Cerium in high-energy magnets
The need to develop cerium-containing alloy is growing in importance, not least because of the substantial benefits the technology offers in terms of production, cost and engineering.
Typically, bastnasite rare earth ores in nature contain 49% cerium (CE), while in monozite deposits, one can expect to find approximately 46% Ce, as detailed in Table 1. Praseodymium (Pr) and Neodymium (Nd) are generally found in twin form in nature, and the combination of these two elements is referred to as didymium (‘di’ means two in Greek, and the names of both elements end with ‘dymium’). Modern high-energy permanent magnets tend to be of a didymium iron boron makeup rather than pure neodymium iron boron.
It is clear that cerium accounts for up to 49% of naturally occurring rare earths, but final high- energy permanent magnets do not contain any cerium at all. So how can cerium be used and what are unit magnetic properties of cerium- containing alloys?
Cerium is more plentiful in terms of availability and is cheaper than neodymium and praseodymium. It is also much cheaper than dysprosium and terbium. The study of cerium- containing alloy is not, however, new. Wallace, Craig and Schaller of the University of Pittsburg looked at the anisotropy of Ce2Co17-xFex as early as July 1972, and discovered that Ce2Fe17 alloy did have c-axis anisotropy while Ce2Co17 had basal- plane type anisotropy. Yet despite this, the anisotropy of the alloy was not conducive to making a high enough coercive force.
The use of cerium in high-energy permanent magnets began in early 1970s when Karl Joseph Strnat (generally considered the father of modern rare earth-transition metal permanent magnets), indicated in his 1976 paper for Dayton University, that Vogel and his colleagues had published the first phase diagram for CeCo5 in 1947.
Buschow and his team studied the change in anisotropy of SmCo5-CeCo5 as a function of
22 TECHNOLOGY INTERNATIONAL 2014
Most Magnet Manufacturers today purchase their alloys from organizations that are equipped siMply to Melt various grades of alloys. their charter is not to supply actual Magnets. to expand their business they Must not lose sight of the need for rare earth Magnets with less robust unit Magnetic properties, at a reasonable cost, using rare earth elements that are More abundant
temperature in mid-1975. By that time, Martin Wells and Dev Ratnam of the Crucible Research Center had produced and documented the manufacturing process for making misch-metal cobalt (MMCo5) magnets. Misch-metal is a naturally occurring ore that contains effectively all the rare elements in combined form. The MMCo5 magnets were made using a natural stream of bastnasite-type rare earths with a host of other light and heavy rare earths including lanthanum (La), mixed to the proper stoichiometric ratio of one to five between rare earth and cobalt. Later Ratnam and Wells varied the content of cerium to achieve a magnet with marketable unit magnetic properties. Nevertheless, the Crucible Research Center soon abandoned the practice of making cerium-containing alloys, for the simple reason
that because the cost of the raw material was only a tiny fraction of the total manufacturing cost (small magnets and highly labor intensive), the final production cost would not be very different between samarium-based magnets and MMCo5 magnets. The lack of repeatability of the magnetic properties from one batch to another, as well as from one part to another part, was a secondary reason for stopping the program in its entirety.
DL Martin of the General Electric Research Center (Schenectady, NY) produced a cerium containing alloy of Sm1-xMMxCo5, with energy capacity of 14.5-18.0MGOe, where x ranged between 0 and 0.4. The magnets exhibited an intrinsic coercivity ranging up to 24kOe. The lack of repeatability of magnetic properties between
material lots, as well as the small difference in terms of the selling price of MMCo5 and SmCo5 magnets, damped the enthusiasm of magnetic circuit designers around the world and therefore magnet manufacturers abandoned the practice for more than a decade.
KHJ Buschow of Philips Research Laboratories determined that the magnetic properties of several compounds of RCo5 (cerium and samarium) are strongly related to magnetocrystalline anisotropy (Figure 1). When Buschow studied the characteristics of the CeCo5 system, cerium was considered a rare earth element with potentially high anisotropy when combined with transition metals such as iron and cobalt.
Cerium in NdFeB alloys
In 1985 M Okada and M Homma of Tohoku University in Sendai, Japan, observed the process of making pure rare earth alloys by using a rare ore containing 49% Ce and a host of other light and heavy rare earth oxides. The separation of pure Nd and pure Pr was more complex, and therefore very expensive. Figure 1 shows the typical process of generating pure Nd and Pr.
Rare earth ore typically has a host of light and heavy rare earths. The extraneous elements are removed as the first order of priority and then La has to be removed in its entirety. Leaving a small percentage of Ce would reduce the cost of the processing substantially, especially as bringing the content of Ce down to almost zero in the alloys will need many more expensive steps. Nevertheless,
the remaining Ce content will reduce the intrinsic coercive force of the alloy and this reduction will be proportional to how much is left in the final alloy.
Making didymium the base alloy
To determine the effect of cerium on rare earth iron boron magnets, Okada and Homma selected three Ce-containing alloys with the following compositions:
NdFeB alloy with no cerium-Nd – 10%wt Pr
NdFeB alloy with 5% cerium-Nd – 15%wt Pr, 5% Ce (5Ce-didymium)
NdFeB alloy with 40% cerium-Nd – 10%wt Pr,
40% Ce (40Ce-didymium)
The range of the unit magnetic properties achieved with the above Ce-containing alloys ranged from from 27-40MGOe with various values of the intrinsic coercive force depending on the content of Ce-didymium. It is important to note that the alloys were made using an induction vacuum melt process, crushed into 258µm (average) particle size, and ball milled to a 3-4µm average particle size with the oxygen content being substantially higher than with the modern
Figure 1: Anistrophy in SmC05
methods of manufacturing. In other words, the vacuum-melted buttons may have had some inhomogeneity across their depth and thus the microstructure may have been less conducive to making high magnetic properties. Despite this, the result of the process was immensely encouraging and the future seemed to be very promising for ceruim containing alloys. A typical NdFeB alloy today is made of strip cast alloy that is subsequently hydrogen-decrepitated and jet milled with a much lower oxygen content and much improved particle distribution. As a
result, enhanced unit magnetic properties have been realized.
One important observation that can be drawn is that the distribution of Ce and Nd in this alloy is impressively even, but the distribution becomes uneven as the content of Ce increases and the magnetic properties deteriorate.
Manufacturing of cerium
A modern approach to producing Ce-didymium permanent magnets includes strip casting the
alloy; hydrogen decrepitation of the alloy; jet milling to an average of 3µm particle size with a reasonable Gaussian distribution; hydrostatically
The saturation magnetization of 13.7kG for the Ce-didymium alloy is far lower than that of the didymium base alloy. Furthermore, the curie temperature of the modified alloy could be as much as 15-20°C lower than the didymium alloy.
The didymium alloy exhibits a curie point of around
310°C. However there are applications in use today – as well as potential future examples – where the intrinsic coercive force of the modified alloy will be more than sufficient. Figure 2 shows the relationship between a typical NdFeB magnet and a Ce-didymium magnet produced using the above manufacturing steps.
Most magnet manufacturers today purchase their alloys from organizations that are equipped simply to melt various grades of alloys. Their charter is not to supply actual magnets. To expand their business they must not lose sight of the fact that there is a need for rare earth magnets with
less robust unit magnetic properties, at a reasonable cost, using rare earth elements that are more abundant.
They will need to develop Ce-containing alloy to essentially accomplish two goals: first, to maintain the cost of the rare earth alloys at a price level where it will not choke the current and future use of these magnets in applications
where super high-magnetic characteristics may not be needed; second, to use the most abundant rare earth element and help ensure the proper usage balance between the heavy and light rare earths.
pressing at a magnetic field of 20kOe, and subsequently cold iso-statically pressing the part; and sintering at a temperature of around 1,020°C and annealing at 520°C.
These key manufacturing steps apply to an alloy of generic form, such as (Nd1-xCex)30FebalanceB1.0 with X being 0, 0.1, 0.15 and 0.2. The best result was achieved for X=0.2, with unit magnet properties of Br = 13.7kG, Hci of 12.0kOe and BHmax of
41.0MGOe. The pure Ce-based alloy with the C2Fe14B structure will have a much lower Br and intrinsic coercive force and therefore a lower energy product. This is because the magnetic moment and the anisotropy of Ce2Fe14B are lower than Ce-enriched alloy, as well as being much lower than the pure NdFe14B alloy.
The important factor in achieving a high Br and a higher density magnet has been the use of a dual
component alloy. Ce2Fe14 alloy has very low anisotropy of less than 40kOe, while Ce-enriched didymium alloy will achieve a high densification at a low temperature of around 1,020°C.
Figure 2: Schematic illustration of extraction process of the rare earth elements from ore
Karl Joseph Strnat: Introduction to the Workshop, Rare Earth Cobalt Permanent Magnet And Their Applications, Dayton, June,1976
Karl Joseph Strnat: Review of Rare Earth Magnets, Rare Earth
Cobalt Permanent Magnets And Their Applications, June 1976
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HG Schaller, RS Craig and WE Wallace, J.Appl.Phys. Vol.143, No.7
H Domazer: New Substituted SmCo, Workshop Rare Earth Cobalt, June 1976
M Okada, M Homma: The Properties and Microstructure of
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and Wei Li, Fabrication of High performance NdCe-containing magnets, International Workshop on Rare Magnets and their Applications,September 2012, Nagasaki, Japan