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Development of HoBCO coated conductor by PLD method
Physica C 412–414 (2004) 931–936 www.elsevier.com/locate/physc
Development of HoBCO coated conductor by PLD method S. Hahakura *, K. Fujino, M. Konishi, K. Ohmatsu Energy and Environmental Technology Research Laboratories, Sumitomo Electric Industries, Ltd., Shimaya 1-1-3, Konohana-ku, Osaka, Osaka 554-0024, Japan Received 29 October 2003; accepted 15 December 2003 Available online 24 June 2004
Abstract We have developed RE-123 (RE: rare earth element) thin films using pulsed laser deposition (PLD) method, and adopted HoBa2 Cu3 Oy (HoBCO) as a RE-123 superconducting layer. HoBCO shows stable and high critical current density (Jc ) over 3 MA/cm2 in the case deposited on single crystal substrates such as sapphire and LAO. In this work, based on our PLD technique, HoBCO coated conductors have been developed on flexible metal textured substrates. Structure of oxide multi-buffer layers was investigated and fine crystal orientation of delta phi value below 10 was obtained by hetero-epitaxial growth. Furthermore, 10 m long multi-buffer tape was fabricated by continuous reel-toreel system, and the uniformity of crystal alignment and surface flatness was confirmed. Fabricated HoBCO tape also showed excellent in-plane alignment and smooth surface morphology. Jc at 77 K, in self-field of HoBCO superconducting films were around 1 MA/cm2 . 2004 Elsevier B.V. All rights reserved. PACS: 74.76.Bz; 85.25.Kx Keywords: Superconducting film; Pulse laser deposition; HoBCO
1. Introduction Recently, next generation coated conductors, which consist of RE1 Ba2 Cu3 O7x (RE: rare earth element and Y) thin films and flexible metal tape substrates, have been developed for electrical power equipments such as proto-type underground cable and magnet [1,2]. High temperature superconducting (HTS) thin films on single crystal substrates have also been developed for electrical
*
Corresponding author. Tel.: +81-6-6466-5639; fax: +81-66466-5705. E-mail address: [email protected] (S. Hahakura).
power devices such as microwave filter [3] and SN transition type fault current limiter (FCL) [4]. In our group, large-size HoBCO thin films on sapphire substrates with high and homogeneous critical current density (Jc ) values have been developed by 2-dimentional scanning PLD method [4]. The distribution of Jc measured by the inductive method at 77 K and self-field was almost uniform, ranging from 3.3 to 4.5 MA/cm2 . Based on our PLD technique, HoBCO coated conductors have been prepared on tape substrates [5,6]. We previously reported some merits of HoBCO than YBCO from the viewpoint of application [3,7]. Firstly, high rate deposition of HoBCO was
0921-4534/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2003.12.086
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precisely inspected using pulse laser deposition method, and revealed that the deposition rate of HoBCO reached to 5 lm/min., which was about twice faster than that of YBCO [7]. The HoBCO also showed excellent superconducting properties as same as YBCO [7]. Secondly, HoBCO thin film on a single crystal showed better durable, which means less Jc degradation, than YBa2 Cu3 Oy (YBCO) in the ambience of high temperature and high humidity [3]. Considering future mass production, high speed and long length are desired, so we installed largesize PLD equipment [7]. The PLD equipment has productive capacity to fabricate long coated conductor of more than 100 m by industrial laser with 200 W (1 J · 200 Hz) and continuous reel-to-reel system.
2. Experimental Multi-layer buffer layers and HoBCO superconducting layer were deposited on textured substrates, with the cross section of 1 cm in width and 0.1 mm in thickness, using PLD method. A KrF excimer laser with a wavelength of 248 nm was used for the film deposition. Critical current (Ic ) was measured at 77 K, in self-field by DC four-probe method, and a criterion of 1 lV/cm was used. Critical current density (Jc ) was measured by both transport method and contact-less inductive method. Standard X-ray diffraction (XRD) was utilized to evaluate crystal orientation of the thin film, and the microstructure was observed using scanning electron microscope (SEM) and atomic force microscope (AFM).
3. Results and discussion 3.1. Buffer layer on the textured substrate A 0.1 mm thickness textured (0 0 1) Ni alloy tape with the delta phi value of 9–12 was used as the substrates. Textured substrates were highly mirror-polished by using mechanochemical polishing to the level of surface roughness (Ra) of less than 5 nm. To achieve high Jc superconducting layer, it becomes very important for the surface of the buffer layer to satisfy both fine crystal alignment and flatness, simultaneously. So buffer layer structure was precisely investigated and several kinds of the combination of multi-buffer layer materials were tried by PLD method. Fig. 1 shows in-plane phi scan pattern of homo-epitaxial buffer layers. In this case, both 1st and 2nd layers were YSZ. On the other hand, Fig. 2 shows in-plane phi scan pattern of hetero-epitaxial buffer layers. In this structure, 1st and 2nd layers consisted CeO2 and YSZ, respectively. As is seen in these figures, fine (0 0 1) oriented epitaxial growth with the delta phi value of 9–10 was obtained for the heteroepitaxial buffer layers with the structure of YSZ/ CeO2 /Ni alloy. These results imply that CeO2 easily epitaxially grow as a seed layer on the textured substrate than YSZ. AFM analysis of YSZ 2nd buffer layer was investigated and the surface roughness (Ra) measured in a scan area of 20*20 lm2 was around 5 nm. This indicated the surface was almost flat as the same level of substrate. 3.2. Long length buffer layer Based on these basic results of buffer layer, we have developed 10 m long multi-layer buffer films
Fig. 1. X-ray phi scan pattern of homo-epitaxial buffer layer; 1st buffer layer (YSZ) and 2nd buffer layer (YSZ).
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Fig. 2. X-ray phi scan pattern of hetero-epitaxial buffer layer; 1st buffer layer (YSZ) and 2nd buffer layer (CeO2 ).
Table 1 Parameters of 10 m multi-buffer layers Parameters Material Tape speed Thickness Length
1st buffer layer
2nd buffer layer
3rd buffer layer
CeO2 7.7 m/H 150 nm 10 m
YSZ 0.9 m/H 2 lm 10 m
CeO2 Stationary 40 nm Short
longitudinal distribution of the ratio of (0 0 1) orientation, calculated by X-ray diffraction peak intensity ratio of (200)/((200)+(111)). The ratio of (0 0 1) orientation for both YSZ 2nd layer and CeO2 3rd buffer layer exceeded 90% over 10 m long tape. In considering the epitaxial crystal growth, crystal structure becomes very important. Both YSZ and CeO2 have crystal structure of cubic system. Therefore, this result indicates buffer layer materials were mainly c-axis orientated. Furthermore, the ratio of (0 0 1) orientation of the 3rd buffer layer slightly increased than that of the 2nd buffer layer that implies the effect of developing the epitaxial growth of CeO2 . In-plane texture measurement over 10 m buffer tape is shown in Fig. 4. In-plane texture of the 2nd layer of YSZ (111) over 10 m varied from 12 to 16. However, that of 3rd layer of CeO2 (111) decreased to the value of 10 to 12. From this result, CeO2 cap layer also showed the effect to develop texturing degree.
(200)/ {(200)+(111)} (%)
on the Ni alloy textured tapes using large-size pulse laser deposition equipment as is mentioned above. The parameters of each buffer layers were summarized in Table 1. 10 m long buffer layer consists of 3 multi-layers of CeO2 1st layer, YSZ 2nd layer, and CeO2 3rd layer. As is mentioned above, CeO2 1st layer was grown as a seed layer on the textured substrate. YSZ 2nd layer was grown to prevent the inter-diffusion between the substrate and superconducting layer. Both 1st and 2nd buffer layers were formed by continuous deposition. 1st buffer layer of 150 nm thickness CeO2 was deposited at the high moving speed of 7.7 m/H. Furthermore, 2nd buffer layer of 2 lm thickness YSZ was deposited at the moving speed of 0.9 m/ H. From the viewpoint of high production speed, the moving speed of CeO2 is promising, however, that of YSZ is still low. Further inspection to optimize the film thickness of 2nd buffer layer will be necessary to increase the production speed. On the other hand, thin 3rd buffer layer of 40 nm thickness CeO2 was formed by stationary deposition as a cap layer to investigate the property of HoBCO superconducting layer. As for the crystal growth of buffer layer, it is necessary to show both fine (0 0 1) orientated epitaxial growth and in-plane alignment. Fig. 3 shows
100 90 80 70 60 50 40 30 20 10 0
YSZ: 2nd buffer CeO2: 3rd buffer
0
2
4 6 Length (m)
8
10
Fig. 3. Longitudinal distribution of the ratio of (0 0 1) orientation, calculated by X-ray diffraction peak ratio of (2 0 0)/ ((2 0 0)+(1 1 1)).
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Ave. FWHM (deg.)
30 YSZ: 2nd buffer CeO2: 3rd buffer
25 20 15 10 5 0 0
2
4 6 Length (m)
8
10
Fig. 4. In-plane texture measurement over 10 m buffer tape.
Moreover, AFM and SEM analysis were employed to investigate surface morphology of the buffer films. Fig. 5(a) and (b) show the AFM
Fig. 6. Surface SEM images of (a) YSZ 2nd buffer and (b) CeO2 3rd buffer.
images of YSZ 2nd buffer layer and CeO2 3rd buffer layer, respectively. Surface roughness (Ra) measured in the scan area of 5 · 5 lm2 was 1.5 and 6.3 nm for YSZ and CeO2 , respectively that indicated the both surfaces were very flat as the same level of substrate. Surface SEM images of YSZ 2nd buffer layer and CeO2 3rd buffer layer are shown in Fig. 6(a) and (b). It can be seen that both of them indicated excellent smooth surface morphology with no particles, corresponding to the results of AFM analysis. 3.3. HoBCO films on buffered textured substrate Fig. 5. AFM image of (a) YSZ 2nd buffer and (b) CeO2 3rd buffer.
HoBCO films were deposited on multi-buffered textured substrates by PLD method. Experiments
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Table 2 Parameters during the HoBCO deposition Parameters Laser pulse energy Laser repetition rate Heater temperature O2 atmosphere HoBCO film thickness
600 mJ 20–150 Hz 700–800 C 100–200 mTorr 0.23–1.0 lm
were carried out using a small-size PLD system that consisted of an eximer laser with the maximum power of 100 W and small chamber. The main parameters of HoBCO deposition are summarized in Table 2. Heater temperature was 700– 800 C, where the oxygen pressure was 100–200 mTorr. HoBCO films with the thickness from 0.23 to 1.0 lm were prepared. Sample A with 0.54 lm in thickness showed Ic of 30 A cmw at 77 K, in self-field, which corresponds to the Jc of 0.58 MA/cm2 . Sample B with 0.23 lm in thickness showed Ic of 18 A cmw at 77 K, in self-field, which corresponds to the Jc of 0.9 MA/cm2 . Jc versus full width at half maximum (FWHM) of HoBCO layer is shown in Fig. 7. Jc increased with the improvement of FWHM value. The sample with Jc of 0.9 MA/cm2 showed FWHM of 8.6. This trend in Fig. 7 implies that further improvement of in-plane alignment is necessary to acquire Jc property over 1 MA/cm2 . Surface morphology of HoBCO layer was also investigated by SEM analysis. In this experiment, the dependence of laser energy density on the 1.0
Jc (MA/cm2)
0.8 0.6 0.4 0.2 0.0 8
9
10
11
12
13
FWHM (°)
Fig. 7. X-ray (1 0 3) pole figure of HoBCO layer on buffered textured substrate by PLD method.
Fig. 8. SEM images of HoBCO layer on buffered textured substrate by PLD method. (a) The surface morphology when the laser energy density was 2.0 J/cm2 (Jc < 0:1 MA/cm2 ), (b) The surface morphology when the laser energy density was 2.5 J/cm2 (Jc ¼ 0:9 MA/cm2 ), (c) The surface morphology when the laser energy density was 3.7 J/cm2 (Jc ¼ 0:2 MA/cm2 ).
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target was inspected as one of the most important factors of PLD by varied the illuminated size on the target. Laser energy density on the target was varied from 2.0 to 3.7 J/cm2 . Fig. 8(a), (b) and (c) show SEM images of HoBCO layer on buffered textured substrate by PLD method. Each photograph shows the surface morphology of the sample irradiated by the laser energy densities of 2.0, 2.5, and 3.7 J/cm2 . And Jc of each sample was less than 0.1, 0.9, and 0.2 MA/cm2 , respectively. In the case of 3.7 J/cm2 (Fig. 8(c)), a lot of particles were obviously recognized and the surface looked bumpy. From this result, it could be guessed that the energy density was so strong that crystal growth at the film surface became drastic. On the other hand, in the cases of lower energy densities of 2.0–2.5 J/cm2 (Fig. 8(a) and (b)), the features of the films revealed relatively smooth. However, even on the sample that showed Jc of 0.9 MA/cm2 , some particles were still observed (Fig. 8(b)). These particles are considered as the one reason of suppressing Jc of HoBCO layer. Therefore, the Jc of HoBCO is expected to improve by leveling the surface flatness. Furthermore, it can be said that laser energy density could be one of the most important factor during the pulse laser deposition.
4. Conclusion HoBCO coated conductors have been developed on flexible metal textured substrates. Both buffer layer and HoBCO layer were fabricated by using PLD method. Multi-buffer layer structure on the textured substrate was investigated, and fine crystal alignment was obtained in the case of the hetero-epitaxial growth of CeO2 and YSZ. Furthermore, 10 m long multi-buffer tape was fabricated by continuous reel-to-reel system. In the top
surface layer of CeO2 , the uniformity of in-plane crystal alignment around 10 and surface flatness around Ra of 6 nm was obtained. HoBCO coated conductors on multi-buffered textured substrates have been developed. HoBCO superconducting layer showed fine in-plane alignment of FWHM ¼ 8.6 and relative smooth surface morphology. HoBCO film showed high Jc around 1 MA/cm2 . Acknowledgements A part of this work was supported by the New Energy and Industrial Technology Development Organization (NEDO) as Collaborative Research and Development of Fundamental Technology for Superconductivity applications.
References [1] K. Fujino, K. Muranaka, T. Taneda, K. Ohmatsu, H. Takei, Y. Sato, K. Matsuo, Y. Takahashi, Extended abstract of 2001 International Workshop on Superconductivity, Hawaii, 2001, pp. 61–64. [2] K. Fujino, S. Hahakura, K. Ohmatsu, H. Takei, Y. Sato, S. Honjo, Y. Iwata, SEI Technical Review 49 (2000) 182. [3] S. Hahakura, K. Fujino, K. Ohmatsu, H. Takei, IEE Japan B 10 (2001) 1339. [4] K. Ohmatsu, S. Hahakura, H. Takei, Y. Ozawa, in: Proceedings of IEEE/PES T & D 2002 Asia Pacific, Yokohama, 2002, pp. 2318–2321. [5] S. Hahakura, K. Fujino, M. Konishi, K. Ohmatsu, Extended abstract of International Workshop on Coated Conductors for Applications (CCA 2003), Lago d’Orta, 2003. [6] S. Hahakura, K. Fujino, M. Konishi, K. Ohmatsu, in: Proceedings of 6th European Conference on Applied Superconductivity (EUCAS 2003), Sorrento, 2003, to be published. [7] K. Fujino, M. Konishi, K. Muranaka, S. Hahakura, K. Ohmatsu, K. Hayashi, N. Hobara, S. Honjo, Y. Takahashi, Physica C 392–396 (2003) 815.
Development of HoBCO coated conductor by PLD method S. Hahakura *, K. Fujino, M. Konishi, K. Ohmatsu Energy and Environmental Technology Research Laboratories, Sumitomo Electric Industries, Ltd., Shimaya 1-1-3, Konohana-ku, Osaka, Osaka 554-0024, Japan Received 29 October 2003; accepted 15 December 2003 Available online 24 June 2004
Abstract We have developed RE-123 (RE: rare earth element) thin films using pulsed laser deposition (PLD) method, and adopted HoBa2 Cu3 Oy (HoBCO) as a RE-123 superconducting layer. HoBCO shows stable and high critical current density (Jc ) over 3 MA/cm2 in the case deposited on single crystal substrates such as sapphire and LAO. In this work, based on our PLD technique, HoBCO coated conductors have been developed on flexible metal textured substrates. Structure of oxide multi-buffer layers was investigated and fine crystal orientation of delta phi value below 10 was obtained by hetero-epitaxial growth. Furthermore, 10 m long multi-buffer tape was fabricated by continuous reel-toreel system, and the uniformity of crystal alignment and surface flatness was confirmed. Fabricated HoBCO tape also showed excellent in-plane alignment and smooth surface morphology. Jc at 77 K, in self-field of HoBCO superconducting films were around 1 MA/cm2 . 2004 Elsevier B.V. All rights reserved. PACS: 74.76.Bz; 85.25.Kx Keywords: Superconducting film; Pulse laser deposition; HoBCO
1. Introduction Recently, next generation coated conductors, which consist of RE1 Ba2 Cu3 O7x (RE: rare earth element and Y) thin films and flexible metal tape substrates, have been developed for electrical power equipments such as proto-type underground cable and magnet [1,2]. High temperature superconducting (HTS) thin films on single crystal substrates have also been developed for electrical
*
Corresponding author. Tel.: +81-6-6466-5639; fax: +81-66466-5705. E-mail address: [email protected] (S. Hahakura).
power devices such as microwave filter [3] and SN transition type fault current limiter (FCL) [4]. In our group, large-size HoBCO thin films on sapphire substrates with high and homogeneous critical current density (Jc ) values have been developed by 2-dimentional scanning PLD method [4]. The distribution of Jc measured by the inductive method at 77 K and self-field was almost uniform, ranging from 3.3 to 4.5 MA/cm2 . Based on our PLD technique, HoBCO coated conductors have been prepared on tape substrates [5,6]. We previously reported some merits of HoBCO than YBCO from the viewpoint of application [3,7]. Firstly, high rate deposition of HoBCO was
0921-4534/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2003.12.086
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precisely inspected using pulse laser deposition method, and revealed that the deposition rate of HoBCO reached to 5 lm/min., which was about twice faster than that of YBCO [7]. The HoBCO also showed excellent superconducting properties as same as YBCO [7]. Secondly, HoBCO thin film on a single crystal showed better durable, which means less Jc degradation, than YBa2 Cu3 Oy (YBCO) in the ambience of high temperature and high humidity [3]. Considering future mass production, high speed and long length are desired, so we installed largesize PLD equipment [7]. The PLD equipment has productive capacity to fabricate long coated conductor of more than 100 m by industrial laser with 200 W (1 J · 200 Hz) and continuous reel-to-reel system.
2. Experimental Multi-layer buffer layers and HoBCO superconducting layer were deposited on textured substrates, with the cross section of 1 cm in width and 0.1 mm in thickness, using PLD method. A KrF excimer laser with a wavelength of 248 nm was used for the film deposition. Critical current (Ic ) was measured at 77 K, in self-field by DC four-probe method, and a criterion of 1 lV/cm was used. Critical current density (Jc ) was measured by both transport method and contact-less inductive method. Standard X-ray diffraction (XRD) was utilized to evaluate crystal orientation of the thin film, and the microstructure was observed using scanning electron microscope (SEM) and atomic force microscope (AFM).
3. Results and discussion 3.1. Buffer layer on the textured substrate A 0.1 mm thickness textured (0 0 1) Ni alloy tape with the delta phi value of 9–12 was used as the substrates. Textured substrates were highly mirror-polished by using mechanochemical polishing to the level of surface roughness (Ra) of less than 5 nm. To achieve high Jc superconducting layer, it becomes very important for the surface of the buffer layer to satisfy both fine crystal alignment and flatness, simultaneously. So buffer layer structure was precisely investigated and several kinds of the combination of multi-buffer layer materials were tried by PLD method. Fig. 1 shows in-plane phi scan pattern of homo-epitaxial buffer layers. In this case, both 1st and 2nd layers were YSZ. On the other hand, Fig. 2 shows in-plane phi scan pattern of hetero-epitaxial buffer layers. In this structure, 1st and 2nd layers consisted CeO2 and YSZ, respectively. As is seen in these figures, fine (0 0 1) oriented epitaxial growth with the delta phi value of 9–10 was obtained for the heteroepitaxial buffer layers with the structure of YSZ/ CeO2 /Ni alloy. These results imply that CeO2 easily epitaxially grow as a seed layer on the textured substrate than YSZ. AFM analysis of YSZ 2nd buffer layer was investigated and the surface roughness (Ra) measured in a scan area of 20*20 lm2 was around 5 nm. This indicated the surface was almost flat as the same level of substrate. 3.2. Long length buffer layer Based on these basic results of buffer layer, we have developed 10 m long multi-layer buffer films
Fig. 1. X-ray phi scan pattern of homo-epitaxial buffer layer; 1st buffer layer (YSZ) and 2nd buffer layer (YSZ).
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Fig. 2. X-ray phi scan pattern of hetero-epitaxial buffer layer; 1st buffer layer (YSZ) and 2nd buffer layer (CeO2 ).
Table 1 Parameters of 10 m multi-buffer layers Parameters Material Tape speed Thickness Length
1st buffer layer
2nd buffer layer
3rd buffer layer
CeO2 7.7 m/H 150 nm 10 m
YSZ 0.9 m/H 2 lm 10 m
CeO2 Stationary 40 nm Short
longitudinal distribution of the ratio of (0 0 1) orientation, calculated by X-ray diffraction peak intensity ratio of (200)/((200)+(111)). The ratio of (0 0 1) orientation for both YSZ 2nd layer and CeO2 3rd buffer layer exceeded 90% over 10 m long tape. In considering the epitaxial crystal growth, crystal structure becomes very important. Both YSZ and CeO2 have crystal structure of cubic system. Therefore, this result indicates buffer layer materials were mainly c-axis orientated. Furthermore, the ratio of (0 0 1) orientation of the 3rd buffer layer slightly increased than that of the 2nd buffer layer that implies the effect of developing the epitaxial growth of CeO2 . In-plane texture measurement over 10 m buffer tape is shown in Fig. 4. In-plane texture of the 2nd layer of YSZ (111) over 10 m varied from 12 to 16. However, that of 3rd layer of CeO2 (111) decreased to the value of 10 to 12. From this result, CeO2 cap layer also showed the effect to develop texturing degree.
(200)/ {(200)+(111)} (%)
on the Ni alloy textured tapes using large-size pulse laser deposition equipment as is mentioned above. The parameters of each buffer layers were summarized in Table 1. 10 m long buffer layer consists of 3 multi-layers of CeO2 1st layer, YSZ 2nd layer, and CeO2 3rd layer. As is mentioned above, CeO2 1st layer was grown as a seed layer on the textured substrate. YSZ 2nd layer was grown to prevent the inter-diffusion between the substrate and superconducting layer. Both 1st and 2nd buffer layers were formed by continuous deposition. 1st buffer layer of 150 nm thickness CeO2 was deposited at the high moving speed of 7.7 m/H. Furthermore, 2nd buffer layer of 2 lm thickness YSZ was deposited at the moving speed of 0.9 m/ H. From the viewpoint of high production speed, the moving speed of CeO2 is promising, however, that of YSZ is still low. Further inspection to optimize the film thickness of 2nd buffer layer will be necessary to increase the production speed. On the other hand, thin 3rd buffer layer of 40 nm thickness CeO2 was formed by stationary deposition as a cap layer to investigate the property of HoBCO superconducting layer. As for the crystal growth of buffer layer, it is necessary to show both fine (0 0 1) orientated epitaxial growth and in-plane alignment. Fig. 3 shows
100 90 80 70 60 50 40 30 20 10 0
YSZ: 2nd buffer CeO2: 3rd buffer
0
2
4 6 Length (m)
8
10
Fig. 3. Longitudinal distribution of the ratio of (0 0 1) orientation, calculated by X-ray diffraction peak ratio of (2 0 0)/ ((2 0 0)+(1 1 1)).
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Ave. FWHM (deg.)
30 YSZ: 2nd buffer CeO2: 3rd buffer
25 20 15 10 5 0 0
2
4 6 Length (m)
8
10
Fig. 4. In-plane texture measurement over 10 m buffer tape.
Moreover, AFM and SEM analysis were employed to investigate surface morphology of the buffer films. Fig. 5(a) and (b) show the AFM
Fig. 6. Surface SEM images of (a) YSZ 2nd buffer and (b) CeO2 3rd buffer.
images of YSZ 2nd buffer layer and CeO2 3rd buffer layer, respectively. Surface roughness (Ra) measured in the scan area of 5 · 5 lm2 was 1.5 and 6.3 nm for YSZ and CeO2 , respectively that indicated the both surfaces were very flat as the same level of substrate. Surface SEM images of YSZ 2nd buffer layer and CeO2 3rd buffer layer are shown in Fig. 6(a) and (b). It can be seen that both of them indicated excellent smooth surface morphology with no particles, corresponding to the results of AFM analysis. 3.3. HoBCO films on buffered textured substrate Fig. 5. AFM image of (a) YSZ 2nd buffer and (b) CeO2 3rd buffer.
HoBCO films were deposited on multi-buffered textured substrates by PLD method. Experiments
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Table 2 Parameters during the HoBCO deposition Parameters Laser pulse energy Laser repetition rate Heater temperature O2 atmosphere HoBCO film thickness
600 mJ 20–150 Hz 700–800 C 100–200 mTorr 0.23–1.0 lm
were carried out using a small-size PLD system that consisted of an eximer laser with the maximum power of 100 W and small chamber. The main parameters of HoBCO deposition are summarized in Table 2. Heater temperature was 700– 800 C, where the oxygen pressure was 100–200 mTorr. HoBCO films with the thickness from 0.23 to 1.0 lm were prepared. Sample A with 0.54 lm in thickness showed Ic of 30 A cmw at 77 K, in self-field, which corresponds to the Jc of 0.58 MA/cm2 . Sample B with 0.23 lm in thickness showed Ic of 18 A cmw at 77 K, in self-field, which corresponds to the Jc of 0.9 MA/cm2 . Jc versus full width at half maximum (FWHM) of HoBCO layer is shown in Fig. 7. Jc increased with the improvement of FWHM value. The sample with Jc of 0.9 MA/cm2 showed FWHM of 8.6. This trend in Fig. 7 implies that further improvement of in-plane alignment is necessary to acquire Jc property over 1 MA/cm2 . Surface morphology of HoBCO layer was also investigated by SEM analysis. In this experiment, the dependence of laser energy density on the 1.0
Jc (MA/cm2)
0.8 0.6 0.4 0.2 0.0 8
9
10
11
12
13
FWHM (°)
Fig. 7. X-ray (1 0 3) pole figure of HoBCO layer on buffered textured substrate by PLD method.
Fig. 8. SEM images of HoBCO layer on buffered textured substrate by PLD method. (a) The surface morphology when the laser energy density was 2.0 J/cm2 (Jc < 0:1 MA/cm2 ), (b) The surface morphology when the laser energy density was 2.5 J/cm2 (Jc ¼ 0:9 MA/cm2 ), (c) The surface morphology when the laser energy density was 3.7 J/cm2 (Jc ¼ 0:2 MA/cm2 ).
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target was inspected as one of the most important factors of PLD by varied the illuminated size on the target. Laser energy density on the target was varied from 2.0 to 3.7 J/cm2 . Fig. 8(a), (b) and (c) show SEM images of HoBCO layer on buffered textured substrate by PLD method. Each photograph shows the surface morphology of the sample irradiated by the laser energy densities of 2.0, 2.5, and 3.7 J/cm2 . And Jc of each sample was less than 0.1, 0.9, and 0.2 MA/cm2 , respectively. In the case of 3.7 J/cm2 (Fig. 8(c)), a lot of particles were obviously recognized and the surface looked bumpy. From this result, it could be guessed that the energy density was so strong that crystal growth at the film surface became drastic. On the other hand, in the cases of lower energy densities of 2.0–2.5 J/cm2 (Fig. 8(a) and (b)), the features of the films revealed relatively smooth. However, even on the sample that showed Jc of 0.9 MA/cm2 , some particles were still observed (Fig. 8(b)). These particles are considered as the one reason of suppressing Jc of HoBCO layer. Therefore, the Jc of HoBCO is expected to improve by leveling the surface flatness. Furthermore, it can be said that laser energy density could be one of the most important factor during the pulse laser deposition.
4. Conclusion HoBCO coated conductors have been developed on flexible metal textured substrates. Both buffer layer and HoBCO layer were fabricated by using PLD method. Multi-buffer layer structure on the textured substrate was investigated, and fine crystal alignment was obtained in the case of the hetero-epitaxial growth of CeO2 and YSZ. Furthermore, 10 m long multi-buffer tape was fabricated by continuous reel-to-reel system. In the top
surface layer of CeO2 , the uniformity of in-plane crystal alignment around 10 and surface flatness around Ra of 6 nm was obtained. HoBCO coated conductors on multi-buffered textured substrates have been developed. HoBCO superconducting layer showed fine in-plane alignment of FWHM ¼ 8.6 and relative smooth surface morphology. HoBCO film showed high Jc around 1 MA/cm2 . Acknowledgements A part of this work was supported by the New Energy and Industrial Technology Development Organization (NEDO) as Collaborative Research and Development of Fundamental Technology for Superconductivity applications.
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