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Transmission electron microscopic studies on crystallization of YBa2Cu3O7−y films deposited by advanced TFA-MOD method
Physica C 426–431 (2005) 1051–1055 www.elsevier.com/locate/physc
Transmission electron microscopic studies on crystallization of YBa2Cu3O7y films deposited by advanced TFA-MOD method J.S. Matsuda a,*, Y. Tokunaga a,1, K. Nakaoka a, R. Teranishi a, Y. Aoki a, H. Fuji a, A. Yajima b, Y. Yamada c, T. Izumi a, Y. Shiohara a a
Superconductivity Research Laboratory, ISTEC, Shinonome 1-10-13, Koto-ku, Tokyo 135-0062, Japan b Asahi Denka Kogyo Co. Ltd., Higashi-Ogu 7-2-35, Arakawa-ku, Tokyo 116-8553, Japan c Superconductivity Research Laboratory, ISTEC, Mutsuno 2-4-1, Atsuta-ku, Nagoya 456-8587, Japan Received 23 November 2004; accepted 26 January 2005 Available online 25 July 2005
Abstract We have prepared Y123 quenched and fully crystallized films by the conventional and the advanced TFA-MOD processes. Then, we have investigated microstructures of the films by means of transmission electron microscopy, in order to understand the initial stage of the Y123 growth. As a result, it is found that Y–Ba–O–F grains firstly nucleate on LaAlO3 single crystals as well as on CeO2 buffered metal substrates both in the TFA-MOD processes. The Y–Ba– O–F is grown with epitaxially orientated relationship to the substrate. A BaCeO3 reaction product layer is formed due to Ba diffusion from Y123 to CeO2 mainly through the grain boundaries. At the crystallization temperature of 760 °C, the diffusion rate of Ba is slow. Therefore, it was found that the thickness of the BaCeO3 layer does not increase much further with increasing the holding time until about 1000 min. Ó 2005 Elsevier B.V. All rights reserved. PACS: 74.72.B; 61.16.B Keywords: Metal organic deposition; Nucleation; Transmission electron microscopy
1. Introduction *
Corresponding author. Tel.: +81 3 3536 5711; fax: +81 3 3536 5717. E-mail address: [email protected] (J.S. Matsuda). 1 Present address: R&D Center, Tokyo Electric Power Company, 4-1 Egasaki-cho, Tsurumi-ku, Yokohama 2308510, Japan.
Metal organic deposition using trifluoroacetates (TFA-MOD) is one of the most promising methods for producing coated conductors, since it enables us to fabricate them with high quality at
0921-4534/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2005.01.086
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a low cost. In this method, superconducting films are formed by coating substrates with the solution containing TFA salts, and the two heat-treatments for both calcination and crystallization. In the case of the MOD process using the TFA salts for all cationic elements, the calcination process requires a long time about ten hours for a single coated film [1,2]. In order to shorten the calcination time, we have developed a new combination of starting materials [3]. We call the method using the new solution ‘‘the advanced TFA-MOD method’’. Generally, in the MOD process, heat-treatment conditions for calcination and crystallization have a great influence on microstructures and superconducting properties of the films. As for TFA-MOD process, Honjo et al. have derived a formula relating the partial pressure of water vapor (PH2O) in the crystallization process to the growth rate of YBa2Cu3O7y (Y123) films [4]. It was also reported that the growth rate of Y123 films deposited by the BaF2 ex-situ process is proportional to the square root of the PH2O by Vyacheslav et al. [5,6]. In addition, the rate limiting stage in the overall process for the Y123 growth is considered to be the HF-removal rate in these BaF2 processes. However, there are only a little understanding for reactions and mechanism of the Y123 formation. Therefore, in this study we focus on the interfacial structure evolution and the initial stage in the growth of Y123 films deposited by the conventional and the advanced TFA-MOD processes. By means of transmission electron microscopy (TEM), we have investigated the microstructures of the Y123 quenched and fully crystallized films, in order to understand the Y123 growth mechanism and to contribute to obtain the Y123 films with higher IC.
2. Experimental procedure The Y123 precursor and fully crystallized films were fabricated on LaAlO3(LAO)(0 0 1) single crystals or PLD–CeO2/IBAD–Gd2Zr2O7/Hastelloy tapes by the TFA-MOD methods. LAO substrates and the buffered metal tapes were spin-coated or dip-coated with a solution contain-
ing Y-, Ba-TFA salts and a Cu-naphthenate. Then, these films were heat-treated to 400 °C in an O2 gas flow with PH2O of 2.1%. In the case of using CeO2/Gd2Zr2O7/Hastelloy, we repeated the spin-coating and the calcination processes several times in order to fabricate thick precursor films; it is called ‘‘multi-coating’’. Finally, these precursor films were heated at 775 °C or 760 °C in an Ar/O2 gas flow with PH2O of 4.2% or 6.3%. Y123 quenched films were prepared by cooling rapidly during heating up to the crystallization temperature or after holding at the temperature for various times. TEM specimens were prepared by ion-milling. We used JEM-2010F operating at 200 kV and JEM-4000EX operating at 400 kV for microstructural observations.
3. Results and discussion 3.1. Y123 nucleation on LAO single crystal We have prepared Y123 quenched films by cooling rapidly during crystallization, and observed the films by means of TEM. Fig. 1(a) and (b) show cross-sectional TEM images of the Y123 film prepared by cooling rapidly at 650 °C during heating up for the crystallization in the conventional TFA-MOD process. As shown in these images, crystal nuclei different from the Y123 were crystallized on the LAO substrate. In the high-resolution TEM (HRTEM) image, the lattice spacing parallel to (0 0 1)LAO was 0.52 nm, and that parallel to (1 0 0)LAO was 0.38 nm. From energy dispersive X-ray spectroscopy (EDS) analysis, Y, Ba, O and F were detected in these crystal nuclei. Through these results, it was found that the island-shaped nuclei crystallized on the surface of the LAO substrate are mainly YOF–Ba solid solution. The Y–Ba–O–F is epitaxially crystallized, the orientation relationship is as follows: ½1 1 0YBOF==½1 0 0LAO;
ð0 0 1ÞYBOF==ð0 0 1ÞLAO
In this case, the misfit parameter between the Y–Ba–O–F and the LAO single crystal substrate is about 0.9%. On the other hand, the misfit
J.S. Matsuda et al. / Physica C 426–431 (2005) 1051–1055
Fig. 1. Cross-sectional TEM images of Y123 quenched film prepared by cooling rapidly from 650 °C during crystallization in the conventional TFA-MOD process: (a) low-magnification image and (b) high-resolution image obtained in the vicinity of the interface between the Y123 precursor film and the LAO substrate.
parameter between (1 0 0)Y123 and (1 0 0)LAO is 2.4%. Therefore, we suggest that the lattice matching of Y–Ba–O–F to LAO makes it easy to nucleate on the substrate during crystallization. However, we should investigate the dependence of the interfacial energies on the temperature and the phase stabilities in the total free energy change. Fig. 2(a) and (b) show the cross-sections of the Y123 film prepared by quenching at 730 °C in the advanced TFA-MOD process. Island-shaped Y– Ba–O–F grains were crystallized as well as in the film deposited by the conventional TFA-MOD. We suggest that the nucleation of Y–Ba–O–F occurs when we use Y, Ba-TFA as starting materials. Fig. 3(a)–(c) show the cross-sections of the Y123 film prepared by quenching at 775 °C in the advanced TFA-MOD process. In this film, the growth of the Y123 progresses, however, residuals of Y–Ba–O–F in the Y123 crystals were observed in the vicinity of the interface. As shown in Fig. 3(c), in the amorphous matrix including microcrystalline at the growth front, non-superconducting inclusions such as BaF2, CuO and Y2Cu2O5 were confirmed to exist. These were identified from EDS analysis and measurements of lattice spacings in HRTEM images [7].
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Fig. 2. Cross-sectional TEM images of Y123 quenched film prepared by cooling rapidly from 730 °C during crystallization in the advanced TFA-MOD process: (a) low-magnification image and (b) high-resolution image obtained in the vicinity of the interface between the precursor film and the LAO substrate.
Fig. 3. Cross-sectional TEM images of Y123 quenched film prepared by cooling rapidly from 775 °C during crystallization in the advanced TFA-MOD process: (a) low-magnification image; (b) high-resolution image in the vicinity of the interface between the film and the substrate and (c) high-resolution image at the growth front of the Y123 film.
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From above-mentioned results, two different kinds of reaction processes for Y123 formation are suggested as follows: ðIÞ Y Ba O F þ xBaF2 þ CuO þ H2 O ! YBa2 Cu3 O7y þ HF ðIIÞ 1=2Y2 Cu2 O5 þ 2BaF2 þ 2CuO þ 2H2 O ! YBa2 Cu3 O7y þ 4HF Y–Ba–O–F crystal nuclei are observed only in the initial stage of the Y123 growth. The Y–Ba– O–F grains disappear due to the reaction to form Y123. The rate limiting reaction of the Y123 growth is considered to be in the stage of HF gas removal. Therefore, identifications of these reactions for Y123 formation may not be significantly necessary from a viewpoint of controlling the growth rate of the Y123 film. However, it is important to understand the mechanism of Y123 crystal nucleation and crystallization, in order to prevent crack formation in the films and to fabricate thicker films in long tapes. 3.2. Y123 nucleation on CeO2 buffer layer Fig. 4(a) and (b) show the cross-sectional TEM images of Y123 films prepared by quenching after holding at 760 °C for 10 and 20 min, respectively. As shown in these figures, nucleation of Y–Ba– O–F crystals has occurred in the film quenched after holding at 760 °C for 10 min. Y–Ba–O–F is epitaxially grown on a CeO2 buffer layer with the lattice misfit parameter of 0.7%. ½1 0 0YBOF==½1 0 0CeO2 ;
ð0 0 1ÞYBOF==ð0 0 1ÞCeO2
After holding at 760 °C for 20 min, the Y123 nucleation occurred on the CeO2 buffer. The orientation relationship between the Y123 film and the CeO2 buffer with the lattice misfit parameter of 0.8% is as follows: ½1 0 0Y123==½1 0 0CeO2 ;
ð0 0 1ÞY123==ð0 0 1ÞCeO2
The nucleation rate of Y123 film deposited on CeO2 buffer tends to be slow compared with that of the film on LAO. We should investigate effects of bufferÕs crystallinity and the surface roughness on the growth rate in detail.
Fig. 4. Cross-sectional TEM images of Y123 quenched films prepared by cooling rapidly after holding at 760 °C for (a) 10 min and (b) 20 min, respectively. These films were deposited on CeO2 buffered metal substrates.
Fig. 5(a) and (b) show the cross-sections of Y123 films prepared by quenching after holding at 760 °C for 100 and 180 min, respectively. In the film held at 760 °C for 100 min, the c-axis oriented Y123 film covered almost all over the buffer layer. However, reaction product phases such as BaCeO3 were not observed at the interface. In contrast, in the film held at 760 °C for 180 min, the BaCeO3 phase was detected in the grain boundaries of CeO2. The BaCeO3 at the grain boundaries in the CeO2 buffer grows with increasing of the holding time in the crystallization. Fig. 6 shows the relationship between the thickness of the BaCeO3 layer and the holding time in the crystallization. As shown in this figure, the BaCeO3 thickness is almost invariable about 0.2 lm, until the holding time reaches 1000 min. Therefore, the reaction of Y123 with CeO2 does not much influence on decreasing of Jc values. Actually, we have recently obtained the 2 lm-thick Y123 film with higher Ic value of 413 A by heattreating the sample for about 700 min in the crystallization. On the other hand, Jc values tend to decrease more rapidly with increasing the holding time longer than 1000 min. We should investigate the effect of crystallization time on Ic values and the growth mechanism of BaCeO3 in more detail.
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4. Conclusion Y123 quenched and fully crystallized films were fabricated by the conventional and the advanced TFA-MOD processes. Then, microstructures of the films were investigated by means of TEM, in order to understand the initial stage of the Y123 growth. As a result, it was found that Y–Ba– O–F grains firstly nucleate both on LAO single crystals and CeO2 buffered metal substrates in the TFA-MOD processes. Y–Ba–O–F is grown with the epitaxial orientation relationships to the substrate. Furthermore, the BaCeO3 reaction product layer is formed due to Ba diffusion from Y123 to CeO2 through the grain boundaries. At the crystallization temperature of 760 °C, the diffusion rate of Ba is slow. Therefore, it is found that the thickness of BaCeO3 does not increase much further with increasing the holding time until about 1000 min.
Acknowledgements
Fig. 5. Cross-sectional TEM images of Y123 quenched films prepared by cooling rapidly after holding at 760 °C for (a) 100 min and (b) 180 min, respectively.
This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) as Collaborative Research and Development of Fundamental Technologies for Superconductivity Applications.
References
Fig. 6. Relationship between thickness of BaCeO3 layer and holding time in crystallization.
[1] P.C. McIntyre, M.J. Cima, J.A. Smith Jr., R.B. Hallock, M.P. Siegal, J.M. Phillips, J. Appl. Phys. 71 (1992) 1868. [2] J. Shibata, Y. Tokunaga, R. Teranishi, H. Fuji, T. Honjo, T. Izumi, Y. Shiohara, Physica C 392–396 (2003) 922. [3] Y. Tokunaga, H. Fuji, R. Teranishi, J. Matsuda, S. Asada, T. Honjo, T. Izumi, Y. Shiohara, Y. Iijima, T. Saitoh, T. Goto and A. Yajima, Proceedings of EUCAS 2003, 1756. [4] T. Honjo, Y. Nakamura, R. Teranishi, H. Fuji, J. Shibata, T. Izumi, Y. Shiohara, IEEE Trans. Appl. Supercond. 13 (2003) 2516. [5] Vyacheslav, F. Solovyov, Harold J. Wiesmann, Li-jun Wu, Y. Zhu, M. Suenaga, Appl. Phys. Lett. 76 (2000) 1911. [6] Vyacheslav, F. Solovyov, Harold J. Wiesmann, M. Suenaga, Physica C 353 (2001) 14. [7] J. Shibata, T. Honjo, H. Fuji, T. Araki, I. Hirabayashi, T. Hirayama, T. Izuni, Y. Shiohara, T. Yamamoto, Y. Ikuhara, J. Mater. Res. 17 (2002) 1266.
Transmission electron microscopic studies on crystallization of YBa2Cu3O7y films deposited by advanced TFA-MOD method J.S. Matsuda a,*, Y. Tokunaga a,1, K. Nakaoka a, R. Teranishi a, Y. Aoki a, H. Fuji a, A. Yajima b, Y. Yamada c, T. Izumi a, Y. Shiohara a a
Superconductivity Research Laboratory, ISTEC, Shinonome 1-10-13, Koto-ku, Tokyo 135-0062, Japan b Asahi Denka Kogyo Co. Ltd., Higashi-Ogu 7-2-35, Arakawa-ku, Tokyo 116-8553, Japan c Superconductivity Research Laboratory, ISTEC, Mutsuno 2-4-1, Atsuta-ku, Nagoya 456-8587, Japan Received 23 November 2004; accepted 26 January 2005 Available online 25 July 2005
Abstract We have prepared Y123 quenched and fully crystallized films by the conventional and the advanced TFA-MOD processes. Then, we have investigated microstructures of the films by means of transmission electron microscopy, in order to understand the initial stage of the Y123 growth. As a result, it is found that Y–Ba–O–F grains firstly nucleate on LaAlO3 single crystals as well as on CeO2 buffered metal substrates both in the TFA-MOD processes. The Y–Ba– O–F is grown with epitaxially orientated relationship to the substrate. A BaCeO3 reaction product layer is formed due to Ba diffusion from Y123 to CeO2 mainly through the grain boundaries. At the crystallization temperature of 760 °C, the diffusion rate of Ba is slow. Therefore, it was found that the thickness of the BaCeO3 layer does not increase much further with increasing the holding time until about 1000 min. Ó 2005 Elsevier B.V. All rights reserved. PACS: 74.72.B; 61.16.B Keywords: Metal organic deposition; Nucleation; Transmission electron microscopy
1. Introduction *
Corresponding author. Tel.: +81 3 3536 5711; fax: +81 3 3536 5717. E-mail address: [email protected] (J.S. Matsuda). 1 Present address: R&D Center, Tokyo Electric Power Company, 4-1 Egasaki-cho, Tsurumi-ku, Yokohama 2308510, Japan.
Metal organic deposition using trifluoroacetates (TFA-MOD) is one of the most promising methods for producing coated conductors, since it enables us to fabricate them with high quality at
0921-4534/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2005.01.086
1052
J.S. Matsuda et al. / Physica C 426–431 (2005) 1051–1055
a low cost. In this method, superconducting films are formed by coating substrates with the solution containing TFA salts, and the two heat-treatments for both calcination and crystallization. In the case of the MOD process using the TFA salts for all cationic elements, the calcination process requires a long time about ten hours for a single coated film [1,2]. In order to shorten the calcination time, we have developed a new combination of starting materials [3]. We call the method using the new solution ‘‘the advanced TFA-MOD method’’. Generally, in the MOD process, heat-treatment conditions for calcination and crystallization have a great influence on microstructures and superconducting properties of the films. As for TFA-MOD process, Honjo et al. have derived a formula relating the partial pressure of water vapor (PH2O) in the crystallization process to the growth rate of YBa2Cu3O7y (Y123) films [4]. It was also reported that the growth rate of Y123 films deposited by the BaF2 ex-situ process is proportional to the square root of the PH2O by Vyacheslav et al. [5,6]. In addition, the rate limiting stage in the overall process for the Y123 growth is considered to be the HF-removal rate in these BaF2 processes. However, there are only a little understanding for reactions and mechanism of the Y123 formation. Therefore, in this study we focus on the interfacial structure evolution and the initial stage in the growth of Y123 films deposited by the conventional and the advanced TFA-MOD processes. By means of transmission electron microscopy (TEM), we have investigated the microstructures of the Y123 quenched and fully crystallized films, in order to understand the Y123 growth mechanism and to contribute to obtain the Y123 films with higher IC.
2. Experimental procedure The Y123 precursor and fully crystallized films were fabricated on LaAlO3(LAO)(0 0 1) single crystals or PLD–CeO2/IBAD–Gd2Zr2O7/Hastelloy tapes by the TFA-MOD methods. LAO substrates and the buffered metal tapes were spin-coated or dip-coated with a solution contain-
ing Y-, Ba-TFA salts and a Cu-naphthenate. Then, these films were heat-treated to 400 °C in an O2 gas flow with PH2O of 2.1%. In the case of using CeO2/Gd2Zr2O7/Hastelloy, we repeated the spin-coating and the calcination processes several times in order to fabricate thick precursor films; it is called ‘‘multi-coating’’. Finally, these precursor films were heated at 775 °C or 760 °C in an Ar/O2 gas flow with PH2O of 4.2% or 6.3%. Y123 quenched films were prepared by cooling rapidly during heating up to the crystallization temperature or after holding at the temperature for various times. TEM specimens were prepared by ion-milling. We used JEM-2010F operating at 200 kV and JEM-4000EX operating at 400 kV for microstructural observations.
3. Results and discussion 3.1. Y123 nucleation on LAO single crystal We have prepared Y123 quenched films by cooling rapidly during crystallization, and observed the films by means of TEM. Fig. 1(a) and (b) show cross-sectional TEM images of the Y123 film prepared by cooling rapidly at 650 °C during heating up for the crystallization in the conventional TFA-MOD process. As shown in these images, crystal nuclei different from the Y123 were crystallized on the LAO substrate. In the high-resolution TEM (HRTEM) image, the lattice spacing parallel to (0 0 1)LAO was 0.52 nm, and that parallel to (1 0 0)LAO was 0.38 nm. From energy dispersive X-ray spectroscopy (EDS) analysis, Y, Ba, O and F were detected in these crystal nuclei. Through these results, it was found that the island-shaped nuclei crystallized on the surface of the LAO substrate are mainly YOF–Ba solid solution. The Y–Ba–O–F is epitaxially crystallized, the orientation relationship is as follows: ½1 1 0YBOF==½1 0 0LAO;
ð0 0 1ÞYBOF==ð0 0 1ÞLAO
In this case, the misfit parameter between the Y–Ba–O–F and the LAO single crystal substrate is about 0.9%. On the other hand, the misfit
J.S. Matsuda et al. / Physica C 426–431 (2005) 1051–1055
Fig. 1. Cross-sectional TEM images of Y123 quenched film prepared by cooling rapidly from 650 °C during crystallization in the conventional TFA-MOD process: (a) low-magnification image and (b) high-resolution image obtained in the vicinity of the interface between the Y123 precursor film and the LAO substrate.
parameter between (1 0 0)Y123 and (1 0 0)LAO is 2.4%. Therefore, we suggest that the lattice matching of Y–Ba–O–F to LAO makes it easy to nucleate on the substrate during crystallization. However, we should investigate the dependence of the interfacial energies on the temperature and the phase stabilities in the total free energy change. Fig. 2(a) and (b) show the cross-sections of the Y123 film prepared by quenching at 730 °C in the advanced TFA-MOD process. Island-shaped Y– Ba–O–F grains were crystallized as well as in the film deposited by the conventional TFA-MOD. We suggest that the nucleation of Y–Ba–O–F occurs when we use Y, Ba-TFA as starting materials. Fig. 3(a)–(c) show the cross-sections of the Y123 film prepared by quenching at 775 °C in the advanced TFA-MOD process. In this film, the growth of the Y123 progresses, however, residuals of Y–Ba–O–F in the Y123 crystals were observed in the vicinity of the interface. As shown in Fig. 3(c), in the amorphous matrix including microcrystalline at the growth front, non-superconducting inclusions such as BaF2, CuO and Y2Cu2O5 were confirmed to exist. These were identified from EDS analysis and measurements of lattice spacings in HRTEM images [7].
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Fig. 2. Cross-sectional TEM images of Y123 quenched film prepared by cooling rapidly from 730 °C during crystallization in the advanced TFA-MOD process: (a) low-magnification image and (b) high-resolution image obtained in the vicinity of the interface between the precursor film and the LAO substrate.
Fig. 3. Cross-sectional TEM images of Y123 quenched film prepared by cooling rapidly from 775 °C during crystallization in the advanced TFA-MOD process: (a) low-magnification image; (b) high-resolution image in the vicinity of the interface between the film and the substrate and (c) high-resolution image at the growth front of the Y123 film.
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From above-mentioned results, two different kinds of reaction processes for Y123 formation are suggested as follows: ðIÞ Y Ba O F þ xBaF2 þ CuO þ H2 O ! YBa2 Cu3 O7y þ HF ðIIÞ 1=2Y2 Cu2 O5 þ 2BaF2 þ 2CuO þ 2H2 O ! YBa2 Cu3 O7y þ 4HF Y–Ba–O–F crystal nuclei are observed only in the initial stage of the Y123 growth. The Y–Ba– O–F grains disappear due to the reaction to form Y123. The rate limiting reaction of the Y123 growth is considered to be in the stage of HF gas removal. Therefore, identifications of these reactions for Y123 formation may not be significantly necessary from a viewpoint of controlling the growth rate of the Y123 film. However, it is important to understand the mechanism of Y123 crystal nucleation and crystallization, in order to prevent crack formation in the films and to fabricate thicker films in long tapes. 3.2. Y123 nucleation on CeO2 buffer layer Fig. 4(a) and (b) show the cross-sectional TEM images of Y123 films prepared by quenching after holding at 760 °C for 10 and 20 min, respectively. As shown in these figures, nucleation of Y–Ba– O–F crystals has occurred in the film quenched after holding at 760 °C for 10 min. Y–Ba–O–F is epitaxially grown on a CeO2 buffer layer with the lattice misfit parameter of 0.7%. ½1 0 0YBOF==½1 0 0CeO2 ;
ð0 0 1ÞYBOF==ð0 0 1ÞCeO2
After holding at 760 °C for 20 min, the Y123 nucleation occurred on the CeO2 buffer. The orientation relationship between the Y123 film and the CeO2 buffer with the lattice misfit parameter of 0.8% is as follows: ½1 0 0Y123==½1 0 0CeO2 ;
ð0 0 1ÞY123==ð0 0 1ÞCeO2
The nucleation rate of Y123 film deposited on CeO2 buffer tends to be slow compared with that of the film on LAO. We should investigate effects of bufferÕs crystallinity and the surface roughness on the growth rate in detail.
Fig. 4. Cross-sectional TEM images of Y123 quenched films prepared by cooling rapidly after holding at 760 °C for (a) 10 min and (b) 20 min, respectively. These films were deposited on CeO2 buffered metal substrates.
Fig. 5(a) and (b) show the cross-sections of Y123 films prepared by quenching after holding at 760 °C for 100 and 180 min, respectively. In the film held at 760 °C for 100 min, the c-axis oriented Y123 film covered almost all over the buffer layer. However, reaction product phases such as BaCeO3 were not observed at the interface. In contrast, in the film held at 760 °C for 180 min, the BaCeO3 phase was detected in the grain boundaries of CeO2. The BaCeO3 at the grain boundaries in the CeO2 buffer grows with increasing of the holding time in the crystallization. Fig. 6 shows the relationship between the thickness of the BaCeO3 layer and the holding time in the crystallization. As shown in this figure, the BaCeO3 thickness is almost invariable about 0.2 lm, until the holding time reaches 1000 min. Therefore, the reaction of Y123 with CeO2 does not much influence on decreasing of Jc values. Actually, we have recently obtained the 2 lm-thick Y123 film with higher Ic value of 413 A by heattreating the sample for about 700 min in the crystallization. On the other hand, Jc values tend to decrease more rapidly with increasing the holding time longer than 1000 min. We should investigate the effect of crystallization time on Ic values and the growth mechanism of BaCeO3 in more detail.
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4. Conclusion Y123 quenched and fully crystallized films were fabricated by the conventional and the advanced TFA-MOD processes. Then, microstructures of the films were investigated by means of TEM, in order to understand the initial stage of the Y123 growth. As a result, it was found that Y–Ba– O–F grains firstly nucleate both on LAO single crystals and CeO2 buffered metal substrates in the TFA-MOD processes. Y–Ba–O–F is grown with the epitaxial orientation relationships to the substrate. Furthermore, the BaCeO3 reaction product layer is formed due to Ba diffusion from Y123 to CeO2 through the grain boundaries. At the crystallization temperature of 760 °C, the diffusion rate of Ba is slow. Therefore, it is found that the thickness of BaCeO3 does not increase much further with increasing the holding time until about 1000 min.
Acknowledgements
Fig. 5. Cross-sectional TEM images of Y123 quenched films prepared by cooling rapidly after holding at 760 °C for (a) 100 min and (b) 180 min, respectively.
This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) as Collaborative Research and Development of Fundamental Technologies for Superconductivity Applications.
References
Fig. 6. Relationship between thickness of BaCeO3 layer and holding time in crystallization.
[1] P.C. McIntyre, M.J. Cima, J.A. Smith Jr., R.B. Hallock, M.P. Siegal, J.M. Phillips, J. Appl. Phys. 71 (1992) 1868. [2] J. Shibata, Y. Tokunaga, R. Teranishi, H. Fuji, T. Honjo, T. Izumi, Y. Shiohara, Physica C 392–396 (2003) 922. [3] Y. Tokunaga, H. Fuji, R. Teranishi, J. Matsuda, S. Asada, T. Honjo, T. Izumi, Y. Shiohara, Y. Iijima, T. Saitoh, T. Goto and A. Yajima, Proceedings of EUCAS 2003, 1756. [4] T. Honjo, Y. Nakamura, R. Teranishi, H. Fuji, J. Shibata, T. Izumi, Y. Shiohara, IEEE Trans. Appl. Supercond. 13 (2003) 2516. [5] Vyacheslav, F. Solovyov, Harold J. Wiesmann, Li-jun Wu, Y. Zhu, M. Suenaga, Appl. Phys. Lett. 76 (2000) 1911. [6] Vyacheslav, F. Solovyov, Harold J. Wiesmann, M. Suenaga, Physica C 353 (2001) 14. [7] J. Shibata, T. Honjo, H. Fuji, T. Araki, I. Hirabayashi, T. Hirayama, T. Izuni, Y. Shiohara, T. Yamamoto, Y. Ikuhara, J. Mater. Res. 17 (2002) 1266.