Hydrogen decrepitation of Sm₂TM₁₇ sintered magnets from scrap rotor assemblies

Sm₂TM₁₇ sintered magnets (TM = Co, Fe, Cu, Zr) have been utilised in high temperature electrical machine rotor applications as they have exceptionally stable magnetic properties at 200–350 °C. Sm and Co are critical elements and reliance on the virgin material supply chain must be reduced. Recycling Sm₂TM₁₇ magnets from end-of-life applications (EoL) via a hydrogen decrepitation (HD) processing route could be a potential solution and was the focus of this study.

Sm₂TM₁₇ magnets were exposed to 2–10 bar pressure at 100 °C for 72 h either loose or constrained within a rotor assembly. The hydrogen absorption/desorption behaviour, unit cell expansion and magnetic properties of the resulting powders were investigated.

Loose and constrained magnets absorbed similar amounts of hydrogen (0.195–0.233 Wt%), with unit cell volume increasing by 1.35–1.87 %, and all samples reached maximum hydrogen desorption at 180–210 °C. Magnets constrained within the rotor assembly were embrittled after HD but required further mechanical agitation to form a powder. Therefore, hydrogen uptake was not impeded by physical constraint, but crack propagation was hampered. Magnetised rotors did not demagnetise during HD but did exhibit reduced magnetic properties. Lattice strain caused by hydrogen uptake may have interfered with magnetic exchange coupling without destroying the domain wall pinning coercivity mechanism.

This study illustrated that HD processing is a potentially viable recycling route for Sm₂TM₁₇ sintered magnets, however a combination of mechanical agitation and a demagnetisation step will be required for the most efficient recovery of Sm₂TM₁₇ sintered magnets from EoL applications.