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参考文献

1 Introduction▲
2 Experimental▲
3 Results and discussion▲
3.1 Phase formation and structure
3.2 Curie temperature and hyperfine structure of as-annealed ribbons
3.3 Magnetic properties after nitridation
4 Conclusion▲
图表▲

Rare Metals2018年第3期

Structure and magnetic properties of melt-spun Sm-Fe-Nb ribbons and their nitrides

Wen-Long Yan Yang Luo Dun-Bo Yu Gui-Yong Wu Ning-Tao Quan Yuan-Fei Yang Hai-Jun Peng Zi-Long Wang

National Engineering Research Center for Rare Earth Materials,General Research Institute for Nonferrous Metals,Grirem Advanced Materials Co. Ltd.

收稿日期：13 November 2017

基金：financially supported by the National Natural Science Foundation of China (No.51501016);the National Key Research and Development Program (No.2016YFB0700902);

Structure and magnetic properties of melt-spun Sm-Fe-Nb ribbons and their nitrides

Wen-Long Yan Yang Luo Dun-Bo Yu Gui-Yong Wu Ning-Tao Quan Yuan-Fei Yang Hai-Jun Peng Zi-Long Wang

National Engineering Research Center for Rare Earth Materials,General Research Institute for Nonferrous Metals,Grirem Advanced Materials Co. Ltd.

Abstract：

SmFe_10-xNb_x(x = 0, 0.1, 0.2, 0.3) ribbons and their nitrides were prepared by melt-spinning, followed by annealing and subsequent nitriding. The structure and magnetic properties were investigated by means of X-ray diffraction(XRD) using the Rietveld method, vibrating sample magnetometer(VSM), transmission electron microscope(TEM) and Mossbauer spectroscopy. XRD analysis shows that the addition of Nb can impede the precipitation of Sm₂ Fe₁₇ and a-Fe phases and the Nb atoms occupy 2 e site in the alloys. At 300 K, the mean hyperfine fields of 2 e site are 29.58 T with Nb doping at x = 0.1 and the corresponding Curie temperature is 552 K. The optimal properties of remanence of B_r = 0.9 T, intrinsic coercivity of H_cj= 741.5 kA·m^-1, and maximum magnetic energy product of(BH)_max= 124.2 kJ·m^-3 are gained at x = 0.10 in the nitrides.

Keyword：

Melt-spinning; SmFe_10-xNb_x powders; Magnetic properties; XRD Rietveld; Mssbauer spectroscopy;

Author： Yang Luo e-mail:eluoyang@foxmail.com;

Received： 13 November 2017

1 Introduction

The SmFe-based alloys have been considered as a potential materials for the permanent magnet application,such as Th₂Zn_17-type Sm₂Fe₁₇N_x [ 1] ,ThMn₁₂-type SmFe₁₁Ti [ 2] and TbCu₇-type SmFe₉N_x [ 3] .The hexagonal TbCu₇-type phase SmFe₉N has received significant attention on account of their novel magnetic properties and potential technological applications since its first discovery by Katter et al. [ 3] .But the TbCu₇-type phase is a metastable phase;many efforts have been made to stabilize the TbCu₇-type metastable phase and to enhance the magnetic properties of the SmFe₉ alloys.In general,there are mostly three methods.The first way is to add one or more 3d elements M like Ga,Co,V,Ti,Al,W,Cr [ 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14] to substitute the iron atoms,and the appropriate doping contents M for SmFe₉N_x compounds with TbCu₇-type structure are very effective to improve coercivity and Curie temperature.The second way is to add Zr,Hf [ 15, 16, 17, 18] to substitute the Sm atoms,and this method can stabilize TbCu₇-type structure and improve magnetic.The last one is to add Si,B [ 19, 20, 21, 22, 23] elements,which could greatly increase the amorphous forming ability.Besides,the single TbCu₇-type phase can be obtained at a low wheel surface velocity.Moreover,Harris et al.reported that the Curie temperature can increase with Nb doping [ 24] in the Sm₂Fe₁₇ alloys.However,few researches have been devoted to the SmFe₉Nb alloy.In this paper,we focus on the crystal structure,hyperfine parameters,hard magnetic properties of the SmFe₉Nb_x alloys and their nitrides.

2 Experimental

Various master alloys with nominal composition of SmFe_10-xNb_x (x=0,0.1,0.2,0.3) were prepared by induction melting Sm (99.9%),Fe (99.9%),and Nb(99.9%) in Ar gas atmosphere.An extra amount of 10 wt%Sm was added to compensate the weight losses during melt-spinning and induction melting process.The molten ingots were ejected through an orifice of 0.9 mm in diameter at the bottom of a quartz crucible.All ribbons were prepared by melt-spinning onto a rotating Cu-disk at50 m·s^-1 surface velocity.Subsequently,the ribbons were annealed at 750℃for 1 h in vacuum and rapidly quenched.Then,the annealed SmFe_10-xNb_x powders were nitrided at 460℃for 16 h in pure nitrogen atmosphere.The crystal structure of ribbons was measured using X-ray diffractometer (XRD,Rigaku SmartLab) with Co Kαradiation,and the data were processed by Rietveld refinement with Rigaku plus software.The Mossbauer spectra were carried out by Mossbauer spectroscope (MS-500).The thermomagnetic curves of the samples were tested using a vibrating sample magnetometer (VSM,Quantum Design VersaLab) with an applied field of 79.6 kA·m^-1The magnetic hysteresis loop of the samples was measured at 300 K using a VSM with a maximum field of2388 kA·m^-1.Microstructures of as-annealed ribbons were observed by transmission electron microscope (TEM,FEI TECNAI F20).

3 Results and discussion

3.1 Phase formation and structure

Figure 1 shows XRD patterns of SmFe_10-xNb_x (x=0,0.1,0.2,0.3) alloys quenched at wheel velocities of 50 m·s^-1and annealed in vacuum at 750℃for 1 h.For SmFe_10-xNb_x (x=0) alloy,as it can be seen from Fig.1b,the main phase is the hexagonal TbCu₇-type structure,accompanying with a small amount of Sm₂Fe₁₇ andα-Fe phases.With the addition of the Nb,Sm₂Fe₁₇ andα-Fe phases are faded away.It can be inferred that the addition of Nb can effectively impede the precipitation of Sm₂Fe₁₇andα-Fe phases and stabilize the TbCu₇-type metastable SmFe₉ phase in the melt-spun alloys.Some amorphous phases can be seen from Fig.1a when Nb doping at x=0.3,because the addition of Nb could increase the ability of forming the amorphous phases in the melt-spun alloys.Figure 1c shows that the main diffraction peaks shift toward lower Bragg angles because Nb enters the lattice of SmFe₉ alloys.However,with the content of Nb increasing (x≥0.2),the main peaks no longer shift to lower angles due to saturation of Nb in the alloys.

Rietveld refinement method was used to analyze the occupation of Nb atoms in alloys.The results of SmFe_10-xNb_x (x=0.1) as a representative sample are shown in Fig.2 with good fitting between the measured and calculated profiles.The results displayed in Table 1show that the Nb atoms enter the lattice of SmFe₉ alloys and share the 2e lattice site with Fe atoms,which is consistent with the results in Fig.1c,whereas the 3g and 61sites are totally occupied by Fe atoms.

3.2 Curie temperature and hyperfine structure of as-annealed ribbons

As shown in Fig.3,the typical thermomagnetic behaviors of as-annealed SmFe_10-xNb_x (x=0,0.1,0.2,0.3) samples were measured at a magnetic field of 79.6 kA·m^-1.The Curie temperature of the SmFe_10-xNb_x (x=0.1) sample increases from 452 K (x=0) to 552 K (x=0.1).Clearly,this can be attributed to the enhancement of the Fe_2e-Fe_2eexchange interactions [ 25] ,because Nb atoms share the 2e lattice site with Fe atoms in the alloys.

Figure 4 shows the Mossbauer spectra of SmFe_10-xNb_x(x=0,0.1,0.2) samples measured at 300 K.The spectrum profiles are similar for the three samples,and the hyperfine field of three Fe atom lattice sites increases firstly and then decreases with the increment of Nb content which can be seen from Fig.5a.However,the hyperfine field of 2e dumbbell sites is obviously larger than that of 3g and 61when Nb doing at x=0.1,which can be concluded that the added Nb atoms share the 2e lattice site with Fe atoms in the alloys.The hyperfine field has the greatest value of29.58 T when Nb doping at x=0.10 because the Fe-Fe exchange interactions can be enhanced by the addition of Nb,which is consistent with the change in Curie temperature.Moreover,the larger the mean hyperfine field is,the greater the iron moment is,because the iron moment can be deduced using the conversion factor 15.60 [ 26] .Besides,the increase in the iron moment will lead to the increase in the Curie temperature.Conversely,the iron moment of SmFe_10-xNb_x samples decreases when Nb doping at x=0.2,which is consistent with the values obtained from M-T curve.Figure 5b,c shows that the isomer shift decreases firstly and then increases,while the quadrupole splitting increases firstly and then decreases with Nb doping,which can be concluded that the 4s conduction electron density is changed [ 27] due to Nb doping.

Fig.1 XRD patterns of SmFe_10-xNb_x (x=0,0.1,0.2,0.3) ribbons:a melt-spun,b annealed,and c main peak amplification of annealed

Fig.2 Rietveld analysis for sample SmFe_10-xNb_x (x=0.1)

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Table 1 TbCu₇-type structure characteristics resulting from Rietveld refinement for samples

Fig.3 M-T curves of as-annealed SmFe_10-xNb_x (x=0,0.1,0.2,0.3) ribbons with TbCu₇-type structure

Fig.4 Mossbauer spectra of SmFe_10-xNb_x (x=0,0.1,0.2) samples measured at 300 K

3.3 Magnetic properties after nitridation

Nitrides of SmFe_10-xNb_x (x=0,0.1,0.2,0.3) alloys are obtained by melt-spun,annealing at 750℃for 1 h in vacuum and nitriding at 460℃for 16 h in nitrogen atmosphere.Figure 6a shows the hysteresis loop of the nitride powders of SmFe_10-xNb_x (x=0,0.1,0.2,0.3),which were measured in a maximum applied field of+2388 kA·m^-1.Figure 6b shows XRD patterns of SmFe_9.9Nb_0.1 alloys with different conditions treatment.As expected,the diffraction peaks of annealed and melt-spun ribbons are similar,and the diffraction peaks of the nitrides shift toward the lower Bragg angles due to lattice expansion [ 28, 29, 30] .The optimum magnetic properties of B_r=0.9 T,H_cj=741.5 kA·m^-1,(BH)_max=124.2 kJ·m^-3 are obtained in the SmFe_10-xNb_xnitride powders at x=0.1,which can be explained by the grain refinement when Nb doping at x=0.1.According to Fig.7b,the average grain size of sample with Nb addition of x=0.10 is reduced to about 35 nm,much smaller than that of Nb-free ribbons,and the micros tructure of the as-annealed S mFe_9.9Nb_0.1 ribbons is more homogeneous than that of Nbfree ribbons.However,with excess Nb content doping at x=0.2,the magnetic properties drop down significantly,which can be explained that the iron moment of SmFe_10-xNb_x samples decreases with Nb doping (x≥0.20)and the“step”in the demagnetization curve as is shown in Fig.6a,which also means the decline of magnetic properties.

Fig.5 Mossbauer parameters of SmFe_10-xNb_x (x=0,0.1,0.2) samples measured at 300 K:a hyperfine field,b isomer shift,and c quadrupole splitting (Q.S.)

Fig.6 a Hysteresis loop of SmFe_10-xNb_xN_δ(x=0,0.1,0.2,0.3) powders and b XRD patterns of melt-spun,annealed and nitrided of SmFe_10-xNb_x (x=0.1)

Fig.7 TEM images of as-annealed SmFe_10-xNb_x ribbons:a x=0 and b x=0.1

4 Conclusion

In this study,Rietveld analysis shows that the Nb atoms occupy 2e site in the alloys and the addition of Nb can impede the precipitation of Sm₂Fe₁₇ andα-Fe phases,which can stabilize the TbCu₇ structure.Mossbauer analysis shows that the mean hyperfine fields of 2e site are29.58 T at 300 K under Nb doping at x=0.1 and the corresponding Curie temperature is 552 K.The optimal properties of B_r=0.9 T,H_cj=741.5 kA·m^-1,(BH)_max=124.2 kJ·m^-3 are obtained at x=0.10 in the nitrides.