Synthesis of Ce-Based RE2Fe14B by Solid-State Reaction and Reduction-Diffusion Process

Abstract

Rare-earth permanent magnets, such as Nd₂Fe₁₄B, have been widely used in electric vehicle and wind turbine motors due to their high anisotropy field (H_a), saturation magnetization (M_s) and coercivity (H_c). Cerium (Ce) has gained attention as a potential alternative to neodymium (Nd) due to its high abundance and low cost. The relatively poor intrinsic magnetic properties of Ce magnets, however, remain a significant challenge for their industrial applications. In this study, the synthesis of Ce-based RE₂Fe₁₄B (2-14-1) phases was achieved by a modified reduction-diffusion (R-D) process using REFeO₃ (RE = Ce, Nd) as a precursor. The precursor was prepared by a solid-state reaction with CeO₂, Nd₂O₃, Fe₂O₃ and Fe powders, which is a much more suitable process for mass production and cost-effectiveness. Optimal composition and heat treatment conditions enabled the formation of single-phase Ce-based 2-14-1 particles. The as-synthesized single-phase Ce₂Fe₁₄B particles exhibited an M_s value of ~120 emu/g and an intrinsic coercivity (H_ci) value of ~85 Oe, which can be attributed to the large particle size as observed by FE-SEM.

1. Introduction

The demand for high performance magnets has been growing due to the increased use of motors in electric/hybrid vehicles and wind turbines [1,2,3,4,5,6]. While rare-earth (RE) permanent magnets, such as Nd₂Fe₁₄B, exhibit excellent magnetic properties, including a high anisotropy field (H_a), high saturation magnetization (M_s), and coercivity (H_c), the supply risk and high price of RE elements have become significant concerns. Ce, which is a byproduct of the Nd extraction process, has been overstocked, and, so, the price of Ce elements is very low compared to Nd, Pr and Dy [7,8,9]. For this reason, Ce-based magnets have emerged as a promising candidate that can replace or reduce the use of Nd in RE permanent magnets, although the inferior intrinsic magnetic properties still pose a significant challenge for industrial applications. As a result, it is expected that developing Ce-based low-cost permanent magnets, with a performance comparable to Nd-based magnets, can bridge the performance gap between ferrite and Nd-Fe-B magnets [8,9].

In addition, the fabrication processes for Nd-Fe-B magnets, such as the rapid solidification process (RSP) or melt-spinning, require the use of pure metals for raw materials, complex equipment and the careful consideration of oxidation of metal alloys during the process, which can lead to high production costs. Compared to conventional Nd-Fe-B magnet fabrication processes, the reduction-diffusion (R-D) process, which uses reduction agents of calcium or calcium compounds, exhibits many advantages over the RSP and melt-spinning, such as (1) low production costs due to the use of inexpensive raw materials such as rare-earth oxides and relatively simple equipment, (2) low energy consumption and (3) the direct production of alloy powder form suitable for the subsequent milling process [10,11,12,13,14,15,16,17,18]. Prior reports on the fabrication of Nd-Fe-B powders by R-D processes have used precursors (composed of NdFeO₃ and Fe phases) prepared by wet chemical methods such as the spray-drying process [10,11,12,13], the sol–gel process [14,15] or the co-precipitation method [16,17,18].

In this study, 2-14-1 phases, which represent RE₂Fe₁₄B phases, were synthesized by a modified reduction-diffusion process, which uses an REFeO₃ precursor synthesized by a solid-state reaction using oxide raw materials. The solid-state reaction has been well known for the synthesis of inorganic solid materials, such as ferrites and perovskites, and widely used in mass production processes [19]. To the best of our knowledge, no previous studies have reported the synthesis of the 2-14-1 phase by a reduction-diffusion process using precursors prepared by the solid-state reaction. The solid-state reaction offers many advantages, which include (1) the use of oxide raw materials (which are relatively not costly and easy to store/handle) and (2) easy control of the elemental composition. Additionally, compared to other processes, it requires simple equipment, which can lead to low production costs.

Therefore, the modified R-D process proposed in this study, utilizing precursors prepared by the solid-state reaction, offers significant advantages over the conventional R-D process due to the low cost of raw materials and simple processes for production, making it a cost-effective approach for large-scale manufacturing. Moreover, precise control of the elemental composition can be achieved in the process. In this study, not only the Ce₂Fe₁₄B phase (Ce 2-14-1) but also (Ce_0.5Nd_0.5)₂Fe₁₄B (Ce_0.5Nd_0.5 2-14-1) and Nd₂Fe₁₄B (Nd 2-14-1) phases were synthesized using the modified R-D process. To obtain single-phase Ce-based 2-14-1 compounds, the elemental composition and heat-treatment condition were optimized. The magnetic properties and microstructure of the 2-14-1 particles were investigated and compared.

2. Materials and Methods

The REFeO₃ (RE = Ce, Nd) precursors were prepared by a solid-state reaction. The CeO₂, Nd₂O₃, Fe₂O₃ and Fe powders of 99.9% purity were used as raw materials. The powders were weighed and ball-milled for 24 h with zirconia balls in ethanol. After drying in an oven at 75 °C, the powder mixture was pelletized with a uniaxial press and then heat-treated at 850–1050 °C for 6 h in Ar. After the heat treatment, the pellets were crushed into powder using a mortar. To make a homogeneous mixture, the powders were mixed with H₃BO₃ powder, which is the boron source of the 2-14-1 phase, ball-milled for 24 h and then dried in an oven. Calcium granule particles with a purity of 99% were added to the powder mixture at a 1:1 wt.% ratio, and, then, the whole mixture was pressed into a green compact with a diameter of 1 cm using the uniaxial press. For the R-D process, the pressed compacts were heat-treated at 1050 °C for 6 h in Ar, which was followed by furnace cooling to room temperature. The final products were crushed into powder and then washed with deionized water and acetic acid several times to remove Ca and CaO. Finally, the washed powder was dried at 120 °C for 30 min in vacuum. The preparation process of RE₂Fe₁₄B magnetic powder through the modified R-D method is illustrated in Figure 1.

The phase analyses were carried out using X-ray diffraction (XRD, D8 Advance, Bruker Corporation, Billerica, MA, USA) with Cu Kα radiation. The magnetic properties were measured at room temperature using a vibrating sample measurement system (VSM, VSM-7410, Lake Shore Cryotronics Inc., Westerville, OH, USA), and the microstructure and particle size were observed by a field emission scanning electron microscope (FE-SEM, MERLIN Compact, Carl Zeiss AG, Oberkochen, Germany).

3. Results and Discussion

3.1. Preparation of REFeO₃ Precursor

To prepare precursors for the R-D process, the CeFeO₃ phase was synthesized by a solid-state reaction at 850 °C for 6 h in an Ar atmosphere using CeO₂, Fe₂O₃ and Fe powders (Ce:Fe = 1:1). Figure 2 shows the XRD results of the samples with different raw material ratios. The XRD results show that a CeFeO₃ phase was successfully formed when Fe powder was substituted for two-thirds of the Fe₂O₃ in the precursor mixture, as shown in Figure 2, but residual CeO₂ and Fe₂O₃ phases were also present. To eliminate these unreacted phases, the Fe content was increased to a Ce:Fe ratio of 1:7, corresponding to the stoichiometric composition of the 2-14-1 phase. Although the XRD pattern of the sample with 1:7 of Ce:Fe exhibited distinct peaks of CeFeO₃, minor amounts of CeO₂ and Fe₂O₃ phases remained. Interestingly, Fe peaks were also observed, which can be potentially utilized in the subsequent synthesis of the 2-14-1 phase through a reduction-diffusion process.

To promote the formation of CeFeO₃ and eliminate residual CeO₂ and Fe₂O₃ phases, the heat-treatment temperature was progressively increased. As shown in Figure 3, as the heat-treatment temperature was increased, the amount of residual CeO₂ and Fe₂O₃ was gradually decreased. At 1050 °C, the precursor composed of pure CeFeO₃ and Fe phases was achieved. This result demonstrates that a CeFeO₃-Fe precursor with a Ce:Fe ratio of 1:7 can be successfully synthesized by the heat treatment of a mixture of CeO₂, Fe₂O₃ and Fe at 1050 °C for 6 h under an Ar atmosphere.

3.2. Synthesis of 2-14-1 Phase Through Reduction-Diffusion Process

Figure 4 shows the XRD results of samples prepared by the modified R-D process with different Ce/Fe ratios (Ce:Fe = 1:5 (a), 1:5.5 (b), 1:6 (c), 1:6.5 (d), and 1:7 (e)). When a Ce:Fe ratio of 1:7 was used, which corresponds to the stoichiometric composition of the 2-14-1 phase, XRD peaks from residual Fe were observed. To decrease the amount of residual Fe, the Ce content was increased. As the amount of Fe was decreased, the intensity of the peak from Fe was decreased, leading to the formation of a pure 2-14-1 phase at a Ce:Fe ratio of 1:5.5 (Figure 4 (b)).

Therefore, it can be confirmed that the Ce 2-14-1 phase can be successfully synthesized by the modified R-D process using Ca granules as a reducing agent, followed by washing with distilled water and acetic acid, as shown in Figure 4. It should be noted that no XRD peaks from residual Ca or CaO (the unreacted reducing agent and byproducts of the R-D process, respectively) were observed in the XRD patterns in Figure 4, which indicates that they were completely removed during the washing steps.

The significant temperature difference between the peritectic reaction of Ce₂Fe₁₄B (982 °C) and the eutectic reaction of γ-Fe (1173 °C) presents a major challenge in synthesizing Ce₂Fe₁₄B alloys without the formation of α-Fe using rapid solidification techniques [8,20,21]. However, precise compositional control through solid-state reactions has enabled the synthesis of a pure 2-14-1 phase. This suggests that the stoichiometry of metal alloys produced through the R-D process can be effectively adjusted by manipulating the composition during solid-state reactions.

When the Fe content was further reduced, a CeFe₂ phase appeared as a secondary phase. As shown in Figure 5, the Ce-Fe-B system is considerably different from the Nd-Fe-B system in terms of phase formation. RE-rich phases play a crucial role in the sintering process, facilitating the densification and the magnetic decoupling of 2-14-1 grains through grain boundary wetting. In Nd-Fe-B magnets, the low melting point of the Nd phase allows for the continuous distribution of the grain boundaries, which enhances coercivity. In contrast, minimizing CeFe₂ formation is crucial, as its high melting temperature of 925 °C leads to poor wettability at grain boundaries. This results in the weakened magnetic decoupling of grains, ultimately deteriorating the magnetic properties of Ce₂Fe₁₄B [8,22].

Rietveld refinement analysis was applied for calculation and quantification of the Ce 2-14-1 phase in the sample of Ce:Fe = 1:5.5, and the results are shown in Figure 6. It can be verified that a single phase of Ce₂Fe₁₄B can be synthesized by the modified R-D process with precise compositional control, since the calculated result is in good agreement with the experimental XRD pattern, and there was a negligible amount of secondary phases.

Field-emission scanning electron microscopy (FE-SEM) analyses revealed a polygonal particle morphology of the Ce 2-14-1 particle with sizes in the micrometer range as shown in Figure 7, which is consistent with previously reported Nd-Fe-B particles synthesized by the R-D process [13].

The solid-state reactions have advantages in the precise elemental control, which can be utilized in the control of the amount of substituted elements. (Ce_0.5Nd_0.5)₂Fe₁₄B and Nd₂Fe₁₄B phases were synthesized by the modified R-D process using optimized conditions for the Ce 2-14-1 synthesis, and the XRD results of the synthesized samples are shown in Figure 8. It was confirmed that single phases of (Ce_0.5Nd_0.5)₂Fe₁₄B and Nd₂Fe₁₄B can be successfully synthesized by the modified R-D process. This suggests that the modified R-D process can be extended to incorporate other light RE elements and heavy RE elements and even transition metal substitutions for Fe (e.g., Co, Ga and Cu), which are known to enhance the magnetic properties of 2-14-1 magnets.

3.3. Magnetic Properties and Microstructure of Ce 2-14-1 Magnetic Particles

Figure 9 and Table 1 show the M-H curves and the magnetic properties of samples prepared by the modified R-D process with different Ce/Fe ratios (Ce:Fe = 1:5, 5.5, 6, 6.5 and 7), respectively.

The single-phase Ce₂Fe₁₄B sample exhibits a magnetization of 118.5 emu/g at 2.6 T and a H_ci of 85.7 Oe. Samples with higher Fe content show increased magnetization at 2.6 T, likely due to the presence of the Fe phase as observed in the XRD results. Conversely, the decrease in magnetization for the 1:5 sample can be attributed to the presence of CeFe₂, a paramagnetic material at room temperature, which reduces the volume fraction of Ce₂Fe₁₄B [8,22]. Moreover, the formation of CeFe₂ must be suppressed for the development of high-performance Ce-based magnets, as its high melting point (925 °C) can result in precipitation at triple junctions. The CeFe₂ phase exhibits poor wettability and restricts grain alignment. The paramagnetic CeFe₂ phase can also act as the nucleation sites for domain reversal, resulting in greatly reduced coercivity [23], and would lead to the consumption of excess Ce that is required for the formation of the Ce-rich grain boundary phase [24]. All samples exhibited H_ci below 100 Oe, which can be attributed to the size of Ce 2-14-1 particles synthesized by the R-D process. As shown in Figure 7, the particle size significantly exceeded the single-domain size of the 2-14-1 phase, and this large particle size can be responsible for the low H_ci value [25]. Therefore, it is expected that the coercivity values of 2-14-1 particles synthesized by the modified R-D process can be improved by the particle size refinement process such as ball-milling, which can reduce the particle size from micrometer-scale dimensions to single-domain size.

4. Conclusions

Ce-based RE₂Fe₁₄B magnetic materials were successfully synthesized by a modified reduction-diffusion (R-D) process using REFeO₃ precursors prepared through the solid-state reaction, using rare-earth oxides, Fe₂O₃ and Fe as raw materials. The single-phase Ce₂Fe₁₄B was synthesized at a Ce:Fe ratio of 1:5.5, and (Ce_0.5Nd_0.5)₂Fe₁₄B and Nd₂Fe₁₄B phases were also successfully synthesized by the modified R-D process. The single-phase Ce₂Fe₁₄B sample exhibited a coercivity of 89 Oe, primarily due to the relatively large particle size.

The modified R-D process can provide significant advantages in rare-earth magnet production, such as a cost-effective approach through the use of oxide raw materials and simple equipment. Furthermore, the successful synthesis of various RE₂Fe₁₄B compounds demonstrates the ability of the modified R-D process for incorporating different rare-earth elements. While further optimization is needed to improve the magnetic properties, such as particle size refinement and the sintering process for bulk magnet fabrication, the results of this work provide an approach to a more economical and sustainable production process for rare-earth permanent magnets.

Author Contributions

S.L., K.L. and S.-I.Y.; methodology, S.L.; data curation, S.L.; writing—original draft preparation, S.L.; writing—review and editing, S.L., Y.-M.K., J.-W.L., J.P., S.-I.Y. and C.P.; supervision, S.-I.Y. and C.P. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available upon request from the corresponding author. The data are not publicly available due to privacy.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

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Figures and Table

Figure 1. Flowchart of preparation of RE2Fe14B magnetic powder by the modified reduction-diffusion process.

Figure 2. XRD analysis results of precursor samples prepared by solid-state reaction (850 °C, 6 h, Ar).

Figure 3. XRD results of precursor samples prepared by solid-state reaction with different heat-treatment temperatures (Ce:Fe = 1:7; 6 h, Ar).

Figure 4. XRD analysis results of samples prepared by the modified reduction-diffusion process with different Ce/Fe ratios (Ce:Fe = 1:5 (a), 1:5.5 (b), 1:6 (c), 1:6.5 (d) and 1:7 (e)).

Figure 5. Phase diagrams of (a) Ce–Fe–B and (b) Nd–Fe–B ternary systems [20].

Figure 6. Rietveld refinement of the XRD pattern of the sample with the Fe/Ce of 5.5 prepared by the modified reduction-diffusion process.

Figure 7. FE-SEM micrographs of the sample with the Fe/Ce ratio of 5.5 prepared by the modified reduction-diffusion process.

Figure 8. XRD analysis results of Ce, Ce0.5Nd0.5 and Nd 2-14-1 samples prepared by the modified reduction-diffusion process (1050 °C, 6 h, Ar).

Figure 9. M−H curves of samples prepared by the modified reduction-diffusion process with different Ce/Fe ratios (Ce:Fe = 1:5, 5.5, 6, 6.5 and 7).

Table 1

Magnetic properties of samples prepared by the modified reduction-diffusion process with different Ce/Fe ratios.

Ce:Fe	M at 2.6 T (emu/g)	Hc (Oe)
1:5	117.2	86.5
1:5.5	118.5	85.7
1:6	123.5	89.9
1:6.5	146.1	87.0
1:7	150.0	72.7

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