Flux Pinning Properties of REBa2Cu3Oy Films Fabricated by a Metal-organic Deposition Using Metal-naphthenates and 2-ethylhexanates

Available online at www.sciencedirect.com

Physics Procedia 36 (2012) 649 – 654

Superconductivity Centennial Conference

Flux pinning properties of REBa2Cu3Oy films fabricated by a metal-organic deposition using metal-naphthenates and 2ethylhexanates Kazuya Matsumoto a, Osuke Miura a, Ryusuke Kita b a

Tokyo Metropolitan University, 1-1Minamiosawa, Hachioji 192-0397, Japan b Shizuoka University, 3-5-1 Johoku,hamamatsu,432-8561, Japan

Abstract We studied the relationship between microstructure and flux pinning properties of REBCO films fabricated by a MOD technique using metal-naphthenate and 2-ethylhexanate (2-EH) solutions. 2-EH-(Sm0.3, Ho0.7) Ba2Cu3Oy film achieved high Jc of 2.19 MA/cm2 at 77 K due to introduction of effective pinning sites. GdBa2Cu3Oy film prepared using complex solutions (CS) obtained high Jc of 30 MA/cm2 at 4.2 K in spite of having low Jc at 77 K. Anomalous temperature dependence of Jc was also observed for CS-GdBa2Cu3Oy film. It is speculated that lower-Tc regions act as strong pinning centers in the CS-GdBa2Cu3Oy film by comparison analysis with flux pinning for NbTi with artificial pins.

© 2012 2011 Published Published by by Elsevier Elsevier B.V. Ltd. Selection Rogalla and © Selection and/or and/or peer-review peer-review under under responsibility responsibility of of Horst the Guest Editors. Peter Kes. Keywords REBCO; fluxpinning; MOD; Lower-Tc region; Superconducting pins

1. Introduction REBa2Cu3Oy (REBCO) coated conductors have been under development by many research institutes for high engineering Jc and high magnetic field applications at liquid nitrogen temperature [1-3]. Several kinds of fabrication method, such as pulsed laser deposition (PLD), chemical vapour deposition (CVD) and metal-organic deposition (MOD) have been studied actively [4-8]. Among them, MOD technique may be more suitable for mass production of REBCO coated conductors than others because this technique is a non-vacuum and cost effective process [9]. Especially a MOD technique using metalnaphthenates (MN) and 2-ethylhexanates (2-EH) has an advantage to avoid the use of fluorine during the process [10]. In this paper we studied the relationship between micro structure and flux pinning properties of several kinds of REBCO films fabricated by the MOD technique using MN and 2-EH solutions. (Sm0.3, Ho0.7) Ba2 Cu3 Oy film prepared by 2-ethylhexanates achieved maximum Jc of 2.19 MA/cm2 at 77 K under self-field due to introduction of effective pinning sites. GdBa2Cu3Oy film prepared by 2-ethylhexanates showed high Jc of 0.92 MA/cm2 at 77 K and 3.3 MA/cm2 at 4.2 K under self-field due to its excellent crystallization. On the other hand, GdBa2Cu3Oy film prepared by complex solutions (CS) achieved much 1875-3892 © 2012 Published by Elsevier B.V. Selection and/or peer-review under responsibility of the Guest Editors. doi:10.1016/j.phpro.2012.06.261

650

Kazuya Matsumoto et al. / Physics Procedia 36 (2012) 649 – 654

higher Jc of 30 MA/cm2 at 4.2 K in spite of having low Jc of 0.51 MA/cm2 at 77 K. Anomalous temperature dependence of Jc was also observed for CS-GdBa2Cu3Oy film. It suggests that strong pinning centers are induced into CS-GdBa2Cu3Oy film at low temperatures. It is speculated that lower-Tc regions act as effective pinning centers in the CS-GdBa2Cu3Oy film by comparison analysis with flux pinning for NbTi with artificial pinning centers.  Experimental detail Several kinds of superconducting films of REBCO were fabricated by the MOD technique. The solutions were prepared by mixing metal MN salts and 2-EH salts or only 2-EH salts with a nominal atomic ratio of Gd: Ba: Cu = 1: 2: 3, or of Sm: Ho: Ba: Cu = 0.3: 0.7: 2: 3. The solutions were coated on substrates of LaAlO3 single crystal (100) by spin coating at 3,500 rpm for 30 sec. And then the films were dried at 120㷄for 30 min, and calcined at 450 or 600㷄for 20 or 30 min in air. This process was repeated 3-5 times. The precursor films were fired at 850-860㷄for 30 min in a nitrogen (N2) gas flow containing partial oxygen of 10 -6 atm at a flowing rate of 0.1 lmin -1. These films were annealed 350 or 500㷄for 2 hour in a oxygen (O2) flow at a flowing rate of 0.1 lmin -1 , and finally the REBCO films were obtained. Table 1 shows the fabrication process of the films. CS-Gd film was fabricated using complex solution mixed MN and 2-EH. 2-EH-Gd film was fabricated using 2-EH solution. 2-EH-Gd film was fabricated using solution mixed 2-EH and colloidal solutions with Pt. 2-EH-HS film was fabricated using 2-EH solution with a nominal atomic ratio of Sm: Ho: Ba: Cu = 0.3: 0.7: 2: 3. Table.1 fabrication process of films Films

Solutions

Composition

Calcination 㷄] temperature[㷄

Firing 㷄] temperature[㷄

Firing rate 㷄 /min] [㷄

O 2 annealing 㷄] temperature[㷄

CS-Gd

CS

Complex solution

450

850

10

500

450

860

7.5

350

600

860

5

350

450

860

5

350

GdBa 2 Cu 3 O y 2-EH-Gd

2-EH

2-ethylhexanates GdBa 2 Cu 3 O y

2-EH-GP

2-EH

2-ethylhexanates GdBa 2 Cu 3 O y +Pt

2-EH-HS

2-EH

2-ethylhexanates ( Sm 0.3 , Ho 0.7 )Ba 2 Cu 3 O y

Surface morphology of the films was observed by scanning electron microscope (SEM), and crystallization of films was observed by x-ray diffraction (XRD) using ș-2ș method with Cu-KĮ1 radiation. Temperature dependence of magnetic susceptibility at 0.001 T in ZFC-FC conditions was measured by superconducting quantum interference device (SQUID) magnetometer. Applied field dependence of magnetization was also measured by SQUID magnetometer. Temperature dependence of critical current density (Jc) was estimated from the magnetization using the Bean critical state model. [11] 3. Result Figures1 (a)-(d) show SEM images of the surface for the CS-Gd, the 2-EH-Gd, the 2-EH-GP and the 2EH-HS films, respectively. For 2-EH-Gd, 2-EH-GP, 2-EH-HS, small pores were observed on the surfaces. In contrast, the surface of CS-Gd seems relatively smooth, and there are little pores on the surface. However, the round micron-size precipitates exist on the surface of CS-Gd. The composition analysis using energy dispersive x-ray spectroscopy (EDS) indicated that the precipitates contained Ba and Cu ions. [10]

651

Kazuya Matsumoto et al. / Physics Procedia 36 (2012) 649 – 654

     

(a)

(b)

(c)

(d)

      Fig. 1 SEM images for (a) the surface of CS-Gd film, (b) the surface of 2-EH-Gd film, (c) the surface of 2-EH-GP film, (d) the surface of 2-EH-HS film, respectively.

Figure 2 (a) shows XRD pattern for all films. The intensity is normalized by (006) peak amplitude. Peaks of c-axis oriented phase are remarkably observed for all films. In contrast, peak of a/b-axis oriented phase is not observed, because higher temperature process inhibits a/b-axis oriented phase growth [12]. Thus high-quality crystallization was achieved for all films. Figure 2 (b) shows an enlargement of (006) peaks. The (006) peak of CS-Gd film slightly shifts to lower angle side in comparison with that of other films and the half width is broader than that of others. This suggests that different phases may be introduced into CS-Gd film. 2-EH-HS

(a)

(b)

2-EH-HS

10

(006)

2-EH-Gd

(006)

30

40 2T[deg.]

CS-Gd

(007)

(004)

20

(005)

(003) (002)

(001)

2-EH-Gd

2-EH-GP

Intensity[a.u.]

Intensity[a.u.]

2-EH-GP

50

60

70

CS-Gd 45

46

47 48 2T [deg.]

49

50

Fig.2 (a) XRD pattern of all films (The number in the figure indicates mirror indexes.), (b) The enlargements of (006) peaks of all films

Figure 3 shows the temperature dependence of magnetic susceptibility for all films. The Tcs are estimated to be 84.8, 90.7, 90.0 and 89.1 K for CS-Gd, 2-EH-Gd, 2-EH-GP and 2-EH-HS films,

652

Kazuya Matsumoto et al. / Physics Procedia 36 (2012) 649 – 654

8

10

200

CS-Gd 2-EH-Gd 2-EH-GP 2-EH-HS

Applied field : 10 [Oe]

0 7

10

-200

Jc(0) [A/cm 2]

Magnetic susceptibility[emu/(cm 3*Oe)]

respectively. The Tc of CS-Gd film is lowest among all films. This is considered to come from the proximity effect due to introduced pinning centers. The magnitude of a magnetic susceptibility at 77 K for each film is corresponding to the magnitude of Jc as shown in figure 4.

-400 CS-Gd 2-EH-Gd 2-EH-GP 2-EH-HS

-600 -800 -1000 60

6

10

5

10

65

70

75

80

85

90

95

Temperature [K]  Fig.3 Temperature dependence of magnetic susceptibility  for all films.

0

10

20

30

40

50

60

70

80

Temperature [K] Fig.4 Temperature dependence of Jc (0) for all films

Figure 4 shows the temperature dependence of Jc at self-field for all films. The CS-Gd film shows the maximum Jc of 30 MA/cm2 at 4.2 K. The Jc of other films are 17.8, 7.4, 3.31 MA/cm2 at 4.2 K, for 2-EHHS, 2-EH-GP and 2-EH-Gd films, respectively. In contrast, the 2-EH-HS film shows the maximum Jc of 2.19 MA/cm2 at 77K. The Jc of other films are 0.92, 0.88, 0.51 MA/cm2 at 77 K for 2-EH-Gd, 2-EH-GP and CS-Gd, respectively. The reason that the Jc of 2-EH-GP showed high value was considered that Pt  acted as an effective pining center. The reason that the Jc of 2-EH-HS showed high value was considered  that defects introduced by disorder at the RE site acted as effective pinning center. The Jc of 2-EH-HS is about 2.38 times as large as Jc of 2-EH-Gd at 77K. The Jc of CS-Gd film is about 9.1 times as large as the Jc of 2-EH-Gd at 4.2K. In contrast, the CS-Gd film is about 0.55 times as small as Jc of 2-EH-Gd at 77K. As shown in fig.4, anomalous behaviour of Jc is observed only in CS-Gd. 4. Discussion In general superconducting films having high Jc at 77 K have also high Jc at low temperatures as 2-EHHS film derived maximum Jc of 2.19 MA/cm2 at 77 K and high Jc of 17.8 MA/cm2 at 4.2 K and self-field. On the contrary CS- GdBCO film achieved much higher Jc of 30 MA/cm2 at 4.2 K in spite of remaining low Jc of 0.51 MA/cm2 at 77 K. An anomalous temperature dependence of Jc was also observed for CSGdBCO film.

653

Kazuya Matsumoto et al. / Physics Procedia 36 (2012) 649 – 654

6

2.5 10

35

Applied Field : 0.1 [T]

Jc [A/cm2]

2 10

6

1.5 10

6

1 10

Fp(Ta0.794) Fp (Nb0.794) Fp (NbTa0.794) Fp (NP0.794)

30 Fp[GN/m3]

CS-Gd 2-EH-Gd 2-EH-GP 2-EH-HS

6

25 20 15

5

5 10

10

0 60 62 64 66 68 70 72 74 76 Temperature [K] Fig.5 Temperature dependence of critical current density at 60-77 [K] (The lines in the figure indicate extrapolations of the data points.)

5 3

3.5 4 4.5 Temperature [K]

5

Fig.6 Temperature dependence of Fp at d f = 0.518 ȝm and B = 1 T [15] (The lines in the figure indicate extrapolations of the data points.)

Figure 5 shows the temperature dependence of Jc at 0.1 T and high temperature range (60-77K) for all films. It is noteworthy that the slope of the temperature dependence of Jc changes dramatically at 70.5 K only for CS-Gd film. This suggests the flux pinning mechanism changes at around 70.5 K for the CS-Gd film. It is reported that lower-Tc regions such as substituted regions or oxygen-deficient regions act as pinning centers in GdBa2Cu3Oy films [13]. As shown in figure 1(a), CS-Gd film has Ba- and Cu-rich precipitates in comparison with other 2-EH-films. Thus it is considered that some Ba sites substituted by Gd ions exist in the CS-Gd film as flux pinning sites. This is also supported by observation of low Tc for CS-Gd films (see figure 3) according to proximity effect of the lower Tc regions in the superconducting matrix. The elementary pinning force fp for very thin superconducting pins in which the spatial variation of the order parameter of superconducting matrix is very small due to the proximity effect [14] can be expressed by sum of potential energy interaction fpp and kinetic energy interaction fpk, d p[ 2 Bcp  Bc2 , f pp 0.479S (1) P0

dp

f pk

0.296S

fp

f pp  f pk .

P 0[

B

2 cp

[ p2  Bc2[ 2 ,

(2) (3)

Where dp is the pin size, [, [p and Bc, Bcp are the coherence lengths and thermodynamic critical fields of superconducting matrix and superconducting pins, respectively [15]. In this case kinetic energy interaction is dominant pinning energy and generates repulsive pinning force. Above Tc for lower-Tc region S-N transition occurs, and thus the lower Tc region becomes normal conductor with condensation energy interaction. Figure 6 shows the temperature dependence of flux pinning force density Fp for several kinds of NbTi wires with artificial superconducting pins introduced into NbTi superconductor [15]. The Fp increases with increasing kinetic energy interaction of each superconducting pin. Here, it is clearly observed that the slope of temperature dependence of Fp changes at 4.4 K in which S-N transition

654

Kazuya Matsumoto et al. / Physics Procedia 36 (2012) 649 – 654

occurs for Ta pins. This behaviour is remarkably similar to that for CS-Gd film. This consistency is speculated that lower-Tc regions act as strong pinning centers at low temperatures for the CS-Gd film. 5. Conclusion We studied the relationship between micro structure and flux pinning properties for several kinds of REBCO films fabricated by a MOD technique using MN and 2-EH solutions. (Sm0.3, Ho0.7) Ba2 Cu3 Oy film prepared by 2-EH solution achieved the maximum Jc of 2.19 MA/cm2 at 77 K under self-field due to introduction of effective pinning sites. GdBa2Cu3Oy film prepared using complex solutions (CS) obtained high Jc of 30 MA/cm2 at 4.2 K in spite of having low Jc of 0.51 MA/cm2 at 77 K. Anomalous temperature dependence of Jc was also observed for CS-GdBa2Cu3Oy film. It suggests that strong pinning centers are induced into CS- GdBa2Cu3Oy film at low temperatures. It is speculated that lower-Tc regions such as substituted regions act as effective pinning centers in the CS-GdBa2Cu3Oy film by comparison analysis with flux pinning for NbTi with different superconducting artificial pins.

References [1] R. Teranishi, T. Izumi, and Y. Shiohara, Supercond. Sci. Technol. 19,(2006), S4-S12 [2] Y. Shiohara,Y. Kitoh, T. Izumi, Physica C, 445-448, (2006), 496-503 [3] Y. Shiohara, N. Fujiwara, H. Hayashi, S. Nagaya, T. Izumi, and M. Yoshizumi, Physica C, 469, (2009), 863-867 [4] A. Sheth, H. Schmidt, V. Lasrado, Appl. Supercond. 6, (1998), 855-873 [5] D. Dijkkamp, T. Venkatesan, X. D. Wu, S. A. Shaheen, N. Jisrawi, Y. H. Min-Lee, W. L. McLean and M. Croft: Appl. Phys. Lett. 51 (1987) 619 [6]M. Ottosson, T. Andersson, J. -O. Carlsson, A. Harsta, and U. Jansson, Appl. Phys. Lett.54, (1989), 2476 [7]A. Gupta, R. Jagannathan, E. I. Cooper, E. A. Geiss, J. I. Landman and B. W. Hussey, Appl. Phys. Lett. 52, (1988), 2077 [8] P. C. Mclntyre, M. J. Cima, and M. F. Ng, J. Appl. Phys, 68, (1990), 4183 [9] M. W. Rupich, S. Annavarapu, C. Thieme, W. Zhang, V. Prunier, M. Paranthaman, A. Goyal, D. F. Lee, E. D. Specht, and F. A. List, IEEE Trans. Appl. Supercond, 11, (2001), 2927 [10] T. Nakamura, R. Kita, O. Miura, A. Ichinose, K. Matsumoto, Y. Yoshida, M. Mukaida, S. Horii, Physica C, 468 (2008) 15421545 [11] C. P. Bean, Rev. Mod. Phys. 36, (1964), 31 [12] M. Mukaida, S. Miyazawa, Jpn. J. Appl. Phys.32 (1993) 4521-4528. [13] T. Matsushita Supercond. Sci. Technol. 13(2000) 730-737. [14] Y.Zhu, O.Miura, K.Hayakawa, and D.Ito, IEEE Trans. Appl. Supercond, 11, (2001), 3808 [15] Y. Zhu, O.Miura, T.Okubo, D.Ito, and S.Endo, IEEE Trans. Appl. Supercond, 10, (2000), 1046