Materials Chemistry and Physics98(2006)
90–94
Influence of heat treatment conditions on the structure and magnetic properties of barium ferrite BaFe1
2O19hollow microspheres of low density
Ping Ren a,b,JianGuo Guan b,∗,XuDong Cheng b
a Department of Energy Sources and Environment Engineering,Shanghai University of Electric Power,Shanghai200093,PR China
b State Key Laboratory of Advanced Technology for Materials Synthesis,Wuhan University of Technology,Wuhan430070,PR China
Received22May2005;received in revised form28August2005;accepted28August2005
Abstract
Barium ferrite BaFe12O19hollow microspheres of low density(about2.50g cm−3)were synthesized without BaFe2O4or other intermediate phase by spray pyrolysis technique,combined with co-precipitation precursor and followed further heat treatment.The relationships between the microstructure,magnetic properties and the sintering temperature,time were investigated.Hollow microspheres,constituted by nanometer particles,showed a broad particle distribution of2–15␮m.When the sample was sintered at900◦C for3h or at1100◦C for2h,pure BaFe12O19 ferrite was formed.With the
increase of the sintering temperature or time,the grain size and preferential growth orientation were influenced,the saturation magnetization(M s)had a small increase,and the coercive force(H c)decreased obviously.
©2005Elsevier B.V.All rights reserved.
Keywords:Barium ferrite;Hollow microspheres;Spray pyrolysis;Microstructure;Magnetic properties
1.Introduction
Barium ferrite(BaFe12O19)has been well-known for perma-nent magnets since its development by Philips researchers in the beginning of the1950s because of their high-coercivity,high-saturation magnetization,high resistance,low cost,associated with their excellent chemical stability and resistance to corrosion [1–3].In recent years,hexagonal ferrite has also caused wide interest in high-density recording media and radar-absorbing coatings[4,5].Its frequency of resonance absorption in the range of GHz is more than one thousand times as high as that of spinel ferrite.Therefore,it is promising for a kind of absorption mate-rial in thefield of high frequency.However,they are quite heavy, which restricts their applications requiring lightweight mass. Moreover,they have difficulties in increasing the permeability in GHz region because of Snoek limit[6,7].As one of the ways to overcome these problem
s,hollow microspheres of low den-sity are suggested.At the same time hollow microspheres also exhibit different catalytic,optical,electric and magnetic prop-erties and have important applications in materials,chemistry, catalysis and biologyfield[8].It is expected that microwave ∗Corresponding author.Tel.:+862787218832;fax:+862787879468.
E-mail address:guanjg@mail.whut.edu(J.G.Guan).absorbing ability can be improved by adjusting the diameter and wall thickness of hollow microspheres or composition and grain size of ferrite nanometer particles.
A number of fabrication methods have been used to produce hollow microspheres which are comprised of polymer,metal and ceramic materials,including nozzle reactor approaches (spray drying or pyrolysis)[9,10],emulsion/phase separation techniques(often combined with sol–gel processing)[11], emulsion/interfacial polymerization strategies[12],deposition method on sacrificial cores[13]and self-assembly process [14,15].Among these routes,spray pyrolysis technique was considered simplest and suitable for the synthesis of inorganic ferrite hollow microspheres,and large quantity production is likely by using the route.In this paper,we synthesized ferrite precursor with co-precipitation method and obtained BaFe12O19 hollow microspheres by spray pyrolysis technique followed fur-ther heat treatment,and reported the influence of heat treatment conditions on thefinal structure and magnetic pr
operties of the samples.
2.Experimental
Barium ferrite hollow microspheres were produced by three steps,the inves-tigated variables were:calcination temperature(T)and calcination time(t). Firstly,an aqueous solution of Fe(NO3)3·6H2O and Ba(NO3)3·2H2O(Fe/Ba
0254-0584/$–see front matter©2005Elsevier B.V.All rights reserved. doi:10.1016/j.matchemphys.2005.08.070
P.Ren et al./Materials Chemistry and Physics98(2006)90–9491
atomic ratio equal to12)was added to a stirring aqueous solution of NaOH and Na2CO3(OH−/CO32−ion ratio equal to5).After they reacted for2h,the resulting precipitates standed for24h,were then washed with deionized water and dried at110◦C.The dry precipitate was grinded in order to obtainfine and uniform barium ferrite precursor powder.Secondly,fine barium ferrite precursor powder was atomized into small particles by negative pressure of the FS-4Flame Spray Dryer.At the same time they were heated quickly at the high temperature flame,which was produced by acetylene a
nd oxygen as combustion gas.They passed the different holes of the Flame Spray Dryer respectively,mixed and burnt in the nozzle.Therefore,spherical droplets,caused by surface tension, decomposed to obtain hollow microspheres.Finally,the hollow microspheres were colleted and further calcined to form BaFe12O19phase or promote crystal-lization under air for3h at several different temperatures(600,800,1000,1100, 1200◦C)and at1100◦C for different time(2,3,4h).
X-ray diffraction with Cu K␣radiation(XRD)was used for phase analysis. The surface morphology and size of the grains were studied by scanning electron microscope(SEM).The thermal curves were presented by thermo gravimetric analysis(TG)and differential scanning calorimeter(DSC).The magnetic mea-surements are performed using a vibrating sample magnetometer(VSM)to an appliedfield of16000Oe,hysteresis loops are measured at room temperature.
3.Results and discussion
3.1.Structural analysis of pyrolysis products without heat treatment
Fig.1shows the micrograph,phase constitution and the thermal curves for the pyrolyzed sample by SEM,XRD,TG and DSC.Spherical particles with a broad particle distribu-tion of2–15␮m were observed in Fig.1(a).This was prob-ably related to uneven solid ferrite precursor powder from co-precipitation me
thod or the uniform atomization of the equip-ment.It demonstrated microspheres could be formed by spray pyrolysis method.Although hollow structure could not directly be seen in Fig.1(a),the average density of the sample(about 2.50g cm−3),which represented a50%decrease of hollow microspheres compared to solid powder of(about5.31g cm−3), could explain it to some extent.A small number of slight diffrac-tion peaks␣-Fe2O3and BaCO3were observed in Fig.1(b), which indicated the sample was constituted by a mixture of␣-Fe2O3and BaCO3phase of low crystallinity and also implied time of pyrolysis was not long enough to completely decom-pose iron hydroxide and barium carbonate or to form BaFe12O19 phase.Therefore,its necessary for further heat-treatment to form ferrite phase.Fig.1(c)was evident,an endothermic reaction accompanying with5.2%weight loss was observed at the tem-perature between78and138◦C,which showed the precursor did not decompose completely during the pyrolysis process. Between838and938◦C,the sample undergone exothermic reaction without weight loss.At the temperature range by com-paring XRD patterns of the samples after being heated at600 and1200◦C,it revealed that this change could be ascribed to the reaction of oxides to form barium ferrite.
3.2.Influence of the heating temperature on the structure
and magnetic properties
The phase constitution of the samples heat-treated at differ-ent temperatures for3h with a heating rate of5K min−1was obtained by XRD and is shown in Fig.2.At600◦C,a small number of␣-Fe2O3diffraction peaks and some slight peaks of BaFe12O19phase were observed.Single phase barium fer-rite had started to form by800◦C(except for the only slight peak at2.69˚A which corresponded to␣-Fe2O3).At900◦C,the sample presented a series of peaks all assigned to BaFe12O
19 Fig.1.(a)SEM image,(b)XRD pattern and(c)TG and DSC curves of the sample after spray pyrolysis without heat treatment.
92P.Ren et al./Materials Chemistry and Physics98(2006)
90–94
Fig.2.XRD patterns of the samples calcined at different temperature for3h: (a)600◦C,(b)800◦C,(c)900◦C,(d)1100◦C,(e)1200◦C.
phase.When the temperature increased further,the values of diffraction peak intensities improved obviously,which could be associated with the amount and grain coarsening of BaFe12O19 phase.There was a slight change between thefirst and second strongest peaks at1200◦C due to preferential grain growth.In this work,pure barium ferrite BaFe12O19was formed directly from iron oxides and barium oxides as the only
components Fig.4.Static magnetic properties of the samples calcined at different tempera-tures for3h.
detected.Other intermediate phase such as␣-BaFe2O4,␥-Fe2O3 were never observed during the heat treatment.This is consistent with the results obtained in the previous work[16].
As expected from XRD data,change in grain morphology with temperature was observed,which is presented in Fig.3. Hollow microspheres had kept intact during the calcination pro-cess.The surface of the sample without heat treatment was relatively smooth.The surface became rougher with the increase of sintering temperature.By1100◦C,the spheres were consti-tuted by crystals for hexagonal like platelet of about several hundred nanometer.These clearly indicated heat treatment was beneficial for ferrite formation and crystallization.
Fig.4shows the effect of the sintering temperature on the magnetic properties between600and1200◦C for3h.The sat-uration magnetization(M s)increased obviously from600to 1000◦C,increased slightly between1000and1200◦C,and reached a maximum of55emu g−1at1200◦C.The value is lower than the theoretical limit(72emu g−1)[17],which might be associated with the density and porosity of hollow micro-spheres.A drastic increase of M s between800and1000◦C
was Fig.3.SEM images of the samples calcined at different temperatures for3h:(a)without heat treatment,(b)800◦C,(c)1000◦C,(d)1100◦C.
P .Ren et al./Materials Chemistry and Physics 98(2006)90–9493
related to the increasing amount of BaFe 12O 19phase and a small increase from 1000to 1200◦C for grain coarsening according to XRD and SEM results.The hematite formation in the samples reduced the volumetric fraction of barium ferrite and conse-quently reduced M s before 800◦C.This result is in accordance with that previously described in literature [18].The rema-nent magnetization (M r )reached a maximum of 28emu g −1at 1000◦C,decreasing slightly for higher temperature.The intrin-sic coercive force (H c )basically showed a contrary behavior as M s ,increased obviously before 800◦C due to the increas-ing amount of BaFe 12O 19phase,decreased slightly from 3.4to 3.2kOe between 800and 1000◦C which is close to the coer-civity value of typical hard magnetic materials,and decreased drastically from 3.2to 1.3kOe between 1000and 1200◦C owing to grain coarsening.This indicated the coercive force exhibited strong dependency on the sintering temperature.Similar results were reported by Joonghoe et al.[19],who studied effects of the grain boundary on the coercivity of barium ferrite BaFe 12O 19.The results were also supported by Janasi et al.[18]who con-ducted an investigation on the effect of heat treatment conditions and routes on the magnetic properties of co-precipitated ferrite powders.
3.3.Influence of the heating time on the structure and magnetic properties
Fig.5shows XRD patterns of the samples calcined at 1100◦C for different heat treatment times.In all ca
ses,the samples were virtually single phase BaFe 12O 19ferrite.With the extending of sintering time,the relative intensities of diffraction peaks increased.At the same time the average grain size,measured from the full-width at half-maximum (FWHM)of (107)peak using Scherrer formula,was found to increase from about 450nm for the heated powder for 2h,520nm for 3h,to 550nm for 4h.According to the XRD intensity formula
I =I 0λ3e 432πR 3m 2c 4 Vq V 20|F h k l |2 1+cos 22θsin 2θcos θ
Fig.5.XRD patterns of the samples calcined at 1100◦C for different times:(a)2h,(b)3h,(c)4
h.
Fig.6.Hysteresis curve of the samples calcined at 1100◦C for different time.
where  V is the grain volume of (h k l )plain-oriented,q the
grain number of (h k l )plain-oriented,V 0the volume of unit cell,the relative intensities of diffraction peaks were consis-tent with the calculated intensities for the powder heat treated at 1100◦C for 3h.However,when the sintering time was 4h,(107)diffraction peak were dominant whereas relative intensity of (114)diffraction peak was diminished because the grain size of (107)plain-oriented increased quicker than that of (114)plain-oriented based on the above formula and Scherrer for-mula.These implicated that preferential grain growth probably occurred when the samples were sintered at long time.
Fig.6shows the magnetization hysteresis loops at room tem-perature of the samples sintered at 1100◦C between 2and 4h.All the samples had almost identical saturation magnetization and remanent magnetization values,indicating that M s and M r are independent on the sintering time,because the sa
mples were all pure BaFe 12O 19phase and their densities are basically same.On the other hand,the coercivity was changed with the sinter-ing time,which decreased initially from 3.0kOe for 2h and then recovered to 2.5kOe for 4h and reached the minimum of 1.9kOe for 3h because of grain size and preferential grain growth according to the XRD results in Fig.5.This clearly illus-trated the well-known fact that the coercivity strongly depended on the calcining time.4.Conclusions
We had prepared barium ferrite BaFe 12O 19hollow micro-spheres mainly distributed between 2and 15␮m by spray pyrolysis technique,combined with co-precipitation precursor and followed further calcinations,and studied the effect of heat treatment conditions.We found that hollow microspheres kept unbroken and the average density nearly kept unchange-able (about 2.50g cm −3)during the heat treatment.When the sample was sintered at 900◦C for 3h or at 1100◦C for 2h,the single phase BaFe 12O 19ferrite was formed,directly by iron oxide and barium oxide without BaFe 2O 4or ␥-Fe 2O 3intermediate phase.Increasing the temperature was beneficial in increasing the crystalline,improving the saturation mag-netization and eliminating the coercive force.The saturation
94P.Ren et al./Materials Chemistry and Physics98(2006)90–94
magnetization(M s)increased obviously before1000◦C,and increased slightly between1000and1200◦C
and reached the maximum of55emu g−1at1200◦C.Increasing the heat treat-ment time influenced grain size,preferential grain growth ori-entation and magnetic properties.H c reached a minimum of 1.9kOe for3h and a maximum of3.0kOe for2h,whereas M s was little affected.In a conclusive manner we can say that H c was strongly dependent on heat treatment conditions owing to grain size and preferential growth and M s was closely related to the materials composition.Preparation of narrow distribution of smaller hollow microspheres and study of their structure and properties will be our following work.
Acknowledgement
This work is supported by863HI-TECH project of China (no.2002AA305302).
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