Magnetics Business & Technology - Spring 2013 - (Page 8)

FEATURE ARTICLE Dysprosium-Free Rare Earth Magnets for High Temperature Applications By Dr. Nimitkumar Sheth, Applications Technology Manager • Molycorp Magnequench Heavy rare earths have been most affected by the escalation of rare earth prices. This has driven motor manufacturers to look for lower cost alternatives that avoid heavy rare earths, especially Dysprosium (Dy) and Terbium (Tb). In sintered NdFeB magnets, Dy or Tb is often added to replace Neodymium (Nd) to preserve the magnetic performance of the magnet at elevated temperature. Due to the scarcity and high demand of these heavy rare earths, Dy and Tb have been facing high price volatility which has lead to a concerted effort by manufacturers to design new applications around Dy and Tb free alloys. The MQ2 Solution Hot pressed nanocrystalline NdFeB magnets (MQ2) provide improved temperature stability without costly heavy rare earths like Terbium and Dysprosium as shown in Figure 2. Produced by a net shape manufacturing technique, limiting the waste and environmental impact, MQ2 magnet is an effective low cost solution for higher flux applications. With its energy product of 14 to 16 MGOe and high temperature stability, MQ2 material is suitable for use in applications like traction motors and air-conditioner compressors. Explanation for MQ2: The Grain Size Advantage Other than heavy rare earth substitution, the intrinsic coercivity of a magnet is strongly influenced by its internal microstructure. Reducing the grain size increases the Hci and thermal stability of a magnet. This is due to the movement of magnetic domains being impeded by a denser network of grain boundaries and also due to individual domains being isolated within single grains if the grains are fine enough. This relationship between grain size and sintered NdFeB magnet Hci has been illustrated by Sagawa and Hono in Figure 3. MQ2 has the smallest grain size of any fully dense rare earth magnet. Figure 4 illustrates this feature, showing MQ2 with a grain size in the order of 80 nm, and a typical sintered NdFeB magnet with grains between 5 to 10 µm. Despite the relatively low remanence of MQ2 magnets in comparison to anisotropic MQ3 and sintered NdFeB magnets (Figure 5), its nano-scale grain structure gives MQ2 the highest Hci and the best possible temperature stability achievable in NdFeB magnets without using dysprosium. Figure 6 compares the flux aging loss for the three types of magnet. These Pc=0.5, 1.0 and 2.0 magnets were held at temperature between 50°C and 175°C for 1 hour, and the irreversible flux loss was measured. Figure 6 also indicates that the Dy-free sintered NdFeB magnet is not suitable to be operated above 80°C (Pc=2), while the Dy-free MQ2 exhibits extremely low flux aging loss and is able to operate up to and beyond 175°C. (a) (b) (c) Figure 4. Fracture surfaces of (a) MQ2 (b) MQ3 and (c) sintered NdFeB magnet showing the individual Nd2Fe14B grains. Please note the difference in scale between these images. 8 Magnetics Business & Technology • Spring 2013 Figure 1. Relative portions of rare earth elements show scarcity of Dysprosium and Terbium. Figure 2. MQ2 retains 98 percent flux after 1,000 hours at 200°C without dysprosium Figure 3. Grain size dependence of Hci. A plot of Hci against the logarithm of the square of grain sizes. Figure 5. Demagnetization curves of the three types of NdFeB magnet. The sintered NdFeB magnet is VACODYM-722 HR from VAC.

Table of Contents for the Digital Edition of Magnetics Business & Technology - Spring 2013

Magnetics Business & Technology - Spring 2013
Editor's Choice
Dysprosium-Free Rare Earth Magnets for High Temperature Applications
Research & Development
Simulating the Toyota Prius Electric Motor
Magnets • Materials • Measurement
Application • Component Developments
Industry News
Marketplace/Advertising Index
Spontaneous Thoughts: Dysprosium 2.0

Magnetics Business & Technology - Spring 2013