1,720,957 research outputs found
Topology-dependent scaling laws for the stiffness and strength of nanoporous gold
No full textNanoporous gold (np-Au) differs from its macroporous counterparts through its ligament and pore length scales, its high relative density, and its very distinct mesoscale cellular architecture. When reexamining the applicability of conventional macroporous foam scaling laws to np-Au, difficulties persist not only in determining the solid properties of nanoscale ligaments, but also because np-Au structure is not self-similar as the relative density changes. Thus, a clear distinction of the effects of relative density and structure is required. This paper aims to capture the role of topology and morphology into the scaling laws by comparing the overall mechanical response of real np-Au structures with the behavior of spinodal and gyroid structures. Quantitative morphological and topological characterization of these structures has been carried out and their role on the macroscopic elastoplastic response of np-Au has been studied using finite element (FE) simulations. The predicted elastic modulus of real np-Au structures from FE simulations is in remarkable agreement with the nanoindentation measurements, and validates the numerical simulations. Quantitative structural analysis reveals that np-Au and spinodal structures are topologically very distinct, but similar in their morphology. On the other hand, gyroids are both morphologically and topologically very distinct from np-Au. The results suggest that the macroscopic stiffness and strength are highly sensitive to the topology, while being relatively much less sensitive to the morphology. The effects of structural topology are captured into modified scaling laws where the geometric pre-factors for the stiffness and strength are found to vary linearly with the scaled genus. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.DFG [MA 5785/1-1, VO 928/8-1
Characterization of Nanoporous Materials with Atom Probe Tomography
No full textA method to characterize open-cell nanoporous materials with atom probe tomography (APT) has been developed. For this, open-cell nanoporous gold with pore diameters of around 50 nm was used as a model system, and filled by electron beam-induced deposition (EBID) to obtain a compact material. Two different EBID precursors were successfully tested-dicobalt octacarbonyl [Co-2(CO)(8)] and diiron nonacarbonyl [Fe-2(CO)(9)]. Penetration and filling depth are sufficient for focused ion beam-based APT sample preparation. With this approach, stable APT analysis of the nanoporous material can be performed. Reconstruction reveals the composition of the deposited precursor and the nanoporous material, as well as chemical information of the interfaces between them. Thus, it is shown that, using an appropriate EBID process, local chemical information in three dimensions with sub-nanometer resolution can be obtained from nanoporous materials using APT
Crystal plasticity study of monocrystalline stochastic honeycombs under in-plane compression
No full textWe present a study on the plastic deformation of single crystalline stochastic honeycombs under in-plane compression using a crystal plasticity constitutive description for face-centered cubic (fcc) materials, focusing on the very early stage of plastic deformation, and identifying the interplay between the crystallographic orientation and the cellular structure during plastic deformation. We observe that despite the stochastic structure, surprisingly, the slip system activations in the honeycombs are almost identical to their corresponding bulk single crystals at the early stage of the plastic deformation. On the other hand, however, the yield stresses of the honeycombs are nearly independent of their crystallographic orientations. Similar mechanical response is found in compression testing of nanoporous gold micro-pillars aligned with various crystallographic orientations. The macroscopic stress tensors of the honeycombs show the same anisotropy as their respective bulk single crystals. Locally, however, there is an appreciable fluctuation in the local stresses, which are even larger than for polycrystals. This explains why the Taylor/Schmid factor associated with the crystallographic orientation is less useful to estimate the yield stresses of the honeycombs than the bulk single crystals and polycrystals, and why the plastic deformation occurs at smaller strains in the honeycombs than their corresponding bulk single crystals. Besides these findings, the observations of the crystallographic reorientation suggest that conventional orientation analysis tools, such as inverse pole figure and related tools, would in general fail to study the plastic deformation mechanism of monocrystalline cellular materials.</p
Mechanical Properties of Lithiumion Batteryelectrodematerials at Different States of Charge
Full text linkAufgrund von signifikanten Volumenänderungen der Elektrodenmaterialien von Lithiumionen Batterien im Betrieb nimmt auch das Verständnis des mechanischen Verhaltens eine essentielle Rolle ein. Zum einen gibt es noch immer offene Fragen zum mechanischen Verhalten von kommerziell bereits genutzten Materialien. Zum anderen wurde für potentielle neue Materialien, welche sonst exzellente Kennwerte für den Einsatz als Elektrodenmaterialien aufweisen, gezeigt, dass die mechanische Degradation bei der Zyklisierung den Flaschenhals für eine erfolgreiche Einführung darstellt. In dieser Arbeit wurde die Veränderung der mechanischen Eigenschaften von geordnetem (HOPG/Graphit) und ungeordnetem (Glassy Carbon) Kohlenstoff, Silizium und Aluminium gemessen. Hierfür wurde eigens ein neuer experimenteller in-situ Aufbau entwickelt. Er erlaubt die elektrochemische Manipulation der Probe in einer elektrochemischen Zelle und kombiniert erstmals die quantitative Messung der mechanischen Eigenschaften µm-skaliger Einkristalle durch einen MTS G200 XP Nanoindenter bei zeitlicher Synchronisation. Per Nanoindentierung und Mikrodruckversuchen konnten elastischer Modul, Härte und Fließ- bzw. Bruchspannung der Elektrodenmaterialien bei verschiedenen Beladungszuständen gemessen werden. Zusätzlich erlaubt der Aufbau eine neuartige experimentelle Untersuchung der Kopplung zwischen elektrochemischem Potential des Lithiums und dem mechanischen Spannungszustand in einer Elektrode. Silizium zeigte eine klare Absenkung von elastischen Modul und Härte bei der Bildung von amorphem LixSi. Es wurden belastbare Werte für den elastischen Modul und die Härte von LixSi gemessen, welche eine gute Übereinstimmung mit verfügbaren Literaturwerten zeigen. Zusätzlich wurden mehrere Aspekte zur elektrochemischen Lithiierung enthüllt. In frühem Stadium der Lithiierung wurden auf (111)-Oberflächen vereinzelt amorphe Inseln gefunden und diesbezüglich ein Modell für heterogene Keimbildung vorgeschlagen. Zusätzlich konnte ein, der Amorphisierung vorgelagerter, Zwischenschritt der Lithiierungsreaktion identifiziert werden. Gleichzeitig verhinderte dieser Zwischenschritt aber eine quantitative Analyse des Kopplungseffekts zwischen mechanischer Spannung und elektrochemischem Potential des Lithiums. Für Graphit (HOPG) wurde unter anwendungsnahen Bedingungen bei vergleichsweise schneller Interkalation mit Lithium erstmals eine bisher nicht bekannte signifikante Reduzierung der Festigkeit festgestellt. Auch ungeordneter Kohlenstoff (Glassy Carbon) zeigte nach der Interkalation mit Lithium eine Festigkeits- und Modulabsenkung. Bezüglich der Anwendung dieser Materialien ist von einer hohen Relevanz dieser Ergebnisse auszugehen. Auch Aluminium zeigte bei der elektrochemischen Legierung mit Lithium eine Reduzierung von elastischem Modul und Härte. Vergleichbare Messungen aus der Literatur sind nicht bekannt. Die in dieser Arbeit gewonnenen Ergebnisse bedeuten einen wichtigen Erkenntnisgewinn im Hinblick auf eine Verbesserung bekannter und eine erfolgreiche Einführung neuer Elektrodenmaterialien für Lithiumionen Batterien. Somit unterstützt diese Arbeit bei der Umsetzung einer signifikanten Erhöhung der Energie- und Leistungsdichten dieses Energiespeichers.Due to significant volume changes of lithium ion battery electrode materials under operation, it is essential to understand the mechanical behavior of these materials. On the one hand there still are open questions about the mechanical behavior of commercially available materials. On the other hand, it has been shown, that mechanical degradation during cycling is the bottleneck for a successful introduction of new materials, which exhibit excellent characteristics otherwise. In this work, the change in mechanical properties of ordered (HOPG/graphite) and non-ordered (Glassy Carbon) carbon, silicon and aluminum has been evaluated. For this purpose a self-designed experimental setup has been constructed. It allows for the electrochemical manipulation of the sample and combines a synchronized quantitative measurement of the mechanical properties of small volumes on the µm-scale in a MTS G200 XP nanoindenter for the first time. Young’s modulus, hardness and strength of the electrode materials can be measured quantitatively at different states of charge. Moreover, the setup enables a novel experimental investigation of the coupling of mechanical stress and the electrochemical potential of lithium in the material. Silicon showed a clear decrease in elastic modulus and hardness during formation of amorphous LixSi. Reliable values for the modulus and hardness of LixSi have been obtained, which are in good agreement with literature values. Also additional aspects of electrochemical lithiation have been revealed. In the early stages of lithiation, separated amorphous islands were found and a model for heterogeneous nucleation has been proposed. An intermediate reaction step prior to lithiation has been identified as well. At the same time, this intermediate reaction step prevented a proper analysis of the coupling between mechanical stress and the electrochemical potential of the lithium. For fast lithium intercalation and conditions close to application, graphite (HOPG) exhibited a significant reduction in strength, which was unknown so far. Non-ordered carbon (Glassy Carbon) also showed a reduction in strength and modulus after lithium intercalation. With respect to application, high relevance of these results is assumed. During electrochemical lithiation, aluminium exhibited a reduction of elastic modulus and hardness as well. So far, comparable results for this material are not known from the literature. Results that were obtained in this work represent an important gain of knowledge in the light of improvement of already used materials and a successful introduction of new electrode materials for lithium ion batteries. Thus, this work assists the route towards a significant raise of energy and power densities for this type of energy storage
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
- …
