Presentation Information
[P2-50]Ultrafine Sm2Fe17N3 hard magnetic particles synthesized by mechanochemical process
*Zhi Yang1, Hongguo Zhang1 (1. Beijing University of Technology (China))
Keywords:
Sm2Fe17N3,Mechanochemistry,Reduction-diffusion
Sm2Fe17N3 exhibits intrinsic magnetic properties that are comparable to or even superior to those of Nd2Fe14B, making it a promising candidate for a new generation of cost-effective permanent magnet materials. In the typical reduction-diffusion process, it is believed that melting Ca is necessary to bring the Sm oxide into contact with Ca and establish a diffusion path for the formation of Sm2Fe17. Consequently, the temperature of the reduction-diffusion reaction must exceed the melting point of Ca (842℃). However, the high temperature of the reduction-diffusion reaction negatively impacts size-dependent coercivity due to the overgrowth of products, leading to a broad particle size distribution and coarse morphology.
The precursors milled at 470 rpm for 8 h, followed by reduction-diffusion annealing at different temperatures respectively. Fig. 1a shows the XRD patterns of the samples prepared by annealing at 750℃, 800℃, 850℃, 900℃, and 950℃. When the reduction-diffusion temperature is 750℃, no Sm-Fe phases were seen in the product, whereas there are some impurities including unreacted α-Fe and SmOx phases. In contrast, the samples produced by annealing at 800℃ or higher exhibit a single-phase of Sm2Fe17. To clarify the difference between the reduction-diffusion reactions with the assistance of the mechanochemical milling and the conventional reduction-diffusion processes, Fig. 1b presents the XRD patterns of the powders synthesized through conventional reduction-diffusion at temperatures ranging from 750℃ to 1050℃ without prior mechanochemical milling. It is evident that only the sample synthesized at 1050℃ matches well with the single phase of Sm2Fe17 when using the conventional reduction-diffusion method. No Sm-Fe phases were observed in the products synthesized through the conventional reduction-diffusion at temperatures below the melting point of Ca, i.e., 750℃ and 800℃. The samples synthesized through conventional reduction-diffusion at 900℃ and 950℃ revealed the coexistence of Sm2Fe17 and residual α-Fe phases. Our results demonstrate that the mechanochemical milling enables complete reduction-diffusion at a temperature below the melting point of Ca, which is 250℃ lower than the conventional reduction-diffusion method.
The SEM images of Sm2Fe17N3 powders, which were synthesized through mechanochemical milling, followed by reduction-diffusion at different temperatures ranging from 800℃ to 950℃, nitridation, and byproduct removal, are shown in Fig. 2. The mechanochemical milling of the precursors was conducted at a speed of 470 rpm for 8 h. It can be seen from Fig. 2 that the particle size decreases significantly with lower reduction-diffusion temperatures. The average particle size was determined by manually measuring 100 particles from SEM images. The average diameters of the Sm2Fe17N3 particles synthesized at reduction-diffusion temperatures of 800℃, 850℃, 900℃, and 950℃ are about 0.30 μm, 0.51 μm, 0.82 μm, and 1.05 μm, respectively. It is noteworthy that the particle size of the Sm2Fe17N3 synthesized at a reduction-diffusion temperature of 800℃ is slightly below the theoretically critical single-domain diameter (~0.35 μm).
Our results demonstrate that the mechanochemical milling of precursors significantly reduces the temperature required for the subsequent reduction-diffusion reaction. This enables the synthesis of Sm2Fe17N3 hard magnetic particles at temperatures below the melting point of calcium, which are 250℃ lower than those required for conventional reduction-diffusion processes.
The precursors milled at 470 rpm for 8 h, followed by reduction-diffusion annealing at different temperatures respectively. Fig. 1a shows the XRD patterns of the samples prepared by annealing at 750℃, 800℃, 850℃, 900℃, and 950℃. When the reduction-diffusion temperature is 750℃, no Sm-Fe phases were seen in the product, whereas there are some impurities including unreacted α-Fe and SmOx phases. In contrast, the samples produced by annealing at 800℃ or higher exhibit a single-phase of Sm2Fe17. To clarify the difference between the reduction-diffusion reactions with the assistance of the mechanochemical milling and the conventional reduction-diffusion processes, Fig. 1b presents the XRD patterns of the powders synthesized through conventional reduction-diffusion at temperatures ranging from 750℃ to 1050℃ without prior mechanochemical milling. It is evident that only the sample synthesized at 1050℃ matches well with the single phase of Sm2Fe17 when using the conventional reduction-diffusion method. No Sm-Fe phases were observed in the products synthesized through the conventional reduction-diffusion at temperatures below the melting point of Ca, i.e., 750℃ and 800℃. The samples synthesized through conventional reduction-diffusion at 900℃ and 950℃ revealed the coexistence of Sm2Fe17 and residual α-Fe phases. Our results demonstrate that the mechanochemical milling enables complete reduction-diffusion at a temperature below the melting point of Ca, which is 250℃ lower than the conventional reduction-diffusion method.
The SEM images of Sm2Fe17N3 powders, which were synthesized through mechanochemical milling, followed by reduction-diffusion at different temperatures ranging from 800℃ to 950℃, nitridation, and byproduct removal, are shown in Fig. 2. The mechanochemical milling of the precursors was conducted at a speed of 470 rpm for 8 h. It can be seen from Fig. 2 that the particle size decreases significantly with lower reduction-diffusion temperatures. The average particle size was determined by manually measuring 100 particles from SEM images. The average diameters of the Sm2Fe17N3 particles synthesized at reduction-diffusion temperatures of 800℃, 850℃, 900℃, and 950℃ are about 0.30 μm, 0.51 μm, 0.82 μm, and 1.05 μm, respectively. It is noteworthy that the particle size of the Sm2Fe17N3 synthesized at a reduction-diffusion temperature of 800℃ is slightly below the theoretically critical single-domain diameter (~0.35 μm).
Our results demonstrate that the mechanochemical milling of precursors significantly reduces the temperature required for the subsequent reduction-diffusion reaction. This enables the synthesis of Sm2Fe17N3 hard magnetic particles at temperatures below the melting point of calcium, which are 250℃ lower than those required for conventional reduction-diffusion processes.
