924 research outputs found
Zündung durch Hochspannungs-Kondensatorentladungen nahe der Mindestzündenergie
No full textIm Rahmen der DFG-Forschergruppe FOR 1447 "Safety Relevant Ignition Processes" wird die Zündung durch elektrische Entladungen untersucht. Klassischerweise wird für die Beurteilung der Gefährdung durch Funkenzündungen die sicherheitstechnische Kenngröße Mindestzündenergie (MZE) verwendet, welche den Charakter eines Schwellwertes hat. Aktuelle Studien weisen darauf hin, dass statistische Verfahren bei der Untersuchung von Zündungen besser für die Abbildung des stochastischen Verhaltens geeignet sind. In dieser Arbeit werden Funkenzündungen bei Propan/Luft- und Ethen/Luft Gemischen untersucht. Dazu werden in einem Kugel/Kugel-Elektroden-Aufbau zunächst die Zündgrenzkurven bestimmt. Diese stimmen gut mit den Daten anderer Autoren überein. Für das jeweils zündwilligste Gemisch wird die Zündwahrscheinlichkeit in Abhängigkeit von der Funkenenergie durch logistische Regression bestimmt. Die Zündwahrscheinlichkeit bei der MZE beträgt demnach für Ethen/Luft weniger als 1 %. Zur zuverlässigen Detektion von Zündungen wird ein Hochgeschwindigkeits-Schlieren-Aufbau verwendet. Aus den hierbei erzeugten Bilderserien ergeben sich die Flammenfrontgeschwindigkeiten. Die Abhängigkeit der Flammenfrontgeschwindigkeit von der Gemischzusammensetzung kann mit dem Versuchsaufbau bestätigt werden, während die Funkenenergie keinen messbaren Einfluss auf die Flammenausbreitung hat. Der Versuchsaufbau soll in Verbindung mit numerischen Simulationen ein besseres Verständnis der physikalisch-chemischen Prozesse, insbesondere der Verluste, ermöglichen, die für die Zündung bei Energien nahe der MZE relevant sind
HiN : Alexander von Humboldt im Netz = Humboldt Digital Library
No full text- Ulrike Leitner: Einleitung
- Frank Baron: From Alexander von Humboldt to Frederic Edwin Church: Voyages of Scientific Exploration and Artistic Creativity
- Rex Clark: Alexander von Humboldt‘s Images of Landscape and the ‚Chaos of the Poets
- Detlev Doherr: The Humboldt Digital Library: Exploring Innovative Structures
- Wolfgang Griep: Die Bedeutung der Umkreisquellen für Alexander von Humboldts südamerikanische Reise
- Ulrike Leitner: Vielschichtigkeit und Komplexität im Reisewerk Alexander von Humboldts – Bibliographischer Hintergrund
- Markus Schnoepf: El Proyecto Humboldt: Una biblioteca digital para las expediciones científicas a las Islas Canaria
Explosionsdruckentlastung durch permeable Werkstoffe
No full textDas bisherige Schutzkonzept der Zündschutzart "Druckfeste Kapselung" beruht auf massiv ausgelegten Gehäusen und hinreichend kleinen Spaltweiten bei großen Spaltlängen, damit sich eine im Gehäuseinneren auftretende Explosion nicht außerhalb des Gehäuses fortsetzen kann. In dieser Arbeit wird ein innovatives Schutzkonzept zur Weiterentwicklung dieser Zündschutzart hin zu schlankeren Gehäusekonstruktionen vorgestellt. Hiermit ist zukünftig eine wirtschaftlichere und individuellere Produktentwicklung explosionsgeschützter Geräte möglich, ohne das Sicherheitsniveau der Zündschutzart zu gefährden. Die Grundlage hierfür bildet die konstruktive Kombination zweier bewährter Schutzprinzipien: In die Wände druckfester Gehäuse werden als Flammensperren fungierende permeable Werkstoffe integriert, die den bei einer Explosion im Inneren des Gehäuses entstehenden Druck zünddurchschlagsicher entlasten. Durch diese Funktionsintegration der Einzelfunktionen "Druckentlastung" und "Verhinderung eines Zünddurchschlags" in einem Konstruktionselement – dem Druckentlastungselement aus einem permeablen Werkstoff – wird hierbei im Unterschied zur herkömmlichen flammenlosen Explosionsdruckentlastung eine kontinuierliche Entlastung ab dem Beginn der Explosion ermöglicht. Die durchgeführten Untersuchungen belegen einen für jeden Werkstoff charakteristischen funktionalen Zusammenhang zwischen der erreichbaren Druckentlastung und der eingebauten Entlastungsfläche, mit dem die Druckentlastungsfähigkeit des Werkstoffs exakt beschrieben werden kann. Dieses neue Schutzkonzept wurde in einer kürzlich abgeschlossenen Dissertation entwickelt und aufgrund der Anwendung im Rahmen der "Druckfesten Kapselung" und zur Abgrenzung von allen bisherigen Arten der Druckentlastung als "Zünddurchschlagsichere Explosionsdruckentlastung" bezeichnet
Late Triassic (Norian-Rhaetian) brackish to freshwater habitats at a fluvial-dominated delta plain (Seinstedt, Lower Saxony, Germany)
No full textEinsatzpotenziale erneuerbarer Energien für Verkehr und Infrastruktur verstärkt erschließen. Ergebnisbericht des Themenfeldes 5 im BMVI-Expertennetzwerk für die Forschungsphase 2016 – 2019
No full textBundesanstalt für Gewässerkunde, Bundesanstalt für Straßenwesen, Deutscher Wetterdienst, Deutsches Zentrum für Schienenverkehrsforschung beim Eisenbahn-BundesamtAutoren: Markus Auerbach (BASt), Anna-Dorothea Ebner von Eschenbach (BfG), Dörthe Eichler (WSV), Felix Gersdorf (BASt), Frank Kaspar (DWD),
Detlev Majewski (DWD), Deborah Niermann (DWD), Benjamin Schima (BfG), Philipp Streek (DZSF/EBA
Indigenous knowledge systems or practical everyday performances? A theoretical reconsideration of indigenous knowledge in anthropology and development studies
No full textThis article was written by Detlev Krige before he joined the University of Pretoria.Much recent writing in the social sciences - boosted by inter alia substantial
funding programmes and valid questions concerning the relationship between
identity politics, knowledge and power - have applauded the formulation of
and research into various forms of Indigenous Knowledge Systems (lKS).
This interest in and advocacy for IKS, at times formulated in the context of
debates on postcolonial identity as an alternative to paradigms constructed on
particular Western-scientific assumptions, has run parallel to a more general
shift in thinking about development initiatives as requiring participatory
research methodologies and bottom-up implementation strategies. Within
development studies and anthropology, this shift has stimulated much
research on localised knowledge practices. There is, however, little evidence
that this body of anthropological (and ethnographic) literature has informed
the thinking of those writing and working within the theoretical paradigm of
Indigenous Knowledge Systems (lKS). Making use of a number of recent
ethnographic studies on Africa, the author argues for a theoretical
reconsideration of the IKS paradigm. He highlights important criticisms of the
ways in which many indigenous knowledge systems proponents essentialise
concepts such as knowledge and culture, as well as the methodological
limitations of much current IKS research. It is argued that a focus on the nonverbal
and local knowledge embodied in everyday practices, as well as the
performance of such knowledge, signals not only the limitations of much IKS
research but also redirects our attention to reformulating and invigorating
ideas about much needed local research
Evolution of Vision / Evolution de la vision
No full textSéminaire FSER organisé par Claude Desplan (University of New York, USA) et Walter Gehring (Biozentrum, Bâle, Suisse) du 14 au 19 octobre 2013 Participants Detlev Arendt, Nansi Jo Colley, Tiffany Cook, Claude Desplan, Markus Friedrich, Walter Gehring, Takashi Gojobori, William Harris, Samer Hattar, Gaspar Jekely, Zbynek Kozmik, Michael F. Land, Dan-E. Nilsson, David C.Plachetzki, Ilaria Rebay, Sylvie Rétaux, Simon Sprecher, Jonathan Tang, Veronica van Heyningen, Shozo Yokoyama -- Twenty exper..
Selektives MOVPE-Wachstum von lateralen InGaAs Nanostrukturen auf Silizium Substraten
No full textIn this work it was possible to find a way to grow novel lateral In1-xGaxAs nanowire structures in a SA-growth process directly on Si substrates of different crystal orientations. The analysis of crystals formed during a high-temperature sample preparation step without the use of Group-III source material initially led to the selective growth of unique lateral In0.2Ga0.8As nanostructures on Si (110) templates. These nanostructures, which could become interesting for numerous (opto)electronic applications, grow in the planar [-110], [-112] and [1 12] orientations. Despite the high lattice mismatch and the polar/nonpolar interface, TEM analyses show a perfect zinc blende structure, which is also nearly defect-free. It could also be demonstrated that these lateral nanostructures can be used as a basis for lateral InAs nanowires. In addition, InAs structures could be grown on Si (110) substrates, which grow like clock hands laterally on the substrate surface. The hands of these structures could also be grown selectively in nanostructured trenches. The selective In0.2Ga0.8As growth method was transferred to the technological important Si (100) substrates. Here, lateral nanowire structures could be grown which also exhibit an almost defect-free zinc blende crystal structure. Similar nanowires could be realized for the In0.6Ga0.4As and the InAs semiconductor material. It was shown that with increasing indium concentration the stacking fault density increased due to the increasing ionicity in these structures. Interestingly, these stacking fault planes are parallel to the electronic transport direction and hence are not intersecting the current transport in a future device. The lateral InAs nanowires were processed to field-effect transistors. From the transistor characteristics a field effect electron mobility of µFE = 4412 cm²/Vs could be estimated. This shows the potential of these selectively grown lateral In1 xGaxAs nanowires
Exploring proximity induced superconductivity in topological insulator based hybrid devices
No full textAs a proposed platform to realize elusive Majorana zero modes (MZMs), the superconducting proximity effect of an s-wave superconductor (S) in the surface states of a 3D topological insulator (TI) forms an attractive field of research driven by both, the fundamental interest in the mere existence of these non-Abelian modes and their envisioned application for fault-tolerant quantum computing. While prior research on several S-TI hybrid devices has already found experimental signatures which are assumed to be related to the unconventional induced superconductivity and MZMs, extensive experimental work is required to reinforce this interpretation and enhance the technological readiness of the S-TI platform. This thesis contributes to this field of research by exploring the realization and characterization of S-TI hybrid devices based on molecular beam epitaxy (MBE) grown tetradymite TIs. For a pristine fabrication of the respective S-TI hybrid devices, this work relies on a scalable ultra-high vacuum (UHV) process. This fabrication technique, which combines selective area growth of TIs and stencil lithography metallization, is further developed in the scope of this thesis to meet the experimental requirements of the respective devices. At first, the integration of topological insulator nanoribbons in superconducting qubit circuits is showcased which is motivated by recent proposal for the detection of MZMs via circuit quantum electrodynamic (cQED) techniques. In order to mitigate dielectric microwave losses in these qubits a local stencil lithography approach is developed. Using this process, a first generation of superconducting transmon qubits based on (Bi,Sb)Te nanoribbons is fabricated. The characterization of these devices finds qubit frequencies tuned by fabrication to the desired transmon regime and yields coherent qubit control and temporal quantum coherence. Thus, important capabilities for future cQED experiments based on S-TI hybrid qubit devices are demonstrated. In these TI nanoribbons, the proximity-induced superconductivity is moreover investigated in transport spectroscopy experiment. The respective devices are fabricated via an extensive multi-step stencil lithography process, which allows for the additional UHV fabrication of the required barriers and normal probes. In electrical transport experiments, signatures of the induced superconductivity in the (Bi,Sb)Te nanoribbons are found, including low energy conductance features assigned to the induced superconductivity in spurious bulk states. These features hamper the analysis of the proximity effect in the surface states. For a mitigation in future experiments, device optimizations are presented to enable junctions in an effectively shorter limit. This thesis further studies the general use of Al as parent superconductor for UHV fabricated tetradymite TIs via the characterization of Josephson junction based on the binary TI BiTe. As a potential substitute for Nb, Al offers some advantageous characteristics and could thus help to improve S-TI hybrid devices. While the application of pure Al on BiTe is found to inhibit induced superconductivity, thin interlayers of 3 nm Nb, Pd, Pt, or Ti are applied for an optimization of the interface. With the finding of supercurrents, excess currents, and subharmonic gap structures on respective Josephson junctions, all interlayers demonstrate to improve the interface quality. These junction characteristics are analyzed for a qualitative comparison of the interlayers. This discussion also covers an observed anomalous temperature dependence of the multiple Andreev reflections, which is assigned to distinct transport contributions from bulk and surface states
Growth and structural characterization of III-V semiconductor nanowires
No full textIn this thesis, the growth and structural properties of III-V semiconductor nanowires and nanowire heterostructures are studied. These nanowires represent structures suitable for both fundamental physics and applications in electronic devices such as (tunnel) field effect transistors. The III-V nanowires are grown with molecular beam epitaxy, high κ dielectric layers are deposited conformally around the nanowires by atomic layer deposition. The morphological and structural characteristics of the obtained structures are analyzed by scanning and transmission electron microscopy as well as x-ray diffraction.InAs nanowires are grown via two different growth mechanisms on both GaAs and Si (111) substrates. The growth proceeds either in the vapor-liquid-solid mechanism involving a liquid In droplet or via the vapor-solid growth mechanism without the use of any catalyst particle. A thorough analyzes of the impact of the substrate preparation on the nanowire growth is conducted and optimal parameters for the in situ and ex situ substrate preparations are identified. The vapor-solid grown InAs NWs exhibit a high density of stacking defects while the growth via the vapor-liquid-solid mechanism results in zinc blende twinning super lattices with a short wurtzite segment below the catalyzing In droplet. This wurtzite segment is attributed to a nucleation at the triple phase line between the vapor, the liquid droplet and the solid crystal. After developing a kinetic model for the presence of the wurtzite phase below the droplet, it becomes possible to include wurtzite segment of various lengths in GaAs and InAs nanowires grown via the vapor-liquid-solid mechanism.The growth of InAs and GaAs nanowires is successfully transferred to Si (100) substrates applying a texturing process to the Si substrate. This produces pyramids bound by {111} facets. An alignment of the pyramids with respect to the effusion cells allows us to control the nanowire growth direction and to study the impact of the growth parameters within a single growth experiment. Furthermore, the textured substrates enable a simultaneous integration of GaAs and InAs nanowires on the very same sample.Nanowire heterostructures are investigated by means of almost lattice matched combinations (InAs, GaSb and AlSb) as well as highly lattice mismatched heterostructures (GaAs/InAs and GaAs/InSb). The latter exhibit a lattice mismatch of 7% and 14%, respectively, resulting in the presence of misfit dislocations already for very thin shells. For zinc blende core-shell nanowires, three types of dislocations are identified: perfect dislocation, Frank partial dislocations and Shockley partial dislocations. Contrary, for the wurtzite core-shell segments only Frank partial dislocations are observed. GaAs/InAs core-shell NWs exhibit a pronounced morphological characteristic at the nanowire tip: the absence of the growth of the InAs shell. This is correlated with the presence of the wurtzite crystal phase. In combination with the control of the crystal phases in GaAs nanowires, we study the growth of the InAs shell in the vicinity of the wurtzite segment. A model is developed to describe the absence of the InAs shell on wurtzite GaAs and the impact of stacking faults and twin boundaries on the growth. GaAs/InSb heterostructure nanowires exhibit two growth regimes: an axial growth at high substrate temperatures and a radial growth at low temperatures. The axial growth is governed by the nucleation of an In droplet on the flat top facet of a GaAs nanowire stem. The radial growth evidences an additional mechanism of strain relaxation via tiled lattice planes. For both GaAs/InSb heterostructures, geometric phase analyzes reveal an abrupt and complete strain relaxation. Core-shell nanowires composed of InAs, GaSb and AlSb are free of misfit dislocations, the shell thicknesses significantly exceeds the critical thicknesses of planar heterostructures. InAs nanowire junctions based on the combination of several individual nanowires are obtained via the growth on Si (100) substrates with V-grooves. Three basic junctions are identified: L-shaped, T-shaped and X-shaped. In the junctions, the crystal structure is modified from a stacking fault rich structure to a zinc blende crystal structure via a solid phase transformation involving Shockley partial dislocations. A passivation of the nanowires is obtained by depositing Al2O3 and HfO2 high κ dielectrics by atomic layer deposition. The mismatch in thermal expansion coefficients between the semiconductor and the Al2O3 induces a compressive strain in the III-V nanowire. HfO2 covering InAs nanowires contain significant amounts of crystalline phases which are expected to influence the dielectric properties. The presence of crystalline phases is suppressed by laminate structures based on Al2O3 and HfO2
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