Journal. Control of Interface Migration in Melt-Infiltrated Niobium-Doped Strontium Titanate by Solute Species and Atmosphere. Jae-Ho Jeon.

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1 Journal J. Am. Ceram. Soc., 81 [3] (1998) Control of Interface Migration in Melt-Infiltrated Niobium-Doped Strontium Titanate by Solute Species and Atmosphere Jae-Ho Jeon Department of Materials Processing, Korea Institute of Machinery and Materials, Changwon , Korea Jung Ho Je Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang , Korea Suk-Joong L. Kang* Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Taejon , Korea The solid/liquid interface migration of niobium-doped strontium titanate (SrTiO 3 ) has been investigated by using barium and calcium solute sources in the form of a Cu-Ba- Ca-O liquid. The solubilities of barium and calcium in SrTiO 3 that is in equilibrium with Cu-Ba-Ca-O melts can be determined by measuring the solute concentration in the migrated Sr(Ba,Ca)TiO 3 regions of SrTiO 3 grains. No interface migration is observed for a certain composition range of Cu-Ba-Ca-O infiltrant. For SrTiO 3 specimens that have been sintered in air, the zero driving force for the interface migration is found at a Ba:Ca ratio of 0.75 in Sr(Ba,Ca)TiO 3 solid solution. This value is in good agreement with a predicted value of zero coherency strain in a thin diffusional Sr(Ba,Ca)TiO 3 layer on bulk SrTiO 3. For the specimens that have been sintered in 5H 2 95N 2, however, a zero driving force results when the ratio is The difference between the value for the specimens sintered in air and that for the specimens sintered in 5H 2 95N 2 is attributed to the change in defect concentration via the change in atmosphere. The estimated difference in the lattice parameters of SrTiO 3 sintered in air and those of SrTiO 3 sintered in 5H 2 95N 2 is Å( nm). The difference in lattice parameters has been further confirmed by using synchrotron X-ray scattering experiment, which has revealed the lattice parameter to be Å ( nm) in air and Å ( nm) in 5H 2 95N 2. I. Introduction IN OUR previous investigations, 1 4 it was demonstrated that the interface migration of niobium-doped strontium titanate (SrTiO 3 ) could be induced by the solute species, as well as by a change in atmosphere. In the case of interface migration that is caused by the solute species, barium and calcium ions induced the interface migration by replacing some of the strontium ions. 1,2 No migration occurred when the estimated lattice parameter of the solid solution that formed at the SrTiO 3 grain surfaces was approximately the same as that of bulk SrTiO 3. However, when a solid phase is used as a solute source, as in the previous investigations, 1,2 the composition of the diffusion zone is not fixed by any thermodynamic condition, although the driving force for the interface migration is thought to be the coherency strain energy that is stored in a thin diffusion zone of receding grains, as in other materials systems. 5 8 Then, it is difficult to achieve the lattice-matching condition of the almost-zero coherency strain in such a ternary solid system of complete solid solubility. 9 However, such difficulty can be overcome by using a solute source in the form of a liquid. In the case of interface migration that was caused by an atmosphere effect, solid/liquid interface migration of SrTiO 3 was observed to occur when an oxidizing atmosphere (air) was used for melt-infiltration treatment after sintering a niobiumdoped SrTiO 3 compact in a reducing atmosphere (5H 2 95N 2 ). Based on the defect chemistry of niobium-doped SrTiO 3 with excess titania (TiO 2 ), the migration was attributed to the coherency strain energy that was stored in a thin diffusional zone of receding grains, via a lattice-parameter change, although the parameter change was not easy to measure via ordinary X-ray diffractometry (XRD) analysis. In the present study, a liquid phase has been used as a source of barium and calcium to overcome the difficulty in achieving the lattice-matching condition. A copper(ii) oxide (CuO) liquid, which contained baria (BaO) and calcia (CaO) in different ratios and formed a liquid film between the SrTiO 3 grains, infiltrated into sintered niobium-doped SrTiO 3 in an air or 5H 2 95N 2 atmosphere. By measuring the change in the latticematching condition with sintering atmosphere, it was also possible to estimate the sign and amount of coherency strain via the change in atmosphere, from 5H 2 95N 2 to air. II. Experimental Procedure Y.-M. Chiang contributing editor Manuscript No Received May 22, 1996; approved June 16, Supported in part by the Center for Interface Science and Engineering of Materials at KAIST and also by the Korea Atomic Energy Institute. Presented at the 97th Annual Meeting of the American Ceramic Society, Cincinnati, OH, May 1, 1995 (Interfaces Symposium, Paper No. SXIII-22-5). *Member, American Ceramic Society. Formerly with the Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology. The specimens were prepared from commercial SrTiO 3 (Ferro Corp., Penn Yan, NY), niobium oxide (Nb 2 O 5 ) (Hermann C. Starck, Berlin, Germany), CuO (Alfa Products, Danvers, MA), barium carbonate (BaCO 3 ) (Aldrich Chemical Co., Milwaukee, WI), and calcium carbonate (CaCO 3 ) (Kojundo, Saitama, Japan). According to the data provided by the producers, the purity and average size of SrTiO 3 were 99.8 wt% and 1.3 m, respectively; the purity of Nb 2 O 5 was 99.8 wt%, and that of other powders was >99.9 wt%. Approximately 50 g of a SrTiO mol% Nb 2 O 5 powder mixture was wet milled 624

2 March 1998 Control of Interface Migration in Melt-Infiltrated Nb-doped SrTiO for 20 h in a polyethylene bottle with ethyl alcohol and polyurethane balls. The dried slurry was crushed in an agate bowl and sieved to 125 m. The mixed powder was slightly pressed into disks that were 9 mm in diameter and 2 mm thick and were isostatically pressed under a pressure of 200 MPa. The compacts were sintered at 1480 C for 5 h in air. To determine the effect of the sintering atmosphere, some compacts were sintered in flowing 5H 2 95N 2, with an oxygen partial pressure of atm ( Pa). The infiltrants for the sintered specimens were made by mixing 80CuO 20(xCaCO 3 (1 x)baco 3 ) (in units of mol%) powder mixtures (where x 0, 0.2, 0.25, 0.3, 0.4, 0.5, 0.54, 0.7, and 1) for 5 h in a polyethylene bottle with ethyl alcohol and zirconia balls. The dried slurry was crushed and mixed with propylene glycol. The infiltrant powder was then coated onto the sintered specimens and an infiltration treatment was performed at 1300 C for 4 h in air. The surfaces of the sintered and infiltrated specimens were polished up to a 0.05 m finish. The sintered specimens were thermally etched at 1350 C for 30 min in air and the infiltrated specimens were chemically etched in a 50H 2 O 45HNO 3 5HF (vol%) solution for s. The microstructures were observed via optical microscopy and scanning electron microscopy (SEM). The chemical analysis was performed via wavelength-dispersive spectroscopy (WDS). The lattice parameter of the specimens sintered in 5H 2 95N 2 and that of the specimens sintered in air were each measured via X-ray scattering experiment at the Beamline X10A facility of the National Synchrotron Light Source (NSLS) at the Brookhaven National Laboratory (Upton, NY). The incident X-rays were focused vertically by using a focusing mirror. A horizontally bent Si(111) crystal was used to monochromatize the X-rays to 9.57 kev (a wavelength of Å ( nm)) and also focus them horizontally. A Ge(111) crystal analyzer was used to achieve a high-resolution configuration (the half width at the half maximum (HWHM) in the longitudinal direction was Å 1 ( nm 1 )). III. Results and Discussion (1) Effect of Solute Species When the specimens sintered in air were held at 1300 C for 4 h with 80CuO 20(xCaO (1 x)bao), the infiltrants melted and penetrated along the grain boundaries of the sintered specimens to form intergranular liquid films, as in the previous investigations. 3,4 However, the penetration depth was limited to 200 m from the surface, irrespective of the infiltrant composition. Figure 1 shows the surface microstructures of specimens after infiltration treatment with various oxide melts at 1300 C for 4 h. When x 0, interface migration occurred too extensively to distinguish the sintered microstructure clearly, as shown in Fig. 1(A). As x increased to 0.2 and 0.25 (Figs. 1(B) and (C), respectively), the migration rate decreased. When x was in the range of , the initially sintered microstructure was not changed (i.e., no migration occurred), as shown in Figs. 1(D) and (E). As x increased further to 0.54, 0.7, and 1 (Figs. 1(F), (G), and (H), respectively), the migration rate increased again. For the specimen with x 1, the migration was just as extensive as that shown in Fig. 1(A), where x 0. The average thickness of the migration region with the infiltrant composition was determined by measuring more than one hundred boundaries and is shown in Fig. 2 (denoted by the solid line); data for the specimens sintered in 5H 2 95N 2 are also shown (denoted by the dashed line). Data for specimens with x values of 0 and 1 are excluded because the migration is too extensive to distinguish the original boundaries. The migration thickness seems to increase as the composition difference between the infiltrant composition and the composition range that has no migration increases. Such shapes of migration-distance curves are typical for chemically induced interface migration, as previously observed in the Mo-Ni-(Co-Sn) system. 10 Figure 3 shows an example of composition analysis. In this particular case of an 80CuO 4CaO 16BaO infiltrant, 31 at.% of the strontium ions were substituted by 10.5 at.% of calcium ions and 20.5 at.% of barium ions in migrated regions. The titanium concentration was almost unchanged in the migrated regions, as expected. 2 This composition analysis confirms that the observed interface migration was chemically induced in a solid liquid two-phase mixture that is similar to those previously observed in some metal and ceramic systems. 5 8 Table I lists the results of the composition analysis of the migration regions that were formed during the infiltration treatment of the sintered specimens. Figure 4 is a diagram that shows the solubilities of barium and calcium ions in SrTiO 3 for a given infiltrant composition x. The solid lines connect the newly formed solid solutions and the infiltrant liquids, and the dotted line is estimated by considering the slope of the measured ones. From the measured composition in Table I, the coherency strains of thin, oxidized Sr(Ba,Ca)TiO 3 layers that are formed on receding SrTiO 3 grains can be calculated, as shown in the last column of the table. For an accurate calculation of the coherency strains, the lattice parameters of the solid solutions that are formed at an infiltration treatment temperature of 1300 C must be used. Because of the unavailability of the data, however, the lattice parameters at 1430 C 11 of 3.90, 4.00, and 3.82 Å (0.390, 0.400, and nm, respectively), for SrTiO 3, BaTiO 3, and calcium titanate (CaTiO 3 ), respectively, were used in the present calculation. The lattice parameters of the solid solutions were also assumed to follow Vegard s law, because CaTiO 3 and BaTiO 3 form complete solid solutions with SrTiO 3 and the change in lattice parameter with substitution is approximately linear. The lattice-misfit parameter ( ), given by = 1 a 0 C a for BaTiO 3 ( BaTiO3 ) and CaTiO 3 ( CaTiO3 ) are then and , respectively. Here, a 0 is the lattice parameter of SrTiO 3 and C is the concentration of solute species (in atomic percent). The observed immobility of the interface in Fig. 2 for the liquid composition range of the specimens sintered in air is somewhat unexpected, because the zero driving force for the interface migration would be located at a specific composition, as in a previous investigation. 6 In our case of SrTiO 3 ceramics, however, an appreciable critical driving force seems to exist for the migration and the driving force for the composition range is below the critical value. 12 The liquid composition, x, for zero driving force in Sr(Ba,Ca)TiO 3 solid solution may then be found by extending the two solid curves in Fig. 2 and determining the minimum point, as shown by the dotted curve. The value of x for zero driving force is then Using this value, the chemical composition of a solid solution with zero driving force can be read as being approximately Sr 0.65 Ba 0.15 Ca 0.20 TiO 3 in Fig. 4 by considering the slope of the solid lines. The Ba:Ca ratio in the solid solution (0.75) is essentially the same as the previously estimated value of the zero driving force (0.8) 2 from an experiment that used a solid-state solute source. The experimentally determined value of 0.75 is also in agreement with the predicted value of 0.8 for zero coherency strain, i.e., lattice matching by using Vegard s law. This agreement again confirms that the observed interface migration is induced by the coherency strain energy that is stored in a thin diffusional layer of receding SrTiO 3 grains. Thus, the interface migration of SrTiO 3 during melt infiltration can be controlled by using solute elements that can change the lattice parameter of a newly formed SrTiO 3 solid solution. (2) Effect of Atmosphere The effect of atmosphere was observed by sintering niobium-doped specimens in 5H 2 95N 2 and then infiltrating vari-

3 626 Journal of the American Ceramic Society Jeon et al. Vol. 81, No. 3 Fig. 1. Microstructures of SrTiO mol% Nb 2 O 5 specimens sintered at 1480 C for 5hinairandinfiltrated with 80CuO 20(xCaO (1 x)bao) at 1300 C for 4hinair, for x values of (A) 0, (B) 0.2, (C) 0.25, (D) 0.3, (E) 0.5, (F) 0.54, (G) 0.7, and (H) 1.

4 March 1998 Control of Interface Migration in Melt-Infiltrated Nb-doped SrTiO Table I. Measured Solute Concentrations in Sr(Ba,Ca)TiO 3 Migrated Regions Formed during Infiltration of 80CuO 20(xCaO (1 x)bao) Melts and Estimated Strains in Thin Diffusional Sr(Ba,Ca)TiO 3 Solid-Solution Layers Coherent with Bulk SrTiO 3 Sintered in Air Solute concentration (at.%) x Calcium Barium Coherency strain, ( 10 3 ) Fig. 2. Observed variation of average migration-layer thickness with the infiltrant composition x of 80CuO 20(xCaO (1 x)bao) during the infiltration and heat treatment of SrTiO mol% Nb 2 O 5 specimens sintered in ( ) air and ( ) 5H 2 95N 2. Infiltration and heat treatment were performed at 1300 C for 4hinair. ous 80CuO 20(xCaO (1 x)bao) infiltrants in air. The experimental procedure was identical to that for the study of the solute effect, with the exception of the sintering atmosphere. Interface migration also occurred for all values of x, except x 0.3, as shown by the dashed lines in Fig. 2. The migration rate in the specimens sintered in 5H 2 95N 2 seems to be higher than that of the specimens sintered in air. Greater interface mobility in the specimens sintered in 5H 2 95N 2 seems to reduce the composition range for interface immobility. Figure 2 shows that the lattice-matching condition shifted from x 0.37 to x 0.34, as indicated by an arrow, when the sintering atmosphere was changed from air to 5H 2 95N 2. For a given composition of infiltrant, the chemical composition of migrated regions in the specimen sintered in 5H 2 95N 2 has been measured to be the same as that in the specimen sintered in air, within the limit of measurement accuracy. This result seems to be reasonable, because the migration layers that are formed during the infiltration in air are oxidized, regardless of the sintering atmosphere. Therefore, the change in the latticematching condition must be a reflection of the effect of the sintering atmosphere. When a thin solid solution layer forms at the surface of SrTiO 3 grains sintered in 5H 2 95N 2, the coherency strain ( ) must be induced in the layer via the substitution of barium and calcium ions in air, as BaTiO3 C BaTiO3 + CaTiO3 C CaTiO3 + 3 C 3 (1) where and C are the lattice-misfit parameter and the concentration of solute, respectively. The last term on the righthand side denotes the effect of the change in defect concentration that is due to the atmosphere change from 5H 2 95N 2 to air. For Nb 2 O 5 -doped SrTiO 3, two types of charge-compensation mechanisms are known to be operative, depending on the atmosphere. These mechanisms are (i) electronic compensation in a reducing atmosphere, which is described by SrTiO Nb 2 O 5 Sr Sr Ti Ti Nb Ti + 3 x O O x O 2 g x e + xv Ö TiO 2 (2) and (ii) ionic compensation in an oxidizing atmosphere, which is described by Fig. 3. Measured cation concentration (( ) strontium, ( ) calcium, ( ) barium, and ( ) titanium) in a migrated region and original SrTiO mol% Nb 2 O 5 grains infiltrated with 80CuO 4CaO 16BaO at 1300 C for 4hinair; the dotted and solid vertical lines respectively indicate the initial and final positions of a liquid film. Fig. 4. Diagram showing the compositions of Sr(Ba,Ca)TiO 3 solid solutions formed during infiltration and heat treatment with 80CuO 20(xCaO (1 x)bao) at 1300 C in air.

5 628 Journal of the American Ceramic Society Jeon et al. Vol. 81, No. 3 SrTiO Nb 2 O Sr Sr Ti Ti Nb Ti + 3O O V Sr SrTiO TiO 2 (3) Here, x denotes the concentration of oxygen vacancies that are formed in a reducing atmosphere. When a CuO-rich liquid infiltrates in air into the grain boundaries of a specimen sintered in 5H 2 95N 2, the electronic compensation at the grain interface must be translated into an ionic compensation, which results in the formation of additional Sr vacancies and a change in the electron concentration in the conduction band This effect of defect-concentration change must be additional to the effect of solute substitution. Currently, it is difficult to accurately determine the equilibrium solute concentration in the solid solution that has no interface migration in the specimen sintered in 5H 2 95N 2, because no appreciable solid-solution layer forms. Nevertheless, by drawing a line from x 0.34 to the solid-solution boundary in Fig. 4, the Ba:Ca ratio in the solid solution can be estimated to be This value is more than 0.09 greater than that for the specimen sintered in air. Therefore, by changing the atmosphere from 5H 2 95N 2 to air, the strain that is introduced in the thin oxidized layer on reduced SrTiO 3 grains is calculated to be by using BaTiO and CaTiO In terms of the difference in lattice parameters, the strain corresponds to Å( nm). The estimated difference was further confirmed via a synchrotron X-ray scattering experiment that used specimens that were sintered in different atmospheres, either air or 5H 2 95N 2. Figure 5 shows the X-ray scattering profile that was obtained on both the samples sintered in air and the samples sintered in 5H 2 95N 2 by varying the momentum transfer, q z, along the surface normal. From the Gaussian fittings of the detailed (110) reflections, as delineated by the solid lines in the figure, the lattice parameter was determined to be Å ( nm) in air and Å ( nm) in 5H 2 95N 2. The latticeparameter value of Å in air agrees well with the reported lattice-parameter value of Å ( nm). 18 The difference between the two measured values, Å ( nm), is also in good agreement with the estimated difference of Å( nm) from the present lattice-matching experiments. These measured and estimated differences in the values of the lattice parameters confirm, in turn, that interface migration can also occur via changes in the defect structures and, hence, the lattice parameter. Fig. 5. X-ray scattering profiles of the SrTiO 3 (110) reflection of the specimens sintered in ( ) 5H 2 95N 2 and ( ) air. IV. Conclusions In the previous investigations 1,2 on the effect of solute species on interface migration in SrTiO 3 sintered in air, clear definition of the thermodynamic condition for the migration was difficult, because the concentration of solute species at the grain boundary could not be determined. By using an inert liquid phase of CuO, the concentration of solute barium and calcium in the liquid was easily controlled and the thermodynamic condition of the solute source was well defined. Thus, the previous difficulty was overcome. The solubility ratio of barium to calcium in SrTiO 3 that has no interface migration (i.e., the lattice-matching condition) was 0.75, which was similar to that which was estimated by a previous solid-state experiment 2 and also was similar to that predicted by an equilattice diagram. 11 The effect of changing the atmosphere, from a reducing one to an oxidizing one, on interface migration could also be quantified by using two types of specimens (one type was sintered in 5H 2 95N 2 and one type was sintered in air). The strain introduced by changes in the lattice defect, when the atmosphere was changed from 5H 2 95N 2 to air, was estimated to be (tensile). The estimated strain could also be confirmed by a precise measurement of the lattice parameters of SrTiO 3 in air and in 5H 2 95N 2 ( Å ( nm) and Å ( nm), respectively), using synchrotron X-ray scattering. Acknowledgment: The authors are grateful to Mr. Joo Seon Kim and Mr. Byoung Ki Lee, for their assistance in experimentation and manuscript preparation, and to Prof. Y.-M. Chiang, for helpful comments. References 1 K. J. Yoon, D. N. Yoon, and S.-J. L. Kang, Chemically Induced Grain Boundary Migration in SrTiO 3, Ceram. Int., 16 [3] (1990). 2 K. J. Yoon and S.-J. L. Kang, Chemical Control of the Grain-Boundary Migration of SrTiO 3 in the SrTiO 3 BaTiO 3 CaTiO 3 System, J. Am. Ceram. Soc., 76 [6] (1993). 3 J.-H Jeon and S.-J. L. Kang, Effect of Sintering Atmosphere on Interface Migration of Niobium-Doped Strontium Titanate during Infiltration of Oxide Melts, J. Am. Ceram. Soc., 77 [6] (1994). 4 J.-H. Jeon, J. S. Kim, S.-J. L. Kang, and M. S. Yang, Atmosphere Control of Interface Migration and Its Effect on Dielectric Property of CuO-Infiltrated Strontium Titanate, J. Am. Ceram. Soc., 79 [6] (1996). 5 Y.-J. Baik and D. N. 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Soc., 78 [4] (1995). 18 Powder Diffraction File Card No , Joint Committee on Powder Diffraction Standards (JCPDS), Swarthmore, PA (now International Centre for Diffraction Data, Newtowne Square, PA), 1994.