- Email: [email protected]
Development of productive process for long coated conductors by EB evaporation
Physica C 463–465 (2007) 594–599 www.elsevier.com/locate/physc
Development of productive process for long coated conductors by EB evaporation M. Mori
a,*
, T. Watanabe a, N. Suda a, N. Kashima a, S. Nagaya a, T. Izumi b, Y. Shiohara a
b
Chubu Electric Power Co., Inc., 20-1 Kitasekiyama, Ohdaka-cho, Midori-ku, Nagoya 459-8522, Japan b ISTEC-SRL, 1-10-13, Shinonome, Koto-ku, Tokyo 135-0062, Japan Received 30 October 2006; accepted 24 January 2007 Available online 24 May 2007
Abstract We have installed original EB evaporation system and verified the performance of CeO2 buffer layers fabricated by EB evaporation method in the YBCO coated conductor in comparison with those fabricated by PLD method. The Ic values of YBCO coated conductors obtained by same deposition for CVD-YBCO were 135 A (EB-CeO2 sample) and 126 A (PLD-CeO2 sample). Though the values of D/ for the EB-CeO2 layer and PLD-CeO2 layer were different, the Ic values of the YBCO were about the same. This result was shown that the CeO2 layer of D/ = 10° obtained by EB evaporation was equivalent in performance for YBCO coated conductors to the CeO2 layer of D/ = 4° obtained by PLD method. The influence of CeO2 film thickness for high speed production was examined. The Ic did not show the remarkable difference with the reduction of CeO2 film thickness. The range of the Ic was only about 20% of the maximum Ic value, regardless of the progress by 5 times in the production speed of CeO2 buffer layers. Ó 2007 Elsevier B.V. All rights reserved. PACS: 74.72.Bk; 81.15.Ef; 81.15.Gh; 84.71.Mn Keywords: CeO2; EB evaporation; IBAD; YBCO coated conductor; MOCVD
1. Introduction In recent years, several efforts to develop YBCO (YBa2Cu3O7 d) coated conductors were made, so that over 400 A of Ic and over 100 m of length were attained [1–6]. Thus, the development of YBCO coated conductors has ushered in the phase of the utilization of technical development for the application apparatus. In this phase, manufacturing technology for high speed, stability in manufacturing process and stable quality on the YBCO coated conductors become important with its performance. The CeO2 buffer layer which constitutes YBCO coated conductor is bearing the important role which influences the YBCO performance. Although many of buffer layers has
*
Corresponding author. Tel.: +81 52 621 6101; fax: +81 52 624 9339. E-mail address: [email protected] (M. Mori).
0921-4534/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2007.01.044
be fabricated by the pulsed laser deposition (PLD) method, if mass production of YBCO coated conductor is assumed, new fabrication method will be searched for which enables us improvement in the manufacturing speed and in the stability of manufacturing process. We have paid our attention to the electron beam (EB) evaporation method for demonstrating high productivity in other fields. The EB evaporation method enables us to obtain homogeneous film quality and wide deposition area. In this paper, we have installed original EB evaporation system and verified the performance of CeO2 buffer layers fabricated by EB evaporation method in the YBCO coated conductor in comparison with those fabricated by PLD method. The EB evaporation system is introduced the mechanism for high speed fabrication to make the substrate tape turn 10 times spirally in the deposition area. The substrate tape is moved in the deposition chamber by going through some guideless rolls. The guideless roll
M. Mori et al. / Physica C 463–465 (2007) 594–599
is valuable protection against particles which originate out of tape’s friction. The roll is our original. We expected that no particles lead to good quality in the CeO2 buffer layers. Furthermore, the possibility of improvement in the manufacturing speed and in the quality of CeO2 buffer layers was examined.
595
tem by passing through the deposition area at each condition in tape moving speeds. The substrate temperature was kept at any temperature among 650 °C, 700 °C and 850 °C. The standard PLD-CeO2 layer [7] on the IBAD-GZO substrate was used for comparison of Ic value. 2.3. Fabrication process of YBCO coated conductors
2. Experiment 2.1. Original EB evaporation system We employed the original EB evaporation system in the fabrication of CeO2 buffer layer for YBCO coated conductors. The schematic diagram of the EB evaporation system is shown in Fig. 1. This system is consisted of a deposition chamber and a pair of reel-to-reel chambers. The deposition chamber has heater units for temperature rising of the tape substrates and has enough space to layout three EB gun units. But an EB gun unit was used in the experience. And the substrate tape is be able to move at the maximum speed of 25 m/h and is accepted by a length of 200 m. The EB evaporation system was introduced the mechanism for high speed fabrication to make the substrate tape turn 10 times spirally in the deposition area. The substrate tape is moved in the deposition chamber by going through some guideless rolls. The guideless roll is valuable protection against particles which originate out of tape’s friction. The roll is our original. We expected that no particles lead to good quality in the CeO2 buffer layers.
The YBCO coated conductor on the CeO2 buffer layers was fabricated by the multi-stage MOCVD system [8–14]. The YBCO layer was deposited by the film thickness of 1 lm at substrate temperature of 845 °C with a tape moving at the speed of 25 m/h. Silver cap layer was deposited on the YBCO layer, and the oxygen treatment was carried out at 500 °C for 3 h in the oxygen gas flow. 2.4. Characterization of CeO2 buffer layers and YBCO coated conductors The critical current of the YBCO coated conductor was measured by employing a DC four terminal method with the criterion of 1 lV/cm at 77 K in a self-magnetic field. Crystallization was checked by employing X-ray diffraction (XRD). The c-axis orientation was examined from h–2h scan and the in-plane orientation was examined from / scan of CeO2(2 2 0) plane. Cross-section TEM observation was realized in the some samples. The surface roughness on the CeO2 layer was evaluated by AFM. 3. Results and discussion
2.2. Fabrication process of CeO2 buffer layer We used ion-beam-assisted-deposition Gd2Zr2O7 layer on Hastelloy-C276 (IBAD-GZO) as substrate tapes for EB evaporation. The substrates were with dimensions of 10 mm in width, 0.1 mm in thickness and 10 cm in length. The CeO2 layer was fabricated by the EB evaporation sys-
3.1. Verification of low-Ic sample on EB-CeO2/IBAD-GZO substrate We have fabricated the sample of YBCO coated conductors on EB-CeO2/BAD-GZO substrates and compared the Ic of the sample 1 with that of the sample of YBCO coated No guide
High Speed
10 turn
Guideless roll
No Particle
EB
EB
Fig. 1. Schematic diagram of original EB evaporation system.
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conductors fabricated on PLD-CeO2/IBAD-GZO substrate. The Ic of the sample on the EB-CeO2/IBAD-GZO substrate was 45 A which was 63% of Ic obtained at the sample on PLD-CeO2/IBAD-GZO substrate. The difference of the Ic was considered to be responsible for the difference of the CeO2 buffer layers, since the YBCO layer on the CeO2 buffer layers was deposited at the same time. To clear the reason against the low-Ic on the EB-CeO2 sample, we have realized a cross-section TEM observation. The results are shown in Fig. 2. The irregular phase and the rough interface were observed between the CeO2 layer and the YBCO layer. Unevenness existed on the surface of CeO2 layer and the irregular phases were formed in the gaps of the unevenness. The irregular phases were presumed to be Y2O3, CuO, BaCeO3, etc. by EDS. It is known that the deviance of composition in CVD-YBCO layer lead the decrease in Ic. We considered the two reasons in the generation of the irregular phase and the rough interface. One is the chemical reaction between YBCO layer and CeO2 layer. The other is initial surface of CeO2 layer. To clear the reason, we have realized a cross-section TEM observation and an AFM observation on the initial CeO2 surface. The results are shown in Fig. 3. In the cross-section TEM image, the unevenness also existed on the initial CeO2 surface with the wide period of 200–300 nm. And
the rough surface with the wide period of the same kind was observed in the AFM image. The average roughness of the initial surface on the EB-CeO2 layer was Ra = 39 nm. Though the causal relation between the generation of the irregular phase and the rough surface on the CeO2 layer was unrevealed against the low-Ic sample, we would assume that the rough surface on the CeO2 layer was a principal reason for decreasing the Ic of YBCO coated conductors. 3.2. Improved EB-CeO2 surface and optimization of the CeO2 formation To improve the rough CeO2 surface, we have realized the optimization of CeO2 formation by EB evaporation. The followings are quoted as the process factors at EB evaporation: (1) substrate temperature, (2) evaporation rate, (3) film thickness, (4) tape traveling speed. We have realized several depositions of CeO2 layer to compare the process factor. Therefore it was found that the most effective factor in the EB evaporation for improving the CeO2 surface was the substrate temperature. The improved CeO2 surface measured by AFM was shown in Fig. 4. In contradistinction with the rough CeO2 surface shown in Fig. 4a, the smooth CeO2 surface shown in Fig. 4b-1 and
Fig. 2. Cross-section TEM images of the YBCO sample with low-Ic fabricated on EB-CeO2/IBAD-GZO substrate. (a) Wide scale image, (b) zoom scale image around the interface between the CeO2 layer and the YBCO layer.
Fig. 3. (a) Cross-section TEM image of EB-CeO2 sample fabricated on IBAD-GZO substrate, (b) AFM image (full height scale 440 nm) on the surface of the EB-CeO2 sample. The average surface roughness is 39 nm.
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Fig. 4. AFM images of initial surface and improved surface on EB-CeO2 samples. (a) CeO2 film thickness = 1 lm, D/ = 7.3° deposited at 850 °C, (b) CeO2 film thickness = 1 lm, D/ = 8.9° deposited at 700 °C, (c) CeO2 film thickness = 0.3 lm, D/ = 10° deposited at 650 °C. The full height scale of AFM image is (a) and (b)-1 = 440 nm, (b)-2 and (c) = 50 nm, respectively.
Fig. 4b-2 was realized in substrate temperature of 700 °C which was 150 °C lower. The average surface roughness at the CeO2 in 850 °C and at the CeO2 in 700 °C were Ra = 39 nm and Ra = 3 nm, respectively. Furthermore, though the substrate temperature was reduced to 650 °C, the surface of CeO2 layer was similar to that of the CeO2 in 700 °C. On the contrary, the AFM image of the CeO2 surface decreased the film thickness in 300 nm was shown in Fig. 4c. Compared with Fig. 4b-2, both samples had the same average roughness (Ra) and had hardly the difference in the surface image by AFM. The result is rather welcome and shows the CeO2 buffer layer by EB evaporation has the promising possibility to realize high speed production. We could realize the smoother CeO2 surface with the thinner film thickness, which enables us to realize the high speed production of the CeO2 buffer layer.
Fig. 5. Comparison of YBCO samples with EB-CeO2 layer and with PLD-CeO2 layer. EB-CeO2: CeO2 film thickness = 300 nm, D/ = 10°, Ic = 135 A. PLD-CeO2: CeO2 film thickness = 500 nm, D/ = 4°, Ic = 126 A.
3.3. Comparison of Ic between EB-CeO2 and PLD-CeO2 We have verified the change of Ic on the improved CeO2 surface by EB evaporation. The EB-CeO2 sample shown in Fig. 4c and the standard PLD-CeO2 sample were used for comparison of Ic on YBCO coated conductors. The details of the samples were shown in Fig. 5. The film thickness and D/ value with the EB-CeO2 layer and the PLD-CeO2 layer were 300 nm, D/EB = 10° and 500 nm, D/PLD = 4°, respectively. The Ic values of YBCO coated conductors obtained by same deposition for CVD-YBCO were 135 A (EB-CeO2 sample) and 126 A (PLD-CeO2 sample). Though the values of D/ for the EB-CeO2 layers and PLD-CeO2 layer were
different, the Ic values of the YBCO were about the same. This result was shown that the CeO2 layer of D/ = 10° obtained by EB evaporation was equivalent in performance for YBCO coated conductors to the CeO2 layer of D/ = 4° obtained by PLD method. We have considered that the difference in the deposition process of the CeO2 layers has an effect on the film quality of CeO2 layers and YBCO layers such as the c-axis orientation and in-plane orientation. The evaluation results in the c-axis orientation and in-plane orientation were shown in Fig. 6a and b. The D/ value of YBCO layer on the EB-CeO2 layer decreased in comparison with the D/ value of CeO2 layer. The results represented the following improvement: The self-orientation
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5
12
EB-CeO2 PLD-CeO2
EB-CeO2 PLD-CeO2
10
4 8 3
6 4
2 2 0
1 CeO2
YBC
CeO2
YBCO
Fig. 6. (a) The change of D/ values in the CeO2 layers and in the YBCO layers, (b) the change of Dx values in the CeO2 layers and in the YBCO layers.
would take place in the YBCO layer on the EB-CeO2 layer and the difference of initial performance with the CeO2 layer would shrink between the EB-CeO2 and the PLDCeO2. On the contrary, the Dx value of the EB-CeO2 layer was very small than that of the PLD-CeO2 layer. The Dx values of the YBCO layers on the CeO2 layers reflected the Dx value of the CeO2 layer. It was found that the CeO2 layer by EB evaporation method have been improved toward good orientation, especially in the Dx value, for the Ic of YBCO coated conductors. We have also realized the cross-section TEM observation in the YBCO coated conductor with the Ic of 135 A obtained on the EB-CeO2 layer. Fig. 7 shows the cross-section TEM image of the sample. It was confirmed the clear interface between the CeO2 layer and the YBCO layer according to the smooth CeO2 surface obtained by AFM observation. We have successfully realized the fabrication of the CeO2 buffer layers by EB evaporation with the practical properties which is equivalent to the CeO2 buffer layer of D/ = 4° by PLD method. New potentials were found out that the CeO2 layer by EB evaporation could realize high Ic of YBCO coated conductors by further improvement in the D/ value of CeO2 layers.
on the IBAD-GZO substrates. The CeO2 film thickness were 300 nm by tape traveling speed of 5 m/h, 150 nm by the speed of 10 m/h and 60 nm by the speed of 25 m/h, respectively. The CVD-YBCO layer was deposited on the obtained CeO2 buffer layer at the simultaneous deposition. The Ic of the samples was shown in Fig. 8 with the CeO2 film thickness. The Ic did not show the remarkable difference with the reduction of CeO2 film thickness. The range of the Ic was only about 20% of the maximum Ic value, regardless of the progress by 5 times in the production speed of CeO2 buffer layers. However the Ic values was recognized to decline gradually with the reduction of the CeO2 film thickness. As for reasons, we consider that the quality of CeO2 layer in case of thin film thickness is influence by the surface roughness on IBAD-GZO substrate. The surface on IBAD-GZO substrate was observed by AFM. The average surface roughness (Ra) was 12.7 nm and the peak–valley surface roughness (Rp–v) was 124 nm. The sample of CeO2 layer in 60 nm was not covered enough on the surface of IBAD-GZO substrate. Therefore we assumed that the Ic of the sample should be slightly decreased. It was found that the film thickness of CeO2 for buffer layer was necessary to be more than the value of Rp–v on IBAD-GZO substrates.
3.4. Influence of CeO2 film thickness for high speed production We have fabricated the CeO2 buffer layers with the different film thickness by changing the tape traveling speed 120 100
Ic (A)
80 60 40 20 0 0 Fig. 7. Cross-section TEM image of the YBCO sample fabricated on the improved CeO2 surface (Fig. 4c). The interface between the CeO2 layer and the YBCO layer was clear.
100 200 300 CeO2 film thickness (nm)
Fig. 8. CeO2 film thickness versus Ic value of YBCO sample fabricated on.
M. Mori et al. / Physica C 463–465 (2007) 594–599
4. Conclusion We have installed original EB evaporation system for CeO2 buffer layers in the YBCO coated conductors and have successfully fabricated the better CeO2 buffer layer in comparison with the CeO2 buffer layer fabricated by PLD method. It was found that the EB evaporation method by the original EB evaporation system was promising to improve in the manufacturing speed and in the quality of CeO2 buffer layers. Acknowledgements The authors thank Dr. T. Kato, Dr. Y. Sasaki and Dr. T. Hirayama of Japan Fine Ceramics Center (JFCC) for TEM observation and helpful discussion. This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) through the International Superconductivity Technology Center (ISTEC) as the Collaborative Research and Development of Fundamental Technologies for Superconductivity Applications. References [1] A. Goyal, D.P. Norton, J.D. Budai, M. Paranthaman, E.D. Specht, D.M. Kroeger, D.K. Christen, Q. He, B. Saffian, F.A. List, D.F. Lee, P.M. Martin, C.E. Klabunde, E. Hartfield, V.K. Sikka, Appl. Phys. Lett. 69 (1996) 1795. [2] K. Hasegawa, K. Fujino, H. Mukai, M. Konishi, K. Hayashi, K. Sato, S. Honjo, Y. Sato, H. Ishii, Y. Iwata, Appl. Supercond. 4 (1996) 487.
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[3] K. Matsumoto, S.B. Kim, J.G. Wen, I. Hirabayashi, T. Watanabe, N. Uno, M. Ikeda, IEEE Trans. Appl. Supercond. 9 (1999) 1539. [4] T. Doi, K. Inoue, Y. Hakuraku, K. Onabe, M. Okada, N. Kashima, S. Nagaya, Physica C 392–396 (2003) 853. [5] T. Izumi, H. Fuji, Y. Aoki, R. Teranishi, K. Nakaoka, J.S. Matsuda, Y. Kitoh, S. Nomoto, Y. Shiohara, A. Yajima, Y. Iijima, T. Saitoh, Ext. Abstr. Int. Workshop on Coated Conductors for Applications (CCA2004), O2-8. [6] Y. Iijima, K. Kakimoto, Y. Sutoh, N. Kaneko, T. Saitoh, Ext. Abstr. Int. Workshop on Coated Conductors for Applications (CCA2004), O1-12. [7] Y. Yamada, T. Muroga, H. Iwai, T. Izumi, Y. Shiohara, Physica C 392–396 (2003) 777. [8] K. Onabe, K. Higashiyama, S. Nagaya, T. Saito, IEEE Trans. Appl. Supercond. 11 (1) (2000) 3150. [9] K. Onabe, T. Doi, N. Kashima, S. Nagaya, T. Saitoh, Physica C 392– 396 (2003) 863. [10] N. Kashima, T. Niwa, S. Nagaya, K. Onabe, T. Saito, T. Muroga, S. Miyata, T. Watanabe, Y. Yamada, Physica C 412–414 (2004) 944. [11] N. Kashima, T. Niwa, M. Mori, S. Nagaya, T. Muroga, S. Miyata, T. Watanabe, Y. Yamada, T. Izumi, Y. Shiohara, IEEE Trans. Appl. Supercond. 15 (2005) 763. [12] M. Mori, N. Kashima, S. Nagaya, T. Muroga, S. Miyata, T. Watanabe, Y. Yamada, T. Izumi, Y. Shiohara, Ext. Abstr. Int. Workshop on Coated Conductors for Applications (CCA2004), O213. [13] N. Kashima, T. Niwa, M. Mori, S. Nagaya, T. Muroga, S. Miyata, T. Watanabe, Y. Yamada, T. Izumi, Y. Shiohara, Physica C 426–431 (2005) 887. [14] M. Mori, T. Watanabe, N. Kashima, S. Nagaya, T. Muroga, S. Miyata, Y. Yamada, T. Izumi, Y. Shiohara, Physica C 445–448 (2006) 515.
Development of productive process for long coated conductors by EB evaporation M. Mori
a,*
, T. Watanabe a, N. Suda a, N. Kashima a, S. Nagaya a, T. Izumi b, Y. Shiohara a
b
Chubu Electric Power Co., Inc., 20-1 Kitasekiyama, Ohdaka-cho, Midori-ku, Nagoya 459-8522, Japan b ISTEC-SRL, 1-10-13, Shinonome, Koto-ku, Tokyo 135-0062, Japan Received 30 October 2006; accepted 24 January 2007 Available online 24 May 2007
Abstract We have installed original EB evaporation system and verified the performance of CeO2 buffer layers fabricated by EB evaporation method in the YBCO coated conductor in comparison with those fabricated by PLD method. The Ic values of YBCO coated conductors obtained by same deposition for CVD-YBCO were 135 A (EB-CeO2 sample) and 126 A (PLD-CeO2 sample). Though the values of D/ for the EB-CeO2 layer and PLD-CeO2 layer were different, the Ic values of the YBCO were about the same. This result was shown that the CeO2 layer of D/ = 10° obtained by EB evaporation was equivalent in performance for YBCO coated conductors to the CeO2 layer of D/ = 4° obtained by PLD method. The influence of CeO2 film thickness for high speed production was examined. The Ic did not show the remarkable difference with the reduction of CeO2 film thickness. The range of the Ic was only about 20% of the maximum Ic value, regardless of the progress by 5 times in the production speed of CeO2 buffer layers. Ó 2007 Elsevier B.V. All rights reserved. PACS: 74.72.Bk; 81.15.Ef; 81.15.Gh; 84.71.Mn Keywords: CeO2; EB evaporation; IBAD; YBCO coated conductor; MOCVD
1. Introduction In recent years, several efforts to develop YBCO (YBa2Cu3O7 d) coated conductors were made, so that over 400 A of Ic and over 100 m of length were attained [1–6]. Thus, the development of YBCO coated conductors has ushered in the phase of the utilization of technical development for the application apparatus. In this phase, manufacturing technology for high speed, stability in manufacturing process and stable quality on the YBCO coated conductors become important with its performance. The CeO2 buffer layer which constitutes YBCO coated conductor is bearing the important role which influences the YBCO performance. Although many of buffer layers has
*
Corresponding author. Tel.: +81 52 621 6101; fax: +81 52 624 9339. E-mail address: [email protected] (M. Mori).
0921-4534/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2007.01.044
be fabricated by the pulsed laser deposition (PLD) method, if mass production of YBCO coated conductor is assumed, new fabrication method will be searched for which enables us improvement in the manufacturing speed and in the stability of manufacturing process. We have paid our attention to the electron beam (EB) evaporation method for demonstrating high productivity in other fields. The EB evaporation method enables us to obtain homogeneous film quality and wide deposition area. In this paper, we have installed original EB evaporation system and verified the performance of CeO2 buffer layers fabricated by EB evaporation method in the YBCO coated conductor in comparison with those fabricated by PLD method. The EB evaporation system is introduced the mechanism for high speed fabrication to make the substrate tape turn 10 times spirally in the deposition area. The substrate tape is moved in the deposition chamber by going through some guideless rolls. The guideless roll
M. Mori et al. / Physica C 463–465 (2007) 594–599
is valuable protection against particles which originate out of tape’s friction. The roll is our original. We expected that no particles lead to good quality in the CeO2 buffer layers. Furthermore, the possibility of improvement in the manufacturing speed and in the quality of CeO2 buffer layers was examined.
595
tem by passing through the deposition area at each condition in tape moving speeds. The substrate temperature was kept at any temperature among 650 °C, 700 °C and 850 °C. The standard PLD-CeO2 layer [7] on the IBAD-GZO substrate was used for comparison of Ic value. 2.3. Fabrication process of YBCO coated conductors
2. Experiment 2.1. Original EB evaporation system We employed the original EB evaporation system in the fabrication of CeO2 buffer layer for YBCO coated conductors. The schematic diagram of the EB evaporation system is shown in Fig. 1. This system is consisted of a deposition chamber and a pair of reel-to-reel chambers. The deposition chamber has heater units for temperature rising of the tape substrates and has enough space to layout three EB gun units. But an EB gun unit was used in the experience. And the substrate tape is be able to move at the maximum speed of 25 m/h and is accepted by a length of 200 m. The EB evaporation system was introduced the mechanism for high speed fabrication to make the substrate tape turn 10 times spirally in the deposition area. The substrate tape is moved in the deposition chamber by going through some guideless rolls. The guideless roll is valuable protection against particles which originate out of tape’s friction. The roll is our original. We expected that no particles lead to good quality in the CeO2 buffer layers.
The YBCO coated conductor on the CeO2 buffer layers was fabricated by the multi-stage MOCVD system [8–14]. The YBCO layer was deposited by the film thickness of 1 lm at substrate temperature of 845 °C with a tape moving at the speed of 25 m/h. Silver cap layer was deposited on the YBCO layer, and the oxygen treatment was carried out at 500 °C for 3 h in the oxygen gas flow. 2.4. Characterization of CeO2 buffer layers and YBCO coated conductors The critical current of the YBCO coated conductor was measured by employing a DC four terminal method with the criterion of 1 lV/cm at 77 K in a self-magnetic field. Crystallization was checked by employing X-ray diffraction (XRD). The c-axis orientation was examined from h–2h scan and the in-plane orientation was examined from / scan of CeO2(2 2 0) plane. Cross-section TEM observation was realized in the some samples. The surface roughness on the CeO2 layer was evaluated by AFM. 3. Results and discussion
2.2. Fabrication process of CeO2 buffer layer We used ion-beam-assisted-deposition Gd2Zr2O7 layer on Hastelloy-C276 (IBAD-GZO) as substrate tapes for EB evaporation. The substrates were with dimensions of 10 mm in width, 0.1 mm in thickness and 10 cm in length. The CeO2 layer was fabricated by the EB evaporation sys-
3.1. Verification of low-Ic sample on EB-CeO2/IBAD-GZO substrate We have fabricated the sample of YBCO coated conductors on EB-CeO2/BAD-GZO substrates and compared the Ic of the sample 1 with that of the sample of YBCO coated No guide
High Speed
10 turn
Guideless roll
No Particle
EB
EB
Fig. 1. Schematic diagram of original EB evaporation system.
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conductors fabricated on PLD-CeO2/IBAD-GZO substrate. The Ic of the sample on the EB-CeO2/IBAD-GZO substrate was 45 A which was 63% of Ic obtained at the sample on PLD-CeO2/IBAD-GZO substrate. The difference of the Ic was considered to be responsible for the difference of the CeO2 buffer layers, since the YBCO layer on the CeO2 buffer layers was deposited at the same time. To clear the reason against the low-Ic on the EB-CeO2 sample, we have realized a cross-section TEM observation. The results are shown in Fig. 2. The irregular phase and the rough interface were observed between the CeO2 layer and the YBCO layer. Unevenness existed on the surface of CeO2 layer and the irregular phases were formed in the gaps of the unevenness. The irregular phases were presumed to be Y2O3, CuO, BaCeO3, etc. by EDS. It is known that the deviance of composition in CVD-YBCO layer lead the decrease in Ic. We considered the two reasons in the generation of the irregular phase and the rough interface. One is the chemical reaction between YBCO layer and CeO2 layer. The other is initial surface of CeO2 layer. To clear the reason, we have realized a cross-section TEM observation and an AFM observation on the initial CeO2 surface. The results are shown in Fig. 3. In the cross-section TEM image, the unevenness also existed on the initial CeO2 surface with the wide period of 200–300 nm. And
the rough surface with the wide period of the same kind was observed in the AFM image. The average roughness of the initial surface on the EB-CeO2 layer was Ra = 39 nm. Though the causal relation between the generation of the irregular phase and the rough surface on the CeO2 layer was unrevealed against the low-Ic sample, we would assume that the rough surface on the CeO2 layer was a principal reason for decreasing the Ic of YBCO coated conductors. 3.2. Improved EB-CeO2 surface and optimization of the CeO2 formation To improve the rough CeO2 surface, we have realized the optimization of CeO2 formation by EB evaporation. The followings are quoted as the process factors at EB evaporation: (1) substrate temperature, (2) evaporation rate, (3) film thickness, (4) tape traveling speed. We have realized several depositions of CeO2 layer to compare the process factor. Therefore it was found that the most effective factor in the EB evaporation for improving the CeO2 surface was the substrate temperature. The improved CeO2 surface measured by AFM was shown in Fig. 4. In contradistinction with the rough CeO2 surface shown in Fig. 4a, the smooth CeO2 surface shown in Fig. 4b-1 and
Fig. 2. Cross-section TEM images of the YBCO sample with low-Ic fabricated on EB-CeO2/IBAD-GZO substrate. (a) Wide scale image, (b) zoom scale image around the interface between the CeO2 layer and the YBCO layer.
Fig. 3. (a) Cross-section TEM image of EB-CeO2 sample fabricated on IBAD-GZO substrate, (b) AFM image (full height scale 440 nm) on the surface of the EB-CeO2 sample. The average surface roughness is 39 nm.
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Fig. 4. AFM images of initial surface and improved surface on EB-CeO2 samples. (a) CeO2 film thickness = 1 lm, D/ = 7.3° deposited at 850 °C, (b) CeO2 film thickness = 1 lm, D/ = 8.9° deposited at 700 °C, (c) CeO2 film thickness = 0.3 lm, D/ = 10° deposited at 650 °C. The full height scale of AFM image is (a) and (b)-1 = 440 nm, (b)-2 and (c) = 50 nm, respectively.
Fig. 4b-2 was realized in substrate temperature of 700 °C which was 150 °C lower. The average surface roughness at the CeO2 in 850 °C and at the CeO2 in 700 °C were Ra = 39 nm and Ra = 3 nm, respectively. Furthermore, though the substrate temperature was reduced to 650 °C, the surface of CeO2 layer was similar to that of the CeO2 in 700 °C. On the contrary, the AFM image of the CeO2 surface decreased the film thickness in 300 nm was shown in Fig. 4c. Compared with Fig. 4b-2, both samples had the same average roughness (Ra) and had hardly the difference in the surface image by AFM. The result is rather welcome and shows the CeO2 buffer layer by EB evaporation has the promising possibility to realize high speed production. We could realize the smoother CeO2 surface with the thinner film thickness, which enables us to realize the high speed production of the CeO2 buffer layer.
Fig. 5. Comparison of YBCO samples with EB-CeO2 layer and with PLD-CeO2 layer. EB-CeO2: CeO2 film thickness = 300 nm, D/ = 10°, Ic = 135 A. PLD-CeO2: CeO2 film thickness = 500 nm, D/ = 4°, Ic = 126 A.
3.3. Comparison of Ic between EB-CeO2 and PLD-CeO2 We have verified the change of Ic on the improved CeO2 surface by EB evaporation. The EB-CeO2 sample shown in Fig. 4c and the standard PLD-CeO2 sample were used for comparison of Ic on YBCO coated conductors. The details of the samples were shown in Fig. 5. The film thickness and D/ value with the EB-CeO2 layer and the PLD-CeO2 layer were 300 nm, D/EB = 10° and 500 nm, D/PLD = 4°, respectively. The Ic values of YBCO coated conductors obtained by same deposition for CVD-YBCO were 135 A (EB-CeO2 sample) and 126 A (PLD-CeO2 sample). Though the values of D/ for the EB-CeO2 layers and PLD-CeO2 layer were
different, the Ic values of the YBCO were about the same. This result was shown that the CeO2 layer of D/ = 10° obtained by EB evaporation was equivalent in performance for YBCO coated conductors to the CeO2 layer of D/ = 4° obtained by PLD method. We have considered that the difference in the deposition process of the CeO2 layers has an effect on the film quality of CeO2 layers and YBCO layers such as the c-axis orientation and in-plane orientation. The evaluation results in the c-axis orientation and in-plane orientation were shown in Fig. 6a and b. The D/ value of YBCO layer on the EB-CeO2 layer decreased in comparison with the D/ value of CeO2 layer. The results represented the following improvement: The self-orientation
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5
12
EB-CeO2 PLD-CeO2
EB-CeO2 PLD-CeO2
10
4 8 3
6 4
2 2 0
1 CeO2
YBC
CeO2
YBCO
Fig. 6. (a) The change of D/ values in the CeO2 layers and in the YBCO layers, (b) the change of Dx values in the CeO2 layers and in the YBCO layers.
would take place in the YBCO layer on the EB-CeO2 layer and the difference of initial performance with the CeO2 layer would shrink between the EB-CeO2 and the PLDCeO2. On the contrary, the Dx value of the EB-CeO2 layer was very small than that of the PLD-CeO2 layer. The Dx values of the YBCO layers on the CeO2 layers reflected the Dx value of the CeO2 layer. It was found that the CeO2 layer by EB evaporation method have been improved toward good orientation, especially in the Dx value, for the Ic of YBCO coated conductors. We have also realized the cross-section TEM observation in the YBCO coated conductor with the Ic of 135 A obtained on the EB-CeO2 layer. Fig. 7 shows the cross-section TEM image of the sample. It was confirmed the clear interface between the CeO2 layer and the YBCO layer according to the smooth CeO2 surface obtained by AFM observation. We have successfully realized the fabrication of the CeO2 buffer layers by EB evaporation with the practical properties which is equivalent to the CeO2 buffer layer of D/ = 4° by PLD method. New potentials were found out that the CeO2 layer by EB evaporation could realize high Ic of YBCO coated conductors by further improvement in the D/ value of CeO2 layers.
on the IBAD-GZO substrates. The CeO2 film thickness were 300 nm by tape traveling speed of 5 m/h, 150 nm by the speed of 10 m/h and 60 nm by the speed of 25 m/h, respectively. The CVD-YBCO layer was deposited on the obtained CeO2 buffer layer at the simultaneous deposition. The Ic of the samples was shown in Fig. 8 with the CeO2 film thickness. The Ic did not show the remarkable difference with the reduction of CeO2 film thickness. The range of the Ic was only about 20% of the maximum Ic value, regardless of the progress by 5 times in the production speed of CeO2 buffer layers. However the Ic values was recognized to decline gradually with the reduction of the CeO2 film thickness. As for reasons, we consider that the quality of CeO2 layer in case of thin film thickness is influence by the surface roughness on IBAD-GZO substrate. The surface on IBAD-GZO substrate was observed by AFM. The average surface roughness (Ra) was 12.7 nm and the peak–valley surface roughness (Rp–v) was 124 nm. The sample of CeO2 layer in 60 nm was not covered enough on the surface of IBAD-GZO substrate. Therefore we assumed that the Ic of the sample should be slightly decreased. It was found that the film thickness of CeO2 for buffer layer was necessary to be more than the value of Rp–v on IBAD-GZO substrates.
3.4. Influence of CeO2 film thickness for high speed production We have fabricated the CeO2 buffer layers with the different film thickness by changing the tape traveling speed 120 100
Ic (A)
80 60 40 20 0 0 Fig. 7. Cross-section TEM image of the YBCO sample fabricated on the improved CeO2 surface (Fig. 4c). The interface between the CeO2 layer and the YBCO layer was clear.
100 200 300 CeO2 film thickness (nm)
Fig. 8. CeO2 film thickness versus Ic value of YBCO sample fabricated on.
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4. Conclusion We have installed original EB evaporation system for CeO2 buffer layers in the YBCO coated conductors and have successfully fabricated the better CeO2 buffer layer in comparison with the CeO2 buffer layer fabricated by PLD method. It was found that the EB evaporation method by the original EB evaporation system was promising to improve in the manufacturing speed and in the quality of CeO2 buffer layers. Acknowledgements The authors thank Dr. T. Kato, Dr. Y. Sasaki and Dr. T. Hirayama of Japan Fine Ceramics Center (JFCC) for TEM observation and helpful discussion. This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) through the International Superconductivity Technology Center (ISTEC) as the Collaborative Research and Development of Fundamental Technologies for Superconductivity Applications. References [1] A. Goyal, D.P. Norton, J.D. Budai, M. Paranthaman, E.D. Specht, D.M. Kroeger, D.K. Christen, Q. He, B. Saffian, F.A. List, D.F. Lee, P.M. Martin, C.E. Klabunde, E. Hartfield, V.K. Sikka, Appl. Phys. Lett. 69 (1996) 1795. [2] K. Hasegawa, K. Fujino, H. Mukai, M. Konishi, K. Hayashi, K. Sato, S. Honjo, Y. Sato, H. Ishii, Y. Iwata, Appl. Supercond. 4 (1996) 487.
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