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Hankuk Jeon,Jung Tae Lim,Hui-Dong Qian,Jihoon Park,Hyojun Ahn,Chul-Jin Choi 한국자기학회 2021 한국자기학회 학술연구발표회 논문개요집 Vol.31 No.2
Permanent magnets are increasingly used in transportation technology and sustainable energy production for EVs, hybrid vehicles and wind turbines. High performance permanent magnets, such as Nd-based magnets, have problems with stability at high temperature, supply and high price. Therefore, developing rare-earth free or rare-earth lean permanent magnets is becoming an important task to solve the abovementioned issues. Herein, iron-rich rare-earth alloys with tetragonal ThMn12 structure, which can replace rare earth permanent magnets, is drawing attention due to its high saturation magnetization of 1.43 T, anisotropy field of 10.9 T, and Curie temperature of 800 K. Although the magnetic properties of SmFe12 with ThMn12 structure have been already demonstrated, a number of studies to enhance coercivity are still underway. The coercivity increases as grain sizes approach to the single domain size. Therefore, in this work, we conducted experiments to obtain SmFe12 particles with single domain sized grains to acquire high coercivity through the high energy ball milling and the reduction diffusion process. In this experiment, Sm₂O₃(Samarium Oxide), Fe₂O₃(Iron Oxide), Co, TiO₂(Titanium Dioxide), Ca were used as starting materials. Sm₂O₃ and Ca, as a reducing agent, were excessively added in consideration of vaporization. The starting materials except Ca were crushed and homogeneously mixed for 4 hours using a high energy ball milling process. Then, the powders were processed by low energy ball milling with Ca powder for 2 hours. The resulting mixture was sealed in a graphite crucible and then heat treated in argon atmosphere. This heat treatment process including heat treatment temperature and time were optimized. Then, it was washed using a detergent that dissolves unreacted Ca and remaining CaO, followed by drying in vacuum. It was concluded that the purity of the samples varied with the initial contents of Sm and heat treatment time. The resultant particle size and magnetic properties of the products were also affected by the ball milling conditions. The detailed experimental procedures and physical and magnetic properties will be discussed.
Hankuk Jeon,JungTae Lim,Hui-Dong Qian,Jihoon Park,Hyo-Jun Ahn,Chul-Jin Choi 한국자기학회 2021 한국자기학회 학술연구발표회 논문개요집 Vol.31 No.1
Permanent magnets are increasingly used in transportation technology and sustainable energy production for EVs, hybrid vehicles and wind turbines. High performance permanent magnets, such as Nd-based magnets, have problems with supply and high price. Therefore, developing rare-earth free or rare-earth lean permanent magnets is becoming an important task to solve the abovementioned issues. Herein, iron-rich rare-earth alloys with tetragonal ThMn<sub>12</sub> structure, which can replace rare earth permanent magnets, is drawing attention due to its high saturation magnetization of 1.43 T, anisotropy field of 10.9 T, and Curie temperature of 800 K [1]. Although the magnetic properties of SmFe<sub>12</sub> with ThMn<sub>12</sub> structure have been already demonstrated, a number of studies to enhance coercivity are still underway. The coercivity increases as grain sizes approach to the single domain size. Therefore, in this work, we conducted experiments to obtain SmFe<sub>12</sub> particles with single domain sized grains to acquire high coercivity through the reduction diffusion process. In this experiment, Sm<sub>2</sub>O3(Samarium Oxide), ZrO<sub>2</sub>(Zirconium Dioxide), Fe2O3(Iron Oxide), Co, TiO2(Titanium Dioxide), Ca and CaO(Calcium Oxide) were used as starting materials. Sm<sub>2</sub>O3 and Ca were added in excess in consideration of vaporization, and CaO was added three times of the remaining substances excluding Ca. Here, Ca is used as a reducing agent, and CaO acts as a dispersant that alleviates the harmful effect on the coercivity caused by grain growth at the reduction temperature. The starting materials were homogeneously mixed using a ball milling process. The resulting mixture was heat treated in argon atmosphere. Then, it was washed in 9 steps using a detergent that dissolves unreacted Ca and remaining CaO, followed by drying in vacuum. We studied the magnetic properties and microstructure of the produced particles via VSM, XRD, and SEM. The magnetic properties of Sm-Zr-Fe-Co-Ti alloy has been enhanced by the fine particles by reduction diffusion process.
Hui-Dong Qian,Jung Tae Lim,Yang Yang,Jong-Woo Kim,Tian Hong Zhou,Su Yeon Ahn,Hankuk-Jeon,Kyung Mox Cho,Jihoon Park,Chul-Jin Choi 한국자기학회 2021 한국자기학회 학술연구발표회 논문개요집 Vol.31 No.2
Rare-earth intermetallic compounds of R(Fe,M)12 (R = rare earth elements, M = transition metals) with ThMn12 structure have been known to be promising permanent magnetic materials since the 1980s. Recently, increasing rare earth price has pushed the industry to seek ways to reduce the R-content in the hard magnetic materials. In case, strong magnets with the ThMn12 type of structure received much attention. However, during the several tens of years, the research about ThMn12 magnetic materials was not made a breakthrough. As a turning point of the ThMn12-type Fe-rich compounds research, ThMn12-type Sm(Fe1-xCox)12 compound films with a saturation magnetization of 1.78 T, an anisotropy field of 12 T, and a Curie temperature of 586 °C, all of which are superior to those for Nd₂Fe14B, were successfully produced. However, it still has difficulty in stabilizing the unstable ThMn12 phase in magnetic powders and bulks. In previous research, the ThMn12 structure is also unstable and partial Fe atoms must be substituted with phase stabilizing element(s), such as Ti, V, Cr, Mn, Mo, W, Al, and Si, which results in magnetization reduction. So, decreasing magnetization or coercivity with the non-magnetic elements substitution is a new challenge for the ThMn12-type Sm(Fe1-xCox)12 compound research. Therefore, we have developed a new fabrication method to produce a high-density Sm(Fe0.8Co0.2)11Ti bulk with high purity and magnetic properties and investigated Si substitution or doping effects on this work"s magnetic and physical properties. The purity of the hard magnetic ThMn12 phase in the bulk magnet reached higher than 97 wt.%. The remanent magnetization and maximum energy product of the prepared Sm(Fe0.8Co0.2)11Ti bulk reached high values of 96.0 emu/g and 12.22 MGOe, respectively. The phase transformation behavior from amorphous to ThMn12 phase during heat treatment was systematically investigated by transmission electron microscopy. The magnetic properties and grain sizes of Sm(Fe0.8Co0.2)11Ti bulk magnets with different annealing times were shown in Fig. 1 (a). To investigate the effect of substituted elements in the ThMn12-type Fe-rich compounds and compare with the Ti substitution, Si was selected to dop into the ThMn12-type Fe-rich compounds. Sm(Fe0.8Co0.2)10Si₂ and Sm(Fe0.8Co0.2)11Ti+Six (x = 0, 0.5, and 1) ribbons were produced using a melt spinning method. The magnetic properties of the Sm(Fe0.8Co0.2)10Si₂ ribbons with different melt spinning speeds and the Sm(Fe0.8Co0.2)11Ti+Six ribbons with melt spinning speed of 39 m/s are shown in Fig. 1 (b). The maximum coercivity of the Sm(Fe0.8Co0.2)10Si₂ and Sm(Fe0.8Co0.2)11Ti+Six ribbons reached 1745 and 3140 Oe, respectively. The details of the fabrication procedure, microstructure, and magnetic properties of as mentioned compounds will be discussed. 〈그림 본문참조〉