Oberflächentechnik/Werkstoffkunde
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Li plating on the anode is the predominant ageing effect that can occur when a lithium-ion battery is charged with high current rates or subjected to cyclic long-term tests, especially at low temperatures. Several studies report reduced Li plating of structured anodes during cyclic ageing. In this study, a closer look into Li plating generation is taken by single charging of fresh cells. Several graphite anodes with an area capacity of 3.0 mAh/cm2 are investigated, both, standard calendered anodes as well as anodes with superficial laser modification (2D) and hole patterning (3D). Thereby, typical Li plating features in the charging and relaxation voltage curves are correlated with subsequently conducted microstructural analysis under Ar atmosphere by means of optical microscopy. First, a series of anodes with different densities (1.6–1.2 g/cm3) were investigated. Based on that, anodes with a density of 1.3 g/cm3 were selected for further studies including laser treatment. When the
reference anode is charged at 1C to 80 % SoC, Li plating occurs, which is significantly reduced by the superficial treatment and even avoided by hole patterning. 2C charging leads to Li plating for all anode types with lower extend for the structured ones. However, the roughened surface due to the quasi 2D approach seems to hinder reintercalation and dissolution of the plated Li. Overall, the hole patterned anode shows the lowest intensity of Li plating, which, however occurs preferentially at the hole edges.
For permanent magnetic materials, anisotropic microstructures are crucial for maximizing remanence Jr and maximum energy product (BH)max. This also applies to additive manufacturing processes such as laser powder bed fusion (PBF-LB). In PBF-LB processing, the solidification behavior is determined by the crystal structure of the material, the substrate, and the melt-pool morphology, resulting from the laser power PL and scanning speed vs. To study the impact of these parameters on the textured growth of grains in the melt-pool, experiments were conducted using single laser tracks on (CoCuFeZr)17Sm2 sintered magnets. A method was developed to quantify this grain shape anisotropy from electron backscatter diffraction (EBSD) analysis. For all grains in the melt-pool, the grain shape aspect ratio (GSAR) is calculated to distinguish columnar (GSAR < 0.5) and equiaxed (GSAR > 0.5) grains. For columnar grains, the grain shape orientation (GSO) is determined. The GSO represents the preferred growth direction of each grain. This method can also be used to reconstruct the temperature gradients present during solidification in the melt-pool. A dependence of the melt-pool aspect ratio (depth/width) on energy input was observed, where increasing energy input (increasing PL, decreasing vs) led to higher aspect ratios. For aspect ratios around 0.3, an optimum for directional columnar growth (93% area fraction) with predominantly vertical growth direction (mean angular deviation of 23.1◦ from vertical) was observed. The resulting crystallographic orientation is beyond the scope of this publication and will be investigated in future work.
Noninvasive characterization methods for ultra-short laser pulse induced volume modifications
(2024)
We present two noninvasive characterization methods to investigate laser induced modifications in bulk fused silica glasses. The methods discussed are immersion microscopy and scanning acoustic microscopy (SAM). SAM shows merits in measuring the distance from sample surface to the first detectable density change of the modification, while immersion microscopy offers a look into the modification. Both noninvasive methods are preferred over conventional polishing or etching techniques due to the facts, that multiple investigations can be done with only one sample and lower time expenditure. The type II modifications were introduced by focusing laser pulses with high repetition rates into the fused silica.