- Email: [email protected]
Fabrication and study of thin Bi-2223 round wires
Physica C 463–465 (2007) 837–840 www.elsevier.com/locate/physc
Fabrication and study of thin Bi-2223 round wires P. Li *, L. Zhao, T.-M. Qu, X.-C. Wang, Z. Han Applied Superconductivity Research Center, Department of Physics, Tsinghua University, Beijing 100084, China Accepted 27 February 2007 Available online 25 May 2007
Abstract In this paper, experimental works in fabrication of Bi-2223 round wires are presented. Seven filament Bi-2223 round wires ranging from 1.0 mm to 0.3 mm in diameter were fabricated and studied. The critical current density Jc (77 K, self field) of round wires was of 103A/cm2 order. A correlation between Jc and wire diameter was observed. For round Bi-2223 wires, there is an optimal diameter at which Jc reaches peak value and the value of optimal diameter was found to be related to coarseness of precursor powder. Ó 2007 Elsevier B.V. All rights reserved. PACS: 74.72.Hs Keywords: Bi-2223; Round wires; Outgrowth; Interface
1. Introduction Among various HTS materials, Bi-2223 is the most widely used one. Bi-2223 tapes are now used in power transportation, fault current limiter, and current leads. However, application of Bi-2223 wires is limited for several reasons. For one reason, composed of brittle ceramic materials, Bi-2223 tapes and wires cannot be subjected to bending strain more than 0.4% [1]. This means Bi-2223 coils cannot be incorporated into smaller systems, such as electronic devices. For another reason, due to geometry, Bi-2223 tapes show Jc anisotropy in magnetic field [2] and are not very convenient for practical application. In this view, thin Bi-2223 round wire could be an alternative of Bi-2223 tapes. Some results about Bi-2223 wires of round section have been reported by several groups [3–5]. Most of these works concentrated on AC losses. In these works, round wires were fabricated by stacking or stranding Bi-2223 tapes and have large diameter. In this paper, we present our
*
Corresponding author. Tel.: +86 10 62789339x210; fax: +86 10 62785913. E-mail address: [email protected] (P. Li). 0921-4534/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2007.02.033
experimental attempts to produce applicable thin Bi-2223 round wires. 2. Experimental In our work, we used commercial precursor powder for standard powder in tube (PIT) process and the powder was of nominal composition of Bi1.8Pb0.34Sr1.9Ca2Cu3.1Ox. After a primary heat treatment at 800 °C, we ground a part of the treated powder manually with mortar and pestle in a glove box of inertia atmosphere. Both ground precursor powder and precursor powder without any grinding were pressed into rods and put into silver tubes. Following standard PIT process, the tubes were drawn to make seven filament wires. In each step of drawing, a 13.5% [6] area reduction rate was adopted. Samples of different diameters were cut from the wires during the process for further study. Without intermediate rolling (IR) in standard PIT process, wires were annealed for about 80 h. The process we used was similar to that used by Yuan et al. in a previous work [7]. Three different annealing temperatures, 822 °C, 825 °C and 828 °C were chosen. Oxygen partial pressure was chosen to be 8.2%.
838
P. Li et al. / Physica C 463–465 (2007) 837–840
Critical current density (77 K, self field) of round wires were measured with four-probe method. 1 lV/cm was used as criteria. Grain size of precursor powder was studied with SEM observation. SEM and Energy Dispersive Spectroanalysis (EDS) study were used to study microstructure of wire filaments. 3. Results After heat treatment at 800 °C, the grain size of powder was 50–100 lm and the powder after grinding had an grain size of 5–10 lm. In this paper, powder without grinding was called coarse powder while ground powder was called coarse powder. Among three different annealing temperatures, 825 °C was found to be the optimal temperature. In this paper, discussion would be concentrated on samples annealed at 825 °C. Fig. 1 is a SEM figure of one filament in a wire of 0.7 mm in diameter fabricated with fine powder. The annealing temperature of this sample was 825 °C. EDS study showed that the phase formed in the filament was mainly Bi-2223. In most part of the filament, orientation of Bi-2223 grains was random and some cavities can be seen. However, grains in a layer at the superconductor– silver interface were dense and well textured. This layer grew continuously along the superconductor–silver interface. The thickness of this layer was about 5–10 lm. It can be also seen that the filament had a polygon-like pro-
file, which was composed of smooth superconductor–silver interfaces and some sharp polygon angle areas. Bi2223 grain outgrowths can be observed at polygon angles areas. Fig. 2 is a SEM figure of one filament in a wire of 0.4 mm in diameter fabricated with fine powder. The annealing temperature of the wire was 825 °C. EDS study showed that the phase formation of the filament was Bi2223. More cavities between grains can be seen. The profile of the filament was much more irregular and the polygon angles were sharper. At the polygon angles, some dark areas that extended into silver matrix could be seen. A careful study of the figure shows that these areas were cracks in the silver matrix. Some Bi-2223 grains could be found in these cracks. The well textured grain layer can still be seen in Fig. 2, but the layer was not continuous and fractured at polygon angles of the filament profile. Fig. 3 is a SEM figure of a filament of a wire fabricated with coarse powder and annealed at 825 °C. The wire was 0.7 mm in diameter. Many large dark particles can be seen and EDS showed that these particles were mainly CuO. While the sample in Fig. 1 and the sample in Fig. 3 are of the same diameter, more Bi-2223 grain outgrowths can be seen in Fig. 3. Some of these outgrowths occur at polygon angles of the filament profile. In some other cases, outgrowths also appeared near CuO particles. As shown in Fig. 3B, near a CuO particle, outgrowth of neighboring Bi-2223 grains can be observed.
Fig. 1. SEM image of a filament in a wire fabricated with fine powder and annealed at 825 °C. The diameter of the wire is 0.7 mm. The circle marks a typical polygon angle area of the superconductor–silver interface.
P. Li et al. / Physica C 463–465 (2007) 837–840
Fig. 2. SEM image of a filament in wire fabricated with fine powder and annealed at 825 °C. The diameter of the wire is 0.4 mm. Some cracks in silver are magnified in part B.
The relationship between Jc value and diameter of round wires annealed at 825 °C are as shown in Fig. 4. For round wires with different diameter, Jc ranges from 4500 A/cm2 to 500 A/cm2. As the wire diameter decreases, Jc first increases and then decreases. For wires fabricated with different powder (fine and coarse), the maximum Jc is reached at different diameter, about 0.8 mm for wires fabricated with coarse powder and 0.6 mm fabricated with fine powder. 4. Discussion Superconductor–silver filament is known to promote the texture of Bi-2223 grains. In the results published by many groups [8–12], the phenomenon of silver-induced texture has been reported and its mechanism has been explained
839
Fig. 3. SEM image of a filament in a wire fabricated with coarse powder and annealed at 825 °C. The diameter of the wire is 0.7 mm. Part B is a magnification of a CuO particle and its neighboring area.
[9,10]. In our samples, the well textured layer of Bi-2223 grains is supposed to be the result of silver-induced texture. The texture of grains is important for current transporting capacity of Bi-2223 wires. The part of filament near interface was reported to have Jc as much as five times higher than the average Jc [13]. As the total section of filaments decreases, the proportion of highly textured grains in the filament section increases and the overall Jc of round wires are hence increased. As observed in our experiments, during drawing process, the filament sections became polygons and the polygons angles grew sharper as diameter of wires decreased. This phenomenon has been reported in a numerical modeling [15]. In our results, cracks are found near polygon angles. Since the superconductor–silver interface promotes the formation of Bi-2223, grains might grow into these cracks during heat treatment. The well textured grain lay-
840
P. Li et al. / Physica C 463–465 (2007) 837–840
5. Conclusions Our experiments of round wires showed that, as diameter decreases, Jc first increased because of increased proportion of well textured grains near superconductor–silver interface. Meanwhile, cracks appeared during drawing process near polygon angle areas of the filament profile, which could induce grain outgrowth. Bi-2223 grains outgrow into these cracks and interrupt the grain layer near filament–silver interface. Fabricated with fine precursor powder, applicable Bi-2223 round wires of small diameter could be produced. References
Fig. 4. Relation between diameter of round wires and critical current density (Jc). The wires were annealed at 825 °C.
ers were fractured at polygon angles. For wires fabricated with coarse powder, cracks appeared at an earlier stage of the drawing process. After the primary heat treatment at 800 °C, the precursor powder did not contain CuO, so the large CuO particles were supposed to appear during cooling period of heat treatment. Since the formation of Bi-2223 in wires is considered to be a nonequilibrium reaction and only local regions are involved, only limited amount of liquid phase is required. Thus, the phase formation depends on the precursor powder’s stoichiometry homogeneity within a local area. Hence, more second phases would appear in wires fabricated with coarse precursor powder [14]. Since CuO particles are very hard compared with silver matrix, if they appear at superconductor–silver interface, they might cause cracks in silver matrix, which are sources of Bi-2223 outgrowth.
[1] Z. Han, P. Skov-Hansen, P. Vase, in: Proceedings of the 4th World Congress on Magnet Technology, 1998, p. 956. [2] Q. Zhu, D.-X. Chen, Z. Han, Supercond. Sci. Technol. 17 (2004) 756. [3] Xiaodong Su, Gre´goire Witz, Reynald Passerini, K. Kwasnitza, Rene´ Flu¨kiger, Physica C 372–376 (2002) 942. [4] X.D. Su, G. Witz, K. Kwasnitza, R. Flu¨kiger, Supercond. Sci. Technol. 15 (2002) 1184. [5] J. Fujikami, K. Hayashi, H. Takei, S. Honjo, T. Mimura, K. Matsuo, Y. Takahashi, Physica C 357–360 (2001) 1267. [6] M. Malberg, J. Bech, N. Bay, P. Skov-Hansen, G. Cualbu, IEEE Trans. Appl. Supercond. 9 (1999) 2577. [7] Y. Yuan, J. Jiang, X.Y. Cai, S. Patnaik, A.A. Polyanskii, E.E. Hellstrom, D.C. Larbalestier, R.K. Williams, Y. Huang, IEEE Trans. Appl. Supercond. 13 (2003) 2921. [8] V. Garnier, R. Passerini, E. Giannini, R. Flu¨kiger, Supercond. Sci. Technol. 16 (2003) 820. [9] Chuanbin Mao, Lian Zhou, Xiaozu Wu, Xiangyun Sun, Physica C 281 (1997) 159. [10] W. Eichenauaer, Z. Metallkd. 53 (1962) 321. [11] S.X. Dou, Y.C. Guo, R.K. Wang, M. Ionescu, H.K. Liu, IEEE Trans. Appl. Supercond. 5 (1995) 1830. [12] G. Grasso, B. Hensel, A. Jeremie, R. Flu¨kiger, EUCAS Conf. (1995) 463. [13] D.C. Larbalestier, X.Y. Cai, Y. Feng, H. Edelman, A. Umezawa, G.N. Riley, W.L. Carter, Physica C 221 (1994) 299. [14] J. Jiang, J.S. Abell, Physica C 296 (1998) 13. [15] Li-Ping Lei, Ying-Hong Zhao, Pan Zeng, Han-Ping Yi, Supercond. Sci. Technol. 18 (2005) 818.
Fabrication and study of thin Bi-2223 round wires P. Li *, L. Zhao, T.-M. Qu, X.-C. Wang, Z. Han Applied Superconductivity Research Center, Department of Physics, Tsinghua University, Beijing 100084, China Accepted 27 February 2007 Available online 25 May 2007
Abstract In this paper, experimental works in fabrication of Bi-2223 round wires are presented. Seven filament Bi-2223 round wires ranging from 1.0 mm to 0.3 mm in diameter were fabricated and studied. The critical current density Jc (77 K, self field) of round wires was of 103A/cm2 order. A correlation between Jc and wire diameter was observed. For round Bi-2223 wires, there is an optimal diameter at which Jc reaches peak value and the value of optimal diameter was found to be related to coarseness of precursor powder. Ó 2007 Elsevier B.V. All rights reserved. PACS: 74.72.Hs Keywords: Bi-2223; Round wires; Outgrowth; Interface
1. Introduction Among various HTS materials, Bi-2223 is the most widely used one. Bi-2223 tapes are now used in power transportation, fault current limiter, and current leads. However, application of Bi-2223 wires is limited for several reasons. For one reason, composed of brittle ceramic materials, Bi-2223 tapes and wires cannot be subjected to bending strain more than 0.4% [1]. This means Bi-2223 coils cannot be incorporated into smaller systems, such as electronic devices. For another reason, due to geometry, Bi-2223 tapes show Jc anisotropy in magnetic field [2] and are not very convenient for practical application. In this view, thin Bi-2223 round wire could be an alternative of Bi-2223 tapes. Some results about Bi-2223 wires of round section have been reported by several groups [3–5]. Most of these works concentrated on AC losses. In these works, round wires were fabricated by stacking or stranding Bi-2223 tapes and have large diameter. In this paper, we present our
*
Corresponding author. Tel.: +86 10 62789339x210; fax: +86 10 62785913. E-mail address: [email protected] (P. Li). 0921-4534/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2007.02.033
experimental attempts to produce applicable thin Bi-2223 round wires. 2. Experimental In our work, we used commercial precursor powder for standard powder in tube (PIT) process and the powder was of nominal composition of Bi1.8Pb0.34Sr1.9Ca2Cu3.1Ox. After a primary heat treatment at 800 °C, we ground a part of the treated powder manually with mortar and pestle in a glove box of inertia atmosphere. Both ground precursor powder and precursor powder without any grinding were pressed into rods and put into silver tubes. Following standard PIT process, the tubes were drawn to make seven filament wires. In each step of drawing, a 13.5% [6] area reduction rate was adopted. Samples of different diameters were cut from the wires during the process for further study. Without intermediate rolling (IR) in standard PIT process, wires were annealed for about 80 h. The process we used was similar to that used by Yuan et al. in a previous work [7]. Three different annealing temperatures, 822 °C, 825 °C and 828 °C were chosen. Oxygen partial pressure was chosen to be 8.2%.
838
P. Li et al. / Physica C 463–465 (2007) 837–840
Critical current density (77 K, self field) of round wires were measured with four-probe method. 1 lV/cm was used as criteria. Grain size of precursor powder was studied with SEM observation. SEM and Energy Dispersive Spectroanalysis (EDS) study were used to study microstructure of wire filaments. 3. Results After heat treatment at 800 °C, the grain size of powder was 50–100 lm and the powder after grinding had an grain size of 5–10 lm. In this paper, powder without grinding was called coarse powder while ground powder was called coarse powder. Among three different annealing temperatures, 825 °C was found to be the optimal temperature. In this paper, discussion would be concentrated on samples annealed at 825 °C. Fig. 1 is a SEM figure of one filament in a wire of 0.7 mm in diameter fabricated with fine powder. The annealing temperature of this sample was 825 °C. EDS study showed that the phase formed in the filament was mainly Bi-2223. In most part of the filament, orientation of Bi-2223 grains was random and some cavities can be seen. However, grains in a layer at the superconductor– silver interface were dense and well textured. This layer grew continuously along the superconductor–silver interface. The thickness of this layer was about 5–10 lm. It can be also seen that the filament had a polygon-like pro-
file, which was composed of smooth superconductor–silver interfaces and some sharp polygon angle areas. Bi2223 grain outgrowths can be observed at polygon angles areas. Fig. 2 is a SEM figure of one filament in a wire of 0.4 mm in diameter fabricated with fine powder. The annealing temperature of the wire was 825 °C. EDS study showed that the phase formation of the filament was Bi2223. More cavities between grains can be seen. The profile of the filament was much more irregular and the polygon angles were sharper. At the polygon angles, some dark areas that extended into silver matrix could be seen. A careful study of the figure shows that these areas were cracks in the silver matrix. Some Bi-2223 grains could be found in these cracks. The well textured grain layer can still be seen in Fig. 2, but the layer was not continuous and fractured at polygon angles of the filament profile. Fig. 3 is a SEM figure of a filament of a wire fabricated with coarse powder and annealed at 825 °C. The wire was 0.7 mm in diameter. Many large dark particles can be seen and EDS showed that these particles were mainly CuO. While the sample in Fig. 1 and the sample in Fig. 3 are of the same diameter, more Bi-2223 grain outgrowths can be seen in Fig. 3. Some of these outgrowths occur at polygon angles of the filament profile. In some other cases, outgrowths also appeared near CuO particles. As shown in Fig. 3B, near a CuO particle, outgrowth of neighboring Bi-2223 grains can be observed.
Fig. 1. SEM image of a filament in a wire fabricated with fine powder and annealed at 825 °C. The diameter of the wire is 0.7 mm. The circle marks a typical polygon angle area of the superconductor–silver interface.
P. Li et al. / Physica C 463–465 (2007) 837–840
Fig. 2. SEM image of a filament in wire fabricated with fine powder and annealed at 825 °C. The diameter of the wire is 0.4 mm. Some cracks in silver are magnified in part B.
The relationship between Jc value and diameter of round wires annealed at 825 °C are as shown in Fig. 4. For round wires with different diameter, Jc ranges from 4500 A/cm2 to 500 A/cm2. As the wire diameter decreases, Jc first increases and then decreases. For wires fabricated with different powder (fine and coarse), the maximum Jc is reached at different diameter, about 0.8 mm for wires fabricated with coarse powder and 0.6 mm fabricated with fine powder. 4. Discussion Superconductor–silver filament is known to promote the texture of Bi-2223 grains. In the results published by many groups [8–12], the phenomenon of silver-induced texture has been reported and its mechanism has been explained
839
Fig. 3. SEM image of a filament in a wire fabricated with coarse powder and annealed at 825 °C. The diameter of the wire is 0.7 mm. Part B is a magnification of a CuO particle and its neighboring area.
[9,10]. In our samples, the well textured layer of Bi-2223 grains is supposed to be the result of silver-induced texture. The texture of grains is important for current transporting capacity of Bi-2223 wires. The part of filament near interface was reported to have Jc as much as five times higher than the average Jc [13]. As the total section of filaments decreases, the proportion of highly textured grains in the filament section increases and the overall Jc of round wires are hence increased. As observed in our experiments, during drawing process, the filament sections became polygons and the polygons angles grew sharper as diameter of wires decreased. This phenomenon has been reported in a numerical modeling [15]. In our results, cracks are found near polygon angles. Since the superconductor–silver interface promotes the formation of Bi-2223, grains might grow into these cracks during heat treatment. The well textured grain lay-
840
P. Li et al. / Physica C 463–465 (2007) 837–840
5. Conclusions Our experiments of round wires showed that, as diameter decreases, Jc first increased because of increased proportion of well textured grains near superconductor–silver interface. Meanwhile, cracks appeared during drawing process near polygon angle areas of the filament profile, which could induce grain outgrowth. Bi-2223 grains outgrow into these cracks and interrupt the grain layer near filament–silver interface. Fabricated with fine precursor powder, applicable Bi-2223 round wires of small diameter could be produced. References
Fig. 4. Relation between diameter of round wires and critical current density (Jc). The wires were annealed at 825 °C.
ers were fractured at polygon angles. For wires fabricated with coarse powder, cracks appeared at an earlier stage of the drawing process. After the primary heat treatment at 800 °C, the precursor powder did not contain CuO, so the large CuO particles were supposed to appear during cooling period of heat treatment. Since the formation of Bi-2223 in wires is considered to be a nonequilibrium reaction and only local regions are involved, only limited amount of liquid phase is required. Thus, the phase formation depends on the precursor powder’s stoichiometry homogeneity within a local area. Hence, more second phases would appear in wires fabricated with coarse precursor powder [14]. Since CuO particles are very hard compared with silver matrix, if they appear at superconductor–silver interface, they might cause cracks in silver matrix, which are sources of Bi-2223 outgrowth.
[1] Z. Han, P. Skov-Hansen, P. Vase, in: Proceedings of the 4th World Congress on Magnet Technology, 1998, p. 956. [2] Q. Zhu, D.-X. Chen, Z. Han, Supercond. Sci. Technol. 17 (2004) 756. [3] Xiaodong Su, Gre´goire Witz, Reynald Passerini, K. Kwasnitza, Rene´ Flu¨kiger, Physica C 372–376 (2002) 942. [4] X.D. Su, G. Witz, K. Kwasnitza, R. Flu¨kiger, Supercond. Sci. Technol. 15 (2002) 1184. [5] J. Fujikami, K. Hayashi, H. Takei, S. Honjo, T. Mimura, K. Matsuo, Y. Takahashi, Physica C 357–360 (2001) 1267. [6] M. Malberg, J. Bech, N. Bay, P. Skov-Hansen, G. Cualbu, IEEE Trans. Appl. Supercond. 9 (1999) 2577. [7] Y. Yuan, J. Jiang, X.Y. Cai, S. Patnaik, A.A. Polyanskii, E.E. Hellstrom, D.C. Larbalestier, R.K. Williams, Y. Huang, IEEE Trans. Appl. Supercond. 13 (2003) 2921. [8] V. Garnier, R. Passerini, E. Giannini, R. Flu¨kiger, Supercond. Sci. Technol. 16 (2003) 820. [9] Chuanbin Mao, Lian Zhou, Xiaozu Wu, Xiangyun Sun, Physica C 281 (1997) 159. [10] W. Eichenauaer, Z. Metallkd. 53 (1962) 321. [11] S.X. Dou, Y.C. Guo, R.K. Wang, M. Ionescu, H.K. Liu, IEEE Trans. Appl. Supercond. 5 (1995) 1830. [12] G. Grasso, B. Hensel, A. Jeremie, R. Flu¨kiger, EUCAS Conf. (1995) 463. [13] D.C. Larbalestier, X.Y. Cai, Y. Feng, H. Edelman, A. Umezawa, G.N. Riley, W.L. Carter, Physica C 221 (1994) 299. [14] J. Jiang, J.S. Abell, Physica C 296 (1998) 13. [15] Li-Ping Lei, Ying-Hong Zhao, Pan Zeng, Han-Ping Yi, Supercond. Sci. Technol. 18 (2005) 818.