- Email: [email protected]
Chemically induced grain boundary migration in SrTiO3
Ceramics International
16 (1990) 151-155
Chemically Induced Grain Boundary Migration in SrTiO 3 Kyung J. Yoon, Duk N. Yoon & Suk-Joong L. Kang Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, P.O. Box 150, Cheongryang, 130-650 Seoul, Republic of South Korea (Received 29 May 1989; accepted 24 July 1989) Abstract: The boundary migration induced by chemical composition change has been investigated in SrTiO 3. Compacts of SrTiO 3 powder have initially been sintered at 1450°C for 4 h in air. The sintered specimens have been packed in CaO or BaTiO 3 powder, and annealed in air at various temperatures between 1200°C and 1400°C and for various times from 20 h to 160 h. In most of the specimens, the grain boundary migrates, forming a new solid solution containing cations of the packing powder. In the specimens packed with CaO powder, the migration distance increases linearly with the annealing time. The migration of some grain boundaries results in a corrugated boundary shape. The driving force for the observed grain boundary migration is attributed to the coherency strain energy in solute diffusion zone in front of the moving boundary.
1 INTRODUCTION Since the observation of grain b o u n d a r y and liquid film migration induced by composition change, similar phenomena have been observed in various alloy systems. 1-6 F o r liquid film migration (LFM) in metal alloys, several experiments ~- 9 and theoretical analyses 1° have shown that the driving force of the migration is a coherency strain energy during the solute diffusion at the surface o f shrinking grains, as proposed by Hillert. 11 In case o f grain b o u n d a r y migration, the coherency strain energy mechanism was also attributed to the migration in some alloys.l 2 - 14 In recent investigations on ceramic systems, it has been demonstrated that the interface can also migrate by changing chemical composition o f the alloy 15'16 or by phase d e c o m p o s i t i o n . 17 K i m e t al. ~ 5 showed that addition or depletion o f P b O in (Pb, La)(Zr, Ti)O 3 resulted in migration of grain boundaries and chemically induced recrystallization (CIR). In this system, however, as in other ceramic systems investigated, the valency problem o f solute Ceramics International
atoms at grain b o u n d a r y layers o f shrinking and growing grains can be important in the migration process. In the present investigation, grain b o u n d a r y migration in SrTiO3 has been studied. The Sr ions in sintered SrTiO 3 compacts have been replaced by Ba or Ca ions by packing in BaTiO 3 or C a O powders and annealing at high temperatures. During the experiments in which the total Sr + Ba or Sr + Ca concentration can be kept constant, possible concentration variation in defects and other ion species near grain b o u n d a r y can be minimized and a strain energy due to the size difference o f atoms is expected to be produced.
2 EXPERIMENTAL PROCEDURE Specimens were prepared from SrTiO 3 (1.55#m, Sr/Ti = 0.993, T A M Ceramics), BaTiO 3 ((Ba + Sr)/ T i = 0 . 9 9 2 , Ferro Co.), and C a O (Junsei Chemical Co.) powders. The SrTiO 3 powder was isostatically pressed under 200 M P a into a compact of 13 mm in diameter and 3 mm in height. The powder
151 0272-8842/90/$03"50 © 1990 Elsevier Science Publishers Ltd, England. Printed in Great Britain
152
Kyung J. Yoon, Duk N. Yoon, Suk-Joong L. Kang
compact was sintered at 1450°C for 4 h in air. The heating rate from 600°C to the sintering temperature was 100°C/h and the cooling rate to 1000°C was about 700°C/h. The sintered specimen was cut into six pieces. Each piece was packed in CaO or BaTiO3 powder and pressed isostatically under 200MPa into a compact of 13 mm in diameter and 6 mm in height. The pellets containing the sintered pieces were then heat-treated between 1200 and 1400°C for various times in air. The heat-treated specimens were polished and etched in a 5 0 H 2 O q 5 H N O a - 5 H F (vol%) solution. Microstructures were observed under SEM and the composition was determined by EDX and WDX. 3 RESULTS A N D DISCUSSION
increasing annealing temperature, pronounced grain boundary movement (indicated by A in Fig. 2) and corrugated boundaries (B) were noticed. In some specimens annealed at higher temperatures, such as the BaTiO3 packed specimen annealed at 1400°C (Fig.2(e)), considerable grain growth was observed. The boundary migration against boundary curvature in the specimens packed in solute sources is believed to be a chemically induced grain boundary migration caused by compositional change. In many metallic alloy systems,1 - ~o. t 2 14,20,21 the interface migration by composition or temperature change was shown to stem from the coherency strain energy in the diffusion layer in front of the migrating grain boundaries. On the contrary, relatively few experimental results have been reported in ceramic systems. Kim et aL ~5 demonstrated that addition or depletion of PbO in the PLZT system induced migration of grain boundaries. The cause of the migration was suggested to be the coherency strain energy, as in metallic systems, produced by variation of the equilibrium content of PbO near the interface due to a change of the external chemical potential of the species. In such a system, however, the solute cation should be found only in a thin boundary layer, order of several atomic distances, of the shrinking grains, compared to its distribution in the thick solid-solution layer on the growing grains. The defect concentrations and the electric charge potential gradients between boundary layers of growing and shrinking grains can thus be different. 22 Hence, in addition to the coherency strain due to the lattice mismatch, the difference in defect concentration and electric potential gradient may affect the driving force of the boundary migration. In contrast, under the present experimental conditions, only Sr concentration is expected to vary by replacing Sr 2 ÷ with Ca 2 + or Ba 2 +, while keeping total S r + C a or S r + B a concentration constant. Possible concentration variations in defects and other ion species can thus be avoided. The coherency strain due to the difference of cation size will be produced by diffusion of solute cation into SrTiO 3. Figure 3 is a scanning electron micrograph of a migrated region in the CaO packed specimen annealed at 1200°C for 40 h. In this figure the initial position of the grain boundary can be distinguished by a straight line revealed after etching. An EDX analysis showed that the migrated region contained 1.1 at% C a a n d 54.6at% Sr, and the other regions less than 0.2 at% Ca and 55.8 at% Sr. The migration -
15
Figure 1 shows a typical microstructure of a SrTiO3 specimen sintered at 1450°C for 4 h. The grains had grown to about 20/~m in diameter. When this specimen was annealed at 1400°C for 40h without packing in CaO or BaTiO 3 powder, negligible grain growth occurred. In contrast, the grain growth during sintering at 1450°C is fast, probably because of the liquid formed due to excess TiO2 in the raw powder. The liquid is expected to form above 1440°C, the eutectic temperature in SrTiO3-TiO 2. Similar phenomena of fast grain growth in the presence of a liquid phase have also been reported previously by several authors. 1s'19 After the heat treatment of the sintered specimen packed in BaTiO3 or CaO, irregular movement of grain boundaries or enhanced grain growth was observed near the surface region of the specimen. Figure 2 shows the grain structure of the specimens heat-treated at various temperatures for 40 h. With
Fig. 1. Microstructure of SrTiO3 specimensintered at 1450°C
for 4h.
-
17
Chemically induced grain boundary migration in Sr TiO 3
153
1300
Fig. 2. Microstructures of BaTiOa-packed ((a), (c) and (e)) and CaO-packed ((b), (d) and (f)) specimens heat treated at 1200°C ((a) and (b)), 1300°C ((c) and (d)) and 1400°C ((e) and (f)) for 40 h. The second phase (arrow) on (a) is mounting material (bakelite) impregnated during the specimen preparation.
154
Kyung J. Yoon, Duk N. Yoon, Suk-Joong L. Kang 2.5 t , S"
E•.
s//
2.0
js
J
j~
1.5
•
/i
•
I
"1o r
c: 1.0 O •
.-~ 0.5
st
/
• sI ii t Ij
0.0
~o
B'o
,~o
,~o
Heat treatment time, h
Fig. 4. Observed increase of migration distance of grain boundaries with heat-treatment time in SrTiOa-CaO specimens heat-treated at 1200°C in air.
Fig. 3. SEM micrograph showing grain boundary migration in the CaO-packed SrTiO 3 specimen heat-treated at 1200°C for 40h.
of the boundary is thus accompanied by a replacement of a part of the Sr atoms by Ca atoms, i.e. solution of Ca ions and removal of Sr ions. Similarly, in the migrated region of the BaTiO3 packed specimens, Ba was detected by WDX about 0.5 at% at 1200°C and 6.5 at% at 1400°C. The crystal structure of SrTiO3 is a nearly ideal perovskite with the tolerance factor of 0.99 and SrTiO 3 makes a complete solid solution with BaTiO a and a limited solid solution with CaO. The ionic radii of Sr, Ba and Ca are 1.4, 1-6 and 1-35/~, respectively.23 Since the grain boundary migration occurred by replacing Sr by either larger atom (Ba) or smaller atom (Ca), it can be due to the coherency strain energy established in a thin diffusional interface layer ahead of the moving boundary. Boundary migration occurred in two different ways: migration in one direction such as the regions A in Fig. 2(c) and (d), and boundary corrugation such as the regions B in Fig. 2(d) and (f). More corrugation of boundaries is observed with increasing temperature. The preference between the two different modes may be related to the nucleation and growth rate of the newly formed solid solution. Detailed analysis, however, is not available yet in both metals and ceramics. 24'25 Figure 4 plots the migration distance with annealing time of CaO packed specimens at 1200°C. The distance has been taken as that of the highest frequency in measurements of maximum migration distances of over 30 migrated areas at surface region of the specimens. (This distance can represent the real migration distance. 26) It appears that the migration distance increases linearly with the
annealing time; the migration velocity is constant of approximately 1.4 x 10 -2/~m/h from the slope of the figure. The constancy in the migration velocity implies in turn that the concentration of Ca ion reaches a nearly constant and limited value during the migration. The enhanced grain growth in the BaTiO3 packed specimens sintered at higher temperature (Fig. 2(e)) may result from the formation of a liquid phase owing to excess TiO2 in SrTiO3 (about 0"8 at%) and BaTiO 3 (about 0.7 a t e ) powders. In the presence of excess TiO2, BaTiO3 forms a eutectic at 1320°C, 27'28 and the ternary eutectic in SrTiOa-BaTiOa-TiO2 would be at a lower temperature than the binary eutectic; the liquid formed is thought to enhance the growth of (Sr, Ba)TiO 3 grains, as in other titanate systems. 1s, 19 4 CONCLUSION
For the most of the previous investigations 15'16 on grain boundary migration in ceramics, it seems to be difficult to distinguish the contribution of coherency strain energy to the migration owing to differences in valency of the cations in solution or depleted. In the present investigation on the SrTiO3 system, such a possible valency problem could be eliminated by replacing Sr ions with Ba or Ca ions of equal valency. Substitution of either smaller ions (Ca) or larger ions (Ba) results in pronounced grain boundary migration. In Ca ion added specimens, the migration distance is linearly proportional to the annealing time, in accordance with a prediction of the coherency strain energy model. REFERENCES 1. DEN BROEDER, J. A., Interface reaction and a special form of grain boundary diffusion in the Cr-W system, Acta Metall., 20 (1972) 319-32.
Chemically induced grain boundary migration in S r T i O 3 2. YOON, D. N. & HUPPMANN, W. J., Chemically driven growth of tungsten grains during sintering in liquid nickel, Acta Metall., 27 (1979) 973-7. 3. HILLERT, M. & PURDY, G. R., Chemically induced migration, Acta Metall., 26 (1978) 333--40. 4. CAHN, J. W., PAN, J. D. & BALLUFFI, R. W., Diffusioninduced grain boundary migration, Scripta Metall., 13 (1979) 503-9. 5. PAN, J. D. & BALLUFFI, R. W., Diffusion-induced grain boundary migration in Au/Cu and Au/Ag thin film, Acta Metall., 30 (1982) 861-70. 6. PICCONE, T. J., BUTRYMOWICZ, D. B., NEWBURY, D. E., MANNING, J. R. & CAHN, J. W., Diffusioninduced grain boundary migration in the Cu-Zn system, Scripta Metall., 16 (1982) 839-43. 7. SONG, Y. D., AHN, S. T. & YOON, D. N., Chemically induced migration of liquid film in W-Ni-Fe alloy, Acta Metall., 33 (1985) 1907-10. 8. BAIK, Y. J. & YOON, D. N., Migration of liquid film and grain boundary in Mo-Ni induced by temperature change, Acta Metall., 33 (1985) 1911-7. 9. RHEE, W. H., SONG, Y. D. & YOON, D. N., A critical test for the coherency strain effect on liquid film and grain boundary in Mo-Ni-(Co-Sn) alloy, Acta Metall., 35 (1987) 57-60. 10. YOON, D. N., CAHN, J. W., HANDWERKER, C. A., BLENDELL, J. E. & BAIK, Y. J., Coherency strain induced migration on liquid film through solid, In Interface Migration and Control of Microstructure, C. S. Pande (Ed.), ASM, Columbia, Ohio, 1986, pp. 19-31. 11. HILLERT, M., On the driving force for diffusion-induced grain boundary migration, Scripta Metall., 17 (1983) 237-40. 12. BAIK, Y. J., Effect of coherency strain on the interface migration in Mo-Ni system. PhD Thesis, KAIST, Korea, 1986. 13. KANG, H. K., HACKNEY, S. & YOON, D. N., Migration of liquid and grain boundary in Mo-Ni induced by W diffusion, Acta MetaU., 36 (1988) 695-9. 14. RHEE, W. H. & YOON, D. N., The grain boundary migration induced by diffusional coherency strain in Mo-Ni alloy, Aeta Metall., 37 (1989) 221-8. 15. KIM, J. J., SONG, B. M., KIM, D. Y. & YOON, D. N., Chemically induced grain boundary migration and recrys-
155
16. 17.
18.
19.
20. 21. 22. 23. 24. 25. 26. 27. 28.
tallization in PLZT ceramics. Ceram. Bull., 65 (1986) 1390-2. CHAIM, R., HEUER, A. H. & BRANDON, D. G., Phase equilibriumZrO2-Y203alloys by liquid film migration, J. Am. Ceram. Soc., 69 (1986) 243-8. BUTLER, Z. P. & HEUER, A. H., Grain boundary phase transformation during aging of a partially stabilized ZrO2--a liquid phase analogue of diffusion induced grain boundary migration (DIGM), J. Am. Ceram. Soc., 68 (1985) 197-202. YAMAOKA, N. & MATSU, T., Properties of SrTiO3based boundary layer capacitors, In Advances in Ceramics, Vol. 1, L. M. Levinson & D. C. Hill (Eds), The American Ceramic Society Inc., Columbus, 1981, pp. 232-41. HENNINGS, D. F. K., JANSSEN, R. & REYNEN, P. J. L., Control of liquid-phase-enhanced discontinuous grain growth in barium titanate, J. Am. Ceram. Soc., 70 (1987) 23-7. CHONGMO, LI & HILLERT, M., A metallographic study of diffusion-induced grain boundary migration in the Fe-Zn system, Acta Metall., 29 (1981) 1949-60. CHONGMO, LI & HILLERT, M., Diffusion-induced grain boundary migration Cu-Zn, Acta Metall., 30 (1982) 1133-45. BLAKELY, J. M. & DANYLUK, S., Space charge regions at silver halide surfaces: Effect of divalent impurities and halogen pressure, Surface Science, 40 (1973) 37-60. MEGAW, H. D. (Ed.), Crystal Structure: A Working Approach, W. B. Saunders Company, London, 1973, pp. 26-7. MATTHEWS, J. W., Effect of coherency strain and misfit dislocation on the mode of growth of thin films, Thin Solid Film, 26 (1975) 129-34. HAY, R. S. & EVANS, B., Chemically induced migration in low and high angle calcite grain boundaries, Acta Metall., 35 (1987) 2049-62. SONG, Y. D., Chemically induced liquid film migration in W-Ni-Fe system. PhD Thesis, KAIST, Korea, 1978. NEGAS, T., ROTH, R. S., PARKER, H. S. & MINOR, D., Subsolidus phase relations in the BaTiO3-TiO 2 system, J. Solid State Chem., 9 (1974) 297-307. O'BRYAN, H. M., Jr., & THOMSON, J., Jr., Phase equilibria in the TiO2-rich region of the system BaO-TiO2, J. Am. Ceram. Soc., 57 (1974) 522-6.
16 (1990) 151-155
Chemically Induced Grain Boundary Migration in SrTiO 3 Kyung J. Yoon, Duk N. Yoon & Suk-Joong L. Kang Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, P.O. Box 150, Cheongryang, 130-650 Seoul, Republic of South Korea (Received 29 May 1989; accepted 24 July 1989) Abstract: The boundary migration induced by chemical composition change has been investigated in SrTiO 3. Compacts of SrTiO 3 powder have initially been sintered at 1450°C for 4 h in air. The sintered specimens have been packed in CaO or BaTiO 3 powder, and annealed in air at various temperatures between 1200°C and 1400°C and for various times from 20 h to 160 h. In most of the specimens, the grain boundary migrates, forming a new solid solution containing cations of the packing powder. In the specimens packed with CaO powder, the migration distance increases linearly with the annealing time. The migration of some grain boundaries results in a corrugated boundary shape. The driving force for the observed grain boundary migration is attributed to the coherency strain energy in solute diffusion zone in front of the moving boundary.
1 INTRODUCTION Since the observation of grain b o u n d a r y and liquid film migration induced by composition change, similar phenomena have been observed in various alloy systems. 1-6 F o r liquid film migration (LFM) in metal alloys, several experiments ~- 9 and theoretical analyses 1° have shown that the driving force of the migration is a coherency strain energy during the solute diffusion at the surface o f shrinking grains, as proposed by Hillert. 11 In case o f grain b o u n d a r y migration, the coherency strain energy mechanism was also attributed to the migration in some alloys.l 2 - 14 In recent investigations on ceramic systems, it has been demonstrated that the interface can also migrate by changing chemical composition o f the alloy 15'16 or by phase d e c o m p o s i t i o n . 17 K i m e t al. ~ 5 showed that addition or depletion o f P b O in (Pb, La)(Zr, Ti)O 3 resulted in migration of grain boundaries and chemically induced recrystallization (CIR). In this system, however, as in other ceramic systems investigated, the valency problem o f solute Ceramics International
atoms at grain b o u n d a r y layers o f shrinking and growing grains can be important in the migration process. In the present investigation, grain b o u n d a r y migration in SrTiO3 has been studied. The Sr ions in sintered SrTiO 3 compacts have been replaced by Ba or Ca ions by packing in BaTiO 3 or C a O powders and annealing at high temperatures. During the experiments in which the total Sr + Ba or Sr + Ca concentration can be kept constant, possible concentration variation in defects and other ion species near grain b o u n d a r y can be minimized and a strain energy due to the size difference o f atoms is expected to be produced.
2 EXPERIMENTAL PROCEDURE Specimens were prepared from SrTiO 3 (1.55#m, Sr/Ti = 0.993, T A M Ceramics), BaTiO 3 ((Ba + Sr)/ T i = 0 . 9 9 2 , Ferro Co.), and C a O (Junsei Chemical Co.) powders. The SrTiO 3 powder was isostatically pressed under 200 M P a into a compact of 13 mm in diameter and 3 mm in height. The powder
151 0272-8842/90/$03"50 © 1990 Elsevier Science Publishers Ltd, England. Printed in Great Britain
152
Kyung J. Yoon, Duk N. Yoon, Suk-Joong L. Kang
compact was sintered at 1450°C for 4 h in air. The heating rate from 600°C to the sintering temperature was 100°C/h and the cooling rate to 1000°C was about 700°C/h. The sintered specimen was cut into six pieces. Each piece was packed in CaO or BaTiO3 powder and pressed isostatically under 200MPa into a compact of 13 mm in diameter and 6 mm in height. The pellets containing the sintered pieces were then heat-treated between 1200 and 1400°C for various times in air. The heat-treated specimens were polished and etched in a 5 0 H 2 O q 5 H N O a - 5 H F (vol%) solution. Microstructures were observed under SEM and the composition was determined by EDX and WDX. 3 RESULTS A N D DISCUSSION
increasing annealing temperature, pronounced grain boundary movement (indicated by A in Fig. 2) and corrugated boundaries (B) were noticed. In some specimens annealed at higher temperatures, such as the BaTiO3 packed specimen annealed at 1400°C (Fig.2(e)), considerable grain growth was observed. The boundary migration against boundary curvature in the specimens packed in solute sources is believed to be a chemically induced grain boundary migration caused by compositional change. In many metallic alloy systems,1 - ~o. t 2 14,20,21 the interface migration by composition or temperature change was shown to stem from the coherency strain energy in the diffusion layer in front of the migrating grain boundaries. On the contrary, relatively few experimental results have been reported in ceramic systems. Kim et aL ~5 demonstrated that addition or depletion of PbO in the PLZT system induced migration of grain boundaries. The cause of the migration was suggested to be the coherency strain energy, as in metallic systems, produced by variation of the equilibrium content of PbO near the interface due to a change of the external chemical potential of the species. In such a system, however, the solute cation should be found only in a thin boundary layer, order of several atomic distances, of the shrinking grains, compared to its distribution in the thick solid-solution layer on the growing grains. The defect concentrations and the electric charge potential gradients between boundary layers of growing and shrinking grains can thus be different. 22 Hence, in addition to the coherency strain due to the lattice mismatch, the difference in defect concentration and electric potential gradient may affect the driving force of the boundary migration. In contrast, under the present experimental conditions, only Sr concentration is expected to vary by replacing Sr 2 ÷ with Ca 2 + or Ba 2 +, while keeping total S r + C a or S r + B a concentration constant. Possible concentration variations in defects and other ion species can thus be avoided. The coherency strain due to the difference of cation size will be produced by diffusion of solute cation into SrTiO 3. Figure 3 is a scanning electron micrograph of a migrated region in the CaO packed specimen annealed at 1200°C for 40 h. In this figure the initial position of the grain boundary can be distinguished by a straight line revealed after etching. An EDX analysis showed that the migrated region contained 1.1 at% C a a n d 54.6at% Sr, and the other regions less than 0.2 at% Ca and 55.8 at% Sr. The migration -
15
Figure 1 shows a typical microstructure of a SrTiO3 specimen sintered at 1450°C for 4 h. The grains had grown to about 20/~m in diameter. When this specimen was annealed at 1400°C for 40h without packing in CaO or BaTiO 3 powder, negligible grain growth occurred. In contrast, the grain growth during sintering at 1450°C is fast, probably because of the liquid formed due to excess TiO2 in the raw powder. The liquid is expected to form above 1440°C, the eutectic temperature in SrTiO3-TiO 2. Similar phenomena of fast grain growth in the presence of a liquid phase have also been reported previously by several authors. 1s'19 After the heat treatment of the sintered specimen packed in BaTiO3 or CaO, irregular movement of grain boundaries or enhanced grain growth was observed near the surface region of the specimen. Figure 2 shows the grain structure of the specimens heat-treated at various temperatures for 40 h. With
Fig. 1. Microstructure of SrTiO3 specimensintered at 1450°C
for 4h.
-
17
Chemically induced grain boundary migration in Sr TiO 3
153
1300
Fig. 2. Microstructures of BaTiOa-packed ((a), (c) and (e)) and CaO-packed ((b), (d) and (f)) specimens heat treated at 1200°C ((a) and (b)), 1300°C ((c) and (d)) and 1400°C ((e) and (f)) for 40 h. The second phase (arrow) on (a) is mounting material (bakelite) impregnated during the specimen preparation.
154
Kyung J. Yoon, Duk N. Yoon, Suk-Joong L. Kang 2.5 t , S"
E•.
s//
2.0
js
J
j~
1.5
•
/i
•
I
"1o r
c: 1.0 O •
.-~ 0.5
st
/
• sI ii t Ij
0.0
~o
B'o
,~o
,~o
Heat treatment time, h
Fig. 4. Observed increase of migration distance of grain boundaries with heat-treatment time in SrTiOa-CaO specimens heat-treated at 1200°C in air.
Fig. 3. SEM micrograph showing grain boundary migration in the CaO-packed SrTiO 3 specimen heat-treated at 1200°C for 40h.
of the boundary is thus accompanied by a replacement of a part of the Sr atoms by Ca atoms, i.e. solution of Ca ions and removal of Sr ions. Similarly, in the migrated region of the BaTiO3 packed specimens, Ba was detected by WDX about 0.5 at% at 1200°C and 6.5 at% at 1400°C. The crystal structure of SrTiO3 is a nearly ideal perovskite with the tolerance factor of 0.99 and SrTiO 3 makes a complete solid solution with BaTiO a and a limited solid solution with CaO. The ionic radii of Sr, Ba and Ca are 1.4, 1-6 and 1-35/~, respectively.23 Since the grain boundary migration occurred by replacing Sr by either larger atom (Ba) or smaller atom (Ca), it can be due to the coherency strain energy established in a thin diffusional interface layer ahead of the moving boundary. Boundary migration occurred in two different ways: migration in one direction such as the regions A in Fig. 2(c) and (d), and boundary corrugation such as the regions B in Fig. 2(d) and (f). More corrugation of boundaries is observed with increasing temperature. The preference between the two different modes may be related to the nucleation and growth rate of the newly formed solid solution. Detailed analysis, however, is not available yet in both metals and ceramics. 24'25 Figure 4 plots the migration distance with annealing time of CaO packed specimens at 1200°C. The distance has been taken as that of the highest frequency in measurements of maximum migration distances of over 30 migrated areas at surface region of the specimens. (This distance can represent the real migration distance. 26) It appears that the migration distance increases linearly with the
annealing time; the migration velocity is constant of approximately 1.4 x 10 -2/~m/h from the slope of the figure. The constancy in the migration velocity implies in turn that the concentration of Ca ion reaches a nearly constant and limited value during the migration. The enhanced grain growth in the BaTiO3 packed specimens sintered at higher temperature (Fig. 2(e)) may result from the formation of a liquid phase owing to excess TiO2 in SrTiO3 (about 0"8 at%) and BaTiO 3 (about 0.7 a t e ) powders. In the presence of excess TiO2, BaTiO3 forms a eutectic at 1320°C, 27'28 and the ternary eutectic in SrTiOa-BaTiOa-TiO2 would be at a lower temperature than the binary eutectic; the liquid formed is thought to enhance the growth of (Sr, Ba)TiO 3 grains, as in other titanate systems. 1s, 19 4 CONCLUSION
For the most of the previous investigations 15'16 on grain boundary migration in ceramics, it seems to be difficult to distinguish the contribution of coherency strain energy to the migration owing to differences in valency of the cations in solution or depleted. In the present investigation on the SrTiO3 system, such a possible valency problem could be eliminated by replacing Sr ions with Ba or Ca ions of equal valency. Substitution of either smaller ions (Ca) or larger ions (Ba) results in pronounced grain boundary migration. In Ca ion added specimens, the migration distance is linearly proportional to the annealing time, in accordance with a prediction of the coherency strain energy model. REFERENCES 1. DEN BROEDER, J. A., Interface reaction and a special form of grain boundary diffusion in the Cr-W system, Acta Metall., 20 (1972) 319-32.
Chemically induced grain boundary migration in S r T i O 3 2. YOON, D. N. & HUPPMANN, W. J., Chemically driven growth of tungsten grains during sintering in liquid nickel, Acta Metall., 27 (1979) 973-7. 3. HILLERT, M. & PURDY, G. R., Chemically induced migration, Acta Metall., 26 (1978) 333--40. 4. CAHN, J. W., PAN, J. D. & BALLUFFI, R. W., Diffusioninduced grain boundary migration, Scripta Metall., 13 (1979) 503-9. 5. PAN, J. D. & BALLUFFI, R. W., Diffusion-induced grain boundary migration in Au/Cu and Au/Ag thin film, Acta Metall., 30 (1982) 861-70. 6. PICCONE, T. J., BUTRYMOWICZ, D. B., NEWBURY, D. E., MANNING, J. R. & CAHN, J. W., Diffusioninduced grain boundary migration in the Cu-Zn system, Scripta Metall., 16 (1982) 839-43. 7. SONG, Y. D., AHN, S. T. & YOON, D. N., Chemically induced migration of liquid film in W-Ni-Fe alloy, Acta Metall., 33 (1985) 1907-10. 8. BAIK, Y. J. & YOON, D. N., Migration of liquid film and grain boundary in Mo-Ni induced by temperature change, Acta Metall., 33 (1985) 1911-7. 9. RHEE, W. H., SONG, Y. D. & YOON, D. N., A critical test for the coherency strain effect on liquid film and grain boundary in Mo-Ni-(Co-Sn) alloy, Acta Metall., 35 (1987) 57-60. 10. YOON, D. N., CAHN, J. W., HANDWERKER, C. A., BLENDELL, J. E. & BAIK, Y. J., Coherency strain induced migration on liquid film through solid, In Interface Migration and Control of Microstructure, C. S. Pande (Ed.), ASM, Columbia, Ohio, 1986, pp. 19-31. 11. HILLERT, M., On the driving force for diffusion-induced grain boundary migration, Scripta Metall., 17 (1983) 237-40. 12. BAIK, Y. J., Effect of coherency strain on the interface migration in Mo-Ni system. PhD Thesis, KAIST, Korea, 1986. 13. KANG, H. K., HACKNEY, S. & YOON, D. N., Migration of liquid and grain boundary in Mo-Ni induced by W diffusion, Acta MetaU., 36 (1988) 695-9. 14. RHEE, W. H. & YOON, D. N., The grain boundary migration induced by diffusional coherency strain in Mo-Ni alloy, Aeta Metall., 37 (1989) 221-8. 15. KIM, J. J., SONG, B. M., KIM, D. Y. & YOON, D. N., Chemically induced grain boundary migration and recrys-
155
16. 17.
18.
19.
20. 21. 22. 23. 24. 25. 26. 27. 28.
tallization in PLZT ceramics. Ceram. Bull., 65 (1986) 1390-2. CHAIM, R., HEUER, A. H. & BRANDON, D. G., Phase equilibriumZrO2-Y203alloys by liquid film migration, J. Am. Ceram. Soc., 69 (1986) 243-8. BUTLER, Z. P. & HEUER, A. H., Grain boundary phase transformation during aging of a partially stabilized ZrO2--a liquid phase analogue of diffusion induced grain boundary migration (DIGM), J. Am. Ceram. Soc., 68 (1985) 197-202. YAMAOKA, N. & MATSU, T., Properties of SrTiO3based boundary layer capacitors, In Advances in Ceramics, Vol. 1, L. M. Levinson & D. C. Hill (Eds), The American Ceramic Society Inc., Columbus, 1981, pp. 232-41. HENNINGS, D. F. K., JANSSEN, R. & REYNEN, P. J. L., Control of liquid-phase-enhanced discontinuous grain growth in barium titanate, J. Am. Ceram. Soc., 70 (1987) 23-7. CHONGMO, LI & HILLERT, M., A metallographic study of diffusion-induced grain boundary migration in the Fe-Zn system, Acta Metall., 29 (1981) 1949-60. CHONGMO, LI & HILLERT, M., Diffusion-induced grain boundary migration Cu-Zn, Acta Metall., 30 (1982) 1133-45. BLAKELY, J. M. & DANYLUK, S., Space charge regions at silver halide surfaces: Effect of divalent impurities and halogen pressure, Surface Science, 40 (1973) 37-60. MEGAW, H. D. (Ed.), Crystal Structure: A Working Approach, W. B. Saunders Company, London, 1973, pp. 26-7. MATTHEWS, J. W., Effect of coherency strain and misfit dislocation on the mode of growth of thin films, Thin Solid Film, 26 (1975) 129-34. HAY, R. S. & EVANS, B., Chemically induced migration in low and high angle calcite grain boundaries, Acta Metall., 35 (1987) 2049-62. SONG, Y. D., Chemically induced liquid film migration in W-Ni-Fe system. PhD Thesis, KAIST, Korea, 1978. NEGAS, T., ROTH, R. S., PARKER, H. S. & MINOR, D., Subsolidus phase relations in the BaTiO3-TiO 2 system, J. Solid State Chem., 9 (1974) 297-307. O'BRYAN, H. M., Jr., & THOMSON, J., Jr., Phase equilibria in the TiO2-rich region of the system BaO-TiO2, J. Am. Ceram. Soc., 57 (1974) 522-6.