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

Abstract

The hydrogenation-disproportionation-desorption-recombination (HDDR) process was applied to SmCo5 using extreme conditions, namely high hydrogen pressures and reactive milling under hydrogen. Investigations on the hydrogen absorption behavior of SmCo5 by differential scanning calorimetry under hydrogen pressures between 1 and 7 MPa showed absorption events due to an interstitial absorption at about 100 °C and a disproportionation reaction at about 600 °C. X-ray diffraction showed the disproportionation of SmCo5 into Sm hydride and fcc-Co. A favorable effect of high hydrogen pressures 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 successful for both methods. However, Sm2O3, Sm2Co17, and Sm2Co7 were detected as minor phases. Maximum coercivities μ0JHC of 2.1 and 4.7 T were achieved for high pressure and reactively milled HDDR powders, respectively. The high coercivities originate from the high anisotropy field of the SmCo5 phase in combination with the grain refinement due to the HDDR treatment.