稀有金属(英文版) 2020,39(01),41-47

High coercivity Pr₂Fe₁₄B/α-Fe nanocomposite permanent magnets with Zr addition

Mehran Khan Alam Guang-Bing Han Shi-Shou Kang

School of Physics,Shandong University

作者简介：*Guang-Bing Han e-mail:hangb@sdu.edu.cn;

收稿日期：28 September 2018

基金：financially supported by the National Natural Science Foundation of China(Nos.11474184 and 11627805);Shandong Province Natural Science Foundation of China(No.ZR2016EMM14);

High coercivity Pr₂Fe₁₄B/α-Fe nanocomposite permanent magnets with Zr addition

Mehran Khan Alam Guang-Bing Han Shi-Shou Kang

School of Physics,Shandong University

Abstract：

The ingots with nominal composition Pr_9.5Fe_84-xB_6.4P_0.1Zr_x(x=0,1,2,3) were prepared by an electric arc furnace under purified argon atmosphere.The ribbons were obtained by melt spinning at a wheel speed of16-33 m·s^-1.X-ray diffraction(XRD) results show that P addition decreases crystallinity of hard phase,but further Zr addition increases the amorphous-forming ability of soft phase.The intrinsic coercivity largely increases from 502(Zr-free) to 945 kA·m^-1(2 at% Zr),which is among the highest value reported so far in this poor rare earth nanocomposite magnets.The hysteresis loops of the alloys with addition of 1 at% and 2 at% Zr show good squareness with single-phase characteristic,indicating well exchange coupling between hard and soft magnetic grains.Transmission electron microscope(TEM) results reveal small grain size and uniformity in the microstructure in the Zradded samples,which is the reason for high coercivity.

Keyword：

Magnetic properties; Coercivity; Zr addition; Nanocomposite magnets;

Received： 28 September 2018

1 Introduction

Permanent magnets based on Nd₂Fe₁₄B are important tothe industrial development in electric technology [ 1, 2] .Nanocomposite exchange coupled magnets comprise offine mixture of hard magnetic phase with high coercivity and soft magnetic phase with high magnetization [ 3] .This new paradigm in hard and soft magnetic material hasattracted research interest in recent years.The theoretical maximum energy product of more than 1000 kJ·m^-3 for anisotropic nanocomposite magnets is far higher than the value obtained in experiments [ 4, 5, 6, 7, 8] .The important reason for this mismatch is that it is very hard to obtain ideal microstructure.The magnetic behavior of the permanentmagnets is sensitive to microstructural properties such as grain size,particle shape,grain boundary type and the uniform distribution of the magnetically hard and softphases [ 9, 10, 11, 12, 13] .The microstructure is one of the crucial factors to attain high magnetic properties of Nd-Fe-B-based nanocomposite magnets [ 14] .

For this purpose,Zr is the good candidate for improving the coercivity via reduction of the grain size [ 15, 16, 17, 18] .It was reported that Zr is effective for the improvement in the structural order of single-phase Nd₂Fe₁₄B and reduction of the area of the second phase [ 19, 20] .Wang et al. [ 21] reported that 1 at%Zr addition in Nd₁₀Fe₈₄B₆ increases the coercivity from 600 (Zr-free) to 780 kA·m^-1 (Zr-doped)and maximum energy product from 105 to 130 kJ·m-3.It was also observed that 1 at%Zr in Pr₂Fe₁₄B/α-Fe increases the coercivity from 470.7 to 793.2 kA·m^-1 and the maximum energy product ((BH)_max) from 66.8 to 90.8 kJ·m^-3 [ 22] .The coercivity was significantly improved when the Zr atoms enrich between the grain boundaries and do not enter into the 2:14:1 phase [ 23] .Sheng et al. [ 24] reported coercivity of 650 kA·m^-1 and (BH)_max of 160 kJ·m^-3 for as-spun Nd_9.5Fe₈₁B_6.5Zr₃ alloy.Zhang et al. [ 25] reported that addition of phosphorous increases the amorphous-forming ability in as-spun ribbon Fe₅₅Pt₂₅B₂₀ alloy.In addition,0.1 at%P (phosphorous) was added in the master alloy of Pr_9.5Fe₈₄B_6.5 to improve the remanence and rectangularity of the demagnetization curve,and large energy product of 190 kJ·m^-3 and high remanence of 1.18 T were observed perpendicular to the ribbon plane direction [ 26] .In this article,we focus on experimental investigation of the effect of Zr addition on the magnetic properties ofdirectly quenched Pr_9.5Fe_82-xB_6.4P_0.1Zr_x(x=0,1,2,3)ribbons.The magnetic properties were improved signifi-cantly with Zr addition in the Pr_9.5Fe₈₄B_6.4P_0.1 alloy ribbon because of small grain size and refined microstructure.

Fig.1 XRD patterns of as-spun P_r9.5Fe_84-xB_6.4P_0.1Zr_x (x=0,1,2,3) at different speeds and Pr_9.sFe₈₄B_6.5 at 18 m·s^-1:ax=0,bx=1,cx=2,and dx=3

2 Experimental

Alloy ingots with nominal composition ofPr_9.5Fe_84-xB_6.4P_0.1Zr_x(x=0,1,2,3)were prepared by arcmelting under purified argon atmosphere.The constituent materials praseodymium (Pr),iron (Fe),phosphorus(P) and zirconium (Zr) with purity of at least 99.99%and boron (B) in the form of compound FeB (22 wt%B) wereused.Further 5 wt%Pr was added to compensate theevaporation loss during the melting process.The ingotswere melted three times at a chamber pressure of 0.01 MPa and each time inverted to ensure the uniformity ofnanocomposite.The ingots were crushed into small pieces according to quartz tube size and the oxides surface was removed using grinder machine.The ribbons were obtainedby melt spinning at a chamber pressure of 0.06 MPa in anargon atmosphere at different wheel speed of 16-33 m·s^-1.The ribbons with length of 4-5 mm and width of 2-3 mmwere used to carry out the measurement of magneticproperties through vibrating sample magnetometer (VSM,JDM-13) with magnetic field of 1.5 T at room temperature.The phase constitution and microstructure were character-ized by X-ray diffraction (XRD,Smartlab-se) with Cu Kαradiation and transmission electron microscopy (TEM,FEI Tecnai G2 F20).

3 Results and discussion

XRD patterns of some as-spun ribbons are shown in Fig.1.By comparing the patterns of Pr_9.5Fe₈₄B_6.4P_0.1 with thoseof Pr_9.5Fe₈₄B_6.5 ribbons quenched at 18 m·s^-1 in Fig.1a,we can find that P addition can inhibit the crystallization of hard phase (Pr₂Fe₁₄B) greatly.According to the Senkovmodel [ 27] ,due to size effects,i.e.,R_B>0.6R_Fe and R_P<0.85R_Fe (where R_P is the radius of P atom,R_B is the radius of B atom,and R_Fe is the radius of Fe atom),P creates a large compressive lattice strain at the substitution site,while B causes a large tensile lattice strain,and the amorphization is enhanced in P-added alloys.So,P may substitute the B in the main phase Pr2Fe14B in this work.

Fig.2 Hysteresis loops of as-spun P_r9.5F_e84-xB_6.4P_0.1Zr_x alloys for ax=0,bx=1,cx=2 and dx=3

The Pr_9.5Fe₈₄B_6.4P_0.1 melt spun ribbons quenched at wheel speed of 18-30 m·s^-1 have phases of Pr,Fe14B andα-Fe (Fig.1a).The average grain sizes of 65,45 and 32 nm were calculated from Scherrer equation for the ribbons quenched at wheel speed of 18,24 and 30 m·s^-1,respectively.Figure 1b represents XRD results of the ribbons with 1 at%Zr addition,which shows decreased peak intensity of soft phase compared to Zr-free alloy.This indicates that Zr addition inhibits the formation of the soft phase.The volume of soft phase is also decreased with the increase in cooling rate becauseα-Fe has a higher nucleation rate,and it is characterized by lower growth rate with wheel speed increasing.The decrease in peak intensity at higher quenching speed indicates the amorphization of the Pr_9.5Fe₈₃B_6.4P_0.1Zr₁ alloy.The prominent peak (410) for2:14:1 hard phase was also reported in Nd-Fe-Al-Co-B alloy [ 28] .The intensity of peak (410) increases in the1 at%Zr-added alloy,which indicates that Zr addition increases the crystalline of hard phase (or decreases that of soft phase) in nanocomposite magnets.Wang et al. [ 21] argued that addition of 1 at%Zr in Nd2Fe14B/α-Fe prevents the formation of metastable phase Nd3Fe62B14,which is consistent with our results,no metastable phase is found in our XRD patterns.

XRD results of as-spun Pr_9.5Fe₈₂B_6.4P_0.1Zr₂ ribbons in Fig.1c indicate some traces of 2:14:1 andα-Fe phases.The phase structure is seen to become amorphous at high wheel speed,such as 24 and 30 m·s^-1.The average grain sizes of42 nm (18 m·s^-1),30 nm (24 m·s^-1) and 23 nm(30 m·s^-1) were calculated through Scherrer equation.XRD results in Fig.1d reveal that with addition of 3 at%Zr,there are no obvious peaks found except (311),(410),(331) and (110) for hard and soft phases,respectively.The structure becomes almost amorphous compared to Zr-freealloys ribbon shown in Fig.1a.The amorphization alsoincreases with the wheel speed increasing,as can be seen in the XRD patterns.

Figure 2 gives the hysteresis loops of some as-spunPr_9.5Fe_84-xB_6.4P_0.1Zr_x(x=0,1,2,3) ribbons.The values of saturation magnetization (J_s) decrease from 1.05 T(18 m·s^-1) to 0.98 T (24 m·s^-1),0.95 T (27 m·s^-1) and0.92 T (30 m·s^-1) for Pr_9.5Fe₈₄B_6.4P_0.1 ribbon (Fig.2a).The sample quenched at 27 m·s^-1 shows the best loop squareness (J_r/J_s,where J_r is the remanent magnetization)of 0.72 and coercivity of 497 kA·m^-1.The initial magnetization behavior shown in Fig.3a denotes that the coercivity mechanism is mainly based on nucleation of magnetization reverse.The large loop squareness (J_r/J_s)of0.79 (24 m·s^-1),0.83 (27 m·s^-1) and 0.85 (30 m·s^-1) in Pr_9.5F_e83B_6.4P_0.1Zr₁ ribbons (Fig.2b) indicates the strong exchange coupling among the grains.And the magnetic properties are improved enormously by 1 at%Zr addition.The kinks on the loops of the ribbons quenched at18-24 m·s^-1 indicate the weak exchange coupling among the grains.The second quadrant of hysteresis loops at 27and 30 m·s^-1 shows the single-phase characteristic of the hard phase.The maximum energy product of 78 and71 kJ·m^-3,coercivity of 839 and 870 kA·m^-1 and remanence magnetization of 0.70 and 0.68 T are achieved in the ribbon quenched at 27 and 30 m·s^-1,respectively.The coercivity mechanism is dominated by nucleation first and then domain wall pinning mechanism,as shown in Fig.3b.

Fig.3 Initial magnetization curves of as-spun Pr_9.5Fe_84-xB_6.4P_0.1Zr_x alloys for ax=0,bx=1,cx=2 and dx=3

The hysteresis loops of the samples with 2 at%Zr quenched at 18-30 m·s^-1 show single hard phase characteristic (Fig.2c) and indicate the uniformity and refinementof the magnetic grains.The squareness (J_r/J_s) is 0.76,0.72,0.71 for ribbons quenched at 24,27 and 30 m·s^-1,respectively.The maximum energy product of 70 kJ·m^-3was achieved in Pr_9.5Fe₈₂B_6.4P_0.1Zr₂ quenched at 18 m·s^-1with average grain size of 42 nm.Further increase in wheelspeed decreases the energy product because of the highcontent of amorphous volume.The largest coercivity valueof 945 kA·m^-1 was achieved at 30 m·s^-1.This coercivityis among the highest value reported so far for this as-spunalloy composition.The large coercivity is due to uniformand small grain size of 23 nm of 2:14:1 phase and thesuppressed volume and grain size of soft phase,as can beseen in the XRD result in Figs.1c,4d.In XRD patterns,almost all the peaks belong to Pr₂Fe₁₄B hard phase,whichfurther confirms the existence of a hard phase in themajority,leading to higher coercivity.The initial magne-tization curves shown in Fig.3c indicate that the coercivitymechanism is dominated by a domain wall pinningmechanism.This exhibits that the addition of Zr enhancesthe pinning field.The hysteresis loop of the ribbon obtainedat 18 m·s^-1 is smooth and uniform for the alloy with 3 at%Zr addition (Fig.2d),but the magnetic properties are worse for the ribbons prepared at 16,24 and 30 m·s^-1.The straitened loops with a sharp shoulder in the demagnetization curve are developed,and the magnetic properties are considerably reduced for the sample prepared at 24 and

Fig.4 TEM images of some as-spun Pr_9.5Fe_84-xB_6.4P_0.1Zr_x alloys for a x=0,bx=1,c x=1,dx=2

30 m·s^-1.The largest energy product of 61 kJ·m^-3 and coercivity of 531 kA·m^-1 were achieved at wheel speed of18 m·s^-1 in Pr_9.5Fe₈₁B_6.4P_0.1Zr₃.Further increase in wheel speed lowers the magnetic properties significantly.

TEM result reveals that the grain size of the sample quenched at wheel speed of 18 m·s^-1 is slightly coarse,with an average grain size of 64 and 35 nm for x=0 and 1,respectively (Fig.4a,b).The grain sizes are uniform as well,with relative standard deviation (σ/d) on the order of0.34 and 0.42,respectively.The nanostructures of thealloys prepared at 30 m·s^-1 consist of fine,randomly distributed grain with average sizes of 16 and 24 nm for x=1and 2,respectively (Fig.4c,d).The grain sizes are not uniform,with old on the order of 0.61 and 0.32,respectively.All the nanostructures consist of fairly equiaxed grains.Figure 2 shows that the coercivity values of the samples quenched at 30 m·s^-1 for x=1 and 2 are 870 and945 kA·m^-1,respectively.The narrower grain size distribution (Fig.4d) is probably the main reason for higher coercivity for x=2.

The magnetic interaction behavior can be clarified by the recoil loops.Figure 5a represents the recoil loops for Zr-Free alloy quenched at wheel speed of 30 m·s^-1.The value of the recoil permeability (u_rec,defined as the slope of the recoil loop) varies from 0.0096 to 0.0027 T·kA^-1-m^-1 from low to high external magnetic field.At low applied field,the recoil loop is slightly open and gradually becomes small with the decrease in recoil permeability.This is because the soft phase reverses at low field earlier than the hard phase.The small openness at higher field is due to concurrent reversal of the hard and soft phase under the interaction of exchange coupling [ 29] .For the sample with 2 at%Zr addition shown in Fig.5b,very small recoil openness is noticed withμ_rec value ranging from 0.00093 to0.00043 T·kA^-1.m^-1 from low to high field.This indicates the improvement in exchange coupling due to the fine grain size.The recoil openness also depends on the soft magneticα-Fe and coercivity (H_cj).The lower soft phaseα-Fe content and larger coercivity H_cj value in Pr_9.5Fe₈₂B_6.4P_0.1Zr₂decreaseμ_rec significantly.μ_rec in Pr_9.5Fe₈₂B_6.4P_0.1Zr₂ is a result of concurrent reversal of both phases which lead to strong exchange coupling between hard and soft magnetic phases [ 30] .

Fig.5 Recoil loops of as-spun ribbons at wheel speed of 30 m·s^-1 for a P_r9.5Fe₈₄B_6.4P_0.1 and b P_r9.5Fe₈₂B_6.4P_0.1Zr₂

Fig.6 Intrinsic coercivity of as-spun Pr_9.5Fe8_4-xB_6.4P_0.1Zr_x(x=0,1,2,3) alloy at 30 m·s^-1

Figure 6 shows the dependence of coercivity on Zr concentration at wheel speed of 30 m·s^-1.It can be seen that the intrinsic coercivity first increases with Zr substitution,from 62 kA·m^-1 (Zr-free) up to highest value of 945kA·m^-1 with 2 at%Zr,and then decreases with further addition of Zr.This indicates that a certain amount of Zraddition favors the coercivity of Pr_9.5Fe₈₄B_6.4P_0.1 alloy.However,this gives the impression that too high concentration of Zr is not favorable for magnetic properties.The considerable enhancement in coercivity value is ascribed to the increase in the 2:14:1 hard phase amount,small grain size (23 nm) and refined microstructure.

4 Conclusion

The melt spun Pr_9.5Fe₈₄B_6.4P_0.1 ribbons were prepared with different additions of Zr in the master alloy.XRD results showed that phosphorous addition decreased crystallinity of hard phases in the ribbons.Further Zr addition decreased soft phase crystallinity greatly and relatively increased hard phase crystallinity.The magnetic properties were improved significantly with Zr addition in the P_r9.5Fe₈₄B_6.4P_0.1 alloy ribbon.The largest maximum energy product of 78 kJ·m^-3was achieved in the sample with 1 at%Zr prepared at27 m·s^-1.The highest coercivity value of 945 kA·m^-1 was achieved in the ribbons with 2 at%Zr quenched at30 m·s^-1.The reduction in recoil loop openness verified the enhancement in coercivity by Zr addition.The domain wall pinning mechanism is mainly responsible for higher coercivity the sample.

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