Presentation Information
[P2-42]Effects of Thermal Damage and Thermal Demagnetisation on the Hydrogen Decrepitation Behaviour of Sm2TM17 Sintered Magnets
*James Thomas Griffiths1, Oliver Peter Brooks1, Gokul Subramanian1, Viktoria Kozak1, Alexander Campbell2, Alexis Lambourne2, Richard Stuart Sheridan1 (1. University of Birmingham (UK), 2. Rolls-Royce Groups plc (UK))
Keywords:
Hydrogen Decrepitation,Samarium Cobalt,Recycling,Thermal Exposure
Sm2TM17 sintered magnets (TM = Co, Fe, Cu and Zr) are typically used in high-temperature magnetic applications, such as permanent magnet motors and actuators in the aerospace sector. This is due to their stable magnetic properties across a wide temperature range e.g. 200 – 550 °C, and their high coercivities > 2000 kA/m. Sm2TM17 sintered magnets rely on a nano-scale cellular structure for their ferromagnetic properties. This structure consists of a 2:17 R (rhombohedral) phase enriched in Fe, a 1:5 H (hexagonal) cell boundary phase enriched in Cu and a Zr-rich lamellar phase. This generates a domain wall pinning coercivity mechanism occurring at the cell boundaries.
Sm and Co are listed as 'critical materials' within the UK and EU due to their high economic importance but high potential of supply risk. Thus, there is an economic and strategic imperative to recycle these materials and reduce reliance on virgin material supply chains. Wet chemical 'long-loop' based recycling approaches exist but are time intensive and high cost. A potential alternative is a Hydrogen Decrepitation (HD) based recycling process, which involves exposing Sm2TM17 magnets to hydrogen at various pressures, temperatures and times to generate a powder. The hydrogen forms an interstitial hydride within the 2:17 R phase causing it to expand. This initiates intergranular and transgranular cracking of the material forming a powder, which could then be used as a feedstock for 'short loop' recycling.
The effects of composition, pressure & temperature on HD processing have been explored, but limited to samples in the as-manufactured, unmagnetised state. This work shows that HD of Sm2TM17 magnets does not result in demagnetisation of the material as it does with NdFeB, due in part to its domain wall pinning coercivity mechanism. This may be an issue for further powder processing as the powder would interact with ferromagnetic processing equipment and clump together, making it difficult to handle. The magnet could be demagnetised by taking the magnet above its Curie temperature, i.e. ∼ 850 °C in the case of Sm2TM17, however extended thermal exposure can lead to damage of the magnet cellular structure which may influence HD behaviour. The impact of cell structure damage on the HD of Sm2TM17 magnets was explored here, additionally the use of thermal exposure before and after HD to circumvent further processing issues was investigated.
Three different commercially available magnet compositions were exposed to a range of temperatures under vacuum conditions to damage their cellular structures. The magnetic behaviour of thermally damaged material was assessed to relate cell structure damage and magnetic properties. Each composition was exposed to HD processing when magnetised, unmagnetised and thermally damaged. Resultant powders were characterised to assess their particle size/morphology, residual hydrogen content, degassing behaviour and phase balance.
Thermal exposure caused cell structure damage at temperatures much lower than the Curie temperature and was influenced by exposure time. It was concluded that the 1:5 H phase was likely disrupted during thermal exposure, evidenced by the large non-recoverable coercivity losses in each magnet composition. Magnet compositions with higher Cu and Zr were more resistant to thermal damage, which was attributed to a more robust Cu-rich 1:5 H cell boundary phase. Remanence values were largely unaffected, likely due to thermal exposure having less impact on the 2:17 R phase. HD behaviour across the thermally damaged samples was consistent with the as-manufactured samples regarding particle size, but hydrogen content was lower in the thermally damaged samples. The consistent particle size was attributed to the 2:17 R phase being undamaged and the differing hydrogen content may be linked to thermal exposure altering available hydrogen trapping sites, e.g. annealing out dislocations.
Sm and Co are listed as 'critical materials' within the UK and EU due to their high economic importance but high potential of supply risk. Thus, there is an economic and strategic imperative to recycle these materials and reduce reliance on virgin material supply chains. Wet chemical 'long-loop' based recycling approaches exist but are time intensive and high cost. A potential alternative is a Hydrogen Decrepitation (HD) based recycling process, which involves exposing Sm2TM17 magnets to hydrogen at various pressures, temperatures and times to generate a powder. The hydrogen forms an interstitial hydride within the 2:17 R phase causing it to expand. This initiates intergranular and transgranular cracking of the material forming a powder, which could then be used as a feedstock for 'short loop' recycling.
The effects of composition, pressure & temperature on HD processing have been explored, but limited to samples in the as-manufactured, unmagnetised state. This work shows that HD of Sm2TM17 magnets does not result in demagnetisation of the material as it does with NdFeB, due in part to its domain wall pinning coercivity mechanism. This may be an issue for further powder processing as the powder would interact with ferromagnetic processing equipment and clump together, making it difficult to handle. The magnet could be demagnetised by taking the magnet above its Curie temperature, i.e. ∼ 850 °C in the case of Sm2TM17, however extended thermal exposure can lead to damage of the magnet cellular structure which may influence HD behaviour. The impact of cell structure damage on the HD of Sm2TM17 magnets was explored here, additionally the use of thermal exposure before and after HD to circumvent further processing issues was investigated.
Three different commercially available magnet compositions were exposed to a range of temperatures under vacuum conditions to damage their cellular structures. The magnetic behaviour of thermally damaged material was assessed to relate cell structure damage and magnetic properties. Each composition was exposed to HD processing when magnetised, unmagnetised and thermally damaged. Resultant powders were characterised to assess their particle size/morphology, residual hydrogen content, degassing behaviour and phase balance.
Thermal exposure caused cell structure damage at temperatures much lower than the Curie temperature and was influenced by exposure time. It was concluded that the 1:5 H phase was likely disrupted during thermal exposure, evidenced by the large non-recoverable coercivity losses in each magnet composition. Magnet compositions with higher Cu and Zr were more resistant to thermal damage, which was attributed to a more robust Cu-rich 1:5 H cell boundary phase. Remanence values were largely unaffected, likely due to thermal exposure having less impact on the 2:17 R phase. HD behaviour across the thermally damaged samples was consistent with the as-manufactured samples regarding particle size, but hydrogen content was lower in the thermally damaged samples. The consistent particle size was attributed to the 2:17 R phase being undamaged and the differing hydrogen content may be linked to thermal exposure altering available hydrogen trapping sites, e.g. annealing out dislocations.