1,680 Works

Size affected dislocation activity in crystals : advanced surface and grain boundary conditions

Edgar Husser, Celal Soyarslan & Swantje Bargmann
Extended crystal plasticity theories are well established to study size-dependent hardening of metals. Surface and inner grain boundary conditions play a significant role for crystals at small scales as they affect the dislocation activity and, hence, alter strength and strain hardening behavior. Conventional micro boundary conditions, i.e., microhard and microfree, are unable to capture the underlying physics as they describe ideal and over-simplified surface/interface conditions. In this work, advanced boundary conditions for gradient extended crystal...

Fourth-order strain-gradient phase mixture model for nanocrystalline fcc materials

Benjamin Klusemann, Swantje Bargmann & Yuri Estrin
The proposed modeling approach for nanocrystalline materials is an extension of the local phase mixture model introduced by Kim et al (2000 Acta Mater. 48 493–504). Local models cannot account for any non-uniformities or strain patterns, i.e. such models describe the behavior correctly only as long as it is homogeneous. In order to capture heterogeneities, the phase mixture model is augmented with gradient terms of higher order, namely second and fourth order. Different deformation mechanisms...

A principle curvatures analysis of the isothermal evolution of nanoporous gold: Quantifying the characteristic length-scales

Markus Ziehmer, Kaixiong Hu, Ke Wang & Erica Lilleodden
A study of the isothermal evolution of a nanoporous gold (npg) microstructure after dealloying has been performed. In order to adequately characterize its complex three-dimensional bicontinuous ligamentring structure, an analysis of the scaled principle curvatures k1 and k2 based on representative volumes of meshed 3D reconstructions was applied. Five npg samples, as obtained from an electrolytical dealloying process, with different mean ligament diameters ranging from ca. 25 nm (as-dealloyed) to ca. 420 nm (from annealing...

Switchable imbibition in nanoporous gold

Yahui Xue, Jürgen Markmann, Huiling Duan, Jörg Weißmüller & Patrick Huber
Spontaneous imbibition enables the elegant propelling of nano-flows because of the dominance of capillarity at small length scales. The imbibition kinetics are, however, solely determined by the static host geometry, the capillarity, and the fluidity of the imbibed liquid. This makes active control particularly challenging. Here we show for aqueous electrolyte imbibition in nanoporous gold that the fluid flow can be reversibly switched on and off through electric potential control of the solid-liquid interfacial tension,...

Interface elasticity effects in polymer-filled nanoporous metals

Jana Wilmers, Andrew McBride & Swantje Bargmann
A continuum formulation for electroactive composites made from nanoporous gold and ionconducting polymer is proposed. A novel extension of surface elasticity theory is developed to account for the high surface-to-volume ratio of nanoporous gold, and to capture the chemoelectromechanical coupling that occurs on the interface between the metal and the polymer. This continuum formulation accounts for the fully non-linear behaviour exhibited by the composite. The balance of linear momentum, Gauß's flux theorem and a relation...

A nanoporous gold-polypyrrole hybrid nanomaterial for actuation

Ke Wang, Charlotte Stenner & Jörg Weißmüller
We discuss actuation with a hybrid nanomaterial that is made by electro-polymerizing pyrrole on the internal surfaces of dealloying-derived nanoporous gold and then letting aqueous electrolyte be imbibed in the remaining pore space. In this way, active polypyrrole films are contacted by two separate but individually contiguous conduction paths, providing efficient transport of ions in the electrolyte channels and of electrons in the metal skeleton. The metal skeleton also serves to enhance the mechanical behavior...

COLIBRI MODULE O17 : documentation of project results : final report

Jens Myrup Pedersen
During 2014 and 2017 the project COLIBRI (Collaboration and Innovation for Better, Personalised and IT-Supported Teaching) was carried out between seven European Universities, two companies, and a national documentation center. The aim was to develop and try out new and innovative teaching methods. You can read more about the project at www.erasmus-colibri.eu . The purpose of this document is to share the experiences we had during the three years, in a way that is useful...

Local flow stresses in interpenetrating-phase composites based on nanoporous gold — in situ diffraction

Ke Wang, Christian Hartig, Malte Blankenburg, Martin Müller, Robert Günther & Jörg Weißmüller
We report a synchrotron in situ diffraction experiment exploring stress evolution during compression of interpenetrating-phase nanocomposites based on nanoporous gold and polymer.While previous experiments provided indirect indication of local flow conditions based on macroscopic effective flow stress and micromechanics models such as Gibson-Ashby scaling law, the lattice parameter data of our experiment access the flow stress directly. At small structure size we find excellent agreement with previous reports, supporting thematch between material andmodel of those...

Nanomechanics of individual aerographite tetrapods

Raimonds Meija, Stefano Signetti, Arnim Schuchardt, Kerstin Meurisch, Daria Smazna, Matthias Mecklenburg, Karl Schulte, Donats Erts, Oleg Lupan, Bodo Fiedler, Yogendra Kumar Mishra, Rainer Adelung & Nicola M. Pugno
Carbon-based three-dimensional aerographite networks, built from interconnected hollow tubular tetrapods of multilayer graphene, are ultra-lightweight materials recently discovered and ideal for advanced multifunctional applications. In order to predict the bulk mechanical behaviour of networks it is very important to understand the mechanics of their individual building blocks. Here we characterize the mechanical response of single aerographite tetrapods via in situ scanning electron and atomic force microscopy measurements. To understand the acquired results, which show that...

Protein engineering and synthetic pathways in Escherichia coli for effective production of 5-hydroxytryptophan and serotonin

José Aníbal Mora Villalobos
Metabolic engineering has improved the production of tryptophan in E. coli during the last two decades, opening itself a plethora of opportunities for the production of tryptophan derivatives. 5-Hydroxytryptophan (5HTP) and serotonin are two important derivatives not only important for their pharmaceutical value but also because they could serve as a precursor of other molecules which includes sleep cycle regulator, anti-migraine medications, sedatives, anticonvulsants, antitumors, antimicrobials, antivirals, among others. To the present, 5HTP is mainly...

Strong light scattering and broadband (UV to IR) photoabsorption in stretchable 3D hybrid architectures based on Aerographite decorated by ZnO nanocrystallites

Ion Tiginyanu, Lidia Ghimpu, Jorit Gröttrup, Vitalie Postolache, Matthias Mecklenburg, Marion A. Stevens-Kalceff, Veaceslav Ursaki, Nader Payami, Robert Feidenhan'L, Karl Schulte, Rainer Adelung & Yogendra Kumar Mishra
In present work, the nano- and microscale tetrapods from zinc oxide were integrated on the surface of Aerographite material (as backbone) in carbon-metal oxide hybrid hierarchical network via a simple and single step magnetron sputtering process. The fabricated hybrid networks are characterized for morphology, microstructural and optical properties. The cathodoluminescence investigations revealed interesting luminescence features related to carbon impurities and inherent host defects in zinc oxide. Because of the wide bandgap of zinc oxide and...

Effect of surface slasticity on the elastic response of nanoporous gold

Celal Soyarslan, Edgar Husser & Swantje Bargmann
In this work, the effect of surface elasticity on the effective elastic properties of nanoporous gold is studied. To this end, a theoretical framework for surface elasticity effects in submicron-sized solids is implemented as a user-defined finite element subroutine. This allows the use of the theory in large-scale engineering problems. The theory suggests a zero-thickness surface accommodating unique energetic properties and surface tension. For the example of ball-and-stick diamond cubic unit cell structures for nanoporous...

Cross-disciplinary communication among Ph.D. candidates : how to make it work

Ulrike Herzog, Alette Winter & Christian H. Kautz
Cross-disciplinary research settings are highly challenging for young scientists. In our research centre, Ph.D. candidates with various disciplinary backgrounds are expected to cooperate. To support cooperation, we designed a Ph.D. programme that fosters cross-disciplinary communication. The programme uses an innovative teaching format that aims to broaden the participants’ understanding of technical basics and to improve their academic teaching skills. Being faced with dissatisfaction among the Ph.D. candidates at first, we introduced a targeted feedback process...

Three-dimensional Aerographite-GaN hybrid networks: single step fabrication of porous and mechanically flexible materials for multifunctional applications

Arnim Schuchardt, Tudor Braniste, Yogendra Kumar Mishra, Mao Deng, Matthias Mecklenburg, Marion A. Stevens-Kalceff, Simion Raevschi, Karl Schulte, Lorenz Kienle, Rainer Adelung & Ion Tiginyanu
Three dimensional (3D) elastic hybrid networks built from interconnected nano- and microstructure building units, in the form of semiconducting-carbonaceous materials, are potential candidates for advanced technological applications. However, fabrication of these 3D hybrid networks by simple and versatile methods is a challenging task due to the involvement of complex and multiple synthesis processes. In this paper, we demonstrate the growth of Aerographite-GaN 3D hybrid networks using ultralight and extremely porous carbon based Aerographite material as...

Automatic three-dimensional geometry and mesh generation of periodic representative volume elements for matrix-inclusion composites

Konrad Schneider, Benjamin Klusemann & Swantje Bargmann
This paper introduces an efficient method to automatically generate and mesh a periodic three- dimensional microstructure for matrix-inclusion composites. Such models are of major importance in the field of computational micromechanics for homogenization purposes utilizing unit cell models. The main focus of this contribution is on the creation of cubic representative volume elements (RVEs) featuring a periodic geometry and a periodic mesh topology suitable for the application of periodic boundary condi- tions in the framework...

Scaling laws of nanoporous metals under uniaxial compression

Norbert Huber, R. N. Viswanath, Nadiia Mameka, Jürgen Markmann & Jörg Weißmüller
This study is motivated by discrepancies between recent experimental compression test data of nanoporus gold and the scaling laws for strength and elasticity by Gibson and Ashby. We present a systematic theoretical investigation of the relationship between microstructure and macroscopic behaviour of nanoporous metals. The microstructure is modelled by four-coordinated spherical nodes interconnected by cylindrical struts. The node positions are randomly displaced from the lattice points of a diamond lattice. We report scaling laws for...

Nanoporous gold : testing macro-scale samples to probe small-scale mechanical behavior

Nadiia Mameka, Ke Wang, Jürgen Markmann, Erica Lilleodden & Jörg Weißmüller
Nanoporous gold made by dealloying exemplifies how the exciting mechanical properties of nanoscale objects can be exploited in designing materials from which macroscopic things can be formed. The homogeneous microstructure and the possibility of adjusting the ligament size, L, between few and few hundred nm, along with the high deformability and reproducible mechanical behavior predestine the material for model studies of small-scale plasticity using reliable macroscopic testing schemes on mm- or cm-size samples. Such experiments...

Reflection from a free carrier front via an intraband indirect photonic transition

Mahmoud A. Gaafar, Dirk Jalas, Liam O'Faolain, Juntao Li, Thomas F. Krauss, Alexander Petrov & Manfred Eich
The reflection of light from moving boundaries is of interest both fundamentally and for applications in frequency conversion, but typically requires high pump power. By using a dispersion-engineered silicon photonic crystal waveguide, we are able to achieve a propagating free carrier front with only a moderate on-chip peak power of 6 W in a 6 ps-long pump pulse. We employ an intraband indirect photonic transition of a co-propagating probe, whereby the probe practically escapes from the front...

Novel, highly-filled ceramic–polymer composites synthesized by a spouted bed spray granulation process

Michael F. H. Wolff, Vitalij Salikov, Sergiy Antonyuk, Stefan Heinrich & Gerold A. Schneider
We present a novel processing route to synthesize homogeneous ceramic polymer composites with ultrahigh ( 78 vol.%) packing density by using the spouted bed granulation technology and subsequent warm pressing. In the granulation process, two ceramic particle size fractions (a-Al2O3) and a thermoplastic polymer (polyvinyl butyral) are assembled to granules. In the process, lm-sized particles are coated with a layer of polymer which contains a second, nm-sized ceramic particles fraction. The mass fractions of each...

Three-dimensional discrete element modeling of micromechanical bending tests of ceramic–polymer composite materials

Michael F. H. Wolff, Vitalij Salikov, Sergiy Antonyuk, Stefan Heinrich & Gerold A. Schneider
Ceramic–polymer composite materials are used throughout materials science, but theoretical models which take into account the microstructural design to predict their mechanical properties are still not fully developed. We present an approach to use the discrete element method to model the mechanical behavior under bending load of dense composite materials made from ceramic particles which are bonded together by polymeric layers. Unlike many other modeling approaches, the internal particulate structure of the material, including the...

Combined FEM-SPH simulations for ice in compression

Niklas Düchting
Within this chapter the outcome of the numerical investigation on a combined FEM-SPH simulation is summed up. The practical approach of this project allows only a rather sim-ple assessment and there are aspects that need to be investigated further. An outlook in-dicates possible aspects for future research. Regarding the correlation of the contact forces the results of the final simulations pre-sented in section 3.3.1 are satisfying. The ability to model the transformation of the ma-terial...

Analysis and engineering of biomolecules and microorganisms: from genome-scale study of pathogens to programming of DNA and cells

Lifu Song
This thesis is consisted of three major but different parts with the general aims of systems level evaluation and engineering of biomolecules and biological systems. In the first part of this thesis, comparative genomic studies of mutans streptococci strains, which are involved in the development of dental caries, were performed for better understanding their pathogenicity at the level of systems biology. A mosaic-like structure of genome arrangement was revealed by genome alignment analysis. Genes related...

Prozessentwicklung zur Integration von enzymatisch-katalysierten Reaktionen in die Reaktivrektifikation

Rene Heils
Die Integration von Reaktion und Trennung in der Reaktivrektifikation ist eine etablierte Methode zur Prozessintensivierung. Aufgrund der thermischen Belastung ist die Verwendung von enzymatischen Katalysatoren in der Reaktivrektifikation weitgehend unerforscht. In dieser Arbeit wurden Methoden zur Immobilisierung und zum Austausch des Katalysators weiterentwickelt, um schließlich in experimentellen und simulationsbasierten Untersuchungen erstmals die Umsetzung von enzymspezifischen Reaktionen in der Reaktivrektifikation aufzuzeigen.

Low powered, tunable and ultra-light aerographite sensor for climate relevant gas monitoring

Oleg Lupan, V. Postica, Matthias Mecklenburg, Karl Schulte, Yogendra Kumar Mishra, Bodo Fiedler & Rainer Adelung
Increasing atmospheric CO2 gas pollution and emergence of new types of green energy sources require continuous environmental monitoring. In this context, fast, efficient, light, robust, and reliable gas sensors that can work at room temperature are in high demand. We report on a low-powered type of ultra-light sensor, based on a 3-D-microtube network from a 2-D graphene/nanographite, called aerographite, and a method to tune the nanosensor’s selectivity by a simple variation of the applied bias...

Integrating technology

Reinhard Bachmann
Review of "Expertise and Innovation - Information Technology Strategies in the Financial Services Sector, R. Fincham, J. Fleck, R. Procter, H. Scarborough, M. Tierney and R. Williams. Oxford, New York: Oxford University Press, 1994. 334 pp. GBP 35.00. ISBN 0-19-828904-9". This book is the result of a multidisciplinary research project on the development and application of information technology (IT) in retail financial services.

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