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In this work porous MgO fibers were investigated as an immobilizing agent in the solid molten salt electrolytes for thermal batteries. Their distribution, uniformity and electrochemical performance were characterized. Compared with ball-milling method, the use of liquid nitrogen as mixing medium resulted in an homogeneous distribution of molten salt particles on the surface of porous MgO fibers. The ionic conductivity of the separator using porous MgO fibers as an immobilizing agent is slightly higher than that of the separator when MgO powders were used as the immobilizing agent. Moreover, the discharge specific capacity of the battery using the porous MgO fibers, 564.39 mAh · g −1 , is significantly greater than that of the battery using MgO powders, 355.67 mAh · g −1 . More importantly, the electrolyte leakage content of the separator based on porous MgO fibers is also the lowest. Porous MgO fibers offer promising application in thermal batteries. Compared with conventional Li-ion batteries using electrolytes made from organic liquids or gels, 1-3 thermal batteries using solid molten salt electrolyte demonstrate some promising and potential applications in fields as diverse as nuclear weapons, 4,5 rocketry and fossil fuel production 6,7 due to high specific energy, wide temperature range and long storage capabilities. 8,9 However, in order to prevent leakage of the molten salt electrolyte during the operating temperature (350∼550• C) some immobilizing agents such as ceramic fibers, e.g., BN, Y 2 O 3 , SiO 2 and kaolin are usually added to the electrolyte. [10][11][12][13][14][15][16] However, these immobilizing agents are hampered by limitations including low specific surface area, high cost and reaction with the electrodes.10,13 Magnesia (MgO) is an efficient material due to its low cost and chemical stability. Moreover, its solubility in molten salts is low: pK(MgO) = 7.2 in the LiCl-KCl eutectic at 450• C. 10 Thus, MgO powders, which are usually obtained by a thermal decomposition of magnesium hydroxide or magnesium carbonate hydroxide, 23 are widely employed as immobilizing agents in molten salt for thermal batteries, at 35-40 wt% loading.17-27 Liu, 6 Masset, 20 Guidotti and Reinhardt et al. 28 used 35 wt% of MgO powders as the immobilizing agent. Czajka et al. 23 found that separator containing 40-50 wt% of modified MgO (MgF 2 -MgO) immobilizing agent demonstrated good effect but the low ionic conductivity is in the range of 0.02 to 0.04 S · cm −1 and the high mass ratio of the immobilizing agent leads to an increasing resistance and mass of the battery.Consequently an immobilizing agent that can enhance the immobilizing effect and does not significantly reduce ionic conductivity is required. Compared with MgO powders, MgO fibers or porous MgO fibers result in an extensive network structure. The latter have additional complex pore structure. In principle porous MgO fibers can result in an enhanced immobilizing effect, decreased electrolyte leakage and usage compared with conventional MgO powders. At pres...

Though the mechanisms of dendrite fragmentation in agitated solidification are of great importance and have been investigated for many years, no accurate calculation of the stress fields of dendrites stirred in the metallic flow has been reported. In this study, the stress field and strain field of an aluminum dendrite model in moderate agitated fluid were calculated using the finite-element method. It was found that the stress concentrations at the roots of dendrite branches are so great that plastic deformation occurs when the fluid force is far from sufficient to bend the dendrite branches plastically. According to this calculation, a mechanism, detaching after fatigue erosion, was suggested. The new mechanism suggests that stress concentrations at the roots of dendrite branches create slip band intrusions and extrusions. These slip band intrusions and extrusions are thermally unstable and are soon remelted. If the erosion rate of the roots is sufficiently large, the roots will be thinned and dendrite branches will detach. Experiments on the effects of stirring intensity, cooling rate, and stirring time on the grain sizes of the A356 rheocasting structure, and reported high-temperature fatigue tests, corroborate the effectiveness of the new mechanism.