Table Of ContentIMPROVING THE PROPERTIES OF
PERMANENT MAGNETS
A Study of Patents, Patent Applications
and Other Literature
IMPROVING THE PROPERTIES OF
PERMANENT MAGNETS
A Study of Patents, Patent Applications
and Other Literature
Edited by
G.H.M. KOPER
and
MARTEN TERPSTRA
The Hague, The Netherlands
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British Library Cataloguing in Publication Data
Improving the properties of permanent magnets.
I. Magnetic properties
I. Koper, G. H. M. II. Terpstra, Marten
538.3
ISBN 1-85166-610-9
Library of Congress CIP data applied for
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v
PREFACE
The present study complements the study on patents, patent applications and other
literature on rare earth metals based permanent magnets by Frits Andriessen and
Marten Terpstra, published by Elsevier Applied Science in 1989, and complements
in part the book on Nd-Fe permanent magnets edited by LV. Mitchell, which was
the result of a workshop organized by the Commission of the European
Communities and held in Brussels on 25 October 1984.
The difference between the content of the first book and that of the present study
is that the first is more specifically directed to various kinds and compositions of
alloys used in newly developed magnets, while the present book emphasises the
improvements obtained when using particular alloys. The study edited by Mitchell
deals more specifically with the economic, physical and chemical aspects of rare
earth metals based magnet alloys, their properties compared with the more common
and classical magnets such as ferro-cobalt alloy magnets, and their applications to
various fields of technology.
From the present study it has become apparent that there exist only a few patents
and patent applications covering a specific use of particular magnets having specific
properties to a circuit, arrangement, device or electric motor. This appears to be due
to the fact that every manufacturer of such circuits or arrangements applying
magnets naturally wants to employ the most effective magnets.
Such a consideration, however, cannot be the subject of a patentable invention. In
connection with their high energy product, hysteresis properties and their relative
low specific gravity, magnets of the Nd-Fe family seem to be preferred to other
magnets used in high power electric motors, but because of their low Curie point
(± 350 K) such motors have to be cooled extremely, and that implies a substantial
increase in manufacturing cost. As a consequence, the trend in the application of
rare earth metals alloy magnets to high power devices looks to be directed to the
employment of Sm-alloyed magnet materials.
The three studies discussed above, either apart or in combination, provide a
comprehensive review of the state of the art on permanent magnets, particularly
rare earth metals based magnets, and may be a contribution to the labour of all
scientists and technologists who are trying to develop new materials for and to
improve the properties of permanent magnets.
MARTEN TERPSTRA
Vll
CONTENTS
Introduction. . . . . . . . . . . . . . . . . . . . . . . .
1. Methods to Enhance the Magnetic Properties of Magnets and Magnetic
Materials. . . . . . . . . . . . . . . . 10
1.1 Improvement of magnetic characteristics in general. 10
1.1.1 By specifically providing magnetic anisotropy . 10
1.1.2 By using specific compositions. . . . . 19
1.1.3 By providing a low average crystal grain size 20
1.1.4 By heat treatment or by hot-working 26
1.1.5 By other measures. . . . . . . 32
1.2 Improvement of coercivity, remanent flux density and energy
product . . . . . . . . 40
1.2.1 By hot working and/or heat treating 40
1.2.2 By addition of specific elements 43
1.2.3 By other measures. . . . . . 44
1.3 Improvement of coercivity and energy product 46
1.4 Improvement of coercivity, saturation magnetization and squareness of
hysteresis loop . . . . . . . . 49
1.5 Improvement of coercivity . . . . 50
1.5.1 By addition of specific elements 50
1.5.2 By other measures. . . . . 57
1.6 Improvement of remanent flux density and energy product 60
1.7 Improvement of remanent flux density . . . . 64
1.8 Improvement of energy product and square ratio 66
1.9 Improvement of magnetostrictive response 69
2. Methods to Improve the Physical and Chemical Characteristics of
Magnets and Magnetic Materials. . . . . . . . . 70
2.1 Improvement of corrosion resistance and thermal stability 70
2.2 Improvement of corrosion resistance 75
2.2.1 By using specific compositions 75
2.2.2 By applying a specific coating . 79
2.2.3 By other measures. . . . . 83
2.3 Improvement of temperature characteristics 91
2.3.1 By using specific compositions. . . 91
2.3.2 By addition of specific elements or compounds 97
2.3.3 By other measures. . . . . . . . . . 101
3. Methods to Improve Other Properties and Characteristics of Magnets and
Magnetic Materials . . . . 104
3.1 Improvement of workability 104
viii
3.2 Improvement of sintering characteristics 105
3.3 Improvement of dimensions IlO
3.4 Improvement of mechanical properties . III
4. Methods to Lower the Production Costs of Magnets and Magnetic
Materials. . . . . . . . . . . . . 112
4.1 By using commercial manufacturing processes • . . • Il2
4.2 By eliminating specific costly production steps • • • • 120
4.3 By substituting inexpensive materials for expensive materials 128
4.4 By other measures. • • • • • • • • • • • • 137
5. Methods to Prevent Risks During the Production of Magnets and
Magnetic Materials . . . . . . . . . . . . . . . . . 141
5.1 To prevent the formation of poisonous volatile boron compounds. 141
5.2 To prevent the risk of explosion . • . • • . . . . . • . 145
References 146
Abbreviations Used 147
List of Patentees . 149
INTRODUCTION
The present literature study is the result of a comprehensive
literature analysis of united states, Japanese, British,
French, German, European (Munich) and PCT (Geneva) patents,
patent applications and other technical and scientific lite
rature, published since January 1986.
For conducting the analysis documents classified in accordance
with international patent classes (I.P.C.) H 01 F 1/04 through
/09 relating to magnetic materials for magnets and magnetic
bodies, metals and metal alloys, were studied, and copies of
the documents were supplied by the Patent Information Service,
TNO, Rijswijk, The Netherlands.
From the study it came apparent that the last years particu
larly Japanese technologists and magnet manufacturers have
made an intensive research in composing rare earth alloys.
Out of the original patents and patent applications discussed
in the present survey, the Japanese cover 67%.
Obviously this is due to the explosive development of the
Japanese electric and electronic industry in the last three
decades.
Though, due to the language barrier, Japanese patents and
other technical specifications are hardly accessible to West
ern researchers, fortunately the Japanese Patent Office is
providing extensive English language abstracts of the contents
of Japanese patents and patent applications, and as a result
the Japanese section of the present study has been mainly
based on those abstracts.
2
since more than 90% of the documents studied for this survey
deal with rare earth metal - iron - boron permanent magnets
and their production, as to the grouping in chapters of this
literature study has been chosen for methods to improve cer
tain properties and production processes, rather than for a
grouping according to composition.
HISTORY
The development of permanent magnet materials can be said to
have started around the beginning of this century. However
novel in appearance, permanent magnets gave a relatively low
performance at that time and in most technological applica
tions use was made of electromagnets. Owing to the steady
increase in their quality and performance, permanent magnets
have gradually superceded electromagnets for most applicat
ions. Nowadays permanent magnets are so widely used that one
could consider them as indispensable prerequisites of modern
technology. By far the largest application of permanent mag
nets is in motors and generators. Next in importance are
applications in telecommunication, data techniques and measur
ing and control devices. Substantial numbers are also used in
acoustic devices and magnetomeqhanical applications.
-1200 kA/m -800 -l.00
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I Tesla
12
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y y 10
8
V
NdFe!LtSm~
/ 5 B
/;V~o t
r--
I..
/
2
V
/
VFxd3~ o
-16000 oers ted -8000 -1..000
-H
G
U
00
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e e
2 3 l. 5 6 7 8 9 10
3
The schematic representation on the previous page may serve to
illustrate the progress that has been made since the change of
the century. The figure shows a number of permanent magnet
bodies, the volume of which has been chosen so that each of
them represents the same total magnetic energy. Furthermore,
the cross-sectional area (~) of these magnets is smaller the
higher the magnetic induction (B) so that the magnetic flux
(~) is the same for all of them. Finally the different
lengths L of the magnets reflect their ability to produce the
same magneto-motive force, which is proportional to HL [5].
There are three important classes of permanent magnets:
alnicos, discovered in the 1930s, ferrites, invented in late
1940s, and rare earth transition metal alloys, found in the
1960s. These three classes constitute the bulk of the perman
ent magnets produced in the world., though their relative
share has varied from time to time over this period. Other
types of permanent magnets, e.g. Pt-Fe, Fe-Cr-Co, single
domain particles etc. are essentially specialty materials [6].
Rare-earth elements is a general term for the 15 elements
between lanthanum, with atomic number 57, and lutetium, with
atomic number 71, and yttrium, which has similar properties.
-
Sm2Co17 family
-
r-- R-Co family
- SmCos family
Rare earth magnet I-
r-- Nd-Fe-B sintered
- -
R-Fe family
Nd-Fe-8 rapid
......
solidification
The above table shows the classification of rare earth magnets
[7] •
4
NEODYMIUM MAGNETS
Neodymium magnetic material is an alloy with boron and iron
discovered by a team at the GM Research Labs in the 1970s.
Neodymium magnets generally have cost more than samarium
cobalt devices because of high neodymium and fabrication
costs. Neodymium ore is plentiful - more so than lead, and
about the same as tin. But demand has been low, so it has been
refined in only small quantities. The ores, which are plenti
ful in the U.S. and many other regions, are widely mined and
refined as catalysts of petroleum cracking, as misch metal (an
alloying agent in metal industries), as flint for cigarette
lighters, and as reagents for the glass and ceramic indust
ries. But because there has been little demand for pure
neodymium, the metal remains combined with other elements in
the refined products. Neodymium can be cheaply refined in
large volumes when demand justifies it [4].
New magnet manufacturing methods are now being developed as
alternatives to sintering. They include rapid quenching of
Nd-Fe-B, which can be followed by hot pressing and
die-upsetting. Bonded magnet types are being rapidly intro
duced that have relatively low energy products. Extrapolating
from the commercial success of "rubber ferrite", such products
should have a bright future [10].
Much of the work that was originally devoted to optimizing the
properties of Sm-Co magnets has been transferred to magnetic
materials based on alloys containing neodymium and iron.
Ideally, a cheap high performance magnet should be iron-rich.
Neodymium has the additional property of being able to
contribute axial anisotropy (directional properties) in a
suitable crystal structure. For technical and metallurgical
reasons, it is necessary to use a small amount of a third
element, boron, in order to stabilise useful phases [9].
