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- ItemSelf-propelled micromotors for cleaning polluted water(Washington, DC : ACS, 2013) Soler, L.; Magdanz, V.; Fomin, V.M.; Sanchez, S.; Schmidt, O.G.We describe the use of catalytically self-propelled microjets (dubbed micromotors) for degrading organic pollutants in water via the Fenton oxidation process. The tubular micromotors are composed of rolled-up functional nanomembranes consisting of Fe/Pt bilayers. The micromotors contain double functionality within their architecture, i.e., the inner Pt for the self-propulsion and the outer Fe for the in situ generation of ferrous ions boosting the remediation of contaminated water.The degradation of organic pollutants takes place in the presence of hydrogen peroxide, which acts as a reagent for the Fenton reaction and as main fuel to propel the micromotors. Factors influencing the efficiency of the Fenton oxidation process, including thickness of the Fe layer, pH, and concentration of hydrogen peroxide, are investigated. The ability of these catalytically self-propelled micromotors to improve intermixing in liquids results in the removal of organic pollutants ca. 12 times faster than when the Fenton oxidation process is carried out without catalytically active micromotors. The enhanced reaction-diffusion provided by micromotors has been theoretically modeled. The synergy between the internal and external functionalities of the micromotors, without the need of further functionalization, results into an enhanced degradation of nonbiodegradable and dangerous organic pollutants at small-scale environments and holds considerable promise for the remediation of contaminated water.
- ItemRoles of hydrogenation, annealing and field in the structure and magnetic entropy change of Tb-based bulk metallic glasses(New York : American Institute of Physics, 2013) Luo, Qiang; Schwarz, Björn; Mattern, Norbert; Shen, Jun; Eckert, JürgenThe reduction of open-volume regions in Tb-based metallic glass (MG) by annealing and hydrogen charging was found to rearrange the atomic structure and tune the magnetic behaviors. After crystallization, the magnetic structure and magnetic entropy change (MEC) alters due to the structural transformation, and a plateau-like-MEC behavior can be obtained. The hydrogen concentration after charging at 1mA/cm2 for 576 h reaches as high as 3290 w-ppm. The magnetization behavior and the MEC change due to the modification of the exchange interaction and the random magnetic anisotropy (RMA) upon hydrogenation. At low temperatures, irreversible positive MEC was obtained, which is related to the internal entropy production. The RMA-to-exchange ratio acts as a switch to control the irreversible entropy production channel and the reversible entropy transfer channel. The field dependence of the MEC is discussed in term of the competition among Zeeman energy, exchange interaction and RMA.
- ItemCorrelation between the microstructures and the deformation mechanisms of CuZr-based bulk metallic glass composites(New York : American Institute of Physics, 2013) Song, K.K.; Pauly, S.; Sun, B.A; Tan, J.; Stoica, M.; Kühn, U.; Eckert, J.The variation of the transformation-mediated deformation behavior with microstructural changes in CuZr-based bulk metallic glass composites is investigated. With increasing crystalline volume fraction, the deformation mechanism gradually changes from a shear-banding dominated process as evidenced by a chaotic serrated flow behavior, to being governed by a martensitic transformation with a pronounced elastic-plastic stage, resulting in different plastic deformations evolving into a self-organized critical state characterized by the power-law distribution of shear avalanches. This is reflected in the stress-strain curves by a single-to-"double"-to-"triple"- double yielding transition and by different mechanical properties with different serrated flow characteristics, which are interpreted based on the microstructural evolutions and a fundamental energy theorem. Our results can assist in understanding deformation behaviors for high-performance metastable alloys.
- ItemChemotactic behavior of catalytic motors in microfluidic channels(Hoboken, NJ : Wiley, 2013) Baraban, Larysa; Harazim, Stefan M.; Sanchez, Samuel; Schmidt, Oliver.G.Chemotaxis in practice: Two different artificial catalytic micromotors (tubular and spherical, see scheme) show chemotactic behavior in microfluidic channels demonstrating that catalytic micromotors can sense the gradient of chemical fuel in their environment and be directed towards desired locations.
- ItemThickness dependent exchange bias in martensitic epitaxial Ni-Mn-Sn thin films(New York : American Institute of Physics, 2013) Behler, Anna; Teichert, Niclas; Dutta, Biswanath; Waske, Anja; Hickel, Tilmann; Auge, Alexander; Hütten, Andreas; Eckert, JürgenA thickness dependent exchange bias in the low temperature martensitic state of epitaxial Ni-Mn-Sn thin films is found. The effect can be retained down to very small thicknesses. For a Ni50Mn32Sn18 thin film, which does not undergo a martensitic transformation, no exchange bias is observed. Our results suggest that a significant interplay between ferromagnetic and antiferromagnetic regions, which is the origin for exchange bias, is only present in the martensite. The finding is supported by ab initio calculations showing that the antiferromagnetic order is stabilized in the phase.
- ItemArtificial micro-cinderella based on self-propelled micromagnets for the active separation of paramagnetic particles(Cambridge : RSC, 2013) Zhao, G.; Wang, H.; Sanchez, S.; Schmidt, O.G.; Pumera, M.In this work, we will show that ferromagnetic microjets can pick-up paramagnetic beads while not showing any interaction with diamagnetic silica microparticles for the active separation of microparticles in solution.
- ItemProduction of porous β-Type Ti–40Nb alloy for biomedical applications: Comparison of selective laser melting and hot pressing(Basel : MDPI, 2013) Zhuravleva, Ksenia; Bönisch, Matthias; Prashanth, Konda Gokuldoss; Hempel, Ute; Helth, Arne; Gemming, Thomas; Calin, Mariana; Scudino, Sergio; Schultz, Ludwig; Eckert, Jürgen; Gebert, AnnettWe used selective laser melting (SLM) and hot pressing of mechanically-alloyed β-type Ti–40Nb powder to fabricate macroporous bulk specimens (solid cylinders). The total porosity, compressive strength, and compressive elastic modulus of the SLM-fabricated material were determined as 17% ± 1%, 968 ± 8 MPa, and 33 ± 2 GPa, respectively. The alloy’s elastic modulus is comparable to that of healthy cancellous bone. The comparable results for the hot-pressed material were 3% ± 2%, 1400 ± 19 MPa, and 77 ± 3 GPa. This difference in mechanical properties results from different porosity and phase composition of the two alloys. Both SLM-fabricated and hot-pressed cylinders demonstrated good in vitro biocompatibility. The presented results suggest that the SLM-fabricated alloy may be preferable to the hot-pressed alloy for biomedical applications, such as the manufacture of load-bearing metallic components for total joint replacements.
- ItemConfined crystals of the smallest phase-change material(Washington, DC : American Chemical Society, 2013) Giusca, C.E.; Stolojan, V.; Sloan, J.; Börrnert, F.; Shiozawa, H.; Sader, K.; Rümmeli, M.H.; Büchner, B.; Silva, S.R.P.The demand for high-density memory in tandem with limitations imposed by the minimum feature size of current storage devices has created a need for new materials that can store information in smaller volumes than currently possible. Successfully employed in commercial optical data storage products, phase-change materials, that can reversibly and rapidly change from an amorphous phase to a crystalline phase when subject to heating or cooling have been identified for the development of the next generation electronic memories. There are limitations to the miniaturization of these devices due to current synthesis and theoretical considerations that place a lower limit of 2 nm on the minimum bit size, below which the material does not transform in the structural phase. We show here that by using carbon nanotubes of less than 2 nm diameter as templates phase-change nanowires confined to their smallest conceivable scale are obtained. Contrary to previous experimental evidence and theoretical expectations, the nanowires are found to crystallize at this scale and display amorphous-to-crystalline phase changes, fulfilling an important prerequisite of a memory element. We show evidence for the smallest phase-change material, extending thus the size limit to explore phase-change memory devices at extreme scales.
- ItemProcessing metallic glasses by selective laser melting(Amsterdam [u.a.] : Elsevier, 2013) Pauly, S.; Löber, L.; Petters, R.; Stoica, M.; Scudino, S.; Kühn, U.; Eckert, J.Metallic glasses and their descendants, the so-called bulk metallic glasses (BMGs), can be regarded as frozen liquids with a high resistance to crystallization. The lack of a conventional structure turns them into a material exhibiting near-theoretical strength, low Young's modulus and large elasticity. These unique mechanical properties can be only obtained when the metallic melts are rapidly cooled to bypass the nucleation and growth of crystals. Most of the commonly known and used processing routes, such as casting, melt spinning or gas atomization, have intrinsic limitations regarding the complexity and dimensions of the geometries. Here, it is shown that selective laser melting (SLM), which is usually used to process conventional metallic alloys and polymers, can be applied to implement complex geometries and components from an Fe-base metallic glass. This approach is in principle viable for a large variety of metallic alloys and paves the way for the novel synthesis of materials and the development of parts with advanced functional and structural properties without limitations in size and intricacy.
- ItemQuasi-static and dynamic deformation behaviour of Zr-based bulk metallic glass(Milton Park : Taylor & Francis, 2013) Nekouie, V.; Kühn, U.; Roy, A.; Silberschmidt, V.Nano- and micro-indentation studies were carried out to characterise a plasticity mechanism through the evolution of localised shear bands that drive material's deformation at sub-micron length scale. Initial deformation of Zr-based bulk metallic glass (BMG) was investigated with nanoindentation tests using a spherical indenter. The indentation cycle reflects an elastic deformation with the yielding load of approx. 3 mN. For designed cycling indentation, hardening and softening phenomena were observed in nano- and micro-indentations, respectively. High-precision dynamic mechanical relaxation measurements were performed using a Dynamic Mechanical Analyzer (DMA), on decreasing frequency from 160 Hz to 0.1 Hz. A mechanical response of the BMG surface to a concentrated impact load was also studied. The obtained results indicated that the studied Zr-based BMG behaved as an elastic-perfectly plastic material at macroscale with discrete plasticity events at smaller length scales.