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Permanent Magnets based on Fe-Pt Alloys
P.D. Thang, E. Brück, K.H.J. Buschow, F.R. de Boer
Financial support by STW
2
Contents
Introduction
Permanent magnets, motivation
Experimental
Sample preparation, analysis techniques
Results
Structure, magnetic and mechanical properties
Conclusions
3
Introduction. What’s permanent magnet ?
Magnetic materials
Large Mr, high Hc
High (BH)max
e.g:
SmCo5 (1969),
Sm2Co17,
Nd2Fe14B (1983)
• B = µ0(M+H)
BM
4
Applications of permanent magnets
Automobile industry.
Computer industry.
Scientific research.
Biomedical treatment.
and many other applications.
5
Dental applications
Denture retention system
Soft magnetic: Pd-Co
Magnet: NdFeB(DYNA Dental Engineering B.V.)
Aim: to find a magnet and a soft magnetic material with
Good mechanical properties
High corrosion resistance
Magnet problems
Very brittle
Low corrosion resistance
6
Introduction. Fe-Pt alloys
Good mechanical strength and high corrosion resistance
fcc statistical distribution high magnetisation
fct layer structure high magnetic anisotropy
fcc - fct phase transition
7
Experimental
Sample processing
Preparation FexPt100-x and (Fe0.6Pt0.4)100-xMx based alloys
- Arc-melting the pure elements (3N) in Ar
- Casting to cylinder, disc
Heat treatment:
- Homogenisation (as-quenched sample):
1325C/1h, under Ar + quenching in water fcc phase.
- Ageing (aged sample):
500-700C + quench in water fct phase.
8
Analysis techniques
Tensile strength: tensometer
Magnetic properties: hysteresis-loop
Microstructure analysis: TEM ... and … SANS
Applied force
9
Results. Crystallographic structure
As-quenched: presence of the fct phase.
Ageing: fcc-fct transformation.
Fe60Pt40
20 30 40 50 60 70 80 90
10h
5h
2h
625°C, 1h
as-quenched
(002)(202)
(222)(001)
fct
(311)(200)(111)
(110)
(113)
(202)(112)(201)
(222)
(311)fctfct
fct
(220)
(200)(111)
Inte
nsity (arb
. unit)
2 (deg.)
10
Optimal hard-magnetic properties for Fe60Pt40 aged at 625C, 1h:
Br = 0.97 T, BHc = 294 kA/m, (BH)max = 118 kJ/m3
Permanent-magnet properties: FexPt100-x
0.4
0.6
0.8
1.0
1.2
BH
c (kA/m)
600OC 625
OC
650OC
Br (T)
80
160
240
320
0 2 4 6 8 100
30
60
90
120
Ageing time (h)
(BH)max
(kJ/m3)
0.00
0.25
0.50
0.75
1.00
Br (T)
0
80
160
240
320
BHc (kA/m)
50 55 60 65 700
30
60
90
120
(BH)max (kJ/m3)
Fe content x (at. %)
11
Microstructure: Fe60Pt40
Dark field image: Fct particle size increases during the ageing.
Fct nano-size observed.
Selected area diffraction pattern: Degree of atomic order increases during the ageing.
625°C, 1h: 2-5 nmas-quenched: 1-3 nm
12
Magnetic properties: (Fe0.6Pt0.4)100-xMx (M = Nb, Al)
0.5% Nb aged at 625C, 24h: Br= 0.98 T, BHc= 302 kA/m, (BH)max= 125 kJ/m3
0.25% Al aged at 525C, 24h : Br= 1.02 T, BHc= 300 kA/m, (BH)max= 132 kJ/m3
0.8
0.9
1.0
(Fe0.6Pt0.4)100-xNbx
Br (
T)
100
200
300
BH
c (k
A/m
)
0 1 2 3 4 50
40
80
120
(BH
) max
(kJ
/m3 )
Nb content (x)
0.8
0.9
1.0
(Fe0.6
Pt0.4
)100-x
Alx
Br (
T)
200
250
300
BH
c (k
A/m
)
0.00 0.25 0.50 0.75 1.000
40
80
120
(BH
) max
(kJ
/m3 )
Al content (x)
13
Thermomagnetic analysis: (Fe0.6Pt0.4)100-xMx
0.5 at. % Nb:
• Tc (as-quenched) 340C
• Tc (aged) 400C
0.25 at. % Al:
• Tc (as-quenched) 380C
• Tc (aged) 400C
0 100 200 300 400 500
(Fe0.6
Pt0.4
)100-x
Nbx
x=0.5, aged
x=5x=3x=1
x=0.5
x=0.25
M
(arb
. unit)
T (°C)
0 100 200 300 400 500
(Fe0.6Pt0.4)100-xAlx
x=0.25, aged
x=1
x=0.5
x=0.25
x=0.1
M
(ar
b. u
nit)
T (°C)
14
Temperature dependence: (Fe0.6Pt0.4)100-xMx
• Nb: = -0.04 %/K for Br, = -0.12 %/K for BHc and = -0.12 %/K for (BH)max.
• Al: = -0.06 %/K for Br, = -0.15 %/K for BHc and = -0.17 %/K for (BH)max.
0.3
0.6
0.9
1.2
M = Nbx = 0.5
Br (T)
100
200
300
400
500
BHc(kA/m)
0 100 200 300 4000
50
100
150
200
T (K)
(BH)max(kJ/m3)
0.3
0.6
0.9
1.2
M = Alx = 0.25
Br (T)
100
200
300
400
500
BH
c(kA/m)
100 200 300 4000
50
100
150
200
T (K)
(BH)max(kJ/m3)
15
Mechanical properties: (Fe0.6Pt0.4)100-xNbx
• Hardness
• Tensile strength of 0.5% Nb
0.0 0.5 1.0 1.5 2.0 2.5 3.0250
300
350
400
450
500
550(Fe
0.6Pt
0.4)100-x
Nbx
Nb content x (at. %)
Har
dnes
s (H
V)
as-quenched aged
0 50 100 150 200 250 300 350
0
100
200
300
400
----- as-quenched sample----- aged sample
Str
ess
(MP
a)
Load time (s)
16
Mechanical properties: comparison
Hardness: comparable to file band or of cutting tools.
Tensile strength:
0
100
200
300
400
500
600
700
800
900Ni-Cr
64%Au-Ag-Cu
CuAg0.08%
Fe-Pt aged
Au-Pt-Pd
Fe-Pt aq
Alumina
790
550
400350
260
150120
Ten
sile
stren
gth
(MP
a)
Alloys
17
Microstructure: (Fe0.6Pt0.4)99.5Nb0.5
as-quenched: 1 nm 625°C,12h: 1-3 nm
625°C,24h: 3-8 nm 625°C,48h: 8-16 nm
18
Microstructure: (Fe0.6Pt0.4)99.75Al0.25
as-quenched:
2 nm
525°C, 24h:
3-7 nm
100 nm
High coercivity: correlated with the magnetic anisotropy, i.e. the atomic order in the fcc/fct and fct grain growth.High remanence: originated by the exchange coupling of the soft fcc phase with the nano-sized hard fct phase.
19
What’s neutron and why SANS ?
Properties mn = 1.674710-24 g
n = 9.6628610-27 J/T
q = 0, 1/2 = 624 s
Interaction with matter - Scattering from the atomic nucleus
- Magnetic scattering
SANS: Small angle neutron scattering
- 10 Å 102 Å
- Domain and particle size
q = kf - ki
I(q,) = A(q) + B(q)sin2(/2)
nuclear scatterin
g
magnetic scattering
20
2D SANS images: Fe59.7Pt39.8Nb0.5
B = 0 (virgin) B = 1.8 T (in field) B removed (remanent)
As-
que
nch
edA
ged
At 5m
21
Reduced SANS data: Fe59.7Pt39.8Nb0.5
I(Q): difference in the virgin, field and remanent states
I(Q): difference for the as-quenched and aged samples
As-quenched Aged
1E-3 0.01 0.1 110
0
101
102
103
104
105
106
virgin
in field
remanence
I (c
m-1)
Q (nm-1)
1E-3 0.01 0.1 110
0
101
102
103
104
105
106
virgin
in field
remanence
I (c
m-1)
Q (nm-1
)
22
SANS analysis
Model fitting: monodisperse or polydisperse model ?
Virgin state Field state
SANS dominated by particles with different magnetisation:
polydisperse model
SANS dominated by randomly oriented magnetic domains:
monodisperse model
f c t f c c
Magnetic domains
B
Particles
23
Model fitting: Fe59.7Pt39.8Nb0.5 aged
Mono.: domain size ~ 100 nm.
Poly.: fct particles R = 6 nm.
(TEM: fct particles 3-8 nm)
1E-3 0.01 0.1
101
102
103
104
105
virgin & mono. cal. field & poly. cal. rem. & poly. cal.
I (cm
-1)
Q (nm-1)
0 50 100 150
0
20
40
60
80
100
p
r (nm)
monodisperse
Correlation length 105 nm
0 10 20 30 40 50 600.00
0.01
0.02
0.03
0.04
(x2): field remanence
D
R (nm)
6 nm polydisperse
24
Conclusions
Best permanent magnets obtained with (Fe0.6Pt0.4)100-xMx:
M = 0.5 at. % Nb and 0.25 at. % Al.
Good thermal stabilisation.
Fe67Pt33: soft magnetic properties.
Good mechanical properties.
Suitable for biomedical applications, e.g. denture retention.
Coexistent nanostructure observed by TEM and SANS:
High coercivity: correlated with the atomic order in the fcc/fct
structures and the fct grain growth.
High remanence: originated by the exchange coupling of the
soft fcc phase with the nano-sized hard fct phase.