The effects of different levels of strontium on nucleation and growth of the eutectic in a commercial hypoeutectic Al-Si foundry alloy have been investigated by optical microscopy and electron backscattering diffraction (EBSD) mapping by scanning electron microscopy (SEM). The microstructural evolution of each specimen during solidification was studied by a quenching technique at different temperatures and Sr contents. By comparing the orientation of the aluminum in the eutectic to that of the surrounding primary aluminum dendrites by EBSD, the eutectic formation mechanism could be determined. The results of these studies show that the eutectic nucleation mode, and subsequent growth mode, is strongly dependent on Sr level. Three distinctly different eutectic growth modes were found, in isolation or sometimes together, but different for each Sr content. At very low Sr contents, the eutectic nucleated and grew from the primary phase. Increasing the Sr level to between 70 and 110 ppm resulted in nucleation of independent eutectic grains with no relation to the primary dendrites. At a Sr level of 500 ppm, the eutectic again nucleated on and grew from the primary phase while a well-modified eutectic structure was still present. A slight dependency of eutectic growth radially from the mold wall opposite the thermal gradient was observed in all specimens in the early stages of eutectic solidification.
Li-rich layered cathode materials have been considered as a family of promising high-energy density cathode materials for next generation lithium-ion batteries (LIBs). However, although activation of the Li 2 MnO 3 phase is known to play an essential role in providing superior capacity, the mechanism of activation of the Li 2 MnO 3 phase in Li-rich cathode materials is still not fully understood. In this work, an interesting Li-rich cathode material Li 1.87 Mn 0.94 Ni 0.19 O 3 is reported where the Li 2 MnO 3 phase activation process can be effectively controlled due to the relatively low level of Ni doping. Such a unique feature offers the possibility of investigating the detailed activation mechanism by examining the intermediate states and phases of the Li 2 MnO 3 during the controlled activation process. Combining powerful synchrotron in situ X-ray diffraction analysis and observations using advanced scanning transmission electron microscopy equipped with a high angle annular dark fi eld detector, it has been revealed that the subreaction of O 2 generation may feature a much faster kinetics than the transition metal diffusion during the Li 2 MnO 3 activation process, indicating that the latter plays a crucial role in determining the Li 2 MnO 3 activation rate and leading to the unusual stepwise capacity increase over charging cycles.
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