Repository for natural sciences and technology
The repository is used for the free publication of scientific publications from the fields of natural sciences and technology in the sense of Open Access and is open to all authors. The publications placed in the repository are permanently freely available and citable worldwide. The repository is operated by the Technical Information Library (TIB).
17945 results
Search Results
Now showing 1 - 10 of 17945
- ItemReport on the DFG project "Optimization of different strategies for designing an energy harvester based on spin-torque diodes" (BE 2464/21-1)(Hannover : Technische Informationsbibliothek, 2025-06-04) Berkov, DmitrySteadily growing demand for cheap and green energy has caused a rapid development of so called ‘energy harvesting’ devices for producing dc-power from the ambient microwave radiation from various sources like TV and mobile-phone networks, Wi-Fi routers etc. The energy density of this radiation ranges from 1 to 1000 nW/cm^2, so that corresponding technology could be successfully used by low-power applications (digital thermometer, smoke detectors, some sensors in medicine etc.). The main goal of this project was the optimization of various designs for energy harvesters based on spin-torque-diodes (STDs), i.e. devices where dc-voltage is generated when an ac-current flows through a magnetic tunnel junction (MTJ). Using computer simulations, we have studied and optimized three main types of MTJ-based nanodevices: (i) ‘standard’ MTJ nanopillars of the resonant type employing quasi-homogeneous in-plane magnetization oscillations; (ii) MTJs in the out-of-plane precession regime for broadband rectification and (iii) multilayer stacks with the in-plane shape designed for oscillation of domains walls. As the results of this project we have determined optimal geometric and magnetic parameters for all three kinds of spin-torque-based energy harvesters listed above, and predicted corresponding maximal rectification efficiencies in ambient conditions.
- ItemAbschlussbericht zum DFG-Transferprojekt "Simulationsgestützte Werkzeug- und Prozessauslegung beim Microfinishen mit metallisch gebundenen Honwerkzeugen"(Hannover : Technische Informationsbibliothek, 2025) Biermann, Dirk; Wiederkehr, Petra; Heining, Ines; Tilger, MeikInnerhalb des Ergebnistransferprojektes sollten aufbauend auf den bisherigen Ergebnissen Untersuchungen zur Herstellung konvexer Konturen mittels Microfinishen durchgeführt und Simulationsmodelle zur prototypischen Prozess- und Werkzeugauslegung entwickelt werden. Die aus den vorhergehenden Grundlagenprojekten gewonnenen Erkenntnisse sollten im Rahmen des Transferprojektes prototypisch auf eine industrielle Anwendung und ein neues Werkzeugkonzept übertragen werden. Hierzu haben das Institut für Spanende Fertigung (ISF) und der Lehrstuhl Virtual Machining (VM) gemeinsam mit den Firmen NAGEL Maschi- nen- und Werkzeugfabrik GmbH, ELGAN-Diamantwerkzeuge GmbH & Co. KG und der Alicona Imaging GmbH die im Rahmen des Transfervorhabens notwendigen Forschungs- und Entwicklungsarbeiten durchgeführt. Es erfolgten experimentelle Untersuchungen zur Konditionierung von metallisch gebundenen Honwerkzeugen, um eine geeignete Prozess- kette für das Profilieren, Schärfen sowie das Einlaufen der Werkzeuge zu definieren. Parallel zu den technologischen Untersuchungen erfolgte die Entwicklung einer geometrisch- physikalischen Honsimulation. Das Ziel der Simulationsentwicklung war es, unterschiedliche Werkzeugkonfigurationen, bspw. durch eine Variation der Korngröße oder der Werkzeugges- talt, und Prozesskenngrößen im Hinblick auf die resultierende Kontur der bearbeiteten Wel- lenabsätze analysieren und mittels Microfinishbearbeitung gezielt einstellen zu können. Zur detaillierten Abbildung der komplexen Prozessführung wurden neben der Modellierung der Kraftregelung auf Einzelkornbasis neue Modelle entwickelt, welche zum einen bei der Be- rechnung der Einzelkornkraft die Orientierung der Korntopographie in Relation zur Schnitt- richtung berücksichtigten und zum anderen die explizite Beschreibung der Bindung und die Berücksichtigung der Bindungsverschleißmechanismen im Werkzeugmodell ermöglichten. Zur Validierung des entwickelten Simulationssystems wurden verschiedene Parameterkonfi- gurationen zunächst simulativ systematisch untersucht und anschließend experimentelle Untersuchungen für ausgewählte Werkzeug- und Prozesskonfigurationen durchgeführt. Die Ergebnisse der experimentellen Werkzeug- und Prozessanalyse konnten hierbei dazu ein- gesetzt werden, die Simulationsparametrierung zu optimieren und den Einfluss unterschied- licher Werkzeugkonfigurationen auf die resultierende Werkstückgestalt zu analysieren. Durch die gewonnenen Erkenntnisse konnten die Projektpartner zum einen reproduzierbare Ergeb- nisse für die Werkzeugkonditionierung durch die neu entwickelten Methoden erzielen. Zum anderen tragen die Ergebnisse der experimentellen und simulationsgestützten Untersuchun- gen zu einem verbesserten Prozessverständnis bei. Zusätzlich konnte das Potenzial aufge- zeigt werden, durch eine simulationsgestützte Werkzeugauslegung den ökonomische Auf- wand bei der Werkzeugentwicklung für die Projektpartner perspektivisch zu reduzieren.
- ItemCavitation Control using Mesoscale Surface Structuring in Marine Engineering and Hydraulic Systems(Hannover : Technische Informationsbibliothek, 2025-06-02) el Moctar, Bettar Ould; Kadivar, EbrahimWe propose a passive cavitation control method to mitigate undesirable effects, such as structural vibration in the context of marine engineering and hydraulic systems. For this aim, we used different mesoscale surface structures, such as scalloped and sawtooth riblet structures, finned and roughness structures to control the cavitating flow around circular cylinders, and hydrofoils. We performed extensive experiments to investigate the effects of wall roughness and riblet structures on the dynamics of cavitation, cavitation instability and turbulence structures in the cross flow around and in the wake of a circular cylinder, and a hydrofoil at different cavitating regimes and various Reynolds numbers. We used high-speed cameras to visualize the cavitation structure and a Particle Image Velocimetry (PIV) method to measure the velocity fields. In addition, we measured the forces acting on the smooth cylinder and on the cylinders with different mesoscale surface structures, and we also performed an acoustic measurement, using a hydrophone located downstream of the cylinders. Finally, we numerically studied the dynamics of the cavitation flow around a benchmark hydrofoil with and without passive control methods and compared our numerical results with our experimental data. Our results showed that the mesoscale surface structures were very efficient in suppressing or mitigating cavitation. The cavitation-induced vibration exciting forces acting on the cylinders and hydrofoils with riblet structures were significantly reduced compared to the cases without cavitation control. Furthermore, a substantial reduction in the cavitation volume and the sound pressure level in the low- and middle-frequency ranges were observed for the hydrofoils with riblet structures. The large-scale cloud cavity on the hydrofoil with scalloped and sawtooth riblets was changed to a small-scale cavity, which modified the cavitation dynamics on the hydrofoil surface and controlled unsteady cloud cavitation.
- ItemDFG Final Report: Validated computation of patterns in recurrent neural networks(Hannover : Technische Informationsbibliothek, 2025) Queirolo, Elena[no abstract available]
- ItemDFG final report: Smart design of crystal growth furnaces and processes(Hannover : Technische Informationsbibliothek, 2025) Holena, Martin; Linke, DavidThe project-funded research in the co-applicant’s group at LIKAT Rostock was pursued in two directions. The main direction was supporting the research of the applicant’s group at the application of machine learning, statistical, and optimization methods. They were applied to the modelling of Czochralski crystal growth of Ge, Si and GaAs, as well as of vertical gradient freeze growth and floating zone growth. In the preliminary step, the machine learning methods were tuned for small data applications using existing CFD data from other crystal growth techniques, such as vertical gradient freeze and floating zone. As a complementary research direction, research into some machine learning, statistical, and optimization methods has been performed, in particular into some applications of artificial neural networks and classification methods, and into the landscape-analysis aspect of evolutionary optimization.
- ItemDFG final project report: Lattice QCD investigation of a b-bar b-bar u d tetraquark resonance(Hannover : Technische Informationsbibliothek, 2025) Wagner, MarcQuarks typically appear in pairs, known as mesons, or in triplets, known as baryons, with protons and neutrons as well-known representatives of the latter. However, there are also exotic combinations of four quarks, known as tetraquarks, which have gained significant interest, particularly in recent years. This interest arises from the fact that they can be both detected in modern accelerator experiments and increasingly well understood and precisely predicted on a theoretical level with modern numerical calculations. The goal of this project was to study the existence and properties of specific tetraquark systems, consisting of two heavy antiquarks and two light quarks, based on first principles quantum chromodynamics, using numerical lattice field theory calculations. Investigating a $\bar b \bar b u d$ tetraquark resonance with quantum numbers $I(J^P) = 0(1^-)$ proved to be particularly challenging. Within the scope of this project, it was understood through the Born-Oppenheimer approximation that the tetraquark resonance is not located slightly above the lower $B B$ threshold, as previously expected, but considerably higher, above the $B^\ast B^\ast$ threshold. Final results with full lattice QCD calculations beyond the Born-Oppenheimer approximation have yet to be achieved, as this requires the numerical solution of a complex two-channel scattering problem involving a $B B$ and a $B^\ast B^\ast$ channel. However, essential technical steps for such a future computation have been implemented. For the two $\bar b \bar c u d$ tetraquark systems with quantum numbers $I(J^P) = 0(0^+)$ and $I(J^P) = 0(1^+)$, a rigorous finite-volume scattering analysis based on full lattice QCD computations was performed for the first time. This led to the prediction of a weakly bound but stable tetraquark for each of the two systems, as well as a tetraquark resonance approximately $100 \, \text{MeV}$ above the lowest meson-meson threshold ($B D$ or $B^\ast D$, respectively).
- ItemTopological transport control of colloidal particles(Hannover : Technische Informationsbibliothek, 2025) de las Heras, Daniel; Fischer, Thomas M.We have studied experimentally and with computer simulations the transport of magnetic particles on top of magnetic patterns. The motion is driven by either a modulation loop of the orientation of a uniform external magnetic field or by a drift force. The application of an adiabatic modulation loop of the direction of an external magnetic field to magnetic colloids or macroscopic magnetic particles on a periodic pattern offers unprecedented control over the motion and assembly of such colloids or particles. The motion is topologically protected since only those loops that wind around special orientations of the external field induce particle transport. The set of winding numbers around the special orientations is the topological invariant that protects the motion. The colloidal or macroscopic particles are sorted into topological classes and the transport of each class can be controlled independently and simultaneously with the other topological classes. The use of non-periodic patterns facilitates the transport of identical colloidal particles independently and simultaneously. The complexity of the loop can be imprinted in either the pattern or the modulation loop. In twisted magnetic patterns high mobility peaks of non-topologically driven particles emerge at non generic magic angles, but these mo- bility peaks in contrast to topologically driven systems are very fragile and can be easily destroyed via the analogue of an Anderson transition.
- ItemMultifilament fibres of poly(ε-caprolactone)/poly(lactic acid) blends with multiwalled carbon nanotubes as sensor materials for ethyl acetate and acetone(Amsterdam [u.a.] : Elsevier, 2011) Rentenberger, Rosina; Cayla, Aurélie; Villmow, Tobias; Jehnichen, Dieter; Campagne, Christine; Rochery, Maryline; Devaux, Eric; Pötschke, PetraConductive poly(ε-caprolactone) (PCL) + 4% multiwalled carbon nanotubes (MWCNTs)/poly(lactic acid) (PLA) = 50/50 wt% blend multifilament fibres were melt-spun and a woven textile was made by a handloom with the conductive fibres in weft direction. The fibres were tested for cyclic liquid sensing in ethyl acetate and acetone as two moderate solvents and in ethanol as a poor solvent. The liquid sensing responses, namely the relative resistance changes Rrel relating the resistance change to the initial resistance of the samples on contact with ethyl acetate and acetone, were fast (R rel higher than 16 after 100 s), with high amplitudes (R rel higher than 23 after 500 s), and well reproducible. At the same time, the fibres were resistant against these solvents. The response to ethanol was also reproducible, however, very slow and with low amplitude. PLA was found to crystallize during the immersion process, whereas in PCL the crystalline domains transformed into amorphous ones as studied by Wide Angle X-ray Diffraction. The crystallization of PLA does not influence negatively the liquid sensing properties which can be assigned to the finding that the MWCNT are predominantly localized in the PCL phase as confirmed by Scanning Electron Microscopy. In the final step, a textile based on those fibres was prepared and its sensing behaviour was investigated on ethyl acetate and acetone clearly showing that such textiles are suitable to detect these solvents.
- ItemVapor sensing properties of thermoplastic polyurethane multifilament covered with carbon nanotube networks(Amsterdam [u.a.] : Elsevier, 2011) Fan, Qingqing; Qin, Zongyi; Villmow, Tobias; Pionteck, Jürgen; Pötschke, Petra; Wu, Yongtao; Voit, Brigitte; Zhu, MeifangThe volatile organic compound (VOC) vapor sensing properties of a novel kind of thermoplastic polyurethane multifilament - carbon nanotubes (TPU-CNTs) composites is studied. And the sensing is based on changes in the electrical resistance of the composites due to vapor contact. The composites were readily obtained by adhering CNTs on the surface layer of TPU by means of simply immersing pure TPU multifilament into CNT dispersion. The uniformly formed nanotube networks on the outer layer of composite multifilament are favorable for providing efficient conductive pathways. The resulting TPU-CNTs composites show good reproducibility and fast response (within seconds) of electrical resistance change in cyclic exposure to diluted VOC and pure dry air. The vapor sensing behaviors of the composites are related to CNT content, vapor concentration, and polar solubility parameters of the target vapors. A relatively low vapor concentration of 0.5% is detectable, and a maximum relative resistance change of 900% is obtained for the composite with 0.8 wt.% CNT loading when sensing 7.0% chloroform. It is proposed that both the disconnection of CNT networks caused by swelling effects of the TPU matrix and the adsorption of VOC molecules on the CNTs are responsible for the vapor sensing behavior of TPU-CNTs composite, while the former effect plays the major role.
- ItemDuctile-to-semiductile transition in PP-MWNT nanocomposites(Weinheim : Wiley-VCH, 2007) Satapathy, Bhabani K.; Ganß, Martin; Weidisch, Roland; Pötschke, Petra; Jehnichen, Dieter; Keller, Thomas; Jandt, Klaus D.A ductile-to-semiductile transition in the crack resistance behaviour of PP/MWNT composites is discussed, using an essential work of fracture approach based on a post yield fracture mechanics concept and its possible interrelation to the structural attributes studied by TEM, SEM, and WAXD. A maximum in the non-essential work of fracture is observed at 0.5 wt-% MWNT content, which demonstrates the enhanced resistance to crack propagation compared to pure PP, followed by a sharp decline with the increase in MWNT content to 1.5 wt.-%, which reveals a ductile-to-semiductile transition. Fracture kinetic studies present a qualitative picture of the nature of such a transition in terms of a) switch over from non-steady (in pure PP) to steady-state crack tip opening displacement rate (in nanocomposites), and b) a ductile-to-semiductile transition largely as a result of delayed- yielding of the nanocomposites.