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Switching characteristics of MgB2 wires subjected to transient application of magnetic field
Physica C 426–431 (2005) 1261–1266 www.elsevier.com/locate/physc
Switching characteristics of MgB2 wires subjected to transient application of magnetic field K. Higashikawa
a,*
, T. Nakamura a, K. Osamura b, M. Takahashi c, M. Okada
c
a
Department of Electrical Engineering, Graduate School of Engineering, Kyoto University, Kyoto-Daigaku-Katsura, Nishikyo-Ku, Kyoto 615-8510, Japan b Department of Materials Science and Engineering, Kyoto University, Yoshida-Honmachi, Sakyo-Ku, Kyoto 606-8501, Japan c Department of Materials Research for Power Plants, Materials Research Laboratory, Hitachi Ltd., 1-1 Omika-cho 7-chome, Hitachi, Ibaraki 319-1292, Japan Received 23 November 2004; accepted 17 January 2005 Available online 14 July 2005
Abstract We investigated current transport properties in MgB2 wires subjected to transient application of magnetic field, and then discussed their applicability to magnetically controlled switching elements. These wires were fabricated by powderin-tube (PIT) technique, and Cu–Ni as well as stainless was utilized for their sheath material. A short sample of the wire was installed in a Gifford–McMahon (GM) cryocooler, and was impregnated with solid nitrogen for the sake of thermal stabilization. After stabilizing the temperature of the sample at a certain bias current, the voltage and the temperature in between potential taps were monitored with a transient application of magnetic field. Such measurements were repeated systematically as functions of bias current, operating temperature and magnetic field intensity. It was shown that the sample quickly generated resistance without temperature increment by applying the transient and weak magnetic field. This indicates the applicability of MgB2 wires to persistent current switches (PCS) with quick response. Ó 2005 Elsevier B.V. All rights reserved. PACS: 74.70.Ad; 85.25.Kx; 74.25. q; 74.25.Ha; 07.20.Mc Keywords: MgB2; Persistent current switch; Quick response; Solid nitrogen
1. Introduction *
Corresponding author. Tel.: +81 75 383 2225; fax: +81 75 383 2224. E-mail address: [email protected] (K. Higashikawa).
The discovery of superconductivity in MgB2 [1] has provided us the potential for fabricating economical superconducting wires, that possesses higher performance compared to conventional
0921-4534/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2005.01.067
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K. Higashikawa et al. / Physica C 426–431 (2005) 1261–1266
low-Tc superconducting (LTS) ones, e.g., NbTi and Nb3Sn. That is, MgB2, has much higher Tc i.e., Tc 39 K, and then the possibility to be introduced to conduction cooled applications at the temperature range, for instance, 10–30 K. Although it is considered at the moment that the MgB2 wires are not suitable for coil applications such as superconducting magnetic energy storage (SMES) at 20–40 K because of poor critical current density against external magnetic field in this temperature range, we explore the applicability of the wires to persistent current switches (PCS) by taking advantage of such sensitive dependency of current carrying characteristics upon magnetic field, i.e., magnetically controlled PCS. Magnetically controlled switching elements with LTS conductors have already been proposed by Nitta et al. [2]. It has been expected that such type of PCS can perform much faster switching operation than conventional thermally controlled counterparts, because its switching speed theoretically depends on the sweep rate of controlling magnetic field [2–5]. In addition, so-called n-value of MgB2 wires is much higher than that of high-Tc superconducting (HTS) ones, e.g., Bi-2212, Bi-2223, and then such a clear boundary between superconducting and normal conducting states is also considered to be appropriate for switching elements. In this paper, we examine experimentally the current transport properties in MgB2 wires subjected to pulsed application of the magnetic field. Solid nitrogen is also utilized as additional cryogen [6–14] for the sake of thermal stabilization of the wires. And then, applicability of the wires to the magnetically controlled switching elements is to be discussed based on the experimental results.
Table 1 Specifications of the samples of MgB2 wires Items
Sample A
Sample B
Type
MgB2 monofilament
Sheath material Process Annealing Cross-sectional Shape
Cu–Ni In situ 630 °C, 1 h Circle (0.96 mm in diameter)
Length
100 mm
MgB2 monofilament Stainless Ex situ As-rolled Rectangular (2.4 mm in width, 0.36 mm in thickness) 100 mm
cross-sectional shape is circle (0.96 mm in diameter). Sample B, on the other hand, employs stainless as the sheath material and ex situ process without annealing (as-rolled). Its cross-sectional shape is rectangular (2.4 mm in width and 0.36 mm in thickness). Their short samples (100 mm in length) are used for the measurements. Schematic diagram of a sample holder is illustrated in Fig. 1. Both ends of the short sample are connected to copper blocks used as current leads as well as thermal anchors. A miniature thermo-sensor is mounted on the sample in between voltage taps by using thermal conducting grease. Furthermore, an air-core copper coil (22 mm in inner diameter, 40 mm in outer diameter, 26 mm in height and 200 turns) for the application of magnetic field to the sample is placed at 1 mm in
40 mm Copper blocks 22 mm Miniature thermo sensor
Copper coil for magnetic field 26 mm
2. Experimental
1mm
2.1. Sample settings 10 mm
Measurements are carried out with the use of two kinds of MgB2 wires, and their specifications are listed in Table 1. Both of them are fabricated by powder-in-tube (PIT) technique. Sample A adopts Cu–Ni as the sheath material and in situ process with annealing at 630 °C for 1 h. Its
Voltage taps for sample A
10 mm
Voltage taps for sample B
50 mm
Short sample of MgB2 wire
100 mm
Fig. 1. Schematic of the sample holder.
K. Higashikawa et al. / Physica C 426–431 (2005) 1261–1266 Vacuum chamber
Observational windows
Sample chamber
Copper cover (50 K shield) Copper cover Nitrogen gas inlets Sample holder Vacuum space
Observational windows
CP
Indium seal
Nitrogen gas outlet Copper leads
Thermal anchors
HTS leads (Bi-2223/Ag tapes) 2nd stage to 1st stage
Fig. 2. Schematic of the experimental system. Temperature at point ‘‘CP’’ is controlled with the use of a GM cryocooler.
distance from the surface of the sample as shown in Fig. 1. The sample holder is attached to the 2nd stage of a Gifford–McMahon (GM) cryocooler as shown in Fig. 2. A sample chamber is surrounded by vacuum space, and sealed with indium in preparation for solid nitrogen utilized as additional cryogen. 2.2. Measurements methods Measurements of critical temperature, Tc transport current versus electric field, I–E, curves and switching characteristics operated by pulsed magnetic field in the wires are carried out in this study. For all measurements, the sample is impregnated with solid nitrogen as follows: Firstly, the temperature at the 2nd stage of the GM cryocooler is decreased down to 64.0 K. Secondly, nitrogen gas (99.9999% in purity) is pressurized into the sample chamber, and then is liquified there. After the liquid nitrogen is adequately produced in the chamber, the temperature at the 2nd stage of the cryocooler is kept to be 62.5 K until the liquid nitrogen is solidified completely. Such temperature is near the triple point, 63.15 K, of nitrogen. It has already been reported that slow solidification process of liquid nitrogen in this way is very important for good thermal contact between cooling objects and solid nitrogen [11–13].
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After the sample is impregnated with solid nitrogen, Tc measurements are carried out by standard four-probe dc-electrical resistivity method with bias current at 10 mA. I–E curves are also obtained at various temperatures by using the same method. In addition, measurements of switching characteristics are performed as follows: Firstly, we have to wait for the temperature of the sample with bias current to be stabilized. Then, during an application of pulsed voltage (4 ns in rising time and 7.3 ms in duration) to the coil, current of the coil, generated voltage and temperature at the surface of the sample are monitored as a function of time. Such measurements are repeated systematically as functions of bias current, operating temperature and supplied voltage of the coil.
3. Results and discussion 3.1. Tc and I–E curves The results of Tc measurements are listed in Table 2. Here, Tc denotes so-called end-point of critical temperature, and DTc the temperature width for superconducting transition, respectively. As can be seen in the figure, sample A has higher Tc as well as narrower DTc compared to sample B does, and this is considered to be the annealing effect in fabricating process of the former sample. I–E curves obtained at various temperatures are shown in Fig. 3. Critical current with electric field criterion at 1 lV/cm (= 10 4V/m), Ic of sample A is 105 A at 31.2 K, while that of sample B is 18.0 A even at 18.4 K. Such great difference also attributes to the annealing effect. In both of the samples, on the other hand, the apparent n-value becomes higher as the measurement temperature decreases. Further, the value at lower temperature
Table 2 Results of Tc measurements for the samples Items
Sample A
Sample B
Tc (K) DTc (K)
36.5 0.5
31.0 7.0
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K. Higashikawa et al. / Physica C 426–431 (2005) 1261–1266 -3
Electric field E / (Vm-1)
10
-4
10
-5
10
-6
10
36.3 K 35.2 K 34.2 K 33.2 K 32.2 K 31.2 K
-7
10 -1 10
0
10
a
1
10
0
10
10 Current I/A
2
10
3
1
10
-3
Electric field E / (Vm-1)
10
-4
10
-5
10
-6
10
30.2 K 28.2 K 26.2 K 24.2 K 22.2 K 20.3 K 18.4 K
-7
b
10 -2 10
10
-1
10 Current I/A
2
Fig. 3. I–E curves at various temperatures: (a) for sample A and (b) for sample B.
of MgB2 wires is much higher than that of HTS conductors, e.g., more than 150 in sample B below 24.2 K. Such high n-value can be obtained without dependence on Tc and/or Ic values at lower temperatures. We believe that such a clear boundary between superconducting and normal conducting states in MgB2 wires is suitable for switching elements. 3.2. Switching characteristics Fig. 4 shows switching characteristics in sample B at T = 24.2 K and I = 3.6 A with coil current of 3.0 A. The corresponding load factor defined with I/Ic, LF, is 0.86, and maximum magnetic flux den-
sity applied to the sample is 12 mT. Traces of the coil current, generated electric field and temperature at the surface of the sample are shown in the figure. After the voltage with the rising time at 4 ns is supplied to the coil, the coil current takes 3.2 ms to increase to 3.0 A. Such time is determined by inductance and resistance of the coil. In the same way, the coil current spends 3.2 ms in order to decrease down to zero after the supplied voltage is stopped. The electric field generated in between the voltage taps on the sample increases and decreases following such variation of the coil current. Rising time of the electric field is 0.6 ms, and then the part between the voltage taps transits to normal conducting state for the coil current at 1.7 A. It should be noted that such transition is occurred without temperature variation (see temperature trace in Fig. 4). Therefore, the sample can swiftly return to superconducting state when the application of magnetic field is stopped. These results indicate the applicability of MgB2 wires to magnetically controlled switching elements with quick response. Fig. 5 shows switching conditions in both of the samples. The conditions in sample B are obtained with respect to two values of coil current. The plots in the figure indicate lower limit of LF at various temperatures, where the above-mentioned switching characteristics can be observed. That is, the samples can be switched off/on in shaded area. The switching operation is available for sample A in the region of larger LF and higher temperature compared to the case for sample B at the same coil current. The reason is that sample A has better performance against external magnetic field than sample B does. Therefore, the wires should have large critical current at self-field as well as moderately sensitive performance against external magnetic field for the application of effective switching elements. In this study, the switching operation can be observed at 5 mT, i.e., weaker magnetic field. However, such switching region will be expanded by increasing magnetic field as shown for sample B in the figure (see the comparison of the cases between 5 mT and 12 mT). Therefore, operational conditions such as LF and intensity of the field should be optimized in practical systems.
K. Higashikawa et al. / Physica C 426–431 (2005) 1261–1266
Coil current Icoil/ A
12.0
1.7 A
2.0
8.0
1.0
4.0
0.0
0.0 3.2 ms
3.2 ms
Maximum magnetic flux density Bmax/ mT
3.0
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Electric field E / (mVm-1)
1.5 1.0 0.5 0.0 0.6 ms
Temperature T/K
24.8 Duration of supplying voltage to the coil
24.6 24.4 24.2 5.0
10.0
15.0 Time t /ms
20.0
25.0
Fig. 4. Switching characteristics in sample B at T = 24.2 K and I = 3.6 A with the coil current of 3.0 A. The corresponding LF (= I/Ic) is 0.86, and maximum magnetic flux density applied to the sample is 12 mT.
Sample A with coil current 1.3 A (B
= 5 mT)
Sample B with coil current 1.3 A (B
= 5 mT)
Sample B with coil current 3.0 A (B
= 12 mT)
max
max max
4. Conclusions
Load factor LF (= I / Ic)
1.0 0.8 0.6 0.4 0.2 Tc in sample B
0.0 20.0
25.0
Tc in sample A
30.0
35.0
40.0
Temperature T/K Fig. 5. Switching conditions in the samples. The conditions in sample B are obtained with respect to two values of coil current.
We measured current transport properties in two kinds of MgB2 wires impregnated with solid nitrogen subjected to pulsed magnetic field. As a result, the wires took less than 1 ms to transit from superconducting to normal conducting state. Such time is much shorter than that in conventional thermally controlled PCS of a few seconds. Furthermore, temperature variation during such transition could be suppressed with the aid of solid nitrogen. Therefore, the wires could quickly return to superconducting state when the application of magnetic field is stopped. These results indicate the applicability of MgB2 wires to magnetically controlled switching elements cooled by cryocoolers.
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K. Higashikawa et al. / Physica C 426–431 (2005) 1261–1266
Acknowledgements This work has been supported by The Iwatani Naoji FoundationÕs Research Grant in Japan. This work has also been carried out as a project of 21st Century COE Program (No. 14213201) from the Ministry of Education, Culture, Sports, Science and Technology in Japan. The authors are grateful to Mr. Fujiwara at Kyoto University in Japan for the support of the experiments.
References [1] J. Nagamatsu, N. Nakagawa, T. Muranaka, Y. Zenitani, J. Akimitsu, Nature 410 (2001) 63. [2] T. Nitta, M. Nakata, T. Okada, IEEE Trans. Appl. Supercond. 28 (1) (1992) 418. [3] N. Sadakata, K. Uchiyama, K. Goto, T. Saito, O. Kohno, Y. Kouno, M. Matsukawa, K. Noto, H. Honma, T. Tatsuki, IEEE Trans. Appl. Supercond. 5 (2) (1995) 282.
[4] K. Noto, Y. Kouno, M. Matsukawa, M. Itagaki, T. Ishida, K. Chiba, T. Tatsuki, H. Honma, N. Sadakata, T. Saito, O. Kohno, IEEE Trans. Appl. Supercond. 5 (2) (1995) 258. [5] K. Goto, N. Sadakata, T. Saitoh, O. Kohno, IEEE Trans. Appl. Supercond. 9 (2) (1999) 173. [6] H. Isogami, B.J. Haid, Y. Iwasa, IEEE Trans. Appl. Supercond. 11 (1) (2001) 1852. [7] B.J. Haid, H. Lee, Y. Iwasa, S.S. Oh, H.S. Ha, Y.K. Kwon, K.S. Ryu, IEEE Trans. Appl. Supercond. 11 (1) (2001) 2244. [8] A. Sugawara, H. Isogami, B.J. Haid, Y. Iwasa, Physica C 372–376 (2002) 1443. [9] B.J. Haid, H. Lee, Y. Iwasa, S.S. Oh, Y.K. Kwon, K.S. Ryu, Cryogenics 42 (3–4) (2002) 229. [10] B.J. Haid, H. Lee, Y. Iwasa, S.S. Oh, Y.K. Kwon, K.S. Ryu, Cryogenics 42 (10) (2002) 617. [11] T. Nakamura, I. Muta, K. Okude, A. Fujio, T. Hoshino, Physica C 372–376 (2002) 1434. [12] T. Nakamura, K. Okude, A. Fujio, I. Muta, T. Hoshino, Cryog. Eng. 37 (9) (2002) 465 (in Japanese). [13] T. Nakamura, K. Higashikawa, I. Muta, A. Fujio, K. Okude, T. Hoshino, Physica C 386 (2003) 415. [14] T. Nakamura, K. Higashikawa, I. Muta, T. Hoshino, Physica C 412–414 (2004) 1221.
Switching characteristics of MgB2 wires subjected to transient application of magnetic field K. Higashikawa
a,*
, T. Nakamura a, K. Osamura b, M. Takahashi c, M. Okada
c
a
Department of Electrical Engineering, Graduate School of Engineering, Kyoto University, Kyoto-Daigaku-Katsura, Nishikyo-Ku, Kyoto 615-8510, Japan b Department of Materials Science and Engineering, Kyoto University, Yoshida-Honmachi, Sakyo-Ku, Kyoto 606-8501, Japan c Department of Materials Research for Power Plants, Materials Research Laboratory, Hitachi Ltd., 1-1 Omika-cho 7-chome, Hitachi, Ibaraki 319-1292, Japan Received 23 November 2004; accepted 17 January 2005 Available online 14 July 2005
Abstract We investigated current transport properties in MgB2 wires subjected to transient application of magnetic field, and then discussed their applicability to magnetically controlled switching elements. These wires were fabricated by powderin-tube (PIT) technique, and Cu–Ni as well as stainless was utilized for their sheath material. A short sample of the wire was installed in a Gifford–McMahon (GM) cryocooler, and was impregnated with solid nitrogen for the sake of thermal stabilization. After stabilizing the temperature of the sample at a certain bias current, the voltage and the temperature in between potential taps were monitored with a transient application of magnetic field. Such measurements were repeated systematically as functions of bias current, operating temperature and magnetic field intensity. It was shown that the sample quickly generated resistance without temperature increment by applying the transient and weak magnetic field. This indicates the applicability of MgB2 wires to persistent current switches (PCS) with quick response. Ó 2005 Elsevier B.V. All rights reserved. PACS: 74.70.Ad; 85.25.Kx; 74.25. q; 74.25.Ha; 07.20.Mc Keywords: MgB2; Persistent current switch; Quick response; Solid nitrogen
1. Introduction *
Corresponding author. Tel.: +81 75 383 2225; fax: +81 75 383 2224. E-mail address: [email protected] (K. Higashikawa).
The discovery of superconductivity in MgB2 [1] has provided us the potential for fabricating economical superconducting wires, that possesses higher performance compared to conventional
0921-4534/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2005.01.067
1262
K. Higashikawa et al. / Physica C 426–431 (2005) 1261–1266
low-Tc superconducting (LTS) ones, e.g., NbTi and Nb3Sn. That is, MgB2, has much higher Tc i.e., Tc 39 K, and then the possibility to be introduced to conduction cooled applications at the temperature range, for instance, 10–30 K. Although it is considered at the moment that the MgB2 wires are not suitable for coil applications such as superconducting magnetic energy storage (SMES) at 20–40 K because of poor critical current density against external magnetic field in this temperature range, we explore the applicability of the wires to persistent current switches (PCS) by taking advantage of such sensitive dependency of current carrying characteristics upon magnetic field, i.e., magnetically controlled PCS. Magnetically controlled switching elements with LTS conductors have already been proposed by Nitta et al. [2]. It has been expected that such type of PCS can perform much faster switching operation than conventional thermally controlled counterparts, because its switching speed theoretically depends on the sweep rate of controlling magnetic field [2–5]. In addition, so-called n-value of MgB2 wires is much higher than that of high-Tc superconducting (HTS) ones, e.g., Bi-2212, Bi-2223, and then such a clear boundary between superconducting and normal conducting states is also considered to be appropriate for switching elements. In this paper, we examine experimentally the current transport properties in MgB2 wires subjected to pulsed application of the magnetic field. Solid nitrogen is also utilized as additional cryogen [6–14] for the sake of thermal stabilization of the wires. And then, applicability of the wires to the magnetically controlled switching elements is to be discussed based on the experimental results.
Table 1 Specifications of the samples of MgB2 wires Items
Sample A
Sample B
Type
MgB2 monofilament
Sheath material Process Annealing Cross-sectional Shape
Cu–Ni In situ 630 °C, 1 h Circle (0.96 mm in diameter)
Length
100 mm
MgB2 monofilament Stainless Ex situ As-rolled Rectangular (2.4 mm in width, 0.36 mm in thickness) 100 mm
cross-sectional shape is circle (0.96 mm in diameter). Sample B, on the other hand, employs stainless as the sheath material and ex situ process without annealing (as-rolled). Its cross-sectional shape is rectangular (2.4 mm in width and 0.36 mm in thickness). Their short samples (100 mm in length) are used for the measurements. Schematic diagram of a sample holder is illustrated in Fig. 1. Both ends of the short sample are connected to copper blocks used as current leads as well as thermal anchors. A miniature thermo-sensor is mounted on the sample in between voltage taps by using thermal conducting grease. Furthermore, an air-core copper coil (22 mm in inner diameter, 40 mm in outer diameter, 26 mm in height and 200 turns) for the application of magnetic field to the sample is placed at 1 mm in
40 mm Copper blocks 22 mm Miniature thermo sensor
Copper coil for magnetic field 26 mm
2. Experimental
1mm
2.1. Sample settings 10 mm
Measurements are carried out with the use of two kinds of MgB2 wires, and their specifications are listed in Table 1. Both of them are fabricated by powder-in-tube (PIT) technique. Sample A adopts Cu–Ni as the sheath material and in situ process with annealing at 630 °C for 1 h. Its
Voltage taps for sample A
10 mm
Voltage taps for sample B
50 mm
Short sample of MgB2 wire
100 mm
Fig. 1. Schematic of the sample holder.
K. Higashikawa et al. / Physica C 426–431 (2005) 1261–1266 Vacuum chamber
Observational windows
Sample chamber
Copper cover (50 K shield) Copper cover Nitrogen gas inlets Sample holder Vacuum space
Observational windows
CP
Indium seal
Nitrogen gas outlet Copper leads
Thermal anchors
HTS leads (Bi-2223/Ag tapes) 2nd stage to 1st stage
Fig. 2. Schematic of the experimental system. Temperature at point ‘‘CP’’ is controlled with the use of a GM cryocooler.
distance from the surface of the sample as shown in Fig. 1. The sample holder is attached to the 2nd stage of a Gifford–McMahon (GM) cryocooler as shown in Fig. 2. A sample chamber is surrounded by vacuum space, and sealed with indium in preparation for solid nitrogen utilized as additional cryogen. 2.2. Measurements methods Measurements of critical temperature, Tc transport current versus electric field, I–E, curves and switching characteristics operated by pulsed magnetic field in the wires are carried out in this study. For all measurements, the sample is impregnated with solid nitrogen as follows: Firstly, the temperature at the 2nd stage of the GM cryocooler is decreased down to 64.0 K. Secondly, nitrogen gas (99.9999% in purity) is pressurized into the sample chamber, and then is liquified there. After the liquid nitrogen is adequately produced in the chamber, the temperature at the 2nd stage of the cryocooler is kept to be 62.5 K until the liquid nitrogen is solidified completely. Such temperature is near the triple point, 63.15 K, of nitrogen. It has already been reported that slow solidification process of liquid nitrogen in this way is very important for good thermal contact between cooling objects and solid nitrogen [11–13].
1263
After the sample is impregnated with solid nitrogen, Tc measurements are carried out by standard four-probe dc-electrical resistivity method with bias current at 10 mA. I–E curves are also obtained at various temperatures by using the same method. In addition, measurements of switching characteristics are performed as follows: Firstly, we have to wait for the temperature of the sample with bias current to be stabilized. Then, during an application of pulsed voltage (4 ns in rising time and 7.3 ms in duration) to the coil, current of the coil, generated voltage and temperature at the surface of the sample are monitored as a function of time. Such measurements are repeated systematically as functions of bias current, operating temperature and supplied voltage of the coil.
3. Results and discussion 3.1. Tc and I–E curves The results of Tc measurements are listed in Table 2. Here, Tc denotes so-called end-point of critical temperature, and DTc the temperature width for superconducting transition, respectively. As can be seen in the figure, sample A has higher Tc as well as narrower DTc compared to sample B does, and this is considered to be the annealing effect in fabricating process of the former sample. I–E curves obtained at various temperatures are shown in Fig. 3. Critical current with electric field criterion at 1 lV/cm (= 10 4V/m), Ic of sample A is 105 A at 31.2 K, while that of sample B is 18.0 A even at 18.4 K. Such great difference also attributes to the annealing effect. In both of the samples, on the other hand, the apparent n-value becomes higher as the measurement temperature decreases. Further, the value at lower temperature
Table 2 Results of Tc measurements for the samples Items
Sample A
Sample B
Tc (K) DTc (K)
36.5 0.5
31.0 7.0
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K. Higashikawa et al. / Physica C 426–431 (2005) 1261–1266 -3
Electric field E / (Vm-1)
10
-4
10
-5
10
-6
10
36.3 K 35.2 K 34.2 K 33.2 K 32.2 K 31.2 K
-7
10 -1 10
0
10
a
1
10
0
10
10 Current I/A
2
10
3
1
10
-3
Electric field E / (Vm-1)
10
-4
10
-5
10
-6
10
30.2 K 28.2 K 26.2 K 24.2 K 22.2 K 20.3 K 18.4 K
-7
b
10 -2 10
10
-1
10 Current I/A
2
Fig. 3. I–E curves at various temperatures: (a) for sample A and (b) for sample B.
of MgB2 wires is much higher than that of HTS conductors, e.g., more than 150 in sample B below 24.2 K. Such high n-value can be obtained without dependence on Tc and/or Ic values at lower temperatures. We believe that such a clear boundary between superconducting and normal conducting states in MgB2 wires is suitable for switching elements. 3.2. Switching characteristics Fig. 4 shows switching characteristics in sample B at T = 24.2 K and I = 3.6 A with coil current of 3.0 A. The corresponding load factor defined with I/Ic, LF, is 0.86, and maximum magnetic flux den-
sity applied to the sample is 12 mT. Traces of the coil current, generated electric field and temperature at the surface of the sample are shown in the figure. After the voltage with the rising time at 4 ns is supplied to the coil, the coil current takes 3.2 ms to increase to 3.0 A. Such time is determined by inductance and resistance of the coil. In the same way, the coil current spends 3.2 ms in order to decrease down to zero after the supplied voltage is stopped. The electric field generated in between the voltage taps on the sample increases and decreases following such variation of the coil current. Rising time of the electric field is 0.6 ms, and then the part between the voltage taps transits to normal conducting state for the coil current at 1.7 A. It should be noted that such transition is occurred without temperature variation (see temperature trace in Fig. 4). Therefore, the sample can swiftly return to superconducting state when the application of magnetic field is stopped. These results indicate the applicability of MgB2 wires to magnetically controlled switching elements with quick response. Fig. 5 shows switching conditions in both of the samples. The conditions in sample B are obtained with respect to two values of coil current. The plots in the figure indicate lower limit of LF at various temperatures, where the above-mentioned switching characteristics can be observed. That is, the samples can be switched off/on in shaded area. The switching operation is available for sample A in the region of larger LF and higher temperature compared to the case for sample B at the same coil current. The reason is that sample A has better performance against external magnetic field than sample B does. Therefore, the wires should have large critical current at self-field as well as moderately sensitive performance against external magnetic field for the application of effective switching elements. In this study, the switching operation can be observed at 5 mT, i.e., weaker magnetic field. However, such switching region will be expanded by increasing magnetic field as shown for sample B in the figure (see the comparison of the cases between 5 mT and 12 mT). Therefore, operational conditions such as LF and intensity of the field should be optimized in practical systems.
K. Higashikawa et al. / Physica C 426–431 (2005) 1261–1266
Coil current Icoil/ A
12.0
1.7 A
2.0
8.0
1.0
4.0
0.0
0.0 3.2 ms
3.2 ms
Maximum magnetic flux density Bmax/ mT
3.0
1265
Electric field E / (mVm-1)
1.5 1.0 0.5 0.0 0.6 ms
Temperature T/K
24.8 Duration of supplying voltage to the coil
24.6 24.4 24.2 5.0
10.0
15.0 Time t /ms
20.0
25.0
Fig. 4. Switching characteristics in sample B at T = 24.2 K and I = 3.6 A with the coil current of 3.0 A. The corresponding LF (= I/Ic) is 0.86, and maximum magnetic flux density applied to the sample is 12 mT.
Sample A with coil current 1.3 A (B
= 5 mT)
Sample B with coil current 1.3 A (B
= 5 mT)
Sample B with coil current 3.0 A (B
= 12 mT)
max
max max
4. Conclusions
Load factor LF (= I / Ic)
1.0 0.8 0.6 0.4 0.2 Tc in sample B
0.0 20.0
25.0
Tc in sample A
30.0
35.0
40.0
Temperature T/K Fig. 5. Switching conditions in the samples. The conditions in sample B are obtained with respect to two values of coil current.
We measured current transport properties in two kinds of MgB2 wires impregnated with solid nitrogen subjected to pulsed magnetic field. As a result, the wires took less than 1 ms to transit from superconducting to normal conducting state. Such time is much shorter than that in conventional thermally controlled PCS of a few seconds. Furthermore, temperature variation during such transition could be suppressed with the aid of solid nitrogen. Therefore, the wires could quickly return to superconducting state when the application of magnetic field is stopped. These results indicate the applicability of MgB2 wires to magnetically controlled switching elements cooled by cryocoolers.
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Acknowledgements This work has been supported by The Iwatani Naoji FoundationÕs Research Grant in Japan. This work has also been carried out as a project of 21st Century COE Program (No. 14213201) from the Ministry of Education, Culture, Sports, Science and Technology in Japan. The authors are grateful to Mr. Fujiwara at Kyoto University in Japan for the support of the experiments.
References [1] J. Nagamatsu, N. Nakagawa, T. Muranaka, Y. Zenitani, J. Akimitsu, Nature 410 (2001) 63. [2] T. Nitta, M. Nakata, T. Okada, IEEE Trans. Appl. Supercond. 28 (1) (1992) 418. [3] N. Sadakata, K. Uchiyama, K. Goto, T. Saito, O. Kohno, Y. Kouno, M. Matsukawa, K. Noto, H. Honma, T. Tatsuki, IEEE Trans. Appl. Supercond. 5 (2) (1995) 282.
[4] K. Noto, Y. Kouno, M. Matsukawa, M. Itagaki, T. Ishida, K. Chiba, T. Tatsuki, H. Honma, N. Sadakata, T. Saito, O. Kohno, IEEE Trans. Appl. Supercond. 5 (2) (1995) 258. [5] K. Goto, N. Sadakata, T. Saitoh, O. Kohno, IEEE Trans. Appl. Supercond. 9 (2) (1999) 173. [6] H. Isogami, B.J. Haid, Y. Iwasa, IEEE Trans. Appl. Supercond. 11 (1) (2001) 1852. [7] B.J. Haid, H. Lee, Y. Iwasa, S.S. Oh, H.S. Ha, Y.K. Kwon, K.S. Ryu, IEEE Trans. Appl. Supercond. 11 (1) (2001) 2244. [8] A. Sugawara, H. Isogami, B.J. Haid, Y. Iwasa, Physica C 372–376 (2002) 1443. [9] B.J. Haid, H. Lee, Y. Iwasa, S.S. Oh, Y.K. Kwon, K.S. Ryu, Cryogenics 42 (3–4) (2002) 229. [10] B.J. Haid, H. Lee, Y. Iwasa, S.S. Oh, Y.K. Kwon, K.S. Ryu, Cryogenics 42 (10) (2002) 617. [11] T. Nakamura, I. Muta, K. Okude, A. Fujio, T. Hoshino, Physica C 372–376 (2002) 1434. [12] T. Nakamura, K. Okude, A. Fujio, I. Muta, T. Hoshino, Cryog. Eng. 37 (9) (2002) 465 (in Japanese). [13] T. Nakamura, K. Higashikawa, I. Muta, A. Fujio, K. Okude, T. Hoshino, Physica C 386 (2003) 415. [14] T. Nakamura, K. Higashikawa, I. Muta, T. Hoshino, Physica C 412–414 (2004) 1221.