Optimizing the magnetic properties of Ce-containing Nd–Fe–B sintered magnets with La substitution

Ce and La–Ce containing magnets with the compositions of [(La0.4Ce0.6)0.4Nd0.6]31.5FebalM2.02B0.96 and (Ce0.4Nd0.6)31.5FebalM2.02B0.96 were prepared by traditional sintering process. The results shows La substitution for Ce occurs in the 2:14:1 matrix phase which suppress the formation of the nonmagnetic CeFe2 phase in the magnet. Moreover, compared to the Ce containing magnet, the grain boundry (GB) phase is thinner and very continuous in the La–Ce containing magnet. The coercivity and the maximum energy product of La–Ce containing magnet increased by 2 kOe and 1.5 MGOe, respecitively.


Introduction
Rare-earth permanent magnets play a critical role in the development of electric motors, hybrid vehicles, wind generators, and electronic communication devices [1][2][3][4][5][6][7][8]. Specially, the rapid growth of Nd-Fe-B sintered permanent magnets consumes a large quantily of rare earth elements, such as Nd and Pr. Meanwhile, it causes the overstock of abundant rare earth elements, such as La and Ce, leading to the imbalance utilization of rareearth resources. Therefore, researchers focus on reducing the cost of Nd-Fe-B magnets by Ce substitution and comprehensive utilization of rare earth resources.
Recently, various attempts have been made to prepare Ce-containing Nd-Fe-B sintered magnets [9][10][11][12][13]. However, the magnetic properties of the magnets deteriorated after Ce substitution due to the poor intrinsic properties of the Ce 2 Fe 14 B phase, compared with the Nd 2 Fe 14 B phase [14]. Moreover, Herbst et al found that the magnetic properties of Ce 17 Fe 78 B 6 ribbons deteriorated due to the existence of the CeFe 2 phase, which decreased the ratio of the 2:14:1 phase in the magnets [15]. It is generally accepted that the CeFe 2 phase forms easily in Nd-Fe-B magnets with high Ce content, undermining the magnetic properties of these magnets [16,17]. In order to eliminate the CeFe 2 phase, La can be introduced into the Ce-Fe-B ribbons and sintered magnets. In a previous study, we found that La substitution for Ce inhibits the formation of the CeFe 2 phase in melt-spun (La/Ce) 3 Fe 14 B ribbons [18]. Liu et al also verified that La substitution for Ce could effectively inhibit the precipitation of the CeFe 2 phase in melt-spun (La x Ce 1−x ) 2 Fe 14 B ribbons [19]. Shi et al [20] prepared Ce-Fe-B and (La 35 Ce 65 )-Fe-B strips, and they found that the CeFe 2 phase was drastically decreased from 67.1% to 38.1% by La substitution for 35% Ce. However, the CeFe 2 phase were not diminated in the sintered magnet.
Most previous studies of La substitution for Ce to inhibits the formation of the CeFe 2 phase have focused on melt-spun ribbons, but very little is known for sintered magnets. Compared to melt-spun ribbons, sintered magnets are more worthy of investigating due to their widespread application. In this work, Ce-containing and (LaCe)-containning Nd-Fe-B sintered magnets with nominal compositions of (Ce 0.4 Nd 0.6 ) 31.5 Fe bal M 2.02 B 0.96 and (La 0.16 Ce 0.24 Nd 0.6 ) 31.5 Fe bal M 2.02 B 0.96 were prepared. Subsequently, the properties and structures of the two sintered magnets were investigated to provide a technical guide for the preparation of La and/or Ce containing Nd-Fe-B sintered magnets with high magnetic properties. The final powders with a mean particle size of 3-5 μm were pressed and aligned under a compressive pressure of 5.5 MPa in a perpendicular magnetic field of 1.5 T followed by isostatic pressing of 225 MPa. The final magnets were prepared by sintering at 1333 K for 3 h, followed by a two-step annealing treatment at 1173 K for 2 h and 753 K for 3 h.

Experimental
X-ray diffraction (XRD) with Cu K α radiation was used to determine the crystal structure. The magnetic properties of the magnets were measured by a close-circuit permanent magnetic measurement system (NIM-500C). The microstructure of the magnets was observed by scanning electron microscope (SEM, Nova Nano200) with energy-dispersive x-ray spectroscopy and electron-probe microanalyzer (EPMA, JEOL JXA-800) with a wavelength-dispersive x-ray detector. Transmission electron microscopy (TEM) was performed using a JEM-2100F microscope. Figure 1 shows the demagnetization curves of magnets A and B. The derived magnetic properties of coercivity H cj , remanence B r and energy product (BH) max are also given in the figure. The B r of magnets A and B are 11.7 kG, but the H cj of 5.3 kOe of magnet A is 2 kOe lower than that of magnet B. As a result, the (BH) max of magnet A is 1.5 MGOe lower than that of magnet B. Clearly the La partial substitution of Ce in the magnet was enhanced the overall magnetic properties. Nevertheless, it is neccssary to determine the origin of the magnetic property enhancement in magnets A and B.

Results and discussion
The phase compositions of the two magnets were analyzed by XRD with the Rietveld method, and the results are shown in figure 2. It can be seen in figure 2(a) that the sharpest diffraction peaks correspond to the tetragonal (Ce, Nd) 2 Fe 14 B matrix phase. However, the peaks at the 2θ angles of 21.0°, 34.6°and 40.8°in the XRD pattern of magnet A correspond to the (111), (022) and (113) peaks of the cubic CeFe 2 phase. With the La substitution, as shown in figure 2(b), the CeFe 2 phase no longer exists in magnet B, indicating that the CeFe 2 phase can be inhibited by La substitution [21].
The lattice parameters and mass fractions of the phases in magnet A and magnet B evaluated by the Rietveld XRD analysis are listed in table 1. The mass fraction of the (Ce, Nd) 2 Fe 14 B matrix phase, CeFe 2 phase, and RErich phase in magnet A are 92.2 wt%, 5.5 wt% and 2.3 wt%, respectively. With the La substitution, the mass fraction of (La-Ce, Nd) 2 Fe 14 B matrix phase and RE-rich phase increases to 96.3 wt% and 3.7 wt%, respectively. More importantly, the mass fraction of CeFe 2 phase decrease to 0 by La substitution in magnet B.
The lattice parameters of the (Ce, Nd) 2 Fe 14 B matrix phase in magnet A are a=8.783(2) Å and c=12.113  larger atomic radius than Ce. Moreover, the change of the lattice parameters of the RE-rich phase is larger compared to the 2:14:1 matrix phase, indicat that the La is more likely to enter the RE-rich phase. The result is in agreement with the calculations of Liu [22]. On the other hand, the lattice parameter of the CeFe 2 phase in magnet A is a=7.338(2) Å. There is no CeFe 2 phase in magnet B, indicating that CeFe 2 phase can be inhibited by La substitution.
The distributions of the elements in magnet A and magnet B analyzed by EPMA are shown in figure 3. Furthermore, the elemental contents of region 1 to 5 were tested by EDS point scanning, and the results are shown in table 2. As shown in figure 3(a), the gray areas, the light gray areas, and the white areas are identified as the 2:14:1 matrix phase, the CeFe 2 phase, and the RE-rich phase, respectively. It can be seen that the CeFe 2 phases are located at region 1 in magnet A, and the contents of Ce and Fe are consistent with the stoichiometry of the CeFe 2 phase. Only the 2:14:1 matrix phase and the RE-rich phase exist in magnet B, as shown in figure 2(b).
To follow the effect of La substitution on the grain boundary (GB) microstructure, SEM and TEM characterizations were carried out, as shown in figure 4. Figures 4(a) and (d) shows the BSE images of magnet A and magnet B. It can be seen that the GB phase in magnet B exhibits more continuity compared to magnet A. As shown in figure 4(b), the 2:14:1 matrix phase in magnet A was identified by the SAED patterns in the illustration, taken along the [001] zone axes. TEM characterizations conducted from magnet B are also shown in figure 4(e). Figures 4(c) and (f) show the different HRTEM pictures of typical GB morphology of the two magnets. The GB phase in figure 4(c) looks poorly distinct. However, the GB phase in figure 4 f is thinner and continuous (4.86 nm). La substitution could result in a coercivity enhancement to 7.3 kOe, which could explain the difference between the high-resolution TEM images, as shown in figures 4(f) and (c). This GB phase magnetically insulates the matrix phase grains and increases the resistance to demagnetization. Even in cases where the layer is extremely thin, the coercivity could be enhanced if the interface is smooth enough, and no soft magnetic phases exist within the GB phase [23].     matrix phase which suppress the formation of the nonmagnetic CeFe 2 phase in the magnet. Moreover, compared to the Ce-containing magnet, the GB phase is thinner and very continuous in the LaCe-containing magnet. Therefore, better magnetic properties of H cj of 7.3 kOe, B r of 11.7 kG and (BH) max of 32.1 MGOe were obtained in the LaCe-containing Nd-Fe-B sintered magnet.