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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. Introduction Permanent magnets, motivation Experimental Sample preparation, analysis techniques Results Structure, magnetic and mechanical properties

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Permanent magnets based on fe pt alloys

Permanent Magnets based on Fe-Pt Alloys

P.D. Thang, E. Brück, K.H.J. Buschow, F.R. de Boer

Financial support by STW


Contents

Introduction

Permanent magnets, motivation

Experimental

Sample preparation, analysis techniques

Results

Structure, magnetic and mechanical properties

Conclusions

Contents


Introduction what s permanent magnet
Introduction. What’s permanent magnet ?

  • Magnetic materials

    • Large Mr, high Hc

    • High (BH)max

      e.g:

      SmCo5 (1969),

      Sm2Co17,

      Nd2Fe14B (1983)

B

M

  • B = µ0(M+H)


Applications of permanent magnets
Applications of permanent magnets

  • Automobile industry.

  • Computer industry.

  • Scientific research.

  • Biomedical treatment.

  • and many other applications.


Dental applications
Dental applications

  • Denture retention system

    • Soft magnetic: Pd-Co

    • Magnet: NdFeB

      (DYNA Dental Engineering B.V.)

  • Magnet problems

    • Very brittle

    • Low corrosion resistance

  • Aim: to find a magnet and a soft magnetic material with

    • Good mechanical properties

    • High corrosion resistance


Introduction fe pt alloys
Introduction. Fe-Pt alloys

  • fcc - fct phase transition

fcc

  • statistical distribution

  • high magnetisation

fct

  • layer structure

  • high magnetic anisotropy

  • Good mechanical strength and high corrosion resistance


Experimental
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):

    1325C/1h, under Ar + quenching in water  fcc phase.

    - Ageing (aged sample):

    500-700C + quench in water  fct phase.


Analysis techniques
Analysis techniques

Applied force

Tensile strength: tensometer

Magnetic properties: hysteresis-loop

and … SANS

Microstructure analysis: TEM ...


Results crystallographic structure
Results. Crystallographic structure

  • Fe60Pt40

  • As-quenched: presence of the fct phase.

  • Ageing: fcc-fct transformation.


Permanent magnet properties fe x pt 100 x
Permanent-magnet properties: FexPt100-x

Optimal hard-magnetic properties for Fe60Pt40 aged at 625C, 1h:

Br = 0.97 T, BHc = 294 kA/m, (BH)max = 118 kJ/m3


Microstructure fe 60 pt 40
Microstructure:Fe60Pt40

625°C, 1h: 2-5 nm

as-quenched: 1-3 nm

  • 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.


Magnetic properties fe 0 6 pt 0 4 100 x m x m nb al
Magnetic properties: (Fe0.6Pt0.4)100-xMx (M = Nb, Al)

  • 0.5% Nb aged at 625C, 24h: Br= 0.98 T, BHc= 302 kA/m, (BH)max= 125 kJ/m3

  • 0.25% Al aged at 525C, 24h : Br= 1.02 T, BHc= 300 kA/m, (BH)max= 132 kJ/m3


Thermomagnetic analysis fe 0 6 pt 0 4 100 x m x
Thermomagnetic analysis: (Fe0.6Pt0.4)100-xMx

  • 0.5 at. % Nb:

  • Tc (as-quenched)  340C

  • Tc (aged)  400C

  • 0.25 at. % Al:

  • Tc (as-quenched)  380C

  • Tc (aged)  400C


Temperature dependence fe 0 6 pt 0 4 100 x m x
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.


Mechanical properties fe 0 6 pt 0 4 100 x nb x
Mechanical properties: (Fe0.6Pt0.4)100-xNbx

  • Hardness

  • Tensile strength of 0.5% Nb


Mechanical properties comparison
Mechanical properties: comparison

  • Hardness: comparable to file band or of cutting tools.

  • Tensile strength:


Microstructure fe 0 6 pt 0 4 99 5 nb 0 5
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


Microstructure fe 0 6 pt 0 4 99 75 al 0 25
Microstructure: (Fe0.6Pt0.4)99.75Al0.25

100 nm

as-quenched:

2 nm

525°C, 24h:

3-7 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.


What s neutron and why sans
What’s neutron and why SANS ?

  • Properties

    mn = 1.674710-24 g

    n = 9.6628610-27 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 scattering

magnetic scattering


2d sans images fe 59 7 pt 39 8 nb 0 5
2D SANS images: Fe59.7Pt39.8Nb0.5

As-quenched

B = 0 (virgin)

B = 1.8 T (in field)

B removed (remanent)

At 5m

Aged


Reduced sans data fe 59 7 pt 39 8 nb 0 5
Reduced SANS data: Fe59.7Pt39.8Nb0.5

As-quenched

Aged

  • I(Q): difference in the virgin, field and remanent states

  • I(Q): difference for the as-quenched and aged samples


Sans analysis
SANS analysis

  • Model fitting: monodisperse or polydisperse model ?

  • Virgin state

  • Field state

Particles

Magnetic domains

SANS dominated by randomly

oriented magnetic domains: monodisperse model

SANS dominated by particles with different magnetisation: polydisperse model


Model fitting fe 59 7 pt 39 8 nb 0 5 a ged
Model fitting:Fe59.7Pt39.8Nb0.5 aged

monodisperse

Correlation length 105 nm

6 nm

polydisperse

  • Mono.: domain size ~ 100 nm.

  • Poly.: fct particles R = 6 nm.

    (TEM: fct particles 3-8 nm)


Conclusions
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.


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