Influence of additional nanoparticles on coercivity of sintered Nd–Fe–B magnets

In this work, we investigated the influence of concentration of the additional nanoparticles of and on the coercivity of the sintered magnets. Composition and concentration of the additional compounds clearly influence magnetic properties of the magnets. The coercivity increases linearly from to with increasing the weight fraction of the nanoparticles from 0 to . Meanwhile, the of the added magnets reaches a maximal value of at the optimum addition of . The quite high maximum energy products, , were also obtained for the magnets added with the nanoparticles.

Original content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. increased because the anisotropy field H A of Dy 2 Fe 14 B of 278 kOe at room temperature is much higher than that of Nd 2 Fe 14 B, H A = 75 kOe. In addition, the adding Dy also avoids oxygenation for the magnets. However, antiferromagn etic coupling with Fe leads to reduction of remanence B r and the maximum energy product (BH) max . It is known that the coercivity of the sintered Nd-Fe-B magnets is sensitive to the microstructure. The addition of Alcontaining compounds can improve microstructure such as smoothness of grain bound aries, uniformity of the particles of the magnets [17][18][19]. Pandian et al reported that the addition of Al of 1 − 2 wt% decreases remanence B r of 5% but increases the coercivity H C of about 20%.
Our previous study reported that, by adding 2 wt% of Dy 40 Nd 30 Al 30 nanoparticles to grain boundaries, the coer civity of the magnets could be considerably improved [20]. In this work, we investigated the influence of weight fractions of the additional nanoparticles of Dy 40 Nd 30 Al 30 and Nd 30 Al 30 on the coercivity of the sintered Nd-Fe-B magnets.

Experimental
The prealloys of Nd 16.5 Fe 77 B 6.5 were prepared from Nd, Fe and FeB by induction melting under Ar gas to avoid oxida tion. The obtained ingots were pulverized for 8 h to obtain powder with grain size of 3 − 5 µm by ball milling method in industrial white gasoline. The addition alloys Dy 40 Nd 30 Al 30 and Nd 40 Al 30 were prepared by arcmelting furnace under argon atmosphere. After that the resulted alloys were pulver ized by high energy ball milling method with milling time of 4 h to obtain nanoparticles with average size of about 50 nm. The solvent/material and ball/powder ratios are 1/1 and 4/1 , respectively. The additional nanoparticles with various weight fractions from 1 to 5% were mixed into the Nd-Fe-B powder thoroughly. The mixed powder was pressed under a pressure of 15 MPa in an oriented magnetic field of about 20 kOe. The pressed magnets were sintered at 1080 • C for 1 h. A twostage heat treatment process was chosen and carried out using a vacuum furnace. At the first stage, the magnets were heattreated at 820 • C for 1 h and then rapidly quenched to room temperature by argon atmosphere. For the second stage, the magnets were heattreated at 540 • C for 1 h and rapidly quenched by argon atmosphere. For both the stages, the heating and quenching rates were 30 • C min −1 and 50 • C min −1 , respectively. The structure of the samples was thoroughly analyzed by using scanning electron microscope (SEM). The specimens of cylinders with 3 mm diameter and 3 mm height were cut to investigate magnetic properties on a pulsed high field magnetometer. In order to determine the maximum energy product (BH) max of the magnets, a demag netization factor was estimated through a semiexperimental data sheet. Figure 1 shows the SEM images of powder of the addi tional compounds with milling time of 4 h. We can see that the particles of the samples are relatively uniform with the average size smaller than 50 nm. However, for both the sam ples still contains region which is difficult to observe indi vidual grains by their coalescence. With high surface energy, the first melting of nanoparticles during sintering process makes homogeneous distribution of the intergranular phase, leading to a decrease of exchange interaction of the Nd 2 Fe 14 B grains [21]. This is one of the reasons for the enhancement of coercivity H C and maximum energy product (BH) max . Nano scale sized particles are desired to mix with the Nd-Fe-B micropowder. Figures 2 and 3 show the hysteresis loops of the magnets added with various weight fractions of Dy 40 Nd 30 Al 30 and Nd 40 Al 30 nanoparticles before and after heat treatment. We can realize that, the coercivity of the magnets depends on both the nanoparticle addition and heat treatment process. The influence of addition of Dy 40 Nd 30 Al 30 nanoparticles is stronger than that of Nd 40 Al 30 ones. With the Nd 30 Al 30 added magnets, the change of the coercivity on large concentration of nanoparticles is not considerably. After heat treatment the coercivity was significantly enhanced. This probably is due to the improvement of microstructure of the magnets after heat treatment such as controlling particles size, creating the suitable grain boundary phase… However, the squareness of the hysteresis loops of the magnets is slightly decreased. A little dip was observed in the second quadrant demagnetiza tion curves of the annealed magnets. This can be explained by the wide distribution of the grain size after heat treatment. At the same time, the addition of elements can change the structure and distribution of phases, leading to the inhomoge neity of demagnetization field. On the other hand, heteroge neous grain boundaries can lead to the formation of the soft magnetic αFe phase which plays a role as nucleation centre of reversal domains to cause magnetization of the magnets at lower external magnetic field [22].

Results and discussion
Magnetic characteristic curves of the heattreated magnets added with 5 wt% of Dy 40 Nd 30 Al 30 and 3 wt% of Nd 30 Al 30 are presented in figure 4. The obtained results show that, the coercivity of the magnets added with Dy 40 Nd 30 Al 30 is higher than that of the one added with Nd 30 Al 30 . However, the squareness of hysteresis loop of the former is worse than that of the latter one, leading to the decrease of maximum energy product (BH) max , which has been obtained to be 36 MGOe and 37 MGOe, respectively. In general, the enhancement both the coercivity H C and the maximum energy product (BH) max of the magnets is difficult. Because the magnetic properties of the magnets are not only dependent on Nd 2 Fe 14 B phase, but also on the microstructure. Optimization of microstructure depends on parameters of technological conditions such as particle size, sintering temperature, sintering time, annealing time, annealing temperature… Controlling of manufacture technology to create the sintered magnets with suitable magn etic properties for practical applications is required. Figure 5 shows the dependences of the coercivity H C of the magnets on various weight fractions of nanoparti cles of Dy 40 Nd 30 Al 30 and Nd 30 Al 30 before and after heat treatment. We can see that H C depends almost linearly on the concentration of Dy 40 Nd 30 Al 30 ( figure 5(a)). Its value    increases from 5.3 to 10kOe for the assintered magnets and from 8 to 13 kOe for the heattreated magnets when weight fractions of Dy 40 Nd 30 Al 30 nanoparticles increases from 0 to 5%. This is agreed with the result reported by Liu et al [6]. The coercivity enhancement of the magnets added with Dycontaining compounds is due to Dy diffusion from the grain boundaries to the 2 : 14 : 1 grain during sintering and heat treatment process, leading to the formation of the (Nd, Dy) 2 Fe 14 B shell. Because H A of Dy 2 Fe 14 B is higher than that of Nd 2 Fe 14 B, the formation (Nd, Dy) 2 Fe 14 B shell might make magnetic anisotropy of the outer layer higher than that of the interior. As a result, the formation and prop agation of a reverse domain would be inhibited more than in the normal grains. When the reverse nucleations are inhib ited at the surface of the grains, an external magnetic field must be large enough for the formation and growth of them, meaning that the magnets have high coercivity. However, the effect of the coercivity enhancement for the magnets by adding Dy 40 Nd 30 Al 30 nanoparticles in this work is weaker than that obtained in our previous investigation [20]. This probably is due to the change of milling solvent, whose contaminations might affect to the quality of the additional nanoparticles.
As for the magnets added with Nd 30 Al 30 nanoparticles, their coercivity slightly increases from 5.3 to 7.3 kOe before heat treatment, and from 8 to 10 kOe after heat treatment as weight fraction of the additional compound increases from 0 to 3% ( figure 5(b)). After that, the H C decreases when the additional fraction is further increased. Thus, the optimal additional weight fraction of Nd 30 Al 30 nanoparticles is 3%. The increase of the coercivity is good agreement with the result reported by Mottram et al [19]. The formation of dis advantage phases at grain boundaries is reason for reduction of the coercivity with additional fractions of 4%. On the other hand, the optimal sintering temperature might be changed by large fraction of the additional compound, which has melting temperature far from that of the Nd-Fe-B phase, leading to the undesired microstructure for the magnets.
The dependence of maximum energy product (BH) max of the heattreated magnets on various additional fractions of the Dy 40 Nd 30 Al 30 and Nd 30 Al 30 nanoparticles is shown in figure 6. We can see that, the (BH) max decreases with increasing the fraction of both the additional compounds, agreeing with the results reported in [12]. The reduction of the (BH) max of the added magnets is due to a decrease of saturation mag netization by additional of nonferromagnetic nanoparticles. Although the (BH) max is reduced but its value is still high enough (> 30 MOe) for practical application. Especially, the enhancement of the coercivity is necessary for electric genera tors and motors. On the other hand, the less use or unused of the heavy rare earth of Dy is important for lowering the cost of the magnets.

Conclusion
The influence of concentration of the additional nanoparticles of Dy 40 Nd 30 Al 30 and Nd 30 Al 30 on the coercivity H C of the sintered Nd 16.5 Fe 77 B 6.5 magnets has been investigated. The H C is considerably improved by adding nanoparticles to the grain boundaries of magnets. The effect of the Dy 40 Nd 30 Al 30 nanoparticles on the coercivity enhancement for the magnets is stronger than that of the Nd 30 Al 30 ones. While the H C reaches a maximal value of 10 kOe at 3 wt% of Nd 30 Al 30 addition, it increases linearly from 8 kOe to 13 kOe with increasing the weight fraction of the Dy 40 Nd 30 Al 30 nanoparticles from 0 to 5%. The (BH) max of the magnets is still retained high enough (> 30 MOe). The obtained hard magnetic parameters of the magnets can be applied in practice.