High Coercivity Neodymium Magnet without Using Heavy Rare Earth Element Dysprosium
National Institute for Materials Science
NIMS developed a method for increasing the coercivity of neodymium magnet powder without using dysprosium (hereinafter, Dy).
Abstract
- The Magnetic Materials Center at the National Institute for Materials Science (President: Sukekatsu Ushioda) developed a method for increasing the coercivity of neodymium magnet powder without using dysprosium (hereinafter, Dy). Dy is a heavy rare earth element that is normally essential for achieving high coercivity in neodymium magnets for traction motors of hybrid vehicles (HV). This research has demonstrated that the coercivity of neodymium magnets powder produced by the hydrogenation-disproportionation-desorption-recombination (HDDR) process can be enhanced by modifying the chemistry of the crystal grain boundaries. The HDDR powder is composed of ultrafine crystals of a Nd2Fe14B composund of about 250 nm, which is more than one order of magnitude smaller than that of the standard neodymium magnets manufactured by the sintering process.
- One of the hard magnetic properties called "coercivity" of neodymium magnets decreases as temperature increases, thus the neodymium magnets comprised of neodymium, iron and boron cannot be used in the traction motors of HV, in which the operating temperature of the magnet rises to around 200°C. To overcome this problem, currently used high coercivity neodymium magnets contain a large amount of Dy, as much as 40% of the neodymium (Nd).
- However, the natural abundance of Dy is only 10% of that of Nd. Thus, it is necessary to reduce the amount of Dy to less than 10% of that of Nd to meet the increasing demand of high coercivity Dy-bearing neodymium magnets.
- One effective method for reducing the amount of Dy in high coercivity neodymium magnets is to diffuse the Dy from the surface of the magnet along the grain boundaries. The amount of required Dy can be reduced considerably since only the Nd at the grain boundaries can be substituted with Dy. However, heavy rare earth elements are still necessary with this diffusion method.
- Based on the systematic nanostructure analysis of existing neodymium magnets, the researchers at NIMS reached a conclusion that the coercivity can be improved by decoupling the ferromagnetic interactions between the crystal grains. Instead of using heavy rare earth elements, they successfully modified the Nd composition at the grain boundaries by diffusing a neodymium-copper (Nd-Cu) alloy along the grain boundaries. Since the Nd-Cu alloy has a lower melting point due to the eutectic reaction, the melt infiltrates into the grain boundaries above 550°C, thereby increasing the Nd concentration at the grain boundaries. This led to an increase in coercivity of more than 20% without using any heavy rare earth elements. This research has demonstrated that high coercivity can be realized by a magnetic isolation of fine crystal grains in a neodymium magnet powder produced by the HDDR method, which has a grain size of approximately one order of magnitude smaller than that of the sintered magnets.
- This work will be presented at the 34th Annual Conference on Magnetics in Japan to be held in Tsukuba on September 5. The paper has been accepted by Scripta Materialia, and has become available on line at http://dx.doi.org/10.1016/j.scriptamat.2010.08.021. This research was carried out as part of the Elements Science and Technology Project (Element Strategy Project), “Project for High Performance Anisotropic Nanocomposite Permanent Magnets with Low Rare-Earth Content” of the Ministry of Education, Culture, Sports, Science and Technology (MEXT).
