目录结构

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参考文献

1 Introduction▲
2 Experimental▲
3 Results and discussion▲
3.1 Physical characteristics after calcination
3.2 Magnetic properties
4 Conclusion▲
图表▲

Rare Metals2020年第1期

Magnetic properties of M-type strontium ferrites with different heat treatment conditions

Namji Oh Seungyeon Park Yongwan Kim Hyukmin Kwon Sangsub Kim Kyoungmook Lim

Korea Institute of Industrial Technology

Department of Materials Science and Engineering,Inha University

Ugimag Korea

作者简介：*Kyoungmook Lim e-mail:mook@kitech.re.kr;

收稿日期：7 June 2018

Magnetic properties of M-type strontium ferrites with different heat treatment conditions

Namji Oh Seungyeon Park Yongwan Kim Hyukmin Kwon Sangsub Kim Kyoungmook Lim

Korea Institute of Industrial Technology

Department of Materials Science and Engineering,Inha University

Ugimag Korea

Abstract：

The effects of heat treatment conditions on the magnetic properties and microstructure of M-type strontium ferrite according to calcination temperature were analyzed.Strontium ferrite Sr_0.06Ca_0.52La_0.52Fe_11.68Co_0.22O₁₉magnetic powder was prepared by a standard ceramic process.During experiments,the calcination temperature was varied from 1180 to 1260℃,and sintering temperature was fixed.While the M-phase(SrFe₁₂O₁₉) existed with hematite(Fe2 O3) in the powder calcined at below 1220℃,the pure M-phase was observed in the powder calcined at over1240℃.With an increase in the calcination temperature,the magnetization of the calcined powder increases,meanwhile,the coercivity decreases.The magnetization is improved by decreasing the lattice constant c and activating the Fe³⁺-OFe3+superexchange interaction,and the coercivity decreases by the large particle sizes due to the grain growth.

Keyword：

M-type ferrites; Standard ceramic process; Phase analysis; Superexchange; Permanent magnet;

Received： 7 June 2018

1 Introduction

M-phase hexagonal ferrites (MFe₁₂O₁₉,M=Ba,Sr) havebeen used for various applications such as in microwave devices,electromagnetic wave absorbers,magneto-optics,and magnetic recording media [ 1, 2, 3, 4, 5, 6, 7, 8, 9] .It is possible to apply it to various fields due to the low price and wide availability of raw materials,excellent chemical stability,corrosion resistivity,high Curie temperature,largemagnetocrystalline anisotropy,and high magnetization [ 10, 11, 12, 13] .

In order to improve the magnetic properties of M-phase hexagonal ferrites such as the remanence,magnetization,coercivity,and maximum energy product,many research-ers have studied La³⁺and Co2⁺substitutions for Sr²⁺or Ba²⁺and Fe³⁺,respectively,such as La only [ 14, 15, 16, 17] or combinations of La-Co [ 18] .Recently,Ca2+substitution for Sr²⁺has been investigated due to the cheap Ca²⁺raw materials [ 19] .Further,heat treatment influences themagnetic properties of the M-phase hexagonal ferrites,and this has been explored by some researchers [ 20, 21, 22, 23] .Theheat treatment at a high temperature for a long duration causes grain growth in the magnetic particles,and then,the magnetic properties begin to decline.

In this study,the effects of the calcination temperature on the physical properties of the M-type strontium ferrites Sr_0.06Ca_0.52La_0.52Fe_11.68Co_0.22O₁₉ were investigated.Furthermore,the effect of the calcination temperature on the magnetic properties was also investigated.

2 Experimental

The M-type strontium hexaferrite was prepared by thefollowing standard process.The following raw powderswere used:Fe₂O₃ (99.5%purity),SrCO³ (99.5%purity),CaCO₃ (99.95%purity),La₂O₃ (99.5%purity),and Co₃O₄(99.7%purity) corresponding to the composition ofSr_0.06Ca_0.52La_0.52Fe_11.68Co_0.22O₁₉.A mixture (75 g) was injected into a polypropylene (PP) container with stainless steel balls (1 kg) with a diameter of 9.5 mm and milled for4h at an angular velocity of 142 r·min^-1 in deionized(D.I.) water of 100 ml.Mixed powder was calcined atvarious temperatures from 1180 to 1260℃for 1 h in air.After calcination,the milling process was performed twice.In the first round of milling,calcined powder was milled for 5 h at an angular velocity of 136 r·min^-1 in D.I.water for obtaining particles with an average particle size of3μm;this promotes effective milling and mixing withfurther additives.The resultant powders were milled again for 24 h at an angular velocity of 136 r·min^-1 with stainless steel balls with a diameter of 4.9 mm.In this secondroun_d of milling,SiO₂,CaCO₃,BaCO₃,and La₂O₃ wereadded into the powder.The additives are for controlling the particle sizes and the magnetic properties of the ferrite after the process of sintering.The milled slurry was pressed into the shape of pellets with a diameter of 40 mm under amagnetic field of 1.5 T,which was parallel to a direction of the pressing.The green body was sintered at 1200℃for1 h in air.In order to check the effect of the calcinationtemperature only,the process of mixing,drying,milling,pressing,and sintering was optimized by the otherexperiments.

Structural properties of the M-type hexaferrites were determined by X-ray diffractometer (XRD,D8ADVANCE,Bruker,USA) using Cu Kαsource(λ=0.15406 nm).Phase content was calculated by an X-ray fluorescence (XRF,ARL PERFORM'X,ThermoFisher Scientific,USA).The microstructures of all specimens were obtained by using a feld-emission scanningelectron microscope (FESEM,JSM-7100F,JEOL,Japan),and the changes in the microstructure with the change in the calcination temperature were analyzed.Particle sizes of the magnetic powders were measured by using a particlesize analyzer (HMK-22,AZO,U.K.) on the basis of principle of air permeability.The magnetic property wasobtained using a vibrating sample magnetometer (VSM,VersaLab VSM,Quantum Design,Inc.,USA) up to 3 T atroom temperature,respectively.

3 Results and discussion

3.1 Physical characteristics after calcination

Figure 1 shows XRD patterns for the powder calcined at the various temperatures from 1180 to 1260℃,andTable 1 shows the phase content determined by XRF.It is observed that the powder calcined at all temperatures is mainly composed of M-phase (SrFe12O19,COD 1008855).As an impurity phase,8.9%,4.2%,and 2.9%hematites(Fe₂O₃,COD 9014880) exist in the powder calcined at1180℃,1200℃,and 1220℃,respectively.Tempera-tures below 1220℃are insufficient for the completedcalcination.The peak of the hematite results from this inadequate reaction.The amount of the hematite decreases with an increase in the calcination temperature.Above1240℃,there is only M-phase (>99.9%) in the magnetic powder.Thus,temperatures over 1240℃are suitable to obtain the pure M-type ferrites.Yang et al. [ 24] have reported that the magnetic powder calcined over 1290℃consisted of the pure M-type hexagonal ferrites.Our results are similar to those of this previous report;in the meantime,it is possible to decrease the calcination temperature to 1240℃for obtaining the pure M-phase with theseexperiments.On the basis of XRD patterns of the powder calcined at 1180-1260℃,the lattice constants a and c can be estimated from the peaks of (107) and (114),which are the main peaks of the M-phase.The calculation of theconstants refers to the following formula [ 25] :

The calculated lattice constants for the magnetic powder calcined at 1180-1260℃are indicated in Fig.2.It can be seen that the lattice constant c decreases from 2.2828 to2.2754 nm with the increase in the calcination temperature from 1180 to 1260℃.The powder synthesized at 1180℃has the maximum value of c,whereas the powders synthesized at 1200 and 1260℃have the minimum c value.In this regard,the magnetocrystalline anisotropy is affected by a value of cla which is relevant to a direction of c.The value of c/a of all powders approximates to 3.9.Moreover,the lattice parameter a is constant.Therefore,it isdemonstrated that there is no distortion in the structure with the change of the calcination temperatures.

Fig.1 XRD patterns of M-type hexaferrite magnetic powders calcined at various temperatures from 1180 to 1260℃

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Table 1 Phase content of M-type hexaferrites Sr_0.06Ca_0.52La_0.52-Fe_11.68Co_0.22O₁₉ calcined at various temperatures (%)

Fig.2 Lattice parameters a and c and cla ratio for M-type hexaferrite magnetic powders calcined at various temperatures from 1180 to1260℃

Figure 3 shows the microstructural images of theM-type magnetic powder calcined at 1180-1260℃.After the calcination,the particle,which is called“clinker,”was observed by FESEM.As seen from Fig.3,the magneticpowder particles have a hexagonal shape.Figure 4 indicates the magnetic powders after the milling of two steps with size of～0.8μm.To verify an accurate particle size,the measurement of size was performed and this result is presented in Fig.5.All calcined powder particles have size of at least 3μm;in particular,the powder calcined at1260℃has 5.70μm in size.It is clear that the grain growth occurs in the particles due to the heat duringcalcination.During the milling of two steps,all samples were crushed by the same condition,e.g.,time,revolu-tions per minute (R.P.M.),and ratio between powder,ball,and water.The same condition for the milling oftwo steps causes a difference in the sizes of the final particle sizes due to a difference in an effect of milling.That is,the large particle can be effectively crushed more than small particle.Meanwhile,for the particle with size of over 5.2μm (at 1260℃),it remains large after the milling.It seems to exist a critical size for the effective milling.

3.2 Magnetic properties

Figure 6 and Table 2 show the influence of the calcination temperature on magnetic properties of the magnetic powder calcined at 1180-1260℃.With the increase in the calcination temperature,the saturation magnetization (M_s)increases,whereas the coercivity (H_c) decreases.Themagnetization relates to the Fe³⁺-O-Fe³⁺superexchange interaction [ 26] .The Fe³⁺are crystallographically located in five interstitial sites of magnetoplumbite structure:three octahedral sites of 2a,12k,and 4f₂,one tetrahedral site of4f₁,and one trigonal by pyramidal site of 2b [ 27, 28] .Among the above sites,the Fe³⁺-O-Fe³⁺superexchange interaction occurs at 12k and 2b sites that are parallel to ferrimagnetic direction;that is,they have the magnetic moments of the up-spin state.If there are causes for activation of the Fe³⁺-O-Fe³⁺superexchange interaction in the sites,the net magnetic moment will be improved,and the magnetization will be also advanced.As seen fromFig.1 and Table 1,the impurity phase,Fe2O3,disappearswith the increase in the temperature.When the amount of the hematite (Fe2O3) phase decreases,the amount of Fe³⁺increases,and this results in the expansion of the region corresponding to the Fe³⁺-O-Fe³⁺superexchange interaction [ 24] .This in turn activates the interaction between Fe³⁺and O^2-and causes the transfer of the magneticmoment among adjacent iron cations.Furthermore,the net magnetic moment is improved by the expanded region ofthe interaction.Moreover,the magnetization is connected with the lattice constants.The small constant c results in theincrease in exchange parameter because the bond angle of Fe³⁺-O-Fe³⁺superexchange approaches to 180° [ 29] .

In terms of the coercivity,the maximum coercivity value of 137.23 kA·m^-1 is obtained at the lowest temperature of1180℃,while the minimum value of 104.16 kA·m^-1 is obtained at the highest temperature of 1260℃.This is in agreement with that reported by Teh et al. [ 23] .They have reported that the coercivity depends on the particle size,and the particle size is affected by the heat treatment.It is well known that the domain size of the ferrite is about 1μm.Several domains can exist in the particle of the present experiments,so there are some troubles in an alignment of domains.That is why the coercivity decreases with theincrease in the calcination temperature.

4 Conclusion

The M-type strontium hexagonal ferrite powder was prepared by the standard ceramic process.The analyses of phase characteristics show that both the M-phase and the impurity phase are detected for the calcination temperatures ranging from 1180 to 1220℃and that the pureM-phase exists in the powder calcined above 1240℃.Above the temperature of 1240℃is suitable for obtaining the pure M-type ferrite.The microstructural images of the magnetic powder indicate that all of the calcined powder has the particles with a size of at least 3μm due to the grain growth during the calcination.Although the calcined powder was crushed by the same condition during twosteps of the milling,the final particle sizes are different from each other because the larger particle can be crushed more effectively than the small particle.

Fig.3 FESEM images of hexaferrites calcined at a 1180℃,b 1200℃,c 1220℃,d 1240℃,and e 1260℃

Fig.4 FESEM images of milled powders calcined at a 1180℃,b 1200℃,c 1220℃,d 1240℃,and e 1260℃after second milling

Fig.5 Particle sizes of hexaferrite powders after process of calcina-tion and process of second milling

Fig.6 Magnetic properties of M-type hexaferrite magnetic powders calcined at various temperatures from 1180 to 1260℃

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Table 2 Values for magnetic properties of calcined powders

With the increase in the calcination temperature,the Ms increases with the decrease in the lattice constant c and theactivation of the Fe³⁺-O-Fe³⁺superexchange interaction.In the meantime,the H_c decreases with the increase in the calcination temperature due to the large particle sizes by the grain growth.