Magnetic properties of M-type strontium ferrites with different heat treatment conditions
来源期刊:Rare Metals2020年第1期
论文作者:Namji Oh Seungyeon Park Sangsub Kim Kyoungmook Lim
文章页码:84 - 88
摘 要: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 Sr0.06Ca0.52La0.52Fe11.68Co0.22O19magnetic 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.
稀有金属(英文版) 2020,39(01),84-88
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
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 Sr0.06Ca0.52La0.52Fe11.68Co0.22O19magnetic 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
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
In this study,the effects of the calcination temperature on the physical properties of the M-type strontium ferrites Sr0.06Ca0.52La0.52Fe11.68Co0.22O19 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 ofSr0.06Ca0.52La0.52Fe11.68Co0.22O19.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 secondround 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.
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℃
Table 1 Phase content of M-type hexaferrites Sr0.06Ca0.52La0.52-Fe11.68Co0.22O19 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 (Ms)increases,whereas the coercivity (Hc) decreases.Themagnetization relates to the Fe3+-O-Fe3+superexchange interaction
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.
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℃
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 Hc decreases with the increase in the calcination temperature due to the large particle sizes by the grain growth.
参考文献
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