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Microstructure and dielectric properties of nano-grained X7R type BaTiO3 ceramic capacitors

sintered by 2.45 GHz microwave

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2007 Phys. Scr. 2007 170

(http://iopscience.iop.org/1402-4896/2007/T129/039)

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Phys. Scr.T129 (2007) 170–174 doi:10.1088/0031-8949/2007/T129/039

Microstructure and dielectric propertiesof nano-grained X7R type BaTiO3ceramic capacitors sintered by2.45 GHz microwaveCheng-Sao Chen1 and Chen-Chia Chou2

1 Department of Mechanical Engineering, Hwa-Hsia Institute of Technology, 111 Gong Jhuan Road,Chung Ho, Taipei 235, Taiwan, Republic of China2 Department of Mechanical Engineering, National Taiwan University of Science and Technology,43 Keelung Road, Section 4, Taipei 10672, Taiwan, Republic of China

E-mail: rickchen@cc.hwh.edu.tw

Received 30 March 2007Accepted for publication 27 July 2007Published 27 November 2007Online atstacks.iop.org/PhysScr/T129/170

AbstractSystematic investigation on the effects of microwave sintering on the characteristics ofnano-grained BaTiO3 (BT) capacitor materials co-doped with yttrium (Y) and magnesium(Mg) elements was carried out. The granular structure in these materials was observed to berelatively insensitive to the sintering temperature and soaking time. The nano-sized BaTiO3

capacitor materials possessing X7R dielectric constant–temperature (K–T) characteristicshave been obtained over a wide range of sintering conditions. Transmission electronmicroscope (TEM) examinations revealed that the detailed microstructural study of thesematerials is extremely complicated. The uniqueK–T properties of the materials are ascribed tothe duplex structure in the samples, namely finer grains of paraelectric phase and larger grainsof ferroelectric phase.

PACS numbers: 77.22.Ch, 87.64.Ee, 78.70.Gq

(Some figures in this article are in colour only in the electronic version.)

1. Introduction

Barium titanate (BaTiO3, BT)-based ceramics have attractedmuch interest and attention, from both fundamental aspectand application point of view such as multilayer ceramiccapacitors (MLCCs) and positive temperature coefficientresistors (PTCRs) [1–4]. The fine grain BT-based MLCCshave shown higher stored energy, flatter temperaturecoefficient of capacitance (TCC) and are non-toxic to theenvironment. Small size multilayer capacitors are physicallycompatible with solid-state semiconductor components andare usually used in hybrid circuits. For this purpose, thetemperature dependent dielectric constant of the capacitor isvery important since the circuits are expected to functionin a variety of ambient conditions and heat output from thesemiconductor components can be high [5].

In X7R type MLCC materials, the relative change incapacitance (1C/C25) must be within±15% in the tem-perature regime between−55 and 125◦C, whereC25 is thecapacitance at 25◦C. The key factor for the small changein capacitance (1C/C25) is presumed to be the presence ofthe core–shell microstructure [6–9], which can be achievedby critically controlling the doping species and its concen-tration [10–12]. However, the microstructure and hence therelated dielectric properties are extremely sensitive to theprocessing parameters, which are supposed to be dependenton the growth of the grains during sintering, altering thecore–shell microstructure for the materials. In view of this, asintering process which can effectively densify the materialswithout inducing the growth of grains is thus demanded.Microwave sintering can significantly reduce the temperaturenecessary for densification of the materials [13–16]. Grain

0031-8949/07/129170+05$30.00 © 2007 The Royal Swedish Academy of Sciences Printed in the UK 170

Microstructure and dielectric properties of nano-grained X7R

Mg(CH3COO)2·4H2O2 mol% (99.6% purity)

Phase and structure analysis

Stirred & dried slowly at 80 °C for 24 h, and then calcined at 1150 °C for 2h

Dielectric property measurement

Dissolved and stirred in alcohol with 0.75 wt% Darvan C for 1 h

Y(NO3)3·6H2O 3 mol% (99.9% purity)

Microstructural observation

2.45 GHz Microwave sintered at 950-1200 °C for 0.1-30 min in air

BaTiO3 (TPL, Inc., USA, average grain size of 70 nm)

Figure 1. Flow chart for the experimental procedure.

growth phenomenon is thus pronouncedly suppressed insuch a way that high-density materials containing finegrain microstructure can be obtained, which is expected toimprove the processing reliability.

Furthermore, one of the most effective means is theuse of fine-grained BaTiO3 for reducing dielectric thicknesswhile retaining good reliability [17]. As the thickness ofthe dielectric layer is reduced to 2µm, after sintering thedielectric grains of ceramic layer must have a uniform grainsize less than 200 nm for the reliability of MLCCs. It is alsoknown that DC degradation, voltage breakdown strength andfracture toughness can be improved by decreasing the grainsize. Moreover, enhanced dielectric performance of BaTiO3

can be expected by shifting the phase transitions near to roomtemperature and flattening the dielectric constant–temperature(K–T) curve of the solid solution formed with suitabledopants such as Y2O3 and MgO. Hence, the temperaturedependence of the permittivity is determined by the degree ofchemical heterogeneity in the doped material [12, 16, 18].

In this paper, a microwave-sintering technique wasadopted to densify the nano-grained BaTiO3 ceramics co-doped with yttrium (Y) and magnesium (Mg) elements. Theeffect of processing parameters on the microstructure anddielectric properties was examined.

2. Experimental

The flow chart for the preparation employed during processingis shown in figure1. The as-calcined mixture was pressed intopellets of about 10 mm in diameter and 1.2 mm in thicknessand then sintered at 950–1200◦C for 10–30 min in a CEMMAX-7000 microwave furnace using 2.45 GHz microwaves.The temperature profile was measured using Pt–13% Rhthermocouple placed near the sample surface. The heating ratewas 30◦C min−1 and the cooling rate was 40◦C min−1.

The sintered density was measured using Archimedes’method. The microstructures of the samples that werepolished and then chemically etched were examined usinga scanning electron microscope (JEOL 6500F). For detailedmicrostructural investigations, as-sintered samples werefurther thinned by mechanical dimple grinding followed bymilling using an ion miller with argon ion beam at a smallincident angle. Image examination was carried out on a JEOL2000FXII scanning transmission electron microscope (TEM)operating at 200 kV to characterize the microstructures of thesamples. Cu-paste was screen printed over the surface of thesintered samples, followed by firing at 900◦C for 10 min toserve as electrodes. The dielectric properties were measuredfrom −55 to 150◦C using an HP 4278 capacitance meter.The capacitance of the samples was measured only at 1 kHzbecause it is the conventionally used frequency in the industryfor evaluating the dielectric properties of materials.

3. Results and discussion

The microwave sintering process can densify the nano-grainedBaTiO3 materials very efficiently. Figure2(a) shows that thedensity of the samples increases up to 90% of theoreticaldensity (TD) by microwave sintering at 1100◦C for 20 min.The density of the samples increases gradually with thesintering temperature, reaching 96% of TD for the samplesintered at 1200◦C. Moreover, figure2(b) shows the variationin density with soaking time for the samples sintered at1150◦C. The density of the samples rises gradually with thesoaking time and reaches a maximum value of 95.5% of TDin the soaking time of 20–30 min.

Figures3(a) and (b) respectively show scanning electronmicroscopy (SEM) images of BaTiO3 samples co-doped with3 mol.% Y and 2 mol.% Mg sintered at 1150◦C for 20 minusing a microwave and sintered at 1250◦C for 120 min

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50

60

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Den

sity

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)(a)

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91

92

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96

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Figure 2. Effect of (a) sintering temperature and (b) soaking timeon the densification behaviour of microwave-sintered BaTiO3

materials.

through conventional sintering. Typically, the microwave-sintered samples contain very fine grains with uniformgrain size distribution, which is desired for applications ofhigh-reliability MLCCs. The microstructure of the samplesmicrowave-sintered at other temperatures is essentiallythe same as that shown in figure3(a). On the other hand,figure 2(b) indicates that the density reaches 95% of TDby sintering at 1150◦C for 20 min, which is sufficient forinsuring high insulating resistance for these materials. In con-trast, it needs at least 1250◦C (120 min) to sinter the BaTiO3material to the same density by conventional sinteringprocess. However, the granular microstructure of theseconventionally sintered samples illustrated in figure3(b)is markedly different from that of the samples sintered bymicrowave sintering process. These conventionally sinteredsamples exhibit markedly larger grain size than that ofmicrowave-sintered ones. According to the above experimen-tal results, the microwave sintering process is observed todensify the BaTiO3 materials effectively without inducing thegrowth of grains at a very rapid rate and at a substantiallylower temperature. That means, the microwave sinteringprocess can markedly reduce the temperature necessary fordensification of the materials. Grain growth phenomenon isthus pronouncedly suppressed such that high-density materi-als containing finer grain microstructure can be obtained.

The microwave-sintered BaTiO3 materials possess highdielectric constant (K = 2000) at room temperature with very

(b)

1µm

(a)

1µm

Figure 3. SEM microstructures of BaTiO3 materials sintered by(a) microwave sintering process at 1150◦C for 20 minand (b) conventional sintering at 1250◦C for 120 min.

flat dielectric constant–temperature (K–T) properties, as longas the samples possess high density. The peak at high dielec-tric constant which in general occurs at the Curie temperatureof BaTiO3 materials is completely suppressed. Figure4(a)shows the variation of the capacitance with sintering tem-perature. It is found that the relative variation in capacitance(1C/C) is small with the increase in sintering temperaturefrom 1100 to 1200◦C, which meets X7R specification. Asmall hump is observed at around 10◦C, which is a typicalcharacteristic of heavily doped materials. For the samplesintered at 1100◦C, however, the1C/C–T characteristicswas modified slightly and the shift of the X7R specificationis ascribed to the insufficient density (i.e. 90% TD).

Soaking time during microwave sintering only slightlyaffects the K–T behaviour of the BaTiO3 samples.Figure 4(b) indicates that the sample soaked for 20 minpossesses a flat1C/C–T characteristics, satisfying theX7R specification. Longer soaking time (30 min) leadsto even flatter1C/C–T characteristics, without inducingany pronounced change in the dielectric properties of thesamples. The sample soaked for 10 min, however, exhibitsslightly inferior dielectric properties which is ascribed to theinsufficient sintered density. These results clearly demonstratethat X7R-type characteristics for BaTiO3 materials canbe achieved in a wide range of sintering temperatures and

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Microstructure and dielectric properties of nano-grained X7R

–30

–20

–10

0

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/C (

%)

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MS1150 °C

–30

–20

–10

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30

–60Temperature (°C)

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1200 °C

1150 °C

1100 °C

X7R Spec.

(a) ∆C/C-T

∆C/C-T

1501209060300–30

–60 1501209060300–30

Figure 4. Capacitance variation versus temperature (1C/C–T) ofthe BaTiO3 samples densified by microwave sintering at(a) 1100–1200◦C for 20 min and (b) 1150◦C for 10, 20 or 30 min.

soaking times in microwave sintering process. In contrast,for the BaTiO3 materials prepared by conventional sinteringprocess, the sintering parameters need to be criticallycontrolled in order to maintain smaller value of1C/C.

TEM micrographs of the BaTiO3 materials densified bymicrowave sintering at 1150◦C for 20 min are shown infigure 5, which reveals that the detailed microstructure ofthe samples is extremely complicated and the compositiondistribution of the samples is very inhomogeneous. Mostof the grains are very small (∼80–100 nm) and are highlystrained (labelled as A). These fine grains contain noferroelectric domains and are supposed to be paraelectric.Large grains of about 150–200 nm which are frequentlyobserved have clear domain structure and are ferroelectric(labelled as B). Core–shell structured grains are seldomobserved. The fine grains probably result from the pronouncedinward diffusion of dopant species, Y and Mg, into theBaTiO3 grains, which suppresses the mobility of graingrowth and induces a large proportion of strain. The largegrains contain a smaller proportion of dopants and cangrow to a larger size, preserving ferroelectric characteristics.The implication of the microstructure observed in figure5indicates that small capacitance variation (1C/C) for BaTiO3

materials can also be achieved by randomly mixing theparaelectric grains with the ferroelectric ones [19]. Core–shellmicrostructure is not really necessary.

The significant feature of such a unique microstructureis the kinetics of interdiffusion between dopants (Y/Mg)

100 nm

AB

Figure 5. TEM images of the nano-grained BaTiO3 samplesdensified by microwave sintering at 1150◦C for 20 min.

and BaTiO3 grains which are markedly enhanced in themicrowave sintering process. Most of the dopants can beincorporated into the grains in a very short period of sinteringwhich results in highly strained paraelectric BaTiO3 grains.Increasing the sintering temperature or extending the soakingtime only increases the sintered density of the samples,without further modifying the microstructure. The desiredflat K–T dielectric properties for these materials can beobtained for a wide range of sintering temperature and soakingtime. In contrast, for the conventionally sintered BaTiO3

materials, core–shell structured grains with large shell-to-corethickness ratio are needed to result in flatK–T properties forthese materials. The core–shell structure is a non-equilibriummicrostructure, which changes with sintering conditionsprofoundly. Therefore, the processing parameters need tobe stringently controlled in order to maintain the desiredmicrostructure and the flatK–T behaviour for the BaTiO3.

4. Conclusions

Nano-grained BaTiO3 materials co-doped simultaneouslywith Y and Mg elements have been successfully sintered bymicrowave sintering. The samples can be efficiently densifiedsuch that the density reaches 95% of TD by sintering at1150◦C for only 20 min. The samples possessing high enoughdensity exhibit small capacitance variation (1C/C). Thematerials meet X7R specification for a wide range of sinteringtemperatures and soaking times. TEM examinations revealthat the microstructures are extremely complicated althoughthe grains are uniformly small for all the samples. TheuniqueK–T characteristics of the samples are ascribed to theduplex structure of the samples, which contain fine grains ofparaelectric phase and large grains of ferroelectric phase.

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