Effects of Sintering Temperature on Structural and Electromagnetic Properties of MgCuZn ferrite...

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RESEARCH PAPER

Effects of sintering temperature on structuraland electromagnetic properties of MgCuZnferrite prepared by microwave sintering

M. Penchal Reddy1,2*, M. Venkata Ramana3, W. Madhuri4, K. Sadhana5,K.V. Siva Kumar2 and R. Ramakrishna Reddy2

Nanocrystalline magnesium–copper–zinc (Mg0.30Cu0.20Zn0.50Fe2O4) ferrites were prepared by

microwave sintering technique. The effects of the sintering temperature on particle size and

magnetic properties were investigated. In this article, optimum sintering temperature required for

MgCuZn ferrite system for obtaining good electromagnetic properties, suitable for applications in

low temperature co-fired ceramics (LTCC) chip components was studied. The grain size, initial

permeability, dielectric constant and saturation magnetisations were found to increase, and

dielectric loss was found to decrease with the increasing sintering temperature. Mg–Cu–Zn fer-

rites with a permeability of m ¼ 1110 (at 1 MHz) were fully densified at the standard LTCC sintering

temperature of 9508C.

Keywords: Ceramics, Microwave sintering, X-ray diffraction, Microstructure, Electrical properties, Magnetic properties

IntroductionCurrently, the ceramic packaging and multilayer sub-strate process have received much worldwide attentionin response to an increasing demand for circuit minia-turisation, and higher performance devices have led tothe development of the low temperature co-fired ceramic(LTCC) technology. Low temperature co-fired ceramicis a multilayer substrate technology for device inte-gration. This technology has been growing continuouslysince the appearance of the first commercial co-firedceramic product for robust capacitors in the early1990s.1 In the standard LTCC technology, the passivecomponents, such as inductors, capacitors and filters,are integrated into multilayer LTCC substrates.In practice, embedded inductors and capacitors requireultra low sintering temperature functional ceramics likesoft ferrites or high k dielectric materials that are com-patible with LTCC process conditions and fired at<960uC to form a highly integrated structure.

Because silver (Ag) is used as metallisation for theinternal coil winding, the co-firing temperature of themultilayer device is limited to T<960uC. Ferrites forLTCC substrates, therefore, have to exhibit a highersintering activity to form dense layers with an optimisedmicrostructure at T<960uC. Further requirements

include high permeability at a given range of operatingfrequencies and good compatibility with the internalwinding and external contact materials.2

Recently, NiCuZn ferrites with high permeability anda high Curie temperature have been widely studiedboth for scientific reasons and for practical, microwaveapplications.3–5 NiCuZn ferrites are standard ferritematerials for the fabrication of multilayer inductivedevices. The application of NiCuZn ferrite ensureseffective sintering at a low temperature (T<960uC) andsoft magnetic properties.6,7

On the other hand, NiCuZn ferrites have successfullybeen produced for multilayer inductors; there are stillunresolved issues that require further invest8igation, forexample stress of internal windings, compatibility withAg, silver mobility in the ferrite and others. Moreover,nickel oxide raw materials are hazardous and, hence, itwould be advantageous to replace Ni with another metalin the ferrite materials. MgCuZn ferrites were proposedas alternative materials with an increasing potential forLTCC microinductors.8

MgCuZn ferrite powders, prepared by the conven-tional solid state reaction method, have a sinteringtemperature higher than 1050uC.9 The high sinteringtemperatures lead to a series of problems, including highlosses, low densities, more power consumption, etc.10–12

Therefore, from the viewpoint of engineering, it will bevery interesting to reduce the sintering temperature.

Recently, microwave sintering (MS) has attractedmuch scientific interest as a technique for ceramicmanufacturing.13,14 The fundamental difference betweenMS and conventional sintering (CS) is in the heatingmechanism.15,16 For CS, heat is generated by heatingelements and is then transferred to the samples viaradiation, conduction and convection. For MS, the heat

1Ningbo Institute of Materials Technology and Engineering, ChineseAcademy of Sciences, Ningbo 315201, Zhejiang, China2Department of Physics, Sri Krishnadevaraya University, Anantapur515 001, India3Department of Physics, National Taiwan Normal University, Taipe11677, Taiwan4School of Advanced Sciences, VIT University, Vellore 632 014, India5Department of Physics, Osmania University, Hyderabad 500007, India

*Corresponding author, email [email protected]

� 2015 Institute of Materials, Minerals and MiningReceived 15 October 2014; accepted 18 January 2015DOI 10.1179/1743676115Y.0000000003 Advances in Applied Ceramics 2015 VOL 00 NO 0 1

Advances in Applied Ceramics aac1550.3d 29/1/2015 11:29:10

is generated internally within the test sample by a rapidoscillation of dipoles. The advantages of MS were foundto include higher energy efficiency, cost savings, higherpost-sintering density17–20 and lower sintering tempera-tures21,22 compared with CS. So, it is believed that usingthe MS technique, one can produce high dense MgCuZnferrites with good magnetic properties of at relativelylow sintering temperatures.

In this paper, the MS technique has been applied tofabricate polycrystalline Mg0.30Cu0.20Zn0.50Fe2O4 fer-rite, and the electromagnetic properties of this systemwere studied as a function of sintering temperature. Ourresults show that using the MS method, the optimalsintering temperature can be lowered to 950uC, and verygood magnetic and dielectric properties of the MgCuZnferrite were achieved.

Experimental

ChemicalsThe high purity (99.995%) chemicals magnesium oxide(MgO), copper oxide (CuO), zinc oxide (ZnO), ironoxide (Fe2O3) and polyvinyl alcohol were used for thepreparation of MgCuZn ferrite sample. All chemicalswere of analytical grade and without any furtherpurification.

Sample preparationThe polycrystalline MgCuZn ferrite sample having thechemical formula Mg0.30Cu0.20Zn0.50Fe2O4 was pre-pared by standard ceramic method.6 High purity indi-vidual oxides (99.995%, Sigma-Aldrich) MgO, CuO,ZnO and Fe2O3 were mixed stoichiometrically and pre-sintered at 600uC for 4 hours in a furnace. Then, thepresintered powders were milled for 30 h at a millingrotation speed of 200 rev min21, using ethanol and agateballs as a milling medium, to obtain fine particle size.The weight ratio of balls to powder was fixed to 5:1. Theresultant slurry was dried in vacuum evaporator andscreened through a 60 mesh screen.

The green powder thus obtained was pressed in theform of toroids having dimensions 12 mm outsidediameter, 8 mm internal diameter and 4 mm height andpellets having dimensions of thickness 2 mm and cross-sectional area 10 mm in diameter with a hydraulic pressat a pressure of 200 MPa using 2% polyvinyl alcoholsolution as a binder with a suitable die.

The pellet and toroid shaped samples were sintered bya heat treatment process using a single mode microwave(MS) furnace with the magnetron frequency of2.45 GHz (sharp; 1.5 kW, 2.45 GHz), the maximumoperating temperature up to 1400uC and the outputpower of 0.5–1.5 kW. During the sintering process, theMS chamber was filled with high purity nitrogen gasflow (99.999%), and the temperatures of the sampleswere monitored continuously with an optical pyrometerat the top of the furnace. The crucible was surroundedby SiC plates, which act as susceptors to provide initialheating of the compact disc samples. Once the materialsare sufficiently hot, they will couple/absorb microwaveeffectively and will get heated directly, includingthe core. The secondary purpose of SiC is to maintainthe surface temperature. The crucible was positioned atthe centre of the furnace, where the microwave radiationis the strongest. An adjustable electrical control system

was used to control the energy to be delivered to thesample at a programmed rate. The general configurationof the microwave furnace has been described else-where.23 The specimens (pellet and toroids) were sin-tered using MS by keeping them inside an aluminacrucible. During the MS of the samples, temperaturewas measured using Pt–Rh thermocouple. The cruciblewas surrounded by SiC plates, which act as susceptors toprovide initial heating of the compact disc samples.Once the materials are sufficiently hot, they will couple/absorb microwave effectively and will get heateddirectly, including the core. The secondary purpose ofSiC is to maintain the surface temperature. The cruciblewas positioned at the centre of the furnace, where themicrowave radiation is the strongest. The specimens(pellet and toroids) were finally sintered by microwaveheating and cooling rate of 20–30uC min21 to 800–950uC for 30 min.

CharacterisationThe phase identification of sintered materials was con-firmed by X-ray diffraction (XRD) technique using anX-ray diffractometer (PW1730 Germany) equipped withCu Ka as a radiation source (l ¼ 0.15406 A). Eachsample was scanned for 2h range (20–70u) with a stepsize of 0.02u and a step time of 5 s. The microstructuralanalysis of the samples was carried out by scanningelectron microscopy (SEM, CRL-ZESIS-EVO-MAI5).The bulk density dB of measurements was carried out onsintered pellets by distilled water with Archimedesmethod. The porosity P (%) of the samples was calcu-lated from X-ray density dx and bulk density dB valuesusing the formula: P¼12(dB/dX).

24 The initial per-meability was calculated using the observed and air coreinductance mi ¼ L/L0, where L is the observed induc-tance in mH and L0 is the air core inductance

L0 ¼ 0 : 004606N2h logOD

ID

� �1026 ð1Þ

where N is the number of turns, h is the thickness of thetoroid in inches and outside diameter and internaldiameter are the outer and inner diameters of the toroid.These measurements were carried out in the temperaturerange 30–2008C at 108C temperature intervals.

Conducting silver paint was applied on both the sidesof the microwave sintered pellets and air dried to havegood ohmic contacts for the dielectric measurements.The capacitance C of the samples was measured usingLCR HiTester in the frequency range of 100 Hze to1 MHz at room temperature from which the dielectricconstant was calculated. The dielectric loss (tan d) wasalso measured directly using the LCR HiTester.

Magnetic properties of sintered pellets were measuredat room temperature using vibrating sample magnet-ometer (Model 7040; Lakeshore, Westerville, OH) withan applied field of ¡6 kOe. From the obtained hyster-esis loops, the saturation magnetisation Ms and coer-civity Hc were determined.

Results and discussionThe effect of sintering temperature on the phase struc-ture of Mg0.30Cu0.20Zn0.50Fe2O4 has been investigatedby the XRD analysis. Figure 1 shows the XRD patternsof MgCuZn ferrite samples fabricated by MS process at

1Penchal Reddy et al. Effects of sintering temperature on MgCuZn ferrite

2 Advances in Applied Ceramics 2015 VOL 00 NO 0


different temperatures. Our results indicate that there isno phase change irrespective of sintering temperature,although there is an increase in the average crystallitesize. The increase in crystallite size manifests as theformation of larger sized particles during the sinteringprocess. Similar results were observed for NiCuZn fer-rite by Yang et al.25 The most intense peaks in all spe-cimens, indexed as (220), (311), (222), (400), (422), (511)and (440), were found to match well with single phasecubic spinel. No additional phase corresponding to anyother structure is found.

The structural parameters as a function of sinteringtemperature are presented in Table 1. The lattice con-stant a is almost same for the four sintered samples. Theobserved lattice constant values are in good agreementwith the reported results.12 It is noticed that the bulkdensity of MgCuZn ferrite increases with increase insintering temperature. On the other hand, porosity hasthe opposite trend. The increase in density with sinteringtemperature was expected. This is because during thesintering process, the thermal energy generates a forcethat drives the grain boundaries to grow over pores,thereby decreasing the pore volume and densifying thematerial. In addition, it was also observed that X-raydensity of each sample is larger than the correspondingbulk density of sintered samples. This may be due to theexistence of pores in the samples. From the resultsobtained in Table 1, it could be concluded that thedensity of MS sintered at 950uC for 30 min is 96%theoretical density (the porosity was ignored due to itssmall content). This sintering condition is optimal forapplications in LTCC components.

For further support, Fig. 2 shows the SEM imagesfor the sintered samples at 800, 850, 900 and 950uC.The SEM images of samples at various sintering tem-peratures show a significant change in particle size.

The values of particle size determined from the SEMimages were given in Table 1. It can be seen that thegrain size seems to be increasing with increase in sin-tering temperature. The sintering temperature had agreat influence on the microstructure.25,26 The densifi-cation mechanism in ferrites is mainly due to diffusion ofoxygen vacancies.27 As pointed out in literature, athigher temperatures of sintering, Fe3þ ions were reducedto Fe2þ ions, creating oxygen vacancies resulting inincreased densification with larger grains.28 In addition,Fe2þ ions diffuse very fast, thereby grain growth takesplace.29 All these factors affect the microstructure of thesample sintered at 950uC, resulting in enlarged grainsize. Compared with the CS (Tsw10 h), MS inducesshort sintering time and low sintering temperature. Theheating rates during sintering process are high, which ismuch higher than the conventional heating rate.In addition, heat is generated internally within thematerial, instead of originating from external sources forMS, and the specimen set rotates continuously, so theuniform heating induces. Compared to conventionallysintered samples, microwave sintered samples arehomogeneously crystallised and has great advantages onenergy efficiency and energy consumption. Hence, theMS technique significantly enhanced the grain size in amuch shorter time and at lower sintering temperatures.30

Figure 3 shows dielectric constant versus frequencyplots for the samples sintered at different sinteringtemperatures. It can be seen that like all other ferritematerials, the samples show frequency dependentphenomenon, i.e. dielectric constant decreases withincreasing frequency exhibiting normal ferrimagneticbehaviour.26

The simplest relation between dielectric constant andangular frequency v is

19 ¼ 4Ps9AC

v tan dð2Þ

where sAC the ac conductivity, v is the angular fre-quency and tan d is the dielectric loss factor.

The mechanism of dielectric polarisation in ferrites isparallel to conduction mechanism. Polarisation is alsoaffected by factors such as structural homogeneity,stoichiometry, density, grain size and porosity of theferrites. The electron exchange through Fe2þ $ Fe3þ

hopping mechanism results in local displacement of theelectrons in the direction of applied electric field; there-fore, polarisation is induced in the ferrites.31 In thepresent study, the dielectric permittivity is observed tobe increasing with increasing sintering temperature, andits values are presented in Table 1. This is due toaccelerated grain growth and improved hopping mech-anism due to the generation of divalent iron (Fe2þ) ions

Table 1 Lattice parameter, X-ray density, bulk density, porosity, average grain size, dielectric constant, loss tangent, satur-2

ation magnetisation (at room temperature), initial permeability and Curie transition temperature of microwave sinteredMg0.30Cu0.20Zn0.50Fe2O4 ferrite

Sinteringtemperature

a(A¡0.002) dx/g cm23 dB/g cm23 P/% Gavg (mm)¡0.36

DC(1 MHz)

DL(1 MHz)

Ms

(emu/g)IP

(1 MHz)Tc

(¡58C)

8008C 8.340 4.87 4.73 5.33 1.21 4.4 0.283 23 684 1128508C 8.344 4.99 4.86 4.86 1.36 6.0 0.161 26 817 1139008C 8.341 5.20 4.94 4.42 1.52 8.7 0.123 31 925 1159508C 8.343 5.27 4.99 4.33 1.98 26 0.082 38 1110 118

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1 X-ray diffraction patterns of MgCuZn ferrite samples sin-

tered at different temperatures

1 Penchal Reddy et al. Effects of sintering temperature on MgCuZn ferrite

Advances in Applied Ceramics 2015 VOL 00 NO 0 3


at elevated sintering temperature. At higher frequency,dielectric permittivity remains unchanged due to the factthat beyond a certain frequency of external alternatingcurrent field, the electron exchange Fe3þ $ Fe2þ cannotfollow the changes in the applied field.32 Similar dielec-tric behaviour was also observed by Sujatha et al.26 inthe case of NiCuZn ferrites.

Figure 4 shows the variation of dielectric loss tangent(tan d) with frequency at room temperature.

The dielectric loss decreases with the increasing fre-quency, which is a normal behaviour of any ferritematerial. The dielectric loss decreases rapidly in the lowfrequency region, while the rate of decrease is slow in thehigh frequency region, and it shows an almost frequencyindependent behaviour in the very high frequencyregion. The low loss values at higher frequencies showthe potential applications of these materials in high

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2 Images (SEM) of MgCuZn ferrite samples sintered at different temperatures

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3 Frequency dependence of dielectric constant for MgCuZn

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4 Frequency dependence of dielectric loss for MgCuZn

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frequency microwave devices. The behaviour can beexplained on the basis that in the low frequency region,which corresponds to a high resistivity (due to the grainboundary), more energy is required for electronexchange between Fe2þ and Fe3þ ions; as a result, theloss is high. In the high frequency region, which corre-sponds to a low resistivity (due to the grains), smallenergy is required for electron transfer between the twoFe ions at the octahedral site. Moreover, the dielectricloss factor also depends on a number of other factors,such as stoichiometry, Fe2þ content and structuralhomogeneity, which in turn depend on the compositionand sintering temperature of the samples.32,33 Moreover,in the MS process, the shorter sintering time restrainsthe evaporation of zinc oxide, leading to the lowercontent of Fe2þ in the material, and the Fe2þ is a kind ofion with low conductivity, so the dielectric loss decreasedcompared to the conventional sintered samples. There-fore, a better dielectric property of the MgCuZn ferritewas obtained with MS process.

Magnetic properties like saturation magnetisation areinfluenced by intrinsic factors such as preferential siteoccupancy of the cations and composition and were alsoinfluenced by extrinsic factors like microstructure andbulk density of the ferrites. The variation of saturationmagnetisation as a function of sintering temperature ispresented in Fig. 5. It can be observed that highersintering temperature leads to superior saturationmagnetisation because both the crystal grains and thedensity increase with the sintering temperature. It can benoticed from the Table 1, the largest Ms of 38 emu g21

is obtained in the sample sintered at 9508C, indicatingthat the MS method is efficient to fabricated high qualityMgCuZn ferrite material for LTCC microinductor.A similar observation is reported by Qinghui25 in thecase of microwave sintered NiCuZn ferrites.

The initial permeability is a very sensitive parameter,which depends on various factors like temperature, grainsize, method of preparation, etc. The magnitudes of in-itial permeability make MgCuZn ferrites technologicallyvery important. Figure 6 shows the initial permeabilityof sintered samples at different temperatures as a func-tion of temperature. It can be seen from the figure thatthe initial permeability remains constant over a widetemperature range for all microwave sintered samples.It can also be noticed from the figure that the microwave

sintered ferrites show good thermal stability. It wasobserved that initial permeability increases graduallywith increase in temperature attaining a peak value justbefore Tc, known as Hopkinson peak. The Curie tran-sition temperature determined from temperaturedependence of permeability of the samples sintered atdifferent temperatures are found to be Tc ¼ (115¡5)8C,and its values were presented in Table 1. It is alsoobserved from Fig. 6 that the magnitude of initial per-meability increases with increasing sintering tempera-tures because of enhancement of grain size (see Fig. 2).

For soft ferrites, the permeability and grain size aredirectly proportional to each other, i.e. m, a and D.34

Equation (3) also shows that permeability due to themotion of the domain wall is remarkably influenced bythe grain size. Since the initial permeability is measuredas a result of the easy reversal of domain wall dis-placement in the direction of the applied field and biggergrains accommodate more domains, the larger thenumber of domain walls, the higher the permeability is.It is the fact that the average grain size of the sampleincreases with the rise in sintering temperature. Thus, fora large grain, permeability will increase as it changesproportionally with the diameter of the grain. Thismeans that higher initial permeability is expected at highTs. However, in our study, it has been noticed that forthe samplesm, the real part of the initial permeabilitywas found to be highest at an optimum sintering tem-perature (see Fig. 6).35 A linear relationship between thegrain size and the permeability is valid until the graingrowth is normal, i.e. if all the grains grow pretty muchat the same time and rate. However, the pores generatedat the grain boundaries are less damaging to the per-meability because it causes less interruption to domainwall motion than that within the grains.

The magnetic initial permeability of softferrites related to the spin rotation and domain wallmotion.36–38 Spin rotation and domain wall motion arerelated as mi ¼ 1þ xw þ xSpin, where xw is the domainwall susceptibility, and xSpin is the intrinsic rotationalsusceptibility. The domain wall susceptibility and theintrinsic rotational susceptibility are given by the fol-lowing equations

xw ¼ 3pM2SD=4c ð3Þ

xw ¼ 2pM2S=K ð4Þ

where Ms, K, D and c are the saturation magnetisation,total anisotropy, average grain diameter and domain wallenergy respectively. Properties of soft ferrites are depen-dent on their compositions, additives andmicrostructures.It is well known that the magnetic properties are greatlyinfluenced by the microstructures; the larger the grainsizes, the higher the saturation magnetisation and largerinitial permeability. Ferrites with lower initial per-meability and saturation magnetisation were suitable forLTCC multilayer substrates and microwave applications.

ConclusionsFerrite samples of Mg0.30Cu0.20Zn0.50Fe2O4 were pre-pared successfully by the MS method. X-ray diffractionpatterns confirm the formation of a cubic spinel struc-ture. The results showed that material properties aresensitive to microstructure, and they were enhanced with

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5 Saturation magnetisation with sintering temperature for

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increase in sintering temperature. Microstructure inves-tigation reveals a strong correlation between per-meability and volume fraction of large ferrite grains. Thedielectric constant and loss tangent both show a normalbehaviour with respect to frequency. The maximumvalue of saturation magnetisation was found to be38 emu g21 for the sample sintered at 950uC. The tem-perature dependence of the initial permeabilitymeasurements has been found to execute the goodpeaking behaviour at the transitions temperatures Tc.The permeability of the samples was found to beincreased significantly with the increasing of sinteringtemperature. MgCuZn ferrites represent an interestingalternative to NiCuZn ferrites with large potential forapplications in LTCC chip components. With MStechnique, the sintering temperature can be decreasefrom conventional 1050 to 950uC, which is the typicalLTCC type processing temperature of multilayers.Electrical and magnetic properties make microwavesintered Mg0.30Cu0.20Zn0.50Fe2O4 ferrite extremely suit-able for LTCC multilayer substrates and microwaveapplications. Therefore, the MS process is a potentiallyimportant technique to fabricate chip components forLTCC technology.

AcknowledgementsOne of the authors, R. Ramakrishna Reddy thanks Uni-versity Grants Commission, New Delhi, for providing

Basic ScienceResearch faculty fellowship during this timewhen part of the work has been carried out.

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