The Influence of Grain Boundary Diffusion of Cu₂₈Ce₇₂ Alloy on the Magnetic Properties of HDDR NdFeB Powders

Highlights

What are the main findings?

• The magnetic properties of HDDR NdFeB can be improved by adding Cu₂₈Ce₇₂ alloy.
• The H_cj, B_r and BH_max of the bonded magnet respectively increased by 12.91%, 12.4% and 27.1%.

What are the implications of the main findings?

• Cu and Ce elements enter the NdFeB particles along the grain boundaries to repair the grain boundaries.

Abstract

In this paper, the magnetic properties of HDDR NdFeB powders were improved by the grain boundary diffusion of Cu₂₈Ce₇₂ alloy. The influence and mechanism of Cu₂₈Ce₇₂ alloy and heating process on the coercivity (H_cj), remanence (B_r), and maximum magnetic energy product (BH_max) of magnetic powders were investigated. The grain boundary diffusion of Cu₂₈Ce₇₂ alloy can effectively improve the H_cj, B_r and BH_max of bonded magnets, exhibiting a trend of first increasing and then decreasing with the increase in diffusion temperature and Cu₂₈Ce₇₂ addition, and the maximum values of 927 kA/m, 0.625 T and 61 kJ/m³ are obtained at the Cu₂₈Ce₇₂ content of 5.0 wt% and the heating temperature of 380 °C. The demagnetization coupling effect is increased by a continuous grain boundary phase formed by the diffusion of Ce and Cu elements into the grain boundaries, and thus the coercivity of magnetic powder is improved. During the diffusion process, Ce element diffuses along the grain boundaries to repair the defective areas around the grains and form a Ce₂Fe₁₄B phase, which improves the H_cj and B_r of magnetic powders; on the other hand, Ce element diffuses into Nd₂Fe₁₄B grains to replace the Nd crystal sites, reducing the H_cj and B_r of magnet; therefore, the H_cj and B_r of HDDR NdFeB powders are comprehensively affected by these two aspects.

1. Introduction

Bonded NdFeB magnets are characterized by a high degree of shape freedom, high dimensional accuracy, the elimination of secondary processing, and high material utilization. They are widely used in the fabrication of micro-motors for applications such as automotive, home appliances, and consumer electronics industries [1,2,3]. With the development of China’s green and low-carbon economy and the national goals of “Carbon Peak” and “Carbon Neutrality”, micro-motors are evolving toward miniaturization, high efficiency, energy conservation, and intelligence. This trend places higher demands on the magnetic properties of bonded NdFeB magnets, particularly concerning coercivity [4,5].

Typically, the fabrication of Bonded NdFeB magnets involves mixing and compounding NdFeB magnetic powder with coupling agents, binders and lubricants in specific proportions and sequences. The mixture is then formed into green compacts using injection molding, calendering, extrusion, or compression molding processes, and finally prepared by surface anti-corrosion treatment and magnetic field orientation [6,7,8,9]. The magnetic properties of bonded NdFeB magnets are influenced by both the magnetic powder and the preparation process. As the foundation of magnet fabrication, the magnetic powder plays a decisive role in determining the magnet performance [10,11,12]. Kurniawan C et al. [13] found that beyond the intrinsic magnetic properties of the powder, factors such as particle size distribution of the magnetic powder also affect the magnetic performance of the final magnets. Liang Chao et al. [14] significantly improved the magnetic properties of NdFeB powder by modifying the HDDR process and combining it with grain boundary diffusion and tempering treatments, achieving results of B_r = 6.24 kGs, H_cj = 14.67 kOe, and BH_max = 7.06 MGOe. Zhang Wen et al. [15] investigated the effect of Zr addition on the structure and magnetic properties of NdFeB powder, and the results showed that Zr addition effectively refines the grain size and enhances the exchange coupling interaction between the hard and soft magnetic phases, thereby improving overall performance. The addition of heavy rare earth elements such as Dy/Tb can effectively enhance the coercivity of the magnetic powder. Honkura et al. [16] introduced Dy element into NdFeB powder prepared by the d-HDDR process, increasing the intrinsic coercivity from 1.12 MA/m to 1.56 MA/m. However, since heavy rare earth elements such as Dy and Tb are non-renewable resources with prohibitive costs, research has prioritized the reduction or complete elimination of heavy rare earth usage in magnet fabrication [17,18]. Conversely, grain boundary diffusion using low-melting-point alloys of light rare earths like Pr/Nd facilitates the formation of a continuous intergranular phase, which increases decoupling between main-phase grains, suppresses intergranular magnetic reversal, and thereby enhances coercivity [19,20,21,22]. Lin Zhong et al. added Pr₆₈Cu₃₂ alloy powder to Pr_9.5Fe₈₃Zr₂B_5.5 [23] and Nd₁₃Fe_79.4B₇Nb_0.3Ga_0.3 [24] HDDR magnetic powders for grain boundary diffusion treatment. As a result, the coercivity of the magnets respectively increased from 6.5 kOe and 13 kOe to 26 kOe and 18 kOe, with no significant decrease in remanence or maximum energy product.

With the significant increase in the price of Pr/Nd rare earth elements in recent years, the substitution of Pr/Nd with lower-cost and more abundant alternatives has become a research hotspot in the industry. As byproducts of Nd/Pr extraction, La/Ce cost less than one-twentieth of Nd/Pr and are far more abundant in nature. Developing Nd-Ce-Fe-B or Nd-La-Fe-B magnets by substituting Pr/Nd with La/Ce is an effective approach to alleviating the rare earth resource crisis and reducing the cost of Nd-Fe-B magnets [25,26,27,28]. This study employed low-melting-point Cu₂₈Ce₇₂ alloy (with a eutectic temperature of 407 °C) [29] as diffusion source for diffusion heat treatment of HDDR NdFeB magnetic powder. The effects of Cu₂₈Ce₇₂ alloy concentration and heat treatment processes on coercivity, remanence and maximum magnetic energy product were investigated, along with the underlying mechanisms.

2. Materials and Methods

In this study, NdFeB magnetic powder, prepared via the HDDR process, was sieved through a 60-mesh screen to obtain particles smaller than 250 μm. The Cu₂₈Ce₇₂ diffusion alloy was prepared using vacuum arc melting furnace under argon atmosphere (Beijing Wuke Optoelectronics, Beijing, China). Cu and Ce metals were weighed according to the atomic ratio of Cu₂₈Ce₇₂, with the surface oxide layer of Ce mechanically removed. An additional 5 wt% of Ce was added to compensate for potential oxidation losses during melting. The melted Cu₂₈Ce₇₂ ingot was mechanically crushed under nitrogen protection and passed through 100-mesh sieve (Shaoxing Shangyu Huafeng Hardware & Instrument, Shaoxing, China) (particle size < 150 μm). Under a nitrogen atmosphere, the NdFeB magnetic powder (20.0 g) and a specific mass ratio of Cu₂₈Ce₇₂ powder were placed into ball milling jar (Changsha Tianchuang Powder Technology, Changsha, China), along with 25 large (9.6 mm diameter) and 50 small (5.8 mm diameter) grinding balls. The mixture was ball-milled at 60 rpm, promoting adhesion of fine Cu₂₈Ce₇₂ particles onto the surface of the NdFeB powders.

The ball-milled powder was pre-pressed and formed under pressure of 200 MPa to ensure tight compaction during the diffusion heat treatment. The pre-pressed compact was placed in a vacuum furnace, evacuated to 5 × 10⁻³ Pa and heated to temperature range of 380 °C to 700 °C for diffusion heat treatment for 1.0 h, then cooled to room temperature. The heat-treated compact was then ground into powder using a mortar and suspended in acetone. Epoxy resin (2.5 wt%) and KH550 coupling agent (0.5 wt%) relative to the magnetic powder mass were added and thoroughly mixed by stirring. After acetone evaporation and drying, the mixture was ground in a mortar for 10 min and left in air for 24 h to ensure complete removal of residual solvent. The magnet was prepared using BCY200 magnetic field press (Baiqida Intelligent Technology, Ningbo, China). The mixed magnetic powder was placed in mold at 140 °C and oriented under 1.8 T alternating magnetic field, the mixture was pressed into 7 mm × 7 mm × 7 mm bonded magnet under 450 MPa pressure, followed by curing at 120 °C for 30 min to yield the final product.

The magnetic properties of the magnets were characterized using a NIM-2000 permanent magnet material measuring instrument (National Institute of Metrology, Beijing, China) to determine the remanence, coercivity, and maximum magnetic energy product. The morphology and composition of the magnets were examined using a field emission scanning electron microscope (Zeiss SUPRATM55, Carl Zeiss AG, Oberkochen, Germany) equipped with energy dispersive spectrometer (EDS) (INCA X-MAX 50, Oxford Instruments, Abingdon, UK).

3. Results and Discussion

Figure 1 shows the variation curves of the coercivity of bonded NdFeB magnets with different Cu₂₈Ce₇₂ addition amounts as a function of the diffusion temperature. For the bonded magnet without Cu₂₈Ce₇₂, the H_cj decreases slightly with increasing diffusion temperature below 380 °C and decreases sharply above 380 °C. This phenomenon occurs because magnetic powders undergo minimal oxidation at temperatures below 380 °C, while higher temperatures accelerate the grain growth and oxidation, resulting in reduced H_cj. For magnets containing Cu₂₈Ce₇₂ as a diffusion agent, increasing the diffusion temperature facilitates the diffusion of Cu and Ce into the interior of the NdFeB particles, effectively repairing grain boundaries and enhancing H_cj. The H_cj reaches its maximum at a diffusion temperature of 380 °C. Beyond this temperature, the H_cj decreases due to oxidation and the bulk diffusion of Ce elements [30,31].

As shown in Figure 1, with the increase in the Cu₂₈Ce₇₂ addition, the effect of diffusion-repaired grain boundaries is significantly enhanced and the H_cj of the bonded magnet increases accordingly, reaching a maximum at an addition amount of 5.0 wt%. With a further increase in the addition amount, Ce element bulk diffusion leads to a decrease in H_cj. At a diffusion temperature of 380 °C, the bonded magnet with 5.0 wt% Cu₂₈Ce₇₂ exhibits the highest H_cj of 927 kA/m, representing a 12.91% improvement over the magnet without Cu₂₈Ce₇₂ diffusion agent (821 kA/m). The demagnetization curve of the magnet is shown in Figure 2.

Figure 3 shows the curves of remanence (Figure 3a) and maximum energy product (Figure 3b) as a function of the diffusion temperature for bonded NdFeB magnets with 5.0 wt% Cu₂₈Ce₇₂ addition. As the diffusion temperature increases, both the B_r and BH_max of the bonded magnets initially increase and then decrease, reaching maximum values of 0.625 T and 61 kJ/m³ at 380 °C, which are 7.2% and 22.0% higher than those of magnets without heat treatment (0.583 T and 50 kJ/m³). The diffusion temperature is determined by the melting point of the Cu₂₈Ce₇₂ alloy at 407 °C. As the pre-pressed compact under 200 MPa presents low density and many pores, the HDDR NdFeB powders and Cu₂₈Ce₇₂ alloy are easily oxidized during high-temperature diffusion process, which results in the decrease in magnetic properties. As the diffusion temperature of 380 °C is lower than the melting point of the Cu₂₈Ce₇₂ alloy, the solid-state diffusion effect has limited effect on improving magnetic properties, but it also greatly reduces the oxidation effect of HDDR magnetic powder. Under the influence of these two aspects, the optimal properties occur at 380 °C. The effect of Cu₂₈Ce₇₂ grain boundary diffusion gradually strengthens with increasing heat treatment temperature. On the one hand, Ce elements diffuse along the grain boundaries into defect regions of the grains to form the (Nd_1−xCe_x)₂Fe₁₄B phase, which repairs grain boundary defects and enhances B_r and BH_max. On the other hand, the volume diffusion of Ce elements substitutes for Nd sites, forming the (Nd_1−xCe_x)₂Fe₁₄B phase, which leads to reduction in B_r and BH_max [32,33]. Additionally, the increased oxidation caused by excessively high temperatures is another important factor contributing to the decrease in B_r and BH_max.

Figure 4 shows the variation curves of remanence (Figure 4a) and maximum magnetic energy product (Figure 4b) of bonded NdFeB magnets at a diffusion temperature of 380 °C with increasing Cu₂₈Ce₇₂ addition. As the Cu₂₈Ce₇₂ content increases, both the B_r and BH_max of the bonded magnets initially increase and then decrease. The maximum values of 0.625 T and 61 kJ/m³ are achieved at an addition of 5.0 wt%, which are 12.4% and 27.1% higher than the magnets without Cu₂₈Ce₇₂ (0.556 T and 48 kJ/m³). The corresponding demagnetization curves are shown in Figure 2. With the increase in Cu₂₈Ce₇₂ addition, the effectiveness of Ce elements in repairing defective grain regions is significantly enhanced, thereby improving the B_r and BH_max. However, as the Cu₂₈Ce₇₂ diffusant is a non-magnetic phase, excessive addition inevitably leads to a reduction in B_r and BH_max.

Figure 5 presents the SEM energy spectrum of the cross-section of bonded NdFeB magnet with 5.0 wt% Cu₂₈Ce₇₂ addition after diffusion treatment at 380 °C for 1.0 h. The melting point of the Cu₂₈Ce₇₂ alloy is 407 °C, which is higher than the diffusion treatment temperature of 380 °C. Therefore, the solid-state Cu₂₈Ce₇₂ alloy can be observed as red blocky regions in the figure. The distribution of Ce within the NdFeB particles indicates that, during diffusion, Ce penetrates into the interior of the magnetic powder along grain boundaries. The irregular atomic arrangement and the presence of various defects at the Nd₂Fe₁4B grain boundaries render them prone to softening and liquefaction at elevated temperatures [34]. During diffusion, Ce and Cu elements infiltrate the NdFeB particles along these boundaries, repairing defects and forming a continuous grain boundary phase. This isolates magnetic decoupling between grains, thereby improving the H_cj of the magnetic powder [23,24]. Furthermore, incomplete defect regions exist around the Nd₂Fe₁₄B grains, preventing the formation of a fully developed Nd₂Fe₁₄B tetragonal phase. Ce atoms diffuse along the grain boundaries into these defect regions and substitute for Nd sites, resulting in the formation of a (Nd_1−xCe_x)₂Fe₁₄B phase [34,35]. Although Ce₂Fe₁₄B has a lower saturation polarization (1.17 T) and anisotropy field (3.0 T) than those of Nd₂Fe₁₄B (1.60 T, 7.3 T), the formation of the (Nd_1−xCe_x)₂Fe₁₄B phase can still fill the non-magnetic phases originally present in the defects of the magnetic powder, thereby enhancing both the H_cj and B_r [36,37].

Increasing the Cu₂₈Ce₇₂ addition and raising the diffusion temperature enhance the diffusion efficiency, promoting bulk diffusion of Ce. When the addition amount is excessive (exceeding 5.0 wt%) or the diffusion temperature is too high (above 380 °C), Ce elements not only occupy the defect regions of the grains but also penetrate into the Nd₂Fe₁₄B phase to substitute for Nd at lattice sites, forming the (Nd_1−xCe_x)₂Fe₁₄B phase, which leads to a reduction in both H_cj and B_r. Additionally, high-temperature oxidation and the doping of the non-magnetic Cu₂₈Ce₇₂ alloy phase also contribute to the decrease in H_cj and B_r.

4. Conclusions

This study investigates the use of Cu₂₈Ce₇₂ alloy as a diffusion agent for grain boundary diffusion heat treatment of HDDR NdFeB magnetic powder. The effects of the Cu₂₈Ce₇₂ addition amount and heat treatment process on the H_cj, B_r and BH_max of the magnetic powder and the associated mechanisms were studied. The main conclusions are summarized as follows:

(1)

The H_cj, B_r, and BH_max of the diffused magnets exhibited a trend of first increasing and then decreasing with increasing Cu₂₈Ce₇₂ addition amount and rising diffusion temperature. The maximum values of 927 kA/m, 0.625 T, and 61 kJ/m³ were achieved at an addition amount of 5.0 wt% and diffusion temperature of 380 °C, which are 12.91%, 12.4%, and 27.1% higher than those of the magnets without Cu₂₈Ce₇₂ addition (821 kA/m, 0.556 T, and 48 kJ/m³).

(2)

The diffusion of Ce and Cu into the grain boundaries facilitates the formation of a continuously distributed grain boundary phase, which enhances the demagnetization coupling between the main-phase grains and improves the coercivity of the magnetic powder.

(3)

During the diffusion process, on the one hand, Ce elements diffuse along grain boundaries to repair incomplete defect regions around the grains, forming the (Nd_1−xCe_x)₂Fe₁₄B phase, which enhances the H_cj and B_r of the magnetic powder. On the other hand, they undergo volume diffusion into the grain interiors to substitute for Nd sites in the Nd₂Fe₁₄B phase, which reduces the H_cj and B_r of the magnetic powder. These two effects collectively influence the H_cj and B_r of the diffused magnetic powder.