1,445 research outputs found
Data-driven exact model order reduction for computational multiscale methods to predict high-cycle fatigue-damage in short-fiber reinforced plastics
Motiviert durch die Entwicklung energieeffizienterer Maschinen und Transportmittel hat der Leichtbau in den letzten Jahren enorm an Wichtigkeit gewonnen. Eine wichtige Klasse der Leichtbaumaterialien sind die faserverstärkten Kunststoffe. In der vorliegenden Arbeit liegt der Fokus auf der Entwicklung und Bereitstellung von Materialmodellen zur Vorhersage des Ermüdungsverhaltens kurzglasfaserverstärkter Thermoplaste. Diese Materialien unterscheiden sich dabei durch ihre Aufschmelzbarkeit und ihrer damit einhergehenden besseren Recyclebarkeit von thermosetbasierten Materialien. Außerdem erlauben die Kurzglasfasern im Gegensatz zu Langfasern eine einfache und zeiteffiziente Herstellung komplexer Komponenten.
Ermüdung ist ein wichtiger Versagensmechanismus in solchen Komponenten, insbesondere für Bauteile z.B. in Fahrzeugen, die vibrationsartigen Belastungen ausgesetzt sind. Durch die inherente Anisotropie des Materials sind die experimentelle Charakterisierung und Vorhersage dieses Versagensmechanismus jedoch äußerst zeitintensiv und stellen somit eine wesentliche Herausforderung im Entwicklungsprozess und für die breitere Anwendung solcher Bauteile dar. Daher ist die Entwicklung komplementärer simulativer Methoden von großem Interesse.
Im Rahmen dieser Arbeit werden Methoden zur Vorhersage der Ermüdungsschädigung kurzglasfaserverstärkter Werkstoffe im Rahmen einer Multiskalenmethode entwickelt. Die in der Arbeit betrachteten Multiskalenmodelle bieten die Möglichkeit, allein anhand der experimentellen Charakterisierungen der Materialparameter der Konstituenten, d.h. Faser und Matrix, komplexe anisotrope Effekte des Verbundmaterials vorherzusagen. Der experimentelle Aufwand kann dadurch enorm reduziert werden. Dazu werden zunächst Materialmodelle für die Konstituenten des Komposits entwickelt. Mithilfe FFT-basierter rechnergestützter Homogenisierung wird daraus das Materialverhalten des Komposits für verschiedene Mikrostrukturen und Lastfälle vorhergesagt. Die vorberechneten Lastfälle auf Mikrostrukturebene werden mit datengetriebenen Methoden auf die Makroskala übertragen. Das ermöglicht eine effiziente Berechnung von Bauteilen in wenigen Stunden, wohingegen eine entsprechende Berechnung mit geometrischer Auflösung aller einzelnen Fasern der Mikrostruktur auf heutigen Computern viele Jahre dauern würden.
Für die Matrix werden unterschiedliche Schädigungsmodelle untersucht. Ihre Vor- und Nachteile werden analysiert. Die Mikrostruktursimulationen geben einen Einblick in den Einfluss verschiedener statistischer Parameter wie Faserlängen und Faservolumengehalt auf das Kompositverhalten. Ein neues Modellordnungsreduktionsverfahren wird entwickelt und zur Simulation des Ermüdungsschädigungsverhaltens auf Bauteilebene angewandt. Weiter werden Modellerweiterungen zur Berücksichtigung des R-Wert-Verhältnisses und viskoelastischer Effekte in der Evolution der Ermüdungsschädigung entwickelt und mit experimentellen Ergebnissen validiert.
Das entstandene Simulationsframework erlaubt nach Vorrechnungen auf einer geringen Menge von Mikrostrukturen und Lastfällen eine effiziente Makrosimulation eines Bauteils vorzunehmen. Dabei können Effekte wie Viskoelastizität und R-Wert-Abhängigkeit je nach gewünschter Modellierungstiefe berücksichtigt oder vernachlässigt werden, um immer das effizientste Modell, das alle relevanten Effekte abbildet, nutzen zu können
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When to Hold and When to Fold: Studies on the topology of origami and linkages
Linkages and mechanisms are pervasive in physics and engineering as models for avariety of structures and systems, from jamming to biomechanics. With the increasein physical realizations of discrete shape-changing materials, such as metamaterials,programmable materials, and self-actuating structures, an increased understandingof mechanisms and how they can be designed is crucial. At a basic level, linkagesor mechanisms can be understood to be rigid bars connected at pivots around whichthey can rotate freely. We will have a particular focus on origami-like materials, anextension to linkages with the added constraint of faces. Self-actuated versions typ-ically start flat and when exposed to an external stimulus - such as a temperaturechange or magnetic field - spontaneously fold. Since these structures fold all at once,and the number of folding patterns accessible to a given origami are exponential, theyare prone to folding to a configuration other than the desired one. Other work hassuggested methods for avoiding this misfolding, but it assumes ideal, rigid origami. Here, we expand on these models to account for the elasticity of real structures andintroduce methods for accounting for Gaussian curvature in them. We also explorehow to find and set an upper bound on minimal forcing sets, or the minimum set offolds required to force an origami, and present a graph theory algorithm for findingthem in arbitrary origami. Taken altogether, these origami studies give insight intohow the physical properties of origami influence folding and a new set of tools foravoiding misfolding. Next, we turn back to a more fundamental study of linkagesand present a new method for finding the manifold of their critical points. We thendemonstrate a design protocol that utilizes this manifold to create linkages with tun-able motions, before turning to several example structures, including the four-barlinkage and the Kane-Lubensky chain
Selected problems of materials science. Vol. 2. Nano-dielectrics metals in electronics. Mеtamaterials. Multiferroics. Nano-magnetics
The textbook examines physical foundations and practical application of current electronics materials. Modern theories are presented, more important experimental data and specifications of basic materials necessary for practical application are given. Contemporary research in the field of microelectronics and nanophysics is taken into account, while special attention is paid to the influence of the internal structure on the physical properties of materials and the prospects for their use. English-language lectures and other classes on the subject of the book are held at Igor Sikorsky Kyiv Polytechnic Institute at the departments of “Applied Physics” and “Microelectronics” on the subject of materials science, which is necessary for students of higher educational institutions when performing scientific works.
For master’s degree applicants in specialty 105 “Applied physics and nanomaterials”.Розглянуто фізичні основи та практичне застосування актуальних матеріалів електроніки. Подано сучасні теорії, наведено найважливіші експериментальні дані та специфікації основних матеріалів, які потрібні для практичного застосування. Враховано сучасні дослідження у галузі мікроелектроніки та нанофізики, при цьому особливу увагу приділено впливу внутрішньої структури на фізичні властивості матеріалів і на перспективи їх використання. Англомовні лекції та інші види занять за тематикою книги проводяться в КПІ ім. Ігоря Сікорського на кафедрах «Прикладна фізика» та «Мікро-електроніка» за напрямом матеріалознавство, що необхідно студентам вищих навчальних закладів при виконанні наукових робіт.
Для здобувачів магістратури за спеціальністю 105 «Прикладна фізика та наноматеріали»
Optimization of niobium oxide-based threshold switches for oscillator-based applications
In niobium oxide-based capacitors non-linear switching characteristics can be observed if the oxide properties are adjusted accordingly. Such non-linear threshold switching characteristics can be utilized in various non-linear circuit applications, which have the potential to pave the way for the application of new computing paradigms. Furthermore, the non-linearity also makes them an interesting candidate for the application as selector devices e.g. for non-volatile memory devices. To satisfy the requirements for those two areas of application, the threshold switching characteristics need to be adjusted to either obtain a maximized voltage extension of the negative differential resistance region in the quasi-static I-V characteristics, which enhances the non-linearity of the devices and results in improved robustness to device-to-device variability or to adapt the threshold voltage to a specific non-volatile memory cell. Those adaptations of the threshold switching characteristics were successfully achieved by deliberate modifications of the niobium oxide stack. Furthermore, the impact of the material stack on the dynamic behavior of the threshold switches in non-linear circuits as well as the impact of the electroforming routine on the threshold switching characteristics were analyzed. The optimized device stack was transferred from the micrometer-sized test structures to submicrometer-sized devices, which were packaged to enable easy integration in complex circuits. Based on those packaged threshold switching devices the behavior of single as well as of coupled relaxation oscillators was analyzed. Subsequently, the obtained results in combination with the measurement results for the statistic device-to-device variability were used as a basis to simulate the pattern formation in coupled relaxation oscillator networks as well as their performance in solving graph coloring problems. Furthermore, strategies to adapt the threshold voltage to the switching characteristics of a tantalum oxide-based non-volatile resistive switch and a non-volatile phase change cell, to enable their application as selector devices for the respective cells, were discussed.:Abstract I
Zusammenfassung II
List of Abbrevations VI
List of Symbols VII
1 Motivation 1
2 Basics 5
2.1 Negative differential resistance and local activity in memristor devices 5
2.2 Threshold switches as selector devices 8
2.3 Switching effects observed in NbOx 13
2.3.1 Threshold switching caused by metal-insulator transition 13
2.3.2 Threshold switching caused by Frenkel-Poole conduction 18
2.3.3 Non-volatile resistive switching 32
3 Sample preparation 35
3.1 Deposition techniques 35
3.1.1 Evaporation 35
3.1.2 Sputtering 36
3.2 Micrometer-sized devices 36
3.3 Submicrometer-sized devices 37
3.3.1 Process flow 37
3.3.2 Reduction of the electrode resistance 39
3.3.3 Transfer from structuring via electron beam lithography to structuring via
laser lithography 48
3.3.4 Packaging procedure 50
4 Investigation and optimization of the electrical device characteristic 51
4.1 Introduction 51
4.2 Measurement setup 52
4.3 Electroforming 53
4.3.1 Optimization of the electroforming process 53
4.3.2 Characterization of the formed filament 62
4.4 Dynamic device characteristics 67
4.4.1 Emergence and measurement of dynamic behavior 67
4.4.2 Impact of the dynamic device characteristics on quasi-static I-V
characteristics 70
5 Optimization of the material stack 81
5.1 Introduction 81
5.2 Adjustment of the oxygen content in the bottom layer 82
5.3 Influence of the thickness of the oxygen-rich niobium oxide layer 92
5.4 Multilayer stacks 96
5.5 Device-to-device and Sample-to-sample variability 110
6 Applications of NbOx-based threshold switching devices 117
6.1 Introduction 117
6.2 Non-linear circuits 117
6.2.1 Coupled relaxation oscillators 117
6.2.2 Memristor Cellular Neural Network 121
6.2.3 Graph Coloring 127
6.3 Selector devices 132
7 Summary and Outlook 138
8 References 141
9 List of publications 154
10 Appendix 155
10.1 Parameter used for the LT Spice simulation of I-V curves for threshold
switches with varying oxide thicknesses 155
10.2 Dependence of the oscillation frequency of the relaxation oscillator circuit
on the capacitance and the applied source voltage 156
10.3 Calculation of the oscillation frequency of the relaxation oscillator circuit 157
10.4 Characteristics of the memristors and the cells utilized in the simulation of
the memristor cellular neural network 164
10.5 Calculation of the impedance of the cell in the memristor cellular network 166
10.6 Example graphs from the 2nd DIMACS series 179
11 List of Figures 182
12 List of Tables 19
Microstructure modeling and crystal plasticity parameter identification for predicting the cyclic mechanical behavior of polycrystalline metals
Computational homogenization permits to capture the influence of the microstructure on the cyclic mechanical behavior of polycrystalline metals. In this work we investigate methods to compute Laguerre tessellations as computational cells of polycrystalline microstructures, propose a new method to assign crystallographic orientations to the Laguerre cells and use Bayesian optimization to find suitable parameters for the underlying micromechanical model from macroscopic experiments
Exactly soluble models in many-body physics
Almost all phenomena in the universe are described, at the fundamental level, by quantum manybody
models. In general, however, a complete understanding of large systems with many degrees of
freedom is impossible. While in general many-body quantum systems are intractable, there are
special cases for which there are techniques that allow for an exact solution.
Exactly soluble models are interesting because they are soluble; beyond this, they can be used to
gain intuition for further reaching many-body systems, including when they can be leveraged to help
with numerical approximations for general models. The work presented in this thesis considers
exactly soluble models of quantum many-body systems.
The first part of this thesis extends the family of many-body spin models for which we can find a freefermion
solution.
A solution method that was developed for a specific free-fermion model is generalized in such a way
that allows application to a broader class of many-body spin system than was previously known to be
free. Models which admit a solution via this method are characterized by a graph theory invariants: in
brief it is shown that a quantum spin system has an exact description via non-interacting fermions if
its frustration graph is claw-free and contains a simplicial clique.
The second part of this thesis gives an explicit example of how the usefulness of exactly soluble
models can extend beyond the solution itself. This chapter pertains to the calculation of the
topological entanglement entropy in topologically ordered loop-gas states. Topological entanglement
entropy gives an understanding of how correlations may extend throughout a system. In this chapter
the topological entanglement entropy of two- and three-dimensional loop-gas states is calculated in
the bulk and at the boundary. We obtain a closed form expression for the topological entanglement in
terms of the anyonic theory that the models support
Quantum Codes on Graphs
We consider some questions related to codes constructed using various graphs,
in particular focusing on graphs which are not lattices in two or three
dimensions. We begin by considering Floquet codes which can be constructed
using ``emergent fermions". Here, we are considering codes that in some sense
generalize the honeycomb code[1] to more general, non-planar graphs. We then
consider a class of these codes that is related to (generalized) toric codes on
-complexes. For (generalized) toric codes on -complexes, the following
question arises: can the distance of these codes grow faster than square-root?
We answer the question negatively, and remark on recent systolic
inequalities[2]. We then turn to the case that of planar codes with vacancies,
or ``dead qubits", and consider the statistical mechanics of decoding in this
setting. Although we do not prove a threshold, our results should be
asymptotically correct for low error probability and high degree decoding
graphs (high degree taken before low error probability). In an appendix, we
discuss a toy model of vacancies in planar quantum codes, giving a
phenomenological discussion of how errors occur when ``super-stabilizers" are
not measured, and in a separate appendix we discuss a relation between Floquet
codes and chain maps.Comment: 25 pages, 1 figur
The long-range Falicov-Kimball model and the amorphous Kitaev model: Quantum many-body systems I have known and loved
Large systems of interacting objects can give rise to a rich array of emergent behaviours. Make those objects quantum and the possibilities only expand. Interacting quantum many-body systems, as such systems are called, include essentially all physical systems. Luckily, we don't usually need to consider this full quantum many-body description. The world at the human scale is essentially classical (not quantum), while at the microscopic scale of condensed matter physics we can often get by without interactions. Strongly correlated materials, however, do require the full description. Some of the most exciting topics in modern condensed matter fall under this umbrella: the spin liquids, the fractional quantum Hall effect, high temperature superconductivity and much more. Unfortunately, strongly correlated materials are notoriously difficult to study, defying many of the established theoretical techniques within the field. Enter exactly solvable models, these are interacting quantum many-body systems with extensively many local symmetries. The symmetries give rise to conserved charges. These charges break the model up into many non-interacting quantum systems which are more amenable to standard theoretical techniques. This thesis will focus on two such exactly solvable models.
The first, the Falicov-Kimball (FK) model is an exactly solvable limit of the famous Hubbard model which describes itinerant fermions interacting with a classical Ising background field. Originally introduced to explain metal-insulator transitions, it has a rich set of ground state and thermodynamic phases. Disorder or interactions can turn metals into insulators and the FK model features both transitions. We will define a generalised FK model in 1D with long-range interactions. This model shows a similarly rich phase diagram to its higher dimensional cousins. We use an exact Markov Chain Monte Carlo method to map the phase diagram and compute the energy resolved localisation properties of the fermions. This allows us to look at how the move to 1D affects the physics of the model. We show that the model can be understood by comparison to a simpler model of fermions coupled to binary disorder.
The second, the Kitaev Honeycomb (KH) model, was the one of the first solvable 2D models with a Quantum Spin Liquid (QSL) ground state. QSLs are generally expected to arise from Mott insulators, when frustration prevents magnetic ordering all the way to zero temperature. The QSL state defies the traditional Landau-Ginzburg-Wilson paradigm of phases being defined by local order parameters. It is instead a topologically ordered phase. Recent work generalising non-interacting topological insulator phases to amorphous lattices raises the question of whether interacting phases like the QSLs can be similarly generalised. We extend the KH model to random lattices with fixed coordination number three generated by Voronoi partitions of the plane. We show that this model remains solvable and hosts a chiral amorphous QSL ground state. The presence of plaquettes with an odd number of sides leads to a spontaneous breaking of time reversal symmetry. We unearth a rich phase diagram displaying Abelian as well as a non-Abelian QSL phases with a remarkably simple ground state flux pattern. Furthermore, we show that the system undergoes a phase transition to a conducting thermal metal state and discuss possible experimental realisations.Open Acces
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