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Lösungs-Epitaktisch gewachsene Metallhalogenid-Perowskit Mikrokristall-Laser
Full text linkEpitaxy is a process to grow high-quality crystalline materials in a highly ordered manner, which is a key requirement for the development of modern electronic and optoelectronic devices. In the last few years, metal halide perovskites have revolutionized the field of optoelectronics due to their astonishing properties. Thus, they are promising to compete against conventional semiconductors for certain applications in electronics or optoelectronics. Moreover, their easy solution processing promises low-cost and facile device fabrication as well as high throughput via printing or coating techniques. Recently, perovskites have been demonstrated as efficient laser gain media with high optical gain coefficients comparable to those of commercial lasers. Furthermore, their self-assembled microcrystals can act as optical resonators, offering the fabrication of compact lasers with the potential for integration and use in a wide range of applications. Therefore, the solution epitaxy of perovskites represents a significant step forward toward economical practical applications. Specifically, the epitaxial perovskite microcrystal lasers will advance the development of on-chip integrated miniaturized coherent light sources.
In this doctoral thesis, the epitaxial growth of oriented high-quality microcrystals from formamidinium lead bromide and methylammonium lead bromide perovskites is demonstrated by facile solution methods. The drop casting method and antisolvent vapor-assisted crystallization are introduced to expand the current solution-epitaxial methods of perovskites. Various crystal geometries are grown from both materials, and the growth methods offer the possibility to control the crystals’ sizes and shapes under certain conditions. The microcrystals serve as optical resonators, producing the first solution-epitaxial perovskite microcavity laser. The lasing is achieved under optical pumping with a record low threshold for the epitaxially grown perovskite microcavities. The different geometry microcavities are demonstrated to support various resonant optical modes. Remarkably, the microcavities exhibit high lasing emission stability, which is a record value for the perovskite lasers tested in the air. Furthermore, single-mode lasing –a desirable operation for optical communications– is successfully realized via increasing the lasing mode spacing by decreasing the size of the pumped microcavities. These promising results emphasize that solution-epitaxial perovskite microlasers are competitive to vapor-epitaxial ones, paving the way toward low-cost and energy-efficient fabrication of high-quality optoelectronics.Bei der Epitaxie handelt es sich um ein Verfahren, mit dem hochwertige kristalline Materialien in einer hochgradig geordneten Weise gezüchtet werden können, was eine wichtige Voraussetzung für die Entwicklung moderner elektronischer und optoelektronischer Geräte ist. In den letzten Jahren haben Metallhalogenid-Perowskite aufgrund ihrer erstaunlichen Eigenschaften den Bereich der Optoelektronik revolutioniert. Damit sind sie vielversprechend, um bei bestimmten Anwendungen in der Elektronik oder Optoelektronik mit herkömmlichen Halbleitern zu konkurrieren. Darüber hinaus verspricht ihre einfache Verarbeitung in Lösung eine kostengünstige und einfache Herstellung von Bauelementen sowie einen hohen Durchsatz durch Druck- oder Beschichtungsverfahren. Kürzlich wurden Perowskite als effiziente Lasergewinnungsmedien mit hohen optischen Gewinnkoeffizienten nachgewiesen, die mit denen kommerzieller Laser vergleichbar sind. Darüber hinaus können ihre selbstorganisierten Mikrokristalle als optische Resonatoren fungieren, was die Herstellung kompakter Laser mit dem Potenzial für die Integration und den Einsatz in einer Vielzahl von Anwendungen ermöglicht. Daher stellt die Lösungsepitaxie von Perowskiten einen bedeutenden Schritt in Richtung wirtschaftlicher praktischer Anwendungen dar. Insbesondere werden die epitaktischen Perowskit-Mikrokristall-Laser die Entwicklung von auf dem Chip integrierten, miniaturisierten kohärenten Lichtquellen vorantreiben.
In dieser Dissertation wird das epitaktische Wachstum von orientierten, hochwertigen Mikrokristallen aus Formamidinium-Bleibromid und Methylammonium-Bleibromid-Perowskiten durch einfache Lösungsmethoden demonstriert. Auftropfen und die dampfunterstützte Kristallisation mit Anti-Lösungsmitteln werden eingeführt, um die derzeitigen lösungs-epitaktischen Methoden für Perowskite zu erweitern. Aus beiden Materialien werden verschiedene Kristallgeometrien gezüchtet, und die Wachstumsmethoden bieten die Möglichkeit, die Größe und Form der Kristalle unter bestimmten Bedingungen zu kontrollieren. Die Mikrokristalle dienen als optische Resonatoren, wodurch der erste lösungs-epitaktische Perowskit-Mikroresonatorlaser entstanden war. Das Lasern wird unter optischem Pumpen mit einer rekordverdächtig niedrigen Schwelle für die epitaktisch gewachsenen Perowskit-Mikrokavitäten erreicht. Die Mikrokavitäten mit unterschiedlicher Geometrie unterstützen nachweislich verschiedene optische Resonanzmoden. Bemerkenswerterweise weisen die Mikrokavitäten eine hohe Laseremissionsstabilität auf, was einen Rekordwert für die an der Luft getesteten Perowskitlaser darstellt. Darüber hinaus wird Single-Mode-Lasertätigkeit - ein wünschenswerter Vorgang für die optische Kommunikation - erfolgreich durch Vergrößerung des Laser-Modenabstands durch Verkleinerung der gepumpten Mikrokavitäten realisiert. Diese vielversprechenden Ergebnisse unterstreichen die Konkurrenzfähigkeit der lösungs-epitaktischen Perowskit-Mikrolaser mit epitaktisch aufgedampften Lasern und ebnen den Weg für eine kostengünstige und energieeffiziente Herstellung hochwertiger Optoelektronik
Effect of coal fly ash addition on the properties of aluminum alloy composites
No full textIn the this PhD study, Hellenic, Class C fly ashes were utilized for the synthesis of Al- and Al-alloy-matrix composites by means of Powder Metallurgy, Pressure Infiltration, and Stir Casting techniques. In the case of Powder Metallurgy (P/M), aluminum–fly ash and aluminum/silicon alloy (Al-12Si)-fly ash mixtures, containing 5, 10, 15, and 20 wt.% fly ash were prepared and compacted. The green products were sintered for 2 and 6 hours in inert atmosphere. The composites are tested for: a) their microstructure, b) chemical composition, c) mineralogy, d) tribological performance, e) hardness, and f) corrosion behavior. The microscopic studies of green and sintered compacts clearly indicated the effectiveness of sintering process. Moreover, it was shown that the addition of both types of ash enhanced the tribo-performance of Al, with the optimum metal powder replacement determined to the point of 15% wt., in the case of high-Si and 10% wt., in the case of high-Ca ash particles. Corrosion resistance of P/M Al- and Al-alloy composites is declined compared to the matrix materials. In the case of pressure infiltration, KFA and MFA were separated into their particle fractions by means of manual screening. Narrow-sized particle fractions were selected so as to better facilitate homogeneity into the composites. Before they are incorporated into composites, the various fractions are characterized in terms of their chemical, mineralogical composition and thermal properties. Moreover, so as to investigate the effect of using ground ash particles on the composite properties, one specific particle fraction (25-40μm) is ground and the final specimens are comparatively examined on the basis of the effect of “as received” and ground ash particles. The composite specimens underwent the same series of characterization tests with the P/M products, with the exception of corrosion behavior. It was concluded that using fine, high-Ca ash particles can improve the properties of composites, and that using ash particles in a ground form can better facilitate the production process of MMCs. In the case of stir casting, it was verified that conducting stir mixing of fine, gradually pre-heated calcareous FA particles at a higher molten alloy-temperature (Tmelt > 900ºC, while previous attempts referred to ~700ºC) can eventually result in the incorporation of high-Ca FA particles into the melt and in the subsequent successful synthesis of lightweight Al alloy composites containing at least 10 wt. % FA, with enhanced tribo properties.Η παρούσα διδακτορική διατριβή πραγματεύεται την παρασκευή συνθέτων υλικών με μητρική φάση Al και κράματά του, και ενισχυτική φάση ιπτάμενες τέφρες από τους λιγνιτικούς ατμοηλεκτρικούς σταθμούς (ΑΗΣ) της Καρδιάς και της Μεγαλόπολης. Για την παρασκευή των συνθέτων υλικών εφαρμόζεται η τεχνική της κονιομεταλλουργίας (powder metallurgy), καθώς επίσης και τεχνικές με τη μητρική (μεταλλική) φάση υπό μορφή τήγματος: η έγχυση-υπό-πίεση (pressure infiltration) και η χύτευση-υπό-ανάδευση (stir casting). Στην περίπτωση της κονιομεταλλουργίας, το κ.β. ποσοστό αντικατάστασης της μητρικής από την ενισχυτική φάση φθάνει στο 20%, στη χύτευση-υπό-ανάδευση το 10%, ενώ στην έγχυση-υπό-πίεση το 50%, όπου πρακτικά δεν αναφερόμαστε πλέον σε καθεστώς μητρικής-ενισχυτικής φάσης, αλλά σε ισόποση συμμετοχή δυο διαφορετικών υλικών εντός του συνθέτου. Τα σύνθετα υλικά που προκύπτουν, υποβάλλονται σε έλεγχο: α) της μικροδομής τους β) της χημικής και ορυκτολογικής τους σύστασης, γ) των τριβολογικών ιδιοτήτων τους, δ) της επιφανειακής σκληρότητάς τους και ε) της ανθεκτικότητάς τους στη διάβρωση. Στην περίπτωση της παρασκευής των συνθέτων με τη χρήση τεχνικών τηγμένου μετάλλου, οι τέφρες διαχωρίζονται στα σωματιδιακά κλάσματά τους με τη χρήση των αντιστοίχων κοσκίνων. Αφότου διαπιστωθεί ότι στις συγκεκριμένες θερμοκρασίες που θα λάβουν χώρα οι διαδικασίες της έγχυσης-υπό-πίεση και της χύτευσης-υπό-ανάδευση δε συμβαίνουν φυσικοχημικές μεταβολές στις τέφρες, κατόπιν αξιολογείται η χημική και ορυκτολογική τους σύσταση σε σχέση με την πιθανή επίδρασή τους στη μορφολογία και τη μικροδομή των συνθέτων, και κατ’ επέκταση στις ιδιότητές τους. Επίσης, διερευνάται ί η επίδραση της χρήσης αλεσμένων τεφρών στα σύνθετα. Με τη χρήση της τεχνικής της έγχυσης-υπό-πίεση, παρασκευάζονται οχτώ διαφορετικές σειρές δοκιμίων με ιπτάμενη τέφρα Καρδιάς και Μεγαλόπολης, εισάγοντας στο κράμα A356 (Al-7Si-0.35Mg) σωματίδια διαμέτρου έως 90 μm. Με τη χρήση της χύτευσης-υπό-ανάδευση, παρασκευάζονται δύο σειρές δοκιμίων, η μία με τα πολύ λεπτά σωματίδια της ιπτάμενης τέφρας Καρδιάς (<25μm) και η άλλη με τα πολύ λεπτά σωματίδια της ιπτάμενης τέφρας Μεγαλόπολης (<25μm). Τα σύνθετα υλικά που παρασκευάσθηκαν με τις τεχνικές τηγμένου μετάλλου υποβλήθηκαν στις ίδιες διαδικασίες χαρακτηρισμού όπως και στην κονιομεταλλουργία, με την εξαίρεση του ελέγχου της διάβρωσης. Διαπιστώνεται ότι η ασβεστούχος σύσταση των ελληνικών ιπταμένων τεφρών δεν συνιστά εμπόδιο στην επιτυχή παρασκευή συνθέτων μήτρας Al και κραμάτων του με τις προαναφερθήσες τεχνικές
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
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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