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End date: 2019 × Start date: 2010 × Has files: Yes × Type: article × Sponsor: Leibniz_Fonds ×

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Now showing 1 - 10 of 28
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    Optical Harmonic Vernier Effect: A New Tool for High Performance Interferometric Fiber Sensors
    (Basel : MDPI AG, 2019) Gomes, André D.; Ferreira, Marta S.; Bierlich, Jörg; Kobelke, Jens; Rothhardt, Manfred; Bartelt, Hartmut; Frazão, Orlando
    The optical Vernier effect magnifies the sensing capabilities of an interferometer, allowing for unprecedented sensitivities and resolutions to be achieved. Just like a caliper uses two different scales to achieve higher resolution measurements, the optical Vernier effect is based on the overlap in the responses of two interferometers with slightly detuned interference signals. Here, we present a novel approach in detail, which introduces optical harmonics to the Vernier effect through Fabry–Perot interferometers, where the two interferometers can have very different frequencies in the interferometric pattern. We demonstrate not only a considerable enhancement compared to current methods, but also better control of the sensitivity magnification factor, which scales up with the order of the harmonics, allowing us to surpass the limits of the conventional Vernier effect as used today. In addition, this novel concept opens also new ways of dimensioning the sensing structures, together with improved fabrication tolerances.
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    High strength nanostructured Al-based alloys through optimized processing of rapidly quenched amorphous precursors
    (London : Nature Publishing Group, 2018) Kim, S.-Y.; Lee, G.-Y.; Park, G.-H.; Kim, H.-A.; Lee, A.-Y.; Scudino, S.; Prashanth, K.G.; Kim, D.-H.; Eckert, J.; Lee, M.-H.
    We report the methods increasing both strength and ductility of aluminum alloys transformed from amorphous precursor. The mechanical properties of bulk samples produced by spark-plasma sintering (SPS) of amorphous Al-Ni-Co-Dy powders at temperatures above 673 K are significantly enhanced by in-situ crystallization of nano-scale intermetallic compounds during the SPS process. The spark plasma sintered Al84Ni7Co3Dy6 bulk specimens exhibit 1433 MPa compressive yield strength and 1773 MPa maximum strength together with 5.6% plastic strain, respectively. The addition of Dy enhances the thermal stability of primary fcc Al in the amorphous Al-TM -RE alloy. The precipitation of intermetallic phases by crystallization of the remaining amorphous matrix plays important role to restrict the growth of the fcc Al phase and contributes to the improvement of the mechanical properties. Such fully crystalline nano- or ultrafine-scale Al-Ni-Co-Dy systems are considered promising for industrial application because their superior mechanical properties in terms of a combination of very high room temperature strength combined with good ductility.
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    Determination of tip transfer function for quantitative MFM using frequency domain filtering and least squares method
    (London : Nature Publishing Group, 2019) Nečas, D.; Klapetek, P.; Neu, V.; Havlíček, M.; Puttock, R.; Kazakova, O.; Hu, X.; Zajíčková, L.
    Magnetic force microscopy has unsurpassed capabilities in analysis of nanoscale and microscale magnetic samples and devices. Similar to other Scanning Probe Microscopy techniques, quantitative analysis remains a challenge. Despite large theoretical and practical progress in this area, present methods are seldom used due to their complexity and lack of systematic understanding of related uncertainties and recommended best practice. Use of the Tip Transfer Function (TTF) is a key concept in making Magnetic Force Microscopy measurements quantitative. We present a numerical study of several aspects of TTF reconstruction using multilayer samples with perpendicular magnetisation. We address the choice of numerical approach, impact of non-periodicity and windowing, suitable conventions for data normalisation and units, criteria for choice of regularisation parameter and experimental effects observed in real measurements. We present a simple regularisation parameter selection method based on TTF width and verify this approach via numerical experiments. Examples of TTF estimation are shown on both 2D and 3D experimental datasets. We give recommendations on best practices for robust TTF estimation, including the choice of windowing function, measurement strategy and dealing with experimental error sources. A method for synthetic MFM data generation, suitable for large scale numerical experiments is also presented.
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    Converse Magnetoelectric Composite Resonator for Sensing Small Magnetic Fields
    (London : Nature Publishing Group, 2019) Hayes, P.; Jovičević Klug, M.; Toxværd, S.; Durdaut, P.; Schell, V.; Teplyuk, A.; Burdin, D.; Winkler, A.; Weser, R.; Fetisov, Y.; Höft, M.; Knöchel, R.; McCord, J.; Quandt, E.
    Magnetoelectric (ME) thin film composites consisting of sputtered piezoelectric (PE) and magnetostrictive (MS) layers enable for measurements of magnetic fields passively, i.e. an AC magnetic field directly generates an ME voltage by mechanical coupling of the MS deformation to the PE phase. In order to achieve high field sensitivities a magnetic bias field is necessary to operate at the maximum piezomagnetic coefficient of the MS phase, harnessing mechanical resonances further enhances this direct ME effect size. Despite being able to detect very small AC field amplitudes, exploiting mechanical resonances directly, implies a limitation to available signal bandwidth along with the inherent inability to detect DC or very low frequency magnetic fields. The presented work demonstrates converse ME modulation of thin film Si cantilever composites of mesoscopic dimensions (25 mm × 2.45 mm × 0.35 mm), employing piezoelectric AlN and magnetostrictive FeCoSiB films of 2 µm thickness each. A high frequency mechanical resonance at about 515 kHz leads to strong induced voltages in a surrounding pickup coil with matched self-resonance, leading to field sensitivities up to 64 kV/T. A DC limit of detection of 210 pT/Hz1/2 as well as about 70 pT/Hz1/2 at 10 Hz, without the need for a magnetic bias field, pave the way towards biomagnetic applications.
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    Mesoscale Dzyaloshinskii-Moriya interaction: Geometrical tailoring of the magnetochirality
    (London : Nature Publishing Group, 2018) Volkov, O.M.; Sheka, D.D.; Gaididei, Y.; Kravchuk, V.P.; Rößler, U.K.; Fassbender, J.; Makarov, D.
    Crystals with broken inversion symmetry can host fundamentally appealing and technologically relevant periodical or localized chiral magnetic textures. The type of the texture as well as its magnetochiral properties are determined by the intrinsic Dzyaloshinskii-Moriya interaction (DMI), which is a material property and can hardly be changed. Here we put forth a method to create new artificial chiral nanoscale objects with tunable magnetochiral properties from standard magnetic materials by using geometrical manipulations. We introduce a mesoscale Dzyaloshinskii-Moriya interaction that combines the intrinsic spin-orbit and extrinsic curvature-driven DMI terms and depends both on the material and geometrical parameters. The vector of the mesoscale DMI determines magnetochiral properties of any curved magnetic system with broken inversion symmetry. The strength and orientation of this vector can be changed by properly choosing the geometry. For a specific example of nanosized magnetic helix, the same material system with different geometrical parameters can acquire one of three zero-temperature magnetic phases, namely, phase with a quasitangential magnetization state, phase with a periodical state and one intermediate phase with a periodical domain wall state. Our approach paves the way towards the realization of a new class of nanoscale spintronic and spinorbitronic devices with the geometrically tunable magnetochirality.
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    Electrodeposited metal-organic framework films as self-assembled hierarchically superstructured supports for stable omniphobic surface coatings
    (London : Nature Publishing Group, 2018) Sablowski, J.; Linnemann, J.; Hempel, S.; Hoffmann, V.; Unz, S.; Beckmann, M.; Giebeler, L.
    Superhierarchically rough films are rapidly synthesised on metal substrates via electrochemically triggered self-assembly of meso/macroporous-structured metal-organic framework (MOF) crystals. These coatings are applied to immobilise a functional oil with low surface energy to provide stable coatings repellent to a wide range of hydrophobic as well as hydrophilic fluids. Such omniphobic surfaces are highly interesting for several applications such as anti-fouling, anti-icing, and dropwise condensation, and become easily scalable with the presented bottom-up fabrication approach. As investigated by environmental scanning electron microscopy (ESEM), the presented perfluorinated oil-infused Cu-BTC coating constitutes of a flat liquid-covered surface with protruding edges of octahedral superstructured MOF crystals. Water and non-polar diiodomethane droplets form considerably high contact angles and even low-surface-tension fluids, e.g. acetone, form droplets on the infused coating. The repellent properties towards the test fluids do not change upon extended water spraying in contrast to oil-infused porous copper oxide or native copper surfaces. It is discussed in detail, how the presented electrodeposited MOF films grow and provide a proficient surface morphology to stabilise the functional oil film due to hemiwicking.
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    Optimizing mechanical properties of Fe26.7Co26.7Ni26.7Si8.9B11 high entropy alloy by inducing hypoeutectic to quasi-duplex microstructural transition
    (London : Nature Publishing Group, 2019) Zhang, Z.-Q.; Song, K.-K.; Guo, S.; Xue, Q.-S.; Xing, H.; Cao, C.-D.; Dai, F.-P.; Völker, B.; Hohenwarter, A.; Maity, T.; Chawake, N.; Kim, J.-T.; Wang, L.; Kaban, I.; Eckert, J.
    High-entropy alloys (HEAs) have inspired considerable interest due to their attractive physical and mechanical properties. In this work, the microstructural evolution induced by different heat treatments on rapidly solidified hypoeutectic precursors of a Fe26.7Co26.7Ni26.7Si8.9B11 HEA is investigated and correlated with the corresponding mechanical properties. The microstructures of the rapidly solidified precursors are composed of primary fcc solid solution dendrites embedded in a eutectic matrix. When the samples are annealed at different temperatures after furnace cooling or quenching, respectively, the eutectic structure gradually decomposes into fcc, tetragonal (Fe,Co)2B, and hexagonal Ni31Si12 crystals with increasing annealing temperature, leading to a gradual increase of the content of the fcc crystals and both their aggregation and coarsening. Then the dominant structural framework gradually transforms from eutectic structures to fcc dendrites and ultimately the (Fe,Co)2B crystals become isolated as dominant reinforcement particles distributed in the interdendritic regions. This gradual microstructural transition from hypoeutectic to quasi-duplex structures leads to the change of the dominant deformation mechanism from crack-controlled to dislocation-dominated deformation, which allows to control both ductility and strength in a wide range. Hence, this study provides some guideline for how to tune the microstructure and mechanical properties of HEAs.
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    Dynamic volume magnetic domain wall imaging in grain oriented electrical steel at power frequencies with accumulative high-frame rate neutron dark-field imaging
    (London : Nature Publishing Group, 2018) Harti, R.P.; Strobl, M.; Schäfer, R.; Kardjilov, N.; Tremsin, A.S.; Grünzweig, C.
    The mobility of magnetic domains forms the link between the basic physical properties of a magnetic material and its global characteristics such as permeability and saturation field. Most commonly, surface domain structure are studied using magneto-optical Kerr microscopy. The limited information depth of approx. 20 nanometers, however, allows only for an indirect interpretation of the internal volume domain structures. Here we show how accumulative high-frame rate dynamic neutron dark-field imaging is able for the first time to visualize the dynamic of the volume magnetic domain structures in grain oriented electrical steel laminations at power frequencies. In particular we studied the volume domain structures with a spatial resolution of ∼100 μm and successfully quantified domain sizes, wall velocities, domain annihilation and its duration and domain wall multiplication in real time recordings at power frequencies of 10, 25 and 50 Hz with ±262.5 A/m and ±525 A/m (peak to peak) applied field.
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    Role of disorder when upscaling magnetocaloric Ni-Co-Mn-Al Heusler alloys from thin films to ribbons
    (London : Nature Publishing Group, 2018) Weise, B.; Dutta, B.; Teichert, N.; Hütten, A.; Hickel, T.; Waske, A.
    Research in functional magnetic materials often employs thin films as model systems for finding new chemical compositions with promising properties. However, the scale-up of thin films towards bulk-like structures is challenging, since the material synthesis conditions are entirely different for thin films and e.g. rapid quenching methods. As one of the consequences, the type and degree of order in thin films and melt-spun ribbons are usually different, leading to different magnetic properties. In this work, using the example of magnetocaloric Ni-Co-Mn-Al melt-spun ribbons and thin films, we show that the excellent functional properties of the films can be reproduced also in ribbons, if an appropriate heat treatment is applied, that installs the right degree of order in the ribbons. We show that some chemical disorder is needed to get a pronounced and sharp martensitic transition. Increasing the order with annealing improves the magnetic properties only up to a point where selected types of disorder survive, which in turn compromise the magnetic properties. These findings allow us to understand the impact of the type and degree of disorder on the functional properties, paving the way for a faster transfer of combinatorial thin film research towards bulk-like materials for magnetic Heusler alloys.
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    Climate change, agriculture, and economic development in Ethiopia
    (Basel : MDPI AG, 2018) Yalew, A.W.; Hirte, G.; Lotze-Campen, H.; Tscharaktschiew, S.
    Quantifying the economic effects of climate change is a crucial step for planning adaptation in developing countries. This study assesses the economy-wide and regional effects of climate change-induced productivity and labor supply shocks in Ethiopian agriculture. We pursue a structural approach that blends biophysical and economic models. We consider different crop yield projections and add a regionalization to the country-wide CGE results. The study shows, in the worst case scenario, the effects on country-wide GDP may add up to -8%. The effects on regional value-added GDP are uneven and range from -10% to +2.5%. However, plausible cost-free exogenous structural change scenarios in labor skills and marketing margins may offset about 20-30% of these general equilibrium effects. As such, the ongoing structural transformation in the country may underpin the resilience of the economy to climate change. This can be regarded as a co-benefit of structural change in the country. Nevertheless, given the role of the sector in the current economic structure and the potency of the projected biophysical impacts, adaptation in agriculture is imperative. Otherwise, climate change may make rural livelihoods unpredictable and strain the country's economic progress.