Sommersemester 2017

Prof. Dr. Tobias Kraus

INM Leibniz Institut für neue Materialien, Saarbrücken

17.05.2017, 17.00h     
H 009, Am Schwarzenberg-Campus 5, TUHH
pdf version


Self‐assembly of particle-based materials: Mechanisms and their application for flexible electronics

Concrete, paint, rubber, and many other important materials are prepared from mixtures of particles, polymers, solvents, and additives. Their microstructures are often heterogeneous and hard to predict; they limit the performance. This talk will discuss how self‐assembly can be used to gain control over microstructure and properties of particlebased materials. We seek self‐assembly mechanisms that work with relevant materials, do not require complex chemistry, and are compatible with established materials manufacturing processes such as spray coating, doctor blading, and inkjet printing.

I will discuss particle‐based electronic materials that illustrate our strategy. Metal spheres, rods, and wires with characteristic dimensions between 2 nm and 50 nm and narrow size distributions were chemically synthesized and coated with organic shells of varying thickness, density, and chemical nature. We determined shape and size using electron microscopy and scattering techniques. Colloidal interactions between the hybrid particles in different solvents were systematically quantified through concentration‐ and temperature‐dependent light and X‐ray‐scattering experiments. We study the onset of agglomeration, agglomeration rates, and the geometry of the agglomerates. Interfaces are used to confine the particles and template self‐assembly. I will show that monolayers and multilayers of nanoparticles, supraparticles and structured nanocomposites can be deposited using the right combination of interactions and confinement.

Figure: I will discuss how the self‐assembly of chemically synthesized nanoparticles (for example, metals) can be tuned through chemistry, confinement, and external stimulation to yield functional structures for electronics

We find that mobility and interaction at different length scales are central features of self‐assembly mechanisms for particle‐based materials. Their interplay affects whether the resulting materials reach equilibrium structures or are kinetically dominated. In practice, viscosity and time scales are often not freely adjustable– there are large differences, for example, between inkjet printing and 3D printing via fused deposition modeling–and rule out certain self‐assembly mechanisms. I will discuss such boundary conditions on the example of transparent electrode layers that self‐assemble from ultrathin gold wires.

As an outlook, I will discuss particle‐based structures that can reconfigure in the material during its lifetime. First examples of “active” nanocomposites based on self‐assembly and on disassembly of particles can change their properties upon stimulation. We explore such materials for a digital world where even materials are connected to networks.


 

Dr. Rebecca Janisch

Interdisciplinary Centre for Advanced Materials Simulation (ICAMS),
Ruhr-Universität Bochum

03.05.2017, 17.00h     
H 009, Am Schwarzenberg-Campus 5, TUHH
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Grain boundary properties: Insights from atomistic simulations and their use in mechanical modeling of materials

Modern structural materials are seldom single crystals, but exhibit a polycrystalline, multiphase, often hierarchical microstructure. The thus occurring interfaces in the microstructure have significant influence on the macroscopic properties. Nowadays even tailored microstructures, containing certain arrangements of grain boundaries with specific properties that can be tuned by segregation engineering, are within experimental reach. This gives additional impetus to the development of predictive material models that bridge between the atomistic details of grain boundaries and the effective properties of the microstructure, and can help to identify microstructures with optimized mechanical properties.

Numerical simulation methods, that either allow the study of relevant processes on their characteristic length scale, or can be used to pass on information from finer to coarser length scales, are common tools in this respect. In the presentation some examples of atomistic studies of grain boundaries will be given that illustrate current developments and the challenges that one has to face when trying to extract effective mechanical behavior and to link it to fundamental physical and geometrical properties of the interfaces. The focus will be on lamellar TiAl alloys, in which the high density of interfaces can rule the overall mechanical behavior. High resolution experimental methods exist to analyze the underlying atomistic processes. However, since these processes are not independent, often several of them occur at the same time. To isolate the intrinsic deformation mechanisms of grain boundaries we have carried out molecular statics and molecular dynamics simulations of bicrystal shear at different boundaries. Four distinct mechanisms could be identified, namely rigid grain sliding, grain boundary migration, coupled sliding and migration, and dislocation nucleation and emission – that could be related to structural features of the grain boundaries as well as physical properties of the material. Their relevance for some of the experimentally observed phenomena will be discussed.


 

Prof. Helmut Cölfen

Physikalische Chemie,
Universität Konstanz

05.04.2017, 17.00h     
H 009, Am Schwarzenberg-Campus 5, TUHH
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Bio-Inspired Organic-Inorganic Hybrid Materials

Biominerals teach excellent lessons about advanced materials design. Their structural design is optimized for the specific materials purpose and often, the beneficial properties are generated on several hierarchy levels. Consequently, Biominerals are an intense subject of research to reveal the design principles. This led to the discovery of amorphous or even liquid precursors to single crystals in Biomineralization and additive controlled crystallization events. Non classical particle mediated crystallization pathways were found to be important besides the classical crystallization path.

This presentation begins with Nacre mimics and self-assembled hierarchical layered materials from anisotropic nanoparticles aligned by modified self-assembling polyoxazoline polymers with mesogens forming liquid crystals as driving force towards crystallization. Furthermore, an attempt towards combination of several advantageous biomineral properties, namely the fracture resistance of Nacre, the wear resistance of chiton teeth and the magnetic properties of magnetotactic bacteria will be reported. The third example is bio-inspired elastic cement synthesized via a non-classical crystallization pathway. Calcium-silicate-hydrate (C-S-H) nanoparticles, the glue in concrete, are stabilized by copolymers with anionic groups and moieties able to form hydrogen bonds. These polymers bind to C-S-H nanoparticles at pH 12 and stabilize them. Further pH increase leads to destabilization and subsequent nanoparticle aggregation in crystallographic register forming a mesocrystal with a similar structure to a sea urchin spine. This mesocrystal is elastic and can be bent without breaking. This is a further demonstration that bio-inspired synthesis and structuration of organic-inorganic hybrid materials can lead to significant materials improvement – even for the most used synthetic material.

Bending by micromanipulation of bio-inspired mesocrystalline elastic cement.


Wintersemester 2016/2017

Prof. Jörg Libuda

Department Chemie und Pharmazie,
Friedrich-Alexander Universität Erlangen-Nürnberg

25.01.2017, 17.00h     
H 009, Am Schwarzenberg-Campus 5, TUHH
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Functional molecular layers on atomically-defined oxide surfaces and nanoparticles

Functional molecular films on oxide surfaces are at the heart of emerging technologies. Potential fields of application include molecular electronics, solar energy conversion, catalysis, sensor development, or biointerfacial engineering. In spite of this future potential, our understanding of molecule-oxide interfaces is still poor at the atomic level. Whereas the surface science approach has provided a wealth of knowledge on organic film growth on metals, organic-oxide interfaces have remained largely unexplo­red, a situation which we denote as the materials gap in organic thin film science.

In this presentation, selected results from the DFG Research Unit FOR 1878 “funCOS – Functional Molecular Structures on Complex Oxide Surfaces” are reviewed. funCOS started in 2013, aiming to fill the above mentioned gap and to provide the fundamental knowledge basis to design tailor-made functional films on oxides. funCOS follows a knowledge-driven strategy, starting from a rigorous surface science approach. The Research Unit comprises a team of 15 research groups from experimental and theoretical surface and interface science. Combining complementary surface spectroscopies and microscopies, we start from ultrahigh vacuum experiments on ordered single crystal surfaces and bridge the gap between ideal and real conditions by exploring anchoring of molecular films up to ambient pressure and in liquid environments. In the first step, the reaction mechanisms, bonding geometries, energetics and the kinetics of molecular anchoring is investigated with simple test molecules on prototype oxides. Subsequently, this knowledge is transferred the anchoring of porphyrin derivatives. Exemplifying reactions with carboxylate linkers, we explore role of substitution patterns, multiple anchoring, and chelating anchors to control the molecular orientation, formation kinetics, and stability of the films. Interestingly, a strong dependence on the surface structure is observed which can be rationalized on the basis of the cation arrangement and their accessibility at the surface. Finally, we discuss the influence of the molecular orientation and surface structure on metalation reactions and the role of water and hydroxyl groups in molecular anchoring reactions.


 

Prof. Christof Wöll

Institute of Functional Interfaces (IFG),
Karlsruhe Institute of Technology (KIT)

18.01.2017, 17.00h     
K 0506, Denickestr. 15, TUHH
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Programmed assembly of molecular frameworks: A new class of designer solids?

The demand for advanced materials with novel combinations of different functionalities requires the development of new types of solids. In this context, supramolecular chemistry holds unique prospects. Self-assembly of one or different types of functional units can be employed to fabricate crystalline arrangements, yielding complex but at the same time structurally well defined, highly ordered “Designer Solids”, which exhibit functionalities going well beyond that provided by the individual building blocks. In this presentation, it will become evident that a recently introduced class of supramolecular materials, metal-organic frameworks, or MOFs, carry an enormous potential with regard to the fabrication of solids with unusual physical properties [1]. MOFs are stable materials, with decomposition temperatures well above 200°C (in some cases > 500°C). Using selected examples, we will demonstrate the interesting, and often surprising (e.g. negative thermal expansion coefficient), mechanical, electronic, magnetic and optical properties of these molecular, crystalline materials.
This fairly recent class of porous solids, introduced in the 1990s, is very large in number, already more than 50.000 different structures have been reported. In order to exploit the properties of these materials for applications in solid state physics, we have developed a liquid phase epitaxy (LPE) process, which allows growing MOFs on modified substrates using a layer-by-layer procedure [1]. The availability of cm-sized, highly oriented MOF thin films with thickness in the µm-regime allows to determine the basic physical properties (mechanical [2], optical [3], electronic [4], magnetic [5]) of these porous, molecular solids using standard methods. A unique feature of the LPE-process is the ability to use heteroepitaxy [6] to add further functionality to these materials by creating multilayer systems [7].
The porous nature of these crystalline solids opens up the prospect of adding additional functionality by placing molecules [8] or nanoobjects inside the voids within the MOFs, e.g. metal clusters or dye molecules [9]. We will demonstrate the potential of this approach by loading the three-dimensional porous scaffolds, or “designer solids”, with metal-containing molecules such as ferrocene and then determining the change in conductivity using electrochemistry [10].

[1] H. Gliemann und Ch. Wöll, Materials Today 15, 110 (2012)
[2] S. Bundschuh, O. Kraft, H. Arslan, H. Gliemann, P. Weidler, C. Wöll, Appl. Phys. Lett. 101, 101910 (2012)
[3] E. Redel, Z. Wang, S. Walheim, J. Liu, H. Gliemann, Ch. Wöll, Appl. Phys. Lett., 103, 091903 (2013)
[4] J.Liu, W.Zhou, J.Liu, I.Howard, G.Kilibarda, S.Schlabach, D.Coupry, M.Addicoat, S.Yoneda, Y.Tsutsui, T. Sakurai, S.Seki, Z.Wang, P.Lindemann, E.Redel, T.Heine, C.Wöll, Angew. Chemie, 54, 7441 (2015)
[5] M.E. Silvestre, M. Franzreb, P.G. Weidler, O.Shekhah, Ch. Wöll, Adv. Funct. Materials, 23, 1093 (2013)
[6] Z.Wang, J. Liu, B. Lukose, Z. Gu, P.Weidler, H.Gliemann, T. Heine, C. Wöll, Nano Letters, 14, 1526 (2014)
[7]. J.P. Best, J. Michler, J. Liu, Zh. Wang, M. Tsotsalas, X. Maeder, S. Röse, V. Oberst, J. Liu, S. Walheim, H. Gliemann, P.G. Weidler, E. Redel, Ch. Wöll, Appl. Phys. Lett., 107, 101902 (2015)
[8] L. Heinke, Z. Gu, Ch. Wöll, Nature Comm., 5, 4562 (2014)
[9] W. Guo, J. Liu, P.G. Weidler, J. Liu, T. Neumann, D. Danilov, W. Wenzel, C. Feldmann, Ch. Wöll, Phys.Chem.Chem.Phys., 16, 17918 (2014)
[10] A. Dragässer, O. Shekhah, O. Zybaylo, C. Shen, M. Buck, C. Wöll, D. Schlettwein, Chem.Comm. 48, 663 (2012)



Prof. Andreas Walther

A3BMS Lab – Adaptive, Active and Autonomous Bioinspired Material Systems,
Institute for Macromolecular Chemistry,
Freiburg Materials Research Center (FMF), and Freiburg Institute for Interactive Materials and Bioinspired Technologies (FIT),
Albert-Ludwigs-University Freiburg

11.01.2017, 17.00h     
H 009, Am Schwarzenberg-Campus 5, TUHH
pdf version

Static and dynamic bioinspired self-assembled material systems

Biology is a source of inspiration for materials science by demonstrating macroscopic materials with advanced functionalities and excellent mechanical properties. However, although these rather static architectures stimulate large interest, even more thoughtprovoking are the dynamic, kinetically controlled processes and temporal evolution of structures in complex biological systems. These are orchestrated through feedback loops and require energy input and dissipation to allow non-equilibrium materials and full spatiotemporal control.
In man made self-assemblies we have mastered to a large extent near-equilibrium structure formation in space and have gained an increasing understanding of how to construct very complex, hierarchically structured soft matter by using co-assemblies, competing interactions and hierarchical length scales. This has allowed to create real-life materials with unprecedented functionalities, inaccessible without delicate control over molecular interactions and sophisticated nano- and mesostructuration. The next step is to master temporal control in self-assemblies. This requires kinetic control of opposing reactions (builtup/ destruction), internal feedback systems or the use of energy dissipation to sustain structures only as long as a chemical fuel is available. These approaches keep systems forcefully away from equilibrium and potentially allow neat access to temporal control.
In this talk I will present concepts for bioinspired materials formed in both static and dynamic conditions. The first part will deal with rather static, nacre-inspired high-performance nanocomposites, in which lightadaptive properties can be encoded through co-assembly and energy transfer approaches. The second part will focus on a platform concept, which allows to program self-assembling systems outside equilibrium with a lifetime by kinetic control of promoter/deactivator pairs and a simple internal feedback system. This will be showcased for different self-assembling systems.

Recent references:
[1] Self-Assembled Artificial Nacre: Nano Lett. 2016, 16, 5167, Angew. Chem. Int. Ed. 2015, 54, 8653; Nat. Commun. 2015, 6, 5967; ACS Appl. Mater. Interfaces 2013, 5, 3738; Adv. Mater. 2013, 25, 5055;
[2] Time Programmed Dynamic Materials: Nano Lett., 2015, 15, 2213, Angew. Chem. 2015, 54, 13258 , Review: Soft Matter 2015, 11, 7857.



Prof. Robin N. Klupp Taylor

Nanostructured Particles Group,
Friedrich-Alexander Universität Erlangen-Nürnberg, Germany

07.12.2016, 17.00h     
H 009, Am Schwarzenberg-Campus 5, TUHH
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Crystal growth on curved surfaces: Novel approaches for the synthesis of anisotropic nanostructured materials

Small particles are widely exploited in a broad range of functional materials, ranging from dense, close-packed layers to dilute dispersions. The interactions involved in such particle ensembles are generally isotropic and the properties of the resulting material can often be regarded as equivalent to those of an effective, isotropic medium. There is however, a rapidly growing interest in the development of particles with anisotropic character. This is driven, in part, by the promise of using such particles as components of adaptive devices or as building blocks for the self- or directed assembly of complex and functionally optimized hierarchical structures in applications as diverse as catalysts, special effect pigments, sensors and biomedical diagnostics and therapeutics.

In this presentation I will focus on the creation of interfacial anisotropy, a topic which has received rapidly growing attention in recent years. In particular the exciting promise for fundamental and applied research of so-called patchy and Janus particles will be introduced. In this regard, my own research group’s activities to synthesise patchy particles by extremely simple and scalable approaches will be highlighted. In contrast to most other reported methods, we avoid the use of templates and phase boundaries but rather employ electroless metallization reactions. Here we rely on the enrichment of the metal precursor and reducing agent at the core particle surface and subsequent heterogeneous nucleation and surface diffusion driven conformal growth of the metal. To ensure a narrow distribution of metal patch numbers and coverages, we have undertaken a programme of replacing the initially-developed batch processes1,2 with setups based on a continuous flow static mixers3. On the one hand, this has enabled systematic studies of the materials chemistry behind the surface conformal crystal growth. Here I will illustrate our use of advanced characterisation techniques such as analytical ultracentrifugation. On the other hand, we have expanded beyond the original core particle systems of colloidal silica1 and polystyrene2 to demonstrate our fabrication methodology for technical substrates of various compositions. Target applications are wide ranging, from plasmonic sensors and photovoltaic enhancement to catalysis and biomaterials.

[1] H. Bao, R. N. Klupp Taylor, W. Peukert, One-pot Colloidal Synthesis of Plasmonic Patchy Particles, Adv. Mater. 2011, 23, 2644
[2] H. Bao, T. Bihr, A.-S. Smith, R. N. Klupp Taylor, Facile colloidal coating of polystyrene nanospheres with tunable gold dendritic patches, Nanoscale 2014, 6, 3954.
[3] T. Meincke, H. Bao, L. Pflug, M. Stingl, R.N. Klupp Taylor, Heterogeneous nucleation and surface conformal growth of silver nanocoatings on colloidal silica in a continuous flow static T-mixer Chem. Eng. J. 2017, 308, 89.

 


Sommersemester 2016

Prof. Dr. Siegfried Schmauder

Institute for Materials Testing, Materials Science and Strength of Materials (IMWF),
University of Stuttgart, Germany

01.06.2016, 16.30h     
O 018, Eißendorfer Str. 38, TUHH
pdf version

Multiscale Materials Modelling
Procedures-Examples-Challenges

In the recent past, multiscale materials modelling became a central idea in understanding present day complex composites and in making progress in the development of advanced materials. There exists, however, a discrepancy between available results described in literature and the expression “multiscale modelling”, because typically cases are treated with two length scales only and some­times additional one or two time scales.

This presentation will describe several successfully running multiscale examples which are used in ana­lyzing pipeline steel weldments, metal/ceramic interfaces and fatigue problems of metals which are employed for a better understanding of physical phenomena in the materials leading to their de­for­mation and fracture behavior. It will be shown that not only several length and time scales are re­quired but also several methods have to be involved for performing successful hierarchical analy­ses with quantitative results in the field of modern materials research.

In addition, studies will be shown which provide the basis for the development of new material alloys when taking ab initio, Monte Carlo or Molecular Dynamics modelling approaches into account. As examples solid solution hardening is considered for Fe-base ma­te­rials or fatigue loading for polycrystalline steels where property predictions are in close agree­ment to experimental findings.

S. Schmauder, D. Uhlmann, G. Zies, "Experimental and numerical investigations of two material states of the material 15 NiCuMoNb (WB 36)", Computational Materials Science 25, pp. 174-192 (2002).
S. Schmauder, "Simulation als Instrument der Werkstoffentwicklung", Metall 63, pp. 295-297 (2009).
S. Schmauder, C. Kohler, "Atomistic simulations of solid solution strengthening of α-iron", Computational Materials Science 50, pp. 1238-1243 (2011).
S. Schmauder, U. Weber, A. Reuschel, M. Willert, "Simulation of the Mechanical Behaviour of Metal Matrix Composites", Materials Science Forum 678, pp. 49-60 (2011).
Z. Bozic, S. Schmauder, M. Mlikota, M. Hummel, „Multiscale fatigue crack growth modelling for welded stiffened panels”, Fatigue & Fracture of Engineering Materials & Structures 00, pp. 1-12 (2014).


Dr. Gennady Gor

Center for Materials Physics and Technology, Naval Research Laboratory, Wahington, DC, USA

25.05.2016, 17.00h     
O 018, Eißendorfer Str. 38, TUHH
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Mechanical effects of fluids adsorption by nanoporous materials

In my talk I will focus on two mechanical effects related to fluid adsorption by porous materials: adsorption-induced deformation and change of elastic properties of fluids during adsorption. I will start from reviewing recent experimental findings and continue with presenting my theoretical models for them. The development of quantitative theories of these effects will provide the opportunity to employ them for new sensing technologies and new approaches for the characterization of nanoporous materials [1].

Adsorption-induced deformation is a change of shape or volume of a porous sample during adsorption. It could be either expansion or contraction, and while the former is well understood, the latter is still puzzling due to the apparent contradiction with the Gibbs adsorption equation [2]. I will show how this apparent contradiction can be resolved by relating the strain of the solid to the change of the surface stress due to adsorption. I will present the results of the surface stress calculations based on ab initio molecular dynamics simulations.

In the second part of my talk I will focus on the effects of nanoconfinement on the mechanical properties of the adsorbed fluid. Recent ultrasonic experiments have shown that the elastic modulus of argon adsorbed in nanoporous glass noticeably differs from the elastic modulus of bulk liquid argon. I will present the results of a molecular modeling study which explains these experimental observations and sheds light on possibilities of ultrasonic investigation of nanoporous materials [3].

1. Gor, G. Y.; Bertinetti, L.; Bernstein, N.; Hofmann, T.; Fratzl, P. & Huber, P. Appl. Phys. Lett., 2015, 106, 261901
2. Gor, G. Y. & Bernstein, N. Phys. Chem. Chem. Phys., 2016, 18, 9788-9798
3. Gor, G. Y.; Siderius, D. W.; Rasmussen, C. J.; Krekelberg, W. P.; Shen, V. K. & Bernstein, N. J. Chem. Phys., 2015, 143, 194506


Dr. Varun P. Rajan

Laboratory for Multiscale Mechanics Modeling, EPFL, Switzerland

27.04.2016, 17.00h     
O 018, Eißendorfer Str. 38, TUHH
pdf version

 

Microstructural design of fiber composites

Composites reinforced with ceramic fibers are often brittle and fail without warning. One route for improving the composite response involves design of the composite microstructure using multiple fiber types, directions of reinforcement, and/or length scales. Some of these concepts can now be realized in the lab, via techniques such as additive manufacturing. However, the space of possible microstructures is too large to be explored by trial-and-error alone; thus, mechanics models are needed to guide composite manufacturing efforts. These models should connect the overall composite properties to the constituent properties and composite microstructure.

In this talk, I will present one such class of mechanics models, known as global load sharing (GLS) theory. I will apply GLS theory to two types of fiber composites --- “hybrid” composites, which use fibers of multiple types (e.g., carbon and glass), and “hierarchical” composites, in which fibers span multiple length scales. In both cases, I show that the added microstructural complexity allows different properties, such as strength and toughness, to be traded-off. In the case of hybrid composites, I have identified optimal composites that are stronger, stiffer, and tougher than the corresponding single-fiber-type composite. As an added benefit, these composites are “pseudo-ductile,” exhibiting non-linear behavior before fracture. Preliminary comparisons of the model with experimental results on discontinuous-fiber hybrid composites are also generally good. In the case of hierarchical composites, I have demonstrated that the optimal composites are quite complex, comprising long fibers at lower scales and discontinuous fibers at the highest. Although making such composites in the lab will be challenging, they promise to be substantially tougher and more damage-tolerant than their non-hierarchical counterparts.


Wintersemester 2015/2016

Prof. Narayanan Ravishankar

Materials Research Centre, Indian Institute of Science, Banglore, India

27.01.2016, 16.15h     
K 0506, Denickestr. 15, TUHH
pdf version

 

Structural, microstructural and interfacial engineering of nanostructures and hybrids for applications

Nucleation and growth processes play a key role in controlling the structure, microstructure and chemistry and consequently every conceivable property of advanced functional materials. Our group has been working on wet-chemical methods for the synthesis of nanostructures and hybrids. While these methods are simple and undoubtedly very powerful, the mechanisms of nucleation and growth are poorly understood. In particular, there is an over-emphasis on the role of specific reagents rather than broad principles that are applicable for a wide variety of systems. My talk will focus on three specific issues. In the first part, I will discuss some general principles of morphology evolution during wet chemical synthesis. In particular, the formation of anisotropic structures of high symmetry materials and the associated symmetry breaking mechanisms will be discussed. Specific examples include the growth of ultrathin single crystalline Au nanowires and the formation of plate-shaped structures. I will present some of the newer results on the intriguing structure and properties of the ultrathin metal nanowires. In the second part, I will discuss a general method for the synthesis of nanoporous materials and discuss some of their applications. Some unexpected and interesting results on the stability of these nanoporous systems will be presented. In the third part, I will discuss about the role of heterogeneous nucleation for controlled synthesis of nanoscale hybrids for a variety of applications including catalysis and photovoltaic applications.The overall emphasis will be on illustrating general principles that we have been able to extract based on our research over the past few years and also some thoughts on future directions, applications and possible collaborations.


Prof. Maurizio Fermeglia

Simulation Engineering (MOSE) Laboratory, Department of Engineering and Architecture, University of Trieste, Italy

13.01.2016, 16h     
K 0506, Denickestr. 15, TUHH
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Materials by design: multiscale molecular modeling of nanostructured materials

One of the major goals of computational material science is the rapid and accurate prediction of properties of new materials. In order to develop new materials and compositions with designed new properties, it is essential that these properties can be predicted before preparation, processing, and characterization. Despite the tremendous advances made in the modeling of structural, thermal, mechanical and transport properties of materials at the macroscopic level (finite element (FE) analysis of complicated structures), there remains a tremendous uncertainty about how to predict many critical properties related to performance. The fundamental problem here is that these properties depend on the structure that the material exhibits at a length scale ranging from few to some dozens of nanometers, and this structure depends strongly on the interactions at atomistic scale. In order to substantially advance the ability to design useful high performance materials, it is essential that we insert the chemistry into the mesoscopic (MS) modeling. Currently, atomistic level simulations such as molecular dynamics (MD) or Monte Carlo (MC) techniques allows to predict the structure and properties for systems of considerably large number of atoms and time scales of the order of microseconds. Although this can lead to many relevant results in material design, many critical issues in materials design still require time and length scales far too large for practical MD/MC simulations. Given these concepts, it is than necessary to carry out calculations for realistic time scales fast enough to be useful in design. This requires developing techniques useful to design engineers, by incorporating the methods and results of the lower scales (e.g., MD) to mesoscale simulations [1]. To this aim, we have developed a multiscale molecular modeling protocol, based on the combination of different techniques each of them suitable for the simulation at a given time and length scale. The protocol is able to predict macroscopic properties taking into account the nanostructure and the effect of the interphases/interfaces at nanoscale, thus resulting in a powerful tool for designing nanostructured systems [2]. The talk will describe the details of the multiscale molecular simulation framework, and will focus on some examples of industrial relevance both in material science and in life science [3].

References:

[1] Fermeglia M., Pricl S., Multiscale molecular modeling in nanostructured material design and process system engineering, Computers & Chem.Eng, 33:1701-1710 (2009).

[2]Posocco P., Pricl S., Fermeglia M., In "Modeling and Prediction of Polymer Nanocomposite Properties" edited by Vikas Mittal, Wiley-VCH Verlag GmbH & Co, 1:95-128 (2013)

[3] Scocchi G., Posocco P., Handgraaf J.W., Fraaije J.G.E.M., Fermeglia M., Pricl, S., A complete multiscale modelling approach for polymer-clay nanocomposites, Chemistry - A European Journal, 15:7586-7592(2009).


Dr. Jonathan Berger

Mechanical Engineering Department, University of California, Santa Barbara, USA

11.11.2015, 17ct      
K0506, Denickestr. 15, TUHH
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Metamaterials with extreme stiffness and strength

Metamaterials have mechanical properties that are a function of their geometry on the mesoscale, the intermediate scale between microsctructural features of the constituent material(s) and the macroscale on which loads and parts are described. These materials can fill an otherwise unpopulated region of property space in terms of low density and high stiffness and strength. A material geometry has been identified that achieves theoretical bounds for isotropic stiffness, over a wide range of relative densities, giving it nearly ideal properties, and is capable of filling this hole in property space. This closed cell geometry, while being relatively simple, presents challenges in terms of fabricablity when compared to open cell geometries which allow for fluid transport, facilitating infiltration and exfiltration of materials during and after processing. A second disparate material system has been measured to have the highest specific stiffness and strength for its density. This material is formed by self-assembly on the nanoscale to produce an inverse FCC geometry. These opaline structures are then coated with TiO2 by atomic layer deposition. The addition of this second stiff phase greatly increases the ultimate performance, while having a mixed influence on structural efficiency.  While this material has been measure to have extremal properties its performance is poor compared to an ideal geometry. We investigate the morphological features associated with high performance by comparing these two material systems. Finite element homogenization is used to compute the properties and resolve strain energy distributions. Uniform strain energy distributions in stiff networks of members aligned with principle stresses are key to high performance. Two and three phase theoretical bounds are used as metrics for performance, with three phase systems having a significant advantage in ultimate performance when compared to the Voight bound. Compromises between constituent properties, fabrication techniques, mesoscale geometry, scale, and other factors are currently necessary in realizing materials that can achieve extremal properties beyond current possibilities.


Sommersemester 2015

Prof. Ulrike Diebold

Institute of Applied Physic, TU Vienna, Austria

24.06.2015, 17ct      
N 0008, Eißendorfer Str. 40, TUHH

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Surface Science Studies of Magnetite Fe3O4

Magnetite, Fe3O4, is an abundant material with interesting electronic, magnetic, and chemical properties that make it promising for many applications. The talk will focus on the (001) surface of Fe3O4 single crystals.  This surface forms a reconstruction with (√2x√2)R45° symmetry that was solved recently [1].  Subsurface iron atoms are missing in a regular fashion, which gives rise to a peculiar – and rather useful – adsorption behavior on the surface:  vapor-deposited metal atoms, e.g., Au [2], Ag [3] Pd [4], Pt , Fe, Co, Ti , etc., stick strongly to one specific site within the reconstructed unit cell.  Noble metals do not diffuse until the reconstruction is lifted around 700 K [5], while more reactive metals move subsurface filling the cation vacancy sites. This property enables one to follow aggregation and sintering phenomena at the atomic scale [3, 4] with Scanning Tunneling Microscopy (STM).  The Fe3O4(001)–(√2x√2)R45°) is also an excellent model system for observing surface reactions that are relevant in catalysis. 

[1] R.  Bliem, et al. Science, 346 (2014) 1215
[2] Z. Novotný, et al. Physical Review Letters, 108 (2012) 216103
[3] R. Bliem, et al. ACS Nano 8(7) (2014), 7531–7537
[4] G. S. Parkinson, et al., Nature Materials, 12 (2013) 724 - 728
[5] N.C. Bartelt, Phys. Rev. B 88, 235436 (2013).


Prof. Ingo Burgert

Institut für Baustoffe (IfB), ETH Zürich, Switzerland

10.06.2015, 17ct      
N 0008, Eißendorfer Str. 40, TUHH

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Bio-inspired wood materials

The fabrication of renewable materials with superior properties and novel functionalities is one of the key challenges for a transition to sustainable societies. Among the materials available from nature, wood is a rather exceptional material that has been used for thousands of years. The mechanical performance of trees predominately originates from a sophisticated cell wall assembly, the hierarchical structure of the wood body as well as adaptive growth processes. An unravelling of the underlying structure-function relationships of wood allows for transferring the crucial principles and mechanisms for the design of bio-inspired materials. The bulk wood structure can be functionalized at the nano-and microscale to improve wood properties or to add new functions to the engineering material for novel applications. This concept is presented by means of various wood modification approaches such as in-situ polymerization techniques and mineralization processes at the cell wall level or magnetic hybrid wood composites with an anisotropic material profile. Principles of hydro-actuation in plants are presented and transferred for the design of autonomously deforming wood structures, which are driven by daily humidity changes and can be utilized as bio-based and bio-inspired facade elements.


Wintersemester 2014/2015

Prof. Andreas Stierle

DESY NanoLab and University of Hamburg, Physics Department, Hamburg, Germany

12.11.2014, 17ct      
K 0506, Denickestr. 15, TUHH

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The Atomic Structure of Nano-Objects:
What X-Rays Can Tell

The atomic structure determination of nano-objects with dimensions in the sub-100 nm regime is a formidable task for today’s diffraction, imaging and scanning probe techniques. Such a detailed structural and compositional analysis is mandatory for a correlation with the nano- object’s functionality e.g. as heterogeneous catalysts, magnetic storage material or light emitting device. In conventional x-ray diffraction experiments on powder samples the structural analysis is hampered by a random nanoparticle orientation and often by background scattering from the supporting material. Here I will present different ensemble averaging in-situ synchrotron radiation based x-ray diffraction schemes delivering quantitative information on the nanoparticle size, shape and facet surface structures under varying gas surroundings:
First I will discuss high resolution reciprocal space mapping from epitaxial Rh and Pt-Rh nanoparticles under oxidizing and reducing conditions, as well as during CO oxidation at near ambient pressures [1]. As a second approach I will present a combinatorial high energy x-ray diffraction scheme (85 keV photon energy) allowing a systematical screening of particle size or composition under identical reaction conditions, which we used to follow the CO oxidation induced sintering process of PtRh nanoparticles as a function of their composition [2]. Finally, I will demonstrate how graphene templated growth of nanoparticles with diameter < 2 nm opens the door for x-ray diffraction experiments with high crystallographic precision and monitoring of nanoparticle / gas molecule interactions [3].

[1] P. Nolte, A. Stierle, N. Y. Jin-Phillipp, N. Kasper, T. U. Schulli, H. Dosch, Science 321, 1654-1658 (2008).

[2] P. Müller, U. Hejral, U. Rütt and A. Stierle, Phys. Chem. Chem. Phys. 16, 13866 (2014).
[3] D. Franz, S. Runte, C. Busse, S. Schumacher, T. Gerber, T. Michely, M. Mantilla, V. Kilic, J. Zegenhagen, und A. Stierle, Phys. Rev. Lett. 110, 065503 (2013).


Sommersemester 2014

Prof. Matthew R. Begley

ENTFÄLLT

Materials Department and Department of Mechanical Engineering, University of California, Santa Barbara, USA

09.07.2014, 17ct
K 0506, Denickestr. 15, TUHH

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GPU-Based Simulations of Fracture in Idealized Brick and Mortar Microstructures
This talk will describe simulations of fracture in idealized brick and mortar microstructures comprising extremely stiff bricks bonded together with compliant, ductile mortar. The objective is to guide the development of ‘synthetic nacres’ by providing quantitative connections between brick hierarchy (i.e. size distributions, stacking sequences, etc.), interface behaviors, and macroscopically-defined fracture toughness. The simulations are created using an idealized framework tracks individual brick displacements and rotations and accounts for brick interactions using a non-linear cohesive law that spans elastic response, perfectly plastic yielding, and rupture. Micromechanical models will be presented that indicate the range of constituent properties for which microstructural idealization is expected to be valid. A novel incremental Monte-Carlo minimization scheme will be described to simulate cracking without a priori assumptions of the interaction between crack path and brick arrangement; the framework is specifically tailored to using graphical processing units (GPUs) to exploit highly parallel computations. Simulations of fracture in specimens with various brick/interface alignments, size distributions, strength distributions, etc. are used to extract the relationships between these features and the macroscopically-defined intitation toughness, strength and modulus. The results demonstrate that the fracture toughness and strength are a strong function of the orientation between microstructural features and loading direction, which dictates active fracture mechanisms observed elsewhere in experiments (e.g. splitting, staircases, bridging). The results also demonstrate that statistical distributions in constituent properties can have a profound impact on inferred macropscopic properties, even though the latter are essentially deterministic.


Prof. Zhong Lin Wang

School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, USA

25.06.2014, 17ct      
K 0506, Denickestr. 15, TUHH

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press release (in German)

Nanogenerators as new energy technology and piezotronics for functional systems
Developing wireless nanodevices and nanosystems is of critical importance for sensing, medical science, environmental/infrastructure monitoring, defense technology and even personal electronics. It is highly desirable for wireless devices to be self-powered without using battery. Nanogenerators (NGs) have been developed based on piezoelectric, trioboelectric and pyroelectric effect, aiming at building self-sufficient power sources for mico/nano-systems. The output of the nanogenerators now is high enough to drive a wireless sensor system and charge a battery for a cell phone, and they are becoming a vital technology for sustainable, independent and maintenance free operation of micro/nano-systems and mobile/portable electronics. This talk will focus on the fundamentals and novel applications of NGs.

For Wurtzite and zinc blend structures that have non-central symmetry, such as ZnO, GaN and InN, a piezoelectric potential (piezopotential) is created in the crystal by applying a strain. Such piezopotential can serve as a “gate” voltage that can effectively tune/control the charge transport across an interface/junction; electronics fabricated based on such a mechanism is coined as piezotronics, with applications in force/pressure triggered/controlled electronic devices, sensors, logic units and memory. By using the piezotronic effect, we show that the optoelectronc devices fabricated using wurtzite materials can have superior performance as solar cell, photon detector and light emitting diode. Piezotronics is likely to serve as a “mechanosensation” for directly interfacing biomechanical action with silicon based technology and active flexible electronics. This lecture will focus on the fundamental science and novel applications of piezotronics in sensors, touch pad technology, functional devices and energy science.


Prof. Dr. Stanislav N. Gorb

Zoological Institute, Kiel University, Germany

18.06.2014, 17ct
K0506, Denickestr. 15, TUHH

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press release (in German)

Fly on the ceiling:
Animal attachment devices and biologically-inspired adhesives
Why does the fly not fall from the ceiling? Animal attachment systems demonstrate their excellent adhesion and high reliability of contact. The structural background of this functional effect will be discussed. It will be demonstrated how comparative experimental biological approach can aid in development of novel tribological materials and systems. Biomimetic mushroom-shaped fibrillar adhesive microstructure inspired by these systems was characterized using a variety of measurement techniques and compared with a control flat surface made of the same material. Results revealed that pull-off force and peel strength of the structured specimens are more than twice those of the flat specimens. Based on the combination of several geometrical principles found in biological attachment devices, the presented microstructure exhibits a considerable step towards the development of an industrial dry adhesive.


Prof. Paul A. Midgley

Department of Materials Science and Metallurgy, University of Cambridge, UK

07.05.2014, 17ct
K0506, Denickestr. 15, TUHH

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press release (in German)

Electron Tomography in Materials Science: 3D Imaging at the Nanoscale
Although its origins lie in the life sciences as a technique to investigate the 3D ultrastructure of cells, viruses and bacteria, electron tomography has become a method used by many in materials science to routinely characterise 3D morphology at the nanoscale. The need to study ever-more complex materials structures, especially at the nanoscale, led to the introduction first of STEM HAADF tomography and later, through the combination of tomography with other imaging and spectroscopy techniques, methods for 3D mapping of composition, dislocation networks and electromagnetic potentials.
More recently, there is a growing need not only for higher spatial resolution but also for improved quantification of tomograms which has led to novel reconstruction algorithms yielding more reliable and robust 3D information. Developments in the efficiency and speed of detectors and spectrometers has led to advances in ‘spectrum-tomography’ where 4D data sets (with spatial and energy dimensions) contain a wealth of information not only about 3D morphology but also the composition and chemistry at the nanoscale.
This seminar will discuss progress made to date, highlighting key advances with illustrations from a broad spectrum of materials science. Challenges and opportunities that lie ahead will also be considered, focussing on how recent technical developments, both hardware and software, should allow new insights into the understanding of materials at the nanoscale.


Wintersemester 2013/2014

Prof. Francois Barthelat

Department of Mechanical Engineering McGill University, Montreal, Canada

29.01.2014, 17ct
K0506, Denickestr. 15, TUHH

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Overcoming brittleness trough bio-inspiration and microarchitecture

Natural structural materials such as bone and seashells are made of weak and brittle “building blocks”, yet they exhibit unique combinations of mechanical properties currently unmatched by their engineering counterparts. This performance can be largely explained by their “staggered microstructure”, a brick wall-like arrangement of stiff inclusions of high aspect ratio parallel to each other with some overlap, and bonded by a softer matrix. Here I will discuss how this seemingly simple microstructure generates high stiffness, high strength and attractive post-yielding behaviors by gliding of the inclusions on one another, and how this attractive micro-mechanism propagates over large volumes within the material ensuring high properties at the macroscale. I will also discuss, through experiments and modeling, how the staggered structure “amplifies” the toughness of its constituents through crack deflection along weak interfaces, crack bridging and process zone mechanisms. As a result, a material like nacre is several orders of magnitude tougher than calcium carbonate, its main consistent.
Duplicating the structures and mechanisms of natural materials into “biomimetic materials” represents formidable challenges in terms of fabrication, which we are addressing using two different approaches. In the “inclusion-based” approach we align and assemble microscopic ceramic inclusions using a simple doctor blading technique. We used this method to produce nacre-like films, and cylinders with a structural hierarchy similar to bone osteons. On the other hand, the “interface-based” approach is a topdown approach where weaker interfaces are carved directly within the bulk of glass using threedimensional laser engraving. The architecture and the toughness of the interfaces is designed to channel cracks into controlled sliding and toughening configurations, which resulted in a deformable material made of 99% glass, but 300 tougher. These micro-architectured glasses can be further combined with elastomers and other polymers with attractive rheologies to produce nacre-like glasses and other twoand three dimensional bio-inspired materials with unusual behavior and mechanical performance.


Prof. Reinhold H. Dauskardt

Department of Materials Science and Engineering, Stanford University, USA

30.10.2013, 17ct
K0506, Denickestr. 15, TUHH

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Hybrid Films in Nano- and Bio-Technologies:
Molecular Design and Thermo-Mechanical Properties

Hybrid films comprising inorganic and organic components tailored at molecular length scales are used in a wide range of emerging nano-technologies. These range from protective transparent (conductive) coatings in display and photovoltaic devices, membranes in fuel cells, dielectric layers in microelectronics and adhesive layers in high-performance laminates.  They operate near the envelope of their mechanical and adhesive properties with remarkably high levels of film stress.  Reliability integrating new multi-functional hybrid films requires a new understanding of their mechanical properties and how they are related to underlying molecular structure. Similarly, some biological tissues like human skin are layered structures in which biomechanical properties of component layers are crucial in understanding biophysical function.  Skin hybrid constructs that incorporate inorganic UV absorbing nanoparticles can be designed to prevent damage after solar exposure.

We will describe some of our research by selecting several examples involving hybrid materials in emerging nanoscience, energy and bioscience technologies.  Specifically, we will discuss molecular design of multi-functional hybrids for resistance to moisture assisted cracking, the fracture behavior of materials in active layers and modules of photovoltaic devices exposed to hostile solar conditions, and finally, discuss the biomechanical function of human skin and the effects of treatments and technologies to reduce skin damage and promote regeneration.


Sommersemester 2013

Prof. Dr. Dierk Raabe

Max-Planck-Institut für Eisenforschung, Düsseldorf, Germany

03.07.2013, 17ct
K0506, Denickestr. 15, TUHH

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Intrinsic Nanostructures in 1 Billon Tons:
Interface Engineering in Complex Steels and Biological Nanocomposites

Two topics will be addressed in this contribution: advanced near-atomic scale interface analysis in Fe-Mn steels and in biological chitin-based nanocomposites.
First, we present novel approaches to the atomic-scale understanding and design of advanced steels. Our scientific interest follows 3 directions: (1) Steels are complex alloys where minor chemical or structural changes can dramatically alter their behavior. (2) Steels can undergo multiple phase transformations that lead to specific nanostructure and property profiles. (3) We increasingly observe that steels can be bottom-up designed by exploiting and designing partitioning, equilibrium defect segregation, and displacive transformation at an atomic scale. Smart use of these effects allows us to better understand and tailor mankind’s most important mass produced material via self organization, lattice defect-, phase-, and interface-design from an atomic perspective. We give exemplary examples from the fields of maraging TRIP steels, TWIP steels, pearlite, and soft magnetic steels.
In the second part of the talk a cross-hierarchical analysis and theoretical treatment of calcite-reinforced chitin-based biological nanocomposites will be presented. The background of this work is the analysis of the structure and the mechanical properties of the arthropod cuticle, which serves as the main structural material for more than 90% of all species on the planet.


Prof. David J. Norris

Optical Materials Engineering Laboratory ETH Zürich, Switzerland

26.06.2013, 17ct
K0506, Denickestr. 15, TUHH

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Template-Stripped Plasmonic Films For Energy Conversion

In general, template stripping utilizes the fact that coinage metals (e.g., silver, gold, and copper) will wet silicon substrates well but adhere poorly. Thus, by depositing such a metal on a patterned silicon wafer, the metal film can then be “stripped” off to reveal a smooth patterned interface that was templated by the substrate. Previously, we demonstrated that silver interfaces obtained via this approach could be ultra-smooth and exhibit improved properties for photonic applications. In particular, such metallic films can be patterned to manipulate electromagnetic waves known as surface plasmons that exist at the interface of the metal. The field of plasmonics has been using patterned metals to channel, concentrate, or otherwise manipulate these waves. However, surface roughness and other inhomogeneities can impede the propagation of the surface plasmons, limiting performance. Template stripping can provide a simple high-throughput method for obtaining high quality patterned metals to avoid these issues. Moreover, we have recently demonstrated that template stripping can be extended beyond the coinage metals to refractory metals, semiconductors, and oxides, enabling a variety of structures. In this talk, we will discuss the use of these template-stripped films for photovoltaic applications. For example, because heat can be used to generate surface plasmons, we have been studying hot plasmonic structures for obtaining new and useful optical behavior. We have shown that metallic bull's eye patterns can lead to thermal emission that is amazingly narrow, both in terms of its spectrum and its angular divergence. Thus, a simple metallic foil can generate a highly directional beam of monochromatic light by a thermal process. This effect has implications for creating efficient thermophotovoltaic devices, which convert heat into electricity. Structured stacks of metallic and semiconductor layers can also have implications for photovoltaic devices.


Prof. Dr. Helena Van Swygenhofen-Moens

Ecole Polytechnique Fédérale de Lausanne, Paul Scherrer Institut, Switzerland

12.06.2013, 17ct
K0506, Denickestr. 15, TUHH

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Insitu X-ray and neutron diffraction for mechanical metallurgy

The structure-mechanics relation in metals is truly multi-scale: the atomic to the macro scale are connected by a series of reactions that determine the resulting microstructural evolution under load. The ability to use physics-based computational models for the understanding and prediction of the mechanical performance has revolutionized engineering and contributed to new concepts to make and design new metal-based microstructures. Key in improving the accuracy of these models is the use of appropriate constitutive equations. The latter requires synergetic use of computational and experimental approaches to understand the physics behind the mechanical behavior.
Neutron and X-ray diffraction habe contributed to a great extend to the science and engineering of metals. An X-ray or neutron diffraction pattern is a static footprint of a microstructure. When performed insitu, the footprint of the microstructure can be followed during load and this separately for all constituent phases. Using examples, this talk will illustrate the use of in-situ powder and Laue diffraction to reveal the secrets of basic deformation mechanism, load transfer, elastic and plastic anisotropy and deveolpment of microstress during load path changes.


Wintersemester 2012/13

Prof. Dr. Dr. h.c. Peter Fratzl

Max-Planck-Institut für Kolloid- und Grenzflächenforschung, Golm (Potsdam), Department of Biomaterials, Germany

23.01.2013, 17ct
K 0506, Denickestr. 15, TUHH

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Multi-scale structure and interface design in biological materials

Most load-bearing biological tissues, such as bones, plant stems, glass sponges or protein fibers have a multi-scale architecture resulting from the assembly of building blocks. This allows the fine-tuning of (generally multi-functional) properties but also requires specific interface structures to allow proper load transfer between the building blocks. The lecture reviews recent work on determining structure, composition and functionality of interfaces in a variety of biological materials.


Prof. Dr. Olaf Magnussen

Christian-Albrechts-Universität zu Kiel, Institut für Experimentelle und Angewandte Physik, Germany

28.11.2012, 17ct
K 0506, Denickestr. 15, TUHH

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Surface X-ray scattering studies of deeply buried interfaces in condensed matter: Functional composites, electrochemical growth, and liquid interface structure