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submitted as paper BE 01 to the Intermag 2002 Conference, Amsterdam, April 2002<
Top Nd-Fe-B Magnets: >56 MGOe, Energy
Density, 9.8 kOe Coercivity
W. RODEWALD, B. WALL, M. KATTER, and K. UESTUENER
Abstract—Sintered Nd-Fe-B magnets are prepared by powder
metallurgy. Besides the composition of the alloy, the alignment of
the powder particles is decisive for achieving high remanent
polarizations. The alignment of the powder particles can be
improved by applying field pulses in addition to an alignment in
a constant magnetic field. Alternating field pulses parallel and
antiparallel to the easy axis proved to be more efficient than
unidirectional field pulses. By such a refined processing route
sintered Nd-Fe-B magnets with a remanent polarization of
1.519 T, a coercivity H cJ of 7.8 kA/cm (9.8 kOe) and a maximum
energy density of 451 kJ/m³ (56.7 MGOe) could be prepared.
Index Terms—Alignment coefficient, energy density, magnets,
temperature measurement.
M
I. INTRODUCTION
ODERN permanent magnets enable the design of strong
motors, disk drives for data storage in computers or
various accessories for automobiles. In most applications the
magnets should be as powerful as possible in order to
optimize the efficiency of a magnet assembly and to save
weight or volume, respectively. Up to now sintered Nd-Fe-B
magnets achieve the highest energy densities.
The maximum energy density is mainly determined by the
remanent polarization and the reversible permeability. The
J
remanent polarization at 20 °C can be calculated from:
r
J r ( 20°
C)
= J S ( 20°
C)
⋅ ⋅ ( 1−
Vnonmag
ρ0
⋅ fϕ
J s ( 20 ° C)
, ρ0
Vnonmag fϕ
ρ
)
(1)
ρ , and 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:
Manuscript received February 10, 2002.
W. Rodewald is with Vacuumschmelze GmbH & Co KG, P. O. Box 2253,
D 63412 Hanau, Germany (telephone: + 49 6181 38-2482, e-mail:
werner.rodewald@vacuumschmelze.com).
B. Wall is with Vacuumschmelze GmbH & Co KG, P. O. Box 2253,
D 63412 Hanau, Germany (e-mail: boris.wall@vacuumschmelze.com).
M. Katter is with Vacuumschmelze GmbH & Co KG, P. O. Box 2253,
D 63412 Hanau, Germany (e-mail: matthias.katter @vacuumschmelze.com).
K. Uestuener is with Vacuumschmelze GmbH & Co KG, P. O. Box 2253,
D 63412 Hanau, Germany (e-mail: kaan.uestuener@vacuumschmelze.com).
f = cosϕ
with
ϕ
⎛
⎜ J
= arctan 2
⎜ J
⎝
⎞
⎟
⎟
⎠
r⊥
ϕ (2)
and represents the average misalignment angle ϕ of the
grains with respect to the easy axis [1]. The precondition is a
homogeneous and cylindrical symmetrical distribution of the
misaligned grains. Isostatically pressed magnets meet these
preconditions fairly well and achieve alignment coefficients
up to 98 %. For magnets pressed in a transverse magnetic
field, however, the perpendicular components of the remanent
J
polarization parallel or transverse to the pressing
r⊥
direction are different. Hence for transverse pressed magnets
two alignment coefficients have to be distinguished.
The alignment coefficient can be easily determined from the
remanent polarization measured parallel J r and
J
perpendicular with respect to the easy axis of an
r⊥
anisotropic magnet. The impact of different powder alignment
procedures on the alignment coefficient and hence on the
remanent polarization or maximum energy density,
respectively, are reported.
II. EXPERIMENTAL
Anisotropic magnets were prepared by blending
Nd12.7Dy0.03Fe80.7TM0.8B5.8 and Nd13.7Dy0.03Fe79.8TM0.8B5.7
alloy-powders, TM: Al, Ga, Co, Cu, in different ratios in order
to optimize the RE-concentration in sintered magnets. The
alloy-powders were milled in inert atmosphere to an average
particle size in the range between 3 and 5 µm. The blends
were aligned in a magnetic field of 13 kA/cm and compacted
to about 29 % of the theoretical density. In order to improve
the alignment of the powder particles, magnetic field pulses
with a peak field strength of about 64 kA/cm, t ≈ 10 ms, were
applied parallel to the easy axis of the powder compacts
repeatedly. For comparison another set of powder compacts
got an additional alignment by similar field pulses which were
directed at first antiparallel and afterwards parallel to the easy
axis of the powder compacts. The number of the field pulses
has been varied. All powder compacts are pressed
isostatically.
The green compacts were sintered at temperatures between
1050 and 1100 °C for 4 h and annealed at temperatures
between 500 and 600 °C for 1 h.
r
1

submitted as paper BE 01 to the Intermag 2002 Conference, Amsterdam, April 2002<
The magnetic properties were deduced from
demagnetization curves J(H), recorded in a hysteresisgraph on
test samples 9 mm in diameter, 6 mm in thickness at
temperatures between 20 and 80 °C. The alignment
coefficients have been determined from the remanent
polarizations measured parallel J and perpendicular
r
with respect to the easy axis of the anisotropic magnets.
III. RESULTS
Due to liquid phase sintering, the Nd-Fe-B magnets achieve
easily densities between 7.55 and 7.6 g/cm³ or ρ ρ0
> 99 %,
respectively. The fraction of impurities, for instance Ndoxides
or - nitrides, could be decreased to

submitted as paper BE 01 to the Intermag 2002 Conference, Amsterdam, April 2002<
20 °C 80 °C
-12 -10 -8 -6 -4 -2 0
Fig. 3. Demagnetization curves J(H) and B(H) of a sintered Nd-Fe-B magnet
with a remanent polarization of 1.519 T, a coercivity of 7.8 kA/cm and a
maximum energy density of 451 kJ/m³.
D. Microstructure
magnetic field strength in kOe
Prerequisite for a strong coercivity of a magnet is a
homogeneous fine grained mircostructure. The enhancement
of the coercivity by additions of heavy RE-metals like Dy or
Tb, is not appropriate, since they decrease the saturation
polarization of the Nd2Fe14B compound. Hence the content of
heavy RE additions must be minimized. The average grain
size must be controlled by the sintering conditions in order to
achieve a small grain size. Fig. 4 reveals the microstructure of
a magnet with an energy density of 451 kJ/m³. The average
grain size amounts to 4.6 µm. Only a few grains have
dimensions up to 25 µm. The RE-rich constituents are
homogeneously distributed.
Fig. 4 Microstructure of a sintered Nd-Fe-B magnet with a maximum energy
density of 451 kJ/m³ with an average grains size of 4.6 µm and a few grains
with dimensions up to 25 µm.
However, if the amount of RE-rich constituents is
1,6
1,4
1,2
1,0
0,8
0,6
0,4
0,2
0,0
increased, the grain growth is enforced. That results in an
average grains size of 4.9 µm, see Fig. 5. In addition there are
some large grains with dimensions up to 330 µm, what
increases the reversible permeability and hence decreases the
maximum energy density.
Fig. 5 Microstructure of a sintered Nd-Fe-B magnet with a maximum energy
density of 440 kJ/m³ with an average grains size of 4.9 µm and some large
grains with dimensions up to 330 µm.
IV. CONCLUSIONS
The manufacture of sintered Nd-Fe-B magnets with high
energy densities requires almost ternary Nd-Fe-B alloys with a
negligible amount of impurities. During powder metallurgical
processing the optimum alignment of the powder particles
could be achieved by the application of additional field pulses
with alternating polarity. Probably the powder particles
experience a torque in the direction of the magnetic field and
an impact by magnetic field gradients, so that they can
overcome the friction between the particles and agglomerates
are disintegrated simultaneously.
The minimization of the RE-rich constituents enlarges the
fraction of the hardmagnetic compound, but also promotes a
fine grained mircostructure.
ACKNOWLEDGMENT
The authors are grateful to M. Lemcke for the
metallographic analysis and acknowledge the technical
assistance of R. Langer, S. Stein and J. P. Jacquet.
REFERENCES
[1] W. Fernengel, A. Lehnert, M. Katter, W. Rodewald, B. Wall,
“Examination of the degree of alignment in sintered Nd-Fe-B magnets
by measurements of the remanent polarizations,” J. Magn. Magn. Mat.
vol. 157, pp. 19-20, 1996.
[2] Y. Kaneko, “Highest performance of Nd-Fe-B magnet over 55 MGOe,”
IEEE Trans. Mag. vol. 36, pp. 3275 – 3278, 2000.
[3] M. Sagawa, H. Nagata, “Novel processing technology for permanent
magnets”, IEEE Trans. Mag. vo. 29, pp. 2747 – 2751, 1993.
[4] W. Rodewald, M. Katter, B. Wall, R. Blank, G. W. Reppel, H. D. Zilg,
“Dependence of the coercivity HcJ of heigh energy Nd-Fe-B magnets on
the alignment coefficient”, IEEE Trans. Mag. vol 36, pp. 3279 – 3281,
2000.
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