Recent Developments and Trends in Nd-Fe-B Magnets

Jinfang Liu, President and COO, Electron Energy Corp.
Melania Jasinski, Manager of Process Engineering, Electron Energy Corp.

Nd-Fe-B magnets have a wide variety of applications in aerospace, medical, semiconductor, telecommunications, power generation, oil & gas exploration and automotive industries.  There have been many new developments in recent years including high energy Nd-Fe-B grades, heavy rare earth free high intrinsic coecivity Nd-Fe-B grades, radially oriented anisotropic rings, and improved manufacturing processes. The Nd-Fe-B market has also met a lot of challenges and opportunities in recent years, including volatile raw materials market, overcapacity problems in China, and patent issues. This article will cover some of the recent technical advances and new trends in Nd-Fe-B magnets.

Heavy Rare Earth-Free Nd-Fe-B Magnets
Rare earth elements have been found in a variety of minerals, such as bastnaesite and monazite. Rare earth deposit has been found in many countries including China, United States, Australia, Canada, Brazil, India, Malaysia and South Africa, but China dominates the production of rare earth.

The term rare earth is actually a misnomer. Rare earth is neither “rare” nor “earth”. Rare earth elements are metals that are part of the family of lanthanides. Rare earth elements are separated into two categories, light rare earths and heavy rare earths. Light rare earths, such as neodymium (Nd), are much more abundant than heavy rare earth, such as Dysprosium (Dy).

Dy is commonly used to substitute Nd in the production of neodymium iron boron (Nd-Fe-B) magnets to increase intrinsic coercivity, which is important for many industrial applications such as motors and generators. Dy can cost as much as 5 to 6 times more than Nd, which makes it one of the important cost drivers for the Nd-Fe-B magnets. Moreover, the rare earth market has been very volatile and the availability of heavy rare earth (HRE), such as Dy, could become an issue in the distant future. In order to reduce the cost and support the sustainable growth of the Nd-Fe-B magnet market, many producers started to develop HRE-free high intrinsic coercivity Nd-Fe-B magnets, and have made significant progress in recent years. As shown in Figure 1, with the exception of N46SH, nearly all grades of Nd-Fe-B magnets with intrinsic coercivity less than 20 kOe can be made without heavy rare earths, which can satisfy the majority of requirements for industrial applications. The need for heavy rare earth has also been significantly reduced for Nd-Fe-B magnets with intrinsic coercivity of 25 to 30 kOe.

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Figure 1. Relationship Between Maximum Energy Product, (BH)max, and Intrinsic Coercivity, Hci, for Different Grades of Nd-Fe-B Magnets

 

The magnetic properties of Nd-Fe-B are determined by chemical composition, process parameters, and final microstructure. The following is a list of important factors to consider in the development of HRE-free and HRE-lean Nd-Fe-B sintered magnets:
• Low oxygen and other impurities
• Smaller and more uniform grain size
• Finer and more uniform microstructure of precursor strip cast alloys
• Smaller powder particle size after jet milling
• Better powder particle size distribution
• Better magnetic alignment
• Better sintering equipment and temperature uniformity
• More uniform quenching
• More uniform gain boundary phase
• Grain boundary engineering

There are of course many other process details involved in the successful development of HRE-free magnets. Some of the factors above are actually interrelated. Lowering the impurity level and reducing the material oxidation would help increase the residual induction, Br, and maximum energy product, (BH)max, while smaller grain size can help increase the intrinsic coercivity, Hci, without the need for the extra addition of heavy rare earths such as Dy.

Radial Nd-Fe-B Ring Magnets
A hot pressing and extrusion process was developed first by Daido Steel to produce radially oriented anisotropic Nd-Fe-B ring magnets from rapidly quenched melt-spun ribbons.  The magnets are still isotropic after hot pressing. Die upsetting (extrusion) process leads to a nanocrystalline microstructure with a high degree of radial orientation, as shown in Figure 2. Design engineers are more aware of radial magnets during the design stage in recent years.

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Figure 2. Die Upsetting Process to Produce Textured Radial Nd-Fe-B Magnet Rings

The advantages of single piece multipole Nd-Fe-B magnets include:
1.  Reduced assembly time/costs. Traditional multipole rotors are manufactured by assembling magnet segments onto the shaft. The single radial magnet with multipoles will provide more consistent magnetic properties and reduce significantly the complexity of the assembling process. Manufacturing of radial magnet rings requires special tooling for each application, which could lead to higher initial cost.
2. Better mechanical tolerance
3. More consistent magnetic flux profile
4. Skewed magnetization profile (see Figure 3) possible
5. Improved motor performance due to the potential reduction of air gap

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Figure 3. Sketch Showing Multipole Nd-Fe-B Magnets With or Without Skewing

Cerium Substituted Nd-Fe-B Magnets
Cerium (Ce) is a light rare earth element, the most abundant among the entire rare earth elements family available in the Earth’s crust. Typically, bastnasite rare earth ores contain 49 percent cerium, while monozite contains approximately 46 percent Ce. Cerium is about as abundant as copper and nearly three times as abundant as lead, ranking 28th among the 83 naturally occurring elements. The supply exceeds the demand in current market, and consequently, the price of Ce is much lower than that of Nd.

The cerium dioxide has a number of applications including the fine polishing of glass in the optics industry, cracking catalysts in the petroleum industry, and emission catalyst in the automotive industry. The association of Ce with permanent magnet applications started in the early 70s with the work of Karl Strnat, who opened the era of modern rare earth based permanent magnets. However, SmCo5 magnets prevailed at that time as a better material. Then, soon after the discovery of Nd2Fe14B based magnets, Okada and Homma of Tohoku University in Sendai, Japan, focused their attention in 1985, on determining the effect of cerium on (NdPr)2Fe14B based magnets. They ultimately found out that (BH)max can range from 27 to 40 MGOe with various values of the intrinsic coercivity depending on the content of Ce.

In the last couple of years, the subject of Ce substitution for Nd in 2:14:1 magnets has attracted more attention in the magnetics community due to cost concern of raw materials. Leaving small amounts of Ce coexist with Nd and / or Praseodymium (Pr) would significantly reduce the complexity of the refining process.

Both Ce2Fe14B and Nd2Fe14B phases crystallize in the same tetragonal structure, but there are some differences in their intrinsic magnetic properties due to the fact that Ce has dual valence (3+/4+) characteristics. At room temperature, compared to Nd2Fe14B, the saturation magnetization of Ce2Fe14B (11.7 kG) is 26 percent lower, the anisotropy field is three times lower (26 kOe) and the Curie temperature is lower by 161 K. Acceptable magnetic properties of sintered Nd-Fe-B based magnets with 20 to 30 wt. percent Ce substitution for Nd were reported1 in 2012- 2014, e.g. (BH)max = 42-45 MGOe and Hci = 9.5-12 kOe. Using a so-called dual alloy method, sintered Nd-Fe-B based magnets with 25 percent Ce substitution for Nd have been produced2  with (BH)max = 40.5 MGOe, Hci = 12.1 kOe, Hc = 9.4 kOe, Br = 13.3 kG. Also, the reversible temperature coefficient of Br of – 0.10 percent and reversible temperature coefficient of Hc of – 0.56 percent, were apparently similar or slightly improved compared to magnets without Ce substitutions.
Research has been also done3 on melt spun Ce-added Nd-Fe-B ribbons, which can be used when employing hot pressing and/or hot deformation for the manufacturing of magnets. The decrease in the magnetic properties of the hot pressed magnets was found to be relatively small at 10 percent Ce substitution for Nd. At 20 percent Ce substitution for Nd, (BH)max drops by 10 percent and this deterioration becomes abruptly more pronounced for all hysteresis parameters at increased substitution levels.

The industry sometimes uses Ce substitution in low grades of Nd-Fe-B magnets such as N35 and N38.

Magnet Recycling
Nd-Fe-B type magnets contain roughly 30 wt percent of rare earth elements, primarily Nd. Up to 10 wt percent of the rare earth could be Dy in some Nd-Fe-B grades with high intrinsic coercivity. For Sm-Co magnets, there are two types: one has about 26 wt percent of Sm (based on the Sm2Co17 phase) and another has about 34 wt percent of Sm (based on the SmCo5 phase).

The automotive industry and wind turbines are two major growing applications for rare earth magnets. An electric car like GM’s Chevrolet Volt uses about seven pounds of rare-earth magnets, while each clean-energy wind turbine uses about 1,100 pounds of rare-earth magnets per MW for units of 5+MW output capacity. Considering a life span of 30 years for the offshore wind turbines, one can expect that 10 years from now, tens of tons of rare earth magnets will be at the end of their life cycle.

It is estimated that in a typical magnet manufacturing facility, about 20 to 30 percent of the magnets are wasted as scrap, mostly as leftovers from machining blocks into particular geometries, chipping, cracking, sludge and swarf, with the waste reaching about 1,500 to 2,500 tons/year. As a very rough calculation of the potential value of this waste material, the scrap for one year translates into the equivalent of $1.5 billion. However, to date, only small quantities of magnet material, estimated to be less than 1 percent, are being recycled from pre-consumer scrap.

For post-consumer magnets harvested from equipment at the end of their life cycle, the material composition is unknown and may be different from unit to unit, which creates significant problems in achieving good quality from the recycled product.

There are different recycling routes for rare earth magnets. The simple, direct use of end-of-life magnets recuperated in proper condition (with eventually intact plating) from their host equipment is the most cost effective recycling. Other methods includes remelting to yield alloys, hydrogen decrepitation to yield powder, and liquid metal extraction to yield pure rare earth elements. There are also chemical routes that result in rare earth oxides which need to be further reduced to recover the pure rare earth elements.

Summary
With the exception of N46SH, nearly all Nd-Fe-B magnet grades with intrinsic coercivity less than 20 kOe can be made without heavy rare earths, which can satisfy the majority of requirements for industrial applications. The amount of the heavy rare earth elements has also been significantly reduced in Nd-Fe-B magnets with intrinsic coercivity of 25 to 30 kOe, by employing grain boundary engineering.

Radially oriented Nd-Fe-B magnet rings can be used for multipole rotors, which reduces rotor assembly time and cost, and ultimately, a better motor performance.

The addition of cost effective light rare earth elements, such as Ce, to sintered Nd-Fe-B magnets is acceptable for applications with lower magnetic requirements.

There are a couple of factors which contribute to the currently low recycling rate of rare earth elements from permanent magnets: technological difficulties, lack of regulations and inefficient scrap collection. Development of an economical recycling process to produce pure rare earth elements or magnet alloys using scrap precursors is contingent upon solving problems related to the process scalability, efficiency and product quality.

References
[1]  H. B. Feng, A.H. Li, S.L. Huang et al., The 22nd International Workshop on REPM 2012, Japan and The 23rd International Workshop on REPM 2014, USA
[2]  Fan, S. Guo, K. Chen, R. Chen, D. Lee, C. You, A. Yan, The effect of Ce distribution on the magnetic properties of Nd-Ce-Fe-B sintered magnets, REPM 2016, Darmstadt, Germany,  O5-0900,  p.108
[3]  I. Poenaru, A. Lixandru, A. Dirks, J. Gassmann, R. Hord, O. Diehl, S. Sawatzki, A. Buckow, K. Gueth, R. Gauß, O. Gutfleisch, Cerium substituted nanocrystalline melt-spun NdFeB alloys for resource-efficient permanent magnets, REPM 2016, Darmstadt, Germany, P1-30, p.330

Dr. Jinfang Liu is currently the President and COO at Electron Energy Corporation. He has over 20 years of experience in the field of rare earth permanent magnets and magnetic systems. Dr. Liu is a co-inventor of 12 US patents and co-author of over 200 papers on permanent magnets and magnet systems.

Dr. Melania Jasinski has 10 years of experience in the rare earth permanent magnet industry.  After about a decade spent in research in academic and non-academic organizations, in 2007 she joined Electron Energy Corporation, where she is now the Manager of Process Engineering.  

For more information, visit www.electronenergy.com.

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