| Research Abstract |
In this study, for improvement in performance of batteries used in electric vehicles, we paid attention to Mg_2Ni protium absorbing alloy that has high protium absorption capacity, lightweight and low price as compared with the existing protium absorbing alloys. But Mg_2Ni alloy has high protium absorption and desorption temperature and slow hydriding kinetics that must be improved for practical purposes.
Therefore, for the purpose of decreasing protium absorption and desorption temperature and improving protium absorption rate, we did a partial substitution of Mg_2Ni by third alloying elements. Mn, Fe, Co, Cu, Al, Zn and Ag which are expected to decrease the absolute value of hydride formation enthalpy of Mg_2Ni alloy were selected as third substitutional elements. The ternary alloys were also rapidly solidified, and mechanically ground in order to homogenize their microstructure. The microstructures of specimens obtained by these processes were observed and their protium absorbing and
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desorbing characteristics were investigated. Obtained results are summarized as followed ; (1) All activated specimens absorb protium at room temperature and their protium contents are higher than those of specimens before activation that causes new surface formation and an increase in reaction area. (2) Hydriding kinetics curves measurements show that rapidly solidified specimens of Mg_2Ni_<0.9>Co_<0.1> and Mg2Ni_<0.9>Fe_<0.1> alloys have the highest reaction rate. It is considered that Fe and Co substitution accelerated the diffusion of protium in these two alloys and rapid solidification cause the fine microstructure. (3) As the elapsed time after taking specimens out from hydrogen atmosphere increases, the protium desorption temperature decreases and desorbed protium contents increases. Microstmctural changes of the hydride from high temperature phase to low temperature phase probably causes this phenomenon. Similar results can be obtained when manually hydride of manually ground specimens after activation shows the same microstructural change, resulting in improvement of the desorption characteristics. (4) By addition of third alloying elements and Ni substitution, both activation energy and enthalpy for dehydriding decreases.
From the above results, rapid solidification, mechanical grinding and substitution by 3d transition metals for Ni site in Mg_2Ni crystal structure are effective for improving the protium absorbing and desorbing characteristics of Mg_2Ni alloy. Less
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