Woodhead Publishing Series in Civil and Structural Engineering
Modern Permanent Magnets
Edited by John J. Croat
Naples, Florida, (USA)
John Ormerod
JOC LLC, Loudon, Tennessee, (USA)
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Table of Contents
Cover Image
Title Page
Copyright
Table of Contents
Contributors
Chapter 1 The history of permanent magnets
Abstract
1.1 Introduction
1.2 Lodestones: the first permanent magnets
1.3 Early permanent magnet studies
1.4 The era of steel permanent magnets
1.5 The discovery of alnico permanent magnets
1.6 The discovery of hard ferrite magnets
1.7 The discovery of Sm-Co permanent magnets
1.8 The discovery of NdFeB permanent magnets
1.9 The discovery of Sm-Fe-N permanent magnets
1.10 Future permanent magnet materials
1.11 Summary
References
Chapter 2 Fundamental properties of permanent magnets
Abstract
2.1 Introduction
2.2 The different families and types of permanent magnets
2.3 Key magnetic parameters
2.4 On the origin of magnetism
2.5 The different types of magnetism
2.6 The origin of anisotropy in permanent magnets
2.7 Magnetic domains and domain walls
2.8 Magnetic hysteresis
2.9 Coercivity mechanism in modern permanent magnets
2.10 Stability of permanent magnets
References
Chapter 3 Recent advances in hard ferrite magnets
Abstract
3.1 Introduction
3.2 Historical overview of M-type Sr- and Ba- Hexaferrites
3.3 Crystal structure, intrinsic magnetic properties, microstructure and morphology
3 4 Advances towards the improvement of intrinsic magnetic properties
3.5 Industrial fabrication routes
3.6 Recycling efforts, recovery, and reusability in production line
3.7 Applications of hexaferrites: present and perspectives
References
Chapter 4 Modern Sm-Co permanent magnets
Abstract
4.1 Introduction
4 2 Manufacturing process of Sm-Co magnets
4.3 High (BH)max Sm2Co17 type permanent magnets
4.4 Temperature compensated Sm-Co magnets
4.5 Ultra-high temperature Sm-Co magnets with small reversible temperature coefficient of Br
4.6 Performance of Sm-Co magnets in special environments
4.7 Laminated Sm-Co magnets
4.8 Additive manufacturing
4.9 Small magnets
4.10 Sm-Co nanoparticles and nanoflakes for nanocomposite magnets
4 11 Summary
References
Chapter 5 The status of sintered NdFeB magnets
Abstract
5.1 Introduction
5.2 History of the development of Nd-Fe-B
5.3 Compositions of the NdFeB sintered magnets and their magnetic properties
5.4 Production process for sintered NdFeB magnets
5 5 Progress in the microstructure investigation
5.6 Development of HRE-Free and reduced HRE magnets
5.7 Ultimate NdFeB sintered magnets for EV traction motors
Masato Sagawa Daido Steel Co., Ltd., Nagoya, Japan
Thomas Schliesch Max Baermann GmbH, Bergisch-Gladbach, Germany
Yasuhiro Une Daido Steel Co., Ltd., Nagoya, Japan
Michael Walmer Electron Energy Corporation, Landisville, PA, USA
Yutaka Yoshida Daido Steel (America) Inc., Victoria, BC, Canada
Norio Yoshikawa Daido Electronics Co., Ltd.,
Nakatsugawa, Gifu, Japan
C H A P T E R 1
The history of permanent magnets
John J. Croat a , John Ormerod b
aNaples, Florida, (USA)
bJOC LLC, Loudon, Tennessee, (USA)
Abstract
This chapter traces the historical development of all the important families of permanent magnets that have been developed and produced. Although humans were familiar with naturally occurring lodestone magnets as early as the sixth century BC, it was not until the development of the KS Steel magnet in 1917 that a magnet with an energy product of close to 1 MGOe (7.96 kJ/m3) was produced. Following this, however, improvements in permanent magnet properties was very rapid. The 20th century was a period of great permanent magnet innovation which included the development of alnico, sintered ferrite, Sm-Co and finally, NdFeB magnets, four of the most important discoveries in the history of this industry. Between 1930 and the early 1980s, energy product increased by a factor of 50 and intrinsic coercivity by a factor of 100. As of this date, sintered NdFeB magnets remain the reigning permanent magnet champion with commercial energy products as high as 52 MGOe (414 kJ/m3). Most recently, bonded Sm-Fe-N magnets have been added to the rare earth family of permanent magnets.
Key words
History of permanent magnets; Lodestones; Compound steel magnets; Cast steel magnets; Alnico magnets; Hard ferrite magnets; Rare earth magnets
1.1 Introduction
Modern permanent magnets are now a quintessential component in a wide spectrum of electomechanical devices including motors, generators, sensors, loudspeakers, instruments, traveling wave tubes, bearings and clutches that are used in a wide range of products ranging from automobiles to missiles. Rare earth permanent magnets have become a critical part of many high-tech products, including personal computers, MRI, high-capacity hard disk drives (HDD), wind power electric generators and hybrid and electric vehicle drive motors. In addition, high performance permanent magnets have allowed the miniaturization of many products such as laptop computers and other consumer electronic products. In fact, modern rare earth permanent magnets now play an increasingly important role as an enabler and driver of technology. Without NdFeB permanent magnets such products would not have developed or would not have developed nearly so rapidly. This book provides an overview of all of the commercially important families of permanent magnets that are currently manufactured. Although permanent magnets are now used in large quantities, the development and use of permanent magnets was a relatively slow process. The first permanent magnets known to humans were naturally occurring lodestones which are created when bolts of lightning struck deposits of the mineral magnetite (Fe3O4). Although there are reports about the magical a ractive properties of lodestones that date to as early as the sixth century BC, it was not until about the 11th century AD that they were first used in compasses, the first practical use of a permanent magnet. Although the properties of lodestones are comparatively poor, it was not until the mid-18th century that the first permanent magnets were
produced with higher properties and it was not until the 1930s and the development of alnico magnets that magnetic properties increased to the point where engineers were able to use permanent magnets in electomechanical devices like motors and generators. Prior to this, permanent magnets properties were so low that engineers were forced to use electromagnets for these devices. It was not until the mid-20th century and the development of ferrite magnets that permanent magnets began to be used in significant quantities.
The increasing use and importance of permanent magnets has been driven largely by a significant increase in magnetic properties. This is shown in Fig. 1.1, which displays the chronological development of the various families of permanent magnets over time and presents a very graphic picture of the evolution that has occurred in permanent magnets in the last century. This chronology is expressed in terms of maximum energy product (BH)max, which is the figure of merit most frequently used to rate and compare the various families and grades of magnets. This number is proportional to the energy stored in the magnet and, therefore, the work that can be done by the magnet. As seen in this figure, between about 1917 and 2000 the maximum energy product increased by about fifty times. Over the same time, intrinsic coercivity levels have increased by an almost 100 times, from about 250 Oersted to as high as 25,000 Oersted. The 1950s and 1960s saw the development of sintered ferrite magnets, one of the most important discoveries in the history of permanent magnets. These magnets are still the most commonly used in terms of volume, a testament to their unparalleled economic efficiency and chemical stability. In 2020, an estimated 900,000 tons of sintered ferrite magnets were produced. Another major step in the evolution of permanent magnets was the discovery of Sm-Co magnets in the 1960s and 1970s. The discovery of Sm-Co magnets stimulated a great period of basic research which eventually led to the discovery of NdFeB magnets. As seen in Fig. 1.1, between 1970 and 1990, energy products increased by a factor of 12 compared to those obtained by the best ferrite magnets. The last 50 years has also
seen the development of various grades of bonded magnets produced by compression and injection molding techniques. The status and recent developments in all of these families of magnets are presented in the following chapters. Also included are four chapters of general interest. Chapter 2 provides a discussion of the fundamental properties of permanent magnets so that the nonspecialist readers can more easily follow the discussion of the various families of magnets. In addition, Chapter 10 provides an overview of the situation regarding critical material used in magnets, including rare earths and cobalt. Today you will find many articles in the news about how important key materials are for many high technology products and permanent magnets are no exception. Since coating are required for most rare earth based permanent magnets, a review of current coating and coating evaluation technology is provided in Chapter 11. Finally, a chapter on the markets and major applications for the various families of permanent magnets is provided in Chapter 12.
1.1 The chronological development of permanent magnets since 1917.
FIG.
1.2 Lodestones: the first permanent magnets
As previously mentioned, the first permanent magnets known and used by humans were lodestones, which are naturally occurring magnetized pieces of the iron mineral magnetite (Fe3O4) (Mills 2003; NASA 2020). Lodestones typically contain small amounts of titanium or other elements which slightly increase the coercivity of the magnetite and make them less susceptible to demagnetization over time. It appears that unless these elements are present the magnetite does not have the properties necessary to become or stay permanently magnetized. Lodestones are believed to become magnetized when lightning strikes the surface of magnetite deposits. It is well known that there is a magnetic field associated with a bolt of lightning and the prevailing theory is that fragments of the magnetite rock are ejected from the deposit and these pieces become instantly magnetized by the lightning strike. The typical lodestone has relatively poor magnetic properties with a coercive force of about 50 Oersted and an energy product well below 1 MGOe (7.9 kJ/m3), too low to be included on the chart in Fig. 1.1. However they strongly a ract other iron objects as shown in the photograph in Fig. 1.2. Since this is a natural process, lodestones are still being produced in magnetite deposits around the world. Lodestones also served another important role as the means by which the first steel magnets were magnetized.
FIG. 1.2 A demonstration of the ability of loadstone to attract iron objects (Annet 1921).
It is not known when humans first discovered the propertied of lodestones. One place where lodestones were commonly found was in the prefecture of Magnesia in the Greek province of Thessaly and the name magnet or magnes comes from this region. The earliest known wri en reference to them was made by the sixth century BC Greek philosopher Thales of Miletus who reported on the ability of the lodestone to a ract pieces of iron and other lodestones. Lodestones are also referred to in early Chinese chronicles that date to as early as the fourth century BC. Most historians credit the Chinese with the development of the first compasses but when they actually did this is still being debated. The first incontestable reference to the use of a magnetized needle for navigation appears in the Dream Pool Essay wri en by Song Dynasty writer Shea Kua in 1086 CE. This essay also describes how an iron needle was
magnetized by rubbing it against a lodestone and then used in a compass by suspending it from a single silk thread. This was probably the world’s first practical use of a permanent magnet (Needham 1962; New World Encyclopedia 2020).
At this time historians are unsure if compass technology spread to the rest of the world or was independently discovered. Many historians believe that the technology was probably transferred to both Europe and the Islamic world from China by traders traveling down the Silk Roads. The earliest European reference to the compass used for navigation is found in the book De naturis rerum (on The Nature of Things) wri en by the English scholar Alexander Neckam in about 1190. However, he refers to the compass as a common navigation tool, an indication that it was being used somewhat earlier. One argument used for an indigenous European discovery is that the earliest European compasses had sixteen basic divisions whereas early Chinese versions had twenty-four In the Islamic world, the earliest reference to the use of a compass for navigation is found in The Book of the Merchant’s Treasure, wri en by Baylak alKibjaki in about 1282. However, the author describes having witnessed the use of a compass on a ship some forty years earlier. By the late-12th century the compass became widely used for navigation and is believed to have resulted in a significant increase in world trade.
1.3 Early permanent magnet studies
Humans had long recognized that there appeared to have two distinct poles on lodestone magnets and came to call them southseeking and north-seeking poles. They also tried to find a magnetic monopole by cu ing the magnets in two. Of course, they simply produced two new magnets that seemed to have weaker properties. For some time scholars and navigators did not understand why the magnet rotated into a north-south direction and some, including the Italian explorer Christopher Columbus, speculated that the magnets were being oriented by the north star, which they believed must be a giant celestial magnet. The first serious study of permanent magnets
was carried out by the English physician and scholar William Gilbert near the end of the 15th century. Gilbert had a great interest in magnets and the strange directional properties of the compass needle. Some of his early discoveries were that the magnetic strength of lodestones could be increased by adding iron tips or keepers and reported that doing so could increase the weight of iron that could be lifted by a lodestone by a factor of five. He also discovered that the magnetism of a lodestone could be destroyed by heating to high temperature. He also reinvented the method of magnetizing iron needles by rubbing them with lodestone, a practice first used by the Chinese. Gilbert was also the first to recognize that the best artificial magnets were produced from “hardened iron” which contained higher levels of carbon as opposed to “soft iron” which contained lower levels of carbon. Of course, the higher carbon levels resulted in slighter higher coercivity, enabling the magnet to retain its magnetization.
One of his most important experiments was to trace the magnetic field of a spherical piece of lodestone with a compass. He discovered that magnetic flux lines extended from the poles and were circular in shape and from this he eventually made his most important discovery, that the earth itself was a great magnet with a north and south pole. For the first time it was known why a compass aligned itself in a north-south direction. He published his extensive work in the book De Magnete, Magneticisque Corporibus, et de Magno Magnete Tellure (On the Magnet and Magnetic Bodies and on That Great Magnet the Earth) (Gilbert 1600). This book became the most influential book on magnetism for the next two centuries and sha ered many popular myths and theories that were prevalent at the time. These included the widely held belief that a magnetic field could cure common diseases and that garlic, goat’s blood or diamonds could counteract a magnetic field. In an age when much that was wri en about permanent magnets was li le more than superstition, Gilbert’s work stands out as one of careful scientific inquiry.
The Relationship between Magnetism and Electricity: Gilbert also had an interest in the properties of static electricity. He carefully
p p y y observed the a raction between two pieces of amber that had been rubbed to build up a charge of static electricity but concluded that this force was different than that between two magnetized bodies. Among his observations was that the electric force was a surface phenomenon whereas the magnetic force was a property of the entire body of the magnet. He also noted that the electric force (the spark of electricity) became dissipated when transferred to another body but the magnetic force remained constant when interacting with another iron body. He was the first to use the terms “electric force” and “electric a raction” to describe the a raction between the two pieces of amber. To describe this electric a raction he coined the word electricus which means “like amber” from the Greek word elektron for amber. Albert’s book De Magnete would provide the first tentative studies of both magnetism and electricity and he sensed that they were somehow connected. However, it would be almost two centuries after publication of this book before scientists would discover the close relationship between electricity and magnetism.
One of the most important events leading to this discovery was the development of the first ba ery by the Italian physicist Alessandra Volta in the year 1800. His-early ba eries consisted of two dissimilar metals immersed in an electrolyte. Although he investigated many different combinations, his best results were obtained using plates of zinc and copper with either a weak sulfuric acid solution or salt brine electrolyte. The importance of the development was that it allowed the production of a continuous electric current for the first time. All previous electricity studies were carried out on static electricity obtained by rubbing an object like amber. In 1820 the Danish physicist Hans Christian Oersted discovered that electricity passing through a conductor produced a magnetic field by observing that the flowing current deflected the needle of a compass. This demonstration would not have been possible without the development of the ba ery by Volta. Later in the same year the French physicists Jean-Baptiste Biot and Felix Savant discovered that a current-carrying wire exerts a magnetic force that is inversely proportional to the distance from the wire. This was a very important discovery because it demonstrated for the first time the
p y possibility that electricity and magnetism could be combined to do work. Also during the 1820s, the French physicist Andre-Marie Ampere measured the force between two current carrying conductors and developed Ampere’s Law, the first mathematical equation establishing the relationship between electricity and magnetism. This law provided a way to calculate the magnetic field that is produced as a result of an electric current moving through a wire of any shape, including a solenoid.
These early discoveries eventually led to the invention of the electromagnet by the Englishman William Sturgeon in 1824. The invention of the electromagnet was one of the most important developments in the history of magnetism because it led to the development of the first important electromechanical devices, including the first practical electric motors. A rendering of Sturgeon’s first electromagnet from a paper that he gave to the Royal Society in London in 1824 is shown in Fig. 1.3. Sturgeon activated the electromagnet by turning on the current from a ba er through a switch which consisted of a small cup of liquid mercury. We see again how important the development of the ba ery was to the future understanding of electricity. Without the ba ery to provide a steady current, Sturgeon’s and other experiments involving the study of electricity could not have taken place.
FIG. 1.3 A rendering of William Sturgeon’s first electromagnet (Henry 1824).
In combination these pioneering discoveries led to the first primitive electomechanical devices, including electric motors and generators. Although the electric motor resulted from the combined inventions and innovations of many different scientists, credit for the first working motors go to the Prussian scientist Mori Jacobi in 1834 who built a motor rated at 15 W to power a boat. In the same year the American team of Thomas Davenport and Orange Smiley built a motor rated at 4.5 W. Their motors could rotate at 1000 rpm and were used to power a number of devices, including a printing press. Although these early electric motors used both steel bar magnets and electromagnets for their magnetic circuit, the permanent magnets did not have the strength to provide high performance and were soon replaced entirely by electromagnets. It was not until the discovery of alnico magnets in the 1930s that permanent magnets once again came to be used in motors. Of
