PROPERTIES AND APPLICATIONS OF HIGH ... - Vacuumschmelze

PROPERTIES AND APPLICATIONS OF HIGH PERFORMANCE MAGNETS
W. RODEWALD AND M. KATTER
Vacuumschmelze GmbH & Co. KG, P. O. B. 2253, 63412 Hanau, Germany
Corresponding author e-mail: werner.rodewald@vacuumschmelze.com
Abstract
Neodymium-Iron-Boron (Nd-Fe-B) magnets are produced by powder metallurgy in large quantities
and in many different shapes. Although the Curie temperatures of commercial Nd-Fe-TM-B alloys,
TM: transition metals, range only between 310 and 360 °C, modern Nd-Fe-B magnets can be operated
at temperatures up to 230 °C. The basic features, which determine the temperature stability of
Rare Earth (RE) magnets are discussed in detail.
Today many permanent magnet (pm) motors operate a large variety of machines, propel ships or adjust
small sensors. In particular in motors and positioning devices the cogging torque of pm motors
must be reduced. By novel and economic butterfly-shaped magnets, the cogging torque can be
minimized.
Sometimes RE-magnets need to be operated in hot and humid climates, what may result in corrosion.
In particular the RE-rich constituents in sintered Nd-Fe-B magnets are prone to oxidation and
Hydrogen embrittlement, what may result in a disintegration of surface layers. The basic principles
of the corrosion mechanism in hot and humid climates are reviewed. By replacing the Nd-rich constituents
by more stable compounds with a smaller electronegative potential, the corrosion stability
of the sintered magnets could be improved significantly. New Nd-Fe-B magnet grades have a similar
corrosion stability than common steels. In harsh operating climates, however, the Nd-Fe-B magnets
need protection by proper coatings, which will be reviewed.
In order to save weight or volume of magnet assemblies or in order to optimize efficiency, the maximum
energy density of sintered Nd-Fe-B magnets has been increased by refined processing technologies.
Nd-Fe-B magnets with a typical coercivity HcJ of 9.5 kA/cm achieve maximum energy
densities up to 415 kJ/m³.
Introduction
Permanent magnets have become a key-component in many devices. Due to the high maximum energy
density Rare Earth (RE) magnets are able to drive small motors in watches as well as strong
motors in electric vehicles or huge electric machines for the propulsion of ships. Besides a large variety
of motors high performance magnets are applied for fast and accurate positioning of the read- and
writing heads in hard disc drives, to focus charged particle beams for medical or scientific investigations
or to convert electrical signals into mechanical forces or vice versa.
In many applications, RE magnets have to operate at elevated temperatures or in hot and humid climates.
Hence the continuous maximum operating temperature as well as the temperature and corrosion
stability determine the selection of the permanent magnet material. The basic features, essential
for the temperature and corrosion stability of RE magnets, are reviewed.
In the meantime heavy and large hydraulic devices are going to be replaced by permanent magnet
motors, for instance in electronic power steering of vehicles or electronically assisted braking. In
such motors the cogging torque must be minimized. Novel and economic butterfly-shaped Nd-Fe-B
magnets meet such specifications, what will be demonstrated on permanent magnet motors.
In order to save weight or volume of magnet assemblies or in order to optimize efficiency, the maximum
energy density of sintered Nd-Fe-B magnets has been increased by refined processing technologies
at the expense of reduced operating temperatures between 50 and 80 °C.
1 Temperature dependence of permanent magnets
The maximum continuous operating temperature of permanent magnets depends on the temperature
coefficients TC(Br) of the remanent polarization, TC(HcJ) of the coercivity and the load line B/µ0H of
the magnet or the magnet assembly. In general the maximum operating temperature of the different

permanent magnets increases proportional to the Curie temperature of the magnet materials, see
Fig. 1.
max. operating temperature in °C
600
500
400
300
200
100
0
Hard Ferrites
(Nd,Dy) 2Fe 14B
Sm 2(Co, Cu,Fe,Zr) 17
SmCo 5
AlNiCo
0 200 400 600 800 1000
Curie temperature in °C
Figure 1: Maximum operating temperatures of permanent magnets in dependence on their Curie temperatures. The shaded
lines give the lowest or the highest operating temperatures for different permanent magnet materials, respectively.
Most of the RE magnets are produced by powder metallurgy in many different grades and shapes.
The alloys are melted from RE metals, Fe and Fe-B master-alloys in vacuum-induction furnaces. The
polycrystalline microstructure of the ingots implies, that the magnetic moments are distributed randomly.
In order to achieve a well defined texture, powder metallurgy is applied. The alloys are crushed
and milled to a fine alloy powder, that must consist of single crystals. In general alloy powders
with an average particle size in the range 3 to 5 µm meet this requirement very well. Such alloy
powders can be aligned by a magnetic field, compacted by cold isostatic pressing (CIP) to blocks, by
axial (AP) or transverse (TP) field die-pressing to net-shape parts and sintered to compact anisotropic
magnet materials [1 - 3].
For many applications the maximum energy density is decisive. At room temperature Nd-Fe-B as
well as Nd-Dy-Fe-TM-B magnets with strong coercivities HcJ >24 kA/cm, TM: transition metals,
have got higher maximum energy densities than SmCo5 or Sm2(Co,Cu,Fe,Zr)17 magnets, see Fig. 2.
But at temperatures of approximately 120 °C sintered Sm2(Co,Cu,Fe,Zr)17 magnets are superior.

max. energy density in kJ/m³
450
400
350
300
250
200
150
100
50
0
0 50 100 150 200 250 300 350
temperature in °C
SmCo5 SmCo5 Sm2(Co,Cu,Fe.Zr)17
Sm2(Co,Cu,Fe,Zr) 17
Nd2Fe14B (Nd,Dy)2(Fe,Co)14B
(Nd,Dy) 2(Fe,Co) 14B
Figure 2: Maximum energy density (BH)m of RE magnets in dependence on the temperature.
2 Temperature stability of sintered RE Magnets
The temperature stability of permanent magnets is determined by reversible and irreversible polarization
losses of the magnet material and of the load line B/µ0H of the magnet or of the magnet assembly,
respectively.
2.1 Reversible polarization changes
The reversible polarization changes are represented by the temperature coefficient of the remanent
polarization TC(Br), see Fig. 3. For temperatures between 20 and 100 °C typical temperature coefficients
TC(Br) range from -0.08 to -0.11 %/K for sintered Nd-Dy-Fe-Co-TM-B magnets and amount
to -0.04 or -0.03 %/K for sintered SmCo5 or Sm2(Co,Cu,Fe,Zr)17 magnets, respectively.
2.2 Irreversible polarization losses due to the temperature dependence of the coercivity HcJ .
Irreversible polarization losses are mainly caused by the temperature dependence of the coercivity
HcJ and the thermal aftereffect. With increasing temperature the coercivity HcJ of RE magnets decreases
and as a consequence the magnetization of grains, which have smaller coercivities, may be
reversed. Such irreversible polarization losses do not depend on time, see Fig. 4. Hence irreversible
polarization losses can be anticipated by an annealing treatment of the magnetized magnets. As a rule
of thumb, such ageing treatments are performed at temperatures 10 to 20 K above the maximum operating
temperature in order to stabilize the magnet or the magnet assembly, respectively.
Typical temperature coefficients of the coercivity HcJ range from -0.50 to -0.75 %/K for sintered Nd-
Dy-Fe-Co-TM-B magnets and amount to -0.14 to -0.25 %/K for sintered SmCo5 or -0.15 to
-0.18 %/K for Sm2(Co,Cu,Fe,Zr)17 magnets for temperatures between 20 and 100 °C.
Minor polarization losses are due to the thermal aftereffect or magnetic viscosity, respectively, and
depend on time logarithmically [4]. Fig. 4 gives the relative irreversible polarization losses ∆Jirr/J in
dependence on the exposition time at a temperature of 130 °C for sintered Nd14.3DyFe76.7B8 magnets

[5]. The irreversible polarization losses ∆Jirr due to the temperature dependence of the coercivity HcJ
and due to the thermal aftereffect can be recovered by remagnetization.
TC(B r ) =
TC(H cJ ) =
in %/K, T 0 < T < T 1
20°C 60°C 80°C
100°C
-20 -15 -10 -5 0
magnetic field strength H in kA/cm
120°C
Figure 3: Dependence of the demagnetization curves J(H) and B(H) of VACODYM ®*) 722 HR magnets on the
temperature. The typical magnetic properties amount to: Br = 1.47 T, HcJ = 9.5 kA/cm, (BH)max = 415 kJ/m³ at 20 °C.
rel. polarization losses dJ irr/J in %
0
-5
-10
-15
-20
-25
Nd 14.3DyFe 76.7B 8 magnets
Aging temperature: 130 °C
1 10 100 1000 10000
exposure time in h
B/µoH = -2.0
B/µoH = -1.1
B/µoH = -0.5
Fig. 4: Irreversible polarization losses of sintered Nd14.3DyFe76.7B8 magnets with different load lines B/µoH at a
temperature of 130 °C in dependence of the exposition time.
® registered trade mark of VACUUMSCHMELZE GmbH & Co. KG, *) licenser Sumitomo Special Metals Corp.
1,6
1,2
0,8
0,4
0,0
-0,4
-0,8
polarization J, induction B in T

2.3 Irreversible polarization losses due to microstructural changes
However, irreversible polarization losses due to the deterioration of the magnet surface, for instance
by oxidation, by improper cutting and grinding or by changes of the microstructure due to high temperatures,
cannot be eliminated by remagnetization. Fig. 5 demonstrates the microstructure of an
oxidized surface and the impact on the demagnetization curve J(H) [6]. Due to the oxidized surface
layer the coercivity HcJ within this surface volume is strongly reduced. In particular the surface volume
parallel to the easy axis of the magnet may be demagnetized by stray fields, what results in a
step of the demagnetization curve J(H), see Fig. 5 on the left. The height of the step depends on the
volume of the deteriorated surface layer parallel to the easy axis on relation to the volume of the
magnet. In particular small magnets may experience essential changes in the demagnetization curve
J(H).
surface
interface between
oxidized layer an
undamaged magnet
perfect magnet
selectively oxidized
magnet
polarization M
magnetic field strength H i
step from
side surface
Abb. 5: Influence of the oxidized surface grain layer on the demagnetization curve J(H) of sintered RE magnets. The
enlarged microstructure of an oxidized layer of a Nd-Dy-Fe-TM-B magnet after annealing in air at 500 °C for 23 h
demonstrates, that about one layer of grains is deteriorated [6].
3 Commercial Nd-Dy-Fe-TM-B magnets for high operating temperatures
The temperature stability of sintered Nd-Fe-TM-B magnets can be improved by stronger coercivities
and higher Curie temperatures. Partial substitutions of Nd by Dy and of Fe by Co increase the coercivity
and the Curie temperature, respectively. Besides small additions of Al, Cu, Ga, Sn, Ti, Nb, Mo
a. s. o. promote the coercivity by refining the microstructure of the sintered magnets [1 - 3, 7].
3.1 Magnetic Properties
Optimized additions of Dy and Co results in RE magnets with a remanent polarization of 1.08 T for
net shape magnets and a coercivity of 28.6 kA/cm at 20 °C. The maximum energy density amounts
to 225 kJ/m³, see Fig. 6. With increasing temperatures a decrease of the coercivity cannot be prevented,
but at 150 °C, the coercivity still amounts to about 12 kA/cm.
The irreversible polarization losses in dependence on the temperature are compiled in Fig. 7 for
magnets with different load lines B/µ0H. Due to the high coercivity a maximum operating temperature
of 230 °C results for magnets with a load line B/µoH = -2.

20°C
120°C 150°C 180°C 210°C
-20 -15 -10 -5 0
magnetic field strength H in kA/m
240°C
Figure 6: Demagnetization curves J(H) and B(H) of sintered net shape Nd-Dy-Fe-Co-TM-B magnets, grade
VACODYM 688 AP, in dependence on the temperature. Typical magnetic properties at 20 °C amount to: Br = 1.08 T,
HcJ = 28.6 kA/cm, (BH)max = 225 kJ/m³.
irrev. polarization losses in %
0
-2
-4
-6
-8
-10
temperature in °C
1,6
1,2
0,8
0,4
0,0
-0,4
-0,8
0 50 100 150 200 250 300
B/µ 0 H = 0 - 0,5 -1
-2
Figure 7: Irreversible polarization losses of sintered net shape Nd-Dy-Fe-Co-TM-B magnets, grade VACODYM 688 AP,
in dependence on the temperature for different load lines B/µ0H.
polarization J, induction B in T

3.2 Special shapes for the reduction of the cogging torque in motors
Most of the Nd-Dy-Fe-Co-TM-B magnets with strong coercivities are applied in motors. The rotors
of such motors are assembled from thin platelets of high coercivity magnets, see Fig. 8 a), what can
be performed automatically. After assembly the rotors are magnetized.
Due to the interaction of the magnet poles with the stator slots, a cogging torque results. In order to
minimize the cogging torque skewed arc shape magnet segment may be applied, see Fig 8 b) in the
back. However the grinding of such skewed arc magnets consumes a lot of machining time due to
individual holding tools and hence increases the manufacturing costs. In order to reduce the machining
costs, novel butterfly-shaped magnets have been developed [8], see Fig 8 b) in the front.
Butterfly magnets can be produced with similar skewing angles as skewed arc segments, but the
grinding costs are more economic. Rotors may be assembled from single butterfly magnets or from
axially stapled butterfly magnets, see Fig. 8 b) in the front.
The cogging torques of 8-pole motors with different rotors are presented in Fig. 9. Rotors assembled
from straight segments or platelets achieve a cogging torque of about ±5 N · cm, whereas rotors
manufactured from skewed arc segments or from the novel butterfly magnets have got reduced cogging
torques of

cogging torque in N cm
6
4
2
0
-2
-4
-6
butterfly magnet skewed arc straight segment
180° 190° 200° 210° 220°
angle
230° 240° 250° 260° 270°
Figure 9: Cogging torques of 8-pole brushless dc-motors with rotors assembled from straight segments, skewed arc segments
or novel butterfly magnets, respectively.

4 Corrosion behaviour of sintered RE magnets
In comparison to sintered Sm-Co magnets Nd-Fe-B magnets are more sensitive to corrosion. As a consequence
all finished VACODYM magnets get a final surface treatment. Such magnets have been applied at
ambient conditions (room temperature, humidity up to 50 %, no condensation of moisture) without any
problems for more than 10 years in telephone transducers, for instance.
In humid atmosphere, however, the Nd-rich constituents may be converted to Nd-hydroxides and some
Hydrogen is released. The Hydrogen reacts immediately with the Nd-rich constituents and forms Ndhydrides.
In such a humid environment Nd-hydrides are not stable and are transformed into Ndhydroxides.
Since additional Hydrogen comes into existence the reaction cycle continues [9]. The formation
of Nd-hydrides results in a strong volume increase of the Nd-rich constituents, so that cracks occur
along the grain boundaries. The oxidation is accelerated at high temperatures. In the worst case the surface
of the sintered Nd-Fe-B magnets disintegrates.
As has been demonstrated by A. S. Kim [10] and B. Grieb [11] some time ago, the corrosion behaviour of
Nd-Fe-B magnets can be improved by proper additions of Co, Nb, V, Mo, Cu, Ga etc. By these additions
the Nd-rich constituents are transformed into more noble intermetallic compounds, see Fig. 10 inset, for
instance Nd3(Co,Cu), (Nd,Dy)5(Co,Cu,Ga)3, Nd6(Fe,Co)13Ga [9 - 13]. Since the free electrochemical potential
of these compounds were determined to be about 800 mV higher than that of pure Nd-metal, such
Nd-Fe-B magnets are much more stable in hot and humid climates [9]. The corrosion behaviour is similar
to pure iron or steels.
4.1 Corrosion tests
There are many different corrosion tests. A very common test is the Highly Accelerated Stress Test
(HAST) which is similar to the standard IEC 68-2-66. The test samples are stored at 130 °C, 95 % humidity,
2.6 bar water vapour pressure. Under these conditions no condensation of vapour occurs. The mass
losses per surface area of the magnets are monitored in dependence of the exposition time. Standard Nd-
Fe-B magnets suffer mass-losses of about 10 to 100 mg/cm² after 10 days, whereas the mass-losses of
VACODYM 6xx magnets with an improved corrosion resistance amount to

mass losses in mg/cm²
Matrix
Oxides
Nd-rich constituents
Nd-Fe-B Nd-Dy-Fe-TM-B
Nd 2 Fe 14 B Nd 2 (Fe,Co) 14 B
Nd 2O 3
Nd 2O 3
Nd Nd-(Co,Cu,Al) x
VACODYM 6XX
standard Nd-Fe-B
exposition time in days
Figure 10: Mass losses per surface area of sintered Nd-Fe-B magnets or Nd-Dy-Fe-Co-TM-B magnets with an improved corrosion
resistance in dependence on the exposition time. Highly Accelerated Stress Test (HAST): 130 °C, 95 % relative humidity,
2.6 bar vapour pressure. The inset gives a model of the microstructure of standard sintered Nd-Fe-B magnets or new Nd-Dy-Fe-
Co-TM-B magnets, respectively. The improved corrosion resistance is due to the replacement of the Nd-rich constituents by
more noble Ndx(Co,TM)y compounds.
Surface Thickness Method Colour Hardness Corrosion Temperature
Resistance Range
Sn >15 µm galvanic silver semibright HV 20 humid climate 10 µm galvanic silver semibright HV 350 humid climate 5 µm Ni +
>10 µm Sn galvanic silver semibright HV 20 humid climate 5 µm spray- yellow 4 H excellent climatic, 5 µm IVD yellow 4 H excellent climatic, 15 µm EPP black 4 H excellent climatic, 6 µm EPP black 4 H excellent climatic,

5 Sintered Nd-Fe-B magnets with high energy densities and their applications
The maximum energy density is determined by the remanent polarization and the reversible permeability
µrev:
( BH )
J
2
max = r
4 ⋅ µ o ⋅ µ rev
(1)
In order to achieve a high maximum energy density the remanent polarization has to be optimized. The
remanent polarization Jr is determined by:
J ( ) ( ) ρ
r 20°
C = J S 20°
C ⋅ ⋅ ( 1−
V ) f
ρ
nonmag ⋅ 0.
01⋅
ϕ
(2)
( ° C)
0
J s 20 , ρ ρ0
, Vnonmag and fφ
denote the saturation polarization of the Nd2Fe14B compound at 20 °C,
the density related to the theoretical density of the alloy, the fraction of nonmagnetic constituents and the
alignment coefficient, respectively.
The alignment coefficient is defined by:
⎛ ⎞
f ϕ = (cosϕ ⋅100%)
with ⎜ J r⊥
ϕ = arctan 2 ⎟
⎜ J ⎟
⎝ r ⎠
(3)
and represents the average misalignment angle φ of the grains with respect to the easy axis [14].
The precondition is a cylindrical symmetrical distribution of the misaligned grains. Magnets pressed isostatically
or in an axial magnetic field meet this precondition fairly well. For magnets pressed in a transverse
magnetic field the perpendicular components of the remanent polarization J r⊥
parallel or transverse to the
pressing direction are different. Hence for transverse pressed magnets two alignment coefficients have to
be distinguished. The alignment coefficients can be easily determined from the remanent polarization measured
parallel J and perpendicular J with respect to the easy axis of an anisotropic magnet [14].
r
r⊥
5.1 Magnetic properties
Optimized alloy-composition, proper alignment of the alloy-powders and controlled sintering and heat
treatments result in isostatically pressed Nd-Fe-B blocks with a remanent polarization between 1.42 and
1.47 T, coercivities of about 9.5 kA/cm and maximum energy densities between 380 and 415 kJ/m³, see
Fig. 3. Such magnets achieve alignment coefficients >96 %, what indicates an average misalignment angle
of

emanent polarization B r in T
1,5
1,4
1,3
1,2
1,1
1,0
VACODYM 7xx TP
VACODYM 7xx AP
8 10 12 14 16 18 20 22 24 26 28 30
coercivity H cJ(20°C) in kA/cm
VACODYM 6xx AP
VACODYM 6xx TP
Figure 11: Remanent polarization of sintered net-shape Nd-Fe-B parts, produced by transverse field (TP) or axial field (AP) diepressing
in dependence of the coercivity HcJ(20°C). VACODYM 7xx grades achieve high polarizations and maximum energy
densities. The maximum operating temperatures amount to 50 up to 150°. VACODYM 6xx grades have got stronger coercivities
HcJ and can be operated at temperatures between 110 and 230 °C.
5.2 Applications
Modern permanent magnets enable the design of small and fast disk drives for data storage in computers, a
large number of accessories for automobiles in order to improve safety or comfort, tiny and sensitive sensors,
transducers for acoustic applications, beam guidance assemblies for undulators in order to achieve
brilliant light sources with ultrashort wavelengths, optical insulators for contolling data transmission, and a
large variety of generators and motors. Some small motors with a diameter of 1.9 mm and a length of
5.5 mm speed up to >100 000 rpm and achieve a power of 60 mW for moving microdevices. Besides all
kinds of servo- and linear-motors, there are large machines with a diameter of 3.2 m and a length up to
9.5 m. They are rotating slowly at 120 rpm, but achieve a power of 5 MW for the propulsion of ships. There
are projects for machines with a power up to 18 MW.
Lately more flexible MRI systems have come into application. The patients are no longer confined in a
fixed solenoid or a C-shape magnet system. The magnet assembly resides underneath the patient table
when not in use. At the push of a button a permanent magnet system is elevated to scanning position, so
that intraoperative magnetic resonance imaging has become possible [17]. This procedure enables accurate
imaging during surgery.
Besides part body MRI systems have been developed in order to save time of whole body scanners for major
clinical examinations [18]. Even for pets a variety of MRI systems have come into operation.
Modern computer-controlled magnet assemblies are applied for the navigation of catheters or guidewires,
which contain small permanent magnets at the tip, throughout the cardiovascular system of a patient [19].
The tip of the interventional devices can be digitally controlled within 1 mm by the external fields and
hence facilitates the manipulation of catheters by the physician.
6. Summary
By optimizing the alloy-composition and the processing route Nd-Fe-TM-B magnets with high maximum
energy densities between 305 and 415 kJ/m³ can be produced. The remanent polarizations and coercivities
vary between 1.26 to 1.47 T and 19 to 9.5 kA/cm, grades VACODYM 7xx. Due to the excellent remanent
polarizations, these magnets have got smaller coercivities, what confines the maximum operating tempera-

tures between 50 and 150 °C. For applications which require superior temperature and corrosion stability
Nd-Dy-Fe-Co-TM-B magnets with strong coercivities between 14.4 and 28.6 kA/cm are produced. The
remanent polarizations and the maximum energy densities vary from 1.35 to 1.08 T and from 350 to
225 kJ/m³, grades VACODYM 6xx, for instance. The maximum operating temperatures of these magnet
materials range between 110 and 230 °C.
By refining the processing technology even Nd-Fe-B magnets with a maximum energy density of
453.6 kJ/m³, a remanent polarization of 1.519 T and a coercivity HcJ of 7.8 kOe can be prepared [16]. The
laboratory tests set the pace for refining the production routes. In the last two decades there was a continuous
improvement of the maximum energy density of sintered magnets from about 280 kJ/m³ in 1985 up to
415 kJ/m³ in 2002.
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