- Email: [email protected]
Ag tapes
Cryogenics 36 (1996) 485490 0 1996 Elsevier Science Limited Printed in Great Britain. All tights reserved OOll-2275/96/$15.00
ELSEVIER
Fabrication and properties of superconducting magnets using Bi,Sr2CaCu20JAg tapes Naruaki Tomita, Mikako Arai, Eiji Yanagisawa, Takeshi Morimoto, Hitoshi Kitaguchi”, Hiroaki Kumakura*, Kazumasa Togano* and Katsumi Nomura* Research Center, Asahi Glass Co. Ltd, 1150 Hazawa-cho Kanagawa-ku, Yokohama 221, Japan *National Research Institute for Metals, 1-2-1 Sengen, Tsukuba, lbaraki 305, Japan *Advanced Research Center, Hitachi Cable Ltd, 3350 Kidamari-cho, Tsuchiura, lbaraki 300, Japan Received
19 October
7995; revised
16 November
1995
We fabricated pancake type coils with dimension of 20 mm4 (bore) x 94 mm+ (outer diameter) using Bi-2212/Ag tapes. Bi-2212/Ag tapes were prepared by the combination of a continuous dip-coating process and melt-solidification, and the coils were fabricated by the wind and react method and fixed with epoxy resins. Two coils were stacked and connected in series to construct a magnet. The magnet was tested in bias fields up to 20 T and at temperatures up to 40 K. This magnet generated a magnetic field of 2.6 T (I,, = 385 A ( lo-l3 R m)) under zero bias field, 1.08 T !I,, = 160 A (2 x IO-l3 1Rm)) under a bias field of 20 T in liquid helium. At about 20 K, the generated field of the magnet was 1.53 T (I,, = 225 A (IO-l3 fl m)). The results suggest that a Bi2212 coil is promising as an insert magnet of a high-field superconducting magnet system. The Bi-2212 coil is also expected to operate as a cryogen-free magnet which can be operated at -20 K with cryocooler. Keywords:
superconducting
magnet;
Bi-2212/Ag;
Many studies on the fabrication of superconducting magnets using high T, oxide superconducting tapes and wires are in progress’-9. These superconducting magnets are expected to be used as two types of magnet. One is the inner magnets of a superconducting magnet system which generates higher than 21 T where application of conventional superconducting materials are difficult because of their low upper critical fields. The other is helium-free superconducting magnets which generate higher than 2 T, where the use of an iron core is difficult. These magnets will be operated at around 20 K using refrigerator cooling. Bi,Sr,CaCu,O,( Bi-2212)/Ag tapes prepared by melt-solidification have excellent microstructure of highly oriented and densely stacked Bi-2212 crystals with their &plane parallel to the silver tape surface. They show high J, values, higher than lo5 A cm-*, even in magnetic fields above 20 T at 4.2 K’O. For the formation of Bi-2212 layers on both sides of a long Ag substrate, the dip-coating method was found to be suitable”. In the dip-coated Bi-2212/Ag long tape, the oxide layer stuck firmly to the Ag tape after meltsolidification and showed grain-aligned microstructure, as the case of a short sample. Hence we are developing superconducting magnets using the Bi-2212/Ag composite tapes
melt-solidification
prepared by the combination of a continuous dip-coating process and melt-solidification. Recently, we fabricated several small pancake type coils using Bi-2212/Ag tapes and tested the coils at various temperatures and magnetic fields. A small Bi-2212/Ag pancake coil, used as an insert magnet of the conventional superconducting magnet system, generated 0.7 T under a bias field of 20.8 T, and a total field of 21.5 T was attained in the full superconducting state’. However, the fields generated by the Bi-2212/Ag coil were not high enough because the bore of the superconducting magnet system was limited. In this paper, we report the fabrication and properties of a larger Bi-2212/Ag pancake coil for higher field generation.
Fabrication
of coils
Bi-2212 powder was made by calcination of Bi203, SrCO,, CaCO, and CuO. The nominal composition of the starting powder was fixed to be Ca-poor, [Bi] : [Sr] : [Cal : [Cu] = 2 : 2 : 0.96 : 2, because Bi-2212 with a slightly Capoor composition showed little impurity phases in their microstructure after melt-solidification’*. The slurry for
Cryogenics
1996 Volume
36, Number
6
485
Fabrication
and properties
of superconducting
magnets:
dip-coating was prepared by mixing fine Bi-2212 powder, appropriate amounts of organic solvent, binder and dispersant. This mixture was milled for 2 days. A schematic for the fabrication procedure of the pancake coils is shown in Figure 1. Silver tapes with dimensions of 80 pm x 18 mm x 10.5 m were used in this experiment. The edge of the tape, approximately 5 mm wide, was masked with plastic sticky tape before the dip-coating. The partially masked tapes were dipped continuously into the slurry, pulled upward at a speed of about 70 cm min-’ and dried by a pipe heater using the continuous dipping apparatus. After the dip-coating process, the plastic tapes were removed. The thickness of the as-dip-coated films was approximately 50 pm on each side of the silver tape. Each dip-coated tape was wound into a pancake shape together with polyethylene sheets of 0.8 mm thickness and annealed at 110°C for 1 day. Then, the polyethylene sheets were removed, leaving gaps (0.8 1.O mm) between the turns of the tape. The coiled tapes were put on the powder mixture of Bi,Al,O,, and A1203 with their exposed silver sides in powder and were heated up to 810°C in air to remove organic materials and to decompose some of the carbonates. After cooling to room temperature, the coiled tapes were covered with an alumina crucible to suppress the vaporization of bismuth from Bi-2212 phase during high-temperature heat treatment13. The coiled tapes were melt-solidified with the following heat treatment schedule. The tape was heated up to 891°C kept at 89 1°C for 5 min, slowly cooled down to 840°C at a rate of 5°C h-’ and kept at 840°C for 1 h. Finally, the tape was furnace-cooled down to room temperature. The thickness of the B-2212 layer decreased to about one-fourth of its initial thickness due to meltsolidification.
N. Tomita
et al.
Microstructure and thickness at several points of the Bi2212/Ag tapes were observed by SEM. The fractured cross section of Bi-2212lAg tape prepared by the combination of a continuous dip-coating process and melt-solidification is shown in Figure 2. Highly oriented Bi-2212 layer crystals with their c-axis perpendicular to the tape surface were formed. This microstructure was the same as that of the Bi2212lAg short tapes prepared by doctor-blade casting and melt-solidificationlO. The composition analysis of Bi2212lAg tapes was performed using Inductive Coupled Plasma (ICP) analysis. The ratio of [Bi] : [Sr] : [Cal : [Cu] in the oxide layer was detected as 1.96 : 2.09 : 0.96 : 2. This ratio is almost equal to that of the starting materials, indicating that vaporization of elements during the melt-solidification process is negligibly small. Silver tapes of thickness 300 pm were used for current leads and voltage taps. Current leads were soldered to both ends of the tape at the exposed silver parts. Voltage taps were attached at 50- 100 cm inner positions from both ends of the tape. Three coils were then arranged together by inserting one coil into the other coils to obtain a single pancake coil. Organic films (Mylar tapes) of thickness 50 pm were also inserted into the gaps for insulation. The arranged coil was tightened around the stainless frame of 20 mm+ inner diameter and encased into stainless pipe of 93 mm+ outer diameter. Three tapes were electrically connected in series with silver tapes. The coil was fixed by impregnation using bisphenol A type epoxy resin cured with acid anhydride’. Finally, two pancake coils were stacked and electrically connected in series by silver tapes. Figure 3 shows the fabricated magnet.
Removing Organic Materials t
Melt-Solidification Dip-Coating
ti
pe Figure 1
486
Schematic
Cryogenics
diagram
for fabrication
1996 Volume
procedure
36, Number
Arranging
Fixing
for pancake coil
6
Fabrication
Figure 2 Photograph and melt-solidification
Figure 3
Photograph
and propertiesof superconducting
of fractured cross section of Bi-2212/Ag
of Bi-2212/Ag
tape prepared by the combination
magnets:
N.
fomitaet
of a continuous dip-coating
al.
process
magnet
Cryogenics
1996 Volume
36, Number
6
487
Fabrication
and properties
of superconducting
magnets:
Coil tests and discussion At first the Bi-2212/Ag magnet was tested in liquid helium under zero bias field. The Z-V characteristic of the Bi2212/Ag tape of the coil measured at 4.2 K with a current sweep rate of 300 A min’ is shown in Figure 4a. The ZV curve bends at around 230 A in the superconducting state and shows a clear transition to the normal state around 400 A. The critical current, Z,, and the critical current density, J,, for the oxide layers of the coil depend on a criterion. Z, and J, determined with the criterion of lo-l3 R m are 385 A and 115 000 A cm-‘, respectively. Figure 4b shows the Z-V curve at 4.2 K for a Bi-2212/Ag coil including silver joints. The measured slope is higher than lo-” fl m because of the resistance of silver and solder. If we apply the criterion of 5 x lo-l3 fi m, which is still much lower than the resistivity of silver substrate, the Z, of the Bi-2212/Ag coil is 373 A. The generated magnetic fields of the Bi-2212/Ag coil were detected with the Hall probe set at the centre of the coil. The relationship between the generated field by the double stacked Bi-2212/Ag magnet and the applied current is shown in Figure 4c. From this Hall voltage versus current curve, the coil constant was determined as 0.0678 T A-‘. Using this constant, the generated field of this Bi-2212/Ag magnet was measured as 2.61 T at Z, of the tapes. The relationship between the generated field by the double-stacked Bi-2212/Ag magnet and the applied current is still almost linear up to 400 A, which is larger than Z, for the tape. This suggests that most of the current smaller than
4.2K 0 Bias Field
(a)
s -C-2
J%Y
40 - (b)
N. Tomita
et al.
400 A flows in the superconductor. When the applied current exceeded 400 A, deviation from linearity was observed. The conductors of the magnet consist of Bi-2212 superconductors and Ag substrate tapes whose width is larger than that of the superconductor. Thus, the magnetic constant decreases when the applied current begins to flow in the wider Ag substrate. The I, of the Bi-2212ZAg tape was determined by some weak areas of the superconductor. Most of the local areas of the tape in the magnet are expected to show Z, larger than 385 A. Hence, a higher Z, for the coil is expected if homogeneity of the Bi-2212 is improved and weak areas are eliminated. The coil test with constant applied currents was also performed in liquid helium. With constant currents up to 322 A the voltage of the magnet did not change with time. With a constant current larger than 350 A, on the other hand, the voltage increased with time. Thus, long-time operation of this magnet is possible with the applied current smaller than 322 A. At a current of 322 A, the generated field of the magnet is measured to be 2.17 T. The voltage appearance with time for applied currents greater than 350 A is due to heat generation at the normal areas such as silver leads and solder in the magnet. Figure 5 shows the distribution of the generated field by the magnet along the centre axis of the magnet. Open circles and closed circles represent measured values and calculated values, respectively. Very good agreement is obtained between measured and calculated values. The Z, of the magnet determined with the criterion of lo-l3 Cl m increased gradually with increasing number of Z, measurement times. In other words, a training effect was observed as shown in Figure 6. This phenomenon was observed for every cooling and operation of the magnet. The number of excitation times sometimes exceeded 20 before a saturation of Z, value was obtained. There seem to be two origins for this phenomenon. One possible reason is the large difference in thermal expansion coefficient between the Ag substrate material and the oxide superconductor. Strain is generated in the magnet by this difference after every cooling, and electromagnetic force by self field gradually releases this strain. Another is related to pinning of the oxide superconductor. The temperature dependence of Z, of the coil was also measured up to 40 K under zero bias field in helium gas vaporized from liquid helium. The results are shown in FigB,,,,, : 2.6T (385A)
i f
‘p
I
0
100
I
I
I
I
200 300 Applied Current (A)
I
I
400
Figure4 /-V curve for Bi-2212/Ag tape of coil measured at 4.2 K with current sweep late of 300 A min-’ (a), I-\/curve for Bi2212/Ag coil including silver joints(b) and relationship between applied current and generated field of BL2212/Ag magnet detected by Hall probe set at the centre of coil (c)
488
Cryogenics
1996 Volume
36, Number
6
0 Bias Field :
Position (mm) Figure 5 Distribution of fields generated by Bi-2212/Ag magnet along centre axis of the magnet. Closed circles represent generated fields measured using Hall probe. Open symbols represent fields calculated from dimension of coils and critical current
Fabrication
3 -0
and properties
of superconducting
magnets:
N. Tomita et al.
. J 4.2K
300 L
0 Bias Field
:
1001
, I F 4.2K )// 6 5bl...,.i..,..,.i 10 15
1 O-lsQ*m
3
/ 20I .,., 25O 1
Oo ,..I
Magnetic Field (T) Figure6
Change in 1, by repeat of I, measurement
Figure 8 magnetic
we 7. I, decreased with increasing temperature. However, the decrease in Z, was not crucial up to about 20 K. The Z, and the generated field at 20 K of the coil are estimated to be 225 A and 1.53 T, respectively, under the zero bias field. This result indicates the possibility of generating several tesla at around 20 K by operating a Bi-2212/Ag magnet with a refrigerator. After testing under the zero bias field, this coil was tested using the hybrid magnet system in the High Magnetic Field Laboratory at Grenoble. This magnet system generates a total magnetic field of 20 T: 9 T from a superconducting magnet and 11 T from a resistive magnet, in a room-temperature bore of 130 mm+: After training several times at 0 T, tests on the Bi-2212/Ag magnet were performed in magnetic fields. The dependence of Z, for the magnet on a bias magnetic field at 4.2 K is shown in Figure 8. The ZV curve shows a small slope in the superconducting region and this slope increases with increasing bias magnetic field. Because of the steep slope of the Z-V curve under a high magnetic field, an I, above 8 T could not be determined with the criterion of lo-l3 0 m. With the criterion of 2 x lo-r3 1R m, the I, of the coil under a bias field of 20 T is 160 A, which corresponds to a field generation of 1.08 T. The steeper slope of the I-V curve in magnetic fields is probably due to damage of the oxide layer of the coil during the tests. The dependence of magnetoresistivity of silver on the magnetic field is shown Figure 9. The resistivity of pure silver substrate in 20 T at 4.2 K was about 5 x 10e9 fl m which was about 10 times larger than that at OT. Our Z, criteria, 10-t” R m and 2 x lo-r3 fl m are much smaller than the magnetoresistivity of silver substrate. Thus, below I
”
‘,
I”
”
I”
”
I ”
Dependence of I, of the BiL2212/Ag magnet field at 4.2 K
0.0
Figure 9 magnetic
0
’ ”
*
I ” j ,I”’
5
‘I”’
” c
10 15 20 Magnetic Field (T)
Magnetoresistivity field
on bias
*
25
of silver at 4.2 K as a function of
the Z, criteria, current passing through silver substrate is negligible. After tests at 20 T, the bias magnetic field was rapidly decreased, and the Z-V curve of the coil was no longer reversible, suggesting that the Bi-2212/Ag coil was damaged. After the trouble, degradation of Z, was observed. This coil was fixed with epoxy resins. Epoxy-fixed coils are durable against the electromagnetic stress of high static magnetic field, but not against the stronger electromagnetic stress induced by a rapidly changing magnetic field. Thus, for high-field applications it is necessary to reinforce coils, for example by alloying the Ag substrate.
”
400-
Conclusions
0 Bias Field IO-lsQ*m
300 2 _Q 200 lo&. 0
225A /’ 1.53T
,4;: ’
.;‘:0
-
,,,,: 10
20
30
40
50
Temperature (K) Figure 7 Temperature under zero bias field
dependence
of I, of Bi-2212/Ag
magnet
Pancake type coils with dimension of 20 mm4 x 94 mm+ were fabricated using Bi-2212ZAg tapes, which were prepared by the combination of a continuous dip-coating process and melt-solidification, and the coils were fabricated by the wind and react method and impregnated with epoxy resins. A magnet was fabricated by stacking two coils in series. The magnet was tested in liquid helium in bias fields up to 20 T and in helium gas at temperatures up to 40 K. This magnet generated a magnetic field of 2.6 T (Z, = 385 A ( lo-r3 fl m) under zero bias field, 1.08 T (Z, = 160 A (2 x lo-l3 fl m)) under a bias field of 20 T at 4.2 K. At about 20 K, the generated field of the magnet was 1.53 T
Cryogenics
1996 Volume
36, Number
6
489
Fabrication
and
properties
of superconducting
magnets:
A ( lo-l3 Sz m)). However, mechanical strength is not sufficient to sustain a strong electromagnetic force in high fields. Reinforcement of the coils is essential for highfield applications. (Z, = 225
Acknowledgement The authors are grateful to Dr J.C. Vallier of CNRS, Grenoble for providing them with the opportunity of using the high-field testing facilities.
References Tomita, N., Arai, M., Yanagisawa, E., Morimoto, T., Fujii, H., Kitaguchi, H., Kumakura, H., Inoue, K., Togano, K., Maeda, H. and Nomura, K. Appl Phys Lett ( 1994) 65 898 Tomita, N., Shimoyama, J., Kitaguchi, H., Kumakura, H., Togano, K., Maeda, H. and Nomura, K. Advances in Cryogenic Engineering, Plenum Press, New York, 40 (1994) 297 Shimoyama, J., Morimoto, T., Kitaguchi, H., Kumakura, H.,
490
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1996 Volume 36, Number 6
N.
Tomita
et al.
Togano, K., Maeda, H. and Nomura, K. Jpn .I Appl Phys ( 1992) 31 L163 Balachandran, U., Iyer, A.N., Halder, P., Hoehn, J.G., Jr., Motowidlo, L.R. and Galinski, G. Appl Supercond (1994) 2 25 1 Shihutani, K., Hase, T., Egi, T., Hayashi, S., Ogawa, R. and Kawata, Y. Appl Supercond (1994) 2 237 Sato, K. Physics World (1992) 7 37 Yoshida, M., Endo, A. and Hara, N. Jpn J Appl Phys (1994) 33 L42l Tenbrink, J. and Krauth, H. Advances in Cryogenic Engineering, Plenum Press, New York, 40 (1994) 305 Lue, J.W., Schwenterly, S.W., Lubell, M.S., Luton, J.N., Manlief, M.D., Joshi, C.H., Podtburg, E.R. and Masur, L.J. Advances in Cryogenic Engineering, Plenum Press, New York, 40 ( 1994) 327 Kase, J., Morimoto, T., Togano, K., Kumakura, H., Dietderich, D.R., and Maeda, H. IEEE Trans Maan (1991) 27 1254 Togano, K., Kumakura, H., Kadowal&K., Kitaguchi, H., Maeda, H., Kase, J., Shimoyama, J. and Nomura, K. Advances in Cryogenic Engineering, Plenum Press, New York, 38 (1992) 1081 Tomita, N., Shimoyama, J., Arai, M., Matubara, T., Morimoto, T., Kitaguchi, H., Kumakura, H., Togano, K. and Maeda, H. (unpublished) Shimoyama, J., Tomita, N., Morimoto, T., Kitaguchi, H., Kumakura, H., Togano, K., Maeda, H. and Nomura, K. Jpn J Appl Phys (1992) 31 L1999
ELSEVIER
Fabrication and properties of superconducting magnets using Bi,Sr2CaCu20JAg tapes Naruaki Tomita, Mikako Arai, Eiji Yanagisawa, Takeshi Morimoto, Hitoshi Kitaguchi”, Hiroaki Kumakura*, Kazumasa Togano* and Katsumi Nomura* Research Center, Asahi Glass Co. Ltd, 1150 Hazawa-cho Kanagawa-ku, Yokohama 221, Japan *National Research Institute for Metals, 1-2-1 Sengen, Tsukuba, lbaraki 305, Japan *Advanced Research Center, Hitachi Cable Ltd, 3350 Kidamari-cho, Tsuchiura, lbaraki 300, Japan Received
19 October
7995; revised
16 November
1995
We fabricated pancake type coils with dimension of 20 mm4 (bore) x 94 mm+ (outer diameter) using Bi-2212/Ag tapes. Bi-2212/Ag tapes were prepared by the combination of a continuous dip-coating process and melt-solidification, and the coils were fabricated by the wind and react method and fixed with epoxy resins. Two coils were stacked and connected in series to construct a magnet. The magnet was tested in bias fields up to 20 T and at temperatures up to 40 K. This magnet generated a magnetic field of 2.6 T (I,, = 385 A ( lo-l3 R m)) under zero bias field, 1.08 T !I,, = 160 A (2 x IO-l3 1Rm)) under a bias field of 20 T in liquid helium. At about 20 K, the generated field of the magnet was 1.53 T (I,, = 225 A (IO-l3 fl m)). The results suggest that a Bi2212 coil is promising as an insert magnet of a high-field superconducting magnet system. The Bi-2212 coil is also expected to operate as a cryogen-free magnet which can be operated at -20 K with cryocooler. Keywords:
superconducting
magnet;
Bi-2212/Ag;
Many studies on the fabrication of superconducting magnets using high T, oxide superconducting tapes and wires are in progress’-9. These superconducting magnets are expected to be used as two types of magnet. One is the inner magnets of a superconducting magnet system which generates higher than 21 T where application of conventional superconducting materials are difficult because of their low upper critical fields. The other is helium-free superconducting magnets which generate higher than 2 T, where the use of an iron core is difficult. These magnets will be operated at around 20 K using refrigerator cooling. Bi,Sr,CaCu,O,( Bi-2212)/Ag tapes prepared by melt-solidification have excellent microstructure of highly oriented and densely stacked Bi-2212 crystals with their &plane parallel to the silver tape surface. They show high J, values, higher than lo5 A cm-*, even in magnetic fields above 20 T at 4.2 K’O. For the formation of Bi-2212 layers on both sides of a long Ag substrate, the dip-coating method was found to be suitable”. In the dip-coated Bi-2212/Ag long tape, the oxide layer stuck firmly to the Ag tape after meltsolidification and showed grain-aligned microstructure, as the case of a short sample. Hence we are developing superconducting magnets using the Bi-2212/Ag composite tapes
melt-solidification
prepared by the combination of a continuous dip-coating process and melt-solidification. Recently, we fabricated several small pancake type coils using Bi-2212/Ag tapes and tested the coils at various temperatures and magnetic fields. A small Bi-2212/Ag pancake coil, used as an insert magnet of the conventional superconducting magnet system, generated 0.7 T under a bias field of 20.8 T, and a total field of 21.5 T was attained in the full superconducting state’. However, the fields generated by the Bi-2212/Ag coil were not high enough because the bore of the superconducting magnet system was limited. In this paper, we report the fabrication and properties of a larger Bi-2212/Ag pancake coil for higher field generation.
Fabrication
of coils
Bi-2212 powder was made by calcination of Bi203, SrCO,, CaCO, and CuO. The nominal composition of the starting powder was fixed to be Ca-poor, [Bi] : [Sr] : [Cal : [Cu] = 2 : 2 : 0.96 : 2, because Bi-2212 with a slightly Capoor composition showed little impurity phases in their microstructure after melt-solidification’*. The slurry for
Cryogenics
1996 Volume
36, Number
6
485
Fabrication
and properties
of superconducting
magnets:
dip-coating was prepared by mixing fine Bi-2212 powder, appropriate amounts of organic solvent, binder and dispersant. This mixture was milled for 2 days. A schematic for the fabrication procedure of the pancake coils is shown in Figure 1. Silver tapes with dimensions of 80 pm x 18 mm x 10.5 m were used in this experiment. The edge of the tape, approximately 5 mm wide, was masked with plastic sticky tape before the dip-coating. The partially masked tapes were dipped continuously into the slurry, pulled upward at a speed of about 70 cm min-’ and dried by a pipe heater using the continuous dipping apparatus. After the dip-coating process, the plastic tapes were removed. The thickness of the as-dip-coated films was approximately 50 pm on each side of the silver tape. Each dip-coated tape was wound into a pancake shape together with polyethylene sheets of 0.8 mm thickness and annealed at 110°C for 1 day. Then, the polyethylene sheets were removed, leaving gaps (0.8 1.O mm) between the turns of the tape. The coiled tapes were put on the powder mixture of Bi,Al,O,, and A1203 with their exposed silver sides in powder and were heated up to 810°C in air to remove organic materials and to decompose some of the carbonates. After cooling to room temperature, the coiled tapes were covered with an alumina crucible to suppress the vaporization of bismuth from Bi-2212 phase during high-temperature heat treatment13. The coiled tapes were melt-solidified with the following heat treatment schedule. The tape was heated up to 891°C kept at 89 1°C for 5 min, slowly cooled down to 840°C at a rate of 5°C h-’ and kept at 840°C for 1 h. Finally, the tape was furnace-cooled down to room temperature. The thickness of the B-2212 layer decreased to about one-fourth of its initial thickness due to meltsolidification.
N. Tomita
et al.
Microstructure and thickness at several points of the Bi2212/Ag tapes were observed by SEM. The fractured cross section of Bi-2212lAg tape prepared by the combination of a continuous dip-coating process and melt-solidification is shown in Figure 2. Highly oriented Bi-2212 layer crystals with their c-axis perpendicular to the tape surface were formed. This microstructure was the same as that of the Bi2212lAg short tapes prepared by doctor-blade casting and melt-solidificationlO. The composition analysis of Bi2212lAg tapes was performed using Inductive Coupled Plasma (ICP) analysis. The ratio of [Bi] : [Sr] : [Cal : [Cu] in the oxide layer was detected as 1.96 : 2.09 : 0.96 : 2. This ratio is almost equal to that of the starting materials, indicating that vaporization of elements during the melt-solidification process is negligibly small. Silver tapes of thickness 300 pm were used for current leads and voltage taps. Current leads were soldered to both ends of the tape at the exposed silver parts. Voltage taps were attached at 50- 100 cm inner positions from both ends of the tape. Three coils were then arranged together by inserting one coil into the other coils to obtain a single pancake coil. Organic films (Mylar tapes) of thickness 50 pm were also inserted into the gaps for insulation. The arranged coil was tightened around the stainless frame of 20 mm+ inner diameter and encased into stainless pipe of 93 mm+ outer diameter. Three tapes were electrically connected in series with silver tapes. The coil was fixed by impregnation using bisphenol A type epoxy resin cured with acid anhydride’. Finally, two pancake coils were stacked and electrically connected in series by silver tapes. Figure 3 shows the fabricated magnet.
Removing Organic Materials t
Melt-Solidification Dip-Coating
ti
pe Figure 1
486
Schematic
Cryogenics
diagram
for fabrication
1996 Volume
procedure
36, Number
Arranging
Fixing
for pancake coil
6
Fabrication
Figure 2 Photograph and melt-solidification
Figure 3
Photograph
and propertiesof superconducting
of fractured cross section of Bi-2212/Ag
of Bi-2212/Ag
tape prepared by the combination
magnets:
N.
fomitaet
of a continuous dip-coating
al.
process
magnet
Cryogenics
1996 Volume
36, Number
6
487
Fabrication
and properties
of superconducting
magnets:
Coil tests and discussion At first the Bi-2212/Ag magnet was tested in liquid helium under zero bias field. The Z-V characteristic of the Bi2212/Ag tape of the coil measured at 4.2 K with a current sweep rate of 300 A min’ is shown in Figure 4a. The ZV curve bends at around 230 A in the superconducting state and shows a clear transition to the normal state around 400 A. The critical current, Z,, and the critical current density, J,, for the oxide layers of the coil depend on a criterion. Z, and J, determined with the criterion of lo-l3 R m are 385 A and 115 000 A cm-‘, respectively. Figure 4b shows the Z-V curve at 4.2 K for a Bi-2212/Ag coil including silver joints. The measured slope is higher than lo-” fl m because of the resistance of silver and solder. If we apply the criterion of 5 x lo-l3 fi m, which is still much lower than the resistivity of silver substrate, the Z, of the Bi-2212/Ag coil is 373 A. The generated magnetic fields of the Bi-2212/Ag coil were detected with the Hall probe set at the centre of the coil. The relationship between the generated field by the double stacked Bi-2212/Ag magnet and the applied current is shown in Figure 4c. From this Hall voltage versus current curve, the coil constant was determined as 0.0678 T A-‘. Using this constant, the generated field of this Bi-2212/Ag magnet was measured as 2.61 T at Z, of the tapes. The relationship between the generated field by the double-stacked Bi-2212/Ag magnet and the applied current is still almost linear up to 400 A, which is larger than Z, for the tape. This suggests that most of the current smaller than
4.2K 0 Bias Field
(a)
s -C-2
J%Y
40 - (b)
N. Tomita
et al.
400 A flows in the superconductor. When the applied current exceeded 400 A, deviation from linearity was observed. The conductors of the magnet consist of Bi-2212 superconductors and Ag substrate tapes whose width is larger than that of the superconductor. Thus, the magnetic constant decreases when the applied current begins to flow in the wider Ag substrate. The I, of the Bi-2212ZAg tape was determined by some weak areas of the superconductor. Most of the local areas of the tape in the magnet are expected to show Z, larger than 385 A. Hence, a higher Z, for the coil is expected if homogeneity of the Bi-2212 is improved and weak areas are eliminated. The coil test with constant applied currents was also performed in liquid helium. With constant currents up to 322 A the voltage of the magnet did not change with time. With a constant current larger than 350 A, on the other hand, the voltage increased with time. Thus, long-time operation of this magnet is possible with the applied current smaller than 322 A. At a current of 322 A, the generated field of the magnet is measured to be 2.17 T. The voltage appearance with time for applied currents greater than 350 A is due to heat generation at the normal areas such as silver leads and solder in the magnet. Figure 5 shows the distribution of the generated field by the magnet along the centre axis of the magnet. Open circles and closed circles represent measured values and calculated values, respectively. Very good agreement is obtained between measured and calculated values. The Z, of the magnet determined with the criterion of lo-l3 Cl m increased gradually with increasing number of Z, measurement times. In other words, a training effect was observed as shown in Figure 6. This phenomenon was observed for every cooling and operation of the magnet. The number of excitation times sometimes exceeded 20 before a saturation of Z, value was obtained. There seem to be two origins for this phenomenon. One possible reason is the large difference in thermal expansion coefficient between the Ag substrate material and the oxide superconductor. Strain is generated in the magnet by this difference after every cooling, and electromagnetic force by self field gradually releases this strain. Another is related to pinning of the oxide superconductor. The temperature dependence of Z, of the coil was also measured up to 40 K under zero bias field in helium gas vaporized from liquid helium. The results are shown in FigB,,,,, : 2.6T (385A)
i f
‘p
I
0
100
I
I
I
I
200 300 Applied Current (A)
I
I
400
Figure4 /-V curve for Bi-2212/Ag tape of coil measured at 4.2 K with current sweep late of 300 A min-’ (a), I-\/curve for Bi2212/Ag coil including silver joints(b) and relationship between applied current and generated field of BL2212/Ag magnet detected by Hall probe set at the centre of coil (c)
488
Cryogenics
1996 Volume
36, Number
6
0 Bias Field :
Position (mm) Figure 5 Distribution of fields generated by Bi-2212/Ag magnet along centre axis of the magnet. Closed circles represent generated fields measured using Hall probe. Open symbols represent fields calculated from dimension of coils and critical current
Fabrication
3 -0
and properties
of superconducting
magnets:
N. Tomita et al.
. J 4.2K
300 L
0 Bias Field
:
1001
, I F 4.2K )// 6 5bl...,.i..,..,.i 10 15
1 O-lsQ*m
3
/ 20I .,., 25O 1
Oo ,..I
Magnetic Field (T) Figure6
Change in 1, by repeat of I, measurement
Figure 8 magnetic
we 7. I, decreased with increasing temperature. However, the decrease in Z, was not crucial up to about 20 K. The Z, and the generated field at 20 K of the coil are estimated to be 225 A and 1.53 T, respectively, under the zero bias field. This result indicates the possibility of generating several tesla at around 20 K by operating a Bi-2212/Ag magnet with a refrigerator. After testing under the zero bias field, this coil was tested using the hybrid magnet system in the High Magnetic Field Laboratory at Grenoble. This magnet system generates a total magnetic field of 20 T: 9 T from a superconducting magnet and 11 T from a resistive magnet, in a room-temperature bore of 130 mm+: After training several times at 0 T, tests on the Bi-2212/Ag magnet were performed in magnetic fields. The dependence of Z, for the magnet on a bias magnetic field at 4.2 K is shown in Figure 8. The ZV curve shows a small slope in the superconducting region and this slope increases with increasing bias magnetic field. Because of the steep slope of the Z-V curve under a high magnetic field, an I, above 8 T could not be determined with the criterion of lo-l3 0 m. With the criterion of 2 x lo-r3 1R m, the I, of the coil under a bias field of 20 T is 160 A, which corresponds to a field generation of 1.08 T. The steeper slope of the I-V curve in magnetic fields is probably due to damage of the oxide layer of the coil during the tests. The dependence of magnetoresistivity of silver on the magnetic field is shown Figure 9. The resistivity of pure silver substrate in 20 T at 4.2 K was about 5 x 10e9 fl m which was about 10 times larger than that at OT. Our Z, criteria, 10-t” R m and 2 x lo-r3 fl m are much smaller than the magnetoresistivity of silver substrate. Thus, below I
”
‘,
I”
”
I”
”
I ”
Dependence of I, of the BiL2212/Ag magnet field at 4.2 K
0.0
Figure 9 magnetic
0
’ ”
*
I ” j ,I”’
5
‘I”’
” c
10 15 20 Magnetic Field (T)
Magnetoresistivity field
on bias
*
25
of silver at 4.2 K as a function of
the Z, criteria, current passing through silver substrate is negligible. After tests at 20 T, the bias magnetic field was rapidly decreased, and the Z-V curve of the coil was no longer reversible, suggesting that the Bi-2212/Ag coil was damaged. After the trouble, degradation of Z, was observed. This coil was fixed with epoxy resins. Epoxy-fixed coils are durable against the electromagnetic stress of high static magnetic field, but not against the stronger electromagnetic stress induced by a rapidly changing magnetic field. Thus, for high-field applications it is necessary to reinforce coils, for example by alloying the Ag substrate.
”
400-
Conclusions
0 Bias Field IO-lsQ*m
300 2 _Q 200 lo&. 0
225A /’ 1.53T
,4;: ’
.;‘:0
-
,,,,: 10
20
30
40
50
Temperature (K) Figure 7 Temperature under zero bias field
dependence
of I, of Bi-2212/Ag
magnet
Pancake type coils with dimension of 20 mm4 x 94 mm+ were fabricated using Bi-2212ZAg tapes, which were prepared by the combination of a continuous dip-coating process and melt-solidification, and the coils were fabricated by the wind and react method and impregnated with epoxy resins. A magnet was fabricated by stacking two coils in series. The magnet was tested in liquid helium in bias fields up to 20 T and in helium gas at temperatures up to 40 K. This magnet generated a magnetic field of 2.6 T (Z, = 385 A ( lo-r3 fl m) under zero bias field, 1.08 T (Z, = 160 A (2 x lo-l3 fl m)) under a bias field of 20 T at 4.2 K. At about 20 K, the generated field of the magnet was 1.53 T
Cryogenics
1996 Volume
36, Number
6
489
Fabrication
and
properties
of superconducting
magnets:
A ( lo-l3 Sz m)). However, mechanical strength is not sufficient to sustain a strong electromagnetic force in high fields. Reinforcement of the coils is essential for highfield applications. (Z, = 225
Acknowledgement The authors are grateful to Dr J.C. Vallier of CNRS, Grenoble for providing them with the opportunity of using the high-field testing facilities.
References Tomita, N., Arai, M., Yanagisawa, E., Morimoto, T., Fujii, H., Kitaguchi, H., Kumakura, H., Inoue, K., Togano, K., Maeda, H. and Nomura, K. Appl Phys Lett ( 1994) 65 898 Tomita, N., Shimoyama, J., Kitaguchi, H., Kumakura, H., Togano, K., Maeda, H. and Nomura, K. Advances in Cryogenic Engineering, Plenum Press, New York, 40 (1994) 297 Shimoyama, J., Morimoto, T., Kitaguchi, H., Kumakura, H.,
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Cryogenics
1996 Volume 36, Number 6
N.
Tomita
et al.
Togano, K., Maeda, H. and Nomura, K. Jpn .I Appl Phys ( 1992) 31 L163 Balachandran, U., Iyer, A.N., Halder, P., Hoehn, J.G., Jr., Motowidlo, L.R. and Galinski, G. Appl Supercond (1994) 2 25 1 Shihutani, K., Hase, T., Egi, T., Hayashi, S., Ogawa, R. and Kawata, Y. Appl Supercond (1994) 2 237 Sato, K. Physics World (1992) 7 37 Yoshida, M., Endo, A. and Hara, N. Jpn J Appl Phys (1994) 33 L42l Tenbrink, J. and Krauth, H. Advances in Cryogenic Engineering, Plenum Press, New York, 40 (1994) 305 Lue, J.W., Schwenterly, S.W., Lubell, M.S., Luton, J.N., Manlief, M.D., Joshi, C.H., Podtburg, E.R. and Masur, L.J. Advances in Cryogenic Engineering, Plenum Press, New York, 40 ( 1994) 327 Kase, J., Morimoto, T., Togano, K., Kumakura, H., Dietderich, D.R., and Maeda, H. IEEE Trans Maan (1991) 27 1254 Togano, K., Kumakura, H., Kadowal&K., Kitaguchi, H., Maeda, H., Kase, J., Shimoyama, J. and Nomura, K. Advances in Cryogenic Engineering, Plenum Press, New York, 38 (1992) 1081 Tomita, N., Shimoyama, J., Arai, M., Matubara, T., Morimoto, T., Kitaguchi, H., Kumakura, H., Togano, K. and Maeda, H. (unpublished) Shimoyama, J., Tomita, N., Morimoto, T., Kitaguchi, H., Kumakura, H., Togano, K., Maeda, H. and Nomura, K. Jpn J Appl Phys (1992) 31 L1999