24 research outputs found
Influence of Crystalline Nanoprecipitates on Shear-Band Propagation in Cu-Zr Based Metallic Glasses
The interaction of shear bands with crystalline nanoprecipitates in
Cu-Zr-based metallic glasses is investigated by a combination of
high-resolution TEM imaging and molecular-dynamics computer simulations. Our
results reveal different interaction mechanisms: Shear bands can dissolve
precipitates, can wrap around crystalline obstacles, or can be blocked
depending on size and density of the precipitates. If the crystalline phase has
a low yield strength, we also observe slip transfer through the precipitate.
Based on the computational results and experimental findings, a qualitative
mechanism map is proposed that categorizes the various processes as a function
of the critical stress for dislocation nucleation, precipitate size, and
distance.Comment: 16 pages, 15 figure
Low temperature features in the heat capacity of unary metals and intermetallics for the example of bulk aluminum and AlSc
We explore the competition and coupling of vibrational and electronic
contributions to the heat capacity of Al and AlSc at temperatures below 50
K combining experimental calorimetry with highly converged finite temperature
density functional theory calculations. We find that semilocal exchange
correlation functionals accurately describe the rich feature set observed for
these temperatures, including electron-phonon coupling. Using different
representations of the heat capacity, we are therefore able to identify and
explain deviations from the Debye behaviour in the low-temperature limit and in
the temperature regime 30 - 50 K as well as the reduction of these features due
to the addition of Sc.Comment: 10 pages, 6 figures in total, paper submitted to Physical Review
Differences in structure and dynamics of ternary PdâNi-based bulk metallic glasses containing sulfur or phosphorous
The composition PdNiS has been shown to be the best glass former in the family of recently discovered glass forming PdNiS alloys. In this study, this sample system was systematically investigated using fluctuation- and correlation electron microscopy of which the results are compared to a PdNiP bulk metallic glass that serves as a model system for metallic glasses. Strong differences in the local atomic correlations beyond the short-range order were observed, which are assumed to be a reason for their discrepancy in thermal stability. The relaxation dynamics at room temperature revealed faster dynamics in the sulfur-containing PdNiS glass
Alâdoped ZnOâCoated LiNi1/3Mn1/3Co1/3O2 Powder Electrodes: The Effect of a Coating Layer on The Structural and Chemical Stability of The Electrode / Electrolyte Interface
Abstract LiNi1/3Mn1/3Co1/3O2 (NMCâ111) is one of the most popular cathode materials in Liâion batteries. However, chemical and structural instabilities of the cathode/electrolyte interface at high charge cutâoff voltages cause capacity fading. Surface modifications using metal oxides are promising candidates to suppress capacity fading. Here a systematic study on the degradation mechanism of an uncoated NMCâ111 powder electrode is presented. Moreover, the effect of an Alâdoped ZnO (Al:ZnO) coating layer on the structural and chemical stabilities of NMCâ111 electrode cycled at high charge cutâoff voltages is analyzed using Xâray photoelectron spectroscopy, scanning electron microscopy and analytical transmission electron microscopy as well as electrochemical testing. The coating is applied to commercial NMCâ111 powder using a microwaveâassisted solâgel synthesis method. In the case of uncoated NMCâ111 electrodes, pitting corrosion due to hydrofluoric acid attacking the electrode surface, cation mixing, and an irreversible phase transformation from a trigonal layered to a rockâsalt phase occurs, causing capacity fading. While, in the case of Al:ZnO â coated NMCâ111 electrodes, pitting corrosion, cation mixing, and the irreversible phase transformation are mitigated. Therefore, the capacity retention and rate capability are improved as the coating layer protects the electrode surface from the direct electrolyte exposure
PH-switchable ampholytic supramolecular copolymers
ÎČ-sheet-encoded anionic and cationic dendritic peptide amphiphiles form supramolecular copolymers when self-assembled in a 1:1 feed ratio of the monomers. These ampholytic materials have been designed for on-off polymerization in response to pH triggers. The cooperative supramolecular self-assembly process is switched on at a physiologically relevant pHvalue and can be switched off by increasing or decreasing the pHvalue.</p
Al2O3 protective coating on silicon thin film electrodes and its effect on the aging mechanisms of lithium metal and lithium ion cells
In this work, an investigation of the effect of Al2O3-coating on the aging mechanisms of silicon anode thin films in lithium metal and lithium ion cells is presented. Aging mechanisms, namely: loss of lithium inventory, loss of silicon active material and loss of utilizable capacity due to an increase of cell resistance were determined for both, Li||Si and Si||LiFePO4 cells. Al2O3-coating was shown to be an effective strategy to reduce the loss of lithium inventory, while having a marginal effect on decreasing the loss of silicon active material. Indeed, in case of Si||LiFePO4 cells, where fading is governed by loss of lithium inventory, a 5 nm Al2O3-coating leads to a significant reduction (-64%) of the capacity fade per cycle. On the contrary, in case of Li||Si, where the aging mechanism is governed by the loss of active material, Al2O3-coated and uncoated silicon showed comparable tendencies regarding the capacity fade per cycle. It emerges, also, that loss of silicon active material and loss of lithium inventory are independent of each other. This indicates that the main contribution of loss of lithium inventory is not the lithium trapped in electrically insulated silicon, but rather lithium consumed in the ongoing SEI formation. Al2O3-coating could reduce the latter due the insulating nature of the coating. Ex situ investigations of the SEI by means of X-ray photoelectron spectroscopy confirmed a decrease in solvent decomposition in presence of the Al2O3-coating