Research and Development / New Alloying Process by Grain Boundary Diffusion

  • topics
  • Largest Magnet

New Alloying Process by Grain Boundary Diffusion

"The Alloying Process by Grain Boundary Diffusion" is a new technology developed by Shin-Etsu Chemical, in which the dysprosium is concentrated near the grain boundaries of the Nd-Fe-B sintered magnet. With this new process, further enhancement in coercivity and suppression of reduction in remanence, compared with the conventional "Two Alloy method", has been realized.

In this section, we will explain this new technology.

First of all, let's see the coercivity mechanism of the Nd-Fe-B magnet through the relationship between a magnetization curve and magnetic domain wall movements.

Click the thumbnails for the large figure.

New Alloying Process by Grain Boundary Diffusion

Thus, the coercivity of Nd-Fe-B sintered magnet is governed by a magnetic field which causes nucleation of reversed magnetic domain. Enhancement of the anisotropy field is effective for suppressing the nucleation, so that the higher coercivity can be achieved.

Coercivity can be enhanced by partial substitution of Nd by heavy rare earths such as dysprosium and terbium which increase the anisotropy field. However, as shown in the right-hand figure, the coercivity increases with increasing the amount of substitution, while the remanence decreases.

New Alloying Process by Grain Boundary Diffusion

Consequently, Shin-Etsu Chemical developed an alternative process called "Two Alloy Method". In this process, two alloys with different melting points are mixed and then sintered; one is the main alloy whose composition is close to the stoichiometry of the magnetic compound and free from heavy rare earth elements, another is a sintering aid (i.e. an alloy with a low melting point) containing heavy rare earths. By this process, heavy rare earths such as dysprosium which increase the anisotropy field, are concentrated selectively near the grain boundaries. As a result enhancement of coercivity with suppression of serious reduction in remanence can be achieved and efficient usage of heavy rare earths can be realized.

New Alloying Process by Grain Boundary Diffusion

New Alloying Process by Grain Boundary Diffusion

Furthermore, "New Alloying Process by Grain Boundary Diffusion", newly developed by Shin-Etsu Chemical, has realized a significant enhancement in coercivity with suppression of reduction in remanence due to a sharp distribution of heavy rare earths such as dysprosium and higher concentration of dysprosium compared with the conventional Two Alloy Method.

With the conventional Two Alloy Method, since the dysprosium is diffused during sintering at high temperatures, the dysprosium is distributed widely in the interior of grains, and so an excess of dysprosium is necessary. In the New Alloying Process by Grain Boundary Diffusion, sintered bodies are coated with the rare earth compound and then heat treated at lower temperatures than sintering temperatures (diffusion treatment). Using this new process, the amount of dysprosium diffused into magnets can be appropriate and more efficient usage of heavy rare earths can be realized.

Image diagram

New Alloying Process by Grain Boundary Diffusion

Comparison using actual metallographic structure photographs

New Alloying Process by Grain Boundary Diffusion

As shown in the lower figure, the New Alloying Process by Grain Boundary Diffusion provides an enhancement in coercivity by about 30% without any reduction in remanence compared to the conventional Two Alloy Method.

New Alloying Process by Grain Boundary Diffusion

With the New Alloying Process by Grain Boundary Diffusion, Shin-Etsu Chemical makes it possible to realize the magnets with higher performance than our conventional productions.

New Alloying Process by Grain Boundary Diffusion
  • Topics
  • Si Substlates

Back to Top