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Diffusion joint of YBCO coated conductors using stabilizing silver layers
Physica C 445–448 (2006) 686–688 www.elsevier.com/locate/physc
Diffusion joint of YBCO coated conductors using stabilizing silver layers J.Y. Kato
a,*
, N. Sakai a, S. Tajima a,b, S. Miyata c, M. Konishi c, Y. Yamada c, N. Chikumoto a, K. Nakao a, T. Izumi a, Y. Shiohara a
a
c
Superconductivity Research Laboratory, ISTEC, 1-10-13 Shinonome, Koto-ku, Tokyo 135-0062, Japan b Osaka University, 1-1, Machikaneyama-cho, Toyonaka, Osaka 560-0043, Japan Superconductivity Research Laboratory, ISTEC, 2-4-1 Mutsuno, Atsuta-ku, Nagoya, Aichi 456-8587, Japan Available online 27 June 2006
Abstract We developed a very simple and new joint technique for coated conductors that could achieve a very low resistance less than 0.02 lX across the joint with an area of 2 · 20 mm2 without any degradation of Ic. It is a diffusion joint using the Ag stabilizing layer of YBCO coated conductors. Two tapes were stuck with Ag stabilizing layer in contact in a face to face manner and were pressed uniaxially during a heat treatment from 500 C to room temperature in a pure oxygen atmosphere. The relationship between the joint pressure, the joint structure and Ic were studied. 2006 Elsevier B.V. All rights reserved. PACS: 81.20.Vj; 84.71.Mn; 74.72.Bk; 66.30.h Keywords: Joint; Coated conductor; YBCO; PLD
1. Introduction Joint techniques of coated conductors are essential for their practical applications. There can be two types of joint process for coated conductors. One is a superconducting joint and the other is a non-superconducting joint. The mainstream at present is the latter technique using a low melting point metal such as a soldering material between superconducting tapes [1–3]. Soldering is a very simple joint technique, but resistance across the joint must become high because it includes resistance of Ag stabilizing layer of coated conductors, a soldering material and contact resistance of interfaces between solder, Ag stabilizing layer and YB2Cu3O7d (YBCO). On the other hand, diffusion joint, which does not use soldering material, is expected to reduce resistance across *
Corresponding author. Tel.: +81 3 3536 5707; fax: +81 3 3536 5705. E-mail address: [email protected] (J.Y. Kato).
0921-4534/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2006.05.005
the joint because there is no resistance of a soldering material and contact resistance. In this paper, the diffusion joint process for coated conductors using Ag stabilizing layer was studied.
2. Experimental The YBCO coated conductor was used for joint experiment, which has stacked layers of Gd2Zr2O7 (GZO, with the thickness of 0.8 lm), CeO2 (1.2 lm), YBCO (2.0 lm) and Ag (13 lm) on Hastelloy substrate (100 lm) [4]. The YBCO layer was produced by pulsed laser deposition. V–I properties of samples cut into the size of 2 · 70 mm2 were measured in liquid nitrogen before the joint process as shown in Fig. 1(a). Then the sample was divided into two at the center (Fig. 1(b)), and the two samples were stuck together in a face to face manner with an overlapped area of 2 · 20 mm2 (Fig. 1(c)).
Voltage (x10-6 V)
J.Y. Kato et al. / Physica C 445–448 (2006) 686–688
687
Before joint After joint
12 8 4 0 0
SPL ¼ 2pN where S is the area, P is the pressure, L (m) is the moving length per one revolution of the screw, and N (Nm) is the torque on the screw. It should be noticed that the pressure during the thermal treatment might be different because of the thermal expansion of the holder. Therefore, the number should be regarded as a parameter giving some idea about the pressure dependence of the joint property. The sample held by the holder was set in a tube furnace, and heated to 500 C, held for 1 h and cooled to room temperature in pure oxygen gas flow at the rate of 300 ml/min. Joined-sample was removed from the holder and V–I properties across the joint (Fig. 1(e)) were measured in liquid nitrogen. Furthermore, microstructural observations were performed for the cross section of the joint by optical microscope and SEM. Contact resistance of the interface between Ag stabilizing layer and YBCO was also checked. A small piece of the sample was cut with a size of 2 · 5 mm2 and the Ag stabilizing layer was partly etched in a solution of NH3:H2O2 = 1:1 as shown in Fig. 2. V–I property was measured by a three-probe method in liquid nitrogen after annealing the sample piece at the same condition as the joint process. 3. Results and discussion
35
40
Part
Resistivity
Thickness
Resistance
Ag layer (RAg) Interface between Ag and YBCO (RAg/YBCO) Total of the joint (RAg + RAg/YBCO · 2)
0.3 · 108 X m 0.5 · 1013 X m2
13 lm · 2 –
1.95 · 109 X 1.25 · 109 X 4.45 · 109 X
1.0 0.8 0.6 0.4 0.2 0.0 0
Fig. 3 shows the V–I curves of the sample before and after the joint process with a pressure of 10 MPa. The V–I curve did not change by the joint process. Moreover, the voltage across the joint was less than 5 · 107 V, which is
30
Table 1 Resistivity and resistance for the area of 2 · 20 mm2 at the liquid nitrogen temperature
/ Ic
The overlapped area was held by a holder made of Inconel (Fig. 1(d)) under pressure between 10 and 60 MPa, which was controlled by torque on the screw at room temperature and estimated by the following equation:
15 20 25 Current (A)
the lower limit of our measurement system, up to the critical current Ic. It means that the joint resistance was less than 1.6 · 108 X (6.7 · 1013 X for 1 m2) from estimation at current of 30 A. It was demonstrated that the diffusion joint using Ag stabilizing layer under 10 MPa can achieve low resistance without degrading original property of the YBCO coated conductor. On the other hand, the ideal resistance of this joint was estimated as shown in Table 1. Here, the contact resistivity of the interface between Ag stabilizing layer and YBCO was determined from V–I measurement for the area of 2 · 1.5 mm2 (see Fig. 2). It is found from Table 1 that the resistance of the joint should be 4.45 · 109 X, in an ideal case, and the result of Fig. 3 is consistent within an error. Fig. 4 shows the pressure dependence of I joint =I c . Presc sure was that at room temperature as explained above. I joint and Ic were the critical currents, which were defined c
joint
Fig. 2. Schematic illustration of the configuration for V–I measurement of contact resistance of interface between Ag stabilizing layer and YBCO.
10
Fig. 3. V–I curves of YBCO coated conductors for joined sample (across the joint) in comparison with those of as-cut sample (cut into 2 · 70 mm2).
Ic
Fig. 1. Schematic illustration of the joint procedure and lead wire configuration for V–I measurement. (a)–(e) show the cross section of the sample.
5
10
20
30 40 50 Pressure (MPa)
60
70
Fig. 4. Ic across the joint as a function of the joint pressure. Here I joint =I c c was a ratio of Ic, which was defined by a criterion of 1 lV, before and after joint of the sample.
688
J.Y. Kato et al. / Physica C 445–448 (2006) 686–688
Fig. 5. Optical images of the cross section of the sample joined under the pressure of (a) 10 and (b) 60 MPa.
and CeO2 layer, and interfaces between Ag, YBCO and CeO2. Furthermore, the a-axis oriented grains of YBCO were found on the peeled Ag layer. It suggests that the existence of a-axis grains is the origin of the crack for the joint with the pressure above 30 MPa. 4. Conclusion
Fig. 6. (a) Optical and (b) compositional SEM images of the sample joined under the pressure of 60 MPa.
by a criterion of 1 lV, before and after the joint process. The resistance across the joint was less than 1.6 · 108 X up to Ic irrespective of joint pressure. However Ic was reduced substantially when the joint pressure was above 30 MPa as shown in Fig. 4. Then the microstructure was observed for the cross section of the joint prepared under the pressure of 10 and 60 MPa. As can be seen in Fig. 5, Ag stabilizing layers were well joined so that the boundary is almost invisible for both pressures, whereas a large crack can be seen along the layered structure at the joint with the pressure of 60 MPa (Fig. 5(b)). It is clear that the decreasing of Ic for the high pressure joint was due to a large crack at the joint. Fig. 6 shows the compositional SEM image for further microstructural study. The contrast in this image indicated the composition of the layer, which was checked by wavelength dispersive X-ray spectroscopy (WDX). It was observed that the fractured parts at the joint were YBCO
A diffusion joint technique was developed for the coated conductors using Ag stabilizing layer. The coated conductors were stuck together in a face to face manner, held under pressure and heat-treated from 500 C to room temperature in oxygen atmosphere. It is very simple but realizes a low joint resistance less than 6.7 · 1013 X for 1 m2. Moreover the superconducting property is maintained after the joint process unless the pressure is too high. Ic was decreased after joint due to peeling of Ag layer from YBCO layer and fracture of YBCO and CeO2 layer when the pressure was higher than 30 MPa. It is possible that the presence of large a-axis grains was one of the factors for fractured structure of the joint. Acknowledgements This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) as the Collaborative Research and Development of Fundamental Technologies for Superconductivity Applications. References [1] K. Onabe, S. Nagaya, Y. Iijima, N. Sadakata, T. Saitoh, O. Kohno, in: K. Osamura, I. Hirabayashi (Eds.), Advances in Superconductivity, vol. 10, 1998, p. 603. [2] M.H. Sohn, S.W. Kim, S.K. Baik, Y.S. Jo, M.G. Seo, E.Y. Lee, Y.K. Kwon, IEEE Trans. Appl. Supercond. 13 (2003) 1764. [3] C. Gu, C. Zhuang, T.M. Qu, Z. Han, Physica C 426 (2005) 1385. [4] H. Iwai, T. Muroga, T. Watanabe, S. Miyata, Y. Yamada, Y. Shiohara, T. Kato, T. Hirayama, Supercond. Sci. Technol. 17 (2004) S496.
Diffusion joint of YBCO coated conductors using stabilizing silver layers J.Y. Kato
a,*
, N. Sakai a, S. Tajima a,b, S. Miyata c, M. Konishi c, Y. Yamada c, N. Chikumoto a, K. Nakao a, T. Izumi a, Y. Shiohara a
a
c
Superconductivity Research Laboratory, ISTEC, 1-10-13 Shinonome, Koto-ku, Tokyo 135-0062, Japan b Osaka University, 1-1, Machikaneyama-cho, Toyonaka, Osaka 560-0043, Japan Superconductivity Research Laboratory, ISTEC, 2-4-1 Mutsuno, Atsuta-ku, Nagoya, Aichi 456-8587, Japan Available online 27 June 2006
Abstract We developed a very simple and new joint technique for coated conductors that could achieve a very low resistance less than 0.02 lX across the joint with an area of 2 · 20 mm2 without any degradation of Ic. It is a diffusion joint using the Ag stabilizing layer of YBCO coated conductors. Two tapes were stuck with Ag stabilizing layer in contact in a face to face manner and were pressed uniaxially during a heat treatment from 500 C to room temperature in a pure oxygen atmosphere. The relationship between the joint pressure, the joint structure and Ic were studied. 2006 Elsevier B.V. All rights reserved. PACS: 81.20.Vj; 84.71.Mn; 74.72.Bk; 66.30.h Keywords: Joint; Coated conductor; YBCO; PLD
1. Introduction Joint techniques of coated conductors are essential for their practical applications. There can be two types of joint process for coated conductors. One is a superconducting joint and the other is a non-superconducting joint. The mainstream at present is the latter technique using a low melting point metal such as a soldering material between superconducting tapes [1–3]. Soldering is a very simple joint technique, but resistance across the joint must become high because it includes resistance of Ag stabilizing layer of coated conductors, a soldering material and contact resistance of interfaces between solder, Ag stabilizing layer and YB2Cu3O7d (YBCO). On the other hand, diffusion joint, which does not use soldering material, is expected to reduce resistance across *
Corresponding author. Tel.: +81 3 3536 5707; fax: +81 3 3536 5705. E-mail address: [email protected] (J.Y. Kato).
0921-4534/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2006.05.005
the joint because there is no resistance of a soldering material and contact resistance. In this paper, the diffusion joint process for coated conductors using Ag stabilizing layer was studied.
2. Experimental The YBCO coated conductor was used for joint experiment, which has stacked layers of Gd2Zr2O7 (GZO, with the thickness of 0.8 lm), CeO2 (1.2 lm), YBCO (2.0 lm) and Ag (13 lm) on Hastelloy substrate (100 lm) [4]. The YBCO layer was produced by pulsed laser deposition. V–I properties of samples cut into the size of 2 · 70 mm2 were measured in liquid nitrogen before the joint process as shown in Fig. 1(a). Then the sample was divided into two at the center (Fig. 1(b)), and the two samples were stuck together in a face to face manner with an overlapped area of 2 · 20 mm2 (Fig. 1(c)).
Voltage (x10-6 V)
J.Y. Kato et al. / Physica C 445–448 (2006) 686–688
687
Before joint After joint
12 8 4 0 0
SPL ¼ 2pN where S is the area, P is the pressure, L (m) is the moving length per one revolution of the screw, and N (Nm) is the torque on the screw. It should be noticed that the pressure during the thermal treatment might be different because of the thermal expansion of the holder. Therefore, the number should be regarded as a parameter giving some idea about the pressure dependence of the joint property. The sample held by the holder was set in a tube furnace, and heated to 500 C, held for 1 h and cooled to room temperature in pure oxygen gas flow at the rate of 300 ml/min. Joined-sample was removed from the holder and V–I properties across the joint (Fig. 1(e)) were measured in liquid nitrogen. Furthermore, microstructural observations were performed for the cross section of the joint by optical microscope and SEM. Contact resistance of the interface between Ag stabilizing layer and YBCO was also checked. A small piece of the sample was cut with a size of 2 · 5 mm2 and the Ag stabilizing layer was partly etched in a solution of NH3:H2O2 = 1:1 as shown in Fig. 2. V–I property was measured by a three-probe method in liquid nitrogen after annealing the sample piece at the same condition as the joint process. 3. Results and discussion
35
40
Part
Resistivity
Thickness
Resistance
Ag layer (RAg) Interface between Ag and YBCO (RAg/YBCO) Total of the joint (RAg + RAg/YBCO · 2)
0.3 · 108 X m 0.5 · 1013 X m2
13 lm · 2 –
1.95 · 109 X 1.25 · 109 X 4.45 · 109 X
1.0 0.8 0.6 0.4 0.2 0.0 0
Fig. 3 shows the V–I curves of the sample before and after the joint process with a pressure of 10 MPa. The V–I curve did not change by the joint process. Moreover, the voltage across the joint was less than 5 · 107 V, which is
30
Table 1 Resistivity and resistance for the area of 2 · 20 mm2 at the liquid nitrogen temperature
/ Ic
The overlapped area was held by a holder made of Inconel (Fig. 1(d)) under pressure between 10 and 60 MPa, which was controlled by torque on the screw at room temperature and estimated by the following equation:
15 20 25 Current (A)
the lower limit of our measurement system, up to the critical current Ic. It means that the joint resistance was less than 1.6 · 108 X (6.7 · 1013 X for 1 m2) from estimation at current of 30 A. It was demonstrated that the diffusion joint using Ag stabilizing layer under 10 MPa can achieve low resistance without degrading original property of the YBCO coated conductor. On the other hand, the ideal resistance of this joint was estimated as shown in Table 1. Here, the contact resistivity of the interface between Ag stabilizing layer and YBCO was determined from V–I measurement for the area of 2 · 1.5 mm2 (see Fig. 2). It is found from Table 1 that the resistance of the joint should be 4.45 · 109 X, in an ideal case, and the result of Fig. 3 is consistent within an error. Fig. 4 shows the pressure dependence of I joint =I c . Presc sure was that at room temperature as explained above. I joint and Ic were the critical currents, which were defined c
joint
Fig. 2. Schematic illustration of the configuration for V–I measurement of contact resistance of interface between Ag stabilizing layer and YBCO.
10
Fig. 3. V–I curves of YBCO coated conductors for joined sample (across the joint) in comparison with those of as-cut sample (cut into 2 · 70 mm2).
Ic
Fig. 1. Schematic illustration of the joint procedure and lead wire configuration for V–I measurement. (a)–(e) show the cross section of the sample.
5
10
20
30 40 50 Pressure (MPa)
60
70
Fig. 4. Ic across the joint as a function of the joint pressure. Here I joint =I c c was a ratio of Ic, which was defined by a criterion of 1 lV, before and after joint of the sample.
688
J.Y. Kato et al. / Physica C 445–448 (2006) 686–688
Fig. 5. Optical images of the cross section of the sample joined under the pressure of (a) 10 and (b) 60 MPa.
and CeO2 layer, and interfaces between Ag, YBCO and CeO2. Furthermore, the a-axis oriented grains of YBCO were found on the peeled Ag layer. It suggests that the existence of a-axis grains is the origin of the crack for the joint with the pressure above 30 MPa. 4. Conclusion
Fig. 6. (a) Optical and (b) compositional SEM images of the sample joined under the pressure of 60 MPa.
by a criterion of 1 lV, before and after the joint process. The resistance across the joint was less than 1.6 · 108 X up to Ic irrespective of joint pressure. However Ic was reduced substantially when the joint pressure was above 30 MPa as shown in Fig. 4. Then the microstructure was observed for the cross section of the joint prepared under the pressure of 10 and 60 MPa. As can be seen in Fig. 5, Ag stabilizing layers were well joined so that the boundary is almost invisible for both pressures, whereas a large crack can be seen along the layered structure at the joint with the pressure of 60 MPa (Fig. 5(b)). It is clear that the decreasing of Ic for the high pressure joint was due to a large crack at the joint. Fig. 6 shows the compositional SEM image for further microstructural study. The contrast in this image indicated the composition of the layer, which was checked by wavelength dispersive X-ray spectroscopy (WDX). It was observed that the fractured parts at the joint were YBCO
A diffusion joint technique was developed for the coated conductors using Ag stabilizing layer. The coated conductors were stuck together in a face to face manner, held under pressure and heat-treated from 500 C to room temperature in oxygen atmosphere. It is very simple but realizes a low joint resistance less than 6.7 · 1013 X for 1 m2. Moreover the superconducting property is maintained after the joint process unless the pressure is too high. Ic was decreased after joint due to peeling of Ag layer from YBCO layer and fracture of YBCO and CeO2 layer when the pressure was higher than 30 MPa. It is possible that the presence of large a-axis grains was one of the factors for fractured structure of the joint. Acknowledgements This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) as the Collaborative Research and Development of Fundamental Technologies for Superconductivity Applications. References [1] K. Onabe, S. Nagaya, Y. Iijima, N. Sadakata, T. Saitoh, O. Kohno, in: K. Osamura, I. Hirabayashi (Eds.), Advances in Superconductivity, vol. 10, 1998, p. 603. [2] M.H. Sohn, S.W. Kim, S.K. Baik, Y.S. Jo, M.G. Seo, E.Y. Lee, Y.K. Kwon, IEEE Trans. Appl. Supercond. 13 (2003) 1764. [3] C. Gu, C. Zhuang, T.M. Qu, Z. Han, Physica C 426 (2005) 1385. [4] H. Iwai, T. Muroga, T. Watanabe, S. Miyata, Y. Yamada, Y. Shiohara, T. Kato, T. Hirayama, Supercond. Sci. Technol. 17 (2004) S496.