1,711 Works

Scaling laws of nanoporous gold under uniaxial compression : effects of structural disorder on the solid fraction, elastic Poisson's ratio, Young's modulus and yield strength

Benedikt Roschning & Norbert Huber
In this work the relationship between the structural disorder and the macroscopic mechanical behavior of nanoporous gold under uniaxial compression was investigated, using the finite element method. A recently proposed model based on a microstructure consisting of four-coordinated spherical nodes interconnected by cylindrical struts, whose node positions are randomly displaced from the lattice points of a diamond cubic lattice, was extended. This was done by including the increased density as result of the introduced structural...

Synthesis of uniform bulk nanoporous palladium with tunable structure

Shan Shi, Jürgen Markmann & Jörg Weißmüller
This work presents systematic investigations on the synthesis of hierarchical nanoporous Pd via electrochemical dealloying of CuPd alloys in sulfuric acid. The impact of electrode potential, dealloying temperature, and additional annealing on microstructure and morphology is explored. Dealloying Cu85Pd15 in 1M sulfuric acid at elevated temperature provides a facile strategy to produce bulk nanoporous Pd samples which are uniform, hierarchically nanoporous, and free of macro-scale cracks. The question “Why will one-step template-free dealloying yield a...

A geometric model for the fracture toughness of porous materials

Hans Jelitto & Gerold A. Schneider
Different models for the fracture toughness, KIC, of porous materials have been proposed to describe KIC as a function of the porosity P. They have in common that beside P at least one additional parameter exists that has to be adjusted to the measured data. Based on the cubic structure, we present a geometrical 3D model without any arbitrary parameter, which predicts the KIC the toughness, GC, and the Young's modulus, E, of a porous...

Functionalisation of metal-polymer-nanocomposites : chemoelectromechanical coupling and charge carrier transport

Jana Wilmers & Swantje Bargmann
Electrochemical actuation in nanoporous metals is achieved by impregnation of the material’s pore space with a ionic conductor, typically an aqueous electrolyte. These hybrid actuators exhibit fully reversible deformation and mechanical properties that can be controlled by electric signals. Recently, set-ups have been proposed in which the nanoporous metal’s surface is additionally coated with a conjugated polymer, resulting in a nanocomposite that exhibits strongly increased actuation strains compared to the pure metal while still retaining...

Skeletonization, geometrical analysis, and finite element modeling of nanoporous gold based on 3D tomography data

Claudia Richert & Norbert Huber
Various modeling approaches simplify and parametrize the complex network structure of nanoporous gold (NPG) for studying the structure–property relationship based on artificially generated structures. This paper presents a computational efficient and versatile finite element method (FEM) beam model that is based on skeletonization and diameter information derived from the original 3D focused ion beam-scanning electron microscope (FIB-SEM) tomography data of NPG. The geometrical skeleton network is thoroughly examined for a better understanding of the NPG...

Piezoelectric gold : strong charge-load response in a metal-based hybrid nanomaterial

Charlotte Stenner, Li-Hua Shao, Nadiia Mameka & Jörg Weißmüller
Impregnating the pores of nanoporous gold with aqueous electrolyte yields a hybrid nanomaterial with two separate and interpenetrating charge transport paths, electronic conduction in the metal and ionic conduction in the electrolyte. As the two paths are capacitively connected, space‐charge layers along the internal interfaces are coupled to electric potential differences between the paths and can be controlled or detected thereby. The present experiments show that the space charge couples to mechanical deformation of the...

Performance evaluation of small sized powdered ferric hydroxide as arsenic adsorbent

Muhammad Usman, Ioannis A. Katsoyiannis, Manassis Mitrakas, Anastasios I. Zouboulis & Mathias Ernst
The small sized powdered ferric oxy-hydroxide, termed Dust Ferric Hydroxide (DFH), was applied in batch adsorption experiments to remove arsenic species from water. The DFH was characterized in terms of zero point charge, zeta potential, surface charge density, particle size and moisture content. Batch adsorption isotherm experiments indicated that the Freundlich model described the isothermal adsorption behavior of arsenic species notably well. The results indicated that the adsorption capacity of DFH in deionized ultrapure water,...

Active vibration control using the centrifugal forces of eccentrically rotating masses

Richard Bäumer
In this PhD thesis, active vibration control using eccentrically rotating masses is investigated. The basic layout of the so-called twin rotor damper (TRD) consists of two eccentric control masses rotating about two parallel axes. In a preferred mode of op-eration, the continuous rotation mode, both control masses rotate in opposite direction with a constant angular velocity. Under further operational constraints, the generated centrifugal forces superimpose to a harmonic control force in a single direction. The...

Exploring the acceptance of ubiquitous computing-based Information services in brick and mortar retail environments : an integration of UTAUT2 and media system dependency theory

Sara Kheiravar
Stationäre Einzelhändler setzen zunehmend digitale Technologie ein, um ihren Kunden bei Informationssuche und -bewertung zu unterstützen. Die eingesetzten Technologien werden als In-Store Ubiquitous Computing basierende Informationstechnologien bezeichnet. Erkenntnisse zu Akzeptanz dieser Technologien durch Kunden stehen noch aus. In dieser Dissertation wird ein Kontext spezifisches Modell zur Technologieakzeptanz basierend auf UTAUT2 entwickelt und um Nutzer, Service und Technologie relevante Aspekte erweitert. Das abgeleitete Modell wird mit Hilfe eines Strukturgleichungsmodells (PLS) auf Basis einer auf Szenarien basierenden...

LWE-based encryption schemes and their applications In privacy-friendly data aggregation

Daniela Becker
Wir konstruieren neue Verschlüsselungssysteme basierend auf dem Learning With Errors Problem and lösen die folgenden Probleme: Durch datenschutzfreundliche Aggregation lassen sich Daten von mehreren Nutzern unter Wahrung ihrer Privatsphäre aggregieren ohne dem Aggregator zu vertrauen. Unsere Systeme verbessern sowohl Laufzeit als auch Bandbreite der Verschlüsselung, insbesondere die Entschlüsselung ist ca. 150 Mal schneller als zuvor. Überdies stellen wir die erste Lösung für datenschutzfreundliche Werbung auf sozialen Medien vor - unsere Architektur hat hohe Sicherheits- und...

Approximation of mechanical properties of sintered materials with discrete element method

Maksym Dosta, Robert Besler, Christian Ziehdorn, Rolf Janßen & Stefan Heinrich
Sintering process is a key step in ceramic processing, which has strong influence on quality of final product. The final shape, microstructure and mechanical properties, e.g. density, heat conductivity, strength and hardness are depending on the sintering process. In order to characterize mechanical properties of sintered materials, in this contribution we present a microscale modelling approach. This approach consists of three different stages: simulation of the sintering process, transition to final structure and modelling of...

Fabrication of composites via spouted bed granulation process and simulation of their micromechanical properties

Eduard Eichner, Maksym Dosta, Stefan Heinrich & Gerold A. Schneider
In this contribution numerical simulation of Young’s modulus of copper-polymer composites is presented. For the simulation of the composites the Bonded-Particle-Model was applied. The model allows representing of the structure of composite materials realistically. The polymer matrix, which surrounds the particles, was represented as network of solid bonds connecting copper particles. Simulation results were validated based on mechanical determination of modulus of elasticity. The modulus of elasticity was approximated in experiments as well as in...

Electromagnetic modeling and optimization of through silicon vias

David Dahl
Die vorliegende Arbeit beschreibt die Simulation und den Entwurf von Durchkontaktierungen in Siliziumsubstraten (TSVs), wie sie in der Aufbau- und Verbindungstechnik für integrierte Schaltkreise Anwendung finden. Numerische Verfahren von vergleichsweise hoher numerischer Effizienz werden entwickelt und angewendet, um realistische Anordnungen mit großen Anzahlen von TSVs simulieren zu können. Die vorgestellten Methoden werden mit alternativen Ansätzen korreliert und aus Parametervariationen werden Designempfehlungen abgeleitet. Des Weiteren werden mehrere Messstrukturen mit TSVs entwickelt und Messergebnisse für diese mit...

Einbindung von Brennstoffzellenanlagen in schiffstechnische Systeme

Christoph Gentner
The high temperature polymer electrolyte (HT-PEM) fuel cell technology is investigated with regard to its suitability for the electrical energy supply of marine systems. For this purpose, the dynamic and stationary behavior of a pilot fuel cell system is analyzed. For model calibration, experiments are carried out on a methanol reformer, which supplies synthesis gas for the HT-PEM fuel cell stack. A thermodynamic model of the fuel cell system (evaporator, methanol reformer, cell stack) with...

Numerical investigation of compaction of deformable particles with bonded-particle model

Maksym Dosta, Clara Costa & Hazim Ali Al-Qureshi
In this contribution, a novel approach developed for the microscale modelling of particles which undergo large deformations is presented. The proposed method is based on the bonded-particle model (BPM) and multi-stage strategy to adjust material and model parameters. By the BPM, modelled objects are represented as agglomerates which consist of smaller ideally spherical particles and are connected with cylindrical solid bonds. Each bond is considered as a separate object and in each time step the...

Fermentation of mixed substrates by Clostridium pasteurianum and its physiological, metabolic and proteomic characterizations

Wael Sabra, Wei Wang, Sruthi Surandram, Christin Prescher & An-Ping Zeng
Clostridium pasteurianum is becoming increasingly attractive for the production of chemicals and fuels such as n-butanol and 1,3-propanediol. Previously we have shown that dual substrate fermentation using glucose and glycerol enhanced the cell growth and butanol production significantly. Although C. pasteurianum can grow efficiently with either glucose or glycerol alone, under certain conditions, glucose limitation in the mixed substrate fermentation leads to growth cessation. To understand this phenomenon and for process optimization, fermentation experiments were...

Computability theory

Karl-Heinz Zimmermann
This publication provides an introduction to the theory of computation.

Information optimum design of discrete LDPC decoders for irregular codes

Maximilian Stark
Powerful forward error correction is inevitable for every modern communication system. However, leveraging all error correction capabilities offered by channel codes like low density parity check (LDPC) codes, introduces the burden of computationally demanding decoding at the receiver. Especially in power or latency restricted scenarios, efficient decoding algorithms are of crucial importance. In this thesis a novel decoding approach combining ideas from information theory and machine learning was developed, resulting in a so-called information-optimum LDPC...

Closed form probabilistic analysis of lamination parameters for composite structures

Benedikt Kriegesmann
Probabilistic analyses allow predicting the stochastic distribution of an output variable (e.g., the buckling load of a structure) based on the stochastic distribution of input parameters (e.g., material and geometric properties). In the probabilistic analysis of composite structures, one important quantity that can be subject to scatter is the ply or fiber orientation of the layers. Laminates with a large number of plies lead to a large number of random variables, which makes the probabilistic...

Design of cylindrical shells using the single perturbation load approach : potentials and application limits

Benedikt Kriegesmann, Eelco Luc Jansen & Raimund Rolfes
The Single Perturbation Load Approach (SPLA) is a promising deterministic procedure on the basis of mechanical considerations to determine reasonable design loads for cylindrical shells in axial compression. In this paper, two main issues are identified that should be understood better in order to appreciate the potential and the limits of application of the SPLA. Firstly, the question whether a single perturbation load in general represents a “worst case” imperfection is addressed. Secondly, the influence...

Lurie solution for spherical particle and spring layer model of interphases : its application in analysis of effective properties of composites

Lidiia V. Nazarenko, Swantje Bargmann & Henryk K. Stolarski
A new approach to the determination of equivalent inhomogeneity for spherical particles and the spring layer model of their interphases with the matrix material is developed. To validate this approach the effective properties of random composites containing spherical inhomogeneities surrounded by an interphase material of constant thickness are evaluated. The properties of equivalent inhomogeneity, incorporating only properties of the original inhomogeneity and its interphase, are determined employing a new approach based on the exact Lurie's...

Identification and control of the laser-based synchronization system for the European X-ray Free Electron Laser

Michael Heuer
The European X-ray Free-Electron Laser is currently under construction at theDeutsches ElektronenSynchrotron in Hamburg, Germany. This linear accelerator, with a length of 3.4 km,will generate extremely intense and short X-ray laser light pulses with a duration in the femtosecondrange and wavelengths down to 0.05 nm. These laser pulses provide physicists with alight source to take a closer look into small structures on atomic scale.Those precise measurements require timing with an error margin in the femto-second...

Relation between observed and perceived traffic noise and socio-economic status in urban blocks of different characteristics

Malte Von Szombathely, Myriam Albrecht, Jobst Augustin, Benjamin Bechtel, Isabel Dwinger, Philine Gaffron, Anne Caroline Krefis, Jürgen Oßenbrügge & Anke Strüver
Living in cities offers many benefits and thus more and more people are living in urban areas. However, the concentration of human activities also creates environmental stressors with severe influence on people’s health and well-being. Noise is an environmental stressor with known health impact. Despite this, studies investigating small-scale difference in noise exposure and annoyance are lacking. Against this background, this case study investigates environmental justice empirically, focusing on the distribution of road traffic noise...

Nanoporous-gold-based composites : toward tensile ductility

Ke Wang, Aaron Kobler, Christian Kübel, Hans Jelitto, Gerold A. Schneider & Jörg Weißmüller
We report on mechanical tests on interpenetrating-phase nanocomposite materials made by vacuum impregnation of nanoscale metal networks with a polymer. The metal component is nanoporous gold made by dealloying, whereas two epoxy resins and polyurethane are explored as the polymer component. The composites are strong and deformable in compression. Although previous observations invariably indicate tensile brittleness for nanoporous gold, composite samples made from cm-sized nanoporous samples enable macroscopic tensile and four-point bending tests that show...

Prekäre Privilegien - Wie Ingenieur_innen ihren Alltag gestalten

Jette Hausotter
A qualitative study based on 21 problem-centered interviews examines the impact of precarization on engineers in Germany. An empirical typology is elaborated and leads to a comparative analysis of how precarious privileges affect the everyday life of engineers and how they position themselves in conflicts between their own wishes and the social conditions. The results are evaluated regarding theories of intersectionality and social inequality, gender studies and sociology of work an employment.

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