RESUMO
The cellular automata-lattice Boltzmann method is used to simulate the dendritic growth process of aluminum alloys under the action of hypergravity by performing coupling heat and mass transfer, solidification and flow. The dendrite arm spacing, growth rate, and dendrite morphology vary greatly with the size and direction of hypergravity, and solute segregation occurs. Compared with the gravity of the earth (1 g), hypergravity strongly strengthens the buoyancy-driven flow and considerably affects the morphology of the solidified grain. The dendritic growth rate is also accelerating. According to the direction of hypergravity in relation to the dendritic growth direction, there exist different flow states that show stable or unstable dendritic growth dynamics. For columnar crystal growth, when the hypergravity and growth direction are identical, the dendrite tip undergoes downward melt flow, and the dendrite grows in a stable manner. When the hypergravity and the growth direction are opposite, the dendrite tip undergoes upward melt flow, the dendrite grows in an unstable manner, and the primary dendrite spacing decreases. For the growth of equiaxed crystals, the convection induced by hypergravity causes the equiaxed crystals to be asymmetric, and the solute segregates in the direction of gravity. Channel segregation occurs in the mushy zone in the presence of equiaxed crystal chains.
RESUMO
The research on 811 ternary cathode materials is mainly based on synthesis and modification. However, the preparation process of these materials is accompanied by complex chemical reactions, and the reaction process and corresponding kinetic analysis have not been widely explored. Under different oxygen concentrations, this study analyzed the chemical reaction mechanism of the raw material's (namely, Ni0.8Co0. 1Mn0. 1(OH)2 and LiOH·H2O mixture, which is referred to as the raw material hereinafter) calcination process by non-isothermal thermogravimetry, differential scanning calorimetry, and in situ X-ray diffraction. Based on the obtained data, multiple heating rate methods were used to calculate the reaction mechanism functions and kinetic parameters at each stage as well as the corresponding activation energy and pre-exponential factor. Results showed that four chemical reactions occurred successively during the calcination process of the raw materials with each corresponding to a different kinetic function, pre-exponential factor, and activation energy. Comparing the calcination characteristics under different oxygen concentrations showed that the activation energy was the smallest when the oxygen concentration was 60%.
RESUMO
Recent theoretical and experimental studies have shown that the formation of Li2O2, the main discharge product of nonaqueous Li-O2 batteries, is a complex multistep reaction process. The formation, nucleation, and adsorption of LixOy (x and y = 0, 1, and 2) and (Li2O2)n clusters with n = 1-4 on the surface of carbon nanotubes (CNTs) were investigated by periodic density functional theory calculation. The results showed that both Li2O2 and Li2O on CNT electrodes are preferentially generated by lithiation reaction rather than disproportionation reaction. The free energy profiles demonstrate that the discharge potentials of 2.54 and 1.29 V are the threshold values of spontaneous nucleation of (Li2O2)2 and (Li2O)2 on a CNT surface, respectively. The electronic structure indicates that Li2O2 is a p-type semiconductor, while Li2O exhibits the properties of an insulator. Interestingly, once Li2O2 molecules condense into large clusters, they will be repelled away from the CNT surface and continue to grow into large-sized Li2O2. Our results provide insights into the full understanding of the electrochemical reaction mechanism and product formation processes of lithium oxides in the cathodes of Li-O2 batteries.