简介概要

Phase composition and magnetic properties of Pr-Nd-MM-Fe-B nanocrystalline magnets prepared by spark plasma sintering

来源期刊：Rare Metals2020年第1期

论文作者：Xin Wang Zeng-Ru Zhao Fei Liu Yan-Li Liu Gao-Feng Wang Ming-Gang Zhu Xue-Feng Zhang

文章页码：36 - 40

摘要：The isotropic nanocrystalline [（PrNd）_0.8MM_0.2]_29.8Fe_68.7Al_0.1Cu_0.12Co_0.88B magnets（MM:mischmetal） were prepared by single-main phase and double-main phase methods using spark plasma sintering（SPS）.Melt-spinning method was used to prepare initial powder and avoid component deviations caused by longtime ball milling.The magnetic properties of the magnet prepared by double-main phase method（called double-main phase magnet,DMP magnet） are remanence of B_r=0.75 T,intrinsic coercivity of H_cj=909.83 kA·m^-1,maximum magnetic energy product of（BH）_max=95.48 kJ·m^-3,which are better than those of the magnet prepared by single-main phase method（called singlemain phase magnet,SMP magnet）.The diffraction peaks of the main phase of DMP magnet split in X-ray diffractometer（XRD） pattern,indicating that R₂ Fe₁₄B phases with different distributions of La/Ce elements exist in the magnet.This speculation is confirmed by transmission electron microscopy（TEM） observation.The La/Ce-rich main phase and La/Celean main phase are present in DMP magnets.The heterogeneity of rare earth elements suppresses the magnetic dilution effect in DMP magnet,and the magnetic properties are improved.Though the DMP magnet contains different main phases,it presents unitary Curie temperature（T_C） of 577 K,which is higher than that of SMP magnet.This result suggests that the T_C of the magnets can be promoted by double-main phase method and SPS.

详情信息展示

稀有金属(英文版) 2020,39(01),36-40

Phase composition and magnetic properties of Pr-Nd-MM-Fe-B nanocrystalline magnets prepared by spark plasma sintering

Xin Wang Zeng-Ru Zhao Fei Liu Yan-Li Liu Gao-Feng Wang Ming-Gang Zhu Xue-Feng Zhang

Division of Functional Materials,Central Iron and SteelResearch Institute

School of Science,Inner Mongolia University of Science and Technology

Key Laboratory of Integrated Exploitation of Bayan Obo Multi-Metal Resources,Inner Mongolia University of Science and Technology

作者简介：*Ming-Gang Zhu e-mail:mgzhu@sina.com;*Xue-Feng Zhang e-mail:xuefeng367@163.com;

收稿日期：13 November 2018

基金：financially supported by the National Key Research and Development Program of China (No. 2016YFB0700903);the National Natural Science Foundation of China (Nos.51571064 and 51571126);

Phase composition and magnetic properties of Pr-Nd-MM-Fe-B nanocrystalline magnets prepared by spark plasma sintering

Xin Wang Zeng-Ru Zhao Fei Liu Yan-Li Liu Gao-Feng Wang Ming-Gang Zhu Xue-Feng Zhang

Division of Functional Materials,Central Iron and SteelResearch Institute

School of Science,Inner Mongolia University of Science and Technology

Key Laboratory of Integrated Exploitation of Bayan Obo Multi-Metal Resources,Inner Mongolia University of Science and Technology

Abstract：

The isotropic nanocrystalline [(PrNd)_0.8MM_0.2]_29.8Fe_68.7Al_0.1Cu_0.12Co_0.88B magnets(MM:mischmetal) were prepared by single-main phase and double-main phase methods using spark plasma sintering(SPS).Melt-spinning method was used to prepare initial powder and avoid component deviations caused by longtime ball milling.The magnetic properties of the magnet prepared by double-main phase method(called double-main phase magnet,DMP magnet) are remanence of B_r=0.75 T,intrinsic coercivity of H_cj=909.83 kA·m^-1,maximum magnetic energy product of(BH)_max=95.48 kJ·m^-3,which are better than those of the magnet prepared by single-main phase method(called singlemain phase magnet,SMP magnet).The diffraction peaks of the main phase of DMP magnet split in X-ray diffractometer(XRD) pattern,indicating that R₂ Fe₁₄B phases with different distributions of La/Ce elements exist in the magnet.This speculation is confirmed by transmission electron microscopy(TEM) observation.The La/Ce-rich main phase and La/Celean main phase are present in DMP magnets.The heterogeneity of rare earth elements suppresses the magnetic dilution effect in DMP magnet,and the magnetic properties are improved.Though the DMP magnet contains different main phases,it presents unitary Curie temperature(T_C) of 577 K,which is higher than that of SMP magnet.This result suggests that the T_C of the magnets can be promoted by double-main phase method and SPS.

Keyword：

Mischmetal; Double-main phase; Spark plasma sintering; Magnetic properties; Microstructure;

Received： 13 November 2018

1 Introduction

The 2:14:1-type Nd-Fe-B magnets have been widely u^sed and rapidly developed due to outstanding magnetic performance among various permanent magnets,si_nce they were announced in 1983[1-5]._In _recent decades,the rapidly growing demand for Nd-Fe-B magnets has given rise to the overuse of the rare ear_th (RE) elements Nd and Pr[6_].Therefo_re,RE elements such as Pr,Nd,^Dy aⁿd Tb are facing a criticality issue due to resource limitatio_n_and high price.The P^r and Nd are less abundant in the natu_ral RE resources,compared w_ith L_a and Ce[7].D_eveloping permanent magnets with high abundant RE is requisite in order to solve problems of cost reduction and ^balanced utilization of RE resourc_e.Recently,the resear_ch on substitution of Ce or mischmetal alloy (defined as MM) for ^Pr and Nd in Nd-Fe-B magnets has attracted much attention[8-20].I_t can be understood that the intrinsic magnetic properties of L_a2Fe₁₄B and Ce₂Fe₁₄B are inferior to those of Nd₂Fe₁₄B and Pr₂Fe_14B[21]._The excellent magnetic properties maintained when a small amount of Nd/Pr was substitute^d b^y La/Ce[22,23].Recently,_Zhu e_t al.[₁7]proposed that mixing PrNdCeFeB and PrNdFeB powders with high magnetic properties (called as dual-main phase method) could be used to design the distribution of 2:14:1magnetic phase in the sintered magnets,and they successfully achieved the sintered magnets with the maximu_m magnetic energy product of 342.²⁸ kJ·m^-3 w_hen the Ce content reached 30%of the total ^amount of the RE metals.Lu et al.[24]reported that the compacted (La_0.3Ce_0.7)3-Fe₁₄B magnet with a coercivⁱty of 445.⁷⁶ kA_·m^-1 was successfully prepared by spark plasma sintering (SPS).The SPS process shows some advantages,s_uch as m_ore convenient operation,lower sintering temperature,shorter sintering time,lower energy consumption and higher saf^ety coefficient than traditional sintering process[²5-30].The low sintering temperature and short holding time in SPSmethod make it suitable to sinter nanocrystalline materials without excessive grain growt_h.The melt-spinning method,high_-energy ball milling method and SPS technology have attracte_d extensive attention in the field of preparing isotropic nanocrystalline hot-pressed magnet_s.In this work,we report the phase composition and magnetic properties in the double-main phase (DMP)magnets (PrNd)_29.2Fe68.7Al_0.1Cu_0.12Co_0.88B/MM_29.2Fe_68.7Al_0.1Cu_0.12Co_0.88B,prepared by SPS procedure._We compare the extrinsic magnetic properties and microstructures of the double-main phase magnet with the magnet[(PrNd)_0.8MM_0.2]_29.2Fe_68.7A_l0.1C_u0.12C_o0.88B prepared b^y the single-main method.

2 Experimental

The purities of the starting materials are ⁹9.99 wt%fo_r Fe,99.23 wt%for Fe-B (Fe:B=80.54:19.46 in mass) alloy and 99.72 wt%for ^Pr-Nd a_lloy (Pr:Nd=25.5:74.5 in mass) allo_y.The mischmetal (purity about 99.5 wt%) _from Bayan Obo mine,w_hich co_ntains 28.63 wt%La,50.₁3wt%Ce,4.⁸1 wt%Pr and 16.₃8 wt%Nd,was u^sed in the experiment.The ingots _with nominal compositions of(PrNd)_29.2Fe_68.7Al_0.1Cu_0.12Co_0.88B,MM_29.2Fe_68.7Al_0.1-Cu_0.12Co_0.88B and[(PrNd)_0.8MM_0.2]29.2Fe_68.7Al_0.1Cu_0.12-Co_0.88B were remelted a_t least three times t^o ensure the compositional homogeneity.For the convenience of writing and reading,in the following content,we define the elements of Fe,Al,Cu and Co as (Fe,_TM).Su_bsequently,the ribbons were obtained directly by induction melting the ingot under an argon atmosphere and then ejecting molte_n alloy through the orifice in quartz crucible onto the e^dge of a rotating copper wheel.The surface velocity of copper wheel was set in the speed o^f 60 m·s^-1 in order to optimize the magnetic properties.Then,^the mixture ribbons of(PrNd)_29.2(Fe,TM)_balB and MM_29.2(Fe,_TM)_balB wⁱth mass ratio of 2:8 and[(PrNd_)0.8MM_0.2]29.2(_Fe,TM)_balB ribbons were milled by the high-energy ball milling for 1 _h.The obtained powders were sintered by SPS method.T_he sintering temperature in the range of ⁵50-7⁵0℃wasemployed to improve the magnetic properties,and the pressure of 500 MPa was applied.The phase compositions of single-main phase (SMP) magnet and DMP magnet were examined by X-ray ^diffraction (XRD,PANalytical _X'pert Powder)using C^u Kαradiation.Magnetic measurements were taken using quantum design vibrating sample magnetometer (VSM,Ve_r^salab) at 300 K with the maximum field of 3 _T.The Curie temperatures ₍T_C) were determined from the temperature dependence of magnetization (M) in a field of 0.⁰5 ^T.The grain microstructures of the sam_ples were studied by transmission electron microscope (TEM,JEM-2100).

3 Results and discussion

Figure 1 displays XRD patterns of SMP magnet and DMPmagnet before and after SPS proces_s.It is seen in X^RDpatterns tha^t the diffraction peaks are not smoot_h.XRDpatterns indicate th_at 2:14:1 magnetic phase,unidentif_ied phase and amorphous phase coexist in the samples before SPS process.Through the above experimental process,the grain size of the powder_s is reduced as much as possibl_e,which can effectively inhibit the abnormal growth of grain during the SPS process[26].After SPS process,XRDpatterns show ^that the main phase is 2:14:1 magnetic phase.The peaks of the main phase of the DMP magnet in XRD pattern split,indicating that the phases in _DMPmagnet are different from those in N_d-Fe-B.The doublemain phase structure is not evident in SM^P magnet.Position shifts exist in the Bragg diffraction pea^ks of the main phase for the two magnets,_indicating that crystal lattice parameters are different.Using Jade software and the Scherrer formula,the average crystalline sizes are es_timated to be about 40 nm in the samples.

The hysteresis loops of the two magnets are shown in Fig.2.The values of remanence (Br),intrinsic coercivity(H_cj),maximum magnetic energy product ((BH)_max) and squareness are listed in Table 1.It is shown that H_cj,(BH)_max and the squareness of DMP magnet are higherthan those of SMP magnet.For DMP magnet with 20 wt%MM,(BH)_max reaches 95.48 kJ·m^-3,H_cj is 909.83 kA·m^-1,and B_r is 0.75 T.

Fig.1 XRD patterns of SMP magnet and DMP magnet before and after SPS process

Fig.2 Hysteresis loops of SMP magnet and DMP magnet

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Table 1 Magnetic properties of SMP magnet and DMP magnet determined at room temperature

TEM images of magnets are shown in Fig.3.Theaverage grain size is about 40 nm,which coincides with XRD result.EDS analysis data are listed in Table 2 for SMP magnet and DMP magnet measured in differentregions (corresponding to Regions 1-15 in Fig.3).The result illustrates that inhomogeneous La/Ce/Pr/Nd distribution within or across the grains does not modify thecrystal symmetry of the main phase.The compositions corresponding to different areas in the DMP magnetdemonstrate that the ratio of La to Ce/RE varies,75 wt%-79 wt%(La/Ce-rich) in Area A and 0 wt%-1.2 wt%(La/Ce-lean) in Area B,as shown in Fig.3b.It is obviously different from La/Ce/Pr/Nd distributions in SMP magnet which is homogeneous in the grains.The intrinsic magnetic properties of 2:14:1 magnetic phase have strong localvariations due to the different La/Ce contents.Meanwhile,a few of La/Ce atoms are diffused into the boundary of the La/Ce-lean region,due to the short sintering time and low sintering temperature in the SPS procedure.In addition,as one can see,the boundary of two agglomeration areas is ambiguous,which may be due to elemental diffusion.The compositions of Regions 10-15 in Fig.3c,d correspond to La/Ce-lean region and La/Ce-rich region,respectively.

The thermal-magnetic measurements reveal that theCurie temperatures (Tc) is different between SMP magnet and DMP magnet,as shown in Fig.4.Based on the above discussion,it can be concluded that the microstructure is actually inhomogeneous in the DMP magnet.The Tcvalues of La₂Fe₁₄B and Ce₂Fe₁₄B are 530 and 424 K,respectively,which are lower than Tc (585 K) of Nd₂Fe₁₄B [ 8] .The curve of SMP magnet exhibits a drastic decay at563 K,which is 14 K lower than that of DMP magnet(577 K).For MM substitution for Pr/Nd,the magnetic dilution effect of La/Ce into the main phase lattice leads to the reduction in Tc of the magnet.Moreover,only one peak is observed in dM/dT-T curves,indicating that the two magnets have the essentially single/homogenous hardmagnetic phase (the 2:14:1 phase).Nevertheless,in comparison with SMP magnet,the peak of DMP magnet isbroader,manifesting that the DMP magnet contains double main phases.These conclusions coincide with XRD result.Consequently,higher Tc of DMP magnet than that of SMPone with the same MM concentration (20 wt%MM/RE)may reflect strengthened short-range exchange interactions against thermal perturbation in chemically inhomogeneous DMP magnet [ 21] .The reason is that the Curie tempera-tures (T_C) depend on the differences of phase structure rather than composition.The further researches will be discussed in another paper.

Fig.3 TEM images of a SMP magnet and b-d DMP magnet

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Table 2 Compositions of selected different regions in TEM image of Fig.3 (wt%)

4 Conclusion

In summary,we reported an effective SPS method to prepare high-performance RE-Fe-B magnets based on mis-chmetal.There is no significant change in remanence (B_r),but the intrinsic coercivity (H_cj),maximum magnetic energy product ((BH)_max) and squareness of double-mainphase magnets are obviously better than those of single main phase one with the same composition of[(PrNd)_0.8MM_0.2]_29.2Fe_balB.Most of diffraction peakscorrespond to R₂Fe₁₄B phase,confirming the main phase of R2Fe14B in these magnets.

EDS analysis of DMP magnets shows that La/Ce/Pr/Nddistributes inhomogeneously between La/Ce-rich areas and La/Ce-lean areas.The selection of the optimum sintering temperature and sintering time during the preparationprocess has a crucial effect on the reduction in the transition area of magnets and the improvement of the defect.The Curie temperature (T_C) of the DMP magnet is higherthan that of the SMP magnet,which is associated with differences of phase structure in chemically inhomogeneous DMP magnet.