The use of hydrogen in the processing of permanent magnets: Future prospects

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1 The use of hydrogen in the processing of permanent magnets: Future prospects Andy Williams Dept of Metallurgy and Materials University of Birmingham UK Advanced Magnetic Materials and their Applications 2007, 9-11 October 2007, Pori, Finland

2 University of Birmingham

3 Plan of Talk Hydrogen Decrepitation (HD) HDDR Recycling - Powders Recycling - Sintered Magnets Conclusions

4 The Hydrogen Decrepitation (HD) Process

5 Hydrogen Decrepitation in Nd 16 Fe 76 B 8 Alloy (8x Normal Speed)

7 HD Particle

8 Schematic of a Jet Mill

9 Jet Milled Powder

10 Advantage of HD Powder

11 Vacuum Desorption Behaviour 3000 Nd 2 Fe 14 B Nd 15.5 Fe 77.5 B 7 Nd H 3 to NdH 2 NdH 2 to Nd Hydrogen Partial Pressure Partial Pressure (arbitary units) Temperature ( C)

12 Kerr Micrograph of Sintered Magnet

13 Advantages of using HD

14 The Modified HD-Process Cast Ingot c-axis Hydrogen Decrepitation Expose to Hydrogen 1-5 bar, Room Temperature Intergranular Fracture Large Particles Nd 2 Fe 14 B Nd - rich Jet M Hydrogen Partial Pressure 3000 Partial Pressure (arbitary units) 0 Partial Degas Nd 15.5 Fe 77.5 B Temperature Temperature ( C) Jet Mill Anneal Machine Coat Align / Press Magnetise Vacuum Sinter Permanent Magnet

15 W.Rodewald et al, Proc. 17 th Int. Workshop (2002), Delaware, p.25

16 The Hydrogenation Disproportionation Desorption Recombination (HDDR) Process

17 Free Energy Curves

18 Nd 2 Fe 14 B + (2 ± H 2 ) 2NdH 2 ±x + 12Fe + Fe 2 B + ΔH Exothermic

20 Disproportionated Mixture TEM Micrograph of solid disproportionated Nd 2 Fe 14 B O.Gutfleisch et al, J.Magn.Magn.Mat

21 Fracture Surfaces Sintered Magnet HDDR Powder 10μm 1μm

22 Anisotropic HDDR Powder

23 Highest Energy Product Maximum energy product reported: Mishima 2000 (BH)max = 342kJ/m -3 for a Co-free alloy using the type of HDDR shown above

24 Sintered NdFeB Magnets - Applications

25 Types of Applications Automotive: Starter motors, Anti-lock braking systems (ABS), Motor drives for wipers, Injection pumps, Fans and controls for windows, seats etc, Loudspeakers, Eddy current brakes, Alternators Telecommunications: Loudspeakers, Microphones, Telephone ringers, Electro-acoustic pick-ups, Switches and relays Data Processing: Disc drives and actuators, Stepping motors, Printers Consumer Electronics: DC motors for showers, Washing machines, Drills, Low voltage DC drives for cordless appliances, Loudspeakers for TV and Audio, TV beam correction and focusing device, Compact-disc drives, Home computers, Video Recorders, Clocks Electronic and Instrumentation: Sensors, Contactless switches, NMR spectrometer, Energy meter disc, Electro-mechanical transducers, Crossed field tubes, Flux-transfer trip device, Dampers. Industrial: DC motors for magnetic tools, Robotics, Magnetic separators for extracting metals and ores, Magnetic bearings, Servo-motor drives, Lifting apparatus, Brakes and clutches, Meters and measuring equipment Astro and Aerospace: Frictionless bearings, Stepping motors, Couplings, Instrumentation, Travelling wave tubes, Auto-compass Biosurgical: Dentures, Orthodontics, Orthopaedics, Wound closures, Stomach seals, Repulsion collars, Ferromagnetic probes, Cancer cell separators, Magnetomotive artificial hearts, NMR / MRI body scanner

26 Many of these products have a long lifetime apart from computer disc drives which have a very short lifetime. This application accounts for a high proportion of all sintered NdFeB magnets.

27 Hard Disk Drive

28 Why Recycle? Conserve vital raw material (e.g. Dy) Diminish dependence on imported products Lower overall energy of manufacture and reduce CO 2 emissions

29 How can all these magnets be recycled?

30 Possible Recycling Routes for NdFeB-type Magnets Scrap Magnets HD / Degas HDDR Blend with Fresh Powder LPPS Zn Coating Sintered Magnets Hot Pressed Magnets Polymer Bonded Magnets

31 HD of Sintered Magnets

32 HD Magnet Powder 200 m

33 Particle Size Distribution HD at 25 o C HD at 150 o C HD at 300 o C HD at 450 o C Particle size ( μm)

34 HD NdFeB Powder Soft Magnetic

35 Vacuum Desorption Behaviour 3000 Nd 2 Fe 14 B Nd 15.5 Fe 77.5 B 7 Nd H 3 to NdH 2 NdH 2 to Nd Hydrogen Partial Pressure Partial Pressure (arbitary units) Temperature ( C)

36 Magnetic Properties after Vacuum Desorption Intrinsic Coercivity, Hcj (ka/m) Br Hcj Degassing Temperature ( o C) Remanence, Br (mt)

37 HD & HDDR Powder

38 Stability of Powder at 85ºC and 80% relative humidity

LPPS of MQA-T Powder MQA-T particle LPPS zinc 10μm Secondary electron SEM image Additional considerations: Surface quality of powder particles Reduction in

39 LPPS of MQA-T Powder MQA-T particle LPPS zinc 10μm Secondary electron SEM image Additional considerations: Surface quality of powder particles Reduction in magnetic material per sample volume Reduction in magnetic properties depending on coating thickness Coating conditions: 370 C, 0.5hr Coating mix: 6g MQA-T, 5g sand, 5g Zn

40 Using HD powder to make sintered magnets

41 Sintered Magnet Production from Scrap Scrap Magnets Hydrogen Decrepitation Roller Mill - ½ hour Vacuum Sinter - 1 hour 1080 C Sintered Magnets

42 Chemical Analysis of Scrap Magnet (Atomic %) Nd Dy B Al Fe C O Starting material

43 Milling of Powder Reduce particle size so that alignment and pressing are better. Higher green density gives easier sinter. No need to get to single crystal particles as grains already aligned. Don t want to contaminate further with oxygen.

44 Roller Ball Milling of HD Powder 1 h (a) 200 m (c) 10 m 0.5 h 20 h 10 m 10 m

45 Recycling without blending additions

46 Magnetic Properties of Recyled Magnets Polarisation (mt)

47 Magnetic Properties of Recyled Magnets BHmax (kjm -3 ) Br (mt) ihc (kam -1 ) Density (gcm -3 )

48 Microstructure of Recycled Magnets After Recycling Once After Recycling Twice After Recycling Three Times After Recycling Four Times

49 Image Analysis Area % of Nd-Rich Phase Starting material 1st cycle 2nd cycle 3rd cycle 4th cycle No Addition

50 Chemical Analysis of Recycled Magnets (Atomic %) Nd Dy B Al Fe C O Starting material Cycle Cycle Cycle

51 Recycling with blending additions

52 Sintered Magnet Production from Scrap Scrap Magnets Hydrogen Decrepitation Roller Mill - ½ hour Blend with Nd Hydride Vacuum Sinter - 1 hour 1080 C Sintered Magnets

53 Why use neodymium hydride?

54 Advantages of Using RE Hydride for Blending Expansion of the lattice during hydriding assists the breaking up of the bulk alloy into particles suitable for milling Hydrides are usually brittle and readily milled The rare earth dihydride powders are more stable than the corresponding metal powders, reducing the incorporation of oxygen during processing Once the hydrogen is desorbed, the powder becomes very reactive, which aids in-situ alloying It is possible that the presence of hydrogen enhances solid state diffusion

55 Magnetic Properties of Recyled Magnets 1at% Nd Hydride added on 2nd 3rd and 4th cycle Polarisation (mt)

56 Magnetic Properties of Recyled Magnets 1at% Nd Hydride added on 2nd 3rd and 4th cycle BHmax (kjm -3 ) Br (mt) ihc (kam -1 ) Density (gcm -3 )

57 Microstructure of Recycled Magnets 1at% Nd Hydride added on 2nd 3rd and 4th cycle After Recycling Once After Recycling Twice After Recycling Three Times After Recycling Four Times

58 Image Analysis Area % of Nd-Rich Phase Starting material 1st cycle 2nd cycle 3rd cycle 4th cycle No Addition Nd Addition

59 Chemical Analysis of Recycled Magnets (Atomic %) 1at% Nd Hydride added on 2nd 3rd and 4th cycle Nd Dy B Al Fe C O Starting material Cycle Cycle Cycle Cycle

60 Comparison of non-blended and blended magnets Magnetic Properties Non-blended Blended

61 Comparison of non-blended and blended magnets Magnetic Properties Non-blended Blended

62 Comparison of blended and non-blended magnets Grain Size Average Grain Size (μm)

63 Conclusions Hydrogen decrepitation provides a convenient and effective means of recycling sintered NdFeB-type magnets It is essential to blend with neodymium hydride to maintain magnetic properties The process can be used to remove magnetic powder from assembled devices

64 Future Work The next stage is to build a pilot plant prototype in order to process large numbers of voice coil magnets and/or large assemblies.

65 Acknowledgements Thanks are due to Precision Magnetics for the provision of scrap magnets and to Less Common Metals for the Chemical Analysis Rex Harris and Miha Zakotnik are also acknowledged for assistance in gathering results and preparation of the talk

66 Thank you for your attention

Effect of Nb on The HDDR Behaviour and The magnetic Properties of NdFeB Alloys

Garyounis University Press Journal of Science and Its Applications Vol. 1, No. 2, pp 79-101, September 2007 97 Effect of Nb on The HDDR Behaviour and The magnetic Properties of NdFeB Alloys F. M. Alyamani