Search for New Hydrogen Absorbing Alloys

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In order to suppress the global greenhouse effect caused by CO_2 gas, it is essential to develop new energy sources such as the solar heat and the heat of the earth in place of petroleum and coal. Hydrogen absorbing alloys can storage such energies safely and conveniently. The purpose of the present project is to search and develop new excellent hydrogen absorbing alloys. The first aim is to make clear the guide principle for searching new alloys convenicntly. The present work has demonstrated that hydrogen absorbing properties can bee evaluted by differential thermal analysis(DTA) of intermetallic compounds in a hydrogen atmosphere. From the pressure dependence of the peak temperature of the exothemic and endothermic peaks, we can determine the thermodynamic data such as the enthalpy and entropy change of hydrogen absorption and desorption. According to this guide principle, the structural changes caused by hydrogenation, the amounts of absorbed hydrogen and the thermodynamic parameters for hydrogen absorption and desorption have been evaluated for Ti, Zr and rare earth metal based intermetallic compounds. Especially, hydrogen absorbing properties of the C15 Laves compounds have been investigated in details. The main results are summarized as follows: (1) hydrogen is not dissolved in the intermetallic compounds in which the tetrahedral sites for hydrogen occupation ARE LESS THAN 0.033 nm, (2) the compounds having the atomic size ratio less than 1.37 absorbs about 1.0 (H/M) of hydrogen in the crystalline state, while those having the ratio more than it amorphize by hydrogenation, which do not exhibit the pressure plateau.