Presentation Information
[O3-2]Effect of Desorption Treatment Conditions for Recombination on Magnetic Properties in (Nd,Ce)-Fe-B Based HDDR Magnet Powders
*Ryo Shimbo1,2, Shunsuke Nomura1, Takashi Horikawa1,2, Masao Yamazaki1, Masashi Matsuura2, Ryosuke Kainuma2, Satoshi Sugimoto3 (1. Aichi Steel Corporation (Japan), 2. Department of Materials Science, Graduate School of Engineering, Tohoku University (Japan), 3. Department of Management Science and Technology, Graduate School of Engineering, Tohoku University (Japan))
Keywords:
rare earth magnets,magnet powder,disproportionation,recombination,microstructural changes
High magnetic anisotropy and coercivity (HcJ) can be achieved by applying the dynamic hydrogenation disproportionation desorption recombination (d-HDDR) process to Nd-Fe-B magnet powder.1) Recent concerns regarding Nd supply owing to rising demand have prompted the development of (Nd,Ce)-Fe-B HDDR anisotropic magnet powders, replacing Nd with abundant Ce. We clarified that the relationship between hydrogen pressure and recombination temperature in HDDR (P-T curve) differs between Nd2Fe14B and Ce2Fe14B and that RFe2 present in both Ce-Fe-B and (Nd,Ce)-Fe-B also undergoes HDDR reactions.2,3) Furthermore, (Nd,Ce)-Fe-B HDDR magnet powder after grain boundary diffusion (GBD) with Nd-Cu-Al exhibited demagnetization curves with low squareness (SQ) with a knick, differing from those of Nd-Fe-B.3) A few microstructural factors were thought to induce this low SQ: ferromagnetic RFe2 (R = Nd and Ce) unevenly distributed as the grain boundary phase,3) and the inhomogeneous distribution of Nd and Ce within the R2Fe14B and R-rich phases.3) The aforementioned difference between the P-T curves of Nd-Fe-B and Ce-Fe-B and the inhomogeneous microstructures of (Nd,Ce)-Fe-B suggest that the HDDR conditions for obtaining high SQ in (Nd,Ce)-Fe-B differ from those in Nd-Fe-B. Therefore, in this study, we investigate the effects of the desorption (dehydrogenation) process temperature and time for recombination, which are closely related to the P-T curve, on the magnetic properties and microstructures of (Nd,Ce)-Fe-B to achieve a high SQ.
The starting powders (Nd1-xCex)12.5Febal.B6.5Nb0.2 (x = 0 (Ce0) and 0.5 (Ce50)) were disproportionated by heating at 840 °C under a H2 pressure of 30 kPa for 3 h and subsequently underwent desorption (dehydrogenation) to induce recombination. Desorption was conducted within a process temperature (Tdes.) range of 800–880 °C and time (tdes.) of 1.5–30 h at 1 kPa followed by an additional vacuum process for 0.5 h. The obtained powders were mixed with Nd-Cu-Al powder and subjected to GBD at 800 °C for 1 h under vacuum.
Figure 1(a) depicts the demagnetization curves of Ce50 after GBD when the desorption times were 1.5 (conventional condition3)), 10, 20, and 30 h at 840 °C, revealing that SQ was improved by prolonging tdes. Similarly, a higher Tdes. improved SQ. In contrast, Ce0 exhibited a negligible change in SQ even under high Tdes. and long tdes.. To investigate the differences in microstructures between the low- and high- SQ samples at Ce50, microstructural observations were performed using SEM-EDS. A difference in the amount of RFe2 present on the powder surface (surface-RFe2) was observed after HDDR. Figure 1(b) illustrates the dependence of the area fraction (AF) of the surface-RFe2 after HDDR on tdes. at 840 °C, indicating that the area fraction decreased with increasing tdes. In the case of GBD on the sample after HDDR for tdef. = 1.5 h (AF of surface-RFe2 ≈ 60%), Ce components derived from the surface-RFe2 are likely to infiltrate through grain boundaries into the internal regions of the powder below this surface-RFe2. Thus, RFe2 is more likely to form in these regions, and the Ce/(Nd+Ce) ratio in the R2Fe14B and R-rich phases increases, consequently decreasing HcJ. Conversely, in regions where there is no surface-RFe2, derived Ce does not infiltrate during GBD, reducing RFe2 and Ce/(Nd+Ce) contents in the R2Fe14B and R-rich phases and consequently HcJ. Thus, the cause of the low SQ at tdef. = 1.5 h is considered to be the different regions of HcJ after GBD generated by the presence or absence of surface-RFe2 after HDDR. In contrast, the sample after HDDR for tdef. = 30 h (AF of surface-RFe2 < 10%) is considered to have a higher SQ because there are fewer areas wherein the Ce components derived from the surface-RFe2 have infiltrated.
Acknowledgements
This paper is based on results obtained from a project, JPNP 23017, commissioned by the New Energy and Industrial Technology Development Organization (NEDO).
This work was partially supported by the Advanced Material Engineering Division of Toyota Motor Corporation, Japan.
1) C. Mishima et al., IEEE Trans. Magn., 37 (2001) 2467.
2) R. Shimbo et al., Mater. Trans., 64 (2023) 2665.
3) R. Shimbo et al., 2025 Joint MMM-Intermag Conference, New Orleans, LA., (2025).
The starting powders (Nd1-xCex)12.5Febal.B6.5Nb0.2 (x = 0 (Ce0) and 0.5 (Ce50)) were disproportionated by heating at 840 °C under a H2 pressure of 30 kPa for 3 h and subsequently underwent desorption (dehydrogenation) to induce recombination. Desorption was conducted within a process temperature (Tdes.) range of 800–880 °C and time (tdes.) of 1.5–30 h at 1 kPa followed by an additional vacuum process for 0.5 h. The obtained powders were mixed with Nd-Cu-Al powder and subjected to GBD at 800 °C for 1 h under vacuum.
Figure 1(a) depicts the demagnetization curves of Ce50 after GBD when the desorption times were 1.5 (conventional condition3)), 10, 20, and 30 h at 840 °C, revealing that SQ was improved by prolonging tdes. Similarly, a higher Tdes. improved SQ. In contrast, Ce0 exhibited a negligible change in SQ even under high Tdes. and long tdes.. To investigate the differences in microstructures between the low- and high- SQ samples at Ce50, microstructural observations were performed using SEM-EDS. A difference in the amount of RFe2 present on the powder surface (surface-RFe2) was observed after HDDR. Figure 1(b) illustrates the dependence of the area fraction (AF) of the surface-RFe2 after HDDR on tdes. at 840 °C, indicating that the area fraction decreased with increasing tdes. In the case of GBD on the sample after HDDR for tdef. = 1.5 h (AF of surface-RFe2 ≈ 60%), Ce components derived from the surface-RFe2 are likely to infiltrate through grain boundaries into the internal regions of the powder below this surface-RFe2. Thus, RFe2 is more likely to form in these regions, and the Ce/(Nd+Ce) ratio in the R2Fe14B and R-rich phases increases, consequently decreasing HcJ. Conversely, in regions where there is no surface-RFe2, derived Ce does not infiltrate during GBD, reducing RFe2 and Ce/(Nd+Ce) contents in the R2Fe14B and R-rich phases and consequently HcJ. Thus, the cause of the low SQ at tdef. = 1.5 h is considered to be the different regions of HcJ after GBD generated by the presence or absence of surface-RFe2 after HDDR. In contrast, the sample after HDDR for tdef. = 30 h (AF of surface-RFe2 < 10%) is considered to have a higher SQ because there are fewer areas wherein the Ce components derived from the surface-RFe2 have infiltrated.
Acknowledgements
This paper is based on results obtained from a project, JPNP 23017, commissioned by the New Energy and Industrial Technology Development Organization (NEDO).
This work was partially supported by the Advanced Material Engineering Division of Toyota Motor Corporation, Japan.
1) C. Mishima et al., IEEE Trans. Magn., 37 (2001) 2467.
2) R. Shimbo et al., Mater. Trans., 64 (2023) 2665.
3) R. Shimbo et al., 2025 Joint MMM-Intermag Conference, New Orleans, LA., (2025).
