Study on sintering process and characteristic of nanosized soft magnetic MnZn ferrite powders

RARE METALS .Vol. 25 , Spec. Issue ,Oct 2006 ,p. 531Study on sintering process and characteristic of nanosized soft mag-netic MnZn ferrite powdersWANG Yongming , WANG Xin ,JIANG Yanfei , HAO Shunli , and LIU ChunjingSchool of Material Science and Engineering , Hebei University of Technology , Tianjin 300130 , China( Received 2006-06-25 )Abstract : The effect of sintering process ( especially the sintering temperature ) on the magnetic property and microstruc-ture of sintered sample of nanosized soft magnetic MnZn ferrite powder was investigated. The sintered sample of MnZn fer-rite was prepared by both traditional pressing and cool isostatic pressing on MnZn ferrite nanoparticals. The sinteringprocess of which was segmented. The density , microstructure and phase composition of sintered sample were analyzed byArchimedes' law , scanning electron microscopy ( SEM ) and X-ray diffraction( XRD ). The grain growth and densificationin sintering process of MnZn ferrite were investigated. The magnetic property was measured by vibrating sample magnetom-eter( VSM ) and Nim2000 magnetic material testing system. The results show that the better sintering temperature is 850C ,at which the better magnetic property and microstructure of sintered compact were obtained.Key words : MnZn ferrite ; sintering temperature ; microstructure ; magnetic property[ This work was financially supported by Natural Science Foundation of Hebei Province ( E2005000027 ) and NaturalScience Foundation of Tianjin ( 06YFJMJC02400 ). ]1.Introductionbe taken as follows :( 1 ) researching the new prepara-MnZn soft magnetic ferrites have been widely usedtion methods of MnZn frrite powde[ 3-8 ];( 2 ) mix-in electronic industry such as high-frequency transform-ing proper additive to improve the magnetic propertyers , inductors , recording heads , noise filters etc. be-[ 9-12 ];( 3 ) sintering the nanosized powders to con-cause of their high permeability , low coercivity and lowtrol the grain size of sintered sample and reduce thepower loss[ 1 ] The development of electronic industrypower loss. In this paper , the effects of sinteringis so speedy that more and more high-powered magneticprocess on the magnetic property , density , microstruc-materials are needed. It requires that the MnZn ferriteture and phase composition of sintered sample werehas the higher permeability(μ; ,μ。) , saturation mag-discussed.netic flux density( B, ), Curie temperature( T。) andresistivity( p ) and lower power loss( P. ). So how to2. Experimentalimprove the magnetic property of MnZn ferrite to fit the2.1. Raw material and instrumentfast development is when the scientists focus on. TheyExperimental material and equipment : nanosizedhave done a lot of researches in it.In2 I中国煤化工alcohol ( PVA ) , iso-There are many factors that influence the magnet-tatifMHC N M H Ge-type temperature con-ic property of power ferrite ,such as raw material , ad-trolled furnace , nitrogen atmosphere device.ditive , sintering process etc.[ 2 ] To improve theMeasuring instrument : scanning electron micros-magnetic property of power ferrite , three methods cancopy( SEM ), X-ray diffraction ( XRD ), vibratingCorresponding a! WANG Xin E-mail : wangxin022@ 263. net532RARE METALS , Vol. 25 ,Spec. Issue , Oct 2006sample magnetometer ( VSM )，Nim2000 magneticsamples were measured at room temperature by X-raymaterial testing system.diffraction( XRD ). The XRD patterns of the sinteredsamples were shown in Fig. 1 , in which the curves.2.2. Sintering experimentC,D,E , F corresponding to 700 , 800 , 820 , 850,Nanosized MnZn ferrite powder , whose structural880 C , respectively.formula is Mno.7 Zno,2Fe2.O, , was prepared with theComparing the diffraction peaks of the samplesmethod in Ref.[ 13 ], which was mixed with 3wt. %with standard PDF card ，it showed that all the five( PVA ) uniformly. The powder was pressed into ringsamples had the better spinel structure. But from thebodies φ20 mm x 8 mm by traditional way ，thenXRD patterms of samples D ,E and F ,the extra peakspressed by cool isostatic pressing way. The density ofcould be seen at 20 = 33. 24°. Comparing with thethe sample was about 2.5 g" cm-3.PDF card , the extra peaks might be the characteristicThe samples were sintered in the tube-type tem-peaks of Fe2O3 or Mn,O4 which might be formed be-perature controlled furnace with nitrogen atmosphere.cause the nitrogen pressure fell and the oxygen pres-Sintering process was segmented two steps which weresure rose during the cooling process. So the oxidationdivided into different temperature areas. Each areareaction occurredhad corresponding heating-up speed. The sinteringMnFe2O3 +1/3[ 0]→Fe2O3 +1/3[ β-Mn,O4] ( 1 )temperature was 700 C ( B ) ,800 C( C),820 Cwere β-Mn3 04 is square structure. Extra phase in the( D),850 C( E),880 C ( F ) respectively. Thecrystals might damage the magnetic properties. In (samples were heated from the room temperature to a-der to protect the structure of the sintered samplesbout 400-450 C at the rate of2 C. min-' , then re-both the cooling rate and atmosphere in the furnacemained at the temperature for 1 h. Later the tempera-should be controlled strictly.ture was up to the scheduled sintering temperature.In addition , according to the half-peak breadth ofRemain the samples at the temperature for 2 h and[ 311 ] diffraction peak , the average grain size ofcool down gradually in the furnace. Finally , the sin-MnZn ferrite sintered sample was estimated usingtered samples of MnZn ferrite were obtained.Scherrer formulaAt the first stage of sintering , the moisture , bondD = Kλ/βcos0(2)nd some impurities in the samples were volatilized.In the formula(2) ,K =0.89 ,入=0. 154178 nm ,β is half-Gas was discharged sufficiently. It made preparationspeak breadth , and θ is diffraction angle. The academicfor the later grain growth. At the second stage , thedensity of MnZn ferrite was calculated using the formulagrains grew up gradually with the rising of the sintering12temperature. The grain boundary moved and the rela-110tive density increased rapidly. Relative to the tradi-tional sintering process , the characteristic in the ex-periment is sintering nanosized powders at lower tem-2 afperature. It hoped that the grain growth can be con-trolled efficiently. At the meantime the density andmagnetic property of sintered sample was improved.Ln swthmc中国煤化工Wumw/rhu BThe low power cost in the production was achieved.MYHCNMH G。603. Results and discussionFig.1. XRD pattern of the sintered samples ( The sinte-3.1. X-ray diffraction ( XRD ) analysisring temperature of sample F ,E ,D , B and C was 880 ,The内分数据of MnZn ferrite is spinel style. The850 ,820 ,700 ,800 C respetively ).Wang Y. M. et al. , Sintering process and characteristic of MnZn ferrite powders533_8Mthe density of the sample which was sintered at 700 Cd-Aa(3 )was higher , due to pressing before sintering. The rela-In the formula( 3 ) ,M is molecular weight ,A =6.02tive density was only about 90% when at 800 C , be-x 1023 , and crystal lattice constant a=0.85 nm. The .cause of the restriction of the forming pressing and e-result of calculation wasd, =5.05 g° cm-3. The rela-quipment , the density of biscuit did not reach a hightive density D was calculated byvalue , which influenced the later sintering process andd。the properties of the sintered sample.(4)d,The air gap rate P( P is an important influencing factor3.2. Microstructurefor many properties. ) was calculated byThe microstructure of the fractured surface of sin-p_d-d.(5)tered samples was analyzed by scanning electron mi-dcroscopy( SEM ). The state of the grain growth of dif-The results of calculation are shown in Table 1.ferent samples which were sintered at different temper-From Table 1 ,it can be seen that the density ofature was shown in Fig. 2.most samples do not reach the academic density. ButTable 1. Relationship of grain size and sintered temperature of Mn-Zn ferritesSinteringGrain size/Density of sintered compact/RelativeSampleAir gap ratetemperature/ Cnm(g° cm-3)density7002:5.030.9960. 004800244.210.8340. 16682(294.170.8260.17485024.300.851.0. 149880374. 580. 9070.093(a)民b)(cP (d(e)2 μm2μm2um部公2unFig. 2. Fractured surface of sintered samples with different sintering temperature :( a ) sintered at 700 C ;( b ) sintered at800C;(c)sinteredat820C;(d)sinteredat850C;(e)sinteredat880C.From Fig.2 ,it was found that the grains of sam-According to the density and grain growth of theple B( sintered at 700 C ) had grown , but they weresintered sample , it was found that the best sinteringrelative independent without coupling. At 800 C , thetemperature of nanosized MnZn ferrite powder was a-grains continued to grow up to a higher density. Thebout 850C. But the density was not perfect , and thegrains of sample D( sintered at 820 °C ) to sample Frelative density was only about 85% .( sintered at 880 C ) kept growing and sintering sam-中国煤化工les with higher density were obtained. The grain3.3.MYHCNMHG.growth of sample F was abnormal , and the most grainswere conglutinated together. The growth was incontrol-The magnetic properties of sintered samples werelable that might damage the magnetic properties of themeasured by vibrating sample magnetometer ( VSM )samples.and Nim2000 magnetic material testing system at room534RARE METALS , Vol. 25 ,Spec. Issue , Oct 2006temperature. The testing results are shown in Fig. 3.Fig. 3 , it is found that the saturation magnetizationIn Fig. 3，A is the M-H curve of nanosized( M, )of nanosized MnZn ferrite is better than that ofMnZn ferrite powder. The curves B，C,D , EMnZn ferrite nanopowders. The M, value of sample Cand F in Fig. 3 show the relationship of M-H of( sintered at 800 C ) is maximum , while that of sam-ple B( sintered at 700 C ) is minimum. But there is .sintered samples which were sintered at 7no obvious variation between magnetic properties and00 ,800 ,820 ,850 and880 C ,respectively. Fromsintering temperature by observing from the Fig.3. It10might relate with sintering atmosphere ，density andcrystal structure. These influencing factors must beocontrolled strictly in the experiment. The experimentaldata is shown in Table 2.From Table 2 it is found that the magnetic proper-ty of sample E( sintered at 850 C ) is better. The8magnetic property of sample F( sintered at 880 C )is0000-20002000000080000worse than that of sample E. Because is most grainsField/Gwere conglutinated together and the growth is incon-Fig.3. Charts of M-H ( The curves A shows relationshiptrollable that the magnetic property of the sample F isof M-H of nanosized soft magnetic MnZn ferrite powdersdamaged. Although the density of sample B ( sinteredthe curves B ,C ,D ,E ,F show relationship of M-H of sin-at 700 C ) is good , grain growth is imperfection be-tered samples which were sintered at 700 , 800 , 820 , 850,cause of inadequate sintering temperature. So the per-880 C ).fect grain growth , uniform grain size and high densityare better for property of the sintered sample.Table 2. Magnetic properties of sintered MnZn power ferritesSinteringM./B./B,/H[/Sampletemperature/C_ ( emu" g_1 )_ ( emu. g1 )mT_mT(A m~")_B70043.7186. 0534610085. 1364.28072471161512.382069. 4232.76182361241274. 45075. 4663.1702374129945.88071.9634.53563381791078.4the nanosized MnZn ferrite powder was sintered by4. Conclusionssegmented sintering process at different temperatures.The density and grain growth were measured and ob-Nanosized MnZn ferrite powders are activity.served. It is found that the sample sintered at 850 CPreparation of the MnZn ferrite using the nanosizedhas uniform grain size , higher density and better mag-powders can lower the sintering temperature , controlne中国煤化Irameters did not reach .the chemical composition and microstructure of the fer-theYHC N M H Gving the properties , therite and improve the magnetic properties. The maincalefactive rate and sintering atmosphere will be adjus-processing parameters which influence the propertiested , and grain growth and crystal structure will beare sintering temperature ，calefactive rate ，atmos-controlled strictly.phere，he时月数糖ation time etc. In our experiment ,The further study will be mixing the nanosized535Wang y. M. et al. , Sintering process and charateristic of MnZn frrite powdersfrites ,J Magn. Magn. Mater ,2000 ,215-216 :171.MnZn ferrite powders with microcontent nanosized[7] Chii-Shyang Hwang , and Neng-Cheng Wang , Prepara-SiO2 ,analyzing the efct and mechanism of additivetion and characteristics of frrite catalysts for reduction ofon properties of MnZn frite.CO2. Materials Chemistry and Physis ,2004 ,88 :258.References[8] Thakur A. , and Singh M. , Preparation and character-ization of nanosize Mn。.Zno.o Fe204 frrite by citrate pre-[ 1] Xi Guoxi ,and Lu Maixi. , New development of synthesiscursor method. 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