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End date: 2019 × Start date: 2010 × Publisher: Basel : MDPI AG × WGL Institute: IFWD ×

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Now showing 1 - 10 of 22
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    Fe1-xNix alloy nanoparticles encapsulated inside carbon nanotubes: Controlled synthesis, structure and magnetic properties
    (Basel : MDPI AG, 2018) Ghunaim, R.; Damm, C.; Wolf, D.; Lubk, A.; Büchner, B.; Mertig, M.; Hampel, S.
    In the present work, different synthesis procedures have been demonstrated to fill carbon nanotubes (CNTs) with Fe1-xNix alloy nanoparticles (x = 0.33, 0.5). CNTs act as templates for the encapsulation of magnetic nanoparticles, and provide a protective shield against oxidation as well as prevent nanoparticles agglomeration. By variation of the reaction parameters, the purity of the samples, degree of filling, the composition and size of filling nanoparticles have been tailored and therefore the magnetic properties. The samples were analyzed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Bright-field (BF) TEM tomography, X-ray powder diffraction, superconducting quantum interference device (SQUID) and thermogravimetric analysis (TGA). The Fe1-x Nix-filled CNTs show a huge enhancement in the coercive fields compared to the corresponding bulk materials, which make them excellent candidates for several applications such as magnetic storage devices.
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    Manufacturing of 42SiCr-pipes for quenching and partitioning by longitudinal HFI-Welding
    (Basel : MDPI AG, 2019) Kroll, M.; Birnbaum, P.; Zeisig, J.; Kraeusel, V.; Wagner, M.F.-X.
    In the pipe manufacturing and pipe processing industry, the demand for cost-effective pipes with high strength and good ductility is increasing. In the present study, the inductive longitudinal welding process was combined with a Quenching and Partitioning (Q&P) treatment to manufacture pipes with enhanced mechanical properties. The aim of the Q&P process is to establish a martensitic structure with increased retained austenite content. This allows for the beneficial use of both phases: the strength of martensite as well as the ductility of retained austenite. A 42SiCr steel, developed for Q&P processes, was joined at the longitudinal seam by a high-frequency induction (HFI) welding process and was subsequently heat-treated. The applied heat treatments included normalizing, austenitizing, quenching, and two Q&P strategies (Q&P-A/Q&P-B) with distinct quenching (Tq = 200/150◦ C) and partitioning temperatures (Tp = 300/250◦ C). Investigations of the microstructures revealed that Q&P tubes exhibit increased amounts of retained austenite in the martensitic matrix. Differences between the weld junction and the base material occurred, especially regarding the morphology of the martensite; the martensite found in the weld junction is finer and corresponds more to the lath-type morphology, compared to the base material in the circumference. In all zones of the welded tube circumference, retained austenite has been found in similar distributions. The mechanical testing of the individual tubes demonstrated that the Q&P treatments offer increased strength compared to all other states and significantly improved ductility compared to the quenched condition. Therefore, the approach of Q&P treatment of HFI-welded tubes represents a route for the mass production of high-strength tubular products with improved ductility.
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    Resonant terahertz light absorption by virtue of tunable hybrid interface phonon-plasmon modes in semiconductor nanoshells
    (Basel : MDPI AG, 2019) Nika, D.L.; Pokatilov, E.P.; Fomin, V.M.; Devreese, J.T.; Tempere, J.
    Metallic nanoshells have proven to be particularly versatile, with applications in biomedical imaging and surface-enhanced Raman spectroscopy. Here, we theoretically demonstrate that hybrid phonon-plasmon modes in semiconductor nanoshells offer similar advantages in the terahertz regime. We show that, depending on tm,n,nhe doping of the semiconductor shells, terahertz light absorption in these nanostructures can be resonantly enhanced due to the strong coupling between interface plasmons and phonons. A threefold to fourfold increase in the absorption peak intensity was achieved at specific values of electron concentration. Doping, as well as adapting the nanoshell radius, allowed for fine-tuning of the absorption peak frequencies.
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    Amorphous Li-Al-based compounds: A novel approach for designing high performance electrode materials for Li-ion batteries
    (Basel : MDPI AG, 2013) Thoss, F.; Giebeler, L.; Thomas, J.; Oswald, S.; Potzger, K.; Reuther, H.; Ehrenberg, H.; Eckert, J.
    A new amorphous compound with the initial atomic composition Al43Li43Y6Ni8 applied as electrode material for Li-ion batteries is investigated. Unlike other amorphous compounds so-far investigated as anode materials, it already contains Li as a base element in the uncycled state. The amorphous compound powder is prepared by high energy ball milling of a master alloy. It shows a strongly enhanced specific capacity in contrast to amorphous alloys without Li in the initial state. Therewith, by enabling a reversible (de)lithiation of metallic electrodes without the phase transition caused volume changes it offers the possibility of much increased specific capacities than conventional graphite anodes. According to the charge rate (C-rate), the specific capacity is reversible over 20 cycles at minimum in contrast to conventional crystalline intermetallic phases failing by volume changes. The delithiation process occurs quasi-continuously over a voltage range of nearly 4 V, while the lithiation is mainly observed between 0.1 V and 1.5 V. That way, the electrode is applicable for different potential needs. The electrode stays amorphous during cycling, thus avoiding volume changes. The cycling performance is further enhanced by a significant amount of Fe introduced as wear debris from the milling tools, which acts as a promoting element.
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    Twinning phenomena along and beyond the bain path
    (Basel : MDPI AG, 2013) Kauffmann-Weiss, S.; Kauffmann, A.; Niemann, R.; Freudenberger, J.; Schultz, L.; Fähler, S.
    Twinning is a phenomenon that occurs, e.g., during deformation, martensitic transformation and film growth. The present study shows that the crystallography of twinning can be described by two twinning modes along the complete Bain transformation path and beyond connecting body-centered and face-centered cubic structures. To probe this concept, we used strained epitaxial films of the Fe-Pd magnetic shape memory system. As the substrate acts as an absolute reference frame, we could show by pole figure measurements that all observed twinning can be a body-centered and face-centered cubic twinning mode. This continuously transforms towards identity when approaching the complementary structure.
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    Synthesis and characterization of nanocrystallineMg-7.4%Al powders produced by mechanical alloying
    (Basel : MDPI AG, 2013) Chaubey, A.K.; Scudino, S.; Khoshkhoo, M.S.; Prashanth, K.G.; Mukhopadhyay, N.K.; Mishra, B.K.; Eckert, J.
    Nanocrystalline Mg-7.4%Al powder was prepared by mechanical alloying using a high-energy mill. The evolution of the various phases and their microstructure, including size and morphology of the powder particles in the course of milling and during subsequent annealing, were investigated in detail. Room temperature milling leads to a rather heterogeneous microstructure consisting of two distinct regions: Al-free Mg cores and Mg-Al intermixed areas. As a result, the material is mechanically heterogeneous with the Mg cores displaying low hardness (40–50 HV) and the Mg-Al intermixed regions showing high hardness of about 170 HV. The Mg cores disappear and the microstructure becomes (also mechanically) homogeneous after subsequent cryo-milling. Rietveld structure refinement reveals that the crystallite size of the milled powders decreases with increasing the milling time reaching a minimum value of about 30 nm. This is corroborated by transmission electron microscopy confirming an average grain size of ~25 nm.
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    Spectral linewidth vs. Front facet reflectivity of 780 nm DFB diode lasers at high optical output power
    (Basel : MDPI AG, 2018) Nguyen, T.-P.; Wenzel, H.; Brox, O.; Bugge, F.; Ressel, P.; Schiemangk, M.; Wicht, A.; Tien, T.Q.; Tränkle, G.
    The influence of the front facet reflectivity on the spectral linewidth of high power DFB (distributed feedback) diode lasers emitting at 780 nm has been investigated theoretically and experimentally. Characterization of lasers at various front facet reflections showed substantial reduction of the linewidth. This behavior is in reasonable agreement with simulation results. A minimum linewidth of 8 kHz was achieved at an output power of 85 mW with the laser featuring a front facet reflectivity of 30%. The device with a front facet reflectivity of 5% reached the same linewidth value at an output power of 290 mW.
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    Binding energy referencing for XPS in Alkali metal-based battery materials research (II): Application to complex composite electrodes
    (Basel : MDPI AG, 2018) Oswald, S.; Thoss, F.; Zier, M.; Hoffmann, M.; Jaumann, T.; Herklotz, M.; Nikolowski, K.; Scheiba, F.; Kohl, M.; Giebeler, L.; Mikhailova, D.; Ehrenberg, H.
    X-ray photoelectron spectroscopy (XPS) is a key method for studying (electro-)chemical changes in metal-ion battery electrode materials. In a recent publication, we pointed out a conflict in binding energy (BE) scale referencing at alkali metal samples, which is manifested in systematic deviations of the BEs up to several eV due to a specific interaction between the highly reactive alkali metal in contact with non-conducting surrounding species. The consequences of this phenomenon for XPS data interpretation are discussed in the present manuscript. Investigations of phenomena at surface-electrolyte interphase regions for a wide range of materials for both lithium and sodium-based applications are explained, ranging from oxide-based cathode materials via alloys and carbon-based anodes including appropriate reference chemicals. Depending on material class and alkaline content, specific solutions are proposed for choosing the correct reference BE to accurately define the BE scale. In conclusion, the different approaches for the use of reference elements, such as aliphatic carbon, implanted noble gas or surface metals, partially lack practicability and can lead to misinterpretation for application in battery materials. Thus, this manuscript provides exemplary alternative solutions.
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    Electrodeposition of nanocrystalline Fe-P coatings: Influence of bath temperature and glycine concentration on structure, mechanical and corrosion behavior
    (Basel : MDPI AG, 2019) Kovalska, N.; Tsyntsaru, N.; Cesiulis, H.; Gebert, A.; Fornell, J.; Pellicer, E.; Sort, J.; Hansal, W.; Kautek, W.
    A detailed electrochemical study and investigation of a Fe-P glycine bath as a function of the temperature and glycine concentrations and current density, and their resulting corrosion and mechanical behavior is presented. A low addition of glycine to the electrolyte led to a drastic increase of the P content. At low Fe-P deposition rates, heterogeneous rough deposits with morphological bumps and pores were observed. By increasing the Fe-P deposition rate, the number of pores were reduced drastically, resulting in smooth coatings. Increasing the P content led to the formation of nanocrystalline grains from an "amorphous-like" state. Coatings with higher P contents exhibited better corrosion resistance and hardening, most likely attributed to grain boundary strengthening.
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    Processing of intermetallic titanium aluminide wires
    (Basel : MDPI AG, 2013) Marr, T.; Freudenberger, J.; Kauffmann, A.; Romberg, J.; Okulov, I.; Petters, R.; Scharnweber, J.; Eschke, A.; Oertel, C.-G.; Kühn, U.; Eckert, J.; Skrotzki, W.; Schultz, L.
    This study shows the possibility of processing titanium aluminide wires by cold deformation and annealing. An accumulative swaging and bundling technique is used to co-deform Ti and Al. Subsequently, a two step heat treatment is applied to form the desired intermetallics, which strongly depends on the ratio of Ti and Al in the final composite and therefore on the geometry of the starting composite. In a first step, the whole amount of Al is transformed to TiAl3 by Al diffusion into Ti. This involves the formation of 12% porosity. In a second step, the complete microstructure is transformed into the equilibrium state of γ-TiAl and TiAl3. Using this approach, it is possible to obtain various kinds of gradient materials, since there is an intrinsic concentration gradient installed due to the swaging and bundling technique, but the processing of pure γ-TiAl wires is possible as well.