Magnetic materials
What is the main purpose of adding Dy or Tb to neodymium magnets used at high temperatures, such as in electric-vehicle motors?
- ATo raise the remanent flux density and thus the maximum energy product
- BTo raise the Curie temperature above 1000°C and so prevent thermal demagnetization at high temperature
- CTo compensate for the coercivity that falls as the temperature rises, by raising the anisotropy field
Show answer and explanation
Answer: C. To compensate for the coercivity that falls as the temperature rises, by raising the anisotropy field
Nd₂Fe₁₄B has a rather low Curie temperature of about 310°C, and as the temperature rises its magnetocrystalline anisotropy falls and its coercivity drops sharply. To prevent demagnetization at the motor's operating temperature, part of the Nd is replaced by Dy or Tb to raise the anisotropy field and the coercivity. However, the magnetic moments of Dy and Tb couple antiparallel to those of Fe, so the more is added, the lower the saturation magnetization, and the remanent flux density and maximum energy product actually decrease. The Curie temperature also stays at a few hundred °C with this addition and does not exceed 1000°C. Because Dy and Tb are expensive and their resources are unevenly distributed, methods that diffuse them only into the grain surfaces to reduce the amount used have been put into practice.