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A study on the microstructures and magnetic properties of the multi-seeded melt growth processed YBCO superconductors
Physica C 372–376 (2002) 1159–1162 www.elsevier.com/locate/physc
A study on the microstructures and magnetic properties of the multi-seeded melt growth processed YBCO superconductors Hojin Kim a
a,b,*
, Chanjoong Kim a, Gyewon Hong a, Jinho Joo
b
Functional Materials Laboratory, Korea Atomic Energy Research Institute, P.O. Box 105, Taejon, Yusong 305-600, South Korea b School of Advanced Materials Engineering, Sungkyunkwan University, Suown, Kyounggi-Do 440-756, South Korea
Abstract We studied the microstructures and magnetic properties of the multi-seeded melt growth processed YBCO superconductors. The use of the multi-seeded melt growth process can reduce the processing time remarkably compared with the single-seeded melt growth process, but the levitation forces and the trapped magnetic field value decreased as the number of seeds increased due to the formation of weakly linked grain boundaries at the grain junctions. The degradation of the trapped magnetic field was attributed to the presence of the residual liquid at the grain boundary. To minimize the degradation of the magnetic properties, various processing parameters such as seed distance and grain orientation were tested. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Multi-seeded melt growth process; Seed distance; Grain junction
1. Introduction The seeded melt growth (SMG) process is known to be the most effective way to fabricate a large area single grain YBCO bulk superconductors, so it can be used as the bearing parts of the flywheel energy storage system and the levitation transportation. However, the disadvantage of the seeded melt growth process is the long
*
Corresponding author. Tel.: +82-31-290-7358; fax: +82-31290-7371. E-mail address: [email protected] (H. Kim).
fabrication time that is attributed to the slow growth rate of 123 grain from the Y211/liquid mixture. Recently, various techniques such as grain boundary joining using filler materials [1], diffusion bonding with pressure [2], and multi-seeding [3] have been attempted to fabricate materials with larger grain size as well as to reduce proceeding time. Among them, the multi-seeded melt growth (MSMG) process was regarded as the most effective method that can decrease the processing time. In this technique, YBCO compact containing several seeds on their top of the surface were annealed using melt processing method. This process, however, results in the grain junctions that bring
0921-4534/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 2 ) 0 0 8 8 0 - 8
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H. Kim et al. / Physica C 372–376 (2002) 1159–1162
harmful effects on the magnetic properties. Therefore, it is necessary to find an optimum process parameter to increase the connectivity between grains. In this study, we studied the variable processing parameters such as seed distance and grain orientation. The effects of parameters on the magnetic properties are reported and the method to minimize the degradation of the magnetic properties is suggested.
2. Experimental procedure The composition of the YBCO bulk compacts was Y1:8 Ba2:4 Cu3:4 Ox (Y1.8) with 1 wt.% CeO2 addition. The 40 g of Y1:8 powder compacts was pressed in a steel mould into a 30 30 25 mm3 size and compacted further by cold isostatic press method. The surfaces of the YBCO compacts were coated with Yb2 O3 powder to suppress the possible formation of undesirable surface nucleation. The Sm1:8 Ba2:4 Cu3:4 Ox (Sm1.8) single crystal seeds were placed on the top of the Y1.8 compacts, and the Y1.8 compacts were placed on the MgO single crystal substrates positioned at the center of the electronic box furnace and meltprocessed following the heating cycles of Fig. 1. The melt-processed samples were heat-treated in flowing oxygen at 500 °C for 50 h for oxygen embedding.
Fig. 2. Area fraction of Y123 grains on the top of the surface as the function of the number of grains.
3. Results and discussion 3.1. The effect of the number of seeds To examine the effect of the number of seeds on processing time, the Y1.8 compacts containing from one to six seeds were prepared with holding at 995 °C for 20 h. As can be seen in Fig. 2, the area fraction of Y123 grains at top surface linearly increases as the number of grain increases. It is only 20% for the sample prepared using one seed, while it is almost 100% for the samples prepared using five or six seeds. However, the levitation force and the trapped magnetic field decreased with the number of grain boundaries. The grain boundary has a lot of non-superconducting phase of CuO and BaCuO2 , formed by the separation of the peritectic melt (Ba3 Cu5 O8 ) into the eutectic melt during cooling. The presence of the non-superconducting phase at the grain boundary is considered as the main cause for decreasing the magnetic properties. 3.2. The effect of the distance (d) between seeds
Fig. 1. Heating cycles of MSMG process.
The melt is often entrapped in the gap spacing between two Y123 grains. As reported by Lo and Cardwell [4], Y123 grains make the faceted surfaces prior to the growth from the seed and then grow in a rectangular shape. When the two Y123 grains grow further, it makes a thin melt channel between two growing Y123 grains. The melt
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Fig. 4(a)–(c) shows the microstructure around the grain boundary. At the sample with d ¼ 0, the grain boundary has no second phase, whereas, at other samples with d > 0, the residual phases, such as CuO, were observed at the grain boundary. 3.3. The effect of the seed arrangement
Fig. 3. The effect of the seed distance on the trapped magnetic field profiles crossing the grain boundary.
channel tends to be well formed when the distance (d) between two seeds is large [5]. To confirm it, the seed distance (d) varied between 0 and 20 mm and the melt growth heat treatment followed. The trapped magnetic field profiles crossing the grain boundary are shown in Fig. 3. The sample with d ¼ 0 appears to have a good connectivity at the grain boundary. Moreover, other samples with d > 0 show the poor grain boundary connectivity as the seed distance increases. In addition, the levitation force has the maximum value, 39 N, at d ¼ 0 and the minimum value, 29 N, at d ¼ 20 mm, confirming the quality of grain boundaries is closely related with magnetic properties.
Fig. 5 shows photos of the top of the YBCO sample surface with various grain junctions by the seed arrangement. Among them, (1 1 0)/(1 1 0) grain junction appeared to have the good connectivity at the trapped magnetic field and had the maximum levitation force value, 52 N. Fig. 6 shows the microstructure of the top of the YBCO sample surface with (1 1 0)/(1 1 0) and (1 0 0)/(1 0 0) grain junction. In (1 1 0)/(1 1 0) grain junction, no residual phases were present, whereas they were appeared at (1 0 0)/(1 0 0) grain junction. The possibility of the existence of residual phases is considered to depend on the wetting angle of the melt at the growth front of Y123 grains. The wetting angle is a function of the interfacial energy
Fig. 5. Photos of the top of the surface with various grain junctions.
Fig. 4. Microstructure around the grain boundary; (a) d ¼ 0, (b) d ¼ 5 mm, and (c) d ¼ 10 mm.
Fig. 6. Microstructure of top surface with (a) (1 1 0)/(1 1 0) and (b) (1 0 0)/(1 0 0) grain junction.
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of a solid surface. When the wetting angle (/) is 0°, a melt completely wets a solid interface. When / is between 0° and 180°, the melt wets the solid interface discretely. When / is 180°, no melt wets at the interface and the grain boundary is clean.
by the various processing parameters can be minimized the degradation of the magnetic properties of multi-seeded specimen.
References 4. Conclusions The various processing parameters of the MSMG method were studied. It was confirmed that multi-seeding remarkably reduced the processing time needed for the growth of Y123 grains, but magnetic properties were degraded at the grain boundaries. The presence of the non-superconducting phase at the grain boundary is considered as the main cause for the decrease of magnetic properties. The control of the melt wetting angle
[1] K. Kimura, K. Miyamoto, M. Hashimoto, in: Advances in Superconductivity VII, Processing of ISS’94, 1994, p. 13. [2] Ph. Vanderbemden, A.D. Bradley, R.A. Doyle, W. Ro, D.M. Astill, D.A. Cardwell, A.M. Campell, Physica C 302 (1998) 257. [3] Y.A. Jee, C.-J. Kim, T.-H. Sung, G.-W. Hong, Supercond. Sci. Technol. 13 (2000) 195. [4] W. Lo, D.A. Cardwell, in: D. Dew-Hughes (Ed.), Applied Superconductivity, Inst. Phys. Conf. Ser. 148, Institute of Physics Publishing, Bristol, 1995, p. 139. [5] C.J. Kim, H.-J. Kim, J.-H. Joo, G.-W. Hong, S.-C. Han, Y.-H. Han, T.-H. Sung, S.-J. Kim, Physica C 336 (2000) 233.
A study on the microstructures and magnetic properties of the multi-seeded melt growth processed YBCO superconductors Hojin Kim a
a,b,*
, Chanjoong Kim a, Gyewon Hong a, Jinho Joo
b
Functional Materials Laboratory, Korea Atomic Energy Research Institute, P.O. Box 105, Taejon, Yusong 305-600, South Korea b School of Advanced Materials Engineering, Sungkyunkwan University, Suown, Kyounggi-Do 440-756, South Korea
Abstract We studied the microstructures and magnetic properties of the multi-seeded melt growth processed YBCO superconductors. The use of the multi-seeded melt growth process can reduce the processing time remarkably compared with the single-seeded melt growth process, but the levitation forces and the trapped magnetic field value decreased as the number of seeds increased due to the formation of weakly linked grain boundaries at the grain junctions. The degradation of the trapped magnetic field was attributed to the presence of the residual liquid at the grain boundary. To minimize the degradation of the magnetic properties, various processing parameters such as seed distance and grain orientation were tested. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Multi-seeded melt growth process; Seed distance; Grain junction
1. Introduction The seeded melt growth (SMG) process is known to be the most effective way to fabricate a large area single grain YBCO bulk superconductors, so it can be used as the bearing parts of the flywheel energy storage system and the levitation transportation. However, the disadvantage of the seeded melt growth process is the long
*
Corresponding author. Tel.: +82-31-290-7358; fax: +82-31290-7371. E-mail address: [email protected] (H. Kim).
fabrication time that is attributed to the slow growth rate of 123 grain from the Y211/liquid mixture. Recently, various techniques such as grain boundary joining using filler materials [1], diffusion bonding with pressure [2], and multi-seeding [3] have been attempted to fabricate materials with larger grain size as well as to reduce proceeding time. Among them, the multi-seeded melt growth (MSMG) process was regarded as the most effective method that can decrease the processing time. In this technique, YBCO compact containing several seeds on their top of the surface were annealed using melt processing method. This process, however, results in the grain junctions that bring
0921-4534/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 2 ) 0 0 8 8 0 - 8
1160
H. Kim et al. / Physica C 372–376 (2002) 1159–1162
harmful effects on the magnetic properties. Therefore, it is necessary to find an optimum process parameter to increase the connectivity between grains. In this study, we studied the variable processing parameters such as seed distance and grain orientation. The effects of parameters on the magnetic properties are reported and the method to minimize the degradation of the magnetic properties is suggested.
2. Experimental procedure The composition of the YBCO bulk compacts was Y1:8 Ba2:4 Cu3:4 Ox (Y1.8) with 1 wt.% CeO2 addition. The 40 g of Y1:8 powder compacts was pressed in a steel mould into a 30 30 25 mm3 size and compacted further by cold isostatic press method. The surfaces of the YBCO compacts were coated with Yb2 O3 powder to suppress the possible formation of undesirable surface nucleation. The Sm1:8 Ba2:4 Cu3:4 Ox (Sm1.8) single crystal seeds were placed on the top of the Y1.8 compacts, and the Y1.8 compacts were placed on the MgO single crystal substrates positioned at the center of the electronic box furnace and meltprocessed following the heating cycles of Fig. 1. The melt-processed samples were heat-treated in flowing oxygen at 500 °C for 50 h for oxygen embedding.
Fig. 2. Area fraction of Y123 grains on the top of the surface as the function of the number of grains.
3. Results and discussion 3.1. The effect of the number of seeds To examine the effect of the number of seeds on processing time, the Y1.8 compacts containing from one to six seeds were prepared with holding at 995 °C for 20 h. As can be seen in Fig. 2, the area fraction of Y123 grains at top surface linearly increases as the number of grain increases. It is only 20% for the sample prepared using one seed, while it is almost 100% for the samples prepared using five or six seeds. However, the levitation force and the trapped magnetic field decreased with the number of grain boundaries. The grain boundary has a lot of non-superconducting phase of CuO and BaCuO2 , formed by the separation of the peritectic melt (Ba3 Cu5 O8 ) into the eutectic melt during cooling. The presence of the non-superconducting phase at the grain boundary is considered as the main cause for decreasing the magnetic properties. 3.2. The effect of the distance (d) between seeds
Fig. 1. Heating cycles of MSMG process.
The melt is often entrapped in the gap spacing between two Y123 grains. As reported by Lo and Cardwell [4], Y123 grains make the faceted surfaces prior to the growth from the seed and then grow in a rectangular shape. When the two Y123 grains grow further, it makes a thin melt channel between two growing Y123 grains. The melt
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Fig. 4(a)–(c) shows the microstructure around the grain boundary. At the sample with d ¼ 0, the grain boundary has no second phase, whereas, at other samples with d > 0, the residual phases, such as CuO, were observed at the grain boundary. 3.3. The effect of the seed arrangement
Fig. 3. The effect of the seed distance on the trapped magnetic field profiles crossing the grain boundary.
channel tends to be well formed when the distance (d) between two seeds is large [5]. To confirm it, the seed distance (d) varied between 0 and 20 mm and the melt growth heat treatment followed. The trapped magnetic field profiles crossing the grain boundary are shown in Fig. 3. The sample with d ¼ 0 appears to have a good connectivity at the grain boundary. Moreover, other samples with d > 0 show the poor grain boundary connectivity as the seed distance increases. In addition, the levitation force has the maximum value, 39 N, at d ¼ 0 and the minimum value, 29 N, at d ¼ 20 mm, confirming the quality of grain boundaries is closely related with magnetic properties.
Fig. 5 shows photos of the top of the YBCO sample surface with various grain junctions by the seed arrangement. Among them, (1 1 0)/(1 1 0) grain junction appeared to have the good connectivity at the trapped magnetic field and had the maximum levitation force value, 52 N. Fig. 6 shows the microstructure of the top of the YBCO sample surface with (1 1 0)/(1 1 0) and (1 0 0)/(1 0 0) grain junction. In (1 1 0)/(1 1 0) grain junction, no residual phases were present, whereas they were appeared at (1 0 0)/(1 0 0) grain junction. The possibility of the existence of residual phases is considered to depend on the wetting angle of the melt at the growth front of Y123 grains. The wetting angle is a function of the interfacial energy
Fig. 5. Photos of the top of the surface with various grain junctions.
Fig. 4. Microstructure around the grain boundary; (a) d ¼ 0, (b) d ¼ 5 mm, and (c) d ¼ 10 mm.
Fig. 6. Microstructure of top surface with (a) (1 1 0)/(1 1 0) and (b) (1 0 0)/(1 0 0) grain junction.
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of a solid surface. When the wetting angle (/) is 0°, a melt completely wets a solid interface. When / is between 0° and 180°, the melt wets the solid interface discretely. When / is 180°, no melt wets at the interface and the grain boundary is clean.
by the various processing parameters can be minimized the degradation of the magnetic properties of multi-seeded specimen.
References 4. Conclusions The various processing parameters of the MSMG method were studied. It was confirmed that multi-seeding remarkably reduced the processing time needed for the growth of Y123 grains, but magnetic properties were degraded at the grain boundaries. The presence of the non-superconducting phase at the grain boundary is considered as the main cause for the decrease of magnetic properties. The control of the melt wetting angle
[1] K. Kimura, K. Miyamoto, M. Hashimoto, in: Advances in Superconductivity VII, Processing of ISS’94, 1994, p. 13. [2] Ph. Vanderbemden, A.D. Bradley, R.A. Doyle, W. Ro, D.M. Astill, D.A. Cardwell, A.M. Campell, Physica C 302 (1998) 257. [3] Y.A. Jee, C.-J. Kim, T.-H. Sung, G.-W. Hong, Supercond. Sci. Technol. 13 (2000) 195. [4] W. Lo, D.A. Cardwell, in: D. Dew-Hughes (Ed.), Applied Superconductivity, Inst. Phys. Conf. Ser. 148, Institute of Physics Publishing, Bristol, 1995, p. 139. [5] C.J. Kim, H.-J. Kim, J.-H. Joo, G.-W. Hong, S.-C. Han, Y.-H. Han, T.-H. Sung, S.-J. Kim, Physica C 336 (2000) 233.