Microstructure of the MgH 2 Synthesized by Hydriding Chemical Vapor Deposition
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0971-Z08-10
Microstructure of the MgH2 Synthesized by Hydriding Chemical Vapor Deposition Itoko Saita1, and Tomohiro Akiyama2 1 Energy Media, CAREM, Hokkaido University, Kita 13 Nishi 8, Kita-ku, Sapporo, 060-8628, Japan 2 Energy Media, CAREM, Hokkaido University, Sapporo, 060-8628, Japan
ABSTRACT It is hardly achieved to prepare highly pure MgH2 by the conventional method of solid-gas reaction between solid magnesium and hydrogen; therefore, we proposed and succeeded to synthesize MgH2 by Hydriding Chemical Vapor Deposition (HCVD). Very interestingly, the HCVDed MgH2 was made of single crystals with fibrous figures; however, further detail of the HCVDed product had not been studied. Therefore the aim of this study was to examine the HCVDed MgH2 in hydrogen storage and to observe the microstructure of the HCVDed MgH2 after the hydrogen desorption and absorption. As the results of Pressure-Composition-Isotherm (PCT) measurement, the HCVDed MgH2 reversibly absorbed and desorbed 7.6 mass% hydrogen, as much as the theoretical maximum hydrogen capacity of magnesium, without any activation treatment. The equilibrium pressure was slightly lower than the reported value. Before and after the PCT measurement, the HCVDed MgH2 did not showed noticeable difference in figure; however, MgHx, which was prepared dehydriding the HCVDed MgH2 in the PCT, showed significant difference in figure: It had zebra stripes in the SEM observation. This observation showed that the hydrogen storage and release went in the radious direction and the hydrogen diffusion was no more rate-limiting. The phase boundaries of Mg and MgH2 involving strain should affected on the plateau pressure in PCT.
INTRODUCTION Magnesium hydride, MgH2, is attractive from engineering aspects because of its abundant raw materials and great hydrogen density of 7.6 mass%; however, it usually requires at least ten times or more activation treatments, which results in quite time-consuming and energyconsuming1, including both hydriding and dehydriding treatments under control of temperature and hydrogen pressure. Even though the activation treatment is conducted, magnesium is not hydrided completely. As far as in marketed products, the product purity of MgH2 is up to 98 % at the most. The activation difficulty of MgH2 remains itself unpractical for use as a hydrogen storage material or as a starting material of the other hydrogen storage materials such as magnesium amide2,3. The problem of activation difficulty is because of small rate of hydrogen diffusion and hardlyrecoverable poisoned part. The gas-solid reaction between magnesium and hydrogen includes a reaction step of hydrogen diffusion in solid magnesium, that is so slow to be the rate-limiting step of the reaction; therefore, many effors have been done by repeating activation treatments,
adding second element as a catalyst to increase the hydrogen diffusivity, or decreasing the size to shorten the diffusion path. Recently, we proposed a new method dubbed Hydriding Chemical Vapor Deposition (HCVD) for MgH2 synthesi
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