May 2013

Rare earth-free magnets for energy-efficient motors and generators

The magnet renaissance

TDK is doing pioneering work to eliminate the need for rare earth metals in high-performance permanent magnets, while improving energy efficiency in a broad range of compact and lightweight motors and generators.

Until now, rare earth metals have been an essential material for the highest performance magnets. To remedy this unsatisfactory situation, TDK is reinventing magnet materials. The company’s approach to creating rare earth-free magnets is two-fold:

  • Reduce and eliminate the most scarce and expensive rare earth metals such as dysprosium (Dy) used in the production of neodymium magnets – the class of permanent magnets with the highest performance
  • Make today’s ferrite magnets completely free of lanthanum (La), a rare earth metal, and cobalt (Co), a rare metal with limited natural reserves

Reducing dysprosium with the HAL process

TDK introduced an innovative high anisotropy field layer (HAL) production process that minimizes the use of dysprosium in its NEOREC series of magnets, which consist mainly of neodymium, iron and boron. In these magnets, some of the neodymium in the periphery of the crystalline structure is replaced with particles of dysprosium to increase the intrinsic coercive force (HCJ) and enhance the heat resistance.

Figure 1: TDK NEOREC magnets manufactured with the HAL process

The TDK NEOREC 55 is the strongest magnet currently produced with the HAL process.

In the HAL process, neodymium magnets are heat-treated at relatively low temperatures after the dysprosium diffusion source is applied to the surfaces of the sintered neodymium magnet substrates. This diffuses the dysprosium evenly throughout the neodymium-rich phases surrounding the crystalline particles without allowing it to penetrate into the crystal grains. The result is enhanced coercivity, while saving expensive dysprosium. The improvements in magnet performance enable smaller motors and greater efficiencies, ultimately leading to energy, and thus also cost savings. The HAL method enables TDK to manufacture NEOREC magnets with up to 50 percent less dysprosium, while at the same time improving their residual flux density (Br) by up to 5 percent (Figure 1).

Mass production of dysprosium-free magnets
It is not possible to manufacture an absolutely Dy-free magnet using the HAL method alone. TDK, therefore, reengineered the entire production process, starting with the material composition of the alloy. By employing a special low oxygen process throughout all production steps from powder processing to sintering, which decreases impurities and results in an even more homogenous microstructure (Figure 2), TDK succeeded in developing a Dy-free neodymium magnet with increased intrinsic coercive force. The new magnet, which is the world’s first commercially produced neodymium magnet that is completely free of dysprosium, will be used in the voice coil motors for the read/write heads of hard disk drives. The NEOREC47HF magnet features a Br of 1390 mT (±20%) and HCJ of ≥1273 KA/m.

Figure 2: Alloy microstructure of a TDK neodymium magnet with and without dysprosium

Rare earth-free ferrite magnets
The second part of TDK’s dual approach is to focus on the development of a high-performance ferrite material that is free of rare earth elements. The aim is to achieve both high performance and a competitive cost for use in high-volume applications, such as compressor motors for air-conditioners and the growing number of micromotors in electric vehicles.

Figure 3: TDK’s new La-Co-free ferrite magnets

The properties of the new La-Co-free ferrite magnet that are equivalent to those of the FB9 material.

Today’s high-performance ferrite magnets, however, still employ the additional trace elements lanthanum and cobalt in order to improve the intrinsic coercive force. Although less powerful than neodymium magnets in terms of magnetic energy, ferrite magnets account for over 90 percent of the world's magnet production by weight. For this reason, TDK – as the leader in magnet technology – has pushed forward the development of magnets that are free of both rare earth metals and rare metals.

Similar to Dy-free neodymium magnets, the key to eliminating the need for lanthanum and cobalt in ferrite magnets is to make the crystalline particles of ferrite more refined and uniform. The result is a new La-Co-free ferrite magnet with properties that are equivalent to those of the FB9 material (Figure 3).

The new material further enhances the cost-effectiveness of ferrite magnets while saving resources and will greatly contribute to the size and weight reduction of motors. Mass production of the first new series of this kind began in 2013.

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