1,721,221 research outputs found
Atomistische Simulationen zur Versetzungs-Nukleations kontrollierten Plastizität in kubisch flächenzentrierten metallischen Nanoobjekten: Rolle der Topologie, Oberflächenmorphologie
No full textMetallic nano-objects are regarded as promising candidates for biosensors, catalysts and nanomechanical devices due to their high surface-to-volume ratio and superior mechanical properties compared to their bulk counterparts. Plasticity in confined dimensions is controlled by surface dislocation nucleation. The dislocation activation at the free surfaces and following interaction and multiplication govern the plastic deformation processes of the nano-objects. Computational modeling particularly atomistic simulations plays an essential role in understanding deformation mechanisms at nanoscale, which can provide useful suggestions for material design and development.
The objectives of this thesis are to understand the role of topology, surface morphology and internal interfaces in surface dislocation nucleation-controlled plasticity of FCC metallic nano-objects. For these purposes, large-scale experimentally-informed molecular dynamics simulations were performed on metallic nanowires with parallel and five-fold twin boundaries along the wire axis and nanoporous gold with realistic and artificial network structures. The influence of above-mentioned aspects on the mechanical response of these nano-objects was investigated at the atomic-level.
Parallel twin boundaries along the wire axis show significant effects on strengthening and localization of plastic deformation of nanowires by interacting with surface-nucleated dislocations under tension. Surface roughness and facet orientation have significant effects on the dislocation activation and deformation localization processes of the nanowires during the simulated tensile tests. Single-crystalline and five-fold twinned nanowires show elastic instability under compression resulting in severe and localized plastic deformation. Five-fold twinned nanowires exhibit kink formation and bending mediated plasticity under compression in contrast to the shear slip mediated plasticity in the single-crystalline nanowires. The bent five-fold twinned nanowires after force controlled bending tests show worse reversibility upon bend deformation than the single-crystalline counterparts due to the formation of sessile dislocation locks and grain boundaries instead of reversible twinning dislocations in the single-crystalline nanowires.
Nanoporous gold shows strongly size-dependent elastic and plastic responses under compression. Particularly, the effects of surface-induced stress on elastic behavior and surface dislocation nucleation are pronounced in sub-10 nm dimensions. In NPG with ligament size of sub-ten nanometer dimensions and 30 nm, dislocation starvation and dislocation interaction and multiplication dominate the plastic flow, respectively. Nanoporous gold shows size-dependent deformation mechanisms, i.e., formation of small-angle grain boundaries and stacking fault tetrahedra due to the nucleation and interaction of full dislocations was only observed in the compressed porous structure with ligament size of 30 nm. Moreover, the topology- and surface morphology-dependent mechanical responses in nanoporous structures were explained at the atomic-level. Realistic nanoporous structure shows early yielding due to the heterogeneity of stress and ligament size distributions. The artificial gyroid nanoporous structure exhibits symmetric surface morphology which leads to the symmetric stress states and nucleation events.
In addition, the outcomes of large-scale experimentally informed atomistic simulations on the nano-objects were correlated with experiments. The importance of realistic geometry in modeling mechanical response of nanomaterials with internal interfaces and heterogeneous microstructures was demonstrated.
/ Metallische Nano-Objekte gelten aufgrund ihres hohen Oberflächen-zu-Volumen-Verhältnisses und ihrer überlegenen mechanischen Eigenschaften gegenüber vergleichbaren massiven Proben als vielversprechende Kandidaten für Biosensoren, Katalysatoren und nanomechanische Bauelemente. Die Plastizität in begrenzten Dimensionen wird durch die Nukleation von Oberflächenversetzungen gesteuert. Die Versetzungsaktivierung an den freien Oberflächen und die anschließende Versetzungsinteraktion und -multiplikation bestimmen die plastischen Verformungsprozesse der Nano-Objekte. Die Computermodellierung, insbesondere die atomistische Simulation, liefert einen wesentlichen Beitrag zum Verständnis von Verformungsmechanismen auf der Nanoskala, wodurch nützliche Vorschläge für Materialdesign und -entwicklung abgeleitet werden können.
Die Ziele dieser Arbeit sind es, ein besseres Verständnis über die Rolle von Topologie, Oberflächenmorphologie und von internen Grenzflächen bei der nukleationskontrollierten Plastizität von Oberflächenversetzungen in kfz metallischen Nano-objekten zu erlangen. Zu diesem Zweck wurden groß-skalige, molekulardynamische Simulationen an experimentell-informierten Datensätzen durchgeführt. Die Simulationen umfassen die Untersuchung von metallischen Nanodrähten mit parallelen und fünffachen Zwillingsgrenzen entlang der Drahtachse, sowie von nanoporösem Gold mit realistischen und künstlichen Netzwerkstrukturen. Der Einfluss der oben genannten Aspekte auf die mechanische Reaktion der Nano-Objekte wurde dabei auf atomarer Ebene untersucht.
Die Untersuchungen an den Nanodrähten zeigen, dass parallele Zwillingsgrenzen entlang der Drahtachse unter Zugbelastung durch ihre Wechselwirkung mit Versetzungen, die an der Oberfläche nukleiert wurden, zu einer signifikanten Festigkeitssteigerung und Lokalisierung der plastischen Verformung führen. Die Oberflächenrauhigkeit und Facettenorientierung der Nanodrähte beinflusst dabei maßgeblich die Versetzungsaktivierung und Lokalisierungg der Plastizität während der simulierten Zugversuche. Unter Druckbelastung zeigen einkristalline und fünffach verzwillingte Nanodrähte eine elastische Instabilität, was zu einer starken und lokalisierten plastischen Verformung führt. Die fünffach verzwillingten Nanodrähte versagen dabei durch Knickbildung und biegeinduzierte Plastizität, wohingegen die einkristallinen Nanodrähte durch plastisches Abscheren versagen.. Die, nach der kraftkontrollierten Belastung, gebogenen fünffach verzwillingten Nanodrähte zeigen bei der Entlastungeine geringere Reversibilität der plastischen Verformung als ihre einkristallinen Pendants. Dies ist auf die Bildung von immobilen Versetzungskonfigurationen und Korngrenzen anstelle von reversiblen Zwillingsversetzungen in den einkristallinen Nanodrähten zurückzuführen.
Nanoporöses Gold (NPG) zeigt ein stark größenabhängiges elastisches und plastisches Verformungsverhalten unter Druck. Dabei beeinflusst insbesondere die Oberflächenspannung das elastische Verhalten und die Keimbildung von Oberflächenversetzungen bei Größendimensionen unter 10 nm. Bei NPG mit einer Ligamentgröße unterhalb von 10 nm und bei einer Ligamentgröße von 30 nm ist das plastische Verhalten hauptsächlich durch einen Mangel an Versetzungen und Versetzungsinteraktions- bzw. -multiplikationsmechanismen bestimmt. Nanoporöses Gold zeigt größenabhängige Verformungsmechanismen. So wird bspw. die Bildung von Kleinwinkelkorngrenzen und von Stapelfehlertetraedern, welche auf der Keimbildung und Interaktion von vollständigen Versetzungen beruht, nur in der komprimierten porösen Struktur mit einer Ligamentgröße von 30 nm beobachtet. In der Arbeit wird weiterführend das Topologie- und Oberflächenmorphologie-abhängige mechanische Verhalten der nanoporösen Strukturen auf atomarer Ebene erklärt. Die experimentell-informierte, realistische nanoporöse Struktur zeigt in der simulierten Kompression frühzeitiges plastisches Fließen, was auf die die Heterogenität der Spannungs- und Ligamentgrößenverteilung zurückgeführt werden kann. Die künstliche-erzeugte gyroidale nanoporöse Struktur besitzt dagegen eine symmetrische Oberflächenmorphologie die zusymmetrischen Spannungszuständen und Nukleationsereignissen führt.
Die Ergebnisse der groß-skaligen experimentell informierten atomistischen Simulationen werden in der Arbeit korrelativ mit entsprechenden experimentellen Befunden verglichen. Dadurch wird die Wichtigkeit der Verwendung einer realistischen Probengeometrie bei der Modellierung des mechanischen Verhaltens von Nanomaterialien mit internen Grenzflächen und heterogenen Mikrostrukturen demonstriert
Multiscale modeling of oxygen and vacancy diffusion in dilute ferritic iron alloys
Full text linkIron-based ferritic alloys are used for a plethora of industrial applications. These alloys contain foreign atoms purposely employed to improve certain properties as well as some unwanted impurities introduced during fabrication. Materials properties are decisively influenced by diffusion processes. Very often diffusion cannot be avoided during fabrication and application. Therefore, many efforts are made to understand the underlying atomic-level mechanisms by both experimental and theoretical investigations. In this thesis work a multiscale modelling approach is used to study oxygen and vacancy diffusion in dilute ferritic iron alloys. Due to the extremely low solubility of oxygen the measurement of oxygen diffusion in iron is difficult. Only few experimental data are available. Experimental investigation of vacancy migration is still more complicated. The lack of reliable experimental data is therefore an important motivation for theoretical investigations. Gaining fundamental data on oxygen and vacancy diffusion in dilute iron alloys is essential for many applications. Oxygen plays a crucial role in the corrosion of iron-based alloys. Oxygen and the vacancy are also important in the formation and evolution of Y-Ti-O nanoclusters in oxide dispersion strengthened ferritic Fe-Cr alloys, which are considered as promising candidates for structural materials of future fusion and fission reactors. Furthermore, vacancies are formed during neutron and ion irradiation and their diffusion affects radiation-induced nanostructure formation in ferritic alloys.
In the first part of this thesis work, the diffusion of interstitial oxygen under the influence of substitutional atoms or solutes (Al, Si, P, S, Ti, Cr, Mn, Ni, Y, Mo and W) in bcc Fe is investigated by the combination of Density Functional Theory (DFT) and Atomistic Kinetic Monte Carlo (AKMC) simulations. The substitutional atoms are assumed to be immobile because oxygen diffusion is much faster than that of the solutes. DFT is applied to gain data on binding energies between interstitial oxygen and the substitutional foreign atoms, and to calculate the migration barriers for oxygen in the environment of the solutes. Using the migration barriers obtained by DFT, the diffusion coefficient of oxygen is determined by AKMC simulation. It is found that Si, P, Ni, Mo, and W have negligible influence on the oxygen diffusion coefficient. Al, Cr, Mn, S, Ti, and Y cause a considerable reduction of oxygen mobility. In these cases, the temperature dependence of oxygen diffusivity shows deviations from Arrhenius behavior. This is explained in detail by the significant temperature dependence of the ratio between residence times in the respective states.
In the second part of the work a method is presented which allows for an efficient calculation of the diffusion coefficient of oxygen and other interstitial atoms in dilute alloys. The method is applied to examples considered in the first part of the work. The calculation procedure is based on the separation of the diffusion path into a contribution related to migration in the interaction region between the mobile interstitial and the substitutional solute and another part related to diffusion in perfect bcc Fe. In this manner AKMC simulation must be performed only for one concentration of the substitutional solute, and the obtained results can be employed to obtain data for other concentrations using analytical expressions containing binding energies between the interstitial and the substitutional solute.
The focus of third part of the work is on the mutual dependence of oxygen and vacancy diffusion in bcc Fe and dilute iron alloys. Here both O and v must be considered as mobile while the substitutional atoms are assumed to be immobile. DFT is applied to determine the binding energy between O and v for different distances, the migration barriers for O in the environment of v, and the corresponding barriers of v in the vicinity of O. In agreement with previous work O and v have a very strong binding at the 1st neighbor distance. On the other hand, the calculations show that the Ov pair at the 6th neighbor distance is instable. The newly found simultaneous or coupled jumps of both O and v compensate the lack of jump paths that would occur due to this instability. The DFT results are employed to determine the diffusion coefficient of O and v using the scheme of the AKMC-based calculation method presented in the second part of the thesis work. At first a model system with fixed O and v concentrations is studied. It is found that a small v content of some ppm can already lead to a strong reduction of the O diffusivity. A similar effect is obtained for v diffusion under the influence of O. Furthermore, investigations on the interdependence of O and v diffusion during thermal processing of oxide dispersion strengthened iron alloys are performed, and the influence of the substitutional atoms Y and Ti is studied. A simple thermodynamic model is employed to determine the concentration of O, Y, and Ti monomers as well as the total v concentration, for a typical total content of O, Y, and Ti. These results are used in calculations of the diffusion coefficients of O and v. Not only a strong mutual dependence but also a significant influence of Y on O diffusion is found. Finally, O and v diffusivities in a system with a total O content close to the thermal solubility are calculated. The monomer O concentration as well as the total v concentration was determined using two different models considering equilibrium of O and v with Ov, or equilibrium of O and v with Ov and O2v or Ov2. Despite the very small value of thermal solubility of O in bcc Fe, both the O and v diffusion coefficient are very different from that in pure iron. Even for such a low amount of O in the alloy the diffusion coefficients differ strongly from those in perfect bcc Fe.
The results of the present work have important consequences for planning and performing new experiments on O and v diffusion in dilute iron alloys. In particular, a very precise knowledge of the concentrations of O and v, as well as of other foreign atoms and traps such as dislocations is required
Role of Solid Electrolyte Morphology and Grain Boundaries on the Lithium Transport in All-Solid-State Batteries
No full textThe potential of solid-state batteries (SSBs) is enormous, as they promise to surpass current liquid lithium-ion cell systems in terms of performance and energy density. In addition, SSBs could address safety concerns while providing faster than conventional lithium-ion batteries. These promises drive research for new solid electrolytes and their complementary high-performance active materials. At the same time, a clear frontrunner for future SSB design has yet to be identified. A fundamental understanding of the defining and limiting and limiting cell processes must be established to facilitate the efficient development of new SSB. Therefore, coupled theoretical and experimental studies provide an approach to making joint efforts for an efficient and sustainable development process. The primary research focus of this work revolves around solid electrolytes and their transport description and performance impact in lithium metal solid-state batteries. We describe the SSB through a microstructurally resolved continuum model that accounts for transport and electrochemical reactions. The coupling between electrode microstructure and electrochemistry allows us to investigate limiting and promoting influences on cell performance. We begin our studies by investigating the effects of transport phenomena in the solid electrolyte, particle morphology, grain boundaries, and particle arrangement, i.e., the microstructure of the solid electrolyte. We then proceeded to the analysis of the whole cell, where we investigated general solid electrolyte transport and the performance limitations in combination with the active material phase. We mainly investigate and identify structural and interfacial degradation phenomena and correlate them with the observed power limitation in SSBs
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
Diffusion Modeling in Stressed Chalcogenide Thin-Films
Full text linkDie Effizienz von Verbindungshalbleitern hängt von ihrer lokalen Zusammensetzung und ihrer räumlichen Elementverteilung ab. Um die opto-elektronischen Eigenschaften solcher Bauelemente zu optimieren, ist ein detailliertes Verständnis und die Kontrolle der Zusammensetzungsgradienten entscheidend. Industriell wichtige Bauelemente sind Absorberschichten für Dünnschichtsolarzellen, die eine hohe Effizienz in Kombination mit einem geringen Materialbedarf und einer hohen elastischen Flexibilität bieten. Ein gängiges Herstellungsverfahren für Dünnschicht-Solarzellenabsorber ist das Annealen bei hohen Temperaturen. Im Gegensatz zu dem, was bei Fick'schen Diffusionsprozessen zu erwarten wäre – führt dieses regelmäßig zur Bildung steiler und stabiler Zusammensetzungsgradienten, die oft von den optimalen Profilen für hocheffiziente Absorber abweichen. In dieser Arbeit liegt das Hauptaugenmerk auf den mechanischen Spannungen, die sich im Inneren von Dünnschichten entwickeln, und auf deren Auswirkungen auf Diffusionsprozesse und die mikrostrukturelle Entwicklung des Materials. Es wird gezeigt, dass die Bildung von elastischen Spannungen die endgültigen Elementverteilungen stark beeinflusst und sogar zur Bildung von starken und stabilen Zusammensetzungsgradienten führt. In dieser Arbeit wird weiterhin argumentiert, dass die Wirkung der Spannungen auf die Gleichgewichts-Zusammensetzungsprofile von den mikrostrukturellen Eigenschaften des Materials abhängen kann, insbesondere vom Vorhandensein von Leerstellenquellen. Ein Vergleich numerischer Berechnungen mit Echtzeitdaten der energiedispersiven Röntgenbeugung, die während der Dünnschichtsynthese in-situ erfasst wurden, hilft zu zeigen, dass die so entwickelten Interdiffusionsmodelle die experimentell beobachteten Beugungsspektren und insbesondere die Stagnation der Interdiffusion vor Erreichen der vollständigen Durchmischung teilweise reproduzieren können.The operational efficiency of compound semiconductors regularly depends on their local elemental composition and on the spatial distribution of contained elements. To optimize the opto-electronic properties of such devices, a detailed understanding and control of compositional gradients is crucial. Industrially important devices are thin-film solar cell absorber layers which deliver high photo-conversion efficiencies in combination with a low demand of material and high elastic flexibility. These materials use local variations in composition to tune their opto-electronic properties. A common fabrication process for thin-film solar cell absorbers involves annealing at
high temperatures to achieve specific compositional gradients, which – contrary to what could
be expected from simple Fickian diffusion processes – regularly results in the formation of steep
and stable compositional gradients, often deviating from the optimal profiles for high-efficiency
absorbers. In this work attention is focused especially on mechanical stresses developing inside thin-films and on their effects on diffusion processes and on the material’s micro-structural evolution. It is shown that the formation of elastic stresses strongly influences the final elemental distributions, even leading to the formation of strong and stable final compositional gradients. However, this thesis also argues that their exact effect on equilibrium composition profiles may depend on the detailed micro-structural properties of the material, especially on the presence of vacancy sources and sinks. A comparison of numerical calculations with real-time synchroton-based energy-dispersive X-ray diffraction data acquired in-situ during thin-film synthesis helps to demonstrate that the such developed interdiffusion models can partly reproduce the experimentally observed diffraction spectra and, especially, the stagnation of interdiffusion before total intermixing is achieved
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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