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Text from PDF Page: 001Materials Transactions, Vol. 55, No. 8 (2014) pp. 1161 to 1167 Special Issue on Advanced Materials for Hydrogen Energy Applications II ©2014 The Japan Institute of Metals and Materials Hydrogen Absorption and Desorption Behavior of Magnesium Hydride: Incubation Period and Reaction Mechanism Nobuhiko Takeichi1,+, Yasuhiro Sakaida2, Tetsu Kiyobayashi1 and Hiroyuki T. Takeshita3,+ 1Research Institute for Ubiquitous Energy Device, National Institute of Advanced Industrial Science and Technology (AIST), Ikeda 563-8577, Japan 2Graduate School of Science and Engineering, Kansai University, Suita 564-8680, Japan 3Department of Chemistry and Materials Engineering, Faculty of Chemistry, Materials and Bioengineering, Kansai University, Suita 564-8680, Japan The hydrogen absorption and desorption reactions of pure MgH2 were investigated by pressure-time measurements using a Sieverts’ type instrument in the temperature and pressure ranges of 653683 K and 0.51.7 MPa, respectively. The absorption and desorption behaviors were analyzed using a fraction of the reaction product during the hydrogen absorption and desorption. The fraction was evaluated based on the amount of absorbed and desorbed hydrogen. The hydrogen absorption of pure Mg immediately occurs when the thermodynamic condition in which the reaction can proceed is reached at 653683K, but the hydrogen desorption does not start immediately when it can thermodynamically proceed at the same temperatures. Incubation periods were observed and had varied values in the range from 0.15 to 1.5 ks under the above-mentioned pressure and temperature conditions. In order to clarify the hydrogen desorption mechanism, the data obtained were analyzed by the Kolmogorov-Johnson-Mehl-Avrami (KJMA) equation. The obtained values of the Avrami exponents varied from 3 to 0.6 with the increasing fraction of Mg. The hydrogen desorption process was classified into four stages based on the KJMA plots of the hydrogen desorption curves of MgH2 measured in this study. These values indicated that the Mg nuclei generate and three-dimensional grow during the initial stage, then the growth is restricted to a two- or one-dimensional. [doi:10.2320/matertrans.MG201405] (Received February 4, 2014; Accepted April 23, 2014; Published May 30, 2014) Keywords: magnesium, hydrogen desorption property, Kolmogorov-Johnson-Mehl-Avrami equation 1. Introduction Mg is one of the promising hydrogen storage materials from view point of basic science and application. Mg can absorb a large amount of hydrogen up to 7.6mass%, as MgH2.1) However, the kinetics of the hydrogenation/ dehydrogenation reaction is slow at room temperature. Actually, the reaction of Mg with hydrogen requires a high working temperature, e.g., 3700K. Recently, MgH2 mixed with a small amount of catalysts, metals, metal oxides and carbons have been investigated,210) and some of the modified MgH2 desorbed hydrogen at temperatures about 100 K lower than that of pure MgH2. Also, the hydrogenation kinetics and hydrogen storage capacities of the pure MgH2 with a nano- structure prepared by ball milling and its single crystals with fibrous shapes prepared by the vapor deposition and gas- phase condensation method have been investigated.1115) The hydrogen absorption and desorption kinetics of Mg have to be improved for it to be used as a hydrogen storage medium. It is necessary to obtain information about the phase transition, the reaction site and hydrogen diffusion path during the hydrogen absorption and desorption reaction. From previous studies, there is a two-stage model to explain the hydrogen absorption of Mg.1620) During the first stage, MgH2 nuclei generate and grow on the surface. After MgH2 covers the surface, the thickness of the MgH2 layer increases with the hydrogenation reaction, which is the second stage. The first stage of the reaction can be described by a nucleation and growth model and the second stage is +Corresponding author, E-mail: email@example.com, h-take@kansai-u. ac.jp explained by the shrinking-core model. Information about the nucleation and growth, such as the nucleation rate and growth dimension, is obtained from the parameters in the Kolmogorov-Johnson-Mehl-Avrami (KJMA) equation.2125) In general, the KJMA equation can be described by the kinetics of the phase transformation of metallic materials, e.g., crystallization of metallic glass and the precipitation phase transformation. A parameter of the KJMA equation provides information to determine the conditions of the nucleation and growth such as the nucleation rate, nuclei growth dimension, and rate-controlling step. There are some studies in which the KJMA equation has been used to describe the hydrogen absorption process of Mg based on the nucleation and growth model, such as the nucleation rate and growth dimension.14,2628) However, it is difficult to describe the dehydrogenation process of MgH2 in detail. In this study, we investigated the hydrogen desorption process of MgH2 and analyzed it using the KJMA equation to compare it with the hydrogen absorption process on a fundamental basis. Based on the results obtained from this analysis, the difference between the dehydrogenation process and hydro- genation process of MgH2 was discussed. 2. Experimental Procedure 2.1 Materials The MgH2 powder was purchased from the Johnson Matthey Co. The major impurity of this material is pure Mg that had not reacted with hydrogen. The MgH2 was stored in a sample container in a glove box filled with Ar of which the dew-point and oxygen concentration were maintained below 200 K and 1 ppm, respectively.
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