This document summarizes research on producing crystallographically anisotropic rare-earth magnetic material flakes through surfactant-assisted high-energy ball milling. Key findings include: 1) SmCo5 and Nd2Fe14B flakes were formed through ball milling with surfactants like oleic acid, whereas without surfactants only particles were formed. 2) Increasing milling time decreased flake thickness and increased texture randomization. Optimal milling times yielded single-crystal or textured polycrystalline flakes. 3) SmCo5 flakes had out-of-plane magnetic alignment whereas Nd2Fe14B flakes had in-plane alignment when subjected to a

1. Permanent Magnet Nanoflakes Jinfang Liu Electron Energy Corporation, Landisville, PA, USA jfl@electronenergy.com Collaborators: Baozhi Cui and Melania Marinescu, Electron Energy Corporation Alex Gabay and George Hadjipanayis, University of Delaware presentation for Workshop on Amorphous and Nanostructured Magnetic Materials Iasi, Romania September 2011

2. Outline 1 Introduction Experimental: surfactant-assisted high-energy ball milling and material characterization 2 Crystallographicallyanisotropic SmCo5 flakes 3 Crystallographicallyanisotropic Nd2Fe14B flakes 4 5 Summary

3. 1 Introduction Room-temperature intrinsic magnetic properties of bulk RCo5 (R = Sm, Y), Sm2Co17 and Nd2Fe14B High HA High Tc High Ms

5. The use of surfactants during ball milling influences not only the size of the particles, but also their shape. Flakes of malleable metals and alloys like Ni, Cu, Fe-Co, Fe-Co-Zr, Fe-Si-Al, Sn-Ag-Cu, have been fabricated by surfactant-assisted ball milling.

6. However, SmCo5 and Nd2Fe14B magnetic materials are brittle in nature and, therefore, they are not expected to "flake" during ball milling.

8. Milling solvent: heptane

9. Surfactants: oleic acid (OA), oleylamine (OY), trioctylamine (TOA) (0 - 150 wt.% of starting powders)

10. Milling time: 0 - 8 hrsHardened steel balls:  4 - 12 mm Ball-to-powder weight ratio: ~ 10:1 with SF without SF SmCo5, Nd2Fe14B flakes and/or microparticles SmCo5 or Nd2Fe14B flakes SmCo5 or Nd2Fe14B nanoparticles (after HEBM with OA for 4 - 8 h); < 1 wt.% XRD, SEM, TEM, VSM

12. preserves the crystal structure of magnetic phases and avoids amorphization;

13. lowers the energies of freshly cleaved surfaces, thus enabling long-range capillary force and lowering the energy required for crack propagation;

14. decreases inter-particle friction;

16. 0 (b) 0.25h (c) 0.5h (d) 1h (e) 2h (f) 3h (g) 4h (h) 5 h Typical morphology of powders by HEBM in heptane (without surfactant) (a) single-crystal microparticles (b-c) single-crystal microflakes (d-e) single-crystal microflakes + polycrystalline microparticles (f-h) Isotropic polycrystalline microparticles (grain size: 8-10 nm)

17. 3 Crystallographicallyanisotropic SmCo5flakes (a) 0 h (b) 0.25 (c) 0.5 (d) 1 h (e) 2 h (a) 3 h (b) 4 h (c) 5 h (d) 6 h (e) 8 h

19. Alignment in magnetic field of SmCo5 flakes Milling time (a) 3 h (b) 6 h SEM images of magnetically aligned SmCo5single-crystal submicron flakes and textured nanoflakes prepared by HEBM in heptane with 15 wt.% OA for (a) 3, and (b) 6 h. The arrow bars show the applied magnetic field directions.

21. Evolution of microstructure during HEBM (in heptane+15 wt.% OA) (a) Single-crystal SmCo5 flake (t ≤ 3 h); (b) Polycrystalline flake with small-angle grain boundaries (t = 3–4 h). The dashed lines marked the orientations (~6.5o) and the arrows reveal the grain boundary. Grain size ~ 20 nm. (c) [001] out-of-plane textured polycrystalline nanoflake(t = 5–8 h). Grain size ~ 8 nm. t is the milling time. B.Z. Cui, W.F. Li, G.C. Hadjipanayis, Acta Mater. 59(2011)563

25. SEM images are obtained after HEBM in heptane + OA for 5 h.0 wt.% OA 15 wt.% OA 40 wt.% OA 150 wt.% OA 2 - 30 µm Isotropic microparticles lateral size: 0.5 - 8 µm Flake thickness: 8 - 80 nm B.Z. Cui, A.M. Gabay, W. F. Li, M. Marinescu, J. F. Liu, and G.C. Hadjipanayis, J. Appl. Phys. 107, 09A721 (2010).

27. Effect of different surfactants on morphology of SmCo5 nanoflakes SmCo5 nanoflakes milled in heptane with OA and OY 30 wt.% OY, 5 h 30 wt.% OA, 5 h There is little difference in morphology between the flakes prepared by HEBM for 5 h in heptane with 30 wt.% oleic acid (OA) and oleylamine (OY). SmCo5 nanoflakes have a length of about 0.5-10 μm and a thickness of 8 to 80 nm.

28. Effect of different surfactants on morphology of SmCo5 nanoflakes SmCo5 powders milled in heptane with TOA (Trioctylamine) 60 wt.%, 2.5 h 100 wt.%, 5 h 30 wt.%, 5 h SmCo5 textured nanoflakes with thickness of 80-200 nm SmCo5 textured nanoflakes with thickness of 50-150 nm SmCo5 irregular particles (nearly isotropic) A higher amount of TOA ( ≥ 40 wt.%) is required to obtain textured SmCo5 nanoflakes, compared with OA and OY.

29. Structure and magnetic properties of SmCo5 nanoflakes by HEBM in heptane with 30 wt.% OA Coercivity of the as-milled SmCo5 flakes increased first and then decreased after reaching a maximum value of 16 kOe after milling for 5 h. The flakes preserve SmCo5 structure and have a [001] texture. With increasing the milling time from 3 to 6 h, the average grain size decreased from 21 to 15 nm.

30. Structure and magnetic properties of SmCo5 nanoflakes by HEBM in heptane with 30 wt.% OY Coercivity of the as-milled SmCo5 flakes increased first and then decreased after reaching a maximum value of 15.0 kOe after milling for 5 h. The flakes preserve SmCo5 structure and have a [001] texture. With increasing the milling time from 3 to 6 h, the average grain size decreased from 21 to 10 nm

31. Structure and magnetic properties of SmCo5nanoflakes by HEBM in heptane with 100 wt.% TOA When the amount of TOA was 100 wt.%, both the texture and coercivity decreased with milling time from 2.5 to 6.5 h. iHc = 15.8 kOe after milled for 2 h.

32. 4 Crystallographicallyanisotropic Nd2Fe14B flakes Sample morphology for different milling time in heptane with 40 wt.% OA (a) 0.25, (b) 1 h, (c) 2 h, (d) 3 h, (e) 5 h Nd2Fe14B microflakes submicron flakes and nanoflakes

34. The effects of OA and OY were found to be very similar.

35. DyF3 was markedly less efficient to providing formation of thin anisotropic Nd2Fe14B flakes. We did not observe any additional magnetic hardening after using the DyF3 additive.40% OA, 5 h HEBM 40% DyF3, 5 h HEBM 20% OY, 5 h HEBM

36. Effect of milling time on flake dimension, grain size and texture Flake thickness and length, intensity ratio of I006/I105 and average grain sizes of the Nd2Fe14B hard phase for the as-milled Nd2Fe14B flakes. *: particle size. HEBM in heptane + 40 wt.% OA Nanoflakes isotropic: 0.5

38. Nd2Fe14B flakes exhibit "in-plane" easy magnetization direction.Magnetically aligned nanoflakes prepared by HEBM for 5 h in heptane with 15 or 40 wt.% OA. The arrow bar shows the applied magnetic field direction.

41. Evolution of coercivity and texture in Nd2Fe14B flakes via surfactant-assisted HEBM

43. Post-annealing of the as-milled Nd2Fe14B flakes.

45. The additives of Nd70Cu30 and Pr68Cu32 have very similar effect on morphology and magnetic properties for both as-milled and annealed Nd2Fe14B flakes.

47. Annealed at 450oC for 30 min One flake of Nd2Fe14B+Nd70Cu30 (EDS) Element Weight % Atomic % ---------------------------------------------- NdL 52.1 29.9 FeK 43.0 63.7 CuK 4.9 6.4 Total 100.0 100.0 A mixture of nanocrystalline Nd2Fe14B and disordered Nd-(Cu) rich phases at grain boundary in one annealed flake Poly-nanocrystalline (partly disordered) Nd70Cu30 flakes Nd70Cu30 (EDS) Element Atomic % ---------------------------------------------- NdL 70.5 CuK 29.5 Total 100.0

49. Surfactants play an essential role in the formation of anisotropic flakes by a decrease of cold welding and agglomeration of the flakes. OA and OY have similar effects on the formation of anisotropic flakes. A higher amount of TOA is required to obtain nanoflakes.

50. Both the addition of a low melting-point Nd70Cu30 or Pr68Cu32 alloys and a proper post-annealing treatment can increase the coercivity of Nd2Fe14B nanoflakes.

51. These novel flakes have unique properties, including a high degree of texture, a high stability in air and a unique shape that can be easily coated.

52. Possible applications of the hard magnetic flakes include anisotropic nanocomposite magnets with high energy product, laminated magnets with reduced eddy current loss, etc.