1,720,991 research outputs found
Structural and compositional gradients in alternating current sintered aluminum-doped zinc oxide
No full textFlash sintering revitalizes the exploration of electric field and current effects on the microstructure with novel phenomena including rapid densification, low-temperature sintering, and unique microstructures. The aim of this study is to investigate the influence of electric current on the microstructural development of Al-doped ZnO (nAl=4at.%). We apply two sintering techniques, namely spark plasma sintering (SPS) and flash spark plasma sintering (FSPS), both conducted under identical conditions (AC, f=50Hz, |E→0|=25V/cm, p=180MPa) for three different temperatures 600∘C, 800∘C and 1000∘C. The distinguishing factor between the two techniques lies in the passage of electric current flow through the specimens in the case of FSPS. The resulting microstructures of the sintered pellets are examined by XRD, SEM, EDX, TEM and SAED to identify the process-related differences. While SPS specimens reveal a homogeneous microstructure, FSPS specimens exhibit a complex microstructure which consists of two distinct regions. The major volume of the FSPS pellets exhibits a structural gradient with coarse ZnO grains and ZnAl2O4 spinel precipitates in the center to fine ZnO grains at the periphery. This microstructure results from Joule heating and geometry-dependent heat dissipation, creating a symmetric temperature profile with a hot center. A minor volume of the FSPS specimens shows a gradient in microstructure and composition. Here, ZnAl2O4 spinel precipitates accumulate in the center and the periphery reveals Al-depleted, dense and very coarse grains. Electromigration of Al-cations and the thermodynamically favored formation of ZnAl2O4 in the hot center are proposed as underlying causes for the observed microstructure. The electric current in FSPS significantly alters the microstructure and composition of sintered specimen where dopant electromigration is most significant under high current densities above |J→|=29(5)A/cm2
Entwicklung einer Methode zur in-situ Flugzeit-massenspektrometrischen Untersuchung des Hochtemperatur-Wachstums von Siliziumkarbid
No full textDas Ziel dieser Arbeit war der Aufbau mit anschließendem Testbetrieb eines Reaktor-
Analyse-Systems zur Herstellung und Untersuchung des Wachstums von SiC-Einkristallen.
Die Apparatur besteht aus einem industriell ausgelegten Hochtemperatur-
CVD- (HTCVD-) Reaktor und einem Flugzeit-Massenspektrometer (TOF). Mit dem
Aufbau ist es möglich, alle am beteiligten Prozess beteiligten Spezies ohne vorherige
Kenntnis der Gaszusammensetzung simultan nachzuweisen. Die am Prozess beteiligten
Spezies sind bisher kaum bekannt, obwohl nur bei eindeutiger Identifizierung
der reagierenden Gasphasenspezies eine Modellvorstellung über den Ablauf der Reaktionen
entwickelt werden kann. Ein solches Modell würde eine wissensbasierte
Steuerung des Prozesses ermöglichen. Der Vorteil des HTCVD-Prozesses im Vergleich
zu herkömmlichen CVD-Prozessen ist die höhere Temperatur, durch die eine
Verringerung der Wachstumsrate durch homogene Gasphasenkeimbildung umgangen
wird.
Für die Untersuchung wurden in dieser Arbeit alle Komponenten der HTCVDMassenspektrometrie
konstruiert, aufgebaut und in das Gesamtsystem integriert.
Die Entwicklung geeigneter Untersuchungsprotokolle und die Datenauswertung waren
ebenfalls wesentliche Arbeitspunkte. Zunächst wurden erste „Proof of Principle“-
Experimente durchgeführt. In der ersten Versuchsreihe wurden die Präkursoren bei
konstanter Temperatur in den heißen Reaktor eingeleitet und die Partikelbildung
beobachtet. In einer zweiten Versuchsreihe wurden die Präkursoren in den kalten
Reaktor eingeleitet und die Zersetzung als Funktion der Temperatur beobachtet. In
einer weiteren Versuchsreihe wurde der Druck bei konstanter Gaszusammensetzung
und Temperatur variiert und der Einfluss auf die Größe der detektierten Spezies beobachtet.
Es zeigte sich, dass in den ersten beiden Versuchsreihen die gleichen Spezies
beobachtet wurden. Bei höheren Drücken wurden größere Massen detektiert. Verbesserungspotenzial
besteht hinsichtlich der Probenahmedüse. Sie setzt sich durch
die Partikelbildung im Reaktor zu. Eine mögliche Lösung des Problems besteht darin,
den Prozess bei höheren Temperaturen durchzuführen, da die Partikel dann vor
dem Erreichen der Düse sublimieren.
Die Ergebnisse der ersten Messungen belegen eindeutig, dass das Gesamtsystem
vollständig funktionstüchtig und prinzipiell geeignet ist, den HTCVD-Prozess
in-situ zu untersuchen. Es steht für weitere systematische Untersuchungen zur Verfügung.
Weiterhin belegen die ersten Messungen, dass kleine SiC-Gasphasenspezies
am Prozess des Kristallwachstums beteiligt sind.The objectives of this work were the development and operational tests of a reactor
and analysis system for synthesizing and studying the growth of SiC-crystals. The
apparatus consists of an industrially designed high-temperature CVD reactor (HTCVD)
reactor and a time-of-flight mass spectrometer (TOF). With this setup, it is
possible to detect all species simultaneously, which contribute to the growth process
without any prior knowledge of the gas composition. Even though the species
that are involved in the growth processes are mostly unknown, only unambigous
identification of the reacting gas phase species, allows the development of a model
describing the process. Such a model would allow a knowledge-based control of the
process.
The advantage of the HTCVD process compared to conventional CVD processes
is the higher temperature that prevents the reduction of the growth rate by homogeneous
gas phase nucleation. The design, building and integration of all essential
components were integral parts of this work. In additon, appropriate study protocols
and data analysis routines were developed.
Proof-of-principle experiments were carried out. In the first series of experiments,
the precursors are introduced at a constant temperature into the hot reactor and the
particle formation is observed. In the second series of experiments, the precursors
are fed into the cold reactor and the precursor decomposition as a function of the
temperature is observed. In another series of experiments, the pressure was varied
at a constant temperature and gas composition and the influence of pressure on the
size of the detected species is observed. In the first two experiments the same species
are observed. At high pressures, larger masses are detected.
The experimental observation of the process is limited by blockage of the sampling
nozzle that transfers gas from the reactor to the mass spectrometer due to
particle formation. One of the ways to overcome this problem is to perform the process
at higher temperatures than in the intial experiments where the particles will
sublimate before reaching the nozzle. The results of the first measurements demonstrate
that the complete system is fully functional and is in principle suitable for the
in-situ investigation of the HTCVD process. Further systematic investigations on
the growth process can be carried out building on the intial results. Furthermore,
the initial experiments indicate that small SiC gas phase species are involved in the
process of crystal growth
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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