Study on Hydrogen Absorption and Desorption Mechanisms in Nanostructured Alloys

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We have previously found that, in Mg-based Mg-Ni and Mg-Ni-La alloys, the hydrogen absorption properties (PCT characteristics and kinetics) are significantly improved by nanostructure formation in the alloys. The objective of this research is to make clear the static properties of hydrogen present in the nano-structured alloys and to reveal the kinetics of hydrogen desorption from the alloys. To accomplish these purposes, we have carried out thermal desorption spectroscopy (TDS) measurements and high resolution transmission electron microscope (HRTEM) observation of alloy specimens prepared by melt-spinning and subsequent crystallization. They were hydrogenated ex-situ by reacting with H_2 gas of 1MPa at 300℃.In TDS measurements of hydrogenated alloys, four kinds of peaks (a〜d) are usually observed at temperatures between 200 and 350℃, and they are attributed to release of hydrogen from Mg_2NiH_4,MgH_2,nano-grain boundaries, and LaH_3 hydrides, respectively. By analyzing the TDS spectr … More a, the relative stability of hydrides characterized by the difference of hydrogen potential has been determined.In the HRTEM observation of the hydrogenated alloys, spherical nano-sized (5-10nm) grains of Mg_2NiH_4 and LaH_3 hydrides are usually observed, imbedded homogeneously in MgH_2 nano-grain matrices. After dehydrogenation at 300℃, the LaH_3 nano-grains are still remaining together with nano-sized Mg_2Ni grains imbedded in the Mg nano-grain matrices. This result indicates that the nanostructures formed in these Mg-based alloys are rather stable to both thermal cycling and hydrogenation-dehydrogenation treatments. The enhanced desorption rate of hydrogen in nano-structured alloys is possibly attributed to quick diffusion of hydrogen atoms along nano-grain boundaries toward the alloy surfaces.In addition to these HRTEM observations, we have first succeeded in visualizing the hydrogen distribution in the specimen by EELS mapping of an H-plasmon peak. The thermal desorption process of hydrogen has successfully been observed in-situ in the electron microscope. This technique is expected to develop as a powerful tool for detecting the dynamic behavior of hydrogen in metallic materials. Less