Presentation Information
[P2-51]Production of Sm2Fe17N3 magnets from fine powder produced from reduction diffusion method
*Akihide Hosokawa1, Yusuke Hirayama1, Shusuke Okada1, Yuta Iida1,2 (1. National Institute of Advanced Industrial Science and Technology (Japan), 2. Niterra Co., Ltd. (Japan))
Keywords:
calciothermic,reduction diffusion,Samarium iron nitride,sintering aid,liquid phase sintering
The Sm2Fe17N3 compound is one of the prospective candidates for next-generation permanent magnet materials due to its excellent intrinsic properties [1]. However, consolidation of this compound has been a challenge due to its low thermal decomposition temperature (= 620 °C). The utilization of cemented carbide die-sets made it possible to apply high pressure during direct current pressure sintering, and enabled to produce sintered Sm2Fe17N3 magnets even below the decomposition temperature, but it turned out coercivity degradation occurrs after sintering, mainly because of redox effects of the powder surface oxides. We developed low-oxygen powder metallurgical processes and successfully produced high-coercivity anisotropic Sm2Fe17N3 magnets. However, the (BH)max we could achieve was 24 MGOe, which is far below the value we expect from the powder (BH)max of 42 MGOe. It has been reported that the relative density of the sintered magnets produced by our low oxygen powder metallurgical processes is not higher than 90%, being a critical reason of the low (BH)max.
From this point of view, we began exploration of new sintering aids that makes it possible to densify the magnets by liquid phase sintering. A sintering aid we recently reported is Ba-Cu alloy. This sintering aid successfully improved relative density of the sintered magnet as well as its magnetization [2]. In the upcoming REPM 2025, the colleagues from our group will introduce the update of that work.
The research topic introduced by the current conference contribution is based on a different approach. As mentioned above, the high coercivity Sm2Fe17N3 magnets requires powder pulverized under low-oxygen atmosphere. In other words, once the powder is exposed to air prior to sintering, the magnets consolidated from those exposed powder never exhibits high coercivity. It is well known that Reduction Diffusion (RD) method using Ca as reducing agent allows to produce extremely high coercivity fine Sm2Fe17N3 powder but the powder necessarily hold the oxidized surfaces because of the inevitable rinsing processes [3]. The question that arose here is whether it is possible to remove those oxidized surfaces of the RD powder if we use Ca-based sintering aids. It is known that lines of Ca oxide and Pr oxide are located below the line of Sm oxides in the Ellingham diagram. In principle, the surface oxides of Sm- and Fe- should be reduced and the Ca-based oxides should form instead, leading to removal of the detrimental effects of the oxidized surface of RD powder. Additionally, Ca-Cu and Pr-Cu binary systems have eutectic points below 500 °C. Based on this idea, we decided to explore the possibility of production of sintered magnets using Sm2Fe17N3 powder produced by RD method and Ca- and Pr-based sintering aids.
High coercivity powder Sm2Fe17N3 having a median diameter of 2.6 um was produced by reduction diffusion method in our laboratory, being used as starting material. The detail of the method can be found in previous paper [3]. As sintering aids, Ca-Cu and Pr-Cu coarse flake powders were produced by melt-spinning technique and followed by crushing. The RD powder and the melt-spun powder were mixed together and compacted inside a cemented-carbide die-set by pressure of 600 MPa.
As a consequence, however, it was found that the annealing temperature of 500 °C is too low in terms of diffusion process, and thus the reduction does not finish within the annealing time around 15 minutes or so. Nevertheless, interesting finding is that the sintering aids are well dispersed within the sintered body. When the Pr-Cu sintering aid is mixed with pulverized powder under low-oxygen atmosphere, the sintering aid does not prevail at all. The reason of this is not clear yet. In this presentation, the difference of microstructural difference between the cases of RD and pulverized powders will be demonstrated and the mechanism behind is discussed.
References
[1] T. Iriyama, K. Kobayashi, N. Imaoka, IEEE Trans. Magn., 2326-2331 (1992)
[2] Niterra Co., Ltd., AIST, press release “Development of high-density technology for Sm2Fe17N3 permanent magnet” (2024)
[3] S. Okada, E. Node, K. Takagi, R. Hashimoto, J. Alloys and Compounds, 170726 (2023)
From this point of view, we began exploration of new sintering aids that makes it possible to densify the magnets by liquid phase sintering. A sintering aid we recently reported is Ba-Cu alloy. This sintering aid successfully improved relative density of the sintered magnet as well as its magnetization [2]. In the upcoming REPM 2025, the colleagues from our group will introduce the update of that work.
The research topic introduced by the current conference contribution is based on a different approach. As mentioned above, the high coercivity Sm2Fe17N3 magnets requires powder pulverized under low-oxygen atmosphere. In other words, once the powder is exposed to air prior to sintering, the magnets consolidated from those exposed powder never exhibits high coercivity. It is well known that Reduction Diffusion (RD) method using Ca as reducing agent allows to produce extremely high coercivity fine Sm2Fe17N3 powder but the powder necessarily hold the oxidized surfaces because of the inevitable rinsing processes [3]. The question that arose here is whether it is possible to remove those oxidized surfaces of the RD powder if we use Ca-based sintering aids. It is known that lines of Ca oxide and Pr oxide are located below the line of Sm oxides in the Ellingham diagram. In principle, the surface oxides of Sm- and Fe- should be reduced and the Ca-based oxides should form instead, leading to removal of the detrimental effects of the oxidized surface of RD powder. Additionally, Ca-Cu and Pr-Cu binary systems have eutectic points below 500 °C. Based on this idea, we decided to explore the possibility of production of sintered magnets using Sm2Fe17N3 powder produced by RD method and Ca- and Pr-based sintering aids.
High coercivity powder Sm2Fe17N3 having a median diameter of 2.6 um was produced by reduction diffusion method in our laboratory, being used as starting material. The detail of the method can be found in previous paper [3]. As sintering aids, Ca-Cu and Pr-Cu coarse flake powders were produced by melt-spinning technique and followed by crushing. The RD powder and the melt-spun powder were mixed together and compacted inside a cemented-carbide die-set by pressure of 600 MPa.
As a consequence, however, it was found that the annealing temperature of 500 °C is too low in terms of diffusion process, and thus the reduction does not finish within the annealing time around 15 minutes or so. Nevertheless, interesting finding is that the sintering aids are well dispersed within the sintered body. When the Pr-Cu sintering aid is mixed with pulverized powder under low-oxygen atmosphere, the sintering aid does not prevail at all. The reason of this is not clear yet. In this presentation, the difference of microstructural difference between the cases of RD and pulverized powders will be demonstrated and the mechanism behind is discussed.
References
[1] T. Iriyama, K. Kobayashi, N. Imaoka, IEEE Trans. Magn., 2326-2331 (1992)
[2] Niterra Co., Ltd., AIST, press release “Development of high-density technology for Sm2Fe17N3 permanent magnet” (2024)
[3] S. Okada, E. Node, K. Takagi, R. Hashimoto, J. Alloys and Compounds, 170726 (2023)