Chul-Jin Choi

Novel process of rare-earth free magnet

and thermochemical route for fabrication

of permanent magnet

Korea Institute of Materials Science, 797 Changwondaero,

Changwon, Kyungnam, Korea

Korea Magnetic Society Pyeongchang, Korea, Dec. 7, 2013

Scope

Powder processing of Mn-Al based

materials for rare earth free magnets

Spray drying and reduction /diffusion

route for Nd-Fe-B permanent magnet

Summary

Alloy Design and Powder processing of

Mn-Al based materials for rare earth free

magnets

Advantages

μ0 Mr（T） μ0 Hc (T) (BH)max (KJ/m3)

Ferrite 0.39 0.30 28

Alnico 1.30 0.07 50

MnAl 0.57 0.31 55.7

SmCo 1.08 1.0 225

NdFeB 1.33 1.6 400

Tb Ta Eu Dy CoSmNd Ni Cu La Al Pb ZnMn C Fe0

10

20

30

40

50

60

70

80

90

100

110

400

500

600

Metal

Pri

ce [

$/k

g]

Mn-Al Magnetic Alloy

Phase diagram of MnAl

How to produce the ferromagnetic Mn-Al alloy

wt.% Mn

at.% Mn Al ← Mn

Methods Morphology

Warm-extruded Bulk

Magnetron sputtering Thin films

Melt spinning Micro-powder

Mechanical milling Micro-powder

Water-atomization Micro-powder

Until now

Methods Morphology

Plasma-arc discharge Nano-powder

Gas-atomization Micro-powder

This study

τ phase is usually produced by

1. Rapid quenching of theεphase followed by isothermal annealing between 400-700℃

2. Cooling the εphase at a rate of ∼10℃/min

Objectives: Manufacturing of ε-phase Mn54Al46 powder; Optimization of processing parameters to obtain τ-phase Mn54Al46

powder (theoretical limit: 144 emu/g); Calculation of (BH)max for core-shell nanomagnet with Skomski’s equation.

1300

1200

1100

1000

900

800

700

600

500

30 40 50 60 70 80

Tem

per

atu

re /°C

Atomic Percent Manganese

τ

ßγ2

γε

Liquid

1300

1200

1100

1000

900

800

700

600

500

30 40 50 60 70 80

Tem

per

atu

re /°C

Atomic Percent Manganese

τ

ßγ2

γε

Liquid

1. quenching

2. Annealing

a b

c

No change in the volume: V = 27.16A3

a ≈ b ≈ c ≈ 3.006A a = b = 2.77A, c = 3.54 A

a:c = 1:1.28

Phase transformation of MnAl alloy

ε ε’ τ

Orthorhombic (ε’) structure Tetragonal (τ) structure

6

Manufacturing of MnAl alloy and estimation of (BH)max for MnAl-soft shell nanomagnet

Estimated (BH)max

MAE of 0.259 meV (1.53×106 J/m3) 38 kOe of magnetocrystalline anisotropy field

12.64 MGOe for τ-phase Mn50Al50 (Br = 0.7 Bs)

1.0 1.2 1.4 1.6

1.9

2.0

2.1

2.2

2.3

2.4

2.5

0.00

0.01

0.02

0.03

0.04

0.05

Mag

net

ic m

om

ent

[B]

c/a

Rel

ativ

e T

ota

l E

ner

gy

[R

yd

]

1.0 1.2 1.4 1.6

1.9

2.0

2.1

2.2

2.3

2.4

2.5

0.00

0.01

0.02

0.03

0.04

0.05

Mag

net

ic m

om

ent

[B]

c/a

Rel

ativ

e T

ota

l E

ner

gy

[R

yd

]

2.37 μB (161 emu/g)

1.3

Magnetic moment and total energy versus c/a ratio forτ-MnAl

0.0 0.2 0.4 0.6 0.8 1.00

10

20

30

40

50

60

70

80

90

1

10

100

1000

Fraction of hard magnetic phase (fh)

(BH

) max

(M

GO

e)

1.3 T

1.6 T

1.9 T

2.2 T

s, D

h = 50 nm

s, D

h = 70 nm

s, D

h = 100 nm

s, D

h = 250 nm T

hic

kn

ess

(n

m)

Br of soft magnet [T]

Thickness and diameter

12.3 MGOe for single phase MnAl

τ-MnAl

Soft magnet

Dh

δs

300 K

[5]

[1] Q. Zeng, I. Baker, J. B. Cui, and Z. C. Yan, J. Magn. Magn. Mater. 308, 214 (2007). [2] N. I. Vlasova, G. S. Kandaurova, YA. S. Shur and N. N. Bykhanova, Phys. Met. Metall. 51, 1 (1981). [3] Y. Zhang, and D. G. Ivey, Mat. Sci. Eng. B, 140, 15 (2007). [4] E. F. Kneller and R. Hawig, IEEE Trans.

Mag., 27, 3588 (1991). [5] G. F. Korznikova, Journal of Microscopy, 239, 239 (2010).

nm 15 alExperiment nm, 9.41077.2100.14

6531038.13.0

4

3.0

87

2162

Ka

kTcδs ≤ 2 x domain wall width of hard phase particle [4]

Theoretical (BM)maxτ-MnAl core/shell nanomagnet

1900 1920 1940 1960 1980 2000 20200

10

20

30

40

50

60

(B

H) m

ax (

MG

Oe)

year

*S. Sugimoto, J. Phys. D: Appl. Phys., 44, 064001 (2011).

KS MK

NKS

OP Sr- or Ba-Ferrite

MnAlC

FeCrCo

SmFeN

1-5:Sm-Co

2-17:Sm-Co

Nd-Fe-B

Pt-Fe

Pt-Co Alnico

Core (MnAl) – shell (soft 1.9 T)

300 K

MnBi

Development trend of permanent magnets

circulation

fan

collector

scrapper

collect

jar sample

auto feeding

system

cathode

anode

plasma

arc

gas circulation

vacuum

out

valve

cooling

jacket

H2 Ar

- Synthesis of Metal/Ceramic nanoparticles

- High-purity and Non-agglomeration

- Surface stability of nanoparticles

- Continuous production

Plasma Arc Discharge process

Mn-Al Nano powder by Plasma Arc Discharge

ε-τ phase transition TEM micrograph

Magnetic Property of Mn-Al Nano powder

Methods Morphology Magnetic Properties

Hc (kOe)

Warm-extruded Bulk 3.02

Magnetron sputtering Thin films 3.0

Melt spinning Micro-powder 1.6

Mechanical milling Micro-powder 4.8

Plasma arc

discharge Nanoparticles 5.6

Magnetic properties of the MnAl system alloys prepared by different methods

-8000 -6000 -4000 -2000 0

Ma

gn

etiz

atio

n

H(Oe)

400 oC

500 oC

600 oC

The Highest Coercivity

Gas atomization Condition: Mn-30 wt.%Al ingots; At nitrogen atmosphere;

sieve

S:25-38 m M: 45-75 m B:100-150 m

Annealing ◆ HT: at 500-700 oC for 20 min

◆ τ-phase : S size, HT at 650 oC for 20 min

◆ Ball milling : 5-26 h

◆ Further annealing: at 280 oC for 20 min

P1

P2 ◆ ε-phase : S sized powder, ball milled for 5-26 h

◆ HT: at 650 oC for 20 min

To produce ferromagnetic MnAl powders by gas-atomization

Ball milling process

Gas Atomized Mn-Al Materials

XRD patterns of the gas-atomized Mn-Al alloy powders treated by different two p

rocesses: (a) P1; (b)P2.

P1 P2

Phase change with ball milling

Good Bad

SEM images of gas-atomized Mn-Al powders with different particle size:

(a) 25-38 μm (b) 45-75 μm (c) 100-150 μm.

Gas atomized Mn-Al Powder

1.4 m 4 m

Phase and Magnetic property change with Annealing

XRD patterns of Mn-Al powders annealed for 20 min

At (a) 500, (b) 550, (c) 600, (d) 650, and (e) 700 oC.

5 h 10 h

20 h 26 h

SEM images of powders treated by P1 process

D～ 4.0 m

Dependence of Mr and Hc of the powders on the ball milling time.

Phase change and Magnetic property change

Particle size with milling Magnetic Property Annealing Effect

Comparison of Magnetic Property

Theoretical

Mn50Al50]

Magnetic

moment 2.37 μB (161emu/g)

MAE 0.259 meV

(1.525×106 J/m3)

(BH)max 12.64 MGOe

Experimental

Mn54Al46

Magnetic

moment 1.44 μB (98.3 emu/g)

(BH)max 4.7 MGOe

Curie

temperature

388 ºC

Theoretical Experimental

Thermochemical Route - Spray Drying

and Reduction/Diffusion process - for

permanent magnet

Application of process

Cost effective production of Nd-permanent magnet

with novel process combined spray drying and R/D

Direct application of

Rare-chloride or

oxide

Nd extraction from Ore

Ore of RE Extraction

from RE ore

RE Chloride

Recycling of Nd from wastes

MRI motor

HDD

electronic device

Outline of process

Mixed salt solution Spray drying Nd-Fe-B powders

packing/alignment/forming Magnetization Nd-permanent magnet

NdCl3 6H2O FeCl3 6H2O

H3BO3

Reduction/diffusion

Cost-effective / Fine magnetic powders

Spray drying/ Reduction-diffusion - Relatively cheap Nd-salt of Nd-oxide as starting material

- Applicable to direct use of extracted Nd from ores and

recycling of Nd from wastes

- Preparation of fine magnetic powders under μm size

- Reduction of defects from milling process

1μm 1㎛

NdCl3 · 6H2O

Spray - drying

Desalting

Milling

H2 Reduction

Ca Reduction

Washing

FeCl3 · 6H2O

Nd2Fe14B Particles

H3BO3

Design of process

Air in

(250℃)

Precursor (20㎖/min )

Chamber

Rotary

Atomizer (15000rpm)

Air out (120℃)

Cyclone

Collector

Schematic diagram of spray dryer

(a) Spray-dried precursors (b) Desalted at 750 ℃

(c) Milling for 40h

& H2-reducing at 1000℃

(d) Ca-reducing at 1000℃

& washing

SEM micrographs of the powders

a

d c

b

Debinding for 2 hours at

(a) 500 oC; (b) 750 oC; (c) 900 oC; (d) 1000 oC.

Effects of temperature on debinding process

TG-DTA curves for spray-dried precursor

in air at 10 oC/min of heating rate.

0 200 400 600 800 1000 1200

50

60

70

80

90

100

exo

endo

3.713%

(0.7011mg)

4.349%

(0.821mg)

14.13%

(2.669mg)

8.733%

(1.649mg)

15.48%

(2.922mg)

558oC

1121oC

506oC

95oC

238oC

DT

A (

oC

/mg

)

Temperature (oC)

Weig

ht

(%)

-0.10

-0.05

0.00

0.05

0.10

NdOCl

Fe2O3

Fe2O3.Nd2O3

TG-DTA for debinding process

20 30 40 50 60 70 80

FeNdO3

Fe2O

3

NdOCl

FeB

(d)

(c)

(b)

(a)

Inte

nsity

(arb

. uni

ts)

XRD patterns of the powders debinded at

(a) 500 oC; (b) 750 oC; (c) 900 oC; (d) 1000 oC.

Fe2O3, NdOCl

Fe2O3, NdOCl,

Fe2O3 Nd2O3

Fe2O3, Fe2O3, Nd2O3,

FeB

Fe2O3, NdOCl,

Fe2O3 Nd2O3

Effects of temperature on debinding process

700 800 900 1000 1100 1200

99

100

101

102

103

1075oC

1131oC

1107oC

1060oC

840oC

Temperature (oC)

Weig

ht

(%)

-0.20

-0.15

-0.10

-0.05

0.00

0.05

0.10

exo

endo

DT

A (

oC

/mg

)

TG-DTA curves for the mixture of Ca and H2-reduced powder

in flowing Ar at 10 oC/min of heating rate.

Ca melting

Nd reduction and Rreation

TG-DTA for Ca-reduction process

XRD patterns of the products in all steps of

(a) Spray drying; (b) Desalted at 750 ℃ in air; (c) H2-reducing at 800 ℃;

(d) Ca-reducing at 1000 ℃ in flowing Ar; (e) washing.

20 25 30 35 40 45 50 55 60 65 70 75 80

Nd2Fe

14B

NdOCl

Fe2O

3

-Fe

(e) Washed

CaO

(b) Desalting (750 oC)

Inte

nsity

(arb

. uni

t)

(a) Precursor (spray-dried)

(D) Ca reducing

(C) H2 reducing

(d)

(c)

(b)

(a)

(e)

Phase evolution in the processes

20nm

TEM photograph of Nd2Fe14B particle

Washing and magnetic properties

Ca/powder

(wt. ratio)

condition Hc

(kG)

Mr

(emu/g)

Ms

(emu/g)

Catotal

(wt. %)

0.40 After R/D 10.97 47.67 64.45 22.9

After washing for 1 hr 9.96 60.43 83.08 11.5

After washing for 2 hrs 9.10 64.74 90.44 8.5

After washing for 3 hrs 6.75 66.17 102.33 0.5

◈ washing conditon

• Weight ratio of Ca/powder = 0.40

• De-ionized water washing, 1~3 hrs under sonication

• Increase of washing time oxidation of powders

• Increase of washing time effective removal of impurities

R/D DIW-1hr DIW-2hrs DIW-3hrs0

5

10

15

BH

max (

MG

Oe)

[DIW; De-Ionized water Washing]

Summary

• The magnetic property of Mn-Al alloy was calculated, it showed 12.3 MGO for single phase and 60 MGO for core-shell nanomagnet.

• Mn-Al nanoparticles were successfully prepared by plasma arc discharge and gas atomization processes and the nanopowders exhibited very high coercivity, compared to other processes.

• The fully tau phase of Mn-Al powders were synthesized by gas atomization, and (BH)max was 4.7 MGO. To increase the magnetic properties, the fabrication of nanocomposite is in investigation.

• The ultrafine grained Nd-Fe-B magnetic powders were successfully fabricated by the thermochemical route including spray drying and reduction/diffusion process.

• The magnetic property of powder show 10 MGO of (BH)max after washing. The enhancement of magnetic property and application to magnet is under study.

Prediction of magnetic property by simulation

Mn-Bi (LTP) structure (Vol. 97 Å 3] Mn-Bi-Co structure (Vol. 103 Å 3]

a = 4.287, c = 6.118

a

c c

a

c

a

a = 4.461, c = 5.989 a = 4.566, c = 5.941

Mn-Bi-Co-Fe structure (Vol. 107 Å 3]

Magnet Volume

(Å 3)

Magnetic

Moment

(μB/u.c.)

Magnetization

(emu/cc)

MAE

(meV/u.c.)

K

(anisotropy

constant)

(106 J/m3)

Tc (K)

Mn-Bi

(UA) 97 7.25

693

(0.87 Tesla) 0.925 1.52 628

Mn-Bi-Co

(UA) 103 9.19

827

(1.04 Tesla) 4.748 7.38 600

Mn-Bi-Co-Fe

(UA) 107 10.17

916

(1.51 Tesla) -0.01361 -0.02 527

Table I. Comparison of calculated magnetic data for conventional Mn-Bi,

new Mn-Bi-Co hard magnet, and Mn-Bi-Co-Fe soft magnet.

Computer simulation of Magnetic property

a a

c

a b

c

Al

Mn

Phase transformation of MnAl alloy :

ε ε’ τ

0.0 0.2 0.4 0.6 0.8 1.00

10

20

30

40

50

60

70

80

90

1

10

100

1000

Fraction of hard magnetic phase (fh)

(BH

) max

[M

GO

e]

1.3 T

1.6 T

1.9 T

2.2 T

2.5 T

s, D

h = 10 nm

s, D

h = 25 nm

s, D

h = 50 nm

s, D

h = 70 nm

s, D

h = 100 nm

s, D

h = 250 nm

Th

ick

ness

[n

m]

Mh = 1 T Kh = 1.5 MJ/m3

Ks = 0.003 MJ/m3

Br of soft magnet [T]

δs ≤ 2 x domain wall width of hard phase particle [4]

25 MGOe for pure MnAl

[5] 15nm alExperiment nm, 96.31077.21053.14

6531038.13.0

4

3.0

87

2162

Ka

kTc

Thickness and diameter

τ-MnAl

Soft magnet

Dh

δs

0 K

0.0 0.2 0.4 0.6 0.8 1.00

10

20

30

40

50

60

70

80

90

1

10

100

1000

Fraction of hard magnetic phase (fh)

(BH

) max

[M

GO

e]

Th

ick

ness

[n

m]

300 K

nm 15 alExperiment nm, 9.41077.2100.14

6531038.13.0

4

3.0

87

2162

Ka

kTc

12.3 MGOe for pure MnAl