119092 research outputs found
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Contrasting packing modes for tubular assemblies in chlorosomes
No full textThe largest light-harvesting antenna in nature, the chlorosome, is a heterogeneous helical BChl self-assembly that has evolved in green bacteria to harvest light for performing photosynthesis in low-light environments. Guided by NMR chemical shifts and distance constraints for Chlorobaculum tepidum wild-type chlorosomes, the two contrasting packing modes for syn-anti parallel stacks of BChl c to form polar 2D arrays, with dipole moments adding up, are explored. Layered assemblies were optimized using local orbital density functional and plane wave pseudopotential methods. The packing mode with the lowest energy contains syn-anti and anti-syn H-bonding between stacks. It can accommodate R and S epimers, and side chain variability. For this packing, a match with the available EM data on the subunit axial repeat and optical data is obtained with multiple concentric cylinders for a rolling vector with the stacks running at an angle of 21° to the cylinder axis and with the BChl dipole moments running at an angle ß ∼ 55° to the tube axis, in accordance with optical data. A packing mode involving alternating syn and anti parallel stacks that is at variance with EM appears higher in energy. A weak cross-peak at -6 ppm in the MAS NMR with 50 kHz spinning, assigned to C-18¹, matches the shift of antiparallel dimers, which possibly reflects a minor impurity-type fraction in the self-assembled BChl c
Severe plastic deformation towards magnetic hardening in SmCo based alloys
No full textPermanent magnets play a crucial role in numerous modern technologies. As key components for sustainable electricity generation and electromobility, they are decisive for reducing greenhouse gas emissions and combating climate change. The most commonly used permanent magnets, such as Nd-Fe-B and Sm-Co, are produced by powder metallurgical sintering. In this work, the unconventional process route of high-pressure torsion (HPT) of powder blends is investigated. The application of HPT to powder blends enables the free selection of different magnetic and binder phases. Simultaneously, the process leads to consolidation and thus bulk nanocomposite materials, that are inaccessible via conventional melting-based routes. Using powder blends of SmCo₅ and Cu or Sn, the effect of increasing number of rotations during HPT was studied in detail. Structural refinement was observed, reaching particle sizes within the region of the single domain particle size of SmCo₅ and well below the limits of SmCo₅ sintered magnets. Microstructural analyses revealed that refinement of the SmCo₅ phase is carried by first fragmentation via cracking, before the phase is plastically deformed. The decreasing SmCo₅ particle size, combined with magnetic decoupling of the hard magnetic phase by the diamagnetic Cu or Sn phase, results in increased coercivity. However, further refinement does not lead to further increase of the coercivity caused by partial amorphization of the SmCo₅ phase. The detrimental character of the amorphous hard magnetic phase was revealed by micromagnetic simulations. Furthermore, a deformation induced texture formation was found, leading to anisotropic magnetic properties. By selecting binder phases such as Ni, Co and Cr, different magnetic interaction mechanisms were studied. This work demonstrates that a wide variety of composite materials can be produced by HPT of powder blends, that are not accessible via conventional powder metallurgical sintering
The Fermi energy as common parameter to describe charge compensation mechanisms: A path to Fermi level engineering of oxide electroceramics
No full textChemical substitution, which can be iso- or heterovalent, is the primary strategy to tailor material properties. There are various ways how a material can react to substitution. Isovalent substitution changes the density of states while heterovalent substitution, i.e. doping, can induce electronic compensation, ionic compensation, valence changes of cations or anions, or result in the segregation or neutralization of the dopant. While all these can, in principle, occur simultaneously, it is often desirable to select a certain mechanism in order to determine material properties. Being able to predict and control the individual compensation mechanism should therefore be a key target of materials science. This contribution outlines the perspective that this could be achieved by taking the Fermi energy as a common descriptor for the different compensation mechanisms. This generalization becomes possible since the formation enthalpies of the defects involved in the various compensation mechanisms do all depend on the Fermi energy. In order to control material properties, it is then necessary to adjust the formation enthalpies and charge transition levels of the involved defects. Understanding how these depend on material composition will open up a new path for the design of materials by Fermi level engineering
Extraction of pure component spectra from ex situ illumination UV/Vis and NMR spectroscopy
No full textObtaining understanding of a photochemical reaction relies on the observation, identification and quantification of the compounds involved. The photochemical properties of the individual components are of particular importance, and their determination, however, is not always trivial. This is also true for the quantitative measure on the ability to absorb light, the extinction coefficient εi if more than one species i is present and two or more species absorb light of the same wavelength. In this work, it is demonstrated how pure component spectra can be obtained with a simple combination of successive and repeated ex situ illumination, UV/Vis and NMR spectroscopy. From the complementary information accessible, the wavelength-dependent extinction coefficients of all species can be calculated yielding the pure component spectra. A comparison with published data shows excellent agreement and thus proves that this approach is highly reliable
Parameter estimation for a bivariate beta distribution with arbitrary beta marginals and positive correlation
No full textWe discuss a bivariate beta distribution that can model arbitrary beta-distributed marginals with a positive correlation. The distribution is constructed from six independent gamma-distributed random variates. While previous work used an approximate and sometimes inaccurate method to compute the distribution’s covariance and estimate its parameters, here, we derive all product moments and the exact covariance, which can be computed numerically. Based on this analysis we present an algorithm for estimating the parameters of the distribution using moment matching. We evaluate this inference method in a simulation study and demonstrate its practical use on a data set consisting of predictions from two correlated forecasters. Furthermore, we generalize the bivariate beta distribution to a correlated Dirichlet distribution, for which the proposed parameter estimation method can be used analogously
The K-functional and saturated linear approximation processes
No full textIt is shown that already the knowledge of the saturation class F(Jₜ,Lᵖ) of a saturated convolution approximation process {Jₜ} on Lᵖ(ℝⁿ),1≤p0. Proofs are mainly based on the Fourier transformation
Lahusen, Christian (2020): Europäisches Lobbying. Ein Berufsfeld zwischen Professionalismus und Aktivismus: Frankfurt am Main: Campus Verlag. 460 Seiten. 45,00 €
No full textEuropäisches Lobbying ist ein bekanntes Phänomen und sehr gut erforscht. Informierte Beobachter*innen wissen auch, dass europäisches Lobbying stetig in Bewegung ist: In mehr oder weniger regelmäßigen Abständen findet sich das Thema in der Presse, entweder im Kontext neuer EU-Regulierungen oder gar verbunden mit einem Aufdeckungsskandal. Ebenso dringt die sozialwissenschaftliche Forschung immer tiefer in das europäische Lobbying ein und liefert neue Erkenntnisse zu Akteuren, Instrumenten und Strukturen. Der Soziologe Christian Lahusen hat nun die europäischen Lobbyist*innen selbst zum Objekt der Forschung gemacht und liefert eine umfassende Bestandsaufnahme und Analyse des europäischen Lobbyings aus der Perspektive der Professionssoziologie. Er überprüft „ob Prozesse der Verberuflichung und Professionalisierung durchlaufen wurden, durch die das Tätigkeitsfeld auf Grundlage gemeinsamer beruflicher Laufbahnen, Wissensbestände und Selbstverständnisse konstituiert, organisiert und reguliert wird“ (S. 15)
In-situ scanning transmission electron microscopy study of Al-amorphous SiO₂ layer-SiC interface
No full textHere, we present a comprehensive study on atomic-scale in-situ biasing/heating scanning transmission electron microscopy ((S)TEM) of Al-amorphous SiO₂–SiC interface. The investigation includes electrical, chemical, and structural analysis of the interface at different temperatures (25–600 °C). The results show that at ~ 500 °C the electrical (three-orders of magnitude resistivity drop), chemical (dissolution of SiO₂ amorphous layer), and microstructural features (e.g. formation of Al₂O₃, Si and Al₄C₃) of the interface start to change. According to the results, amorphous SiO₂ dissolves in Al, leading to formation of α-Al₂O₃ and Si within the Al. In contrast, elemental interdiffusion (Al³⁺ ⇄ Si⁴⁺) between Al and SiC occurs resulting in formation of Al₄C₃. From the results, we can infer that reaction mechanism between Al and crystalline SiC is different with that between Al and SiO₂ amorphous phase. It is believed that structural similarities between SiC and Al₄C₃ play an important role in paving the way for elemental interdiffusion
Vor 125 Jahren: Der Physiker und Reformpädagoge Martin Wagenschein wird geboren
No full textMartin Wagenschein wurde am 3. Dezember 1896 in Gießen geboren, wo er 1914 Abitur machte. Im Ersten Weltkrieg war er im Dienst des Roten Kreuzes tätig und studierte an der Gießener Universität Mathematik, Physik und Geografie. 1918 setzte er sein Studium an der Universität Freiburg fort und bestand im Februar 1920 das Erste Staatsexamen. Schon im Juli 1920 schloss Wagenschein seine Promotion »Experimentelle Untersuchung über das Mitschwingen einer Kugel in einer schwingenden Flüssigkeits- und Gasmasse« ab. 1920/21 bekleidete er eine Assistentenstelle am Physikalischen Institut der Universität Gießen. 1921 bis 1923 folgte die pädagogische Ausbildung in Darmstadt, Worms, Friedberg, Gießen und Bad Nauheim, 1923 das Zweite Staatsexamen für das höhere Lehramt. Zwischen 1924 und 1933 war Wagenschein vom Staatsdienst beurlaubt und unterrichtete an der von Paul Geheeb gegründeten reformpädagogischen Odenwaldschule – eine für seine Pädagogik nachhaltig prägende Zeit
Cellular automata rules solving the wireless sensor network coverage problem
No full textThe problem of an optimal coverage of a wireless sensor network area is considered. To solve this problem, a Cellular Automata (CA) approach is proposed. More specifically, the objective is to find CA rules which are able to cover the 2D space by a minimum number of so–called “Sensor Tiles”. A sensor tile consists of a von Neumann neighborhood of range 2 centered at sensor “point” and surrounded by 12 sensing “pixels”. Two probabilistic CA rules were designed that can perform this task. Results of an experimental study show that the first rule evolves very fast stable sub-optimal coverings, starting from a random configuration. The second rule finds optimal coverings, however it needs much more time for their evolution. The results are supported by a theoretical study on von Neumann neighborhoods and borrowing either from heuristics or from the spectral theory of circulant graphs