Physics Procedia 36 ( 2012 ) 582 – 587

1875-3892 © 2012 Published by Elsevier B.V. Selection and/or peer-review under responsibility of the Guest Editors. doi: 10.1016/j.phpro.2012.06.171

Superconductivity Centennial Conference

Two-axis magnetic field orientation dependence of critical current in full-width REBCO coated conductors

S. C. Hopkinsa*, M. Woźniaka, B. A. Glowackia,b, Y. Chenc, I. Kesgind and V. Selvamanickamd

aDepartment of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, UK bInstitute of Power Engineering, ul. Augustowka 6, 02-981 Warsaw, Poland

cSuperPower Inc., 450 Duane Avenue, Schenectady, NY 12304, USA dDepartment of Mechanical Engineering, University of Houston, 4800 Calhoun Road, Houston, TX 77204, USA

Abstract

Considerable progress has been made with the development of YBa2Cu3O7-δ and related rare-earth (REBCO) coated conductors in recent years. The introduction of BaZrO3 (BZO) nanoparticles has resulted in enhanced critical current density (Jc) for magnetic field orientations away from the intrinsic ab peak, with the absolute Jc and the degree of anisotropy depending on the geometry and orientation distribution of the nanoparticles; and REBCO coated conductors with a variety of substitutions and additions (Gd, Sm, Ba, Zr, Sn) are becoming commercially available in kilometre lengths. It is therefore increasingly important to characterise the magnetic field orientation dependence of critical current (Ic) in full-width coated conductors without destructive sample preparation, both for fundamental pinning studies and the design of large-scale applications. A two-axis goniometer has been developed, allowing the critical current to be measured as a function of both the magnitude and orientation of the applied magnetic field, Ic(B,θ,ϕ), on full-width samples. Measurements of Ic(B,θ,ϕ) at 77 K are reported for four REBCO tapes made by metal organic chemical vapour deposition (MOCVD) with different rare earth content and the results interpreted in relation to rare earth substitution and pinning centre populations. The implications of the field orientation dependence for low and intermediate field applications are discussed. © 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of Horst Rogalla and Peter Kes.

Keywords: Critical current, orientation, magnetic field, coated conductor, cuprate, two-axis, goniometer, YBCO, IBAD

* Corresponding author. E-mail address: sch29@cam.ac.uk

Available online at www.sciencedirect.com

© 2012 Published by Elsevier B.V. Selection and/or peer-review under responsibility of the Guest Editors.

Open access under CC BY-NC-ND license.

Open access under CC BY-NC-ND license.

S. C. Hopkins et al. / Physics Procedia 36 ( 2012 ) 582 – 587 583

1. Introduction

Considerable progress has been made with the development of YBa2Cu3O7-δ and related rare-earth (REBCO) coated conductors in recent years. The introduction of nanoparticles, and notably the growth of BaZrO3 (BZO) aligned with the REBCO c axis, has allowed enhancement of the critical current density (Jc) for magnetic field orientations away from the intrinsic ab peak; and progress with controlling the size, geometry and orientation distribution of such additions has resulted in a growing ability to control both the magnitude and anisotropy of the Jc enhancement [1]. Coated conductors with a variety of substitutions and additions (Gd, Sm, Ba, Zr, Sn) are also now becoming commercially available in kilometre lengths [1,2]. It is therefore increasingly important to characterise the magnetic field orientation dependence of critical current (Ic), both for fundamental pinning studies and for the design of large-scale applications; and it is particularly desirable that this should be possible for full-width (typically 12 mm wide) commercial coated conductors without destructive sample preparation.

The orientation-dependence of Ic is typically reported only for rotation of the magnetic field around

one axis, with the applied magnetic field remaining perpendicular to the current direction. For conductors with the c axis perfectly perpendicular to the substrate plane, and for applications in high magnetic field coils, this is sufficient to characterise the anisotropy of the pinning contributions for the orientation range of practical importance. In order to fully characterise the behaviour of the conductor, however, the dependence of Ic on two other angles would ideally be measured. The first, the direction of the current in the plane of the conductor, cannot be controlled for transport measurements on unmodified full-width conductors, but measurements have been reported for tracks patterned parallel and perpendicular to the length of an MOCVD/IBAD tape [3]. The second, the angle between the current and the magnetic field directions, can be adjusted experimentally on unprepared tapes by tilting the sample plane out of the conventional perpendicular field geometry: and the dependence of Ic on this angle is investigated here.

A two-axis goniometer has been developed, allowing the critical current to be measured as a function

of both the magnitude and orientation of the applied magnetic field, Ic(B,θ,ϕ), on full-width samples [4,5]. Measurements of Ic(B,θ,ϕ) at 77 K are reported for four REBCO tapes made by metal organic chemical vapour deposition (MOCVD) with different rare earth content and the results interpreted in relation to rare earth substitution and pinning centre populations. The implications of the field orientation dependence for conductor development and for applications will be discussed.

2. Experimental methods

2.1. Critical current goniometer

A critical current measurement system has been developed based on a two-axis goniometer probe [4,5]. The probe is supported vertically between the poles of a fixed electromagnet, and the sample can be precisely oriented around two perpendicular axes using computer-controlled stepper motors, as illustrated in figure 1(a). The whole probe can rotate about its long axis (‘rotation’), and a moving sample platform can rotate about a perpendicular axis through the mid-point of the sample platform (‘tilt’), as shown in figure 1(b). Current is transferred to the tilting platform by sliding contacts, which limited the maximum current for the present work to 120-140 A.

Tape samples ~36 mm long were mounted on the tilting platform, with the length of the tape parallel

to the current and a typical voltage contact separation of 6 mm (figure 1). The current contacts between

584 S. C. Hopkins et al. / Physics Procedia 36 ( 2012 ) 582 – 587

the sample and the tilting platform were soldered using indium, and voltage taps were attached with silver paint. Measurements were performed in liquid nitrogen in an applied field of 0.592 T. The critical current was measured as a function of two-axis orientation, extracting Ic and the n value from automated power-law fits to the measured voltage-current characteristics using a 1 μV cm-1 criterion. The angular step size was varied with orientation in the range 1–5° for both axes, to combine high-resolution coverage of the regions in which critical current is strongly angle-dependent with a reasonable total scan duration.

(a) (b)

Figure 1: (a) Schematic illustration of the critical current goniometer probe, showing a tape sample mounted for measurement with the typical voltage contact spacing of 6 mm. (b) Illustration showing the definitions of the rotation and tilt angles.

2.2. Samples

Hastelloy C-276 (50-100 μm)

IBAD MgO (10 nm)

MOCVD REBCO (1 μm)

Silver (2 μm)

MgO (30 nm)LMO (30 nm)

Buffer layers(~150 nm)

12 mm

Yttria (7 nm)Alumina (80 nm)

Figure 2: Schematic illustration of the architecture of the coated conductor samples investigated, showing indicative layer thicknesses (not to scale). The REBCO layer is (Gd,Y)xBa2Cu3O7-δ with Zr additions, with the composition varying between samples as described in the text.

Coated conductor samples with the architecture shown in Figure 2 were supplied from Houston for characterisation in Cambridge. In every case, a textured MgO layer was deposited by IBAD on a non-

S. C. Hopkins et al. / Physics Procedia 36 ( 2012 ) 582 – 587 585

magnetic Hastelloy substrate, and Gd-substituted YBCO films with Zr additions were deposited by MOCVD. The conductors had a silver over-layer, to which contact was made for testing, and no additional stabilisation. Four samples were produced with different Gd, Y and Zr compositions, referred to as 2247-3, M3-732-12, 2247-16 and 2247-25, to create a range of pinning architectures and resulting Ic(θ) characteristics for investigation.

3. Results and discussion

3.1. Rotation-dependence of critical current

The critical current was first measured for each sample as a function of rotation in the range -120° < θ < 120° at a tilt angle of 0°, and the results are presented in Figure 3. As expected for this tilt angle, the critical current is very sensitive to rotation, with the expected ab peak near ±90° and in most cases a peak near 0° as a result of c axis aligned pinning.

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Figure 3: Critical current at 0.592 T as a function of rotation for the four coated conductor samples at a tilt angle of 0°. (a) The full measured range of rotation, (b) enlargement around -90° rotation and (c) enlargement around +90° rotation.

This behaviour has been widely investigated and reported, and can be interpreted based on TEM evidence in the literature, so it will be discussed only briefly here. The addition of Zr in these samples gives rise to largely c-aligned BZO columns, resulting in a c-axis peak in Ic; but the varying rare earth content and Gd substitution for Y results in the formation of ab-plane RE2O3 precipitates. Consequently, as the Gd content is increased the ab peaks are enhanced relative to the strong c axis peak, until for M3-732-12, with the highest rare earth content, the c-axis peak is largely smoothed out whilst the minimum Ic remains enhanced [1,2].

3.2. Variation of critical current with two-axis orientation

Figures 4 and 5 present the measured variation of critical current with the orientation about two axes for two samples, 2247-16 and 2247-25 respectively. The former is relatively symmetrical about 0° in rotation, whereas the latter is highly asymmetric in rotation and has significantly higher critical current in almost all orientations. In both cases the critical current is very insensitive to the tilt angle in the low angle range, approximately -30° < ϕ < 30°; whilst at high angles and near the force-free configuration the critical current markedly increases and its variation with rotation decreases in amplitude, as expected.

586 S. C. Hopkins et al. / Physics Procedia 36 ( 2012 ) 582 – 587

The insensitivity at low tilt angles is a positive feature for most applications: for example, when winding coils, a small degree of misalignment between turns (either at the time of winding or in service) is unlikely to have an appreciable effect on performance.

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Figure 4: Variation of critical current at 0.592 T with both rotation and tilt for sample 2247-16, plotted as (a) a three-dimensional surface plot and (b) a two-dimensional contour plot. Both representations use the same colour scale for critical current.

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125-130120-125115-120110-115105-110100-10595-10090-9585-9080-85

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Figure 5: Variation of critical current at 0.592 T with both rotation and tilt for sample 2247-25, plotted as (a) a three-dimensional surface plot and (b) a two-dimensional contour plot. Both representations use the same colour scale for critical current.

The behaviour at intermediate tilt angles may be of some consequence for applications in which the current follows a more complex path, particularly in low external magnetic fields, examples of which might include low AC loss conductor architectures or cables. It is clear for both samples that the rotation-dependence of critical current decreases at these tilt angles; but it is striking that the behaviour is asymmetric, even for sample 2247-16 for which the zero-tilt rotation behaviour is more symmetric (Figure 4). This requires more detailed investigation, in conjunction with information from TEM about the detailed orientation distribution of BZO and other pinning structures. The dependence of these results on the magnitude of the external magnetic field, particularly in the low (0–1 T) field range, is also a key priority for further investigation: as well as the relevance of these results for low-field applications, it is also expected that the contribution of the different defect structures to critical current will vary with field.

S. C. Hopkins et al. / Physics Procedia 36 ( 2012 ) 582 – 587 587

The behaviour at a tilt angle of 90° has not been plotted here, and the rotation dependence could not be measured in full due to the current limit for the present set of experiments. Although this orientation is of limited practical use, as it cannot be achieved in most applications, the dependence of the maximum critical current and its variation with rotation on conductor composition and pinning structures is worthy of study. Modifications to permit a higher current and magnetic field to be applied are planned.

4. Conclusions

A system has been developed to measure critical current (Ic) on full-width coated conductors as a function of the sample orientation around two axes relative to an applied magnetic field, and measurements have been successfully performed on four MOCVD/IBAD coated conductors with different rare earth content. The variation of Ic with rotation is not always symmetrical, and the large differences in this behaviour between samples has been related to the conductor composition and the resulting distributions of defects acting as pinning centres. This behaviour is very insensitive to tilt angle in the low angle range (-30° < ϕ < 30°), suggesting that small misalignments in coil winding or high field applications need not be a concern; but asymmetry and differences between samples do arise at higher angles, which may have implications for low-field cable applications, and will require further investigation.

Acknowledgements

S. C. Hopkins and B. A. Glowacki gratefully acknowledge financial support the EU Seventh Research Framework Programme (FP7) project ‘EFECTS’ (FP7-NMP-2007-SMALL-1, grant number 205854). M. Woźniak’s doctoral research is funded by Siemens Magnet Technology.

References

[1] Y. Zhang, E. D. Specht, C. Cantoni, D. K. Christen, J. R. Thompson, J. W. Sinclair, A. Goyal, Y. L. Zuev, T. Aytug, M. P. Paranthaman, Y. Chen and V. Selvamanickam, Physica C 469 (2009) 2044

[2] V. Selvamanickam, A. Guevara, Y. Zhang, I. Kesgin, Y. Xie, G. Carota, Y. Chen, J. Dackow, Y. Zhang, Y. Zuev, C. Cantoni, A. Goyal, J. Coulter and L. Civale, Supercond. Sci. Technol. 23 (2010) 014014

[3] M. Weigand, N.A. Rutter, S. Sahonta and J.H. Durrell, IEEE Trans. Appl. Supercond. 21 (2011) 3347 [4] M. Chudý, S. C. Hopkins, M. Woźniak, B. A. Glowacki, M. Eisterer and H. W. Weber, Supercond. Sci. Technol. 24 (2011)

075018 [5] T. Janowski, B. A. Glowacki, G. Wojtasiewicz, S. Kozak, J. Kozak, B. Kondratowicz-Kucewicz, M. Majka, M. Wozniak,

IEEE Trans. Appl. Supercond. 24 (2011) 1413