atom-probe-tomography-studies-on-the-cu-inga-se2-grain-boundaries

Atom Probe Tomography Studies on the Cu(In,Ga)Se<sub>2</sub> Grain Boundaries

Compared with the existent techniques, atom probe tomography is a unique technique able to chemically characterize the internal interfaces at the ...

Many biological materials are composed of fibrils arranged according to well-ordered three-dimensional patterns. These materials often show a strong anisotropy in their properties. An essential characteristic of biological structures is their hierarchical organization from the nanometer to the millimeter scale. Lobster cuticle is a good example of this and a suitable model for studying these properties. In this study the structure of untreated as well as chemically and physically treated cuticle from the exoskeleton of the American lobster (Homarus americanus) was investigated using scanning electron microscopy. Fresh samples have been chemically decalcified and deproteinated and thermally treated to evaluate their resistance to degradation. Results showed that their structure is more complex than the commonly assumed model for arthropod cuticles. Stacked chitin-protein planes create the characteristic twisted plywood pattern found in arthropod cuticles. However, due to a well-developed pore canal system these planes are not simple arrays of parallel chitin-protein fibers. In lobster cuticle, interconnected fibers bend around the continuous lenticellate cavities of the pore canals to form a planar honeycomb-like structure. The chemically and thermally treated samples showed that the organic matrix retains its shape and structure despite the attack of chemical compounds or heat. It was also possible to study the distribution of the biominerals after the removal of the organic matrix. The observed residual structure gives a good impression of how the minerals (mainly calcite) are distributed inside the polymeric network.

The exoskeleton of the lobster Homarus americanus as an example of a smart anisotropic biological material.

The multi-purpose experimental endstation of beamline BL9 at the Dortmund Electron Accelerator (DELTA) is dedicated to diffraction experiments in grazing-incidence geometry, reflectivity and powder diffraction measurements. Moreover, fluorescence analysis and inelastic X-ray scattering experiments can be performed. Recently, a new set-up for small-angle and wide-angle X-ray scattering utilizing detection by means of an image-plate scanner was installed and is described in detail here. First small-angle X-ray scattering experiments on aqueous solutions of lysozyme with different cosolvents and of staphylococcal nuclease are discussed. The application of the set-up for texture analysis is emphasized and a study of the crystallographic texture of natural bio-nanocomposites, using lobster and crab cuticles as model materials, is presented.

The small-angle and wide-angle X-ray scattering set-up at beamline BL9 of DELTA.

We model the elastic properties of bone at the level of mineralized collagen fibrils via step-by-step homogenization from the staggered arrangement of collagen molecules up to an array of parallel mineralized fibrils. A new model for extrafibrillar mineralization is proposed, assuming that the extrafibrillar minerals are mechanically equivalent to reinforcing rings coating each individual fibril. Our modeling suggests that no more than 30% of the total mineral content is extrafibrillar and the fraction of extrafibrillar minerals grows linearly with the overall degree of mineralization. It is shown that the extrafibrillar mineralization considerably reinforces the fibrils' mechanical properties in the transverse directions and the fibrils' shear moduli. The model predictions for the elastic moduli and constants are found to be in a good agreement with the experimental data reported in the literature.

Hierarchical modeling of the elastic properties of bone at submicron scales: the role of extrafibrillar mineralization.

This paper reports a computational study of the indentation of a flat punch into a compressible elastic layer (with Poisson's ratio varying from 0 to 0.49) bonded to a rigid substrate. Based on the computational results and using Sneddon's solution [Sneddon IN. The relation between load and penetration in the axisymmetric Boussinesq problem for a punch of arbitrary profile. Int J Eng Sci 1965;3:47] and the asymptotic solution [Jaffar MJ. A general solution to the axisymmetric frictional contact problem of a thin bonded elastic layer. Proc Inst Mech Eng C 1997;211:549; Yang FQ. Asymptotic solution to axisymmetric indentation of a compressible elastic thin film. Thin Solid Films 2006;515:2274] as the two limits, a simple expression of the load-depth curve valid for an arbitrary ratio of the indenter radius to the thickness of the layer is obtained. Further, a correlation between indentation load and depth for a rigid flat punch indenting into linearly viscoelastic layers bonded to a rigid substrate is proposed by using the correspondence principle. Several procedures are suggested based on the results reported in this study to determine the viscoelastic properties of the layer in the time or frequency domains. The findings are verified by numerical examples. The results may facilitate the use of depth-sensing indentation tests to characterize the mechanical properties of polymeric films or functional coatings on hard substrates, and some biological materials, e.g. articular cartilage.

The use of flat punch indentation to determine the viscoelastic properties in the time and frequency domains of a soft layer bonded to a rigid substrate.

A thorough study of shear stress within the lattice Boltzmann method is provided. Via standard multiscale Chapman-Enskog expansion we investigate the dependence of the error in shear stress on grid resolution showing that the shear stress obtained by the lattice Boltzmann method is second-order accurate. This convergence, however, is usually spoiled by the boundary conditions. It is also investigated which value of the relaxation parameter minimizes the error. Furthermore, for simulations using velocity boundary conditions, an artificial mass increase is often observed. This is a consequence of the compressibility of the lattice Boltzmann fluid. We investigate this issue and derive an analytic expression for the time dependence of the fluid density in terms of the Reynolds number, Mach number, and a geometric factor for the case of a Poiseuille flow through a rectangular channel in three dimensions. Comparison of the analytic expression with results of lattice Boltzmann simulations shows excellent agreement.

Shear stress in lattice Boltzmann simulations.

Revealing the design principles of high-performance biological composites using ab initio and multiscale simulations: the example of lobster cuticle.

Arthropoda, which represent nearly 80% of all known animal species, are protected by an exoskeleton formed by their cuticle. The cuticle represents a hierarchically structured multifunctional biocomposite based on chitin and proteins. Some groups, such as Crustacea, reinforce the load-bearing parts of their cuticle with calcite. As the calcite sometimes contains Mg it was speculated that Mg may have a stiffening impact on the mechanical properties of the cuticle (Becker et al., Dalton Trans. (2005) 1814). Motivated by these facts, we present a theoretical parameter-free quantum-mechanical study of the phase stability and structural and elastic properties of Mg-substituted calcite crystals. The Mg-substitutions were chosen as examples of states that occur in complex chemical environments typical for biological systems in which calcite crystals contain impurities, the role of which is still the topic of debate. Density functional theory calculations of bulk (Ca,Mg)COâ‚ƒ were performed employing 30-atom supercells within the generalized gradient approximation as implemented in the Vienna Ab-initio Simulation Package. Based on the calculated thermodynamic results, low concentrations of Mg atoms are predicted to be stable in calcite crystals in agreement with experimental findings. Examining the structural characteristics, Mg additions nearly linearly reduce the volume of substituted crystals. The predicted elastic bulk modulus results reveal that the Mg substitution nearly linearly stiffens the calcite crystals. Due to the quite large size-mismatch of Mg and Ca atoms, Mg substitution results in local distortions such as off-planar tilting of the COâ‚ƒÂ²â» group.

Ab initio study of thermodynamic, structural, and elastic properties of Mg-substituted crystalline calcite.

We investigate the wetting behavior of liquid droplets on rough hydrophobic substrates for the case of droplets that are of comparable size to the surface asperities. Using a simple three-dimensional analytical free-energy model, we have shown in a recent letter [M. Gross, F. Varnik, and D. Raabe, EPL 88, 26002 (2009)] that, in addition to the well-known Cassie-Baxter and Wenzel states, there exists a further metastable wetting state where the droplet is immersed into the texture to a finite depth, yet not touching the bottom of the substrate. Due to this new state, a quasistatically evaporating droplet can be saved from going over to the Wenzel state and instead remains close to the top of the surface. In the present paper, we give an in-depth account of the droplet behavior based on the results of extensive computer simulations and an improved theoretical model. In particular, we show that releasing the assumption that the droplet is pinned at the outer edges of the pillars improves the analytical results for larger droplets. Interestingly, all qualitative aspects, such as the existence of an intermediate minimum and the "reentrant transition," remain unchanged. We also give a detailed description of the evaporation process for droplets of varying sizes. Our results point out the role of droplet size for superhydrophobicity and give hints for achieving the desired wetting properties of technically produced materials.

Small droplets on superhydrophobic substrates.

This study is about the microstructural evolution of TiAlN/CrN multilayers (with a Ti:Al ratio of 0.75:0.25 and average bilayer period of 9 nm) upon thermal treatment. Pulsed laser atom probe analyses were performed in conjunction with transmission electron microscopy and X-ray diffraction. The layers are found to be thermally stable up to 600 Â°C. At 700 Â°C TiAlN layers begin to decompose into Ti- and Al-rich nitride layers in the out-of-plane direction. Further increase in temperature to 1000 Â°C leads to a strong decomposition of the multilayer structure as well as grain coarsening. Layer dissolution and grain coarsening appear to begin at the surface. Domains of AlN and TiCrN larger than 100 nm are found, together with smaller nano-sized AlN precipitates within the TiCrN matrix. Fe and V impurities are detected in the multilayers as well, which diffuse from the steel substrate into the coating along columnar grain boundaries.

Thermal stability of TiAlN/CrN multilayer coatings studied by atom probe tomography.

We report a newly discovered dense microstructure of dendrite-like biocalcite that is formed by marine organisms. High spatial resolution electron backscatter diffraction (EBSD) was carried out under specific analytical conditions (15 and 10 kV) on the primary layer of the modern brachiopod Gryphus vitreus. The primary layer of modern brachiopods, previously termed nanocrystalline, is formed by an array of concave/convex calcite grains with interdigitated recesses and protrusions of abutting crystals without any cavities in or between the dendrites. The interface topology of this structure ranges from a few tens of nanometres to tens of micrometres, giving a nanoscale structure to the material fabric. The dendritic grains show a spread of crystallographic orientation of several degrees and can thus be referred to as mesocrystals. Individual dendritic mesocrystals reach sizes in one dimension larger than 20 Î¼m. The preferred crystallographic orientation is similar in the primary and adjacent fibrous shell layers, even though these two layers show completely different crystal morphologies and grain boundary topologies. This observation indicates that two separate control mechanisms are active when the primary and the fibrous shell layers are formed. We propose a growth model for the interdigitated dendritic calcite grain structure based on a precursor of vesicles filled with amorphous calcium carbonate (ACC).

Interdigitating biocalcite dendrites form a 3-D jigsaw structure in brachiopod shells.

State-of-the-art ab initio methods constitute a solid basis of modern materials science and materials design. The theoretical research is nowadays increasingly combined with mesoscaleÂ and macroscale approaches as well as advanced experimental techniques in order to address complex multiscale and finite-temperature phenomena. The applicability of such a interdisciplinary methodology will be exemplified by our development of novel alloys for human implants, particularly the improvement of hip transplants. The progress in this field is severely hampered by a lack of suitable materials which are biocompatible in terms of non-toxicity and mechanical properties. The aim of our research has been therefore to identify metallurgical trends for non-poisonous Ti-based alloys employing quantum-mechanical calculations. Guided by the theoretical calculations of phase stability and elastic properties, selected alloys were actually melted, cast, and heat treated to a homogeneous state. The samples have been experimentally characterized by x-ray methods and electron microscopy, including crystallographic and chemical analysis, and mechanically tested using ultrasound measurements. The experimental data obtained in these experiments are in excellent agreement with theoretical predictions.

Theory-guided design of Ti-based binaries for human implants (abstract only).

There is growing evidence that the flow of driven amorphous solids is not homogeneous, even if the macroscopic stress is constant across the system. Via event-driven molecular dynamics simulations of a hard sphere glass, we provide the first direct evidence for a correlation between the fluctuations of the local volume fraction and the fluctuations of the local shear rate. Higher shear rates do preferentially occur at regions of lower density and vice versa. The temporal behavior of fluctuations is governed by a characteristic time scale, which, when measured in units of strain, is independent of shear rate in the investigated range. Interestingly, the correlation volume is also roughly constant for the same range of shear rates. A possible connection between these two observations is discussed.

Heterogeneous shear in hard sphere glasses.

The teeth of two different shark species (Isurus oxyrinchus and Galeocerdo cuvier) and a geological fluoroapatite single crystal were structurally and chemically characterized. In contrast to dentin, enameloid showed sharp diffraction peaks which indicated a high crystallinity of the enameloid. The lattice parameters of enameloid were close to those of the geological fluoroapatite single crystal. The inorganic part of shark teeth consisted of fluoroapatite with a fluoride content in the enameloid of 3.1 wt.%, i.e., close to the fluoride content of the geological fluoroapatite single crystal (3.64 wt.%). Scanning electron micrographs showed that the crystals in enameloid were highly ordered with a special topological orientation (perpendicular towards the outside surface and parallel towards the center). By thermogravimetry, water, organic matrix, and biomineral in dentin and enameloid of both shark species were determined. Dentin had a higher content of water, organic matrix, and carbonate than enameloid but contained less fluoride. Nanoindentation and Vicker's microhardness tests showed that the enameloid of the shark teeth was approximately six times harder than the dentin. The hardness of shark teeth and human teeth was comparable, both for dentin and enamel/enameloid. In contrast, the geological fluoroapatite single crystal was much harder than both kinds of teeth due to the absence of an organic matrix. In summary, the different biological functions of the shark teeth ("tearing" for Isurus and "cutting" for Galeocerdo) are controlled by the different geometry and not by the chemical or crystallographic composition.

Article	Year
The exoskeleton of the lobster Homarus americanus as an example of a smart anisotropic biological material. Acta biomaterialia\| PubMed ID: 17208527	2007
The small-angle and wide-angle X-ray scattering set-up at beamline BL9 of DELTA. Journal of synchrotron radiation\| PubMed ID: 17435299	2007
Hierarchical modeling of the elastic properties of bone at submicron scales: the role of extrafibrillar mineralization. Biophysical journal\| PubMed ID: 18310256	2008
The use of flat punch indentation to determine the viscoelastic properties in the time and frequency domains of a soft layer bonded to a rigid substrate. Acta biomaterialia\| PubMed ID: 18722168	2009
Shear stress in lattice Boltzmann simulations. Physical review. E, Statistical, nonlinear, and soft matter physics\| PubMed ID: 19518377	2009
Revealing the design principles of high-performance biological composites using ab initio and multiscale simulations: the example of lobster cuticle. Advanced materials (Deerfield Beach, Fla.)\| PubMed ID: 20217746	2010
Ab initio study of thermodynamic, structural, and elastic properties of Mg-substituted crystalline calcite. Acta biomaterialia\| PubMed ID: 20650336	2010
Small droplets on superhydrophobic substrates. Physical review. E, Statistical, nonlinear, and soft matter physics\| PubMed ID: 20866238	2010
Thermal stability of TiAlN/CrN multilayer coatings studied by atom probe tomography. Ultramicroscopy\| PubMed ID: 21146308	2011
Interdigitating biocalcite dendrites form a 3-D jigsaw structure in brachiopod shells. Acta biomaterialia\| PubMed ID: 21295163	2011
Theory-guided design of Ti-based binaries for human implants (abstract only). Journal of physics. Condensed matter : an Institute of Physics journal\| PubMed ID: 21693883	2008
Heterogeneous shear in hard sphere glasses. Physical review letters\| PubMed ID: 22463672	2012
Structure, composition, and mechanical properties of shark teeth. Journal of structural biology\| PubMed ID: 22503701	2012

Dierk Raabe

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