Study on the grain refinement mechanism of Mg-Al alloy based on carbon addition.

In this study, a comprehensive treatment process based on the rotary injection of Ar+CO2 Mg-Al alloy melt is proposed. The effect of carbon on the grain refinement of Mg-Al alloy is studied according to the proposed integrated treatment process. The regularity of carbon refinement in the Mg-Al alloy is examined by microstructural observation and theoretical calculation. The results show that carbon has no effect on the grain refinement of Mg-Al alloy when the Al content is less than 1wt.%. However, when the Al content reaches 2 wt.%, the refining effect is obvious, and the grain refinement efficiency is 62%. The refining effect increases with the increase in the Al content, and the refinement efficiency becomes 79% when the Al content reaches 9 wt.%. The size of Al-C-O in the matrix is approximately 5µm, which confirms the existence of Al4C3 phase exists as a heterogeneous nucleating agent. The theoretical calculations suggest that the Al4C3 heterogeneous nucleating agent cannot be formed when the Al content in the Mg alloy is less than 1.34%, so there is no thinning effect under such Al content. The crystallographic calculations reveal that the mismatch between the Al4C3 phase and Mg alloy matrix is only 4.05%, and Al4C3 can exist as a heterogeneous nucleating agent for α-Mg phase. Combining the measured solidification curves with the classical nucleation theory, the wetting angle of Mg-Al alloy on Al4C3 is calculated to be 24.3°.

Engineering Hierarchical CoO Nanospheres Wrapped by Graphene via Controllable Sulfur Doping for Superior Li Ion Storage.

The inferior conductivity and large volume expansion impair the widespread applications of metal oxide-based anode materials for lithium-ion batteries. To address these issues, herein an efficient strategy of structural engineering is proposed to improve lithium storage performance of hierarchical CoO nanospheres wrapped by graphene via controllable S-doping (CoOS0.1   @ G). S-doping promotes the Li+ diffusion kinetics of CoO by expanding the interplanar spacing of CoO, lowering the activation energy, and improving the pseudocapacitance contribution. Meanwhile, the electronic structure of CoO is adjusted by S-doping as confirmed by density functional theory calculations, thus enhancing the conductivity. Finite element analysis reveals that the produced Li2 S during lithiation improves the structural stability of the S-doped electrode, which is further confirmed by experimental observation. As expected, CoOS0.1   @ G exhibits excellent lithium storage performance with an initial discharge capacity of 1974 mAh g-1 at 100 mA g-1 , and high discharge capacity of 1573 mAh g-1 after 400 cycles at 500 mA g-1 . It is believed that the insights into the structural doping enlighten research to explore other metal oxides for fast and stable Li ion storage.