Synthesis of ternary alloys Mg 2 Ni 0.75 Pd 0.25 and studies on its surface properties
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I. INTRODUCTION
Magnesium itself, which forms the binary hydride MgH2 corresponding to 7.6 wt. % H, would be a very attractive hydrogen storage material. Unfortunately, it is quite difficult to react magnesium metal directly with hydrogen. The search for cheaper and lighter hydrogen storage materials has led to the investigation of Mgbased alloys. Reilly and Wiswall have investigated the hydrogen absorption reactions of Mg-based alloys.1 The properties of Mg-based alloys and their hydrides are quite different according to the various preparative methods used. At the present time, Mg-based alloys are usually prepared according to a normal metallurgical procedure which was described previously2; however, a brief outline would be helpful here. The desired amount of Mg and other metals is introduced into a stainless steel container under Ar and melted in an induction furnace at the alloying temperature, 850-900 °C. After the alloy was melted and homogenized, it was cooled to room temperature and transferred to a drybox under Ar, pulverized to 25-100 mesh, and introduced into a stainless steel high-pressure reactor. The sample was hydrided under H 2 at 48.4 atm and 350 °C. It took at least 24 h, even several days, to finish the hydriding process, according to the scale of the operation and the properties of the sample. The hydrided alloy could be decomposed by reducing the H2 overpressure and the original alloy could be regenerated. We have prepared Mg2Cu,3 Mg2Ni,4 and Mg2Ni0.75Cuo.25S by the Replacement Diffusion Method (RDM), and have found that the properties of alloys (by RDM) are different from those prepared by the normal metallurgical method (MM). In the RDM method, the desired alloy is made by replacement of metals in the non-aqueous solvent and by interdiffusion of components in the solid state. The alloys (by RDM) have higher activities for hydrogenation, faster hydriding-dehydriding rates, and can be activated more easily. Under the exJ. Mater. Res., Vol. 5, No. 7 Jul 1990
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perimental conditions used, the reaction between Mg2Nio.75Cuo.25 (by RDM) and H 2 is reversible, the dissociation temperature of the resulting hydride is significantly lowered, and its dissociation pressure is increased. All these factors make the ternary alloy Mg2Nio.75Cuo.25 (made by RDM) more favorable as a hydrogen storage material, compared with the alloy made by the metallurgical method. Mg2Nio.75Cuo.25 (by RDM) has a large specific surface, and according to AES analysis, the product shows that the original surface is mainly composed of Mg, Ni, and Cu rather than Mg and O, as is the case for the metallurgically prepared alloy. Thus it may be deduced that the difference of absorption-desorption equilibrium between the two types of alloys is due to the different surface properties. Accordingly, we extended RDM to another ternary system and successfully synthesized the alloy Mg2Ni0.75Pdo.25. We characterized the structure of the alloy and concomitantly studied its surface properti
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