Thermoanalytical investigation of the hydrogen absorption behaviour of Sm2Fe17-xGax at high hydrogen pressures

The complete disproportionation of Sm2Fe17-xGax during annealing in hydrogen is hindered due to an increased stability of the compounds with a higher Ga content (x ≥ 1). Therefore the HD process as the first step of HDDR (hydrogenation-disproportionation-desorption-recombination) has to be carried out at a high hydrogen pressure for x ≥ 1. The hydrogen absorption behaviour of Sm2Fe17-xGax (x = 0, 0.5, 1 and 2) was investigated by means of hydrogen differential thermal analysis (HDTA) and high pressure differential scanning calorimetry (HPDSC) at hydrogen pressures up to 70 bar. A dependency of hydrogenation and disproportionation temperatures on hydrogen pressure and Ga content was found. The comparison with other substituents (M = Al and Si) instead of M = Ga showed an increased stability of Sm2Fe17-xMx compounds against disproportionation by hydrogen in the sequence Al, Ga and Si. The Curie temperatures of the interstitially hydrogenated Th2Zn17-type materials increase with the hydrogen pressure. In order to produce coercive and thermally stable Sm2Fe15Ga2Cy powder by means of the HDDR process, we recombined material disproportionated at different hydrogen pressures. Preliminary results of magnetic properties of this HDDR treated and gas-carburized Sm2Fe15Ga2Cy are discussed. (orig.)