Magnetics Business & Technology - Summer 2016 - (Page 14)

FEATURE ARTICLE Advances in Manufacture of Low or No Heavy Rare Earths NdFeB Magnets By Shuk Rashidi and Tracy Moon, Tridus Magnetics and Assemblies, USA, in cooperation with Zhong Ke San Huan, High Tech Co. Since its commercialization in 1983, Neodymium Iron Boron magnets have undergone immense changes for the better in composition, integrity and unit properties. Prior to the use of NdFeB alloys, rare earth cobalt alloys were used. The Samarium Cobalt family of alloys were almost ideal in that they were optimized sotichiometrically for the highest energy products (BHmax), as well as intrinsic coercive force (Hci). The Curie temperature of the Samarium family of permanent magnet was higher and the Hci almost free. The design engineer of the magnetic circuit did not have to be overly concerned with the loss of unit properties when the device operated continuously or intermittently at higher temperatures. The reason being that the alloy inherently had high anisotropy of more than 300 Kilo-Oersteds and a extremely high Curie temperature. The Emergence of Neodymium Iron Boron or NdFeB Alloy The magnetic and physical properties of NdFeB magnets have improved steadily since becoming commercially available. When first introduced, this family of alloys offered BHmax that ranged between 30 to 40 MGOe. Now, production magnets are offered with energy products in excess of 55 MGOe, and scientists have achieved as high as 59 MGOe in the lab. The whopping increase in the energy product of these magnets is due to a host of factors, including better handling, and more methodical manufacturing processes combined with new methods leading to the enhancement of coercive force as well as saturation magnetization. The polycrystalline body of the family of NdFeB is made up of three phases; the Nd2Fe14B phase, or the main phase, the Nd rich phase, and the phase where grain boundaries separate the two adjacent crystals. To achieve better magnetic properties it is better to increase the ratio of the main phase to other nonmagnetic phases in the body of the alloy. In the last twenty years, the percentage of the magnetic phase, or the Nd2Fe14B, has gone up from 90 percent to 97 percent. This has enhanced the overall magnetic properties of the alloy substantially. The new family of NdFeB alloys offered much higher saturation magnetization Br, reasonably high BHmax, coupled with Hci that depended mostly on the content of heavy rare earths (HRE). There are generally two groups of rare earth metals that are used in many of the NdFeB family of alloys. The light rare earth metals are related primarily to the Br of the magnets while the level of heavy rare earth metals results in progressively higher Hci in a given alloy. The use of the latter reduces the Br of the magnet. Light rare earths couple ferromagnetically with the transition metal, in this case Iron, while the heavy rare earths couple anti-ferromagnetically. Why does the industry get so uptight about the use of light or heavy rare earths in a given alloy? Cost. Rare earth deposits are made up of mostly light elements-Cerium, Lanthanum, Neodymium and Samarium, while the content of the heavies, Dysprosium, Terbium, Holmium and others, represent a tiny fraction of the total. This imbalance makes the price of the heavies substantially higher than the lights. Thus if the design engineer needs high Hci NdFeB magnet, he has to pay more for more heavies. In a nutshell, the coercive force costs more money in this case while in the case of Samarium based alloys this parameter was available for free. Figure 1. Figure 2. Figure 3. Methods of Reducing the Cost of High Hci Alloys There have been joint efforts by both manufacturers and users of NdFeB magnets to reduce the cost of alloys with reasonable unit magnetic properties. Since there is a poor balance in the availability of light and heavy rare earths causing large price differentials, there has been a crusade by manufacturers to reduce the level of heavy rare earths in the manufacture of alloys, even attempting alloys that are virtually free of heavy rare earths. While the latter quest may be just that, much effort has been placed on one of the most effective technique called grain boundary diffusion of heavies into the light rare earthtransition metal alloys, primarily NdFeB magnets. Heavy Rare Earths and Their Effect on the Magnetic Properties of NdFe B Magnets Figures (1) and (2) depict the trend in the pricing of Praseodymium-Neodymium (PrNd), a light twin rare earth metal alloy and Dyprosium-Ferrite (Dy-Fe) alloy. Both are used in 14 Magnetics Business & Technology * Summer 2016 Figure 4. www.MagneticsMagazine.com http://www.MagneticsMagazine.com

Table of Contents for the Digital Edition of Magnetics Business & Technology - Summer 2016

Editor's Choice
Extending the Limits of the Sm2Co17 System
Advances in Manufacture of Low or No Heavy Rare Earths NdFeB magnets
Research & Development
New Measurement Technique Characterizes Permanent Magnets
Electromagnetics
Industry News
The Ambition to Get Highest Flux Densities,Is It Always Reasonable
Marketplace / Advertising Index
Spontaneous Thoughts: Permanent Magnet Mistakes, Part 2

Magnetics Business & Technology - Summer 2016