Hydrogenation-disproportionation-desorption-recombination in Sm2Fe16M (M =Al, Ga and Si) and magnetic properties of their carburized powders

The application of the hydrogenation-disproportionation-desorption-recombin ation (HDDR) process in Sm2Fe16M (M = Al, Ga and Si) was investigated. The hydrogen absorption behaviour was studied by temperature-pressure analysis (TPA). In the temperature range between 500 and 800 degrees C, Sm2Fe16M sam ples with M=Ga and Si show a weaker hydrogen absorption than Sm2Fe16Al. Thi s was confirmed by X-ray diffraction which showed a complete disproportiona tion of Sm2Fe16Al into SmHz (1.9<z<2.8) and alpha-(Fe, Al), while Sm2Fe16Ga and Sm2Fe16Si exhibit a fraction of undecomposed material with the Th2Zn17 -type structure after the disproportionation procedure. These results point to a stabilization of Sm2Fe16M against disproportionation by hydrogen for M=Ga and Si. The magnetization processes of carburized HDDR powders were st udied by VSM measurements and Kerr microscopy. The demagnetization curve of our HDDR processed Sm2Fe16AlCy is well shaped, whereas those of Sm2Fe16GaC y and Sm2Fe16SiCy show a large decrease of the polarization at low fields. The favourable behaviour of Sm2Fe16AlCy is due to a homogeneous submicron g rain structure. In Sm2Fe16GaCy and Sm2Fe16SiCy samples, an additional, magn etically soft phase with larger domains war observed which causes the low c oercivity. As a consequence, HDDR was only favourable for the preparation o f Sm2Fe16MCy hard magnets with M = Al but not for M = Ga and Si. Optimizati on of the HDDR process in Sm2Fe16Al and subsequent nitrogenation or carburi zation led to coercivities of mu(o j)H(c) = 2.9 and 2.5 T, respectively. (C ) 1998 Elsevier Science S.A. All rights reserved.