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

The further enhancement of magnetic properties and optimization of fabrication technology of anisotropic sintered Nd–Fe–B magnets are still concerned to study because this kind of magnets is more and more applied in practice, especially for generators and vehicle motors [1, 2]. However, the temperature of the sintered Nd–Fe–B magnets in the motors and generators is high ($\sim 200\, {}^\circ{\rm C}$) during operating time [3]. When increasing temperature, the coercivity of the magnets is decreased rapidly due to the thermal demagnetization, leading to the reduction of efficiency in the high-temperature environments. In order to enhance the ${{H}_{C}}$ of the Nd–Fe–B magnets to meet the application requirements in these devices, Dy is usually replaced partially for Nd [4–6]. However, Dy is expensive and scarce. Therefore, a number of scientists have been investigating to improve the quality of the sintered Nd–Fe–B magnets without using or using a small amount of the heavy rare earth element. Previous works have indicated that the coercivity of this type of Nd–Fe–B magnets can be enhanced by addition of some elements other than the main components of Nd, Fe and B such as Al, Cu, Co... or by improving technology [7–10]. Especially, the addition of non-ferromagnetic compounds to grain boundary not only enhances the coercivity but also reduces amount of heavy rare earth [11–16]. Each additional compound contains the elements which differently affect to the magnetic properties of the magnets. With Dy-addition, the coercivity is significantly increased because the anisotropy field ${{H}_{A}}$ of ${\rm D}{{{\rm y}}_{2}}{\rm F}{{{\rm e}}_{14}}{\rm B}$ of $278\,{\rm kOe}$ at room temperature is much higher than that of ${\rm N}{{{\rm d}}_{2}}{\rm F}{{{\rm e}}_{14}}{\rm B}$, ${{H}_{A}}=75\,{\rm kOe}$. In addition, the adding Dy also avoids oxygenation for the magnets. However, antiferromagnetic coupling with Fe leads to reduction of remanence ${{B}_{r}}$ and the maximum energy product ${{\left(BH \right)}_{{\rm max}}}$. It is known that the coercivity of the sintered Nd–Fe–B magnets is sensitive to the microstructure. The addition of Al-containing compounds can improve microstructure such as smoothness of grain boundaries, uniformity of the particles of the magnets [17–19]. Pandian et al reported that the addition of Al of $1-2\,{\rm wt }\%$ decreases remanence ${{B}_{r}}$ of $5\%$ but increases the coercivity ${{H}_{C}}$ of about $20 \%$.

Our previous study reported that, by adding $2\,{\rm wt }\%$ of ${\rm D}{{{\rm y}}_{40}}{\rm N}{{{\rm d}}_{30}}{\rm A}{{{\rm l}}_{30}}$ nanoparticles to grain boundaries, the coercivity of the magnets could be considerably improved [20]. In this work, we investigated the influence of weight fractions of the additional nanoparticles of ${\rm D}{{{\rm y}}_{40}}{\rm N}{{{\rm d}}_{30}}{\rm A}{{{\rm l}}_{30}}$ and ${\rm N}{{{\rm d}}_{30}}{\rm A}{{{\rm l}}_{30}}$ on the coercivity of the sintered Nd–Fe–B magnets.

The pre-alloys of ${\rm N}{{{\rm d}}_{16.5}}{\rm F}{{{\rm e}}_{77}}{{{\rm B}}_{6.5}}$ were prepared from Nd, Fe and FeB by induction melting under Ar gas to avoid oxidation. The obtained ingots were pulverized for 8 h to obtain powder with grain size of $3-5\,\mu {\rm m}$ by ball milling method in industrial white gasoline. The addition alloys ${\rm D}{{{\rm y}}_{40}}{\rm N}{{{\rm d}}_{30}}{\rm A}{{{\rm l}}_{30}}$ and ${\rm N}{{{\rm d}}_{40}}{\rm A}{{{\rm l}}_{30}}$ were prepared by arc-melting furnace under argon atmosphere. After that the resulted alloys were pulverized 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 $20\,{\rm kOe}$. The pressed magnets were sintered at $1080\, {}^\circ{\rm C}$ for 1 h. A two-stage heat treatment process was chosen and carried out using a vacuum furnace. At the first stage, the magnets were heat-treated at $820\, {}^\circ{\rm C}$ for 1 h and then rapidly quenched to room temperature by argon atmosphere. For the second stage, the magnets were heat-treated at $540\, {}^\circ{\rm C}$ for 1 h and rapidly quenched by argon atmosphere. For both the stages, the heating and quenching rates were $30\, {}^\circ{\rm C}\,{{\min }^{-1}}$ and $50\, {}^\circ{\rm C}\,{{\min }^{-1}}$, 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 ${{\left(BH \right)}_{{\rm max}}}$ of the magnets, a demagnetization factor was estimated through a semi-experimental data sheet.

Figure 1 shows the SEM images of powder of the additional 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 samples still contains region which is difficult to observe individual 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 ${\rm N}{{{\rm d}}_{2}}{\rm F}{{{\rm e}}_{14}}{\rm B}$ grains [21]. This is one of the reasons for the enhancement of coercivity ${{H}_{C}}$ and maximum energy product ${{\left(BH \right)}_{{\rm max}}}$. Nano-scale sized particles are desired to mix with the Nd–Fe–B micropowder.

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Figures 2 and 3 show the hysteresis loops of the magnets added with various weight fractions of ${\rm D}{{{\rm y}}_{40}}{\rm N}{{{\rm d}}_{30}}{\rm A}{{{\rm l}}_{30}}$ and ${\rm N}{{{\rm d}}_{40}}{\rm A}{{{\rm l}}_{30}}$ 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 ${\rm D}{{{\rm y}}_{40}}{\rm N}{{{\rm d}}_{30}}{\rm A}{{{\rm l}}_{30}}$ nanoparticles is stronger than that of ${\rm N}{{{\rm d}}_{40}}{\rm A}{{{\rm l}}_{30}}$ ones. With the ${\rm N}{{{\rm d}}_{30}}{\rm A}{{{\rm l}}_{30}}$ 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 demagnetization 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 inhomogeneity of demagnetization field. On the other hand, heterogeneous grain boundaries can lead to the formation of the soft magnetic α-Fe phase which plays a role as nucleation centre of reversal domains to cause magnetization of the magnets at lower external magnetic field [22].

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Magnetic characteristic curves of the heat-treated magnets added with $5\,{\rm wt }\%$ of ${\rm D}{{{\rm y}}_{40}}{\rm N}{{{\rm d}}_{30}}{\rm A}{{{\rm l}}_{30}}$ and $3\,{\rm wt}\%$ of ${\rm N}{{{\rm d}}_{30}}{\rm A}{{{\rm l}}_{30}}$ are presented in figure 4. The obtained results show that, the coercivity of the magnets added with ${\rm D}{{{\rm y}}_{40}}{\rm N}{{{\rm d}}_{30}}{\rm A}{{{\rm l}}_{30}}$ is higher than that of the one added with ${\rm N}{{{\rm d}}_{30}}{\rm A}{{{\rm l}}_{30}}$. 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 ${{\left(BH \right)}_{{\rm max}}}$, 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 ${{\left(BH \right)}_{{\rm max}}}$ of the magnets is difficult. Because the magnetic properties of the magnets are not only dependent on ${\rm N}{{{\rm d}}_{{\rm 2}}}{\rm F}{{{\rm e}}_{14}}{\rm B}$ 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 magnetic properties for practical applications is required.

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Figure 5 shows the dependences of the coercivity ${{H}_{C}}$ of the magnets on various weight fractions of nanoparticles of ${\rm D}{{{\rm y}}_{40}}{\rm N}{{{\rm d}}_{30}}{\rm A}{{{\rm l}}_{30}}$ and ${\rm N}{{{\rm d}}_{30}}{\rm A}{{{\rm l}}_{30}}$ before and after heat treatment. We can see that ${{H}_{C}}$ depends almost linearly on the concentration of ${\rm D}{{{\rm y}}_{40}}{\rm N}{{{\rm d}}_{30}}{\rm A}{{{\rm l}}_{30}}$ (figure 5(a)). Its value increases from 5.3 to $10 {\rm kOe}$ for the as-sintered magnets and from 8 to $13\,{\rm kOe}$ for the heat-treated magnets when weight fractions of ${\rm D}{{{\rm y}}_{40}}{\rm N}{{{\rm d}}_{30}}{\rm A}{{{\rm l}}_{30}}$ 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 Dy-containing 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 ${{\left({\rm Nd,Dy} \right)}_{2}}{\rm F}{{{\rm e}}_{14}}{\rm B}$ shell. Because ${{H}_{A}}$ of ${\rm D}{{{\rm y}}_{{\rm 2}}}{\rm F}{{{\rm e}}_{14}}{\rm B}$ is higher than that of ${\rm N}{{{\rm d}}_{{\rm 2}}}{\rm F}{{{\rm e}}_{14}}{\rm B}$, the formation ${{\left({\rm Nd},{\rm Dy} \right)}_{2}}{\rm F}{{{\rm e}}_{14}}{\rm B}$ shell might make magnetic anisotropy of the outer layer higher than that of the interior. As a result, the formation and propagation of a reverse domain would be inhibited more than in the normal grains. When the reverse nucleations are inhibited 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 ${\rm D}{{{\rm y}}_{40}}{\rm N}{{{\rm d}}_{30}}{\rm A}{{{\rm l}}_{30}}$ 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.

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As for the magnets added with ${\rm N}{{{\rm d}}_{30}}{\rm A}{{{\rm l}}_{30}}$ nanoparticles, their coercivity slightly increases from 5.3 to $7.3\,{\rm kOe}$ before heat treatment, and from 8 to $10\,{\rm kOe}$ 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 ${\rm N}{{{\rm d}}_{30}}{\rm A}{{{\rm l}}_{30}}$ nanoparticles is 3%. The increase of the coercivity is good agreement with the result reported by Mottram et al [19]. The formation of disadvantage 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 ${{\left(BH \right)}_{{\rm max}}}$ of the heat-treated magnets on various additional fractions of the ${\rm D}{{{\rm y}}_{40}}{\rm N}{{{\rm d}}_{30}}{\rm A}{{{\rm l}}_{30}}$ and ${\rm N}{{{\rm d}}_{30}}{\rm A}{{{\rm l}}_{30}}$ nanoparticles is shown in figure 6. We can see that, the ${{\left(BH \right)}_{{\rm max}}}$ decreases with increasing the fraction of both the additional compounds, agreeing with the results reported in [12]. The reduction of the ${{\left(BH \right)}_{{\rm max}}}$ of the added magnets is due to a decrease of saturation magnetization by additional of non-ferromagnetic nanoparticles. Although the ${{\left(BH \right)}_{{\rm max}}}$ is reduced but its value is still high enough ($>30\,{\rm MOe}$) for practical application. Especially, the enhancement of the coercivity is necessary for electric generators 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.

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The influence of concentration of the additional nanoparticles of ${\rm D}{{{\rm y}}_{40}}{\rm N}{{{\rm d}}_{30}}{\rm A}{{{\rm l}}_{30}}$ and ${\rm N}{{{\rm d}}_{30}}{\rm A}{{{\rm l}}_{30}}$ on the coercivity ${{H}_{C}}$ of the sintered ${\rm N}{{{\rm d}}_{16.5}}{\rm F}{{{\rm e}}_{77}}{{{\rm B}}_{6.5}}$ magnets has been investigated. The ${{H}_{C}}$ is considerably improved by adding nanoparticles to the grain boundaries of magnets. The effect of the ${\rm D}{{{\rm y}}_{40}}{\rm N}{{{\rm d}}_{30}}{\rm A}{{{\rm l}}_{30}}$ nanoparticles on the coercivity enhancement for the magnets is stronger than that of the ${\rm N}{{{\rm d}}_{30}}{\rm A}{{{\rm l}}_{30}}$ ones. While the ${{H}_{C}}$ reaches a maximal value of $10\,{\rm kOe}$ at $3\,{\rm wt}\%$ of ${\rm N}{{{\rm d}}_{30}}{\rm A}{{{\rm l}}_{30}}$ addition, it increases linearly from $8\,{\rm kOe}$ to $13\,{\rm kOe}$ with increasing the weight fraction of the ${\rm D}{{{\rm y}}_{40}}{\rm N}{{{\rm d}}_{30}}{\rm A}{{{\rm l}}_{30}}$ nanoparticles from 0 to 5%. The ${{\left(BH \right)}_{{\rm max}}}$ of the magnets is still retained high enough ($>30\,{\rm MOe}$). The obtained hard magnetic parameters of the magnets can be applied in practice.

This work was supported by Vietnam Academy of Science and Technology under Grant number of VAST03.05/16-17. Most of experiments were done in the Key Laboratory for Electronic Materials and Devices, Institute of Materials Science, Vietnam Academy of Science and Technology, Viet Nam.