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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