Journal

J. Am. Ceram. Soc., 88 [11] 3267–3269 (2005)

DOI: 10.1111/j.1551-2916.2005.00586.x

r 2005 The American Ceramic Society

Effect of Grain Boundary Structure on Diffusion-Induced GrainBoundary Migration in BaTiO3

Seong-Min Wang and Suk-Joong L. Kang*,w

Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology,Daejeon 305-701, Korea

The effect of grain boundary structure, either rough or faceted,on diffusion-induced grain boundary migration (DIGM) hasbeen investigated in BaTiO3. SrTiO3 particles were scatteredon the polished surfaces of two kinds of BaTiO3 samples withfaceted and rough boundaries and annealed in air for the sam-ples with faceted boundaries and in H2 for those with roughboundaries. In the BaTiO3 samples with rough boundaries, anappreciable grain boundary migration occurred. In contrast,grain-boundary migration hardly occurred in the BaTiO3 sam-ples with faceted boundaries. The migration suppression ob-served in the sample with faceted boundaries was attributed to alow boundary mobility. The present experimental results showthat DIGM is strongly affected by the boundary structure andcan be suppressed by structural transition of boundaries fromrough to faceted.

I. Introduction

RECENT investigations on polycrystalline microstructures1–9

have shown that grain boundaries can be categorized intotwo types: faceted and rough. The faceted boundary has anatomically ordered structure and exhibits, in general, a ‘‘hill-and-valley’’ shape. The rough grain boundary, on the otherhand, has an atomically disordered structure and exhibitsa smoothly curved shape. As the grain boundary energyvaries with the thermodynamic condition, the grain boundarystructure can also be changed by such thermodynamic param-eters as oxygen partial pressure,1,5–7 temperature,2,3,9 and chem-istry.5 In particular, Lee et al.1 showed that the structuraltransition of grain boundaries between faceted and roughcan be induced in BaTiO3 by changing the oxygen partialpressure. The grain boundary shape was smoothly curvedunder a low oxygen partial pressure, while it was well facetedunder a high oxygen partial pressure. In particular, the struc-tural transition was reversible and rapid with the changein oxygen partial pressure.1,7

When a polycrystal becomes chemically unstable, boundarymigration can occur forming a new solid solution behind themigrating boundaries. This phenomenon, referred to as diffu-sion-induced grain boundary migration or diffusion-inducedinterface migration (DIGM or DIIM), has been observed andanalyzed in a number of systems.10–21 All of the results in pre-vious investigations on DIGM, however, have been interpretedand analyzed under the assumption of a constant boundarymobility. Recent investigations on grain boundary structure and

grain growth,1–7,22,23 however, have shown that the boundarymobility is largely dependent on the boundary structure, roughor faceted.

This investigation studies the effect of the boundary structureon DIGM in a model system BaTiO3, where the two types ofboundaries can be easily prepared by changing the oxygen par-tial pressure. SrTiO3 was taken as a solute species.

II. Experimental Procedure

Two kinds of 0.2 mol% TiO2-excess samples with rough andfaceted boundaries were prepared from commercial BaTiO3

(Sakai Chemical Industry Co., Osaka, Japan) and TiO2 (Ald-rich Chemical Co., Milwaukee, WI) powders. The powder mix-ture was ball milled for 24 h in a nalgene bottle with ethanol andzirconia balls. After drying and sieving to 100 mm, the ball-milled powder was compacted uniaxially at 1 MPa into cylin-drical pellets 10 mm in diameter and 5 mm in height and thenpressed hydrostatically at 200 MPa. The power compacts weresintered at 13001C for 100 h in air and annealed at 12501C for 50h in H2 to prepare samples with rough grain boundaries. Aftersintering and annealing, the samples consisted of large grainsB50 mm in size. Some of the air-sintered and H2-annealed sam-ples were reannealed at 12501C for 50 h in air to prepare sampleswith faceted boundaries. During the air reannealing, apparentlynomore grain growth occurred. All of the samples were polishedup to a 0.25 mm finish for further treatment.

The solute source of SrTiO3 (Ferro Corp., Penn Yan, NY)was prepared by annealing powder compacts at 8001C for 1 hand crushing. For DIGM treatment the prepared SrTiO3 par-ticles of a few to a few hundred microns in size were scattered onthe polished surfaces of the two kinds of BaTiO3 samples withfaceted and rough grain boundaries and annealed at 12501 or13001C in air or in H2. The heating and cooling rate was 4 K/min in all experiments. The microstructures were observed un-der a scanning electron microscope (SEM) after the DIGMtreatment. The cation composition was determined by a wave-length-dispersive spectroscopy (WDS) attached to the SEM.The samples for transmission electron microscope (TEM) ob-servation were mechanically ground to a thickness of 100 mm,ultrasonically cut into 3 mm disks, dimpled to a thickness of lessthan 10 mm, and finally ion milled until perforation for electrontransparency.

III. Results and Discussion

During air sintering at 13001C abnormal grain growth occurredand the sample consisted of large abnormal grains of B35 mmaverage size. The boundary shape of the samples, however, wasgoverned by the oxygen partial pressure during annealing as inour previous investigations.1,7,22 When an air-sintered samplehaving faceted boundaries was annealed in H2, the boundarymorphology was changed from faceted to smoothly curved asshown in Fig. 1(a). However, when the H2-annealed sample was

3267

L. Klein—contributing editor

This work was supported by the National Research Laboratory (NRL) program of theMinistry of Science and Technology, Korea, and also by the Air Force Office of ScientificResearch (AFOSR under contract number FA5209-04-9-0396).

*Member, American Ceramic Society.wAuthor to whom correspondence should be addressed. e-mail: sjkang@kaist.ac.kr

Manuscript No. 20056. Received January 21, 2005; approved May 16, 2005.

reannealed in air, the boundary became faceted (Fig. 1(b)). (Thetwo unstepped boundaries in Fig. 1(b) are in fact straight, i.e.faceted boundaries as well.) Our previous TEM investigation22

showed that the boundary was fully faceted (atomically ordered)in air and smoothly curved (atomically disordered) in H2 with avery low oxygen partial pressure.

During subsequent DIGM treatment of the annealed sampleswith SrTiO3 particles on their surface, SrTiO3 dissolved intoBaTiO3. However, the microstructure of the samples was verydifferent depending on the boundary shape. In samples withsmoothly curved boundaries (rough boundaries), considerablemigration of the boundary was observed at the surface of thesamples during annealing at 12501 or 13001C for 40 h in H2, astypically shown in Fig. 2. The average migration distance wasB5 mm. AWDS analysis showed that the average Ba to Sr ratioin the migrated region was 96 to 4, indicating that a new solidsolution formed during the migration, a typical phenomenonof diffusion-induced grain boundary migration. This type of

boundary migration has long been studied and its driving forceis known to be the coherency strain energy stored in the thindiffusional layer of the shrinking grain.10,14–16,24

In contrast, no boundary migration was observed in thesamples with faceted boundaries during annealing at 12501 or13001C for 40 h in air, as shown in Fig. 3. Possible causes for thedifference in boundary migration between the two kinds of sam-ples with rough and faceted boundaries can be differences indriving force and boundary mobility. As the driving force forDIGM is related to the coherency strain caused by the dissolu-tion of solute atoms into the grains from the grain bounda-ries,10,14–16,24 a difference in driving force can result if the solutediffusion along the grain boundary is different in a differentoxygen partial pressure.

To test the possibility of driving force difference, chemicalcomposition analysis was carried out after annealing SrTiO3-packed BaTiO3 samples at 12501C for 40 h either in air or in H2

(Fig. 4(a)). Figure 4 shows the average Sr concentrations in lay-ers of 20 mm thickness from the surface of the samples annealedin air (Fig. 4(b)) and in H2 (Fig. 4(c)). The Sr concentration wasmeasured in five different places for each layer of 100 mm length.The average and maximum/minimum values are plotted in Figs.4(b) and (c). As can be seen in these figures, the average Sr con-centration in each layer is essentially not different between thetwo kinds of samples. The H2-annealed sample with DIGM,however, should contain more Sr than the air-annealed samplewithout DIGM in the surface region. In our case the depth ofthe migration region was several microns and the measurementwas made excluding the surface region of a few microns in orderto exclude the possible contribution of the dissolved SrTiO3 onthe surface. Therefore, the contribution of the migration regionmay be marginal and can be neglected. The similarity observedin Sr depth profile between the two kinds of samples suggeststhat the Sr diffusion along grain boundaries is not affected bythe structure of the grain boundary. We may, therefore, con-clude that the coherency strain in the diffusional layer formed atthe grain interface, i.e. the driving force for boundary migration,was the same irrespective of the boundary structure.

The difference in boundary migration behavior between thetwo kinds of samples should thus result from a difference inboundary mobility. Some recent investigations1,7,22 have shownthat the grain growth in BaTiO3 is strongly affected by thestructure of grain boundaries, either rough or faceted. When theboundary was rough, continuous grain growth occurred. On theother hand, for faceted boundaries, abnormal or stagnant graingrowth occurred.1,7,22 The grain growth behavior observed insamples with faceted boundaries was explained in terms of non-linear mobility of the boundary.1,7,22,23 When the driving forcefor grain growth was below a critical value, no grain growthoccurred, suggesting that the boundary mobility was negligible.

The absence of DIGM in samples with faceted boundariescan therefore be explained by the very low mobility of the fac-eted boundaries. The result itself may, in turn, confirm that the

(a)

(b)

1 nm1 nm

1 nm1 nm

Fig. 1. Transmission electronmicroscopic micrographs of BaTiO3 sam-ples sintered at 13001C for 100 h in air and (a) annealed at 12501C for50 h in H2 and (b) at 12501C for 50 h in H2 and then reannealed at12501C for 50 h in air.

30 30 ∝m m 30 30 µm

Fig. 2. Surface microstructure of the H2-annealed sample reannealedwith SrTiO3 particles on the surface at 12501C for 40 h in H2. Arrowsindicate the sites of SrTiO3 particles and unfilled arrows indicate themigrating boundaries.

30 30 µm

Fig. 3. Surface microstructure of the air-annealed sample reannealedwith SrTiO3 particles at 12501C for 40 h in air. Arrows indicate the sitesof SrTiO3 particles.

3268 Communications of the American Ceramic Society Vol. 88, No. 11

mobility of a faceted boundary can be much lower than that of arough boundary. As the mobility of the boundary decreases,coherency breaking can occur and no boundary migration re-sults.11 The formation of a new solid solution then occurs onlyvia lattice diffusion as in the conventional alloying process.

At the early stage of DIGM studies in the 1980s, DIGM wasrarely observed in ceramics while it was frequently observed inmetals. As the boundary energy is, in general, more anisotropic

in ceramics than in metals at conventional annealing tempera-tures, the boundaries of ceramics have a greater tendency to be-come faceted with a lower mobility. This is probably the reasonwhy it was difficult to observe DIGM in ceramics in early studies.

IV. Conclusions

To investigate the effect of grain boundary structure on diffu-sion-induced grain boundary migration, two kinds of BaTiO3

samples with faceted and rough boundaries were prepared. Withdissolution of SrTiO3 into BaTiO3, grain boundary migrationoccurred in samples with rough boundaries while no boundarymigration occurred in samples with faceted boundaries. The ab-sence of boundary migration in samples with faceted boundarieswas attributed to a significant reduction in boundary mobilityfor faceted boundaries. This is probably the reason why DIGMwas rarely observed in ceramics with mostly faceted boundariesduring the early stage of DIGM studies.

References

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2S. B. Lee, D. Y. Yoon, andM. F. Henry, ‘‘Abnormal Grain Growth and GrainBoundary Faceting in a Ni-Base Superalloy,’’ Acta Mater., 48 [12] 3071–80 (2000).

3J. B. Koo and D. Y. Yoon, ‘‘The Dependence of Normal and Abnormal GrainGrowth in Silver on Annealing Temperature and Atmosphere,’’ Metall. Mater.Trans. A, 32A [3] 469–75 (2001).

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10W. H. Rhee, Y. D. Song, and D. Y. Yoon, ‘‘A Critical Test for the CoherencyStrain Effect on Liquid Film and Grain Boundary Migration in Mo–Ni–(Co–Sn)alloy,’’ Acta Metall., 35, 57–60 (1987).

11Y.-J. Baik and D. Y. Yoon, ‘‘The Effect of Curvature on the Grain BoundaryMigration Induced by Diffusional Coherency Strain in Mo–Ni Alloy,’’ ActaMetall., 35, 2265–71 (1987).

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14K. J. Yoon and S.-J. L. Kang, ‘‘Chemical Control of Grain Boundary Mi-gration of SrTiO3 in the SrTiO3–BaTiO3–CaTiO3 System,’’ J. Am. Ceram. Soc.,76, 1641–4 (1993).

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16J.-H. Jeon and S.-J. L. Kang, ‘‘Control of Interface Migration of Melt-Infil-trated Niobium-Doped Strontium Titanates by Solute Species and Atmosphere,’’J. Am. Ceram. Soc., 81, 624–8 (1998).

17E. Rabkin andW. Gust, ‘‘Criteria for the Occurrence of the Diffusion-InducedGrain Boundary Migration,’’ Interface Sci., 6, 105–11 (1998).

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19Y. Yamamoto, M. Moriyama, M. Kajihara, and T. Mori, ‘‘Kinetics of Dif-fusion Induced Grain Boundary Migration of [100] Twist Boundaries in theCu(Zn) System,’’ Acta Mater., 47, 1757–66 (1999).

20J. –K. Park and D. –Y. Kim, ‘‘Effect of Grain Size on Diffusion-InducedGrain-Boundary Migration in Ba(Zn1/3Nb2/3)O3 Ceramics,’’ J. Am. Ceram. Soc.,79 [5] 1405–8 (1996).

21C. A. Handwerker, ‘‘Diffusion-Induced Grain Boundary Migration in ThinFilm’’; pp. 245–322 in Diffusion Phenomena in Thin Films and MicroelectronicMaterials, Edited by D. Gupta and P. S. Ho. Noyes Publications, Park Ridge,NJ, 1988.

22Y.-I. Jung, S.-Y. Choi, and S.-J. L. Kang, ‘‘Grain Growth Behavior DuringStepwise Sintering of Barium Titanate in Hydrogen Gas and Air,’’ J. Am. Ceram.Soc., 86 [12] 2228–30 (2003).

23S.-J. L. Kang, Sintering: Densification, Grain Growth and Microstructure,pp. 117–24. Elsevier, Oxford, UK, 2005.

24M. Hillert, ‘‘On the Driving Force for Diffusion Induced Grain BoundaryMigration,’’ Scr. Metall., 17 [2] 237–40 (1983). &

20 30 40 50 60 70 800.0

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Fig. 4. (a) Vertical section of a BaTiO3 sample reannealed with SrTiO3

powder packing at 12501C for 40 h in air. Schematic boxes in thefigure indicate the layers of 20 mm thickness for each measurement ofSr concentration. (b and c) Measured average concentrations of Sr(in at.%.) in the layers of 20 mm thickness for each of the BaTiO3

samples packed with SrTiO3 powder and reannealed in air and inH2, respectively are shown.

November 2005 Communications of the American Ceramic Society 3269