permanent magnet nanoflakes iasi

Permanent Magnet Nanoflakes

Jinfang Liu

Collaborators: Baozhi Cui and Melania Marinescu, Electron Energy CorporationAlex Gabay and George Hadjipanayis, University of Delaware

Electron Energy Corporation, Landisville, PA, USAjfl@electronenergy.com

presentation for

Workshop on Amorphous and Nanostructured Magnetic MaterialsIasi, RomaniaSeptember 2011

OutlineOutline

Experimental: surfactant-assistedhigh-energy ball milling and material characterization

Experimental: surfactant-assistedhigh-energy ball milling and material characterization

22

Crystallographically anisotropic SmCo5 flakesCrystallographically anisotropic SmCo5 flakes33

Summary Summary

Crystallographically anisotropic Nd2Fe14B flakesCrystallographically anisotropic Nd2Fe14B flakes

11

44

IntroductionIntroduction

55

Compounds Curie temperature

Tc (oC)

Saturation magnetization

Ms (kGs)

Anisotropy type

Anisotropy field

HA (kOe)

YCo5 640 10.6 uniaxial 130

SmCo5 680 11.0 uniaxial 440

Sm2Co17 920 12.7 uniaxial 65

Sm2(Co0.7Fe0.3)17 840 14.5 uniaxial 52

Nd2Fe14B 312 16.0 uniaxial 67

Sm2Fe17N2.3 476 15.4 uniaxial 140

Room-temperature intrinsic magnetic properties of bulk RCo5 (R = Sm, Y), Sm2Co17 and Nd2Fe14B

　　 High HA High Tc High Ms

11 IntroductionIntroduction

●Microparticles, nanoparticles, ribbons, thin films of the Sm-Co, Pr-Co and Nd-Fe-B compounds have been produced by ball-milling, melt-spinning, and magnetron sputtering.

●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.

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

●This talk will report on the unusual formation mechanism, optimization and the role of surfactants to form textured SmCo5 and Nd2Fe14B flakes by surfactant-assisted high energy ball milling (HEBM).

Shapes of rare-earth-based magnetic materials

Sm17Co83 or Nd15.5Fe78.5B6 (at.%) ingots

One-step high energy ball milling (HEBM)

SmCo5 or Nd2Fe14B flakes

XRD, SEM, TEM, VSM

SmCo5 or Nd2Fe14Bnanoparticles(after HEBM with OA for 4 - 8 h); < 1 wt.%

Hardened steel balls: 4 - 12 mm Ball-to-powder weight ratio: ~ 10:1

Crushed ingot powders with particle size of about 1-100 μm

Milling solvent: heptane Surfactants: oleic acid (OA),

oleylamine (OY), trioctylamine (TOA) (0 - 150 wt.% of starting powders)

Milling time: 0 - 8 hrs

SmCo5, Nd2Fe14B flakes and/or microparticles

with SF without SF

22 ExperimentalExperimental

●Milling in the presence of solvent and surfactants:*The role of surfactants during ball

millingdecreases flake/particle agglomeration during

milling;preserves the crystal structure of magnetic phases

and avoids amorphization;lowers the energies of freshly cleaved surfaces, thus

enabling long-range capillary force and lowering the energy required for crack propagation;

decreases inter-particle friction;protects the fine magnetic flakes (and nanoparticles)

from oxidation during and after the ball milling.

●The decreased cold welding and agglomeration of the particles/flakes is essential to form anisotropic submicron- and nano-flakes!

HEBMwithout surfactants

HEBMwith surfactants

D. Guérard, Rev. Adv. Mater. Sci. 18 (2008) 225;W.A. Kaczmarek, B.W. Ninham, Mater. Chem. Phys. 40 (1995) 21.

P. Somasundaran, I.J. Lin Ind. Eng. Chem. Process Des. Dev. 11 (1972) 321.

Typical morphology of powders byHEBM in heptane (without surfactant)

(a) 0

(b) 0.25h

(c) 0.5h

(d) 1h

(e) 2h

(f) 3h

(g) 4h

(h) 5 h

(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)

(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

Crystallographically anisotropic SmCo5 flakesCrystallographically anisotropic SmCo5 flakes33

Ball-milling

time (h)

Flake

thickness

(nm)

Flake

length

(mm)

I002/I111Average grain

size

(nm)

0 - 1 - 40* 2978 -

0.25 1000 - 4000 1 - 18 1053 -

0.5 900 - 2600 1 - 10 236 -

1 300 - 1200 1 - 10 131 -

2 250 - 420 1 - 8 30 -

3 50 - 280 1 - 8 12 -

4 10 - 160 1 - 8 5.9 12

5 8 - 80 0.5 - 8 3.2 8

6 8 - 70 0.5 - 8 2.5 8

8 6 - 20 0.5 - 8 1.4 7

Flake thickness and length, values of intensity ratio I002/I111 and average grain size

of the SmCo5 phase for the as-milled SmCo5 flakes.

Effect of milling time on flake dimension, grain size and texture

isotropic: 0.19HEBM in heptane with 15 wt.% OA

Nan

ofla

kes

● As the milling time increases, the flake thickness becomes smaller and the texture of the flakes decreases.

SEM images of magnetically aligned SmCo5 single-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.

Alignment in magnetic field of SmCo5 flakes

Milling time

(a) 3 h

(b) 6 h

Effect of milling time on texture

● Distribution intensities of [001] out-of-plane texture component of SmCo5 are 19.0, 6.0, and 4.1 times the random value, in magnetically aligned SmCo5 submicron flakes and nanoflakes prepared by HEBM in heptane with 15 wt.% OA for (a) 3, (b) 5, and (c) 8 h. (a) non- aligned, (b) aligned in 1.9 T

(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

Evolution of microstructure during HEBM (in heptane+15 wt.% OA)

•Schematic evolution and formation mechanism of single-crystal micron, submicron flakes and textured nanocrystalline nanoflakes from SmCo5 ingot

(a) polycrystalline bulk ingot with a grain size of about 40–100 µm; (b) single-crystal irregular particles (sizes of 1–40 µm in this work); (c) single-crystal micron and then submicron (thick) flakes; (d) submicron (thick) flakes with small-angle grain boundaries; (e) textured poly-nanocrystalline nanoflakes.B.Z. Cui, W.F. Li, G.C. Hadjipanayis, Acta Mater. 59 (2011) 563

Proposed model for the formation of SmCo5 textured nanoflakes

●Dependences of coercivity and squareness of the demagnetization curves on milling time for the samples prepared by HEBM in heptane with 15 wt.% OA.

(S*=Area of M(H) at 2nd quadrant /(Mr*Hc)

Effect of milling time on magnetic properties (with 15 wt.% OA)

B.Z. Cui, W.F. Li, G.C. Hadjipanayis, Acta Mater. 59 (2011) 563

Anisotropic magnetic behavior As-milled samples aligned in 1.9 T

●Morphology of the SmCo5 nanoflakes can be efficiently controlled by the amount of surfactant (oleic acid) : “kebab-like” morphology and well-separated flakes are observed.

●SEM images are obtained after HEBM in heptane + OA for 5 h.

0 wt.% OA 15 wt.% OA 150 wt.% OA40 wt.% OA

Effect of amount of oleic acid on sample morphology

2 - 30 µmIsotropic 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).

●Variation of the amount of OA between 15 and 150 wt.% has little effect on the magnetic properties those are distinctly anisotropic. Coercivities: ~ 17-18 kOe.

15 wt.% OA

Effect of amount of oleic acid on magnetic properties

SmCo5 nanoflakes milled in heptane with OA and OY

30 wt.% OA, 5 h 30 wt.% OY, 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.

Effect of different surfactants on morphology of SmCo5 nanoflakes

SmCo5 powders milled in heptane with TOA (Trioctylamine)

30 wt.%, 5 h

100 wt.%, 5 h60 wt.%, 2.5 h

SmCo5 textured nanoflakes with thickness of 80-200 nm SmCo5 irregular particles

(nearly isotropic)

SmCo5 textured nanoflakes with thickness of 50-150 nm

Effect of different surfactants on morphology of SmCo5 nanoflakes

A higher amount of TOA ( ≥ 40 wt.%) is required to obtain textured SmCo5 nanoflakes, compared with OA and OY.

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.

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.

Structure and magnetic properties of SmCo5 nanoflakes by HEBM in heptane with 30 wt.% OA

Structure and magnetic properties of SmCo5 nanoflakes by HEBM in heptane with 30 wt.% OY

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

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.

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.

Structure and magnetic properties of SmCo5 nanoflakes by HEBM in heptane with 100 wt.% TOA

(a) 0.25,

(b) 1 h,

(c) 2 h,

(d) 3 h,

(e) 5 h

Sample morphology for different milling time in heptane with 40 wt.% OA

Nd2Fe14B microflakes

submicron flakes

and nanoflakes

Crystallographically anisotropic Nd2Fe14B flakesCrystallographically anisotropic Nd2Fe14B flakes44

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

● 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.

►The effect of different surfactants, such as OA, OY and DyF3 (which is expected to inhibit cold-welding similarly to the "true" surfactants)

OA OY DyF3

Thickness (nm) 80 - 160 80 - 180 800 - 6000

Length (μm) 0.5 – 10.0 0.5 – 10.0 1 - 40

Hc (kOe) 3.5 3.7 3.2

40% OA, 5 h HEBM 40% DyF3, 5 h HEBM20% OY, 5 h HEBM

Ball-milling

time (h)

Flake

thickness

(nm)

Flake length

(mm)

I006/I105Average grain

size

(nm)

0 - 1 - 100*-

-

0.25 2000 - 4000 1 - 3340

-

0.5 600 - 1800 1 - 3039

-

1 300 - 1650 1 - 1636

-

2 250 - 980 1 - 1434

-

3 200 - 440 1 - 1413

-

4 90 - 360 1 - 1314

14

5 80 - 180 0.5 - 108

10

6 40 - 110 0.5 - 106

10

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

isotropic: 0.5

Effect of milling time on flake dimension, grain size and textureN

anof

lake

s

● SmCo5 flakes exhibit "out-of-plane" easy magnetization direction.

Alignment in magnetic field: comparison

Nd2Fe14BSmCo5

● 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.

Proposed model for the formation of textured Nd2Fe14B nanoflakes

•Schematic evolution and formation mechanism of single-crystal micron, submicron flakes and textured nanocrystalline nanoflakes from NdFeB ingot

(a) polycrystalline bulk ingot; (b) single-crystal irregular particles (sizes of 1–100 µm in this work); (c) single-crystal micron and then submicron (thick) flakes; (d) submicron (thick) flakes with small-angle grain boundaries; (e) textured poly-nanocrystalline nanoflakes.

●SmCo5 flakes exhibit "out-of-plane" easy magnetization direction, whereas Nd2Fe14B flakes show "in-plane" easy magnetization direction.

●The crystalline anisotropy weakens after prolonged milling.

Basal cleavage planes: (001) SmCo5 or (110) Nd2Fe14B

XRD patterns and M-H curves of Nd2Fe14B (a) single-crystal submicron flakes and (b) textured poly-nanocrystalline flakes prepared by HEBM in heptane with 40 wt.% OA for (a) 2 h, and(b) 5 h.

Crystallographically and magnetically anisotropic behaviorof Nd2Fe14B flakes

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

For the as-milled textured Nd2Fe14B flakes, the major problem was the low iHc ≤ 3.7 kOe. Efforts to increase iHc :

►Introduction of low melting-point Nd70Cu30 and Pr68Cu32 additives (eutectic alloys) during the surfactant-assisted HEBM process.

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

►Nd14Dy1.5Fe78.5B6 rather than the Nd15.5Fe78.5B6 alloys

● When milled together with Nd70Cu30 or Pr68Cu32 powders (Tmelt = 520 or 472oC ), the as-milled Nd15.5Fe78.5B6 nanoflakes exhibited an increase in Hc, from 3.7 to 4.7 kOe by an addition of extra 20 wt% of Nd70Cu30.

● The Hc can be further increased to 6.8 kOe by annealing at 450oC for 30 min.

● The additives of Nd70Cu30 and Pr68Cu32

have very similar effect on morphology and magnetic properties for both as-milled and annealed Nd2Fe14B flakes.

Effect of additives and annealing on magnetic properties of Nd2Fe14B flakes

-20 -10 0 10 20

-100

-50

0

50

100 // aligned

7.0 kOe

Nd14

Dy1.5

Fe78.5

B6

+ 20 wt.% Nd70

Cu30

Nd14

Dy1.5

Fe78.5

B6

+ 20 wt.% Pr68

Cu32

HEBM for 5 h in

heptane + 20 wt.% OY

then, annealed at 450oC

for 30 min7.0 kOe

M (

emu/

g)

H (kOe)

Nd15.5Fe78.5B6 + extra 20 wt.% Nd70Cu30 ingot powders, HEBM for 5 h in heptane + 20 wt.% OY

-20 -10 0 10 20-90

-60

-30

0

30

60

90 Magnetic AnisotropyAnnealed at 450oC for 30 min

Nd15.5

Fe78.5

B6

+ 20 wt.% Nd70

Cu30

flakes by HEBM for 5 h inheptane + 20 wt.% OY

M (

emu/

g)

H (kOe)

//, 6.8 kOe

, 6.9 kOe

-20 -10 0 10 20-100

-50

0

50

100 Magnetic Anisotropy

3.9 kOe

//

4.7 kOe

M (

emu/

g)

H (kOe)

Annealed at 450oC for 30 min

A mixture of nanocrystalline Nd2Fe14B and disordered Nd-(Cu) rich phases at grain boundary in one annealed flake

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

Poly-nanocrystalline (partly disordered) Nd70Cu30 flakes

Nd70Cu30 (EDS)Element Atomic %----------------------------------------------NdL 70.5CuK 29.5Total 100.0

● SmCo5 and Nd2Fe14B single-crystal submicron flakes and textured poly-nanocrystalline nanoflakes were fabricated by one-step surfactant-assisted HEBM.

● 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.

● 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.

● 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.

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

55 Summary Summary