Magnetic Properties of Nd-Fe-B Anisotropic Magnets Prepared by Spark Plasma Sintering Method

Abstract

Nd-Fe-B hot-deformed magnets were produced by the spark plasma sintering (SPS) method from bulk materials prepared by the SPS method. The bulk materials, prepared by the SPS method from commercially available Nd-Fe-B melt-spun ribbon (MQ powder), consisted of the Nd₂Fe₁₄B phase and were magnetically isotropic. The subsequent hot deformation induced magnetic anisotropy in the Nd-Fe-B magnets. The optimally hot-deformed magnet showed a high remanence of 1.39 T with a maximum energy product of 240 kJm⁻³.

1 Introduction

Neodymium-iron-boron (Nd-Fe-B) permanent magnets have been widely used in various industrial applications such as voice coil motors for hard disk drives and motors for hybrid and electric vehicles. Nd-Fe-B magnets are prepared either by sintering or melt-spinning^1,2). In the melt-spinning approach, Nd-Fe-B melt-spun ribbons are comminuted then consolidated into bulk form by hot pressing and subsequent hot deformation³⁾. Hot-deformed magnets show excellent magnetic properties comparable to those of sintered magnets^4,5). Moreover, hot-deformed magnets exhibit higher corrosion resistance and thermal stability than their sintered counterparts^6–9). However, sintered Nd-Fe-B magnets are more widely used in various industrial applications, mainly due to the difficulties involved in the production of hot-deformed magnets. It is therefore necessary to develop a new technique for the production of hot-deformed magnets.

In recent years, a new consolidation technique called the spark plasma sintering (SPS) method has been developed¹⁰⁾. Using the SPS method, powder can be easily consolidated at relatively lower temperatures in a short consolidation time. This method has been used in several studies to consolidate Nd-Fe-B powders^11–16), and may also be applicable to the production of hot-deformed magnets¹⁷⁾. It is known that the magnetic properties of Nd-Fe-B magnets depend on the degree of crystallographic alignment of the tetragonal Nd₂Fe₁₄B magnetic phase. In the melt-spinning approach, alignment of the Nd₂Fe₁₄B phase is achieved by hot deformation^18–22). Hot deformation of as-hot-pressed magnets gives rise to the crystallographic alignment of the Nd₂Fe₁₄B phase. Thus, the hot-deformation process and the quality of the as-hot-pressed magnets must be optimized in order to obtain hot-deformed Nd-Fe-B magnets with desirable magnetic properties comparable to those of sintered Nd-Fe-B magnets.

In this study, commercially available Nd-Fe-B melt-spun ribbon (MQ powder) was consolidated into bulk materials then hot deformed by the SPS method. The relationship between the processing conditions and the magnetic properties of the resultant hot-deformed magnets was investigated.

2 Experiment

Commercially available Nd-Fe-B alloy powder (MQP-A, Magnequench, Inc.) was used in the experiment. The powder was placed in a carbon die and consolidated at 673–873 K for 300 s under an applied pressure of 100 MPa in a vacuum by the SPS method. The powder was successfully consolidated into bulk material by this method. Fig. 1 shows a schematic drawing of the setup for hot deformation by the SPS method. The bulk material was placed between the carbon punches and heated to the hot-deformation temperature of 873–1073 K in a vacuum. The heated bulk materials were deformed to an 80 % reduction in height at a rate of 0.02 mm/s. Specimens for property measurements were cut from the bulk materials and the hot-deformed magnets using a low-speed diamond saw. The phases of the specimens were examined by X-ray diffraction (XRD) using Cu K_α radiation. The magnetic properties of the specimens were measured using a vibrating sample magnetometer (VSM).

Fig. 1

Schematic diagram showing the setup for hot deformation by the SPS method.

3 Results and discussion

The MQ powder was consolidated by the SPS method before the implementation of hot deformation. The resultant bulk materials had a columnar shape with a diameter of 10 mm and a height of 10 mm. The densities of the bulk materials are shown in Fig. 2. The density of the bulk material increased with increasing consolidating temperature. The bulk material produced at 673 K had a density of 6.70 Mgm⁻³ (approximately 88 % relative density), whereas that produced at 873 K had a density of 7.30 Mgm⁻³ (approximately 96 % relative density). These results indicate that the MQ powder was successfully consolidated into bulk materials at relatively low consolidation temperatures of 873 K or lower in the short period of 300 s by the SPS method.

Fig. 2

Dependence of the density on the consolidating temperature of the Nd-Fe-B bulk materials produced by the SPS method.

The magnetic properties of these bulk materials were examined. Fig. 3 shows the dependence of the coercivity on the consolidating temperature of the bulk materials produced by the SPS method. The coercivity of the bulk material decreased as the consolidating temperature increased. Although the bulk material produced at 673 K exhibited the highest coercivity in this range of consolidating temperatures, it was found that bulk materials with a relatively low density (less than 90 %) were broken during the subsequent hot-deformation study. Thus, the bulk materials produced at 723 K or higher by the SPS method were selected for the subsequent experiment.

Fig. 3

Dependence of the coercivity on the consolidating temperature of the Nd-Fe-B bulk materials produced by the SPS method.

The hysteresis loops of the bulk material produced at 723 K by the SPS method are shown in Fig. 4. Although only the hysteresis loops of the bulk material produced at 723 K are shown, similar hysteresis loops were obtained from the bulk materials produced at 773–873 K. Virtually identical hysteresis loops were obtained from the bulk material measured both parallel and perpendicularly to the pressing direction, indicating that the bulk material is magnetically isotropic.

Fig. 4

Hysteresis loops of the Nd-Fe-B bulk material produced at 723 K by the SPS method, measured parallel (red line) and perpendicularly (blue line) to the pressing direction.

The isotropic bulk materials produced by the SPS method were subsequently hot deformed by the SPS method. The bulk materials could not be deformed at 873 K by the SPS method, but they were successfully deformed at temperatures of 923 K or higher. Fig. 5 shows the dependence of the coercivity of the Nd-Fe-B magnets hot deformed by the SPS method. Regardless of the coercivity of the bulk materials used for the hot-deformation studies, the coercivity of the hot-deformed Nd-Fe-B magnets gradually decreased as the hot-deformation temperature increased. The coercivity of the hot-deformed Nd-Fe-B magnets was lower than that of the bulk material, but the hot-deformed Nd-Fe-B magnets prepared from the bulk material produced at 723 K by the SPS method still exhibited a coercivity of around 0.4 MAm⁻¹.

Fig. 5

Dependence of the coercivity of the hot-deformed Nd-Fe-B magnets produced by the SPS method. The bulk materials produced at 723–873 K by the SPS method were used for the hot-deformation studies.

It has been reported that hot deformation of Nd-Fe-B magnets results in the formation of anisotropic magnets showing a higher remanence value than their isotropic counterparts²²⁾. The hysteresis loops of the hot-deformed Nd-Fe-B magnets were measured parallel and perpendicularly to the pressing direction, in order to examine the magnetic anisotropy. Fig. 6 shows the hysteresis loops of the Nd-Fe-B magnet hot deformed at 923 K by the SPS method. The hot-deformed Nd-Fe-B magnet shows magnetic anisotropy characterized by a higher remanence in the parallel direction than in the perpendicular direction. This magnet exhibited a high (BH)_max value of 240 kJm⁻³, confirming that hot deformation by the SPS method can be applied to the production of anisotropic Nd-Fe-B magnets.

Fig. 6

Hysteresis loops of the hot deformed Nd-Fe-B magnet produced at 923 K by the SPS method, measured (a) parallel (red line) and (b) perpendicularly (blue line) to the pressing direction as shown in the insert.

The structures of the bulk material produced at 723 K by the SPS method and the Nd-Fe-B magnet hot deformed at 923 K by the SPS method were further studied in order to examine the crystallographic alignment of the tetragonal Nd₂Fe₁₄B phase. Fig. 7 shows XRD patterns of the MQ powders, the bulk material produced at 723 K by the SPS method, and the hot-deformed Nd-Fe-B magnet produced at 923 K by the SPS method. The x-ray was applied to the pressed surface of the bulk material to examine the crystallographic alignment of its Nd₂Fe₁₄B phase. The diffraction peaks of the MQ powder are well indexed to the Nd₂Fe₁₄B phase. No noticeable diffraction peak of any other phase is seen in the XRD pattern, due to the large fraction of the Nd₂Fe₁₄B phase in the MQ powder. The XRD pattern of the pressed surface of the bulk material is virtually the same as that of the MQ powder, suggesting that the Nd₂Fe₁₄B grains in the bulk material are randomly oriented. Therefore, the bulk material is magnetically isotropic. On the other hand, the hot deformed Nd-Fe-B magnet exhibits c-axis alignment of the Nd₂Fe₁₄B phase, which is characterized by the prominent (006) and (008) peaks. This confirms that the hot-deformed Nd-Fe-B magnet possesses the requisite crystallographic alignment of the Nd₂Fe₁₄B phase for it to be an anisotropic magnet. Further studies of the hot-deformation conditions as well as compositional modifications can be expected to lead to further increases in the magnetic anisotropy of hot-deformed Nd-Fe-B magnets.

Fig. 7

XRD patterns of (a) the MQ powder, (b) the bulk material produced at 723 K by the SPS method, and (c) the hot-deformed Nd-Fe-B magnet produced at 923 K by the SPS method.

4 Conclusions

Commercially available Nd-Fe-B melt-spun ribbon (MQ powder) was consolidated into bulk materials at temperatures ranging from 673 K to 873 K by the SPS method. The density of the bulk materials increased but their coercivity decreased as the consolidation temperature increased. The resultant bulk materials consisted of fine Nd₂Fe₁₄B grains and exhibited high coercivity values, but were magnetically isotropic. The isotropic bulk materials were subsequently hot deformed at temperatures ranging from 873 K to 1073 K by the SPS method. It was found that the bulk material produced at 673 K with a relatively low density (less than 90 %) was not suitable for the hot-deformation study, but the bulk materials produced at 723 K or higher were successfully hot deformed by the SPS method. The coercivity of the resultant hot-deformed magnets gradually decreased as the hot-deformation temperature increased, regardless of the original coercivity of the bulk materials. The optimally hot-deformed magnet showed a high remanence of 1.39 T with a maximum energy product of 240 kJm⁻³.

Acknowledgement

This work was supported by a Grant-in-Aid from the Amada Foundation (AF-2013020).

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