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Ultrasonic melt treatment (UST) was applied to Al-7Si-2Cu-1Mg melt at various temperatures of 620, 650, 700 and 785 °C. MgAl2O4 particles which were often found to be densely populated along oxide films, became effectively dispersed and well-wetted by UST. Transmission electron microscopy work combined with crystallography analysis clearly indicates that MgAl2O4 particles can act as α-Al nucleation site with the aid of UST. However, with UST, grain refinement occurred only at temperature of 620 °C and the grain size increased from 97 to 351 μm with increase of melt temperature to 785 °C for UST. In quantitative analysis of grain size and MgAl2O4 particle diameter, it was found that ultrasonic de-agglomeration decreased mean particle size of the MgAl2O4 particles, significantly reducing size from 1.2 to 0.4 μm when temperature increased from 620 to 785 °C. Such a size reduction with increased number of MgAl2O4 particles does not always guarantee grain refinement. Thus, in this work, detailed condition for achieving grain refinement by UST is discussed based on quantitative measurement. Furthermore, we tried to suggest the most valid grain refinement mechanism among the known mechanisms by investigation of the relationship between grain size and particle size with variation of melt temperature. [ABSTRACT FROM AUTHOR]

Coral-like yolk-shell-structured nickel oxide/carbon composite microspheres were synthesized.Phase separation and polystyrene nanobead decomposition affected the structure formation.Coral-like yolk with interconnected mesopores provided excellent Li-ion storage properties. In this study, coral-like yolk-shell-structured NiO/C composite microspheres (denoted as CYS-NiO/C) were prepared using spray pyrolysis. The unique yolk-shell structure was characterized, and the formation mechanism of the structure was proposed. Both the phase separation of the polyvinylpyrrolidone and polystyrene (PS) colloidal solution and the decomposition of the size-controlled PS nanobeads in the droplet played crucial roles in the formation of the unique coral-like yolk-shell structure. The CYS-NiO/C microspheres delivered a reversible discharge capacity of 991 mAh g−1 after 500 cycles at the current density of 1.0 A g−1. The discharge capacity of the CYS-NiO/C microspheres after the 1000th cycle at the current density of 2.0 A g−1 was 635 mAh g−1, and the capacity retention measured from the second cycle was 91%. The final discharge capacities of the CYS-NiO/C microspheres at the current densities of 0.5, 1.5, 3.0, 5.0, 7.0, and 10.0 A g−1 were 753, 648, 560, 490, 440, and 389 mAh g−1, respectively. The synergetic effect of the coral-like yolk-shell structure with well-defined interconnected mesopores and highly conductive carbon resulted in the excellent Li+-ion storage properties of the CYS-NiO/C microspheres. [ABSTRACT FROM AUTHOR]