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Superconducting layer thickness dependence of magnetic relaxation property in CVD processed YGdBCO coated conductors
Physica C 471 (2011) 1025–1028
Contents lists available at SciVerse ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Superconducting layer thickness dependence of magnetic relaxation property in CVD processed YGdBCO coated conductors Y. Takahashi a, M. Kiuchi a, E.S. Otabe a, T. Matsushita a,⇑, K. Shikimachi b, T. Watanabe b, N. Kashima b, S. Nagaya b a b
Faculty of Computer Science and Systems Engineering, Kyushu Institute of Technology, 680-4 Kawazu, Iizuka, Fukuoka 820-8502, Japan Chubu Electric Power Co., Inc., 20-1, Kitasekiyama, Ohdaka-cho, Midori-ku, Nagoya 459-8522, Japan
a r t i c l e
i n f o
Article history: Available online 13 May 2011 Keywords: YBCO tape IBAD/CVD Thickness dependence Flux creep–flow model Apparent pinning potential
a b s t r a c t One of the most important properties of coated conductors for Superconducting Magnetic Energy Storage (SMES) is the relaxation property of persistent superconducting current. This property can be quantitatively characterized by the apparent pinning potential U 0 . In this paper, the dependence of U 0 on the thickness of superconducting layer d is investigated in the range of 0.33–1.43 lm at the temperature range of 20–30 K and in magnetic fields up to 6.5 T for Y0.7Gd0.3Ba2Cu3O7d coated conductors. It was found that the value of critical current density did not appreciably depend on d at 20 K. This indicates that no structural deterioration of superconducting layer occurs during the process of increasing thickness. U 0 increases and then tends to decrease with an increasing magnetic field. The magnetic field at which U 0 starts to decrease increases with increasing thickness. This property was analyzed using the flux creep–flow model. Application of scaling law is examined for the dependence of U 0 on magnetic field and temperature. It was found that the dependence could be expressed using scaling parameters Bpeak ; U 0 peak in the temperature range 20–30 K. Ó 2011 Published by Elsevier B.V.
1. Introduction Application of REBa2Cu3O7d (RE: Rare Earth, REBCO)-coated conductor is expected to various electric power devices because its critical current characteristics are much superior to those of Bi-2223 tapes at high temperatures and/or at high magnetic fields. Recently, multiple-stage chemical vapor deposition (MS-CVD) method attracts attention for fabrication of coated conductors because fabrication time is shorter and cost is lower than pulsed laser deposition (PLD) method [1]. MS-CVD is expected to prepare REBCO-coated conductors in the future. Superconducting Magnetic Energy Storage (SMES) is one of the electric power devices that may be used in power grid systems in the future. Since this device will be commonly operated in the condition that the magnetic energy is stored, a decay of superconducting current is required to be suppressed severely. Hence, the apparent pinning potential, U 0 , a parameter associated with the decay of superconducting current is required to be enhanced. Since U 0 which determines the flux creep properties depends appreciably on the thickness of superconducting layer. For the application ⇑ Corresponding author. Address: Department of Computer Science and Electronics, Kyushu Institute of Technology, 680-4 Kawazu, Iizuka, Fukuoka 820-8502, Japan. Tel.: +81 948 29 7663; fax: +81 948 29 7661. E-mail address: [email protected] (T. Matsushita). 0921-4534/$ - see front matter Ó 2011 Published by Elsevier B.V. doi:10.1016/j.physc.2011.05.115
to SMES, it is important to investigate the superconducting layer thickness dependence of U 0 . SMES is planned to be operated at very high magnetic fields and hence, at low temperatures. However, the measurement of current relaxation at high magnetic fields is not easy. If the dependence of U 0 on magnetic field and temperature can be described in a form of scaling law, it may be useful for estimation of U 0 at such extreme condition. Then, the scaling law was examined for a series of CVD-processed YBCO-coated conductors [2]. In this paper, the thickness dependence of relaxation properties in CVD-processed YGdBCO coated conductors is investigated, and the result is analyzed by using the flux creep–flow model [3]. The scaling of the dependence U 0 on magnetic field and temperature is also examined for a series of new specimens. 2. Experiments The specimens were Y0.7Gd0.3Ba2Cu3O7d(YGdBCO)-coated conductors prepared by MS-CVD process. The superconducting layer was deposited on ion beam assisted deposition (IBAD)-processed substrate with Gd2Zr2O7 (GZO) inner layer and CeO2 cap layer [1]. The thickness of YGdBCO layer was varied in the range of 0.33–1.43 lm. Specifications of these specimens are listed in Table 1. The relaxation of DC magnetization was measured by using a SQUID magnetometer in a magnetic field up to 6.5 T parallel to
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Y. Takahashi et al. / Physica C 471 (2011) 1025–1028
Table 1 Specifications of specimens. Specimen
Thickness d (lm)
Tc (K)
#1 #2 #3 #4 #5 #6
0.33 0.55 0.77 0.99 1.21 1.43
89.3 89.6 90.9 91.3 90.7 89.7
the c-axis of the specimen over the temperature range of 20–60 K. The critical current density was estimated from the DC magnetization using the Bean model:
Jc ¼
6 Dm 2
dw ð3l wÞ
;
ð1Þ
where Dm is the hysteresis of magnetic moment, d is the thickness of the superconducting layer, and w and l are the dimensions of wide surface of specimens (w < l), respectively. E–J characteristics were estimated from the relaxation measurement [4]. The magnetization M is given by D m/wld. From the logarithmic relaxation rate of the magnetization, U 0 was estimated:
d M U ¼ 0 : dlogt M 0 kB T
ð2Þ
The initial magnetization M0 was determined by extrapolating the relaxation curve from the time range of 3 102–2 103 s to t = 1 s.
The magnetic field dependence of U 0 at 20 K obtained from the relaxation rate of magnetization is shown in Fig. 2. It is found that U 0 increases first and then decreases with increasing magnetic field. The magnetic field at which U 0 starts to decrease increases with increasing thickness. The critical current properties were analyzed using the flux creep–flow model [3]. In the virtual flux-creep free case, the critical current density takes a value determined only by the flux pinning and its temperature and magnetic field dependence is assumed as
m 2 T B J c0 ¼ A 1 Bc1 1 ; Tc Bc2
ð3Þ
where A, m, c are pinning parameters. The pinning potential U0 which determines the flux creep property is described in terms of Jc0 as:
U0 ¼
1=2 0:835g 2 kB J c0
ð2pÞ3=2 B1=4
¼
4:23g 2 kB J c0 d 2pB1=2
; d > L;
ð4Þ
; d < L:
ð5Þ
In the above d is the thickness of the superconducting layer and L is the longitudinal flux bundle size given by
L¼
Baf
2pl0 J c0
1=2 ð6Þ
Fig. 1 shows the Jc–B characteristics of the six specimens at 20 and 60 K in the magnetic field regions of 0–6.5 T. The Jc value does not appreciably depend on the thickness at 20 K. That is, it is expected that there is no structural deterioration of superconducting layer during the process of increasing thickness [2]. On the other hand, Jc of the thinner specimen tends to more rapidly decrease with increasing magnetic field at 60 K, which is attributed to the strong influence of the flux creep as will be discussed later.
with af denoting the flux line spacing. It should be noted that even if the pinning is in the three-dimensional case (d > L) at low magnetic fields for fairly thick superconductor, the pinning may change to the two-dimensional one at high magnetic fields due to the increase in L with decreasing Jc0. More detailed discussion of this mechanism is given in [5]. The parameter g2 is the number of flux lines in a flux bundle. In practical superconductors the flux pinning strength is statistically distributed widely [6]. Here it is simply assumed that only the parameter A which represents the pinning strength is distributed as
Fig. 1. Magnetic field dependence of critical current density at 20 and 60 K in the magnetic field range of 0–6.5 T. The lines are guide for eye.
Fig. 2. Magnetic field dependence of observed U 0 at 20 K. The lines are guide for eye.
3. Results and discussion
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Y. Takahashi et al. / Physica C 471 (2011) 1025–1028
" f ðAÞ ¼ K exp
#
ðlog A log Am Þ2 ; 2r 2
ð7Þ
where Am is the most probable value of A, r2 is a parameter representing a distribution width and K is a normalization constant. The electric field E caused by the mechanism of flux creep and flow is calculated as a function of the current density J and averaged statistically with respect to the distributed A. The details of the theoretical calculation are described elsewhere [3]. The theoretical critical current density is determined from the obtained E–J curve with the same electric field criterion of Ec = 109 V/m as done in experiments. The pinning parameters Am, r, m and c are determined so that the calculated E–J curves agree with the experiments. The obtained parameters are listed in Table 2. The E–J curves of specimen #6 obtained from the relaxation of magnetization are shown in Fig. 3. The lines are the theoretical results of flux creep–flow model. The experimental results are explained well by the theory. Fig. 4 shows the theoretical results of U 0 at 20 K. Good agreement with experiments shown in Fig. 2 is obtained. Thus, the relaxation property is describable by the mechanism of flux creep and flow. The possibility of description of U 0 ðB; TÞ in the form of scaling law is examined. It was shown that the magnetic field dependence of U 0 can be approximated by:
U 0 ðB; TÞ ¼ U 0
peak ðTÞ
a½B Bpeak ðTÞ2 ;
and U 0 peak are determined so that good agreement is obtained with experiments at each temperature, while a is assumed to be independent of temperature. The obtained parameters are a = 1.0 103 (eV/T2) for all the specimens and Bpeak and U 0 peak are listed in Table 3. The experimental results of the apparent pinning
ð8Þ
U 0 peak
where a, Bpeak and are fitting parameters [2]. This function is also adopted for the present specimens. The parameters Bpeak
Table 2 Pinning parameters of specimens at 20 K. Specimen #1 #2 #3 #4 #5 #6
r2
Am 11
4.1 10 3.9 1011 4.0 1011 4.0 1011 3.9 1011 4.0 1011
3
5.3 10 6.4 103 7.1 103 6.0 103 6.8 103 6.0 103
c
m
0.54 0.55 0.58 0.63 0.65 0.67
2.5 2.5 2.6 2.3 2.0 1.9
Fig. 3. E–J curves of specimen #6 obtained from the relaxation of magnetization. The lines are theoretical results of flux creep–flow model.
Fig. 4. Magnetic field dependence of U 0 calculated by theoretical relaxation rate at 20 K. The lines are guide for eye.
Table 3 Scaling parameter U 0
peak
of specimens.
20 K
25 K
Specimen
Bpeak
U 0
#4 #5 #6
3.61 4.77 4.84
7.67 102 8.05 102 8.16 102
peak
30 K
Bpeak
U 0
2.61 3.50 4.02
9.29 102 9.94 102 9.93 102
peak
Bpeak
U 0
1.25 2.47 3.13
1.13 101 1.17 101 1.19 101
peak
Fig. 5. Magnetic field dependence of U 0 in specimen #4 in the temperature region 20–30 K. Symbols and lines represent the experimental result and approximation of Eq. (8), respectively.
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Y. Takahashi et al. / Physica C 471 (2011) 1025–1028
specimens #4, #5 and #6 with the temperature region 20–30 K is shown in Fig. 6. It is found that Bpeak decreases and U 0 peak increases with increasing temperature. At the same time, Bpeak and U 0 peak tend to increase with increasing thickness. Furthermore, U 0 was estimated in the higher magnetic field region at 20 K for specimens #4, #5 and #6 by using Eq. (8) in Fig. 7. The open symbols show the results of estimation. This indicates that superconducting coated conductors with thicker superconducting layer is better for high field performance. Hence, much thicker superconducting layer seems to be promising for the practical superconductor in SMES systems, although the phenomenon is complicated and more detailed analysis is necessary for exact understanding of the phenomenon. 4. Summary
Fig. 6. Bpeak dependence of U 0 20–30 K.
peak
in specimen #4, #5 and #6 temperature region
The thickness dependence of the critical current density and relaxation properties in CVD processed YGdBCO coated conductors was investigated. It was found that the value of critical current density does not appreciably depend on d at 20 K, indicating no structural deterioration of superconducting layer during the process of increasing thickness. On the other hand, Jc of the thinner specimen tends to more rapidly decrease with increasing magnetic field at 60 K. At 20 K the apparent pinning potential U 0 increases and then, tends to decrease with increasing magnetic field. The magnetic field at which U 0 starts to decrease increases with increasing thickness. These results were theoretically explained by the flux creep– flow model based on the dimensionality of flux pinning. Application of scaling law is examined for the dependence of U 0 on magnetic field and temperature. It was found that the dependence can be expressed by using the scaling parameters Bpeak ; U 0 peak in the temperature region 20–30 K. It was also found that Bpeak and U 0 peak tend to increase with increasing thickness. The behavior of these parameter is complicated and more detailed analysis is necessary for exact understanding of the phenomenon. Acknowledgements This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) as the Technological Development of Yttrium-based Superconducting Power Equipment. References
Fig. 7. Magnetic field dependence of U 0 in specimen #4, #5 and #6 at 20 K. The solid symbols are experimental result and the open symbols are the scaling formula of Eq. (8) at 20 K.
potential are compared with the scaling formula of Eq. (8) at 20– 30 K for #4 in Fig. 5. It is found that Eq. (8) expresses well the behavior of U 0 ðB; TÞ. The relationship between Bpeak and U 0 peak for
[1] M. Mori, T. Watanabe, N. Kashima, T. Muroga, S. Miyata, Y. Yamada, T. Izumi, Y. Shiohara, Physica C 445 (448) (2006) 515. [2] Y. Takahashi, M. Kiuchi, E.S. Otabe, T. Matsushita, K. Shikimachi, T. Watanabe, N. Kashima, S. Nagaya, Physica C 470 (2010) 1284. [3] M. Kiuchi, K. Noguchi, T. Matsushita, T. Hikata, K. Sato, Physica C 278 (1997) 62. [4] K. Himeki, M. Kiuchi, E.S. Otabe, T. Matsushita, S. Miyata, A. Ibi, Y. Yamada, Y. Shiohara, Physica C 468 (2007) 1674. [5] T. Matsushita, Flux Pinning in Superconductors, Springer, Berlin, 2006, p. 333. [6] T. Matsushita, T. Tohdoh, N. Ihara, Physica C 259 (1996) 321.
Contents lists available at SciVerse ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Superconducting layer thickness dependence of magnetic relaxation property in CVD processed YGdBCO coated conductors Y. Takahashi a, M. Kiuchi a, E.S. Otabe a, T. Matsushita a,⇑, K. Shikimachi b, T. Watanabe b, N. Kashima b, S. Nagaya b a b
Faculty of Computer Science and Systems Engineering, Kyushu Institute of Technology, 680-4 Kawazu, Iizuka, Fukuoka 820-8502, Japan Chubu Electric Power Co., Inc., 20-1, Kitasekiyama, Ohdaka-cho, Midori-ku, Nagoya 459-8522, Japan
a r t i c l e
i n f o
Article history: Available online 13 May 2011 Keywords: YBCO tape IBAD/CVD Thickness dependence Flux creep–flow model Apparent pinning potential
a b s t r a c t One of the most important properties of coated conductors for Superconducting Magnetic Energy Storage (SMES) is the relaxation property of persistent superconducting current. This property can be quantitatively characterized by the apparent pinning potential U 0 . In this paper, the dependence of U 0 on the thickness of superconducting layer d is investigated in the range of 0.33–1.43 lm at the temperature range of 20–30 K and in magnetic fields up to 6.5 T for Y0.7Gd0.3Ba2Cu3O7d coated conductors. It was found that the value of critical current density did not appreciably depend on d at 20 K. This indicates that no structural deterioration of superconducting layer occurs during the process of increasing thickness. U 0 increases and then tends to decrease with an increasing magnetic field. The magnetic field at which U 0 starts to decrease increases with increasing thickness. This property was analyzed using the flux creep–flow model. Application of scaling law is examined for the dependence of U 0 on magnetic field and temperature. It was found that the dependence could be expressed using scaling parameters Bpeak ; U 0 peak in the temperature range 20–30 K. Ó 2011 Published by Elsevier B.V.
1. Introduction Application of REBa2Cu3O7d (RE: Rare Earth, REBCO)-coated conductor is expected to various electric power devices because its critical current characteristics are much superior to those of Bi-2223 tapes at high temperatures and/or at high magnetic fields. Recently, multiple-stage chemical vapor deposition (MS-CVD) method attracts attention for fabrication of coated conductors because fabrication time is shorter and cost is lower than pulsed laser deposition (PLD) method [1]. MS-CVD is expected to prepare REBCO-coated conductors in the future. Superconducting Magnetic Energy Storage (SMES) is one of the electric power devices that may be used in power grid systems in the future. Since this device will be commonly operated in the condition that the magnetic energy is stored, a decay of superconducting current is required to be suppressed severely. Hence, the apparent pinning potential, U 0 , a parameter associated with the decay of superconducting current is required to be enhanced. Since U 0 which determines the flux creep properties depends appreciably on the thickness of superconducting layer. For the application ⇑ Corresponding author. Address: Department of Computer Science and Electronics, Kyushu Institute of Technology, 680-4 Kawazu, Iizuka, Fukuoka 820-8502, Japan. Tel.: +81 948 29 7663; fax: +81 948 29 7661. E-mail address: [email protected] (T. Matsushita). 0921-4534/$ - see front matter Ó 2011 Published by Elsevier B.V. doi:10.1016/j.physc.2011.05.115
to SMES, it is important to investigate the superconducting layer thickness dependence of U 0 . SMES is planned to be operated at very high magnetic fields and hence, at low temperatures. However, the measurement of current relaxation at high magnetic fields is not easy. If the dependence of U 0 on magnetic field and temperature can be described in a form of scaling law, it may be useful for estimation of U 0 at such extreme condition. Then, the scaling law was examined for a series of CVD-processed YBCO-coated conductors [2]. In this paper, the thickness dependence of relaxation properties in CVD-processed YGdBCO coated conductors is investigated, and the result is analyzed by using the flux creep–flow model [3]. The scaling of the dependence U 0 on magnetic field and temperature is also examined for a series of new specimens. 2. Experiments The specimens were Y0.7Gd0.3Ba2Cu3O7d(YGdBCO)-coated conductors prepared by MS-CVD process. The superconducting layer was deposited on ion beam assisted deposition (IBAD)-processed substrate with Gd2Zr2O7 (GZO) inner layer and CeO2 cap layer [1]. The thickness of YGdBCO layer was varied in the range of 0.33–1.43 lm. Specifications of these specimens are listed in Table 1. The relaxation of DC magnetization was measured by using a SQUID magnetometer in a magnetic field up to 6.5 T parallel to
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Y. Takahashi et al. / Physica C 471 (2011) 1025–1028
Table 1 Specifications of specimens. Specimen
Thickness d (lm)
Tc (K)
#1 #2 #3 #4 #5 #6
0.33 0.55 0.77 0.99 1.21 1.43
89.3 89.6 90.9 91.3 90.7 89.7
the c-axis of the specimen over the temperature range of 20–60 K. The critical current density was estimated from the DC magnetization using the Bean model:
Jc ¼
6 Dm 2
dw ð3l wÞ
;
ð1Þ
where Dm is the hysteresis of magnetic moment, d is the thickness of the superconducting layer, and w and l are the dimensions of wide surface of specimens (w < l), respectively. E–J characteristics were estimated from the relaxation measurement [4]. The magnetization M is given by D m/wld. From the logarithmic relaxation rate of the magnetization, U 0 was estimated:
d M U ¼ 0 : dlogt M 0 kB T
ð2Þ
The initial magnetization M0 was determined by extrapolating the relaxation curve from the time range of 3 102–2 103 s to t = 1 s.
The magnetic field dependence of U 0 at 20 K obtained from the relaxation rate of magnetization is shown in Fig. 2. It is found that U 0 increases first and then decreases with increasing magnetic field. The magnetic field at which U 0 starts to decrease increases with increasing thickness. The critical current properties were analyzed using the flux creep–flow model [3]. In the virtual flux-creep free case, the critical current density takes a value determined only by the flux pinning and its temperature and magnetic field dependence is assumed as
m 2 T B J c0 ¼ A 1 Bc1 1 ; Tc Bc2
ð3Þ
where A, m, c are pinning parameters. The pinning potential U0 which determines the flux creep property is described in terms of Jc0 as:
U0 ¼
1=2 0:835g 2 kB J c0
ð2pÞ3=2 B1=4
¼
4:23g 2 kB J c0 d 2pB1=2
; d > L;
ð4Þ
; d < L:
ð5Þ
In the above d is the thickness of the superconducting layer and L is the longitudinal flux bundle size given by
L¼
Baf
2pl0 J c0
1=2 ð6Þ
Fig. 1 shows the Jc–B characteristics of the six specimens at 20 and 60 K in the magnetic field regions of 0–6.5 T. The Jc value does not appreciably depend on the thickness at 20 K. That is, it is expected that there is no structural deterioration of superconducting layer during the process of increasing thickness [2]. On the other hand, Jc of the thinner specimen tends to more rapidly decrease with increasing magnetic field at 60 K, which is attributed to the strong influence of the flux creep as will be discussed later.
with af denoting the flux line spacing. It should be noted that even if the pinning is in the three-dimensional case (d > L) at low magnetic fields for fairly thick superconductor, the pinning may change to the two-dimensional one at high magnetic fields due to the increase in L with decreasing Jc0. More detailed discussion of this mechanism is given in [5]. The parameter g2 is the number of flux lines in a flux bundle. In practical superconductors the flux pinning strength is statistically distributed widely [6]. Here it is simply assumed that only the parameter A which represents the pinning strength is distributed as
Fig. 1. Magnetic field dependence of critical current density at 20 and 60 K in the magnetic field range of 0–6.5 T. The lines are guide for eye.
Fig. 2. Magnetic field dependence of observed U 0 at 20 K. The lines are guide for eye.
3. Results and discussion
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Y. Takahashi et al. / Physica C 471 (2011) 1025–1028
" f ðAÞ ¼ K exp
#
ðlog A log Am Þ2 ; 2r 2
ð7Þ
where Am is the most probable value of A, r2 is a parameter representing a distribution width and K is a normalization constant. The electric field E caused by the mechanism of flux creep and flow is calculated as a function of the current density J and averaged statistically with respect to the distributed A. The details of the theoretical calculation are described elsewhere [3]. The theoretical critical current density is determined from the obtained E–J curve with the same electric field criterion of Ec = 109 V/m as done in experiments. The pinning parameters Am, r, m and c are determined so that the calculated E–J curves agree with the experiments. The obtained parameters are listed in Table 2. The E–J curves of specimen #6 obtained from the relaxation of magnetization are shown in Fig. 3. The lines are the theoretical results of flux creep–flow model. The experimental results are explained well by the theory. Fig. 4 shows the theoretical results of U 0 at 20 K. Good agreement with experiments shown in Fig. 2 is obtained. Thus, the relaxation property is describable by the mechanism of flux creep and flow. The possibility of description of U 0 ðB; TÞ in the form of scaling law is examined. It was shown that the magnetic field dependence of U 0 can be approximated by:
U 0 ðB; TÞ ¼ U 0
peak ðTÞ
a½B Bpeak ðTÞ2 ;
and U 0 peak are determined so that good agreement is obtained with experiments at each temperature, while a is assumed to be independent of temperature. The obtained parameters are a = 1.0 103 (eV/T2) for all the specimens and Bpeak and U 0 peak are listed in Table 3. The experimental results of the apparent pinning
ð8Þ
U 0 peak
where a, Bpeak and are fitting parameters [2]. This function is also adopted for the present specimens. The parameters Bpeak
Table 2 Pinning parameters of specimens at 20 K. Specimen #1 #2 #3 #4 #5 #6
r2
Am 11
4.1 10 3.9 1011 4.0 1011 4.0 1011 3.9 1011 4.0 1011
3
5.3 10 6.4 103 7.1 103 6.0 103 6.8 103 6.0 103
c
m
0.54 0.55 0.58 0.63 0.65 0.67
2.5 2.5 2.6 2.3 2.0 1.9
Fig. 3. E–J curves of specimen #6 obtained from the relaxation of magnetization. The lines are theoretical results of flux creep–flow model.
Fig. 4. Magnetic field dependence of U 0 calculated by theoretical relaxation rate at 20 K. The lines are guide for eye.
Table 3 Scaling parameter U 0
peak
of specimens.
20 K
25 K
Specimen
Bpeak
U 0
#4 #5 #6
3.61 4.77 4.84
7.67 102 8.05 102 8.16 102
peak
30 K
Bpeak
U 0
2.61 3.50 4.02
9.29 102 9.94 102 9.93 102
peak
Bpeak
U 0
1.25 2.47 3.13
1.13 101 1.17 101 1.19 101
peak
Fig. 5. Magnetic field dependence of U 0 in specimen #4 in the temperature region 20–30 K. Symbols and lines represent the experimental result and approximation of Eq. (8), respectively.
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Y. Takahashi et al. / Physica C 471 (2011) 1025–1028
specimens #4, #5 and #6 with the temperature region 20–30 K is shown in Fig. 6. It is found that Bpeak decreases and U 0 peak increases with increasing temperature. At the same time, Bpeak and U 0 peak tend to increase with increasing thickness. Furthermore, U 0 was estimated in the higher magnetic field region at 20 K for specimens #4, #5 and #6 by using Eq. (8) in Fig. 7. The open symbols show the results of estimation. This indicates that superconducting coated conductors with thicker superconducting layer is better for high field performance. Hence, much thicker superconducting layer seems to be promising for the practical superconductor in SMES systems, although the phenomenon is complicated and more detailed analysis is necessary for exact understanding of the phenomenon. 4. Summary
Fig. 6. Bpeak dependence of U 0 20–30 K.
peak
in specimen #4, #5 and #6 temperature region
The thickness dependence of the critical current density and relaxation properties in CVD processed YGdBCO coated conductors was investigated. It was found that the value of critical current density does not appreciably depend on d at 20 K, indicating no structural deterioration of superconducting layer during the process of increasing thickness. On the other hand, Jc of the thinner specimen tends to more rapidly decrease with increasing magnetic field at 60 K. At 20 K the apparent pinning potential U 0 increases and then, tends to decrease with increasing magnetic field. The magnetic field at which U 0 starts to decrease increases with increasing thickness. These results were theoretically explained by the flux creep– flow model based on the dimensionality of flux pinning. Application of scaling law is examined for the dependence of U 0 on magnetic field and temperature. It was found that the dependence can be expressed by using the scaling parameters Bpeak ; U 0 peak in the temperature region 20–30 K. It was also found that Bpeak and U 0 peak tend to increase with increasing thickness. The behavior of these parameter is complicated and more detailed analysis is necessary for exact understanding of the phenomenon. Acknowledgements This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) as the Technological Development of Yttrium-based Superconducting Power Equipment. References
Fig. 7. Magnetic field dependence of U 0 in specimen #4, #5 and #6 at 20 K. The solid symbols are experimental result and the open symbols are the scaling formula of Eq. (8) at 20 K.
potential are compared with the scaling formula of Eq. (8) at 20– 30 K for #4 in Fig. 5. It is found that Eq. (8) expresses well the behavior of U 0 ðB; TÞ. The relationship between Bpeak and U 0 peak for
[1] M. Mori, T. Watanabe, N. Kashima, T. Muroga, S. Miyata, Y. Yamada, T. Izumi, Y. Shiohara, Physica C 445 (448) (2006) 515. [2] Y. Takahashi, M. Kiuchi, E.S. Otabe, T. Matsushita, K. Shikimachi, T. Watanabe, N. Kashima, S. Nagaya, Physica C 470 (2010) 1284. [3] M. Kiuchi, K. Noguchi, T. Matsushita, T. Hikata, K. Sato, Physica C 278 (1997) 62. [4] K. Himeki, M. Kiuchi, E.S. Otabe, T. Matsushita, S. Miyata, A. Ibi, Y. Yamada, Y. Shiohara, Physica C 468 (2007) 1674. [5] T. Matsushita, Flux Pinning in Superconductors, Springer, Berlin, 2006, p. 333. [6] T. Matsushita, T. Tohdoh, N. Ihara, Physica C 259 (1996) 321.