Presentation Information
[P2-46]The correlation between the coercivity and microstructure of the novel 1-12-type sintered magnet under high molding pressure
*Tomoko Kuno1, Kurima Kobayashi1, Hiroshi Fujiwara1 (1. Ritsumeikan University (Japan))
Keywords:
ThMn12 type crystal structure,Sintered magnet,Coercivity,Microstructure,Spark plasma sintering method
Introduction
The novel ThMn12-type magnetic materials with composition of (Sm,Zr)(Fe,Co)11.3Ti0.7 have the high saturation polarization Js ≒ 1.55 T almost equal to that of Nd-Fe-B magnet, and the high anisotropy field Ha ≒ 8.6 MA / m and high Curie temperature TC ≒ 970 K that they exceed those of the Nd2Fe14B phase. 1) Our group successfully prepared the occurrence of relatively high coercivity (Hc ≒ 5 kOe) by the sintering using spark plasma sintering (SPS) method of the amorphous starting alloy of above composition. 2) And it was revealed that relatively high coercivity appears in the sample showing the XRD pattern of the mixed phase of the TbCu7 type (1-9 phase) and the ThMn12 type (1-12 phase) crystal structures. 2) However, it is important to reveal the coercivity mechanism of the sample because higher coercivity is requested for the application of this magnet. In this paper, therefore, we analyze the correlation between the coercivity and microstructure of the magnets that were sintered under 50 to 100 MPa molding pressure.
Experiments and Results
An amorphous powder of starting compound (Sm0.8Zr0.2)1.1(Fe0.9Co0.1)11.3Ti0.7 was prepared via the RQ method by Santoku Co., Ltd., Japan with the rotation velocity of the cooling roller set to about 40 m / s. The pre-milled powder of starting alloy (ca. 1 g) was transferred into a hard alloy die (inside covered with carbon film) of about 10 mm in diameter, which was placed into a SPS furnace (SPS-510L, Sumitomo Metal Mining Co. Ltd.). SPS was then performed at 923 to 1073 K for 0.3 to 3.6 ks, using an initial heating rate of 0.3 K / s and a pressure of approximately 50 to 100 MPa in vacuum (PO2 = 1.5 × 101 Pa). The SPS sintered samples have a thickness of approximately 2 mm and a diameter of about 10 mm. The crystal structures of the samples were evaluated from their XRD patterns (LabX XRD-6100, Shimadzu, Japan, Cu-Kα). Sample’s Hc were measured by vibrating-sample magnetometry (VSM-5 HSC, TOEI Industrial Co. Ltd., Japan). We used the law of approach for ferromagnetic saturation (LAFS) method1) to determine Js and Ha of the magnetically isotropic samples from hysteresis loops of Hmax of 12 MA / m (High Field Laboratory for Superconducting Materials, Institute for Materials Research, Tohoku University, Japan). The observed surfaces of sintered compacts were etched using OP-S suspension (colloidal silica: Struers). The microstructure of the sintered compacts was observed with a scanning electron microscope (SEM: JSM-7200 (JEOL)). Fig.1 shows the backscattered electron images with indicating the formed phase, Hc and Ha of the samples. With the sintering temperature increased, the XRD pattern of prepared samples changed from that of 1-9 phase to 1-12 phase. However, the temperature range forming the mixed phase extended to the low and high temperature sides in the sintered samples under the molding pressure of 100 MPa. The samples composed of 1-9 and 1-12 mixed phases exhibited relatively high coercivity such as Hc ≒ 4.0 to 5.0, they have gray regions with dimensions of less than 5 µm surrounded by white linear regions, that is, the grain boundary phase. By contrast, the observed microstructure of the sample composed of the 1-12 phase of lower coercivity of less than 3 kOe showed that the disappearing of the network structure. These findings suggest that the comparatively high coercivity of 5 kOe originated from the formation of the network structure. In the presentation, we will discuss the effect of SPS pressure increase on the novel 1-12 type sintered magnets, including the amount of α-Fe precipitation and the variation of the maximum energy product (BHmax) values.
Reference
1) T. Kuno et al., Journal of Magnetism and Magnetic Materials, 498 (2020) 166114.
2) K. Kobayashi et al., Materials Transactions, 62 (2021) 1757-1763.
The novel ThMn12-type magnetic materials with composition of (Sm,Zr)(Fe,Co)11.3Ti0.7 have the high saturation polarization Js ≒ 1.55 T almost equal to that of Nd-Fe-B magnet, and the high anisotropy field Ha ≒ 8.6 MA / m and high Curie temperature TC ≒ 970 K that they exceed those of the Nd2Fe14B phase. 1) Our group successfully prepared the occurrence of relatively high coercivity (Hc ≒ 5 kOe) by the sintering using spark plasma sintering (SPS) method of the amorphous starting alloy of above composition. 2) And it was revealed that relatively high coercivity appears in the sample showing the XRD pattern of the mixed phase of the TbCu7 type (1-9 phase) and the ThMn12 type (1-12 phase) crystal structures. 2) However, it is important to reveal the coercivity mechanism of the sample because higher coercivity is requested for the application of this magnet. In this paper, therefore, we analyze the correlation between the coercivity and microstructure of the magnets that were sintered under 50 to 100 MPa molding pressure.
Experiments and Results
An amorphous powder of starting compound (Sm0.8Zr0.2)1.1(Fe0.9Co0.1)11.3Ti0.7 was prepared via the RQ method by Santoku Co., Ltd., Japan with the rotation velocity of the cooling roller set to about 40 m / s. The pre-milled powder of starting alloy (ca. 1 g) was transferred into a hard alloy die (inside covered with carbon film) of about 10 mm in diameter, which was placed into a SPS furnace (SPS-510L, Sumitomo Metal Mining Co. Ltd.). SPS was then performed at 923 to 1073 K for 0.3 to 3.6 ks, using an initial heating rate of 0.3 K / s and a pressure of approximately 50 to 100 MPa in vacuum (PO2 = 1.5 × 101 Pa). The SPS sintered samples have a thickness of approximately 2 mm and a diameter of about 10 mm. The crystal structures of the samples were evaluated from their XRD patterns (LabX XRD-6100, Shimadzu, Japan, Cu-Kα). Sample’s Hc were measured by vibrating-sample magnetometry (VSM-5 HSC, TOEI Industrial Co. Ltd., Japan). We used the law of approach for ferromagnetic saturation (LAFS) method1) to determine Js and Ha of the magnetically isotropic samples from hysteresis loops of Hmax of 12 MA / m (High Field Laboratory for Superconducting Materials, Institute for Materials Research, Tohoku University, Japan). The observed surfaces of sintered compacts were etched using OP-S suspension (colloidal silica: Struers). The microstructure of the sintered compacts was observed with a scanning electron microscope (SEM: JSM-7200 (JEOL)). Fig.1 shows the backscattered electron images with indicating the formed phase, Hc and Ha of the samples. With the sintering temperature increased, the XRD pattern of prepared samples changed from that of 1-9 phase to 1-12 phase. However, the temperature range forming the mixed phase extended to the low and high temperature sides in the sintered samples under the molding pressure of 100 MPa. The samples composed of 1-9 and 1-12 mixed phases exhibited relatively high coercivity such as Hc ≒ 4.0 to 5.0, they have gray regions with dimensions of less than 5 µm surrounded by white linear regions, that is, the grain boundary phase. By contrast, the observed microstructure of the sample composed of the 1-12 phase of lower coercivity of less than 3 kOe showed that the disappearing of the network structure. These findings suggest that the comparatively high coercivity of 5 kOe originated from the formation of the network structure. In the presentation, we will discuss the effect of SPS pressure increase on the novel 1-12 type sintered magnets, including the amount of α-Fe precipitation and the variation of the maximum energy product (BHmax) values.
Reference
1) T. Kuno et al., Journal of Magnetism and Magnetic Materials, 498 (2020) 166114.
2) K. Kobayashi et al., Materials Transactions, 62 (2021) 1757-1763.
