- Email: [email protected]
Research of long IBAD–PLD coated conductors with high quality
Physica C 426–431 (2005) 858–865 www.elsevier.com/locate/physc
Research of long IBAD–PLD coated conductors with high quality Kazuomi Kakimoto *, Yasunori Sutoh, Naoki Kaneko, Yasuhiro Iijima, Takashi Saitoh Fujikura Ltd., 1-5-1 Kiba, Koto-ku, Tokyo 135-8512, Japan Received 23 November 2004; accepted 27 January 2005
Abstract Long YBa2Cu3Ox (Y-123) coated tapes were produced by reel-to-reel continuous processes using Ion-Beam-Assisted Deposition (IBAD) and Pulsed-Laser-Deposition (PLD). Biaxially textured Gd2Zr2O7 buffer layers up to 255 m long were fabricated by IBAD with the production speed of 0.5–1.0 m/h. Y-123 films were formed on them by PLD with the tape speed of 1.0–4.0 m/h. A 105 m long Y-123 film was formed with the uniform thickness of 1.0 lm and the D/ of 3–4. The end-to-end Ic of 126 A was achieved at 77 K, 0 T, whose I–V curve had the n-value of 28.5. The Ic ˚ . Another 70 m long Y-123 tape with the Ic of over 90 A (77 K, 0 T) was wound into times length reached 13,230 A a solenoid type magnet whose inner diameter was 60 mm. The central magnetic field of 0.082 T was obtained at 77 K, with operating current of 42 A and that of 0.27 T was obtained at 66 K, with operating current of 130 A. 2005 Elsevier B.V. All rights reserved. PACS: 74.72.Bk; 85.25.Kx Keywords: IBAD; PLD; Metallic substrate; Y-123; CeO2
1. Introduction To eliminate intergranular weaklinks, the YBa2Cu3Ox (Y-123) coated conductors have quite sophisticated structures, composed of highly tex*
Corresponding author. Fax: +81 3 5606 1512. E-mail address: [email protected] (K. Kakimoto).
tured thin films on flat flexible substrates. Recently, rapid progresses have been made in several kinds of reel-to-reel processing technologies to improve the longitudinal uniformity of the Y-123 tapes long enough to examine the performance as practical superconducting wires. The Ion-Beam-Assisted Deposition (IBAD) template has a lot of advantages to avoid imperfection of Y-123 film growth. The process
0921-4534/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2005.01.089
K. Kakimoto et al. / Physica C 426–431 (2005) 858–865
temperature is so low that the substrate surface has no thermal damage during the deposition of the buffer films. Furthermore, it has excellent biaxial texture, smooth surface, and small grain size (5– 10 nm), which completely hides the large grain boundaries of the underlying metal substrate [1,2]. For the length of over 100 m, a large area deposition apparatus was successfully developed with long lifetime operation even in oxygen atmosphere using the state-of-the-art ion-source technology [3]. The Pulsed-Laser-Deposition (PLD) was used for Y-123 growth and secondary buffer layer of CeO2. It is characterized with stable stoichiometry, easy oxygen pressure control, very high growth rate [4,5], etc. In the demonstration of the magnet using the Y-123 conductor, we examined coiling capability for solenoid, which have many merits as joint-free, good thermal/mechanical stability etc., compared to the pancake type magnet. In this paper we describe the fabrication of the high-performance 100 m class Y-123 conductors by the combination of the IBAD and the PLD method, and a first demonstration of a solenoid type magnet operating at liquid nitrogen temperature by using a 70 m long Y-123 conductor.
2. Experimental Biaxially aligned Gd2Zr2O7 films were deposited by reel-to-reel, dual-ion-beam sputtering system with two sets of radio frequency (RF) discharged 66 cm · 6 cm square-shaped ion sources. Precisely roll-milled Hastelloy C276 tapes, the average roughness of which was below 30 nm, with the 10 mm width and the 100 lm thickness were used as the fundamental metallic substrate. Continuous growth of Gd2Zr2O7 films was performed with the production speed of 0.5–1.0 m/h under the condition of Table 1. CeO2 secondary buffer layers and Y-123 films were deposited by a reelto-reel PLD system with Kr–F (k = 248 nm) excimer laser. The detailed PLD condition is shown in Table 2. The tape speed was 1.0–4.0 m/h. Twenty micrometers thick Ag films were deposited on Y-123 films by RF magnetron sputtering and thereafter annealed for several hours at 500 C in 760 Torr oxygen. Crystalline alignment was mea-
859
Table 1 Deposition parameters of IBAD template films Sputtering ion beam (Ar+)
Beam voltage: 1500 V Beam current: 500–700 mA
Assisting ion beam (Ar+)
Beam voltage: 200 V Beam current: 400–600 mA Ion incident angle: 55 from substrate normal
Target
Sintered stoichiometric target of Gd2O3:ZrO2 = 1:2 200 C 3.0–4.0 · 10 2 Pa
Temperature of growth stage Vacuum pressure (Ar + O2)
Table 2 Deposition parameters of oxide films by PLD Film
CeO2
Laser pulse energy (mJ) Repetation rate (Hz) Vacuum pressure (O2) (Pa) Temperature of growth stage (C) Target
200–300 200–300 70–100 200 1–3 30–40 400–600 900–1000 Sintered stoichiometric target 50–80
Target–substrate distance (mm)
Y-123
sured by X-ray diffraction pole figure. Transport properties were measured at 77 K by DC fourprobe method. For long length measurement, the Y-123 tape was wound around a cylindrical holder. Voltage taps were attached at every 5 m length. The Ic was determined by the criterion of 10 6 V/cm. The n-values of the I–V curves were evaluated in the voltage per length range from 10 7 to 10 6 V/cm. A 70 m long Y-123 tape formed by the combination of IBAD and PLD was wound into a 12 turn · 22 layer solenoid type magnet as shown in Fig. 1. The designed specifications of the coil are shown in Table 3. The coil bobbin was made of fiber reinforced plastic (FRP). The coil was insulated with thin polyimide tapes and thin FRP sheets. In order to avoid edgewise strains in Y-123 tape, it was carefully treated at turning points between layers. A voltage tap was attached to the edge of each layer to monitor partial I–V characteristics. The coil temperature was monitored by several thermocouples. The central magnetic field was monitored by a Hall probe. The
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coil was cooled by immersion in liquid nitrogen, and pumped liquid nitrogen. Due to avoid the partial burnout of the coil by the rapid voltage increase, the transport current was controlled close to the Ic criterion of 10 9 V/cm.
3. Results and discussion Fig. 2 shows a 255 m long textured Gd2Zr2O7 film formed by the reel-to-reel IBAD with the production speed of 0.7 m/h. The operation of ion sources was very stable during continuous deposition over 380 h under the condition of Table 1. The film thickness was 1.0 lm and the in-plane mosaic spread (D/) were 13.9–14.5 by X-ray (2 2 2) pole figure for the both ends of Gd2Zr2O7 film. Table 4 shows in-plane orientation for several textured template films. Secondary buffer layer of CeO2 grown by PLD is quite effective to improve the in-plane orientation rapidly without ion beam
Fig. 1. A 12 · 22 solenoid coil using a 70 m long Y-123 coated conductor made by IBAD and PLD method.
Table 3 Designed specifications of the Y-123 solenoid magnet Turn · layer Inner diameter Coil dimensions Conductor length Expected field with operating current of 100 A Inductance
12 · 22 60 mm Thickness: 23.5 mm Height: 133 mm 70 m Central field: 0.210 T Maximum field: 0.215 T Maximum field normal to tape: 0.069 T 2.2 mH
Fig. 2. A 255 m long Gd2Zr2O7 template film formed by reelto-reel IBAD.
Table 4 Specifications of long Gd2Zr2O7/CeO2 template films Gd2Zr2O7
CeO2
Length (m)
Thickness (lm)
D/ (deg.)
Production speed (m/h)
Thickness (lm)
D/ (deg.)
Production speed (m/h)
100 137 162 180
1.5 1.4 1.0 1.0
10 11 15 13
0.5 0.55 0.7 0.7
0.55 0.7 0.6 0.8
6 5 5.5 4.5
2.5 1.5 1.5 1.1
K. Kakimoto et al. / Physica C 426–431 (2005) 858–865
bombardment [6]. The production speed could also be improved by optimization of the thickness combination of Gd2Zr2O7 and CeO2. As shown in Fig. 3, 1.0 lm thick Gd2Zr2O7 and 0.6–0.8 lm thick CeO2 could achieve the surface D/ of 5, similar to thicker sample. Fig. 4 shows the dependence of Jc on the Y-123 thickness for 10 mm wide and 0.1 m long samples of Y-123/CeO2/IBAD-Gd2Zr2O7/Hastelloy structure. In this figure, the results on samples of Y-123/Y2O3/IBAD-Gd2Zr2O7/Hastelloy structure were also plotted as the reference. The D/ value of 3–4 was obtained for Y-123 films grown on the CeO2 cap layer. The Jc enhanced up to 1.5 times, compared to the Y2O3 cap layer. The Jc values gradually decreased with the increase of the Y-123 thickness. The Ic value close to 300 A
16 IBAD-Gd2Zr2O7
∆φ (deg.)
12 8 4
IBAD-Gd2Zr2O7 1.0µm+PLD-CeO2 IBAD-Gd2Zr2O7 1.6µm+PLD-CeO2
0 0
0.8
1.6
2.4
Total thickness of buffer layers (µm) Fig. 3. Thickness dependence of in-plane mosaic spread for CeO2 films grown on IBAD-Gd2Zr2O template films.
861
was obtained for a 1.5 lm thick YBCO film. The dependence of the Ic on the thickness could be partially understood by the relative amount of the a-axis aligned YBCO grains and the chemical reaction between the YBCO and the buffer layer [7]. Formerly we reported a 100 m long conductor of Y-123/Y2O3/IBAD-Gd2Zr2O7/Hastelloy structure with the 0.5 lm thick Y-123, which had the end-to-end Ic/Jc values of 38 A/0.8 MA/cm2, respectively [8]. In this paper, we applied highly textured CeO2 secondary buffer layer instead of the Y2O3. Furthermore, the deposition conditions of the continuous PLD process were optimized to improve the longitudinal uniformity of the thicker Y-123 film. Fig. 5 shows the I–V curve for a 105 m long Y-123 tape prepared by the optimized PLD processing. The end-to-end Ic of 126 A was obtained by the criterion of 10 6 V/cm. The Ic ˚ . The n-value of the times length reached 13,230 A I–V curve was 28.5. Fig. 6 shows the longitudinal distribution of the Ic for the 105 m long Y-123 tape. The Y-123 thickness was 1.0 lm. The Ic close to 200 A was observed in some 5 m long parts. It indicates that the superconducting transport properties for some 5 m long tapes are reaching the best one for the short samples shown in Fig. 4. Before the preparation of the test coil using the IBAD/PLD tapes, the Ic degradation by the bending strain was investigated in the samples shown in Table 5. Fig. 7 shows the normalized critical current, the Ic (Ic with the bending strain)/Ic0 (initial Ic without the bending strain) on the tensile and compressive bending strain in the samples with
5 Y-123/CeO2/GZO ∆φ : 3-4˚ Y-123/Y2O3/GZO ∆φ : 7˚
77 K 0T
12 at 77K,
3
1µV/cm
0T
Voltage (mV)
Jc (MA/cm2)
4
2 300A/cm 200A/cm 100A/cm
1 0
0
1 2 Thickness (µm)
3
Fig. 4. Y-123 thickness dependence of Jc for 10 mm wide and 0.1 m long samples.
n=28.5
8 105m end-to-end
Ic : 126A Jc : 1.3MA/cm2 Y-123 : 1.0µm
4
0 0
50
100
150
Transport Current (A) Fig. 5. End-to-end I–V curve for a 105 m long Y-123 tape.
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K. Kakimoto et al. / Physica C 426–431 (2005) 858–865
300
10-5
Voltage per length (V/cm)
Critical current (A)
Reaching the best Jc of 1µm thick YBCO
200
100 Ic X L : 13,230 Am
0
0
20
40
60
80
Fig. 6. Longitudinal distribution of Ic for a 105 m long Y-123 tape.
Table 5 Specifications of YBCO tapes used to investigate the bending strain properties Sample
Gd2Zr2O7 (lm)
CeO2 (lm)
YBCO (lm)
Ag (lm)
Ic0 (A)
A B C
1 1 1
0.8 0.8 0.8
0.5 0.5 0.5
10 20 30
40–70 40–60 30–70
the different thick Ag cap layers. The degradation of the Ic/Ic0 was not observed up to the 0.4% tensile strain and the 0.5% compressive strain in the 10 lm thick Ag cap layer. Furthermore, the degradation of the Ic/Ic0 was not observed up to the 0.5% tensile strain and the 0.6% compressive strain in the 20–30 lm thick Ag cap layer. From these re-
1.0
1.0
Ic /Ico
Ic/Ico
0.2 0.0 0.0 (a)
0.8
0.6 0.4
75m end-to-end
Ag 10µm Ag 20µm Ag 30µm 77K
0.2 0.4 0.6 0.8 Bending strain (%)
n=30
10-9 V/cm
10-9
Ic: 72A
100 Transport Current (A)
sults, it was found that thick Ag cap layer reinforced the Y-123 layer effectively. Fig. 8 shows the end-to-end I–V curve for the 75 m with n-value of 30. The 70 m long part of this wire was applied for the examination of the solenoid magnet. The Ic by the criterion of 10 9 V/ cm was also estimated to be 72 A. Fig. 9 shows I–V characteristics of the Y-123 conductor in the solenoid magnet at liquid nitrogen temperature (77.3 K). To avoid the partial burnout of the coil by the rapid voltage increase, the operating current was limited up to 42 A. And then the Ic was determined by the criterion of 10 9 V/cm. The Ic of the coil was 41 A and the central magnetic field of 0.082 T was obtained with the operating current of 42 A. The partial voltages for all 22 layers of
1.2
Compressive Ag YBCO
10-6V/cm
Ic: 93.5A
Fig. 8. End-to-end I–V curve for a 75 m long Y-123 tape used to prepare a test coil.
1.2
Hastelloy
10-7
10-11 10
100
Position (m)
0.8
77K 0T
0.6 0.4 0.2
0T
0.0 0.0
1.0 (b)
Tensile Ag Hastelloy
YBCO
Ag 10µm Ag 20µm Ag 30µm 77K
0.2 0.4 0.6 0.8 Bending strain (%)
0T 1.0
Fig. 7. Dependence of normalized critical current, Ic/Ic0 on (a) the compressive bending strain and (b) the tensile bending strain for the sample A, B and C.
K. Kakimoto et al. / Physica C 426–431 (2005) 858–865
15
0.04
0.00 0
Total voltage of conductor Voltage in 6th layer
Ic : 5 41A
0.25
200 Central field measured by Hall probe
0.20
150
0.15 100 0.10
Total voltage of conductor
0.05
Voltage in 6th layer
0
0 50
10 20 30 40 Operating Current (A)
250 0.27T
66K
0
Voltage (µV)
Central field measured by Hall probe
10
0.02
Central Magnetic Field (T)
0.082T
0.08 0.06
0.30
20
77K
Voltage (µV)
Central Magnetic Field (T)
0.10
863
Ic : 50 114A
40 80 120 Operating Current (A)
0
Fig. 9. I–V characteristics of the Y-123 conductor in the coil at 77 K. Closed squares indicate the total voltage in the conductor and closed circle indicates voltage emerged in the part wound as 6th layer. The central magnetic field measured by Hall probe was also shown.
Fig. 10. I–V characteristics of the Y-123 conductor in the coil at 66 K. Similar to the case for 77 K, most amounts of voltages emerged in the conductor were generated in the part wound as 6th layer. The central magnetic field of 0.27 T was obtained with the operation current of 130 A.
the solenoid were concurrently monitored. The voltage emerged in the 6th layer was quite large and occupied almost 80% of total voltage. Fig. 10 shows the I–V characteristics and the central magnetic field in the coil at pumped liquid nitrogen temperature (66 K). The Ic of the coil was 114 A and the central magnetic field of 0.27 T was obtained with the operating current of 130 A. During holding 130 A, the rise of the conductor temperature could not be confirmed in
the coil. It indicates that the high-Tc superconducting magnet with the liquid nitrogen cooling has the high reliability, compared to the low-Tc superconducting magnet with the liquid helium cooling. Similar to the case at 77 K, the voltage generated in 6th layer occupied the most amounts of the total voltages in the coil. Fig. 11 shows the distribution of the magnetic field and the B–h plot determined by electromagnetic calculation in the 12 · 22 solenoid coil. The
end turns Axial
The end turn of Y-123 conductor with θ around 30degrees experience most sever field.
100
θ
80
Field angle: θ (deg.)
60
6 turns
40 20 0 0 -20 -40 -60
Central field 22 layers
0.10
0.20
Magnetic field: B (T) 6th layer
-80 Radia -100
Fig. 11. A Schematic drawing of the distribution of experienced magnetic field and the B–h plot determined by electromagnetic calculation in a quarter of the cross section for the 12 · 22 solenoid coil.
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K. Kakimoto et al. / Physica C 426–431 (2005) 858–865
Y-123 conductor should experience the obliqueangled magnetic fields near the end turn of the coil. It is well known that the Y-123 coated conductor has apparent angular dependence of the Ic to the axis of the magnetic field [9]. From Fig. 11, it is considered that the angle between the field vector and the a–b plane becomes larger but the magnitude of the field vector becomes smaller from the fist layer to the final layer. Around the end turn of the 6th layer, the angle (about 30) between field vector and axial direction is large enough to decrease the Ic properties down to that in the case of 90, and also the magnitude of field is not so small. It is reasonable to consider that the Ic should decrease mostly in such field configurations. Fig. 12 shows the load line at the end turn of the 6th layer. The field experienced there is about 60% of the central magnetic field. The dotted lines indicate the end-to-end Ic–B characteristics estimated for the magnet conductor. The Ic with criterion of 10 9 V/cm was used, corresponding to the criterion used in coil examination. The Ic–B curves were fit by data measured for a short sample cut from the same tape used for the magnet. The applied field angle against the tape surface was 30. The cross points of the load line and the estimated Ic–B curves were near 40 A for 77 K, and 115 A for 65 K, respectively. Therefore, it agrees quite well with the magnet examinations
200
at both temperatures, indicating no recognizable degradation in the coiling process and the cooling procedure for the solenoid type magnet using the Y-123 coated conductor. 4. Conclusions The long continuous formation of the Y-123 conductor was performed by the IBAD and the PLD method. A 250 m long IBAD template and a 105 m long Y-123 film were fabricated. The end-to-end Ic of 126 A and high n-value of 28.5 was obtained at 77 K, 0 T. The Ic times length ˚ , which should be large enough reached 13,230 A to examine the performance as the superconducting wire for practical applications. A 70 m long Y-123 tape was wound into a 12 · 22 solenoid type magnet whose inner diameter was 60 mm. The central field of 0.27 T was obtained at 66 K, with operating current of 130 A. The Ic of the coil agreed well with the analysis of load line at the end turn of the intermediate layer. The Ic of the coil was determined at the point of the oblique-angled field configuration. The results indicate the coiling process and cooling procedure had no severe mechanical problem for the solenoid type magnet using Y123 coated conductors. For practical magnet applications, thermal stability and ac losses should also be studied to avoid rapid voltage generation at low Ic points, in addition to further improvement of the longitudinal Ic distributions in conductors.
Operating Current (A)
Central magnetic field
160
Load line at the end turn of 6th layer
120
end-to-end Ic=72A (77K 0T)
80
65K Estimated Ic-B line for θ=30˚
40 0
Acknowledgements
77K 0
0.1 0.2 Magnetic Field (T)
This work is/was supported by New Energy and Industrial Technology Development Organization (NEDO) as Collaborative Research and Development of Fundamental Technologies for Superconductivity Application.
0.3
Fig. 12. The load line at the end turn of the 6th layer. The experienced field there is calculated to be about 60% of central magnetic field. The dotted lines indicate the estimated Ic–B characteristics for the magnet conductor by using end-to-end Ic of 72 A with criterion of 10 9 V/cm.
References [1] D.M. Feldmann, J.L. Reeves, A.A. Polyanskii, G. Kozlowski, R.R. Biggers, R.M. Nekkanti, I. Maartense, M. Tomsic, P. Barnes, C.E. Oberly, T.L. Peterson, S.E. Babcock, D.C. Larbalestier, Appl. Phys. Lett. 77 (18) (2000) 2906.
K. Kakimoto et al. / Physica C 426–431 (2005) 858–865 [2] H. Kung, S.R. Foltyn, P.N. Arendt, M.P. Maley, IEEE Trans. Appl. Supercond. 9 (2) (1999) 2034. [3] Y. Iijima, K. Kakimoto, M. Kimura, K. Takeda, T. Saitoh, IEEE Trans. Appl. Supercond. 11 (1) (2001) 2816. [4] A. Usoskin, J. Knoke, F. Garcia-Moreno, A. Issaev, J. Dzick, S. Sievers, H. Freyhardt, IEEE Trans. Appl. Supercond. 11 (3) (2001) 3385. [5] Y. Ueno, N. Sakakibara, H. Hoshizaki, J. Cryst. Growth 197 (1999) 376.
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[6] Y. Yamada, T. Muroga, H. Iwai, T. Watanabe, S. Miyata, Y. Shiohara, Supercond. Sci. Technol. 17 (2) (2004) S70. [7] K. Kakimoto, Y. Iijima, T. Saitoh, Physica C 392–396 (pt. 2) (2003) 783. [8] Y. Iijima, K. Kakimoto, Y. Sutoh, S. Ajimura, T. Saitoh, Physica C 412–414 (pt. 2) (2004) 801. [9] T. Kiss, M. Inoue, S. Nishimura, T. Kuga, T. Matsushita, Y. Iijima, K. Kakimoto, T. Saitoh, S. Awaji, K. Watanabe, Y. Shiohara, Physica C 378–381 (pt. 2) (2002) 1113.
Research of long IBAD–PLD coated conductors with high quality Kazuomi Kakimoto *, Yasunori Sutoh, Naoki Kaneko, Yasuhiro Iijima, Takashi Saitoh Fujikura Ltd., 1-5-1 Kiba, Koto-ku, Tokyo 135-8512, Japan Received 23 November 2004; accepted 27 January 2005
Abstract Long YBa2Cu3Ox (Y-123) coated tapes were produced by reel-to-reel continuous processes using Ion-Beam-Assisted Deposition (IBAD) and Pulsed-Laser-Deposition (PLD). Biaxially textured Gd2Zr2O7 buffer layers up to 255 m long were fabricated by IBAD with the production speed of 0.5–1.0 m/h. Y-123 films were formed on them by PLD with the tape speed of 1.0–4.0 m/h. A 105 m long Y-123 film was formed with the uniform thickness of 1.0 lm and the D/ of 3–4. The end-to-end Ic of 126 A was achieved at 77 K, 0 T, whose I–V curve had the n-value of 28.5. The Ic ˚ . Another 70 m long Y-123 tape with the Ic of over 90 A (77 K, 0 T) was wound into times length reached 13,230 A a solenoid type magnet whose inner diameter was 60 mm. The central magnetic field of 0.082 T was obtained at 77 K, with operating current of 42 A and that of 0.27 T was obtained at 66 K, with operating current of 130 A. 2005 Elsevier B.V. All rights reserved. PACS: 74.72.Bk; 85.25.Kx Keywords: IBAD; PLD; Metallic substrate; Y-123; CeO2
1. Introduction To eliminate intergranular weaklinks, the YBa2Cu3Ox (Y-123) coated conductors have quite sophisticated structures, composed of highly tex*
Corresponding author. Fax: +81 3 5606 1512. E-mail address: [email protected] (K. Kakimoto).
tured thin films on flat flexible substrates. Recently, rapid progresses have been made in several kinds of reel-to-reel processing technologies to improve the longitudinal uniformity of the Y-123 tapes long enough to examine the performance as practical superconducting wires. The Ion-Beam-Assisted Deposition (IBAD) template has a lot of advantages to avoid imperfection of Y-123 film growth. The process
0921-4534/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2005.01.089
K. Kakimoto et al. / Physica C 426–431 (2005) 858–865
temperature is so low that the substrate surface has no thermal damage during the deposition of the buffer films. Furthermore, it has excellent biaxial texture, smooth surface, and small grain size (5– 10 nm), which completely hides the large grain boundaries of the underlying metal substrate [1,2]. For the length of over 100 m, a large area deposition apparatus was successfully developed with long lifetime operation even in oxygen atmosphere using the state-of-the-art ion-source technology [3]. The Pulsed-Laser-Deposition (PLD) was used for Y-123 growth and secondary buffer layer of CeO2. It is characterized with stable stoichiometry, easy oxygen pressure control, very high growth rate [4,5], etc. In the demonstration of the magnet using the Y-123 conductor, we examined coiling capability for solenoid, which have many merits as joint-free, good thermal/mechanical stability etc., compared to the pancake type magnet. In this paper we describe the fabrication of the high-performance 100 m class Y-123 conductors by the combination of the IBAD and the PLD method, and a first demonstration of a solenoid type magnet operating at liquid nitrogen temperature by using a 70 m long Y-123 conductor.
2. Experimental Biaxially aligned Gd2Zr2O7 films were deposited by reel-to-reel, dual-ion-beam sputtering system with two sets of radio frequency (RF) discharged 66 cm · 6 cm square-shaped ion sources. Precisely roll-milled Hastelloy C276 tapes, the average roughness of which was below 30 nm, with the 10 mm width and the 100 lm thickness were used as the fundamental metallic substrate. Continuous growth of Gd2Zr2O7 films was performed with the production speed of 0.5–1.0 m/h under the condition of Table 1. CeO2 secondary buffer layers and Y-123 films were deposited by a reelto-reel PLD system with Kr–F (k = 248 nm) excimer laser. The detailed PLD condition is shown in Table 2. The tape speed was 1.0–4.0 m/h. Twenty micrometers thick Ag films were deposited on Y-123 films by RF magnetron sputtering and thereafter annealed for several hours at 500 C in 760 Torr oxygen. Crystalline alignment was mea-
859
Table 1 Deposition parameters of IBAD template films Sputtering ion beam (Ar+)
Beam voltage: 1500 V Beam current: 500–700 mA
Assisting ion beam (Ar+)
Beam voltage: 200 V Beam current: 400–600 mA Ion incident angle: 55 from substrate normal
Target
Sintered stoichiometric target of Gd2O3:ZrO2 = 1:2 200 C 3.0–4.0 · 10 2 Pa
Temperature of growth stage Vacuum pressure (Ar + O2)
Table 2 Deposition parameters of oxide films by PLD Film
CeO2
Laser pulse energy (mJ) Repetation rate (Hz) Vacuum pressure (O2) (Pa) Temperature of growth stage (C) Target
200–300 200–300 70–100 200 1–3 30–40 400–600 900–1000 Sintered stoichiometric target 50–80
Target–substrate distance (mm)
Y-123
sured by X-ray diffraction pole figure. Transport properties were measured at 77 K by DC fourprobe method. For long length measurement, the Y-123 tape was wound around a cylindrical holder. Voltage taps were attached at every 5 m length. The Ic was determined by the criterion of 10 6 V/cm. The n-values of the I–V curves were evaluated in the voltage per length range from 10 7 to 10 6 V/cm. A 70 m long Y-123 tape formed by the combination of IBAD and PLD was wound into a 12 turn · 22 layer solenoid type magnet as shown in Fig. 1. The designed specifications of the coil are shown in Table 3. The coil bobbin was made of fiber reinforced plastic (FRP). The coil was insulated with thin polyimide tapes and thin FRP sheets. In order to avoid edgewise strains in Y-123 tape, it was carefully treated at turning points between layers. A voltage tap was attached to the edge of each layer to monitor partial I–V characteristics. The coil temperature was monitored by several thermocouples. The central magnetic field was monitored by a Hall probe. The
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K. Kakimoto et al. / Physica C 426–431 (2005) 858–865
coil was cooled by immersion in liquid nitrogen, and pumped liquid nitrogen. Due to avoid the partial burnout of the coil by the rapid voltage increase, the transport current was controlled close to the Ic criterion of 10 9 V/cm.
3. Results and discussion Fig. 2 shows a 255 m long textured Gd2Zr2O7 film formed by the reel-to-reel IBAD with the production speed of 0.7 m/h. The operation of ion sources was very stable during continuous deposition over 380 h under the condition of Table 1. The film thickness was 1.0 lm and the in-plane mosaic spread (D/) were 13.9–14.5 by X-ray (2 2 2) pole figure for the both ends of Gd2Zr2O7 film. Table 4 shows in-plane orientation for several textured template films. Secondary buffer layer of CeO2 grown by PLD is quite effective to improve the in-plane orientation rapidly without ion beam
Fig. 1. A 12 · 22 solenoid coil using a 70 m long Y-123 coated conductor made by IBAD and PLD method.
Table 3 Designed specifications of the Y-123 solenoid magnet Turn · layer Inner diameter Coil dimensions Conductor length Expected field with operating current of 100 A Inductance
12 · 22 60 mm Thickness: 23.5 mm Height: 133 mm 70 m Central field: 0.210 T Maximum field: 0.215 T Maximum field normal to tape: 0.069 T 2.2 mH
Fig. 2. A 255 m long Gd2Zr2O7 template film formed by reelto-reel IBAD.
Table 4 Specifications of long Gd2Zr2O7/CeO2 template films Gd2Zr2O7
CeO2
Length (m)
Thickness (lm)
D/ (deg.)
Production speed (m/h)
Thickness (lm)
D/ (deg.)
Production speed (m/h)
100 137 162 180
1.5 1.4 1.0 1.0
10 11 15 13
0.5 0.55 0.7 0.7
0.55 0.7 0.6 0.8
6 5 5.5 4.5
2.5 1.5 1.5 1.1
K. Kakimoto et al. / Physica C 426–431 (2005) 858–865
bombardment [6]. The production speed could also be improved by optimization of the thickness combination of Gd2Zr2O7 and CeO2. As shown in Fig. 3, 1.0 lm thick Gd2Zr2O7 and 0.6–0.8 lm thick CeO2 could achieve the surface D/ of 5, similar to thicker sample. Fig. 4 shows the dependence of Jc on the Y-123 thickness for 10 mm wide and 0.1 m long samples of Y-123/CeO2/IBAD-Gd2Zr2O7/Hastelloy structure. In this figure, the results on samples of Y-123/Y2O3/IBAD-Gd2Zr2O7/Hastelloy structure were also plotted as the reference. The D/ value of 3–4 was obtained for Y-123 films grown on the CeO2 cap layer. The Jc enhanced up to 1.5 times, compared to the Y2O3 cap layer. The Jc values gradually decreased with the increase of the Y-123 thickness. The Ic value close to 300 A
16 IBAD-Gd2Zr2O7
∆φ (deg.)
12 8 4
IBAD-Gd2Zr2O7 1.0µm+PLD-CeO2 IBAD-Gd2Zr2O7 1.6µm+PLD-CeO2
0 0
0.8
1.6
2.4
Total thickness of buffer layers (µm) Fig. 3. Thickness dependence of in-plane mosaic spread for CeO2 films grown on IBAD-Gd2Zr2O template films.
861
was obtained for a 1.5 lm thick YBCO film. The dependence of the Ic on the thickness could be partially understood by the relative amount of the a-axis aligned YBCO grains and the chemical reaction between the YBCO and the buffer layer [7]. Formerly we reported a 100 m long conductor of Y-123/Y2O3/IBAD-Gd2Zr2O7/Hastelloy structure with the 0.5 lm thick Y-123, which had the end-to-end Ic/Jc values of 38 A/0.8 MA/cm2, respectively [8]. In this paper, we applied highly textured CeO2 secondary buffer layer instead of the Y2O3. Furthermore, the deposition conditions of the continuous PLD process were optimized to improve the longitudinal uniformity of the thicker Y-123 film. Fig. 5 shows the I–V curve for a 105 m long Y-123 tape prepared by the optimized PLD processing. The end-to-end Ic of 126 A was obtained by the criterion of 10 6 V/cm. The Ic ˚ . The n-value of the times length reached 13,230 A I–V curve was 28.5. Fig. 6 shows the longitudinal distribution of the Ic for the 105 m long Y-123 tape. The Y-123 thickness was 1.0 lm. The Ic close to 200 A was observed in some 5 m long parts. It indicates that the superconducting transport properties for some 5 m long tapes are reaching the best one for the short samples shown in Fig. 4. Before the preparation of the test coil using the IBAD/PLD tapes, the Ic degradation by the bending strain was investigated in the samples shown in Table 5. Fig. 7 shows the normalized critical current, the Ic (Ic with the bending strain)/Ic0 (initial Ic without the bending strain) on the tensile and compressive bending strain in the samples with
5 Y-123/CeO2/GZO ∆φ : 3-4˚ Y-123/Y2O3/GZO ∆φ : 7˚
77 K 0T
12 at 77K,
3
1µV/cm
0T
Voltage (mV)
Jc (MA/cm2)
4
2 300A/cm 200A/cm 100A/cm
1 0
0
1 2 Thickness (µm)
3
Fig. 4. Y-123 thickness dependence of Jc for 10 mm wide and 0.1 m long samples.
n=28.5
8 105m end-to-end
Ic : 126A Jc : 1.3MA/cm2 Y-123 : 1.0µm
4
0 0
50
100
150
Transport Current (A) Fig. 5. End-to-end I–V curve for a 105 m long Y-123 tape.
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K. Kakimoto et al. / Physica C 426–431 (2005) 858–865
300
10-5
Voltage per length (V/cm)
Critical current (A)
Reaching the best Jc of 1µm thick YBCO
200
100 Ic X L : 13,230 Am
0
0
20
40
60
80
Fig. 6. Longitudinal distribution of Ic for a 105 m long Y-123 tape.
Table 5 Specifications of YBCO tapes used to investigate the bending strain properties Sample
Gd2Zr2O7 (lm)
CeO2 (lm)
YBCO (lm)
Ag (lm)
Ic0 (A)
A B C
1 1 1
0.8 0.8 0.8
0.5 0.5 0.5
10 20 30
40–70 40–60 30–70
the different thick Ag cap layers. The degradation of the Ic/Ic0 was not observed up to the 0.4% tensile strain and the 0.5% compressive strain in the 10 lm thick Ag cap layer. Furthermore, the degradation of the Ic/Ic0 was not observed up to the 0.5% tensile strain and the 0.6% compressive strain in the 20–30 lm thick Ag cap layer. From these re-
1.0
1.0
Ic /Ico
Ic/Ico
0.2 0.0 0.0 (a)
0.8
0.6 0.4
75m end-to-end
Ag 10µm Ag 20µm Ag 30µm 77K
0.2 0.4 0.6 0.8 Bending strain (%)
n=30
10-9 V/cm
10-9
Ic: 72A
100 Transport Current (A)
sults, it was found that thick Ag cap layer reinforced the Y-123 layer effectively. Fig. 8 shows the end-to-end I–V curve for the 75 m with n-value of 30. The 70 m long part of this wire was applied for the examination of the solenoid magnet. The Ic by the criterion of 10 9 V/ cm was also estimated to be 72 A. Fig. 9 shows I–V characteristics of the Y-123 conductor in the solenoid magnet at liquid nitrogen temperature (77.3 K). To avoid the partial burnout of the coil by the rapid voltage increase, the operating current was limited up to 42 A. And then the Ic was determined by the criterion of 10 9 V/cm. The Ic of the coil was 41 A and the central magnetic field of 0.082 T was obtained with the operating current of 42 A. The partial voltages for all 22 layers of
1.2
Compressive Ag YBCO
10-6V/cm
Ic: 93.5A
Fig. 8. End-to-end I–V curve for a 75 m long Y-123 tape used to prepare a test coil.
1.2
Hastelloy
10-7
10-11 10
100
Position (m)
0.8
77K 0T
0.6 0.4 0.2
0T
0.0 0.0
1.0 (b)
Tensile Ag Hastelloy
YBCO
Ag 10µm Ag 20µm Ag 30µm 77K
0.2 0.4 0.6 0.8 Bending strain (%)
0T 1.0
Fig. 7. Dependence of normalized critical current, Ic/Ic0 on (a) the compressive bending strain and (b) the tensile bending strain for the sample A, B and C.
K. Kakimoto et al. / Physica C 426–431 (2005) 858–865
15
0.04
0.00 0
Total voltage of conductor Voltage in 6th layer
Ic : 5 41A
0.25
200 Central field measured by Hall probe
0.20
150
0.15 100 0.10
Total voltage of conductor
0.05
Voltage in 6th layer
0
0 50
10 20 30 40 Operating Current (A)
250 0.27T
66K
0
Voltage (µV)
Central field measured by Hall probe
10
0.02
Central Magnetic Field (T)
0.082T
0.08 0.06
0.30
20
77K
Voltage (µV)
Central Magnetic Field (T)
0.10
863
Ic : 50 114A
40 80 120 Operating Current (A)
0
Fig. 9. I–V characteristics of the Y-123 conductor in the coil at 77 K. Closed squares indicate the total voltage in the conductor and closed circle indicates voltage emerged in the part wound as 6th layer. The central magnetic field measured by Hall probe was also shown.
Fig. 10. I–V characteristics of the Y-123 conductor in the coil at 66 K. Similar to the case for 77 K, most amounts of voltages emerged in the conductor were generated in the part wound as 6th layer. The central magnetic field of 0.27 T was obtained with the operation current of 130 A.
the solenoid were concurrently monitored. The voltage emerged in the 6th layer was quite large and occupied almost 80% of total voltage. Fig. 10 shows the I–V characteristics and the central magnetic field in the coil at pumped liquid nitrogen temperature (66 K). The Ic of the coil was 114 A and the central magnetic field of 0.27 T was obtained with the operating current of 130 A. During holding 130 A, the rise of the conductor temperature could not be confirmed in
the coil. It indicates that the high-Tc superconducting magnet with the liquid nitrogen cooling has the high reliability, compared to the low-Tc superconducting magnet with the liquid helium cooling. Similar to the case at 77 K, the voltage generated in 6th layer occupied the most amounts of the total voltages in the coil. Fig. 11 shows the distribution of the magnetic field and the B–h plot determined by electromagnetic calculation in the 12 · 22 solenoid coil. The
end turns Axial
The end turn of Y-123 conductor with θ around 30degrees experience most sever field.
100
θ
80
Field angle: θ (deg.)
60
6 turns
40 20 0 0 -20 -40 -60
Central field 22 layers
0.10
0.20
Magnetic field: B (T) 6th layer
-80 Radia -100
Fig. 11. A Schematic drawing of the distribution of experienced magnetic field and the B–h plot determined by electromagnetic calculation in a quarter of the cross section for the 12 · 22 solenoid coil.
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K. Kakimoto et al. / Physica C 426–431 (2005) 858–865
Y-123 conductor should experience the obliqueangled magnetic fields near the end turn of the coil. It is well known that the Y-123 coated conductor has apparent angular dependence of the Ic to the axis of the magnetic field [9]. From Fig. 11, it is considered that the angle between the field vector and the a–b plane becomes larger but the magnitude of the field vector becomes smaller from the fist layer to the final layer. Around the end turn of the 6th layer, the angle (about 30) between field vector and axial direction is large enough to decrease the Ic properties down to that in the case of 90, and also the magnitude of field is not so small. It is reasonable to consider that the Ic should decrease mostly in such field configurations. Fig. 12 shows the load line at the end turn of the 6th layer. The field experienced there is about 60% of the central magnetic field. The dotted lines indicate the end-to-end Ic–B characteristics estimated for the magnet conductor. The Ic with criterion of 10 9 V/cm was used, corresponding to the criterion used in coil examination. The Ic–B curves were fit by data measured for a short sample cut from the same tape used for the magnet. The applied field angle against the tape surface was 30. The cross points of the load line and the estimated Ic–B curves were near 40 A for 77 K, and 115 A for 65 K, respectively. Therefore, it agrees quite well with the magnet examinations
200
at both temperatures, indicating no recognizable degradation in the coiling process and the cooling procedure for the solenoid type magnet using the Y-123 coated conductor. 4. Conclusions The long continuous formation of the Y-123 conductor was performed by the IBAD and the PLD method. A 250 m long IBAD template and a 105 m long Y-123 film were fabricated. The end-to-end Ic of 126 A and high n-value of 28.5 was obtained at 77 K, 0 T. The Ic times length ˚ , which should be large enough reached 13,230 A to examine the performance as the superconducting wire for practical applications. A 70 m long Y-123 tape was wound into a 12 · 22 solenoid type magnet whose inner diameter was 60 mm. The central field of 0.27 T was obtained at 66 K, with operating current of 130 A. The Ic of the coil agreed well with the analysis of load line at the end turn of the intermediate layer. The Ic of the coil was determined at the point of the oblique-angled field configuration. The results indicate the coiling process and cooling procedure had no severe mechanical problem for the solenoid type magnet using Y123 coated conductors. For practical magnet applications, thermal stability and ac losses should also be studied to avoid rapid voltage generation at low Ic points, in addition to further improvement of the longitudinal Ic distributions in conductors.
Operating Current (A)
Central magnetic field
160
Load line at the end turn of 6th layer
120
end-to-end Ic=72A (77K 0T)
80
65K Estimated Ic-B line for θ=30˚
40 0
Acknowledgements
77K 0
0.1 0.2 Magnetic Field (T)
This work is/was supported by New Energy and Industrial Technology Development Organization (NEDO) as Collaborative Research and Development of Fundamental Technologies for Superconductivity Application.
0.3
Fig. 12. The load line at the end turn of the 6th layer. The experienced field there is calculated to be about 60% of central magnetic field. The dotted lines indicate the estimated Ic–B characteristics for the magnet conductor by using end-to-end Ic of 72 A with criterion of 10 9 V/cm.
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