Phase composition and magnetic properties of Pr-Nd-MM-Fe-B nanocrystalline magnets prepared by spark plasma sintering
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)0.8MM0.2]29.8Fe68.7Al0.1Cu0.12Co0.88B 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 Br=0.75 T,intrinsic coercivity of Hcj=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(TC) of 577 K,which is higher than that of SMP magnet.This result suggests that the TC 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 used and rapidly developed due to outstanding magnetic performance among various permanent magnets,since 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 earth (RE) elements Nd and Pr[6].Therefore,RE elements such as Pr,Nd,Dy and Tb are facing a criticality issue due to resource limitationand high price.The Pr and Nd are less abundant in the natural RE resources,compared with La and Ce[7].Developing permanent magnets with high abundant RE is requisite in order to solve problems of cost reduction and balanced utilization of RE resource.Recently,the research 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].It can be understood that the intrinsic magnetic properties of La2Fe14B and Ce2Fe14B are inferior to those of Nd2Fe14B and Pr2Fe14B[21].The excellent magnetic properties maintained when a small amount of Nd/Pr was substituted by La/Ce[22,23].Recently,Zhu et 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 maximum magnetic energy product of 342.28 kJ·m-3 when the Ce content reached 30%of the total amount of the RE metals.Lu et al.[24]reported that the compacted (La0.3Ce0.7)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,such as more convenient operation,lower sintering temperature,shorter sintering time,lower energy consumption and higher safety 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 growth.The melt-spinning method,high-energy ball milling method and SPS technology have attracted extensive attention in the field of preparing isotropic nanocrystalline hot-pressed magnets.In this work,we report the phase composition and magnetic properties in the double-main phase (DMP)magnets (PrNd)29.2Fe68.7Al0.1Cu0.12Co0.88B/MM29.2Fe68.7Al0.1Cu0.12Co0.88B,prepared by SPS procedure.We compare the extrinsic magnetic properties and microstructures of the double-main phase magnet with the magnet[(PrNd)0.8MM0.2]29.2Fe68.7Al0.1Cu0.12Co0.88B prepared by the single-main method.
2 Experimental
The purities of the starting materials are 99.99 wt%for Fe,99.23 wt%for Fe-B (Fe:B=80.54:19.46 in mass) alloy and 99.72 wt%for Pr-Nd alloy (Pr:Nd=25.5:74.5 in mass) alloy.The mischmetal (purity about 99.5 wt%) from Bayan Obo mine,which contains 28.63 wt%La,50.13wt%Ce,4.81 wt%Pr and 16.38 wt%Nd,was used in the experiment.The ingots with nominal compositions of(PrNd)29.2Fe68.7Al0.1Cu0.12Co0.88B,MM29.2Fe68.7Al0.1-Cu0.12Co0.88B and[(PrNd)0.8MM0.2]29.2Fe68.7Al0.1Cu0.12-Co0.88B were remelted at least three times to 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).Subsequently,the ribbons were obtained directly by induction melting the ingot under an argon atmosphere and then ejecting molten alloy through the orifice in quartz crucible onto the edge of a rotating copper wheel.The surface velocity of copper wheel was set in the speed of 60 m·s-1 in order to optimize the magnetic properties.Then,the mixture ribbons of(PrNd)29.2(Fe,TM)balB and MM29.2(Fe,TM)balB with mass ratio of 2:8 and[(PrNd)0.8MM0.2]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.The 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 Cu Kαradiation.Magnetic measurements were taken using quantum design vibrating sample magnetometer (VSM,Versalab) at 300 K with the maximum field of 3 T.The Curie temperatures (TC) were determined from the temperature dependence of magnetization (M) in a field of 0.05 T.The grain microstructures of the samples 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 process.It is seen in XRDpatterns that the diffraction peaks are not smooth.XRDpatterns indicate that 2:14:1 magnetic phase,unidentified phase and amorphous phase coexist in the samples before SPS process.Through the above experimental process,the grain size of the powders is reduced as much as possible,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 Nd-Fe-B.The doublemain phase structure is not evident in SMP magnet.Position shifts exist in the Bragg diffraction peaks 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 estimated 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(Hcj),maximum magnetic energy product ((BH)max) and squareness are listed in Table 1.It is shown that Hcj,(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,Hcj is 909.83 kA·m-1,and Br 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
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
Fig.3 TEM images of a SMP magnet and b-d DMP magnet
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 (Br),but the intrinsic coercivity (Hcj),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)0.8MM0.2]29.2FebalB.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 (TC) of the DMP magnet is higherthan that of the SMP magnet,which is associated with differences of phase structure in chemically inhomogeneous DMP magnet.
Fig.4 a M-T and b dM/dT-T curves of SMP magnet and DMP magnet measured upon a heating rate of 5℃·min-1 in a field of 0.05 T
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