2013, Luo, Qiang, Schwarz, Björn, Mattern, Norbert, Shen, Jun, Eckert, Jürgen

The 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.

2016, Peng, Chuan-Xiao, Song, Kai-Kai, Wang, Li, Şopu, Daniel, Pauly, Simon, Eckert, Jürgen

Unlike crystalline metals, the plastic deformation of metallic glasses (MGs) involves a competition between disordering and structural relaxation ordering, which is not well understood, yet. Molecular dynamics (MD) simulations were performed to investigate the evolutions of strain localizations, short-range order (SRO) as well as the free volume in the glass during compressive deformation of Fe50Cu50 MGs with different degrees of phase separation. Our findings indicate that the free volume in the phase separating MGs decreases while the shear strain localizations increase with increasing degree of phase separation. Cu-centered clusters show higher potential energies and Voronoi volumes, and bear larger local shear strains. On the other hand, Fe-centered pentagon-rich clusters in Cu-rich regions seem to play an important role to resist the shear transformation. The dilatation or annihilation of Voronoi volumes is due to the competition between ordering via structural relaxation and shear stress-induced deformation. The present study could provide a better understanding of the relationship between the structural inhomogeneity and the deformation of MGs.

2015, Bickel, St., Schwab, H., Pauly, S., Kühn, U., Eckert, Jürgen

[no abstract available]

2013, Wang, Jiao, Zeng, Mengqi, Tan, Lifang, Dai, Boya, Deng, Yuan, Rümmeli, Mark, Xu, Haitao, Li, Zishen, Wang, Sheng, Peng, Lianmao, Eckert, Jürgen, Fu, Lei

The high-quality and low-cost of the graphene preparation method decide whether graphene is put into the applications finally. Enormous efforts have been devoted to understand and optimize the CVD process of graphene over various d-block transition metals (e.g. Cu, Ni and Pt). Here we report the growth of uniform high-quality single-layer, single-crystalline graphene flakes and their continuous films over p-block elements (e.g. Ga) liquid films using ambient-pressure chemical vapor deposition. The graphene shows high crystalline quality with electron mobility reaching levels as high as 7400 cm2 V−1s−1 under ambient conditions. Our employed growth strategy is ultra-low-loss. Only trace amounts of Ga are consumed in the production and transfer of the graphene and expensive film deposition or vacuum systems are not needed. We believe that our research will open up new territory in the field of graphene growth and thus promote its practical application.

2012, Liu, Zengqian, Li, Ran, Liu, Gang, Song, Kaikai, Pauly, Simon, Zhang, Tao, Eckert, Jürgen

Microstructures and mechanical properties of as-cast Cu47.5Zr47.5Al5 bulk metallic glass composites are optimized by appropriate remelting treatment of master alloys. With increasing remelting time, the alloys exhibit homogenized size and distribution of in situ formed B2 CuZr crystals. Pronounced tensile ductility of ∼13.6% and work-hardening ability are obtained for the composite with optimized microstructure. The effect of remelting treatment is attributed to the suppressed heterogeneous nucleation and growth of the crystalline phase from undercooled liquid, which may originate from the dissolution of oxides and nitrides as well as from the micro-scale homogenization of the melt.

2016, He, Jie, Kaban, Ivan, Mattern, Norbert, Song, Kaikai, Sun, Baoan, Kim, Do Hyang, Eckert, Jürgen, Greer, A.Lindsay

At room temperature, plastic flow of metallic glasses (MGs) is sharply localized in shear bands, which are a key feature of the plastic deformation in MGs. Despite their clear importance and decades of study, the conditions for formation of shear bands, their structural evolution and multiplication mechanism are still under debate. In this work, we investigate the local conditions at shear bands in new phase-separated bulk MGs containing glassy nanospheres and exhibiting exceptional plasticity under compression. It is found that the glassy nanospheres within the shear band dissolve through mechanical mixing driven by the sharp strain localization there, while those nearby in the matrix coarsen by Ostwald ripening due to the increased atomic mobility. The experimental evidence demonstrates that there exists an affected zone around the shear band. This zone may arise from low-strain plastic deformation in the matrix between the bands. These results suggest that measured property changes originate not only from the shear bands themselves, but also from the affected zones in the adjacent matrix. This work sheds light on direct visualization of deformation-related effects, in particular increased atomic mobility, in the region around shear bands.

2013, Behler, Anna, Teichert, Niclas, Dutta, Biswanath, Waske, Anja, Hickel, Tilmann, Auge, Alexander, Hütten, Andreas, Eckert, Jürgen

A 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.

2016, Eckert, Jürgen, Giebeler, Lars

Die Entwicklung und Umsetzung umfasst eines völlig neuartigen Batteriekonzeptes, der die Vorteile der bisherigen Na-S-Hochtemperaturbatterien (z.B. niedrige Kosten und hohe Verfügbarkeit der notwendigen Rohstoffe) mit der Performance moderner Lithium-Ionenbatterien, jedoch auf Na-Ionenbasis, im Niedertemperaturbereich verknüpft. Dazu müssen neue Elektroden- bzw. Separatormaterialien mit vorteilhafter Interaktion und Degradationsstabilität in verschiedenen neuartigen Elektrolyten entwickelt werden. Darüber hinaus werden geeignete Verfahren zur Herstellung und Fertigung dieser Komponenten zu Niedertemperatur-Na-S-Batterien generiert. Die Ziele sollen durch die außerordentlich enge Vernetzung mehrerer Professuren der TU Dresden mit verschiedenen Instituten der Fraunhofer Gesellschaft, dem Leibniz IFW Dresden e.V. und der TU Bergakademie Freiberg erreicht werden. Der Arbeitsplan sieht eine 'bottom up' Strategie von der Materialentwicklung und Charakterisierung über die Werkstoffprozessierung hin zur Systementwicklung und -charakterisierung vor. Die universitären Einrichtungen arbeiten dabei vorwiegend grundlagenorientiert auf dem Gebiet der Materialentwicklung für einen völlig neuen Batterietyp. Die dabei gewonnenen Erkenntnisse werden unmittelbar in die anwendungsorientierte Forschung überführt. Entscheidend ist die interaktive Zusammenarbeit zu allen Zeitpunkten und auf allen Ebenen des Gesamtvorhabens.

2016, Ramasamy, Parthiban, Szabo, Attila, Borzel, Stefan, Eckert, Jürgen, Stoica, Mihai, Bárdos, András

Soft ferromagnetic Fe-based bulk metallic glass key-shaped specimens with a maximum and minimum width of 25.4 and 5 mm, respectively, were successfully produced using a high pressure die casting (HPDC) method, The influence of die material, alloy temperature and flow rate on the microstructure, thermal stability and soft ferromagnetic properties has been studied. The results suggest that a steel die in which the molten metal flows at low rate and high temperature can be used to produce completely glassy samples. This can be attributed to the laminar filling of the mold and to a lower heat transfer coefficient, which avoids the skin effect in the steel mold. In addition, magnetic measurements reveal that the amorphous structure of the material is maintained throughout the key-shaped samples. Although it is difficult to control the flow and cooling rate of the molten metal in the corners of the key due to different cross sections, this can be overcome by proper tool geometry. The present results confirm that HPDC is a suitable method for the casting of Fe-based bulk glassy alloys even with complex geometries for a broad range of applications.

2016, Sarac, Baran, Zhang, Long, Kosiba, Konrad, Pauly, Simon, Stoica, Mihai, Eckert, Jürgen

Tailoring the intrinsic length-scale effects in bulk metallic glasses (BMGs) via post-heat treatment necessitates a systematic analyzing strategy. Although various achievements were made in the past years to structurally enhance the properties of different BMG alloys, the influence of short-term sub-glass transition annealing on the relaxation kinetics is still not fully covered. Here, we aim for unraveling the connection between the physical, (thermo)mechanical and structural changes as a function of selected pre-annealing temperatures and time scales with an in-house developed Cu46Zr44Al8Hf2 based BMG alloy. The controlled formation of nanocrystals below 50 nm with homogenous distribution inside the matrix phase via thermal treatment increase the material’s resistance to strain softening by almost an order of magnitude. The present work determines the design aspects of metallic glasses with enhanced mechanical properties via nanostructural modifications, while postulating a counter-argument to the intrinsic property degradation accounted for long-term annealing.