NdFeB based magnets prepared from nanocrystalline powders with various compositions and particle sizes by spark plasma sintering

NdFeB magnets spark plasma sintered from melt spun powders with Nd rich composition

Two distinct zones with different grain sizes were noticed in the spark plasma sintered NdFeB magnets in our previous work.11 Spark plasma sintering temperature and pressure had important effects on the widths of coarse and fine grain zones and the grain sizes in both zones. The changes in grain structure led to the variation in the magnetic properties. Here, powders with three different particle sizes, i.e. <45, 45–100 and 100–200 μm, were employed for SPS using the optimised SPS parameters obtained previously (T_SPS = 700°C, t_SPS = 5 min and P_SPS = 50 MPa).11 Table 1 lists the magnetic properties and density for the as prepared magnets. The density increases from 7·33 g cm⁻³ for the magnets spark plasma sintered with powders having particle size of <45 μm to 7·50 g cm⁻³ for the magnets prepared with powders in the size range of 100–200 μm. Figure 1 shows the magnetic hysteresis loops for the starting powder and spark plasma sintered magnets. For the starting powders, the magnetic properties are J_r = 0·78 T, _jH_c = 1624 kA m⁻¹ and (BH)_max = 103 kJ m⁻³. One should note that these values were obtained without considering the demagnetisation factor for powders. Good shape of the demagnetisation curve was obtained for all spark plasma sintered magnets. With increasing initial particle size, the magnet density increases. The magnet prepared with powders <45 μm has the minimum value of intrinsic coercivity _jH_c. The remanence J_r and the maximum energy product (BH)_max change slightly with varied particle size. The magnets spark plasma sintered at 700°C/50 MPa/5 min have good magnetic properties with J_r = 0·83 T, (BH)_max = 117 kJ m⁻³ and _jH_c = 1202 kA m⁻¹. Since there are no heavy rare earth elements, such as Dy or Tb, the obtained magnetic properties with (BH)_max>110 kJ m⁻³ are indeed very promising. The magnetic properties are also slightly better than the previous report on a similar composition.12

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Table 1

Magnetic properties and density for Nd rich NdFeB magnets spark plasma sintered with three different ranges of particle size (T_SPS = 700°C, P_SPS = 50 MPa and t_SPS = 5 min)

Particle size/μm	(BH)max/kJ m−3	jHc/kA m−1	Jr/T	ρ/g cm−3
<45	117	1202	0·83	7·33
45–100	115	1466	0·82	7·46
100–200	116	1434	0·82	7·50

Figure 2 shows the microstructure of the magnets spark plasma sintered using three types of powders. Two distinguished zones with different grain sizes were noticed again for the magnets spark plasma sintered from the particles in the size ranges of 45–100 and 100–200 μm. This phenomenon is attributed to the mechanism of SPS.11 Local high temperature zone was generated by pulsed energy that existed between particle contacting surfaces. The temperature at the centre of the particle was lower than that in the particle boundaries. Song et al. 13 reported that the temperature in the particle contacting surface is nearly 3000 K higher than that at the particle centre during SPS, and the temperature decreased sharply from the particle contacting surface towards the centre. In this study, the high temperature at the particle boundaries led to excessive grain growth, as shown in Fig. 2c–f. The grains located in the interior of the particle have a less growth trend due to the low temperature, resulting in the fine grain zone. However, interestingly, for the magnets spark plasma sintered with the particle size of <45 μm, only a very small amount of coarse grain zones were observed (not shown here), and the grain size was almost uniformly distributed, as shown in Fig. 2a and b. The mean grain size for this sample is almost similar to that in the fine grain zones for the magnet spark plasma sintered with larger particle sizes. The uniform size for the magnets spark plasma sintered with the particle size of <45 μm can be explained as follows. During SPS, momentary electric currents flow through the particle contacting surface. In the spark plasma sintered billet, the volume fraction of particle boundaries for powders with small sizes is higher than those with large particle sizes of 45–100 and 100–200 μm, resulting in weak current intensity in the former. Yanagisawa et al. 14 reported that spark phenomenon can only occur at large current and low pressure. Therefore, it is possible that there is no or little plasma that existed between the particles with particle size of <45 μm due to the weak current intensity. The grain could not grow due to the deficiency of enough temperature produced by spark plasma. This leads to the disappearance of coarse grain zone in the magnet spark plasma sintered with the particle size of <45 μm. On the other hand, low current intensity and low temperature in the particle boundaries may be not beneficial to the densification of the magnets, which explains the low density of the magnets spark plasma sintered by powders <45 μm, as listed in Table 1.

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Figure 2

Images (SEM) for Nd rich NdFeB magnets spark plasma sintered with three different particle size ranges by T_SPS = 700°C, t_SPS = 5 min and P_SPS = 50 MPa

The spark plasma sintered magnets with nanocrystallites generally show exchange coupling between grains. Henkel plots15^– 17 can be used to analyse the exchange interactions between the grains based on the relationship δM(H) = M_d(H)–[1–2M_r(H)], where M_r(H) and M_d(H) are defined as the remanent magnetisation after applying a field H on a thermally demagnetised sample and a reverse field on a previous saturated sample. According to the Stoner–Wohlfarth theory,18 for an assembly of small non-interacting particles, δM = 0. A positive value of δM indicates that the exchange coupling was enhanced, while a negative value of δM was interpreted as magnetostatic interaction dominating when magnetisation reversal occurs.15,19 Figure 3 shows the Henkel plots for spark plasma sintered magnets prepared from various powders. Large positive values of δM verify the strong exchange coupling interaction due to the nanostructure in the magnets. For the spark plasma sintered magnets prepared from small powders (<45 μm), the largest positive value of δM was obtained, which indicated that exchange coupling in the magnet spark plasma sintered from small powders is stronger than that from large powders. This can be manifested by the disappeared coarse grain zone in magnets prepared from smaller powders, as discussed earlier. Meanwhile, a relatively sharp peak obtained in the δM plot indicates a uniform grain size distribution,19 which can be verified by SEM images, as shown in Fig. 2a and b. For the spark plasma sintered magnet prepared from the particle size of <45 μm, possible deficiency of spark plasma and stronger exchange coupling due to the uniform distribution of nanostructure grain size are the main reasons for the reduction in coercivity. In contrast, the high _jH_c of 1466 and 1434 kA m⁻¹ obtained in the spark plasma sintered magnets from large particles (Table 1) possibly resulted from the weak exchange coupling in the magnets. Meanwhile, the uniform distribution of the Nd rich phase due to the activated powder surface by interparticle discharge plasma in the process of SPS is also contributed to the high _jH_c.

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Figure 3

Henkel plots of Nd rich NdFeB magnets prepared by SPS with different powder size ranges by T_SPS = 700°C, t_SPS = 5 min and P_SPS = 50 MPa

NdFeB magnets spark plasma sintered from melt spun powders with single phase composition

For comparison, isotropic magnets with single phase composition of Nd_11·5Co_1·9Fe_81·1B_5·5 were also prepared by SPS using melt spun powders with four different particle sizes, i.e.<45, 45–100, 100–200 and 200–400 μm. Figure 4 shows the magnetic properties and density for the magnets spark plasma sintered with particles of <45 μm. With the increase in T_SPS, the density increases and _jH_c decreases. The J_r increases with increasing T_SPS until 750°C and decreases afterwards. The different dependences of _jH_c and J_r on the temperature lead to a maximum of (BH)_max at 750°C. The best combination of magnetic properties is J_r = 0·70 T, _jH_c = 435 kA m⁻¹ and (BH)_max = 65·0 kJ m⁻³. It is also noted that the density of the spark plasma sintered magnet ρ = 6·68 g cm⁻³ is much lower than that with Nd rich composition. The low density has been contributed to the deficiency of the Nd rich phase. In addition, as discussed above, the Nd rich phase has an important effect on the _jH_c. For these samples, due to the deficiency of the Nd rich phase, _jH_c decreases monotonically with increasing temperature.

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Figure 4

Magnetic properties and density for single phase NdFeB magnets spark plasma sintered with particle size range of <45 μm as function of sintering temperature (P_SPS = 50 MPa and t_SPS = 3 min)

Figure 5 shows the X-ray diffraction patterns of NdFeB magnets spark plasma sintered with particles of <45 μm. With the increase in temperature, the diffraction peaks become strong and narrow, which indicated the growth of grain size. It is also found that there existed an undefined phase in the magnets prepared at 850 and 900°C. This undefined phase possibly resulted in the reduction in J_r after 850°C. All the other peaks in all the patterns are attributed to the tetragonal hard magnetic Nd₂(FeCo)₁₄B phase.

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Figure 5

X-ray diffraction patterns for single phase NdFeB magnets spark plasma sintered with particle size of <45 μm at different temperatures (P_SPS = 50 MPa and t_SPS = 3 min)

Figure 6 shows SEM images of magnets spark plasma sintered with particles of <45 μm. With increasing T_SPS, the porosity reduces, and thus, the density increases. Similar to the Nd rich composition, at the vicinity of particle boundaries, coarse grain zones also formed, as shown in Fig. 6d. Similar results on the variations in magnetic properties and density were obtained for other spark plasma sintered magnets prepared from powders with size ranges of 45–100, 100–200 and 200–400 μm.

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Figure 6

Images (SEM) for single phase NdFeB magnets spark plasma sintered from powders with particle size of <45 μm at different temperatures (t_SPS = 3 min and P_SPS = 50 MPa)

Table 2 lists the density and optimal magnetic properties for magnets spark plasma sintered from powders with four different sizes under the optimal sintering conditions. The low densities of the spark plasma sintered magnets indicate that it is relatively difficult to consolidate microsized melt spun powders into high density bulk magnet for single phase NdFeB ribbons due to the deficiency of the Nd rich phase. The best magnetic properties with J_r = 0·73 T, _jH_c = 604 kA m⁻¹ and (BH) _max = 81 kJ m⁻³ in the magnet spark plasma sintered from powders with size of 200–400 μm are noticed. The (BH)_max for the spark plasma sintered magnet increases monotonically with increasing particle size under the optimal sintering conditions, as listed in Table 2.

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Table 2

Optimal magnetic properties and density for single phase magnets spark plasma sintered from powders with four different particle size ranges under optimal sintering conditions

Particle size/μm	SPS conditions (TSPS–PSPS–tSPS)	(BH)max/kJ m−3	jHc/kA m−1	Jr/T	ρ/g cm−3
<45	750°C–50 MPa–3 min	65	435	0·70	6·68
45–100	700°C–50 MPa–3 min	69	538	0·67	6·41
100–200	750°C–50 MPa–3 min	79	504	0·73	6·69
200–400	700°C–50 MPa–3 min	81	604	0·73	6·42

Figure 7 shows the dependences of magnetic properties on T_SPS for NdFeB magnets spark plasma sintered with different powders. It is found that T_SPS has important effects on the magnetic properties. Good combination of properties has been obtained in the T_SPS range of 700–800°C. At temperatures higher than 800°C, low _jH_c and J_r result from the grain growth and the existing undefined second phase. At low temperatures, low J_r due to low density is the main reason of the bad combination of magnetic properties. Comparing the magnets prepared from various powders, generally, a larger particle size is beneficial to achieve better magnetic properties under the same sintering condition, including higher J_r and (BH)_max. This is also attributed to the sintering mechanism of SPS. For larger particle size, the volume fraction of coarse grain zone in the spark plasma sintered magnet is less than that for the smaller particle size due to less particle boundaries.

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Figure 7

a energy product, b coercivity and c remanent magnetic polarisation for single phase NdFeB magnets spark plasma sintered with different powder sizes as function of sintering temperature (P_SPS = 50 MPa and t_SPS = 3 min)

Table 2 and Fig. 7 indicate that, for the single phase spark plasma sintered NdFeB magnets, the particle size of the starting melt spun powders has a significant effect on the magnetic properties. However, for the Nd rich magnets, as listed in Table 1, the magnetic properties of magnets spark plasma sintered from the starting particles with sizes of 45–100 and 100–200 μm change only slightly. The latter can be possibly attributed to the fact that both Nd rich powders with sizes of 45–100 and 100–200 μm are crushed into almost the same size due to the high brittleness of the Nd rich composition. The results in this work imply that, to achieve good properties in spark plasma sintered magnets, the powder particle size has to be considered and selected accordingly.