Fujikura Technical Review, 2017 19

Yttrium-based Coated Conductors WithArtifitial Pinning Centers

Tomo Yoshida,1 Shogo Muto,1 Wataru Hirata,1 Yutaka Adachi,1 Shinji Fujita,1

Mitsunori Igarashi,1 Kazuomi Kakimoto,1 Yasuhiro Iijima,1

Kunihiro Naoe,1 and Satoshi Awaji2

Yttrium (Y) -based coated conductors (CCs) are known as helium-less materials and are expected to be applied to the systems such as MRIs and accelerators which need high critical currents (Ic). In this work, we fabricated Y-based CCs with artifitial pinning centers (APCs), which are known to improve in-field Ic, and uniform high in-field Jc by Hot-Wall heating PLD system.

1. Introduction

1.1. Yttrium (Y) -based coated conductors (CCs)

Y-based CCs are called as the second-generation high-temperature superconducting CCs which are represented in REBa2Cu3Oy (REBCO, RE = rare earth such as Y, Gd, Eu, Sm, etc.), and are expected to be the next-generation superconducting materials, because they can exhibit the higher critical current densities (Jc) under high temperatures and high magnetic fields (B) compared to the conventional metal-based superconducting wires and the Bismuth (Bi) -based superconducting wires.

Figure 1 shows the structure of Y-based CCs of Fujikura. We deposited a buffer layer, a superconducting layer, a protection layer, and a stabilizer in this order on it. Finaly, we provided insulation on a HastelloyTM C276 tape substrate which consists primarily of Ni by wrapping a polyimide tape. Y-based CCs have high tensiled and bending strength as well as high Jc properties. Furthermore, the longitudinal Jc distribution of the CCs is quite uniform. For example, we successfully fabricated an over 800-m-long Y-based CC with end-to-end measured Ic of 572 A/cm 1).

1.2. Y-based CCs with artifitial pinning centers (APCs)

To target the applications such as MRIs and accelerators which are operated at high currents under magnetic fields, the further improvement of the in-field Jc properties of Y-based CCs is needed. Introduction of APCs like as BaZrO3 (BZO), BaSnO3

(BSO), and BaHfO3 (BHO) nano-rods is effective for the enhancement of the in-field Jc properties, because APCs can control magnetic flux behavior. Especially,

in case of the superconducting with BHO, the degradation of the in-field Jc properties of thicker films is small compared to those with BZO or BSO. It is reported that the high in-field Jc up to 0.3 MA/cm2 @ 77 K, 3 T of GdBCO + BHO films with 2.9 µm in thickness was obtained 2). It is also reported that replacing Gd with Eu enchances the in-field Jc properties. Recent research indicated that the extremely high in-field Jc up to 0.4 MA/cm2 @ 77 K, 3 T of EuBCO + BHO films with 3.6 µm in thickness was obtained 3).

We have tried to fabricate Y-based CCs with APCs expected to be of the high in-field Jc properties by Hot-Wall heating PLD system, which is Fujikura original technology 4) 5) 6). In this paper, based on the past results, we report on the in-field Jc properties of various types of Y-based CCs.

2. Y-based CCs with APCs by Hot-Wall heating PLD system and in-field properties

2.1. Hot-Wall heating PLD system

We have fabricated high-quality long length Y-based CCs by means of IBAD/PLD systems, which are

Fig. 1. The structure of Y-based coated conductor at Fujikura.

Stabilizer [Sn coated Cu / Electroplate Cu] 20 µm

Protection Layer [Ag] 2 µm

Superconducting Layer [GdBa2Cu3Ox] 2 µm

Buffer Layer [MgO, etc.] ~0.7 µm

Substrate [Hastelloy®] 75 µm

Width 4 mm 1 Energy Technology Research Department of Advanced Technology Laboratory2 High Field Laboratory for Superconducting Materials, Institute for Materials

Research, Tohoku University

20

Fujikura original technologies. In particular, we have developed the original PLD system, it is called “Hot-Wall heating PLD system”. Figure 2 shows the schematic of Hot-Wall heating PLD system. This system can fabricate uniform Y-based CCs at a high deposition rate, because it produces a highly stable

temperature environment over a wide range by adiabatically fencing the superconducting deposition region like an electric furnace.

2.2. Sample specifications

Table 1 shows specifications of the CC samples with APCs : GdHf, GdZr, EuHf, GdHf2, and EuHf2. For comparison, table 1 also shows specifications of the long length mass-produced CC without APCs (Pure) . Types of RE are Gd and Eu, and types of APCs are BHO and BZO. The optimized volume which was reported by Tobita et al 2) was selected for the volume of APCs. Regarding the deposition condition, A indicates the mass-prodution condition in long length without APCs, B indicates the high Jc condition optimized at 30 K and 2 T with APCs, and C indicates the mass-production condition in long length with APCs. The deposition rates were over 40 nm/sec, 5 - 6 nm/sec, and 20 - 30 nm/sec, respectively. The CC length, the Tc (offset) value, the superconducting layer

Panel 1. Abbreviations, Acronyms, and Terms.

Technical terms/Abbreviations Description

Critical temperature, Tc Maximum temperature that can keep a supercon-ducting state.

Critical current, Ic Critical current (Ic) is the maximum current value that can flow in a superconducting state. It de-pends on temperatures and magnetic fields.

Critical current density, Jc Critical current density (Jc) equals the critical current (Ic) divided by the cross-section area of a superconducting coated conductor.

Y-based coated conductor An oxide superconducting wire whose supercon-ducting layer contains yttrium (Y), gadolinium (Gd) and other rare earth elements. Compared to the other oxide superconducting wire, It has a high Ic in a magnetic field at a relatively high temperature of 30 K or higher.

Ion beam assisted deposition, IBAD Ion beam assisted deposition (IBAD) is the key original technology when fabricating Y-based coated conductors, and is a method by which the crystalline orientation depending on the super-conducting characteristics is highly controlled. This is applied to a buffer layer which is located between a metal tape and a superconducting layer. Fujikura has basic patents of IBAD in Japan, USA, Europe, and it is widely used for fabricating Y-based coated conductors with high characteris-

tics.Pulsed laser deposition, PLD

Pulsed laser deposition (PLD) is a method when fabricating Y-based coated conductors. This is applied to a superconducting layer in a vacuum by focusing a beam of ultraviolet laser radiation such as excimer laser radiation.

Flux pinning force, Fp Flux pinning force (Fp) equals the critical current density (Jc) multiplied by the magnetic field (B). This means the total pinning force per unit vol-ume.

Angular dependence of Jc at certain B, Jc-B-q Angular dependence of Jc at certain B is the Jc curve by changing the magnetic applied angle. The shape of the Jc curve varies according to the dispersion of the angle of the artificial pinning centers.

Transmission Electron Microscope, TEM Transmission electron microscope (TEM) is a microscopy technique in which a beam of elec-trons is transmitted through a specimen to form an image.

Matching field Matching field is a magnetic field in which the grid spacing of the quantized magnetic fluxes equals the interval between the artificial pinning centers. Matching field is thought that the pin-ning of magnetic fluxes can be exhibited effec-tively.

Fig. 2. The schematic of Hot-Wall heating PLD system.

Multi-lanes

IBADSubstrate

GdBCOTarget

ExcimerLaser

ScanningLaser

Hot-wallHeating

Fujikura Technical Review, 2017 21

thickness and the Ic value (77.3 K in a self field (s.f.) and 30 K, 2 T) are also listed in teble 1.

2.3. Measuring methods of in-field Ic

We clipped the measuring samples from the end of every CC, and a micro-bridge with 30 µm in width and 1 mm in length was formed by photolithography and wet-etching. Then, we inserted a probe with the sample into a superconducting magnet, cooled the magnet by helium gas, and measured I-V characteristics by the conventional four-probe method in which the

electric-field criteria are 1 µV/cm. The measurement temperature is from 4.2 K to 77.3 K and applied external magnetic field is up to 18 T. The magnetic field applied angle (q) of Jc-B-q profiles is 0 - 180°. The measurment was conducted in the coorporation with High Field Laboratory for Superconducting Materials, Institute for Materials Research, Tohoku University.

2.4. Uniformity of longitudinal Ic distribution

Figure 3 shows the Ic uniformity of the CC samples with APCs ; GdHf, GdZr, EuHf, GdHf2 and EuHf2 ; at

Table 1. Specifications of the CCs by Hot-Wall heating PLD system.

Index Type of REType of APCs

Volume

Deposition Condition

(*2)Length [m]

Tc (offset) [K]

Superconducting Thickness

[µm]

Ic

(77.3 K, s.f.) [A/cm]

Ic

(30 K, 2 T) [A/cm]

GdHf GdBaHfO3 3.5mol%

B 10 90.9 1.02 242 989

GdZr GdBaZrO3 5.0mol%

B 10 91.2 1.09 274 1050

EuHf EuBaHfO3

3.5mol%B 10 92.0 1.05 250 1134

GdHf2 GdBaHfO3 3.5mol%

C 50 90.4 1.32 267 1057

EuHf2 EuBaHfO3 3.5mol%

C 300 92.5 2.60 492 1863

Pure(*1) Gd - A 330 92.5 1.92 525 1063

(*1) Mass-produced products in long length.(*2) Deposition conditions are as indicated below.A : Mass-produced condition in long length without artificial pinning centers.B : High Jc condition optimized at 30 K and 2 T with artificial pinning centers.C : Mass-produced condition in long length with artificial pinning centers.

I c(avg.) : 287 ASTDV : 11.01 A

Uniformity : 3.8%

I c(avg.) : 329 ASTDV : 5.29 A

Uniformity : 1.6%

I c(avg.) : 301 ASTDV : 5.1 A

Uniformity : 1.7%

I c(avg.) : 307 ASTDV : 8.29 A

Uniformity : 2.7%

GdZr

EuHf

GdHf2

GdHf

0 2

Tape Length [m]

300

200

100

I c(77

.3K

, sel

f fie

ld)

[A/1

2 m

m]

0

0 2

300

200

100

0

0 2

300

200

100

0

0 10

4

4

4

20

6

6

6

30

8

8

8

40 50

300

200

100

0

Fig. 3. The Ic uniformity of the CCs at 77.3 K in a self field by Hot-Wall heating PLD system.

22

77.3 K in a self field. The uniformity was defined as the standard deviation (STDV) of the longitudinal Ic values divided the average Ic value. The uniformity of the all CCs with APCs showed the low value up to 4%. Especially, despite of the 300-m-long CC, the uniformity of EuHf2 showed the very low value of 2.6%. This is comparable to those of the past CCs without APCs up to 2%, and it is found quite high Ic uniformity of the CCs was obtained 1).

2.5. In-field Jc, flux pinning force (Fp), and Jc-B-q profiles

Figures 4(a)-(g) show the in-field Jc, Fp and Jc-B-q profiles at various environment temperature In consequence, we found that the in-field Jc properties were dramatically varied depending on the the type of RE, the type of pinning centers, and the deposition condition. The measured results at every environment temperature were as follows.

Figures 4(a) and (b) show the measured results at

Fig. 4.(a)-(c) In-field characteristics of the CCs by Hot-Wall heating PLD system (77.3~50 K).

I

θ

a,b

Bc

I

θ

a,b

Bc

1.0E+07

1.0E+05

1.0E+06

1.0E+04

1.0E+03

1.0E+020 10 205 15

Jc [

A/c

m2 ]

B [T](a) 77.3 K

(b) 65 K

1.2E+07

8.0E+06

1.0E+07

6.0E+06

4.0E+06

2.0E+06

0.0E+000 10 205 15

Jc [

A/c

m2 ]

B [T]

5.0E+06

4.0E+064.5E+06

3.5E+063.0E+06

5.0E+05

2.0E+062.5E+06

1.5E+061.0E+06

0.0E+00-50 50 1500 100

Jc [

A/c

m2 ]

θ [deg](c) 50 K

77.3 K, B//c

7.0E+06

5.0E+06

6.0E+06

4.0E+06

3.0E+06

2.0E+06

1.0E+06

0.0E+000 10 205 15

Jc [

A/c

m2 ]

B [T]

65 K, B//c

50 K, B//c

1.4E+10

1.0E+10

1.2E+10

8.0E+09

6.0E+09

4.0E+09

2.0E+09

0.0E+000 10 205 15

Fp [

N/m

3 ]

8.0E+10

6.0E+10

7.0E+10

5.0E+10

4.0E+10

3.0E+10

2.0E+10

1.0E+10

0.0E+000 10 205 15

Fp [

N/m

3 ]

B [T]

2.5E+11

1.5E+11

2.0E+11

1.0E+11

5.0E+10

0.0E+000 10 205 15

Fp [

N/m

3 ]

B [T]

77.3 K, B//c

65 K, B//c

50 K, B//c

1.6E+06

1.2E+06

1.4E+06

1.0E+06

8.0E+05

6.0E+05

4.0E+05

2.0E+05

0.0E+00-50 50 1500 100

Jc [

A/c

m2 ]

θ [deg]

65 K, 5T

B//c B//ab

B//c B//ab

50 K, 5T

B [T]

Pure

GdHf

GdHf2

EuHf

GdZr

Pure GdHf GdZr GdHf2EuHf

Pure GdHf GdZr GdHf2EuHf

Fujikura Technical Review, 2017 23

77.3 K and 65 K respectively. In this temperature range, except at 77.3 K over 10 T and except at near B//ab (q : 70 - 110°) in the Jc-B-q profile, the in-field Jc

values of EuHf, GdZr, GdHf, GdHf2, and Pure were high in this order.

Figures 4(c) and (d) show the measured results at

50 K and 40 K respectively. In this temperature range, the in-field Jc values of GdZr, GdHf and GdHf2 were nearly equal, and the Jc-B-q profiles of them clearly showed a different behavior from that of EuHf. In the Jc-B-q profile of EuHf, the in-field Jc value at B//c was higher compared to those of the Gd series, and that at

Fig. 4.(d)-(f) In-field characteristics of the CCs by Hot-Wall heating PLD system (50~20 K).

I

θ

a,b

B

c

I

θ

a,b

B

c

I

θ

a,b

B

c

(d) 40 K

2.5E+07

1.5E+07

2.0E+07

1.0E+07

5.0E+06

0.0E+000 10 205 15

Jc [

A/c

m2 ]

B [T](e) 30 K

4.0E+07

2.0E+07

3.0E+07

1.0E+07

2.5E+07

3.5E+07

1.5E+07

5.0E+06

0.0E+000 10 205 15

Jc [

A/c

m2 ]

B [T](f) 20 K

1.8E+07

1.0E+07

1.4E+07

6.0E+06

2.0E+06

1.2E+07

1.6E+07

8.0E+06

4.0E+06

0.0E+000 10 205 15

Jc [

A/c

m2 ]

B [T]

40 K, B//c

Pure GdHf GdZr GdHf2EuHf

Pure GdHf GdZr GdHf2EuHf

Pure GdHf GdZr GdHf2EuHf

30 K, B//c

4.0E+11

2.0E+11

3.0E+11

1.0E+11

2.5E+11

3.5E+11

1.5E+11

5.0E+10

0.0E+000 10 205 15

Fp [

N/m

3 ]

B [T]

6.0E+11

5.0E+11

3.0E+11

4.0E+11

2.0E+11

1.0E+11

0.0E+000 10 205 15

Fp [

N/m

3 ]

B [T]

9.0E+11

5.0E+11

7.0E+11

3.0E+11

1.0E+11

6.0E+11

8.0E+11

4.0E+11

2.0E+11

0.0E+000 10 205 15

Fp [

N/m

3 ]

B [T]

40 K, B//c

30 K, B//c

20 K, B//c

20 K, B//c

1.8E+07

1.0E+07

1.4E+07

6.0E+06

2.0E+06

1.2E+07

1.6E+07

8.0E+06

4.0E+06

0.0E+00-50 50 1500 100

Jc [

A/c

m2 ]

θ [deg]

3.0E+07

1.5E+07

2.0E+07

2.5E+07

1.0E+07

5.0E+06

0.0E+00-50 50 1500 100

Jc [

A/c

m2 ]

θ [deg]

9.0E+06

5.0E+06

7.0E+06

3.0E+06

1.0E+06

6.0E+06

8.0E+06

4.0E+06

2.0E+06

0.0E+00-50 50 1500 100

Jc [

A/c

m2 ]

θ [deg]

B//c B//ab

B//c B//ab

B//c B//ab

40 K, 5T

30 K, 5T

20 K, 5T

4.0E+07

3.0E+07

3.5E+07

2.5E+07

2.0E+07

0.5E+06

1.0E+07

1.5E+07

0.0E+000 10 205 15

Jc [

A/c

m2 ]

B [T]

1.8E+12

1.2E+12

1.6E+12

8.0E+11

4.0E+11

1.0E+12

1.4E+12

6.0E+11

2.0E+110.0E+00

0 10 205 15

Fp [

N/m

3 ]

B [T]

4.2 K, B//c

4.2 K, B//c

1.67 TN/m3 @ 18 T

1.20 TN/m3 @ 18 T

0.44 TN/m3 @ 17 T

(g) 4.2 K

Pure

GdHf2

EuHf

24

near B//ab was lower to the contrary .Figures 4(e) and (f) show the measured results at

30 K and 20 K respectively. In this temperature range, the in-field Jc values over 5 T of EuHf, GdHf2, GdHf, GdZr and Pure were high in this order. Especially, the order of the Gd series showed the opposite results to that at 77.3 K and 65 K. According to the Jc-B-q profiles of EuHf, GdHf2, GdHf, and GdZr the in-field Jc peak at B//c was lower, and on the other hand that at B//ab was sharper compared to those at 50 K and 40 K. The in-field Jc values of EuHf, GdHf2, GdHf and GdZr in all angle directions were much higher than that of Pure.

Figure 4(g) shows the measured results at 4.2 K. The maximum of Fp value of EuHf under 18 T was rather high (~1.67 TN/m3).

2.6. Microscopic superconducting texture

Figure 5 shows the planar and cross-sectional TEM images of EuHf. In the planar TEM image, BHO nano-rods appear as black small dots. Meanwhile, in the cross-sectional TEM image, BHO nano-rods appear as black lines in an oblique direction in patches. The gray matrix surrounding BHO nano-rods is EuBCO matrix. In the cross-sectional TEM image, the lateral long black lines are stacking faults, and the vertical long black lines are the grain boundaries or the spiral centers. In the planar TEM image with a detailed observation, the diameter of the BHO nano-rods was 3 - 4 nm, and the interval between BHO nano-rods was 20 - 30 nm. The matching field was 5 - 7 T calculated from the interveal between BHO nano-rods. In the future, we want to consider how the matching field changes by changing the deposition condition or type of APCs.

3. ConclusionsIn this work, we fabricated Y-based CCs with APCs,

by changing the type of RE, the type of APCs, and the deposition condition by Hot-Wall heating PLD system which produces a highly stable temperature environment. We obtained the CCs for attaining quite

high Ic uniformity at the high deposition rate (≧20 nm/sec). Especially, in the case of the combination of EuBCO + BHO, the maximum of Fp value at 4.2 K under 18 T was rather high (~1.67 TN/m3). Furthermore, in the planar and cross-sectional images with a detailed observation, we successfully observed BHO nano-rods that are the origin of the enhancement of the in-field Jc properties.

We are pushing to produce the low-cost Y-based CCs with APCs by the further enhancement of the deposition rate and the in-field Jc properties. Moreover, we would like to contribute the progress of the magnetic applications through the element technologies including coil development.

AcknowledgmentsThis work included results supported by New

Energy and Industrial Technology Development

Plan-view Cross-sectional

3-4nm dia.~20nmdistance

correspondingmatching field:5-7T

100 nm

100 nm

10 nm

Fig. 5. The planar and cross-sectional TEM images of EuHf by Hot-Wall heating PLD system.

Fig. 4.(g) In-field characteristics of the CCs by Hot-Wall heating PLD system (4.2 K).

I

θ

a,b

B

c

I

θ

a,b

B

c

I

θ

a,b

B

c

(d) 40 K

2.5E+07

1.5E+07

2.0E+07

1.0E+07

5.0E+06

0.0E+000 10 205 15

Jc [

A/c

m2 ]

B [T](e) 30 K

4.0E+07

2.0E+07

3.0E+07

1.0E+07

2.5E+07

3.5E+07

1.5E+07

5.0E+06

0.0E+000 10 205 15

Jc [

A/c

m2 ]

B [T](f) 20 K

1.8E+07

1.0E+07

1.4E+07

6.0E+06

2.0E+06

1.2E+07

1.6E+07

8.0E+06

4.0E+06

0.0E+000 10 205 15

Jc [

A/c

m2 ]

B [T]

40 K, B//c

Pure GdHf GdZr GdHf2EuHf

Pure GdHf GdZr GdHf2EuHf

Pure GdHf GdZr GdHf2EuHf

30 K, B//c

4.0E+11

2.0E+11

3.0E+11

1.0E+11

2.5E+11

3.5E+11

1.5E+11

5.0E+10

0.0E+000 10 205 15

Fp [

N/m

3 ]

B [T]

6.0E+11

5.0E+11

3.0E+11

4.0E+11

2.0E+11

1.0E+11

0.0E+000 10 205 15

Fp [

N/m

3 ]

B [T]

9.0E+11

5.0E+11

7.0E+11

3.0E+11

1.0E+11

6.0E+11

8.0E+11

4.0E+11

2.0E+11

0.0E+000 10 205 15

Fp [

N/m

3 ]

B [T]

40 K, B//c

30 K, B//c

20 K, B//c

20 K, B//c

1.8E+07

1.0E+07

1.4E+07

6.0E+06

2.0E+06

1.2E+07

1.6E+07

8.0E+06

4.0E+06

0.0E+00-50 50 1500 100

Jc [

A/c

m2 ]

θ [deg]

3.0E+07

1.5E+07

2.0E+07

2.5E+07

1.0E+07

5.0E+06

0.0E+00-50 50 1500 100

Jc [

A/c

m2 ]

θ [deg]

9.0E+06

5.0E+06

7.0E+06

3.0E+06

1.0E+06

6.0E+06

8.0E+06

4.0E+06

2.0E+06

0.0E+00-50 50 1500 100

Jc [

A/c

m2 ]

θ [deg]

B//c B//ab

B//c B//ab

B//c B//ab

40 K, 5T

30 K, 5T

20 K, 5T

4.0E+07

3.0E+07

3.5E+07

2.5E+07

2.0E+07

0.5E+06

1.0E+07

1.5E+07

0.0E+000 10 205 15

Jc [

A/c

m2 ]

B [T]

1.8E+12

1.2E+12

1.6E+12

8.0E+11

4.0E+11

1.0E+12

1.4E+12

6.0E+11

2.0E+110.0E+00

0 10 205 15

Fp [

N/m

3 ]

B [T]

4.2 K, B//c

4.2 K, B//c

1.67 TN/m3 @ 18 T

1.20 TN/m3 @ 18 T

0.44 TN/m3 @ 17 T

(g) 4.2 K

Pure

GdHf2

EuHf

Fujikura Technical Review, 2017 25

Organization (NEDO) and Ministry of Economy, Trade and Industry (METI). And, we sincerely thank for the coorporation with measurement of in-field characteristics to High Field Laboratory for Superconducting Materials, Institute for Materials Research, Tohoku University.

References

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