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- ItemTransient spin injection efficiencies at ferromagnet/metal interfaces(Weinheim : Wiley-VCH, 2022-10-19) Elliott, Peter; Eschenlohr, Andrea; Chen, Jinghao; Shallcross, Sam; Bovensiepen, Uwe; Dewhurst, John Kay; Sharma, SangeetaSpin injection across interfaces driven by ultrashort optical pulses on femtosecond timescales constitutes a new way to design spintronics applications. Targeted utilization of this phenomenon requires knowledge of the efficiency of non-equilibrium spin injection. From a quantitative comparison of ab initio time-dependent density functional theory and interface-sensitive, time-resolved non-linear optical experiment, the spin injection efficiency (SIE) at the Co/Cu(001) interface is determined, and its microscopic origin, i.e., the influence of spin-orbit coupling and the interface electronic structure, is discussed. Moreover, we theoretically predict that the SIE at ferromagnetic–metal interfaces can be optimized through laser pulse and materials parameters, namely the fluence, pulse duration, and substrate material.
- ItemThe Role of Al2O3 ALD Coating on Sn-Based Intermetallic Anodes for Rate Capability and Long-Term Cycling in Lithium-Ion Batteries(Weinheim : Wiley-VCH, 2022) Soltani, Niloofar; Abbas, Syed Muhammad; Hantusch, Martin; Lehmann, Sebastian; Nielsch, Kornelius; Bahrami, Amin; Mikhailova, DariaThe electrochemical performances of CoSn2 and Ni3Sn4 as potential anode materials in lithium-ion batteries (LIBs) are investigated using varying thicknesses of an alumina layer deposited by the atomic layer deposition (ALD) technique. Rate capability results showed that at high current densities, Al2O3-coated CoSn2 and Ni3Sn4 electrodes after 10-ALD cycles outperformed uncoated materials. The charge capacities of coated CoSn2 and Ni3Sn4 electrodes are 571 and 134 mAh g−1, respectively, at a high current density of 5 A g−1, while the capacities of uncoated electrodes are 363 and 11 mAh g−1. When the current density is reduced to 1 A g−1, however, the cycling performances of Al2O3-coated CoSn2 and Ni3Sn4 electrodes fade faster after almost 40 cycles than uncoated electrodes. The explanation is found in the composition of the solid-electrolyte interface (SEI), which strongly depends on the current rate. Thus, X-ray photoelectron spectroscopy analysis of SEI layers on coated samples cycles at a low current density of 0.1 Ag−1, revealed organic carbonates as major products, which probably have a low ionic conductivity. In contrast, the SEI of coated materials cycled at 5 Ag−1 consists mostly of mixed inorganic/organic fluorine-rich Al-F and C-F species facilitating a higher ionic transport, which improves electrochemical performance.
- ItemA novel approach to fabricate load-bearing Ti6Al4V-Barium titanate piezoelectric bone scaffolds by coupling electron beam melting and field-assisted sintering(Amsterdam [u.a.] : Elsevier Science, 2022) Riaz, Abdullah; Polley, Christian; Lund, Henrik; Springer, Armin; Seitz, HermannA critical-size bone defect in load-bearing areas is a challenging clinical problem in orthopaedic surgery. Titanium alloy (Ti6Al4V) scaffolds have advantages because of their biomechanical stability but lack electrical activity, which hinders their further use. This work is focused on the fabrication of Ti6Al4V-Barium Titanate (BaTiO3) bulk composite scaffolds to combine the biomechanical stability of Ti6Al4V with electrical activity through BaTiO3. For the first time, a hollow cylindrical Ti6Al4V is additively manufactured by electron beam melting and combined with piezoelectric BaTiO3 powder for joint processing in field-assisted sintering. Scanning electron microscope images on the interface of the Ti6Al4V-BaTiO3 composite scaffold showed that after sintering, the Ti6Al4V lattice structure bounded with BaTiO3 matrix without its major deformation. The Ti6Al4V-BaTiO3 scaffold had average piezoelectric constants of (0.63 ± 0.12) pC/N directly after sintering due to partial dipole alignment of the BaTiO3 tetragonal phase, which increased to (4.92 ± 0.75) pC/N after a successful corona poling. Moreover, the nanoindentation values of Ti6Al4V exhibited an average hardness and Young's modulus of (5.9 ± 0.9) GPa and (130 ± 14) GPa, and BaTiO3 showed (4.0 ± 0.6) GPa and (106 ± 10) GPa, respectively. It reveals that the Ti6Al4V is the harder and stiffer part in the Ti6Al4V-BaTiO3 composite scaffold. Such a scaffold has the potential to treat critical-size bone defects in load-bearing areas and guide tissue regeneration by physical stimulation.
- ItemPrecipitate number density determination in microalloyed steels by complementary atom probe tomography and matrix dissolution(Dordrecht [u.a.] : Springer Science + Business Media B.V, 2022) Weber, Louis; Webel, Johannes; Mücklich, Frank; Kraus, TobiasParticle number densities are a crucial parameter in the microstructure engineering of microalloyed steels. We introduce a new method to determine nanoscale precipitate number densities of macroscopic samples that is based on the matrix dissolution technique (MDT) and combine it with atom probe tomography (APT). APT counts precipitates in microscopic samples of niobium and niobium-titanium microalloyed steels. The new method uses MDT combined with analytical ultracentrifugation (AUC) of extracted precipitates, inductively coupled plasma–optical emission spectrometry, and APT. We compare the precipitate number density ranges from APT of 137.81 to 193.56 × 1021 m−3 for the niobium steel and 104.90 to 129.62 × 1021 m−3 for the niobium-titanium steel to the values from MDT of 2.08 × 1021 m−3 and 2.48 × 1021 m−3. We find that systematic errors due to undesired particle loss during extraction and statistical uncertainties due to the small APT volumes explain the differences. The size ranges of precipitates that can be detected via APT and AUC are investigated by comparison of the obtained precipitate size distributions with transmission electron microscopy analyses of carbon extraction replicas. The methods provide overlapping resulting ranges. MDT probes very large numbers of small particles but is limited by errors due to particle etching, while APT can detect particles with diameters below 10 nm but is limited by small-number statistics. The combination of APT and MDT provides comprehensive data which allows for an improved understanding of the interrelation between thermo-mechanical controlled processing parameters, precipitate number densities, and resulting mechanical-technological material properties. Graphical abstract: [Figure not available: see fulltext.]
- ItemMicroscopic Softening Mechanisms of an Ionic Liquid Additive in an Electrically Conductive Carbon-Silicone Composite(Weinheim : Wiley, 2022) Zhang, Long; Schmidt, Dominik S.; González‐García, Lola; Kraus, TobiasThe microstructural changes caused by the addition of the ionic liquid (IL) 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide to polydimethylsiloxane (PDMS) elastomer composites filled with carbon black (CB) are analyzed to explain the electrical, mechanical, rheological, and optical properties of IL-containing precursors and composites. Swelling experiments and optical analysis indicate a limited solubility of the IL in the PDMS matrix that reduces the cross-linking density of PDMS both globally and locally, which reduces the Young's moduli of the composites. A rheological analysis of the precursor mixture shows that the IL reduces the strength of carbon–carbon and carbon–PDMS interactions, thus lowering the filler–matrix coupling and increasing the elongation at break. Electromechanical testing reveals a combination of reversible and irreversible piezoresistive responses that is consistent with the presence of IL at microscopic carbon–carbon interfaces, where it enables re-established electrical connections after stress release but reduces the absolute conductivity.
- ItemTuning the Release Force of Microfibrillar Adhesives by Geometric Design(Weinheim : Wiley-VCH, 2022) Barnefske, Lena; Rundel, Fabian; Moh, Karsten; Hensel, René; Zhang, Xuan; Arzt, EduardSwitchable micropatterned adhesives exhibit high potential as novel resource-efficient grippers in future pick-and-place systems. In contrast with the adhesion acting during the “pick” phase, the release during the “place” phase has received little research attention so far. For objects smaller than typically 1 mm, release may become difficult as gravitational and inertial forces are no longer sufficient to allow shedding of the object. A compressive overload can initiate release by elastic buckling of the fibrils, but the switching ratio (ratio between high and low adhesion force) is typically only 2–3. In this work, new microfibrillar designs are reported exhibiting directional buckling with high switching ratios in the order of 20. Their functionality is illustrated by in situ optical observation of the contact signatures. Such micropatterns can enable the successful release of small objects with high placement accuracy.
- ItemSliding Mechanism for Release of Superlight Objects from Micropatterned Adhesives(Weinheim : Wiley-VCH, 2022) Wang, Yue; Zhang, Xuan; Hensel, René; Arzt, EduardRobotic handling and transfer printing of micrometer-sized superlight objects is a crucial technology in industrial fabrication. In contrast to the precise gripping with micropatterned adhesives, the reliable release of superlight objects with negligible weight is a great challenge. Slanted deformable polymer microstructures, with typical pillar cross-section 150 µm × 50 µm, are introduced with various tilt angles that enable a reduction of adhesion by a switching ratio of up to 500. The experiments demonstrate that the release from a smooth surface involves sliding of the contact during compression and subsequent peeling of the object during retraction. The handling of a 0.5 mg perfluorinated polymer micro-object with high accuracy in repeated pick-and-place cycles is demonstrated. Based on beam theory, the forces and moments acting at the tip of the microstructure are analyzed. As a result, an expression for the pull-off force is proposed as a function of the sliding distance and a guide to an optimized design for these release structures is provided.
- ItemLayered Nano‐Mosaic of Niobium Disulfide Heterostructures by Direct Sulfidation of Niobium Carbide MXenes for Hydrogen Evolution(Weinheim : Wiley-VCH, 2022) Husmann, Samantha; Torkamanzadeh, Mohammad; Liang, Kun; Majed, Ahmad; Dun, Chaochao; Urban, Jeffrey J.; Naguib, Michael; Presser, VolkerMXene-transition metal dichalcogenide (TMD) heterostructures are synthesized through a one-step heat treatment of Nb2C and Nb4C3. These MXenes are used without delamination or any pre-treatment. Heat treatments accomplish the sacrificial transformation of these MXenes into TMD (NbS2) at 700 and 900 °C under H2S. This work investigates, for the first time, the role of starting MXene phase in the derivative morphology. It is shown that while treatment of Nb2C at 700 °C leads to the formation of pillar-like structures on the parent MXene, Nb4C3 produces nano-mosaic layered NbS2. At 900 °C, both MXene phases, of the same transition metal, fully convert into nano-mosaic layered NbS2 preserving the parent MXene's layered morphology. When tested as electrodes for hydrogen evolution reaction, Nb4C3-derived hybrids show better performance than Nb2C derivatives. The Nb4C3-derived heterostructure exhibits a low overpotential of 198 mV at 10 mA cm−2 and a Tafel slope of 122 mV dec−1, with good cycling stability in an acidic electrolyte.
- ItemRobust Magnetic Order Upon Ultrafast Excitation of an Antiferromagnet(Weinheim : Wiley-VCH, 2022) Lee, Sang‐Eun; Windsor, Yoav William; Fedorov, Alexander; Kliemt, Kristin; Krellner, Cornelius; Schüßler‐Langeheine, Christian; Pontius, Niko; Wolf, Martin; Atxitia, Unai; Vyalikh, Denis V.; Rettig, LaurenzThe ultrafast manipulation of magnetic order due to optical excitation is governed by the intricate flow of energy and momentum between the electron, lattice, and spin subsystems. While various models are commonly employed to describe these dynamics, a prominent example being the microscopic three temperature model (M3TM), systematic, quantitative comparisons to both the dynamics of energy flow and magnetic order are scarce. Here, an M3TM was applied to the ultrafast magnetic order dynamics of the layered antiferromagnet GdRh2Si2. The femtosecond dynamics of electronic temperature, surface ferromagnetic order, and bulk antiferromagnetic order were explored at various pump fluences employing time- and angle-resolved photoemission spectroscopy and time-resolved resonant magnetic soft X-ray diffraction, respectively. After optical excitation, both the surface ferromagnetic order and the bulk antiferromagnetic order dynamics exhibit two-step demagnetization behaviors with two similar timescales (<1 ps, ∼10 ps), indicating a strong exchange coupling between localized 4f and itinerant conduction electrons. Despite a good qualitative agreement, the M3TM predicts larger demagnetization than the experimental observation, which can be phenomenologically described by a transient, fluence-dependent increased Néel temperature. The results indicate that effects beyond a mean-field description have to be considered for a quantitative description of ultrafast magnetic order dynamics.