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The formation and dissociation of magnesium alloy hydrides and their use for fuel storage in the hydrogen car

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Abstract

The formation of hydrides by the reaction of high-pressure hydrogen (300 to 800 psi) with Mg-10Al and Mg-25Ni was studied at 400 and 450°C. Although the reaction kinetics for both alloys can be described by the Johnson-Mehl relationship, the morphology and nature of the reaction products were very different for the two alloys. The temperature dependence of the rates suggests that an interfacial reaction, possibly the transfer of hydrogen from the metal to the hydride, may be the rate-controlling step. One hydride, MgH2, formed internally in the Mg-10Al alloy as spherical particles of nearly constant size at any time, indicating that nucleation was rapid. The reaction involving Mg-25Ni resulted in two hydrides, MgH2 and Mg2NiH4, the former being more stable and forming first by consuming the primary magnesium phase of the eutectic structure. The hydride advanced into the alloy as a “front”, after which the eutectic plates of Mg2Ni reacted to form the ternary hydride. Finally, the larger primary plates of Mg2Ni reacted. Dehydriding of the Mg-10Al alloy hydride also followed the Johnson-Mehl relationship and was found to be complete in 90 min at 300°C, these conditions being favorable for the use of this hydride in vehicles combusting hydrogen. On the other hand, dehydriding of the hydrided Mg-25Ni alloy occurred by a twostep process in which some of the Mg2NiH4 dissociated followed by complete dissociation of this hydride and of the MgH2. Only a small fraction, 10 pct of the available hydrogen could be recovered in several hours at 300°C. The Mg-10Al alloy exhibited a much higher resistance to fragmentation during hydriding than did the Mg-25Ni alloy. An analysis of numerous factors required of hydrides for use in vehicular applications showed that Mg-10Al was much better than Mg-25Ni, although the charging time is much too long.

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Douglass, D.L. The formation and dissociation of magnesium alloy hydrides and their use for fuel storage in the hydrogen car. Metall Trans A 6, 2179 (1975). https://doi.org/10.1007/BF02818641

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  • DOI: https://doi.org/10.1007/BF02818641

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