Reactions in AB5-NH3 systems

Abstract

Treatment of intermetallic compounds LaNi 5 and SmCo 5 with ammonia (initial ammonia pressure 0.630.7 MPa) in the presence of 10 wt % NH 4 Cl as an activator results in hydrogenation of these compounds or, in the case of SmCo 5 , in disproportionation to form Sm 2 Co 7 H x . In some cases, the intermetallic matrix decomposes under this treatment.Intermetallic compounds AB 5 (A = La, Sm; B = Ni, Co) are used as metal hydride accumulators of hydrogen and for production of permanent magnets [1].Intermetallic compounds LaNi 5 and SmCo 5 (structural type CaCu 5 ) have a hexagonal lattice with the parameters a 5.017 and c 3.982, and a 5.003 and c 3.978 A, respectively. These compounds readily absorb hydrogen to form LaNi 5 H 6.8 and SmCo 5 H 4 [2, 3]. After hydrogenation completion, LaNi 5 H 6.8 liberates hydrogen to form a-solid solution [2], and SmCo 5 H 4 loses hydrogen to form SmCo 5 H~2. The latter compound is converted into SmCo 5 H 1 at 703 80oC and is completely dehydrogenated at 1903 200oC. The specific surface area of the hydrogenation products is 0.230.3 m 2 g 31 [4]. The particle size of powders prepared by the hydride dispersion procedure is several microns.Our previous study of reactions in the systems Ti3 NH 3 and ScFe(Ni) 2 3NH 3 [5, 6] suggest that introduction of hydrogen and nitrogen into the matrix of the intermetallic compounds AB 5 would allow preparation of nitrogen-containing hydrides and, by disproportionation of the intermetallic compounds under the action of ammonia, of finely divided products: transition metal (Co or Ni) and hydrides of nitrides of metal A, in the form of homogeneous mixtures.In this work we studied experimentally intercalation of nitrogen and hydrogen into the lattice of intermetallic compounds AB 5 during treatment of these compounds with NH 3 at different temperatures in the presence of NH 4 Cl promoter. Reaction of LaNi 5 and SmCo 5 with ammonia (so-called hydronitriding process) was studied at the initial ammonia pressure of 0.5630.70 MPa and 1503500oC and in the presence of ammonium chloride (10 wt % with respect to the amount of the intermetallic compound).As determined by X-ray diffraction, LaNi 5 is a single-phase compound with the lattice constants, a 5.013 and c 3.986 A (CaCu 5 structure); SmCo 5 (CaCu 5 structure, a 5.001 and c 3.971 A) contains a minor amount of Sm 2 Co 7 located in the intergranular space and promoting cracking and crushing of the SmCo 5 sample in the course of its hydrogenation.The conditions of reactions of NaNi 5 and SmCo 5 with ammonia and composition, lattice constants and specific surface area of the reaction products are summarized in the table. As seen from the table, the reaction of LaNi 5 and SmCo 5 with ammonia at 150oC (sample no. 1 and 8) for 25 h yields a solid solution of hydrogen in the AB 5 matrix. As a result, the lattice constants of the intermetallic compounds slightly increase: a 5.015 and c 3.990 A for La 5 Ni 5 H 0.5 and a 5.001 and c 4.034 A for SmCo 5 H 0.3 . The specific surface area of La 5 Ni 5 H 0.5 (1.3 m 2 g 31 ) ...

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