GRAIN BOUNDARY FACETING AND ABNORMAL GRAIN

GROWTH IN BaTiO3

BYOUNG-KI LEE, SUNG-YOON CHUNG and SUK-JOONG L. KANG{Department of Materials Science and Engineering, Korea Advanced Institute of Science and

Technology, Taejon 305-701, South Korea

(Received 3 September 1999; accepted 7 November 1999)

AbstractÐWhen 0.1 mol% TiO2±excess BaTiO3 was sintered below the eutectic temperature in air, abnor-mally large grains formed in the ®ne matrix BaTiO3 grains. The abnormal grains contained {111} twinlamellae, while the matrix grains did not. A TEM observation revealed that almost all the grain boundarieswere faceted. On the other hand, however, when the air-sintered sample with faceted grain boundaries wasannealed in H2, the faceted boundaries became defaceted, and the growth of abnormal grains was sup-pressed while the growth of the matrix grains was enhanced, showing normal grain growth behavior. In ad-dition, the abnormal grains that had been elongated along their twin lamella grew rather isotropically,irrespective of the presence of {111} twins. It appears therefore that {111} twins appear to enhance thegrowth of the abnormal grains along the twin lamellae only when the grain boundary is faceted. After re-annealing the H2-annealed sample in air, however, the grain growth behavior and grain boundary structurewere found to recover those observed in the air-sintered sample. From these observations, it is concludedthat abnormal growth of BaTiO3 grains observed is related to grain boundary faceting and that boundaryfaceting is a necessary condition for abnormal grain growth. 7 2000 Acta Metallurgica Inc. Published byElsevier Science Ltd. All rights reserved.

Keywords: Grain growth; Grain boundaries; Microstructure; Sintering; Barium titanate

1. INTRODUCTION

Barium titanate is a typical material exhibiting

abnormal grain growth during processing for such

electronic components [1±3], for example, as chip

capacitors and positive-temperature-coe�cient

(PTC) resistors. Since grain size and distribution

considerably a�ect electrical properties, investi-

gators have been trying to understand the causes of

abnormal grain growth [1, 2, 4].

The growth behavior of abnormal grains in

TiO2±excess BaTiO3 manifests itself in two di�erent

ways depending on whether the annealing tempera-

ture is above or below the eutectic temperature of

13328C. Above the eutectic temperature, the shape

of abnormal grains is faceted but equiaxed, and

their fast growth has been thought to result from

the fast precipitation of material through a thin Ti-

rich liquid ®lm at the grain boundaries [1]. In con-

trast, below the eutectic temperature, almost all

abnormal grains contain {111} double twins and

are elongated along the twin lamellae [5±7]. Based

on the observation of the preferential growth along

{111} double twins, Schmelz and Meyer [5, 6]

suggested that the re-entrant edges formed by the

twin lamellae can provide ledge sites where atoms

are easily attached for fast grain growth, similar to

the earlier suggestion of DeVries [8] for systems

with a liquid phase. Eibl et al. [9], however, later

questioned the reentrant edge mechanism for the

abnormal growth of BaTiO3 grains in solid state,

because they could not observe any abnormal

grains in Nb-doped BaTiO3 which contained {111}

twin lamellae. They suggested that {111} double

twins do not always give rise to abnormal grain

growth.

Recently, the causes of abnormal grain growth

have been intensively reexamined in various ma-

terial systems [10±13]. For solid/liquid two-phase

systems, Park et al. [10] suggested that abnormal

grain growth can occur only when the solid/liquid

interface is singular, i.e. faceted. For grains with

faceted solid/liquid interface, grains can grow only

by two-dimensional nucleation or with the assist-

ance of screw dislocations. In this condition, some

large grains that have a driving force above the

critical value for growth can grow abnormally. Lee

et al. [13] have extended this idea to the polycrystal-

line single-phase system. They suggested that grain

Acta mater. 48 (2000) 1575±1580

1359-6454/00/$20.00 7 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.

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{ To whom all correspondence should be addressed.

growth behavior may be correlated to grain bound-ary structure. Yoon and co-workers [13±15] also

observed that in some metallic systems, abnormalgrain growth was correlated to grain boundaryfaceting. Since grain growth involves atomic move-

ment across the grain boundary, it seems reasonablethat the boundary structure a�ects the growth beha-vior of polycrystalline materials.

The purpose of the present investigation is tostudy the conditions for abnormal grain growth inBaTiO3, a ceramic material of practical importance

in the electronics industry. The e�ect of {111} twinlamellae on abnormal grain growth has also beenstudied. During this investigation, special attentionhas been paid to the faceting/defaceting transition

of the grain boundary and its e�ect on grain growthbehavior, as in a previous investigation on Ni [13].

2. EXPERIMENTAL PROCEDURE

Specimens were prepared from commercialBaTiO3 (HPBT-1, Fuji Titanium, Kanagawa,Japan) and TiO2 (Aldrich Chemical Company,Milwaukee, WI) powders. According to the data

provided by the producers, the purities of BaTiO3

and TiO2 were 99.8 and 99.9 wt%, respectively. Theaverage particle size of both powders was 0.3 mm. A

0.1 mol% TiO2±added BaTiO3 powder mixture wasball-milled in ethyl alcohol for 24 h using a poly-ethylene bottle and zirconia balls. The dried slurry

was crushed in an agate bowl and sieved to 125 mm.The mixed powder was slightly pressed into disks of9 mm in diameter and 5 mm in thickness and then

isostatically pressed under 200 MPa.The powder compacts were sintered at 12508C,

828C below the eutectic temperature, for 10 h in airand air-quenched to room temperature. For further

grain growth, the sintered bodies were againannealed at the same temperature as the sinteringtemperature, 12508C, for 48 h in air or in H2. The

oxygen-partial pressure in the H2 atmosphere wasapproximately 10ÿ20 atm. In addition, some of theH2-annealed samples were re-annealed for 200 h in

air or in H2. Both the heating and cooling rates ofall the samples were 3008C/min. Before annealing at12508C, all the samples were held at 9008C for 48 hin both corresponding annealing atmospheres. This

treatment may exclude any possible chemical e�ecton grain growth due to the di�erence in defect con-centration that results from the change in oxygen-

partial pressure during the experiment. Theannealed samples were vertically cut, polished up toa 0.25 mm ®nish, and chemically etched in a

95H2O±5HCl (vol.%) solution.To measure the average grain size, more than 300

grains were examined with an image analyser. The

size of each grain was measured with the areaanalysis method where grain size is determinedfrom a cross-sectional area of the grain. The crystalstructures of the samples were also determined by

X-ray di�ractometry with Cu-Ka radiation and a Ni®lter. For TEM observation, the samples were

ultrasonically cut into 3 mm discs, mechanicallyground to a thickness of 100 mm, dimpled to athickness of less than 10 mm, and ®nally ion-milled

until perforation for electron transparency. Themicrostructures were observed under an opticalmicroscope, in a scanning electron microscope

(SEM515, Philips, Eindhoven, Netherlands) and atransmission electron microscope (JEM-3010,JEOL, Tokyo, Japan) operated at 300 kV. The

chemical compositions were determined by wave-length dispersive spectroscopy (WDS).

3. RESULTS

Figure 1 shows the microstructures of 0.1 mol%

TiO2±added BaTiO3 sintered at 12508C for 10 h inair. Abnormally large grains are present in the ®nematrix grains [Fig. 1(a)]. In contrast to the ®ne

matrix grains [Fig. 1(b)], almost all the abnormalgrains contain {111} twin lamellae, as previouslyobserved [5±7]. Twin lamellae have been thought to

assist BaTiO3 grains to grow preferentially alongtheir plates [5±7]. Indeed, it appears that the prefer-

Fig. 1. Microstructures showing (a) abnormal grains and(b) ®ne matrix grains in 0.1 mol% TiO2±excess BaTiO3

sintered at 12508C for 10 h in air.

1576 LEE et al.: GRAIN BOUNDARY FACETING

ential growth of the grains occurred along the twinlamellae. When a single twin lamella was present,

the grains grew parallel to the twin planes, leadingto a plate-like shape, as shown in Fig. 2(a).However, when more than two nonparallel twin

lamellae were present in the grain, the grain shapebecame equiaxed, as shown in Fig. 2(b).During the annealing of the sintered sample at

12508C, the growth behavior of the ®ne grains, aswell as the abnormal grains, was very di�erentdepending on the annealing atmosphere, the oxidiz-

ing or the reducing, as shown in Figs 3 and 4. Inair, it appears that the abnormal grains growfurther to impinge upon each other while thegrowth of the ®ne matrix grains is not appreciable.

In contrast, in H2 �PO2� 10ÿ20 atm), the growth of

the abnormal grains is much suppressed but thegrowth of the ®ne matrix grains is promoted. The

measured sizes of the abnormal and ®ne matrixgrains in Table 1 con®rm the microstructural obser-vation. In addition to the change in grain growth

rate, the shape of abnormal grains (which containeddouble twins) became less elongated when annealingin H2, as shown in Fig. 5 and Table 1. The decrease

in aspect ratio resulted from faster growth in the

direction perpendicular to the twin lamella than inthe direction parallel to the lamella. This result indi-

cates that under H2, the promotion of grain growthalong the twin lamella direction did not occur andthat growth behavior changed to normal.

Figure 6 shows typical TEM micrographs of thesintered samples annealed in air (a) and in H2 (b).In the course of TEM observation, no amorphous

phase was observed in the samples, con®rming thatthe observed grain growth occurred in single-phasepolycrystalline BaTiO3. In addition, ®ne secondary

particles of Ba6Ti17O40 phase, which is stable onlybelow the eutectic temperature, were observed inboth the air-annealed and H2-annealed samples.Therefore, any possible e�ect of the amorphous

phase on grain growth behavior can be discarded.One noticeable feature in the TEM observation, forexample in Fig. 6, was that the grain boundaries in

the air-annealed samples were mostly faceted butthose in the H2-annealed samples were defaceted.Among more than 40 grain boundaries, approxi-

mately 80% were faceted in the air-annealedsamples, showing a hill-and-valley structure, as indi-cated by arrows in Fig. 6(a). In contrast, no faceted

boundaries were observed in the H2-annealed

Fig. 3. Microstructures showing (a) abnormal grains and(b) ®ne matrix grains in 0.1 mol% TiO2±excess BaTiO3

sintered at 12508C in air for 10 h and then annealed in airfor 48 h.

Fig. 2. SEM micrographs showing (a) elongated and (b)equiaxed abnormal grains in 0.1 mol% TiO2±excess

BaTiO3 sintered at 12508C for 10 h in air.

LEE et al.: GRAIN BOUNDARY FACETING 1577

sample. The present TEM observation suggests thehigh possibility of a correlation between grain

growth behavior and grain boundary structure.The possibility of such a correlation was further

con®rmed by doing a re-annealing experiment withthe H2-annealed samples. Table 2 lists the averagesizes of the abnormal and matrix grains as well as

the aspect ratios of the abnormal grains in the re-annealed samples either in air or in H2. As can beseen from Tables 1 and 2, the growth of ®ne matrix

grains occurred appreciably in H2 but was sup-pressed in air. In contrast, the growth of the abnor-mal grains was suppressed in H2 but promoted in

air. In addition, the aspect ratio of abnormal grainsincreased during air-re-annealing, as shown in

Table 2, indicating that the abnormal grains tendedto be elongated again along twin lamella directions.In terms of grain boundary structure, the defaceted

boundaries became faceted with air-re-annealing, asshown in Fig. 7 (indicated by arrows). This graingrowth behavior and the boundary structure in the

air-re-annealed sample are consistent with those ofthe air-annealed sample shown in Figs 3 and 6(a).

4. DISCUSSION

The experimental results clearly show that thegrain growth behavior and grain boundary struc-

ture of BaTiO3 are very di�erent depending on theannealing atmosphere, the oxidizing or the redu-cing. When an air-sintered sample was again

annealed in air, abnormally large grains with twinlamellae grew further preferentially parallel to thetwin planes while ®ne matrix grains did not. The

TEM observation of this sample showed that mostgrain boundaries were faceted. In the case of H2-annealing, however, all the grains appeared to grownormally, regardless of the presence of twins, and

the grain boundaries that had been faceted duringsintering became defaceted.A previous study on grain boundary faceting

by Cahn [16] showed that grain boundary energyis determined by the misorientation between twograins and the inclination of the boundary. If

there are more than two cusps on the boundaryenergy plotted as a function of inclination for a®xed misorientation angle, faceting of the grain

Fig. 4. Microstructures showing (a) abnormal grains and(b) ®ne matrix grain in 0.1 mol% TiO2±excess BaTiO3 sin-tered at 12508C in air for 10 h and then annealed in H2

for 48 h.

Table 1. Average sizes of matrix and abnormal grains, and aspect ratios of abnormal grains in sintered and annealed 0.1 mol% TiO2±excess BaTiO3 samples

Average size (mm) Aspect ratio of abnormal grains

Matrix grains Abnormal grains

Sintered in air for 10 h 0.84 17.3 2:4721:04Sintered in air for 10 h and then annealed in air for 48 h 0.87 29.3 2:3320:99Sintered in air for 10 h and then annealed in H2 for 48 h 2.49 19.6 1:7420:48

Fig. 5. Typical shape of abnormal grains in 0.1 mol%TiO2±excess BaTiO3 sintered at 12508C in air for 10 h and

then annealed in H2 for 48 h.

1578 LEE et al.: GRAIN BOUNDARY FACETING

boundary occurs to reduce the boundary energy

and each facet has an inclination corresponding

to one of the cusp inclinations. The present ex-

perimental result thus indicates that a strong ani-

sotropy in grain boundary energy exists in the

air-annealed BaTO3 and that general grain bound-

aries with high energy should be faceted to have

energetically stable con®gurations. Such boundary

energy anisotropy, however, is thought to disappear

when annealed in H2, because faceted grain bound-

aries became smoothly curved, suggesting that there

are no cusps on the g-plot.It is well documented that intrinsic oxygen

vacancies, whose e�ective charge is compensated

by electrons, form in BaTiO3 in a reducing atmos-

phere [17]. Since an annealing atmosphere change

from air to H2 or vice versa was involved in the

present experiment, it is possible that the grain

growth was chemically driven, by the di�erence in

oxygen vacancy concentration, as in the case of

SrTiO3 [18]. Based on the measured oxygen di�usiv-

ity reported for BaTiO3 by Shirasaki et al. [19], the

oxygen di�usion rate at 12508C is calculated to be

1 mm/h. This value is much larger than the observed

growth rates of both abnormal and matrix grains,

less than 0.1 mm/h. Furthermore, all samples were

held at 9008C for 48 h in a corresponding atmos-

phere before annealing at 12508C. It is therefore

unlikely that the grain growth in the present exper-

iment was a�ected by the above-mentioned chemi-

cal e�ect.

To explain the fast growth of some BaTiO3

grains below the eutectic temperature, Schmelz and

Meyer [5, 6] suggested re-entrant-edgeassisted

growth. When a reentrant edge forms due to the

presence of a double twin, the reentrant edge may

act as a ledge site for the attachment of atoms and

thus promote grain growth. However, this argument

appears to be valid only when the grain boundaries

are faceted. Where the boundaries are faceted,

grains containing twin lamellae can grow fast

Fig. 6. TEM micrographs showing three-grain junctions in0.1 mol% TiO2±excess BaTiO3 sintered at 12508C in airfor 10 h and then annealed (a) in air and (b) in H2 for

48 h. The arrows indicate faceted boundaries.

Table 2. Average sizes of matrix and abnormal grains, and aspect ratios of abnormal grains in annealed and re-annealed 0.1 mol% TiO2±excess BaTiO3 samples

Average size (mm) Aspect ratio of abnormal grains

Matrix grains Abnormal grains

Air-sintered and H2-annealed for 48 h 2.49 19.6 1:7420:48H2-annealed and then re-annealed in H2 for 200 h 3.05 23.0 1:7120:60H2-annealed and then re-annealed in air for 200 h 2.56 29.1 1:9520:59

Fig. 7. SEM micrograph showing faceted boundaries indi-cated by arrows in 0.1 mol% TiO2±excess BaTiO3 sinteredat 12508C in air for 10 h, annealed in H2 for 48 h, and

then re-annealed in air for 200 h.

LEE et al.: GRAIN BOUNDARY FACETING 1579

because the atoms may more easily attach them-selves at the reentrant edges than at the other facets

of boundaries where excess interfacial energy is gen-erated upon atom attachment. In contrast, if grainboundaries that were faceted become smooth, in

other words atomically rough, the atomic level ofthe step sites is generated on any boundary, andconsequently the reentrant edges of twin lamellae

may no more be the preferential sites for atomattachment. Figures 4 and 5 show that, during H2

annealing, not only did the matrix grains grow con-

siderably but the elongated abnormal grains alsogrew further to reduce the aspect ratio in spite ofthe presence of twin lamellae. This result showsthat twin lamellae have no e�ect on grain growth

when the boundaries are defaceted.The transitions in grain growth behavior, between

abnormal and normal, as well as in grain boundary

structure, between faceted and defaceted, appear tobe reversible when the annealing atmospherechanges. In fact, a reversible transition in grain

boundary structure has already been reported insuch metal systems as Cu [20] and Al [21]. Theseobserved reversible transition behaviors amply show

that grain boundary faceting is necessary for theabnormal growth of BaTiO3 grains. The presence ofthe {111} twin lamellae, however, does not appearto be necessary for abnormal grain growth, con-

trary to a previous suggestion [5, 6]. The twinsenhance the growth of abnormal grains along theirplate only when the grain boundary is faceted.

5. CONCLUSIONS

The cause of abnormal grain growth in BaTiO3

was investigated. Bearing in mind the possibility of

a correlation between abnormal grain growth andgrain boundary faceting, a systematic experimentwas carried out using atmosphere change during

subsequent annealing, from oxidizing to reducingand vice versa. The transition from abnormal tonormal grain growth was related to the transitionfrom grain boundary faceting to grain boundary

defaceting. In the case of grain boundary faceting,the {111} twins enhanced the growth of abnormalgrains in a direction parallel to the twin plates. It

appears, therefore, that a necessary condition for

the abnormal growth of BaTiO3 grains is grainboundary faceting. The presence of the {111} twins

does not appear to be necessary for abnormal graingrowth but to be bene®cial for abnormal graingrowth when the grain boundary is faceted.

AcknowledgementsÐThis work was supported by theKorea Research Foundation (KRF). The authors thankProfessor D. Y. Yoon for helpful discussion and com-ment.

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1580 LEE et al.: GRAIN BOUNDARY FACETING