1,720,998 research outputs found
Analisi dinamico meccanica ultrasonora di materiali polimerici
No full textLe onde ultrasonore, vibrazioni meccaniche con frequenza superiore alla soglia dell’udibile umano (16-20 kHz), si propagano nei materiali in conseguenza di una serie di spostamenti continui molto piccoli di atomi e segmenti di catene intorno alle loro posizioni di equilibrio a causa delle forze intramolecolari all’interno di un segmento di catena ed intermolecolari tra segmenti di catena adiacenti.
La propagazione di onde acustiche nei polimeri dipende dalle loro proprietà viscoelastiche e densità, risultando significativamente influenzata dalle transizioni di fase che avvengono al variare di temperatura, pressione, stress, reazioni chimiche, ecc. Pertanto, l’applicazione di ultrasuoni di opportuna frequenza e intensità nei polimeri può essere considerata come una sorta di analisi dinamico-meccanica ad alta frequenza, in grado di monitorare il comportamento viscoelastico dei materiali polimerici, per esempio durante la transizione vetrosa, le reazioni chimiche di reticolazione, la cristallizzazione, la gelificazione fisica o chimica, il processing. Si noti che, a causa dell’elevata frequenza delle oscillazioni periodiche imposte al campione e della loro piccola ampiezza, l’analisi dinamico-meccanica ultrasonora è relativa alla mobilità a corto raggio di piccoli segmenti di catena, che in genere non coinvolge gli entanglements.
Tra i vari tipi di onde ultrasonore, le più adatte per l’analisi dinamico-meccanica dei materiali polimerici risultano essere le onde longitudinali e quelle di shear, con la distinzione che le prime possono propagarsi senza difficoltà anche nei liquidi e quindi risultano di utilizzo più generale.
Durante una misura ultrasonora, si calcolano velocità (c) e attenuazione (), parametri sufficienti a caratterizzare un materiale dal punto di vista acustico. La prima è la velocità di propagazione delle onde acustiche ed è calcolata sulla base del tempo di volo misurato, il tempo necessario all’onda per attraversare il campione, risultando proporzionale alla componente elastica del materiale.
L’attenuazione è una misura delle perdite di energia meccanica (convertita in calore) dovute sia ad assorbimento sia a scattering delle onde ultrasonore nel materiale. Il contributo dello scattering è notevole quando il mezzo è non omogeneo e contiene particelle con dimensioni confrontabili con la lunghezza d’onda. L’assorbimento è invece legato a fenomeni di rilassamento del materiale, che avviene a causa del tempo finito necessario affinché in un solido l’energia assorbita nel semiperiodo positivo dell’onda venga restituita nel semiperiodo negativo, a causa dello scambio energetico tra moto di traslazione e moti vibrazionali e rotazionali delle molecole. Pertanto, in un materiale polimerico l’attenuazione è fortemente influenzata dalle variazioni strutturali delle macromolecole, come transizione vetrosa, fusione, cristallizzazione e reazioni chimiche.
Nel seguito dell’articolo sarà descritto l’apparato sperimentale sviluppato nel laboratorio di Materiali Polimerici dell'Università di Lecce per l’analisi ultrasonora e saranno accennate alcune applicazioni nel campo dei materiali polimerici
Agroecological indicators of cropping systems: the phosphorus indicator in the Sud Milano agricultural park (northern Italy)
Full text linkAgroecological indicators of cropping systems: nutrient budgets in the Sud Milano agricultural park (northern Italy)
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Soil microbiome before and after fertilization with bovine manure
No full textBackground: Dairy manure is commonly used for the fertilization of agricultural soils, and it includes the intestinal microbial community influenced by the feed and possible drug residues used for therapy. The excessive use of antimicrobials in animal husbandry and subsequent land application of animal wastes may introduce massive quantities of antimicrobials and resistant bacteria into the soil environment.
Objective: The objective of this paper is to evaluate the possible change of the soil microbiome after fertilization with manure coming from intensive dairy farms.
Methodology: Samples of manure and soil (before and after 30 days from fertilization) were collected from intensive dairy farms. Microbioma of samples was determined by a culture independent approach based on NGS sequencing. All samples were also screened for detecting and quantifying the most commonly used antimicrobials (i.e. beta-lactams, fluoroquinolones, polymyxins and macrolides) by HPLC methods.
Results: All samples were negative for the antimicrobials screened. The manure microbioma was homogenous among different farms and regions of Northern Italy, but very different from the soil one. In manure, a higher presence of Porphyromonadaceae, Lachnospiraceae, Clostridiaceae, Pseudomonadaceae and Ruminococcaceae families was observed, whereas in soil the Chitinophagaceae family was the most commonly detected. After fertilization, the microbioma composition of soil was not changed, however the absence of drugs concentration in manure may have had a role in this.
Conclusions / Implications for practice: This is one of the first study evaluating the modifications of the microbial communities of dairy manure and agricultural soil before and after fertilization in Northern Italy
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
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