to heat the cell up to the operating temperature. Therefore, if fuel cells could be designed to give a reasonable power output at intermediate temperatures (IT, 400-700°C), tremendous benefits may result. In particular, in the IT range ferrite steel interconnects can be used instead of expensive and brittle ceramic materials. In addition, sealing becomes easier and more reliable; rapid start-up is possible; thermal stresses (namely, those caused by thermal expansion mismatches) are reduced; electrode sintering becomes negligible. Combined together, all these improvements result in reduced initial and operating costs.
Therefore, the major trend in the present research activities on SOFCs is the reduction of the operating temperature. The problem is that lowering the operating temperatures lowers the electrolyte conductivity, whereas the electrode polarization greatly increases, reducing the overall fuel cell performance. Considering the described scenario, it is clear how the study of materials assumes a considerable role in lowering SOFC operating temperature.
Making SOFCs commercially competitive with conventional energy generation methods means developing a highly efficient and environmental friendly energy production device to provide for a global sustainable energy system. IT-SOFCs represent not only a laboratory research activity, but a great challenge for the entire society.
The purpose of the present dissertation is the development of a stable highly-conductive electrolyte and performing electrodes for lower temperature SOFCs.
Chapter 1A presents the physico-chemical principles of SOFCs functioning, the demands imposed on the components materials, together with a literature survey on the state of-the art technology. Starting from more “conventional” oxygen ion conducting electrolytes, the need for reducing the operation temperature leads to a discussion on the properties of proton conducting materials as a feasible alternative to reach the goal of fabricating an IT-SOFCs.
Chapter 2A describes the main properties of ceramic proton conductors. Several perovskite-type oxides, such as doped BaCeO3, SrCeO3, BaZrO3, and SrZrO3, show proton conductivity in the IT range when exposed to hydrogen and/or water vapour containing atmospheres. They are generally known as high temperature proton conductors (HTPCs). The main challenge in the field of HTPC is to find a compound that concurrently satisfies two of the essential requirements for fuel cell application, namely high proton conductivity and good chemical stability under fuel cell operating conditions.
The second part of this dissertation describes the experimental results achieved during the research carried out.
In view of the considerations given in Chapter 2a, Chapter 1B describes the optimization of the sol-gel procedure to prepare BaZr0.8Y0.2O3-δ (BZY) proton conductor electrolyte. Producing BZY powders with controlled compositional homogeneity and microstructure using a proper synthesis method could improve the electrochemical performance of this electrolyte. The optimized sol–gel procedure allowed the reduction of the diffusion path up to a nanometric scale, and thus required lower calcination temperatures. Nanocrystalline single-phase powders of BZY were produced at temperatures as low as 1100 °C. The same sol-gel procedure was also used to synthesize BaCe0.8Y0.2O3-δ (BCY) proton conductor electrolyte achieving also in this case nanometric particles powder at the calcination temperature of 100°C. The performance of the synthesized BZY and BCY proton conductors were examined in terms of chemical stability. After exposure to CO2 at high temperatures, the synthesized BZY powders presented good chemical and microstructural stability, differently from BCY which strongly decomposed after the CO2 treatment. Electrical conductivity and fuel cell performance were investigated only for the stable BZY electrolyte, however without achieving the required performance for practical application.
Chapter 2 presents the application of the optimized synthetic procedure to the preparation of different proton conductor electrolytes. To further improve the electrochemical performance of barium zirconate electrolyte, the B-site of the BZY perovskite structure was doped with Ce producing several BaZr0.8-xCexY0.2O3-δ compounds (0.0≤x≤0.8). The prepared samples were analyzed in terms of chemical stability in CO2 environment, electrical conductivity, microstructural characteristics, and finally under fuel cell tests. Among the tested electrolytes, the BaZr0.5Ce0.3Y0.2O3-δ composition represented the best compromise between electrical performance and chemical stability. In fact it was able to maintain almost the same chemical stability of BZY, but with improved, more than twice, fuel cell performance.
Chapter 3 describes a further improvement of the HTPC electrolyte performance. To obtain a highly conductive and chemically stable proton conductor electrolyte, a sintered Y-doped barium cerate (BCY) pellet was protected with a thin BZY layer, grown by pulsed laser deposition. The overall performance of the bilayer electrolyte turned out to be of great interest for practical use in IT-SOFCs application. The promising performance of this bilayer electrolyte rose from the very good crystallographic matching at the interface between the two materials, as well as the microstructure properties of the protecting layer in terms of uniformity, density and filling factor. However, while the bilayer conductivity was only slightly smaller than the conductivity of the BCY pellet, the measured fuel cell performances were negatively affected by the interface of the Pt electrodes with the BZY layer. For this reason the development of a superior cathode is crucial to make IT-SOFCs based on proton conductors competitive with the more established SOFCs using oxygen-ion conductor electrolytes.
Chapter 4 focuses on the optimization of composite cathodes for application in IT-SOFC based on HTCP electrolytes. To explore different cathode materials with respect to the most commonly used for proton conductor electrolytes, such as platinum or cobalto-ferrites, the area specific resistance (ASR) of composite cathodes was investigated. Firstly, BaCe0.9Yb0.1O3-δ (10YbBC) and SrCe0.9Yb0.1O3-δ (10YbSC) were tested as cathode materials since they show mixed protonic-electronic conductivity. However, the ASR of the interface of these cathode materials with Y-doped barium cerate proton conductor electrolyte was extremely large, probably because of their too low partial electronic conductivity. For this reason, La1-xSrxCo1-yFeyO3-δ (LSCF), which presents high electronic conductivity, was combined with 10YbSC or 10YbBC to form composite cathodes. LSCF was chosen also because it allows faster oxygen surface exchange being a mixed O2-/e- conductor. The lowest ASR values were achieved with the composite cathode made of LSCF and 10YbBC in a1:1 ratio. Single phase Pt and LSCF cathodes were tested and it was found that they showed higher interfacial resistance than LSCF/10YbBC(1:1) composite cathode. This finding clearly suggests the importance of the proton conductor phase within the electrode, which actually should increase the triple phase boundary (TPB) density and so improve the cathode performance. The good performance observed for LSCF/10YbBC(1:1) composite cathode make it a cheaper and more efficient alternative to the Pt cathode that can actually improve the performance of IT-SOFCs based on proton conductor electrolytes
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
Cyclic AMP signaling in bivalve molluscs: an overview.
No full textThe present review aims at examining the information available on bivalve molluscs, from the first studies carried out in the early 1980s to the last progresses made in the present days. The major focus is on the structural and operational characteristics of the main actors of the signaling pathway, i.e., adenylyl cyclase, G proteins, and protein kinase A, and on the role played by the cyclic nucleotide on smooth muscle, heart, gills, gonads, and metabolism regulation. Moreover, recent evidence regarding the cAMP system as a target of environmental stress factors are discussed
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