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Dielectric Properties of Fine-Grained BaTiO3 Ceramics Doped with CaO

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2002 Jpn. J. Appl. Phys. 41 6922

(http://iopscience.iop.org/1347-4065/41/11S/6922)

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Jpn. J. Appl. Phys. Vol. 41 (2002) pp. 6922–6925Part 1, No. 11B, November 2002c©2002 The Japan Society of Applied Physics

Dielectric Properties of Fine-Grained BaTiO3 Ceramics Doped with CaOYukio SAKABE, Nobuyuki WADA, Takashi HIRAMATSU and Tomohisa TONOGAKI

Murata Mfg. Co., Ltd., 2-26-10 Tenjin, Nagaokakyo, Kyoto 617-8555, Japan

(Received May 24, 2002; accepted for publication June 27, 2002)

Fine-grained dielectric ceramics, i.e., of 0.2µm grain diameter, were obtained using Ca-doped BaTiO3 powders consisting ofparticles 0.2µm in diameter. The (Ba1−x Cax )mTiO3 ceramics (x : 0–0.10,m: 1.003–1.009) showed a high dielectric constant andstable temperature dependence, which were provided by interfacial stress due to the grain boundary, even though the ceramicshas no core-shell structure. The failure time under an accelerated life condition was remarkably improved with Ca doping,which may be caused by the decrease in BaTiO3 unit volume with Ca2+ at the Ba2+ lattice site, and Ca2+ acceptor ions at theTi4+ site. [DOI: 10.1143/JJAP.41.6922]

KEYWORDS: barium titanate, Ca, MLCs, microstructure, core-shell, tetragonality, dislocation loop

1. Introduction

Today’s advanced electronics devices require multilayerceramic capacitors (MLCs) to achieve high performance withminiaturized dimensions. In particular, the market for large-capacitance capacitors, more than 10µF, is expanding and re-placing that for Ta and Al electrolytic capacitors. The devel-opment of Ni electrode MLCs enabled this great increase inthe capacitor market, because electrode material costs weregreatly reduced by replacing the conventional noble metalswith base metal. New dielectric materials compatible withNi electrodes have been developed, which enable the elec-trodes to be fired in a reducing atmosphere. Conventional di-electric ceramics based on titanate have poor insulation re-sistance with firing under a low oxygen pressure atmosphere.Many studies have been carried out to find a means of pre-venting the reduction of BaTiO3 ceramics.1–8) Oxygen disso-ciation can be prevented by transition metal ions, i.e., Mn3+,Co3+, and Fe3+, or by the substitution of Ti ions by lowervalency ions such as Ca2+ or Mg2+. The doped BaTiO3 ce-ramic undergoes considerable grain growth and shows a highdielectric constant (12000), but capacitance change with tem-perature becomes seriously large, being characterized by thespecification of Y5V (EIA code:−82% ≤ �c/c ≤ +22%at −30◦C to 85◦C). On the other hand, core-shell structureddielectrics, of which the grain growth is inhibited, were devel-oped as X7R dielectrics (EIA code:�c/c = ±15% at−55◦Cto 125◦C).9) The core-shell structure is composed of a ferro-electric core region and defused paraelectric shell regions sur-rounding the core. This bimodal phase in the grain can pro-vide the BaTiO3 based dielectrics with a stable temperaturedependence of capacitance. A thinner dielectric layer needsfiner BaTiO3 grains, therefore, it became more and more dif-ficult to design X7R materials with the core-shell structure,because the volume of the core region must become smallerand smaller. Therefore, it is increasingly difficult to obtainsufficient dielectric properties for MLCs, because finer grainsshow a lower dielectric constant due to the size effect of thegrain.10–13)

We have previously reported that grain growth controlledBaTiO3 ceramics showed stable temperature dependenceof the dielectric constant, due to internal stress from thegrain boundary.14) Applying this technology, fine-grained(Ba1−xCax )mTiO3 dielectrics were developed to realize high-

performance MLCs with a thin dielectric layer and a highstacking layer count.

2. Experimental

Ca-doped BaTiO3 powders (Ba1−xCax )mTiO3 were syn-thesized by a hydrolysis method using Ba(OH)2, Ti alkox-ide and CaCl2, with x ranging from 0 to 0.15 andm from1.003 to 1.040. They were calcined at 1050◦C in air, result-ing in (Ba1−xCax )mTiO3 powders. Lattice parameters wereinvestigated by X-ray diffraction (XRD) in order to exam-ine Ca substitution for the Ba sites or Ti sites of BaTiO3.(Ba1−xCax )mTiO3 powders withx ranging from 0 to 0.15 andm from 1.003 to 1.009 were prepared to investigate the dielec-tric properties of (Ba1−xCax )mTiO3 ceramics. They were cal-cined at 1050◦C in air then mixed with 1 mol% MgO to con-trol the grain growth at sintering and 2 mol% SiO2 as a sinter-ing aid. Formulated (Ba1−xCax )mTiO3 powders were mixedwith an organic binder, ethyl alcohol and toluene to prepareslurries. Ceramic green films were prepared using the doc-tor blade method, then stacked and pressed. Disk samples of10 mm diameter and 0.7 mm thickness were prepared. MLCsamples were also prepared with 6-µm-thick green sheet andNi paste for the inner electrode. Disk samples and MLCs with50 active layers and a chip size of 2.0 × 1.25 mm were firedin a reducing atmosphere (1.6 × 10−11 MPa O2 at 1150◦C).Bulk densities of ceramics were evaluated by measuring thesize and weight of sintered disk samples. Scanning electronmicroscope (SEM) analyses were performed to measure theparticle size of calcined powders and the grain size of ceram-ics. Transmission electron microscope (TEM) analyses alsorevealed the microstructure and chemical composition of thegrain and the grain boundary of ceramics.

The capacitance and dissipation factor of MLCs were mea-sured at 1 kHz and 0.5 Vrms/µm, using a LCR meter (HP-4284A). A highly accelerated life test was conducted on MLCsamples at 150◦C and 10 V/µm.

3. Results and Discussion

3.1 Characterization of powder and ceramicsA SEM micrograph of the (Ba1−xCax )mTiO3 powder cal-

cined at 1050◦C is shown in Fig. 1(a). Ceramic samples of92 to 95% theoretical density were obtained under sintering

6922

Jpn. J. Appl. Phys. Vol. 41 (2002) Pt. 1, No. 11B Y. SAKABE et al. 6923

Fig. 1. SEM micrographs of (Ba1−x Cax )mTiO3 (a) powders calcined at1050◦C in air and (b) ceramics including 1 mol% MgO and 2 mol% SiO2.

-1.5

-1.0

-0.5

0.0

0.5

0 5 10 15

Ca content / at.%

Lat

tice

volu

me

chan

ge /

%

m=1.003m=1.005m=1.010m=1.025m=1.040

20

Fig. 2. Lattice volume change of perovskite structure in calcined(Ba1−x Cax )m TiO3 powders.

higher than 1150◦C. The average grain sizes of ceramics sin-tered at 1150◦C were about 0.2 µm as shown in Fig. 1(b). Itis confirmed that the grain growth did not take place duringsintering. The values of the c-axis and a-axis constants wereobtained from XRD patterns. Figure 2 is a plot of the changesin lattice volume of calcined (Ba1−x Cax )mTiO3 powders andFig. 3 shows their tetragonality (c/a). Mitui and Westphal15)

reported that the c- and a-axis and the unit cell volume of theCa-doped BaTiO3 ceramics decrease monotonically with the

Fig. 3. Tetragonality (c/a) of perovskite structure in calcined(Ba1−x Cax )mTiO3 powders.

increase of Ca content. However, in our samples, in whichm is larger than unity, the c-axis decreased and the a-axisincreased with increasing Ca content, which resulted in ab-normal increases in the unit cell volume as shown in Fig. 2.Change in c/a vs Ca content also yields unusual behaviors, asplotted in Fig. 3. These abnormalities are evidence that Ti4+can be replaced with the larger Ca2+ ion. The maximum con-tent of Ca on the Ti site is estimated to be at least 2 mol%.Zhuang et al.16) demonstrated the transition temperature Tcdependence on the Ca content at the Ti site of the composi-tion Ba1−x Ti1−yCax+yO3−y with x from 0 to 5% and y from0 to 5%, and concluded that the Curie temperature decreasedwith Ca on the Ti site due to internal stress from the latticedistortion. The decrease in c/a of our samples is consistentwith that found in previous works.

Figure 4(a) shows the microstructure of the ceramics deter-mined through TEM observation. TEM compositional analy-sis confirmed that Si and Mg ions are located mainly at thegrain boundary and the triple granular point. These compo-nents were nondetectable inside the grains due to their lowconcentration. Transmission electron diffraction revealed thatthe material at the triple granular point had a crystalline phasecomposed of Mg, Si and Ba oxides. Ferroelectric domainstructures were observed throughout the entire volume of thefine grain as shown in Fig. 4(b). These analyses suggest thatthe present fine-grained (Ba1−x Cax )mTiO3 ceramics was uni-form in composition and crystal structure. Figure 5 presents(113) and (311) diffractions of the calcined powder, the sur-face of the fired ceramics and the surface of the chemicallyetched ceramics. The etching was conducted using HCl so-lution with 1N HCl at 25◦C for 60 min to 300 min. The cal-cined powder showed a typical tetragonal XRD pattern. Thefired ceramics showed broadened XRD peaks, which may re-sult from the internal stress caused by surface tension or thegrain boundary.14) It is noted that c/a recovered significantlywith chemical etching, which was found for the first time inthis study. This recovery can be explained by assuming theremoval of the stress with etching of the grain boundary.

3.2 Electric properties of (Ba1−xCax)mTiO3 ceramicsDielectric constant vs temperature for the composition of

(Ba1−x Cax )mTiO3 and BaTiO3 ceramics is shown in Fig. 6.(Ba1−x Cax )mTiO3 ceramics provided a high and stable dielec-

6924 Jpn. J. Appl. Phys. Vol. 41 (2002) Pt. 1, No. 11B Y. SAKABE et al.

Fig. 4. (a) Magnified TEM micrograph of (Ba1−x Cax )mTiO3 ceramics,and (b) low-magnification TEM image of the same sample.

78.0 78.5 79.0 79.5 80.0 80.5 81.02

X-r

ay in

tens

ity (

Arb

. Uni

ts)

calcined powder (c/a=1.010)

sintered ceramics (c/a=1.005)

60min etching (c/a=1.008)

300min etching (c/a=1.010)

/

Fig. 5. (113) and (311) XRD patterns of calcined powder, sintered ceram-ics and etched ceramics for 60 min and 300 min.

tric constant of 3000, which was superior to that of the un-doped BaTiO3 sample. As we reported previously, the graingrowth inhibited BaTiO3 ceramics can yield stable tempera-ture dependence even though they have no core-shell struc-ture. That is to say, the core-shell structure is not an essentialcondition for X7R dielectric ceramics. For the fine-grainedBaTiO3 dielectrics, e.g., a diameter less than 200 nm, inter-nal stress can play a role sufficient to flatten the capacitancechange with temperature.14)

The Ca doping tended to decrease the resistance atroom temperature slightly as shown in Fig. 7. Time de-pendence of the insulation resistance of the composition

0

500

1000

1500

2000

2500

3000

3500

-100 -50 0 50 100 150Temperature / C

Die

lect

ric

cons

tant

BaTi1.003O3

(Ba0.94Ca0.06)1.003TiO3

Fig. 6. Temperature dependence of dielectric constant of(Ba0.94Ca0.06)1.003TiO3 and BaTi1.003O3.

6

7

8

9

10

11

0 2 4 6 8 10 12 14 16

Ca content /at.%

Log

R

(

R :

)

Fig. 7. Resistance of (Ba1−x Cax )1.003TiO3 with x ranging from 0 to 0.15.

0

2

4

6

8

10

0.01 0.1 1 10 100 1000

Time / h

Log

R

(

R :

)

BaTi1.003O3

( Ba0.94Ca0.06 )1.003TiO3

Fig. 8. Time dependence of the resistance under the accelerated life test for(Ba0.94Ca0.06)1.003TiO3 and BaTi1.003O3.

(Ba0.94Ca0.06)1.003TiO3 + 1 mol%MgO + 2 mol%SiO2 underthe accelerated life condition is plotted in Fig. 8 in compari-son with the Ca-undoped one. The Ca-doped dielectrics had atime to failure of about 100 h under accelerated life test condi-tions under the DC field stress 10 V/µm at 150◦C. It becomeclear that Ca played an important role in improving the life-

Jpn. J. Appl. Phys. Vol. 41 (2002) Pt. 1, No. 11B Y. SAKABE et al. 6925

Fig. 9. TEM micrograph of (Ba1−x Cax )mTiO3 ceramics.

time of the Ni-electrode X7R MLCs.

3.3 Role of Ca ions in the (Ba1−xCax)mTiO3 ceramicsIt has long been known that MgO is a useful element for

BaTiO3 ceramics as an effective grain growth inhibitor andacceptor ion at the Ti site. Its solubility limit was estimatedto be around 2 mol% from the results of measuring the latticeconstant. In this study, Mg was detected in the grain bound-ary region, which might act as the inhibitor. However, a smallamount of MgO is expected to exist in the grain and to occupythe Ti4+ site to act as acceptor ions.3) The high insulation re-sistance through sintering under a reducing atmosphere mightbe achieved by acceptor ions Ca2+ and Mg2+ at the Ti4+ site.At the same time, the same number of oxygen vacancies isgenerated according to the following equation:

CaOTiO2 → Ca′′Ti + Oo + V o. (1)

In Fig. 9, a TEM micrograph of (Ba0.94Ca0.06)1.003TiO3 ce-ramics shows the dislocation loops called “coffee bean con-trast” ,17) which is significant in the Ca-doped compositions.These dislocation loops in the grains disappeared under re-oxidation or electron beam irradiation. Therefore, the ob-served dislocation is supposed to be an aggregation of theoxygen vacancies presented in eq. (1).

Under the elevated temperature and high voltage stress,electrical conduction develops by a mechanism governedby Schottky emission or space-charge-limited current. It isknown that the oxygen vacancy plays an important role inthe electrical conduction, because the drift of the oxygen va-cancy toward the cathode leads to inhomogeneity in electric

field distribution, resulting in a fatal increase in current. Theresults revealed that Ca-doped compositions provided a muchbetter lifetime compared to the Ca-free BaTiO3 dielectrics inspite of the high concentration of V o. We can interpret thisresult as being due to a low drift mobility of V o caused bylattice shrinkage with Ca at the Ba site, and the internal lat-tice distortion caused by Ca at the Ti site.

4. Conclusions

The dielectric compositions of (Ba1−x Cax )mTiO3 dopedwith 1 mol% MgO and 2 mol% SiO2 had uniform fine grainsof 0.2 µm diameter with no core-shell structure. Internalstress from the grain boundary suppressed their tetragonal-ity, resulting in a high dielectric constant (3,000) having astable temperature dependence. The core-shell structure wasnot an essential condition for achieving fine-grained X7R di-electrics. Change in the lattice constant and dislocation loopswith the addition of Ca ion was evidence of Ti ion replace-ment with larger Ca ions. The lifetime under the acceleratedlife test condition was much improved, as was indicated bylattice shrinkage with Ca at the Ba site, and the internal lat-tice distortion caused by Ca at the Ti site.

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