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- ItemEffect of Selective Laser Melting on Microstructure, Mechanical, and Corrosion Properties of Biodegradable FeMnCS for Implant Applications(Weinheim : Wiley-VCH Verl., 2020) Hufenbach, Julia; Sander, Jan; Kochta, Fabian; Pilz, Stefan; Voss, Andrea; Kühn, Uta; Gebert, AnnettSelective laser melting (SLM) of biodegradable metallic materials offers a great potential for manufacturing customized implants. Herein, SLM processing of a novel Fe–30Mn–1C–0.02S twinning-induced plasticity (TWIP) alloy and the resulting structural, mechanical, and corrosion properties are presented. The occurring rapid solidification results in a fine-grained austenitic microstructure with mainly homogeneous element distribution, which is investigated by scanning electron microscopy (SEM) combined with energy-dispersive X-ray spectroscopy (EDX) and electron backscatter diffraction (EBSD) as well as X-ray diffraction (XRD). By processing the alloy via SLM, significantly higher strengths under tensile and compressive load in comparison with those for the as-cast counterpart and a 316L reference steel are achieved. Electrochemical corrosion tests in a simulated body fluid (SBF) indicate a moderate corrosion activity, and a beneficial uniform degradation is shown in immersion tests in SBF. Regarding the envisaged application for vascular implants, SLM-processed stent prototypes out of the novel alloy are presented and a first functionality test is shown. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
- ItemCalciothermic Synthesis of Very Fine, Hydrogenated Ti and Ti–Nb Powder for Biomedical Applications(Weinheim : Wiley-VCH Verl., 2020) Lindemann, Inge; Gebel, Bernhard; Pilz, Stefan; Uhlemann, Margitta; Gebert, AnnettDue to their excellent biocompatibility, titanium and titanium–niobium alloys are especially interesting for biomedical applications. With regard to favorable near-net shape production, Ti powder synthesis is the key hurdle. Extensive research has been in progress for alternative synthesis methods since decades. Herein, an efficient alternative method to the conventional powder production process to prepare spherical powders with very small sizes (<45 μm) for high-strength materials is shown. Very fine, hydrogenated Ti and Ti–Nb alloy powders are stable in air and are synthesized by calciothermic reduction in hydrogen. The herein presented reduction using CaH2 starts directly from the oxides instead of chlorides. Correlations of size and morphology of the as-synthesized TiH2 and (Ti,Nb)H2 powders with the precursors (TiO2, Nb2O5, and CaH2) are illustrated and are used to tailor the desired powders.
- Item2D/3D Metallic Nano-objects Self-Organized in an Organic Molecular Thin Film(Washington, DC : ACS Publications, 2020) Molodtsova, Olga V.; Aristova, Irina M.; Potorochin, Dmitrii V.; Babenkov, Sergey V.; Khodos, Igor I.; Molodtsov, Serguei L.; Makarova, Anna A.; Smirnov, Dmitry A.; Aristov, Victor Yu.We present the fabrication and investigation of the properties of nanocomposite structures consisting of two-dimensional (2D) and three-dimensional (3D) metallic nano-objects self-organized on the surface and inside of organic molecular thin-film copper tetrafluorophthalocyanine (CuPcF4). Metallic atoms, deposited under ultrahigh vacuum (UHV) conditions onto the organic ultrathin film, diffuse along the surface and self-assemble into a system of 2D metallic overlayers. At the same time, the majority of the metal atoms diffuse into the organic matrix and self-organize into 3D nanoparticles (NPs) in a well-defined manner. The evolution of the morphology and electronic properties of such structures as a function of nominal metal content is studied under UHV conditions using transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HR-TEM), and photoelectron spectroscopy (PES) techniques. Using HR-TEM, we have observed the periodicity of atomic planes of individual silver NPs. The steady formation of agglomerates from individual single nanocrystallites with intercrystallite boundaries is observed as well. PES reveals generally weak chemical interactions between silver and the organic matrix and n-doping of CuPcF4 at the initial stages of silver deposition, which is associated with charge transfer from the 2D wetting layer on the basis of core-level spectra shift analysis. Copyright © 2020 American Chemical Society.
- ItemSegregated Network Polymer Composites with High Electrical Conductivity and Well Mechanical Properties based on PVC, P(VDFTFE), UHMWPE, and rGO(Washington, DC : ACS Publications, 2020) Shiyanova, Kseniya A.; Gudkov, Maksim V.; Gorenberg, Arkady Ya; Rabchinskii, Maxim K.; Smirnov, Dmitry A.; Shapetina, Maria A.; Gurinovich, Tatiana D.; Goncharuk, Galina P.; Kirilenko, Demid A.; Bazhenov, Sergey L.; Melnikov, Valery P.The formation of a segregated network structure (wittingly uneven distribution of a filler) is one of the most promising strategies for the fabrication of electrically conductive polymer composites at present. However, the simultaneous achievement of high values of electrical conductivity with the retention of well mechanical properties within this approach remains a great challenge. Here, by means of X-ray photoelectron spectra (XPS), near-edge X-ray absorption fine structure (NEXAFS) spectra, scanning electron microscopy (SEM), dielectric spectroscopy, and compression engineering stress-strain curve analysis, we have studied the effect of a segregated network structure on the electrical conductivity and mechanical properties of a set of polymer composites. The composites were prepared by applying graphene oxide (GO) with ultralarge basal plane size (up to 150 μm) onto the surface of polymer powder particles, namely, poly(vinyl chloride) (PVC), poly(vinylidene fluoride-co-tetrafluoroethylene) (P(VDF-TFE)), and ultrahigh-molecular-weight poly(ethylene) (UHMWPE) with the subsequent GO reduction and composite hot pressing. A strong dependence of the segregated network polymer composites' physical properties on the polymer matrix was demonstrated. Particularly, 12 orders of magnitude rise of the polymers' electrical conductivity up to 0.7 S/m was found upon the incorporation of the reduced GO (rGO). A 17% increase in the P(VDF-TFE) elastic modulus filled by 1 wt % of rGO was observed. Fracture strength of PVC/rGO at 0.5 wt % content of the filler was demonstrated to decrease by fourfold. At the same time, the change in strength was not significant for P(VDF-TFE) and UHMWPE composites in comparison with pure polymers. Our results show a promise to accelerate the development of new composites for energy applications, such as metal-free supercapacitor plates and current collectors of lithium-ion batteries, bipolar plates of proton-exchange membrane fuel cells, antistatic elements of various electronic devices, etc. © 2020 American Chemical Society.
- ItemOptical Anisotropy and Momentum-Dependent Excitons in Dibenzopentacene Single Crystals(Washington, DC : ACS Publications, 2022) Graf, Lukas; Liu, Fupin; Naumann, Marco; Roth, Friedrich; Debnath, Bipasha; Büchner, Bernd; Krupskaya, Yulia; Popov, Alexey A.; Knupfer, MartinHigh-quality single crystals of the organic semiconductor (1,2;8,9)-dibenzopentacene were grown via physical vapor transport. The crystal structure─unknown before─was determined by single-crystal X-ray diffraction; polarization-dependent optical absorption measurements display a large anisotropy in the ac plane of the crystals. The overall Davydov splitting is ∼110 meV, which is slightly lower than that in the close relative pentacene (120 meV). Momentum-dependent electron energy-loss spectroscopy measurements show a clear exciton dispersion of the Davydov components. An analysis of the dispersion using a simple 1D model indicates smaller electron- and hole-transfer integrals in dibenzopentacene as compared to pentacene. The spectral weight distribution of the excitation spectra is strongly momentum-dependent and demonstrates a strong momentum-dependent admixture of Frenkel excitons, charge-transfer excitons, and vibrational modes.
- ItemVerification of impurity-related photocatalytic activity of insulating oxide supports(Amsterdam : Elsevier, 2021) Kortewille, Bianca; Springer, Armin; Strunk, JenniferPhotocatalysts composed of vanadium oxide species supported on commercial MgO and ZrO2 are investigated in selective methanol oxidation. Both support oxides are insulators, so the vanadium oxide species are expected as sole active component in photocatalysis. However, the pure supports showed considerable activity: Bare MgO was more active than MgO-supported vanadia catalysts, and ZrO2 showed intermediate activity. By various characterization methods, the presence of TiO2 (anatase) in the MgO support, and the presence of Zn, possibly as ZnO, in ZrO2 is demonstrated. The present study highlights that photocatalysts containing commercial supports must be carefully checked for impurity-related photocatalytic performance. © 2021 The Authors
- ItemThe Impact of AlN Templates on Strain Relaxation Mechanisms during the MOVPE Growth of UVB-LED Structures(Weinheim : Wiley-VCH, 2020) Knauer, Arne; Mogilatenko, Anna; Weinrich, Jonas; Hagedorn, Sylvia; Walde, Sebastian; Kolbe, Tim; Cancellara, Leonardo; Weyers, MarkusStrain relaxation mechanisms in AlGaN based light emitting diodes emitting in the ultraviolet B spectral range (UVB-LEDs) grown on different AlN/sapphire templates are analyzed by combining in situ reflectivity and curvature data with transmission electron microscopy. In particular, the impact of dislocation density, surface morphology, and lattice constant of the AlN/sapphire templates is studied. For nonannealed AlN/templates with threading dislocation densities (TDDs) of 4 × 109 and 3 × 109 cm−2 and different surface morphologies strain relaxation takes place mostly by conventional ways, such as inclination of threading dislocation lines and formation of horizontal dislocation bands. In contrast, a TDD reduction down to 1 × 109 cm−2 as well as a reduction of the lattice constant of high temperature annealed AlN template leads to drastic changes in the structure of subsequently grown AlGaN layers, e.g., to transformation to helical dislocations and enhanced surface enlargement by formation of macrofacets. For the growth of strongly compressively strained AlGaN layers for UVB-LEDs the relaxation mechanism is strongly influenced by the absolute values of TDD and the lattice constant of the AlN templates and is less influenced by their surface morphology.