Technology | Highlight
Technology Yields High Coercivity Magnet Powder at Room Temperature
T

he National Institute of Advanced Industrial Science and Technology (AIST), in collaboration with TDK Corporation, has developed a technology that can produce samarium-iron-nitrogen (Sm2Fe17N3)-based magnet powders that have coercivity at room temperature exceeding 30kOe without using heavy rare earth elements. As it has excellent heat resistance, it is expected to realize a magnet that exceeds neodymium-iron-boron (Nd-Fe-B) magnets in high-temperature environments such as drive motors for hybrid vehicles.

Improved Reduction-Diffusion Reaction System
The research group worked on improving the uniformity of the reduction-diffusion reaction system for increasing the coercivity of Sm2Fe17N3 magnet powder. In the conventional reduction-diffusion method, a large number of coarse agglomerated particles are formed in addition to Sm2Fe17N3 fine particles. These agglomerated particles are mainly in Sm-Fe alloy phase (Sm rich phase) containing a large amount of Sm and generally have low-quality magnetic properties. Therefore, the formation of these agglomerated particles was considered to limit the coercive force of the Sm2Fe17N3 magnet.

In the conventional reduction-diffusion method, it is difficult to cause uniform reaction of Sm and Fe down to the micrometer order. It is considered that this non-uniformity produces Sm-rich liquid phase, resulting in the formation of agglomerated particles due to the cross-linking effect. In order to prevent the formation of agglomerated particles, therefore, the research group tried to make the reduction-diffusion reaction itself more uniform.

Relationship between particle size and room temperature coercivity of the Sm2Fe17N3 fine powder synthesized by the newly developed process and the conventional process (left), and photograph of the developed magnet powder (right)
Fig. 1: Relationship between particle size and room temperature coercivity of the Sm2Fe17N3 fine powder synthesized by the newly developed process and the conventional process (left), and photograph of the developed magnet powder (right)
Field-emission scanning electron microscope image of Sm2Fe17N3 magnet powder synthesized by (a) the conventional method and (b) the newly developed rotary heat treatment technology
Fig. 2: Field-emission scanning electron microscope image of Sm2Fe17N3 magnet powder synthesized by (a) the conventional method and (b) the newly developed rotary heat treatment technology
The best way to homogenize the reduction-diffusion reaction is to make heat treatment of the powder while stirring. However, in the reduction-diffusion reaction, very active Sm-Fe liquid phase, calcium (Ca) melt and Ca vapor are generated. Therefore, it is necessary to simultaneously perform stirring and heat treatment of a multi-phase system in an airtight space.

The group developed a rotary heat treatment technology specializing in the reduction-diffusion reaction that can simultaneously perform these two processes. With this technology, the synthesized Sm2Fe17N3 magnet powder is almost free from coarse agglomerated particles. Furthermore, the growth of particles through the liquid of the Sm-rich phase is suppressed, producing finer Sm2Fe17N3 magnet powder.

By both the newly developed technology and the conventional process, various Sm2Fe17N3 magnet powders having different particle sizes were synthesized under different synthesis conditions including the reduction-diffusion temperature, and their magnetic properties were evaluated and compared. As a result, it was found that the present technology can provide Sm2Fe17N3 powder with coercive force higher than that improved by conventional refinement technology.

In addition, the coercivity was measured to be about 11kOe at 200°C, by heating the magnet powder with room temperature, coercivity of about 32kOe was attained. This shows that it has enough coercive force to be used as an automobile drive motor.

The magnet powder with a coercive force of 30kOe or more produced in this research contains some particles strongly bound together. For this reason, the orientation of the magnetization of the particles formed by orientation molding in a magnetic field remains low, and the residual specific magnetization in the easy axis direction of the molded body remains to be 85 to 100emu/g. This is about 60 percent of the specific saturated magnetization of around 160emu/g of Sm2Fe17N3.

In the future, the residual magnetization will be improved by increasing the dispersibility of particles. The technology of sintering of this magnet powder will be also developed, as current vehicles require drive motors made of sintered magnets.