Presentation Information
[P1-38]Improvement of Hk and squareness in d-HDDR-treated Nd-Fe-B powders prepared using modified starting powder
*Takashi Horikawa1,2, Masao Yamazaki1, Masashi Matsuura2, Satoshi Sugimoto2 (1. Aichi Steel Corporation (Japan), 2. Tohoku University (Japan))
Keywords:
Nd-Fe-B magnet powder,HDDR,Hk,squareness,powder diameter
By applying the dynamic hydrogenation-disproportionation-desorption-recombination (d-HDDR) process to Nd-Fe-B magnetic powder, high magnetic anisotropy and coercivity can be obtained [1]. We previously reported that in the preparation of the starting powder, annealing of the mother alloy at 700°C followed by hydrogen decrepitation at 500°C (referred to as the conventional method) can improve the single crystal ratio of the resultant powder and suppress crack formation in the Nd2Fe14B phase, leading to the formation of low anisotropic regions after d-HDDR. As a result, higher anisotropy was observed in the resultant d-HDDR-treated powder and in the bonded magnets fabricated using the powder [2,3]. However, the Hk and squareness (Hk/HcJ) of demagnetization curves were still low compared to those of commercially available sintered magnets, suggesting the existence of variations in coercivity and magnetic polarization among powder particles, as well as local variations of these properties within a single powder particle.
In our anisotropy induction model [2,3], the regions consisting of spherical NdH2+x grains and Fe matrix, which result in low anisotropic regions, tend to form near the surface of the powder particles after the hydrogen disproportionation step, irrespective of the powder size. As a result, the relative volume fraction of the low-anisotropy region varies with powder size, potentially inducing variations in the overall anisotropy and magnetic polarization among the powder particles. A similar trend is observed for coercivity. During the recombination step of the conventional d-HDDR treatment, the Nd-rich components in the starting powder diffuse into the Nd2Fe14B particles and form Nd-rich grain boundaries, which induce the coercivity. However, the distribution of grain boundary triple junctions appeared to show non-uniformity according to particle size, possibly leading to coercivity variations. Therefore, narrowing the size of starting powder particles is expected to contribute to reducing such variations. Furthermore, although an additional grain boundary diffusion process using fine Nd-Cu-Al alloy powder becomes necessary after the d-HDDR treatment, the removal of the Nd-rich components can result in more homogeneous grain boundaries and hence less variation in coercivity. Fig. 1. Demagnetization curves of d-HDDR-treated powders prepared using conventional and modified starting powders.
In this study, to improve the low Hk of d-HDDR-treated powders, the two modifications mentioned above were applied to the starting powder by sieving. First, the size range of the starting powder particles was narrowed from < 212 μm in the conventional method to 75–106 μm. Second, during this narrowing process, the excess Nd-rich grain boundary components present as fine particles (< 53 μm) in the starting powder, which are usually generated during the hydrogen decrepitation treatment, were simultaneously removed.
After these modifications, the Hk and squareness increased from 0.81 MA/m and 61%, respectively, for the conventional d-HDDR to 0.98 MA/m and 73% (Fig. 1).
[1] C. Mishima et al., IEEE Trans. Magn., 37 (2001) 2467.
[2] T. Horikawa et al., Sci. Tech. Adv. Mater., 22 (2021) 729.
[3] T. Horikawa et al., The 170th Annual meeting of JIM, (2022) S7.2.
In our anisotropy induction model [2,3], the regions consisting of spherical NdH2+x grains and Fe matrix, which result in low anisotropic regions, tend to form near the surface of the powder particles after the hydrogen disproportionation step, irrespective of the powder size. As a result, the relative volume fraction of the low-anisotropy region varies with powder size, potentially inducing variations in the overall anisotropy and magnetic polarization among the powder particles. A similar trend is observed for coercivity. During the recombination step of the conventional d-HDDR treatment, the Nd-rich components in the starting powder diffuse into the Nd2Fe14B particles and form Nd-rich grain boundaries, which induce the coercivity. However, the distribution of grain boundary triple junctions appeared to show non-uniformity according to particle size, possibly leading to coercivity variations. Therefore, narrowing the size of starting powder particles is expected to contribute to reducing such variations. Furthermore, although an additional grain boundary diffusion process using fine Nd-Cu-Al alloy powder becomes necessary after the d-HDDR treatment, the removal of the Nd-rich components can result in more homogeneous grain boundaries and hence less variation in coercivity. Fig. 1. Demagnetization curves of d-HDDR-treated powders prepared using conventional and modified starting powders.
In this study, to improve the low Hk of d-HDDR-treated powders, the two modifications mentioned above were applied to the starting powder by sieving. First, the size range of the starting powder particles was narrowed from < 212 μm in the conventional method to 75–106 μm. Second, during this narrowing process, the excess Nd-rich grain boundary components present as fine particles (< 53 μm) in the starting powder, which are usually generated during the hydrogen decrepitation treatment, were simultaneously removed.
After these modifications, the Hk and squareness increased from 0.81 MA/m and 61%, respectively, for the conventional d-HDDR to 0.98 MA/m and 73% (Fig. 1).
[1] C. Mishima et al., IEEE Trans. Magn., 37 (2001) 2467.
[2] T. Horikawa et al., Sci. Tech. Adv. Mater., 22 (2021) 729.
[3] T. Horikawa et al., The 170th Annual meeting of JIM, (2022) S7.2.
