| Nd-free LPC-type hydrogen storage alloys, in which LPC represents the abbreviation of La-Ce-Pr mischmetal after the extraction of most Ce and Nd, were studied, according to the fact that Nd and Ce are respectively hunted for by magnetic material NdFeB and automotive catalyst, and considering the balanced application of rare earth resources. The possibility of the substitution of Nd-free LPC mischmetal for full-component one to make hydrogen storage alloys was demonstrated. And it was verified that this kind of alloy can be used in the manufacturing of Ni-MH batteries.As we know, the thickness of the hydrogen storage alloy ingots in practical production is popularly 10~30mm. In this study, strip casting was introduced, and the effect of the thickness of ingots within 1-10mm on the electrochemical properties of high-Co and low-Co LPC-type hydrogen storage alloys was studied. It was found that the comprehensive electrochemical properties of high-Co hydrogen storage alloy are improved with the increase of the thickness of the ingots. It is contributed to three factors: the expansion of unit cell volume, the decrease of the defects at grain boundaries, and the diminishing of internal stress.In order to reduce the cost of the hydrogen storage alloys, Cu, Fe and Si were applied to replace Co. It was found that the replacement made the electrochemicalproperties of low-Co LPC-type hydrogen storage alloy much insensitive to the thickness of ingots.It was also known that heat treatment at high-temperature ( 1000 ) is usually applied in the production of hydrogen storage alloys. In this work, the effect of heat treatment procedure at mid-temperature on the electrochemical properties of high-Co LPC-type hydrogen storage alloy was studied. It was found that the alloys with different thickness after heat-treated at mid-temperature for 8 hours could show better comprehensive electrochemical properties. The thinner the thickness of the ingots, the better the effect of the heat treatment. And the improvement of the electrochemical properties of hydrogen storage alloys by the anneal is mainly attributed to the diminishing of internal stress and the decrease of element segregation. The thinner the thickness of the ingots, the larger the effect of internal stress.The effect of the proportion of La/Ce in A-side of LPCNi3.55Co0.75Mn0.4Al0.3 alloy on the electrochemical properties was discussed, according to the fact that Pr is difficult to be adjusted in the production of LPC mischmetal. It was found that as-cast and annealed Nd-free LPC-type hydrogen storage alloys with 20-25% Ce in A-side show the best comprehensive electrochemical properties. Different Ce content in the alloys leads to different unit cell volume and equilibrium discomposed pressure of the hydride; And the valence of Ce could be changed in the hydride. Finally, they affect the electrochemical properties of the alloys.Differential thermo-analysis(DTA) and high-temperature XRD methods were applied together in this study to demonstrate the variation process of the hydrogen storage alloys during heating. It was found that there are five exothermic peaks in the DTA curve of LPCNi3.55Co0.75Mn0.4Al0.3 alloy, named a-b-c-d-e in turn. Peak b and c are sensitive to cooling rate. The smaller the grain sizes, the higher the peaks. Peak b is mainly depended on the diminishing of internal stress at grain boundaries.But peak c is primarily caused by the diminishing of internal stress within the grains and the decrease of elements segregation. Peak d and e depend less on cooling rates. Peak d is mostly introduced by re-crystallization. Peak e is mainly introduced by growth of the grains.According to the analysis results of microstructures during heating, the following variations were expected to happen in the hydrogen storage alloys from 245 to 512 : Diminishing of mechanical stress-Decrease of internal stress at grain boundaries-Decrease of internal stress within grains and decrease of elements segregation-Re-crystallization and growth of grains... |
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