Highly coercive SmCo5 magnets prepared by a modified hydrogenation-disproportionation-desorption-recombination process

The hydrogenation-disproportionation-desorption-recombination (HDDR) proces s was applied to SmCo5 using extreme conditions, namely high hydrogen press ures and reactive milling under hydrogen. Investigations on the hydrogen ab sorption behavior of SmCo5 by differential scanning calorimetry under hydro gen pressures between 1 and 7 MPa showed absorption events due to an inters titial absorption at about 100 degrees C and a disproportionation reaction at about 600 degrees C. X-ray diffraction showed the disproportionation of SmCo5 into Sm hydride and fcc-Co. A favorable effect of high hydrogen press ures on the disproportionation reaction was observed which can be explained by a decrease of the free enthalpy of the samarium hydride for increasing hydrogen pressures. Reactively milled SmCo5 showed also the products of the disproportionation reaction. The recombination to the original SmCo5 phase on hydrogen desorption in a subsequent heat treatment in vacuum was succes sful for both methods. However, Sm2O3, Sm2Co17, and Sm2Co7 were detected as minor phases. Maximum coercivities mu(0J)H(C) of 2.1 and 4.7 T were achiev ed for high pressure and reactively milled HDDR powders, respectively. The high coercivities originate from the high anisotropy field of the SmCo5 pha se in combination with the grain refinement due to the HDDR treatment. (C) 1999 American Institute of Physics. [S0021-8979(99)44308-7].