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Ag superconducting tapes
Physica C 355 (2001) 163±171
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Improved uniformity of microstructure and electrical properties of Bi-2223/Ag superconducting tapes Y.C. Guo a,*, W.M. Chen a, H.K. Liu a, S.X. Dou a, A.V. Lukashenko b a
Institute for Superconducting and Electronic Materials, The University of Wollongong, North®elds Avenue, Wollongong, NSW 2522, Australia b B.I. Verkin Institute for Low Temperature Physics and Engineering, 47 Lenin Avenue, 310164 Kharkov, Ukraine Received 7 September 2000; received in revised form 27 November 2000; accepted 6 December 2000
Abstract A special mechanical deformation technique, ÔsandwichÕ rolling (SR), together with the conventional rolling technique, normal rolling (NR), was used to intermediately deform Bi-2223/Ag superconducting tapes. The tapes processed by these two techniques were compared in terms of phase composition, microstructure, critical temperature (Tc ) and critical current density (Jc ). It was found that while there was no dierence in phase composition between the two types of tapes, density, degree of grain alignment and Jc were considerably higher in the SR tapes than in the NR tapes. In particular, the uniformity of microstructure, Tc , and Jc between the ®laments across the width of the tapes were signi®cantly improved by the SR compared to the NR technique. The results show that in terms of mechanical eects on Bi-2223/Ag tapes SR is more like uniaxial pressing, rather than conventional rolling. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Bi-2223/Ag tapes; Jc ; Jc distribution; Mechanical deformation; Microstructure
1. Introduction Silver-sheathed Bi-2223 superconducting tapes are normally fabricated by the powder-in-tube (PIT) method [1±3]. In the PIT method, superconductor precursor powders are packed into silver tubes. The oxide/silver composites are then drawn into small round wires and ®nally rolled into thin tapes. The resulting green tapes are heat treated by a thermomechanical process, which
* Corresponding author. Tel.: +61-2-4221-4773; fax: +61-24221-5731. E-mail address: [email protected] (Y.C. Guo).
typically consists of two steps of sintering with one step of mechanical deformation (intermediate mechanical deformation) between them. During the ®rst sintering, the low-Tc Bi-2212 phase, the major phase in the precursor powders, is converted into high-Tc Bi-2223 phase [4]. At the same time, due to the phase transformation and grain growth, the superconductor core becomes porous and the grains less aligned. The main purpose of the intermediate deformation is to re-densify the core and to enhance the degree of grain alignment. The second sintering step (®nal sintering step) is to further convert any residual Bi-2212 to Bi-2223 phase, to heal the cracks caused by the deformation, and to improve the connectivity between grains.
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While the intermediate mechanical deformation has proven to be a good technique to facilitate densi®cation and grain alignment, it also has some shortcomings. One of the shortcomings is that it produces non-uniform microstructure and electrical properties along the width of the resulting tapes. It has been found that the density and degree of grain alignment in the central ®laments of the tape are higher than that in the two sides of the tape. As a result, the central portion of the tape core carries a higher critical current density (Jc ) than the edge portions [5±8]. Another shortcoming of the intermediate mechanical deformation is that it causes cracks in the core of tapes even after subsequent heat treatment [9±11]. During mechanical deformation, grains and the connection between grains are broken or cracked due to the brittleness of the Bi-2223 compounds at room temperature. Most of the cracks can be healed by the subsequent sintering, but a complete healing of the cracks is dicult to achieve due to the lack of liquid phase in the late stages of heat treatment. Residual cracks are often found in the ®nal heat treated tapes. Even though these cracks can be very small in size, they are harmful to the electrical properties of the tapes. They form networks across the core of tapes, interrupt current ¯ow paths, allow ¯ux penetration through the superconducting region, and consequently limit the Jc of the tapes. Therefore, how to improve uniformity and reduce cracks through improvement of the intermediate mechanical deformation process has become an important topic in the study of Bi-2223 high-Tc superconducting tapes. There are two major methods which are currently used for the intermediate mechanical deformation, namely pressing and rolling. It has been well demonstrated that the pressing technique is superior to the rolling technique for the deformation of Bi-2223/Ag tapes because the former produces a more uniform, denser and better aligned core than the latter [12,13]. However, the main problem is that pressing is only suitable for deforming short tapes, not for long tapes. Therefore, the major challenge is to develop a mechanical deformation technique which has a mechanical eect similar to that of pressing, but is still suitable for continuous processing of long Bi-2223/Ag tapes.
Some time ago, we developed a technique, called ÔsandwichÕ rolling (SR), to reduce the ÔsausageÕ eect [14]. In this technique, tape is put between two rolling strips, forming a ÔsandwichÕ structure, and then rolled on a normal rolling (NR) machine. If the strips are mechanically harder than the tape sheath (i.e. silver) the pressure from the rollers will be passed on to the tape over a larger area than when it is directly rolled between the two rollers, as in the case in NR. This increase in deformation area changes the stress±strain characteristic of rolling and makes SR more similar to uniaxial pressing than NR. The SR technique can be made suitable for deforming long tapes by using very long strips or even using two large rings to replace the strips. In this work, all three deformation techniques, i.e., pressing, rolling and SR, were used to intermediately deform Bi-2223/Ag superconducting tapes. The eects of dierent deformation techniques on the microstructure, cracking and electrical properties were investigated and compared.
2. Experimental Bi-2223/Ag multi®lamentary tapes were fabricated by the standard PIT technique. Superconductor precursor powders with a stoichiometry of Bi/Pb/Sr/Ca/Cu 1.83/0.34/1.91/2.03/3.05 and a major phase of Bi-2212 were ®rst packed into a pure silver tube. The oxide/metal composite was then cold drawn into a wire of 1±2 mm diameter. The thin wire was cut into 37 pieces of equal length. The bundle of 37 wires was inserted into a second silver tube and again cold drawn into wires of 2.0 mm diameter. The wire was ®nally rolled into multi®lamentary tapes of 0.2±0.3 mm thickness. Short tapes with a length of 4 cm each were cut and subjected to a thermomechanical process consisting of two steps of thermal sintering and one step of mechanical deformation (intermediate mechanical deformation). To investigate the eects of various deformation techniques, three methods were used to carry out the intermediate deformation, namely pressing, NR and SR. The two strips used for SR are made of tool steel and are 1 mm
Y.C. Guo et al. / Physica C 355 (2001) 163±171
thick and 30 mm wide. The deformation rate, i.e. the ratio of tape thickness before and after deformation, was varied from 0.0% up to 35% for each method. All intermediate deformations were carried out at room temperature. The ®nal heat treated tapes were characterised in terms of phase composition, critical transition temperature (Tc ), critical current density (Jc ) and microstructure. To study the uniformity of these properties across the transverse section of the tapes, ®laments from both centre and edge portions of tapes were extracted and analysed separately. Phase composition was investigated by X-ray diraction (XRD) techniques. Tc was determined by measuring the resistance as a function of temperature. Microstructures of tapes were examined by scanning electron microscope (SEM). Jc was measured by the standard four-probe d.c. method using a 1 lV/cm criterion.
3. Results and discussion 3.1. Deformation method and phase composition Fig. 1 shows the XRD patterns for three tapes deformed by dierent methods, but heat treated under the same conditions. The three intermediate deformation methods are pressing, normal rolling and ÔsandwichÕ rolling, which are denoted by PR, NR and SR respectively. The specimen for XRD diraction was prepared by cutting the tape open and splitting the tape from the middle. The resulting pieces were put together for XRD diraction. That is, the XRD patterns shown in Fig. 1 were the overall diraction results with contributions from the entire tape. It is seen in Fig. 1 that all tapes have a high percentage of Bi-2223 phase ( P 95%) together with a small amount of Bi-2212 phase and a trace amount of Sr±Ca±Cu±O phase. The (0 0 l) peaks are very strong and sharp in all the patterns, and non-(0 0 l) peaks are small, indicating good grain alignment in all three tapes. There is no noticeable dierence in the phase composition among the three tapes. This result suggests that the mechanical deformation method did not aect the ®nal
165
Fig. 1. XRD patterns for Bi-2223/Ag superconducting tapes processed by dierent mechanical deformation methods: (a) uniaxial pressing, (b) NR, and (c) SR, but all heat treated under the same thermal conditions (i.e. 840°C for 100 h).
phase purity of the tapes, although it may in¯uence the kinetics of the phase formation [15]. The above XRD results were obtained on tapes deformed at the optimum deformation rate, i.e. the rate at which Jc is the maximum for each deformation method. To study the deformation rate eect, XRD diraction was also performed on tapes deformed to an under-deformation condition (i.e. deformation rate < optimum rate) and tapes in an over-deformation condition (i.e. deformation rate > optimum rate). XRD was also performed on centre and edge portions of tapes to study the uniformity of the phase composition across the width of tapes. In both cases, the XRD patterns did not show any noticeable dierences between among tapes with dierent deformation rates or between ®laments from dierent portion of tapes. Therefore, it was concluded that the deformation method did not aect the phase composition of tapes and that there was no dierence in phase composition between the centre and edge ®laments of the same tape. 3.2. Deformation method and microstructure Shown in Fig. 2a and b are SEM images taken on a transverse cross section of the centre portion
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of a NR tape and a SR tape, respectively. Both tapes were rolled with a thickness reduction rate near their optimal rate (i.e. 20%). It is clear that the SR tape contains denser and better aligned ®laments than the NR tape. Fig. 3a and b shows transverse cross-sectional views of NR and SR tapes rolled at a thickness reduction rate well above their optimum rate. The reduction rate for both tapes was about 33%. Cracks are observed in both tapes. However, compared to the NR tape the SR tape has a much lower incidence of cracks. The size of the cracks in the SR tape is also much smaller. For the rolling method, the main cause for the formation of cracking is the shear force on the tapes. A large
Fig. 3. SEM images of a transverse cross-section of the centre portion of tapes processed by (a) NR and (b) SR. Tapes were deformed at a deformation rate well above the optimum deformation rate for each method.
Fig. 2. SEM images of a transverse cross-section of the centre portion of tapes processed by (a) NR and (b) SR. Tapes were deformed at the optimum deformation rate for each method.
reduction in the number and size of cracks in the SR tape compared to the NR tape means that the SR signi®cantly reduces the shear force compared to the NR method. Fig. 4a and b are SEM images for tapes corresponding to those shown in Fig. 2a and b, but of an edge portion of the tapes. In other words, these are the images of the ®laments located close to the edge of the tapes. It is seen that compared to the central ®laments (Fig. 2a and b), the edge ®laments have a lower density and are less aligned. This is true for both the NR tape and the SR tape. But the dierence between the central and edge ®laments for SR tape is smaller than for NR tape.
Y.C. Guo et al. / Physica C 355 (2001) 163±171
Fig. 4. SEM images of a transverse cross-section of an edge portion of tapes (a) corresponding to Fig. 2a (NR) and (b) corresponding to Fig. 2b (SR). Tapes were deformed at the optimum deformation rate for each method.
The above SEM images suggest that the SR method results in tapes with higher density, better grain alignment, less cracking and more homogeneous ®laments. The dierences in the microstructure between the tapes processed by SR and NR will have an impact on the electrical properties of these two types of tapes, as discussed below. 3.3. Deformation method and Tc Tc was measured on ®laments extracted from centre and edge portions of tapes processed by NR and SR methods. The main focus of this work was to compare these two methods, because the dif-
167
ferences between pressing and NR have been well reported in the literature [11,16,17]. The dependence of resistance on temperature obtained for the NR and SR tapes is shown in Fig. 5. Tc values were determined from these curves and the results are summarised in Table 1. The average Tc for SR tape is 107.9 K, which is slightly higher than for NR tape (average Tc 106:7 K). Both are quite normal values for heat treated Bi-2223/Ag tapes. This small dierence could be due to dierence in the grain connectivity between the two types of tapes. As has been seen in the Section 3.2, SR tape has denser and better aligned superconductor ®laments compared to NR tape. The denser and better aligned ®laments result in a better grain connectivity and consequently, a sharper transition from the normal state to the superconducting state in the SR tape compared to the NR tape. Another interesting result to be noticed is that the Tc value is dierent for ®laments extracted from centre and edge portions of the tapes. The Tc for a ®lament from the centre is always higher than for one from the edge of the tapes. This is true for both the NR and SR tapes. More interesting is that the Tc variation from centre to edge is smaller in the SR tape than in the NR rolled tape. For example, while the Tc for the left side and right side ®laments are 1.2 and 1.1 K lower than for the centre ®lament for the NR tape, the corresponding dierence for the SR tape are only 0.6 and 0.5 K, respectively. This again shows that the SR results in a more homogeneous tape than the NR method. 3.4. Deformation method and Jc 3.4.1. Overall Jc Fig. 6 shows the overall Jc as a function of the thickness reduction rate for tapes processed by three deformation methods: pressing, NR and SR. It is evident that all three tapes follow the same trend. Jc initially increases with increasing deformation rate, reaches a maximum and then decreases with further increases in the deformation rate. The deformation rate at which the tape has the maximum Jc value is de®ned as the optimum rate. Surprisingly, the optimum deformation rate is quite similar for all methods, with the rate for
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Fig. 5. Resistance as a function of temperature for ®laments extracted from dierent portion of tapes. Measurements were carried out for both the NR and SR tape.
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Table 1 Results of Tc measured on ®laments extracted from dierent portions of tapes processed by the NR and SR methods Deformation method
Filament number
Filament position
Length (lm)
Width (lm)
Tc0 (K)
Normal rolling
NR1 NR3 NR2
left edge center rightedge
1150 1150 1150
270 350 250
106.3 107.5 106.4
±
1.1
2.27 2.66 2.33
ÔSandwichÕ rolling
SR1 SR3 SR2
left edge center right edge
1250 1000 1000
350 310 300
107.7 108.3 107.8
0.6 ± 0.5
2.68 2.77 2.69
Tc (edge) Tc (centre) (K) 1.2
R300 =R120
The higher Jc value for SR tapes than for NR tapes can be explained by the dierences in their microstructures. As shown in the previous sections, although there is no dierence in phase composition between the two types of tapes, the SR tapes have denser and better aligned microstructure, less cracking and more uniform ®laments than the NR tapes. Also, the Tc for the SR tapes is slightly higher and more homogeneous than for the NR tapes. All these factors contribute to a higher Jc for SR tapes than for the NR tapes.
Fig. 6. Critical current density (Jc ,77 K) vs. deformation rate for Bi-2223/Ag tapes processed by dierent methods: pressing, NR and SR rolling.
pressing (25%) only slightly higher than that for normal pressing and SR (22%). The most noticeable dierence is in the maximum Jc . First, the maximum Jc values for pressed and SR tapes are signi®cantly higher than for NR tape. Second, the maximum Jc for the SR tape is almost identical to that for the pressed tape. As it is well known that pressing usually produces higher Jc than NR [15], the fact that the Jc achieved by SR is similar to that achieved by pressing means that SR has similar mechanical eects to pressing. Since pressing can be used to process only short tapes (a few centimetres to a few tens of centimetres) and SR can be applied to process very long tapes (e.g. 1 km), this result is very signi®cant from the point of view of large scale production of Bi-2223/Ag tapes.
3.4.2. Jc uniformity Fig. 7 compares Jc values measured on ®laments extracted from the left-side edge, centre and right-side edge portions of tapes processed by NR and SR methods. It is noted that for both methods the Jc for a centre ®lament is much higher than for edge ®laments. This is consistent with the microstructural result presented in Sections 3.2 and 3.3, which showed that the centre ®laments have a higher density, better grain alignment and slightly higher Tc than the edge ®laments. It is well known that these factors are all very critical for Jc of the Bi-2223 superconductors [4]. It is also noted that the Jc dierence between a centre ®lament and edge ®laments is smaller for the SR tape than for the NR tape. More speci®cally, while the average ratio of centre ®lament Jc to edge ®lament Jc for the NR tape is 5.5 the ratio for the SR tape is only 2.5. This is again in agreement with the microstructural result. In Section 3.2, we have seen that the microstructural dierences between the centre and the edge ®laments for the SR tape are smaller than for the NR tape. Since Jc is dependent on microstructure, the dierence in ratio
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Fig. 7. Critical current density (Jc ,77 K) measured on ®laments extracted from left-edge, centre and right-edge portions of tapes processed by NR and SR methods.
of centre ®lament Jc to edge ®lament Jc for the NR and SR tapes is understandable. All these results indicate that SR improves the Jc uniformity of Bi2223/Ag tapes compared to NR.
thermore, the Jc distribution across transverse cross the section of SR tapes was much more uniform than for the NR tapes. The Jc value achieved by SR reached the same level as that obtained by pressing.
4. Conclusions A special mechanical deformation technique, SR and two conventional techniques, uniaxial pressing and NR, were used to process Bi-2223/Ag tapes, and their eects on the uniformity of microstructure and electrical properties were investigated and compared. The results obtained showed: 1. The dierent deformation methods did not result in dierences in the phase composition of tapes after ®nal heat treatment. 2. The dierent deformation techniques did result in dierences in tape microstructure. Compared to the conventional rolling method, the SR tape had higher density, better grain alignment, less cracking, and more uniform ®laments across the entire cross-section of the tape. 3. The SR method resulted in slightly higher Tc and more homogeneous Tc among the ®laments than the conventional rolling technique. 4. Compared to the conventional rolling method, SR improved the Jc of tapes signi®cantly. Fur-
Acknowledgements The authors acknowledge the Australian Research Council/Department of Employment, Education, Training and Youth Aairs for ®nancial support. Thanks are also given to Dr. M. Apperley and Mr. G. McCaughey of Metal Manufactures for supplying the green tapes.
References [1] M. Ueyama, T. Hikata, T. Kato, K. Sato, Jpn. J. Appl. Phys. 30 (1991) L1384. [2] Y. Yamada, B. Obst, R. Fl ukiger, Supercond. Sci. Technol. 4 (1991) 165. [3] S.X. Dou, H.K. Liu, Supercond. Sci. Technol. 6 (1993) 297. [4] Y.C. Guo, H.K. Liu, S.X. Dou, Mater. Sci. Eng. B 23 (1994) 58. [5] D.C. Larbalestier, X.Y. Cai, Y. Feng, H. Edelman, A. Umezawa, G.N. Riley Jr., W.L. Carter, Physica C 221 (1994) 299.
Y.C. Guo et al. / Physica C 355 (2001) 163±171 [6] G. Grassp, B. Hensel, A. Jeremie, R. Fl ukiger, Physica C 241 (1995) 45. [7] A.G. Sivakov, A.V. Lukashenko, D. Abraimov, P. Muller, A.V. Ustinov, M. Leghissa, Appl. Phys Lett. 76 (2000) 2597. [8] A.G. Sivakov, A.V. Lukashenko, O.G. Turutanov, I.M. Dmitrenko, D.V. Abraimov, P. Muller, A.V. Ustinov, Physica B 284±288 (2000) 2071. [9] X.Y. Cai, A. Polyanski, Q. Li, G.N. Riley, D.C. Larbalestier, Nature 392 (1998) 906. [10] J.A. Parrell, A. Polyanski, A.E. Pashitski, D.C. Larbalestier, Supercond. Sci. Technol. 9 (1996) 393. [11] M. Polak, J.A. Parrel, A. Polyanski, A. Pashitski, D.C. Larbalestier, Appl. Phys. Lett. 70 (1997) 1034.
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[12] Z. Han, P. Skov-Hansen, T. Freltoft, Supercond. Sci. Technol. 10 (1997) 371. [13] G. Grasso, A. Jeremie, R. Fl ukiger, Supercond. Sci. Technol. 8 (1995) 827. [14] W.G. Wang, H.K. Liu, Y.C. Guo, P.A. Bain, S.X. Dou, Appl. Supercond. 3 (1996) 599. [15] Y.C. Guo, H.K. Liu, S.X. Dou, T. Kuroda, Y. Tanaka, Supercond. Sci. Technol. 11 (1998) 1053±1056. [16] Q. Li, G.N. Riley Jr., R.D. Parrella, S. Fleshler, M.W. Rupich, W.L. Carter, J.O. Willis, J.Y. Coulter, J.F. Bingert, V.K. Sikka, J.A. Parrell, D.C. Larbalestier, IEEE Trans. Appl. Supercond. 7 (1997) 2026. [17] G. Grasso, A. Perin, B. Hensel, R. Fl ukiger, Physica C 217 (1993) 335.
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Improved uniformity of microstructure and electrical properties of Bi-2223/Ag superconducting tapes Y.C. Guo a,*, W.M. Chen a, H.K. Liu a, S.X. Dou a, A.V. Lukashenko b a
Institute for Superconducting and Electronic Materials, The University of Wollongong, North®elds Avenue, Wollongong, NSW 2522, Australia b B.I. Verkin Institute for Low Temperature Physics and Engineering, 47 Lenin Avenue, 310164 Kharkov, Ukraine Received 7 September 2000; received in revised form 27 November 2000; accepted 6 December 2000
Abstract A special mechanical deformation technique, ÔsandwichÕ rolling (SR), together with the conventional rolling technique, normal rolling (NR), was used to intermediately deform Bi-2223/Ag superconducting tapes. The tapes processed by these two techniques were compared in terms of phase composition, microstructure, critical temperature (Tc ) and critical current density (Jc ). It was found that while there was no dierence in phase composition between the two types of tapes, density, degree of grain alignment and Jc were considerably higher in the SR tapes than in the NR tapes. In particular, the uniformity of microstructure, Tc , and Jc between the ®laments across the width of the tapes were signi®cantly improved by the SR compared to the NR technique. The results show that in terms of mechanical eects on Bi-2223/Ag tapes SR is more like uniaxial pressing, rather than conventional rolling. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Bi-2223/Ag tapes; Jc ; Jc distribution; Mechanical deformation; Microstructure
1. Introduction Silver-sheathed Bi-2223 superconducting tapes are normally fabricated by the powder-in-tube (PIT) method [1±3]. In the PIT method, superconductor precursor powders are packed into silver tubes. The oxide/silver composites are then drawn into small round wires and ®nally rolled into thin tapes. The resulting green tapes are heat treated by a thermomechanical process, which
* Corresponding author. Tel.: +61-2-4221-4773; fax: +61-24221-5731. E-mail address: [email protected] (Y.C. Guo).
typically consists of two steps of sintering with one step of mechanical deformation (intermediate mechanical deformation) between them. During the ®rst sintering, the low-Tc Bi-2212 phase, the major phase in the precursor powders, is converted into high-Tc Bi-2223 phase [4]. At the same time, due to the phase transformation and grain growth, the superconductor core becomes porous and the grains less aligned. The main purpose of the intermediate deformation is to re-densify the core and to enhance the degree of grain alignment. The second sintering step (®nal sintering step) is to further convert any residual Bi-2212 to Bi-2223 phase, to heal the cracks caused by the deformation, and to improve the connectivity between grains.
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While the intermediate mechanical deformation has proven to be a good technique to facilitate densi®cation and grain alignment, it also has some shortcomings. One of the shortcomings is that it produces non-uniform microstructure and electrical properties along the width of the resulting tapes. It has been found that the density and degree of grain alignment in the central ®laments of the tape are higher than that in the two sides of the tape. As a result, the central portion of the tape core carries a higher critical current density (Jc ) than the edge portions [5±8]. Another shortcoming of the intermediate mechanical deformation is that it causes cracks in the core of tapes even after subsequent heat treatment [9±11]. During mechanical deformation, grains and the connection between grains are broken or cracked due to the brittleness of the Bi-2223 compounds at room temperature. Most of the cracks can be healed by the subsequent sintering, but a complete healing of the cracks is dicult to achieve due to the lack of liquid phase in the late stages of heat treatment. Residual cracks are often found in the ®nal heat treated tapes. Even though these cracks can be very small in size, they are harmful to the electrical properties of the tapes. They form networks across the core of tapes, interrupt current ¯ow paths, allow ¯ux penetration through the superconducting region, and consequently limit the Jc of the tapes. Therefore, how to improve uniformity and reduce cracks through improvement of the intermediate mechanical deformation process has become an important topic in the study of Bi-2223 high-Tc superconducting tapes. There are two major methods which are currently used for the intermediate mechanical deformation, namely pressing and rolling. It has been well demonstrated that the pressing technique is superior to the rolling technique for the deformation of Bi-2223/Ag tapes because the former produces a more uniform, denser and better aligned core than the latter [12,13]. However, the main problem is that pressing is only suitable for deforming short tapes, not for long tapes. Therefore, the major challenge is to develop a mechanical deformation technique which has a mechanical eect similar to that of pressing, but is still suitable for continuous processing of long Bi-2223/Ag tapes.
Some time ago, we developed a technique, called ÔsandwichÕ rolling (SR), to reduce the ÔsausageÕ eect [14]. In this technique, tape is put between two rolling strips, forming a ÔsandwichÕ structure, and then rolled on a normal rolling (NR) machine. If the strips are mechanically harder than the tape sheath (i.e. silver) the pressure from the rollers will be passed on to the tape over a larger area than when it is directly rolled between the two rollers, as in the case in NR. This increase in deformation area changes the stress±strain characteristic of rolling and makes SR more similar to uniaxial pressing than NR. The SR technique can be made suitable for deforming long tapes by using very long strips or even using two large rings to replace the strips. In this work, all three deformation techniques, i.e., pressing, rolling and SR, were used to intermediately deform Bi-2223/Ag superconducting tapes. The eects of dierent deformation techniques on the microstructure, cracking and electrical properties were investigated and compared.
2. Experimental Bi-2223/Ag multi®lamentary tapes were fabricated by the standard PIT technique. Superconductor precursor powders with a stoichiometry of Bi/Pb/Sr/Ca/Cu 1.83/0.34/1.91/2.03/3.05 and a major phase of Bi-2212 were ®rst packed into a pure silver tube. The oxide/metal composite was then cold drawn into a wire of 1±2 mm diameter. The thin wire was cut into 37 pieces of equal length. The bundle of 37 wires was inserted into a second silver tube and again cold drawn into wires of 2.0 mm diameter. The wire was ®nally rolled into multi®lamentary tapes of 0.2±0.3 mm thickness. Short tapes with a length of 4 cm each were cut and subjected to a thermomechanical process consisting of two steps of thermal sintering and one step of mechanical deformation (intermediate mechanical deformation). To investigate the eects of various deformation techniques, three methods were used to carry out the intermediate deformation, namely pressing, NR and SR. The two strips used for SR are made of tool steel and are 1 mm
Y.C. Guo et al. / Physica C 355 (2001) 163±171
thick and 30 mm wide. The deformation rate, i.e. the ratio of tape thickness before and after deformation, was varied from 0.0% up to 35% for each method. All intermediate deformations were carried out at room temperature. The ®nal heat treated tapes were characterised in terms of phase composition, critical transition temperature (Tc ), critical current density (Jc ) and microstructure. To study the uniformity of these properties across the transverse section of the tapes, ®laments from both centre and edge portions of tapes were extracted and analysed separately. Phase composition was investigated by X-ray diraction (XRD) techniques. Tc was determined by measuring the resistance as a function of temperature. Microstructures of tapes were examined by scanning electron microscope (SEM). Jc was measured by the standard four-probe d.c. method using a 1 lV/cm criterion.
3. Results and discussion 3.1. Deformation method and phase composition Fig. 1 shows the XRD patterns for three tapes deformed by dierent methods, but heat treated under the same conditions. The three intermediate deformation methods are pressing, normal rolling and ÔsandwichÕ rolling, which are denoted by PR, NR and SR respectively. The specimen for XRD diraction was prepared by cutting the tape open and splitting the tape from the middle. The resulting pieces were put together for XRD diraction. That is, the XRD patterns shown in Fig. 1 were the overall diraction results with contributions from the entire tape. It is seen in Fig. 1 that all tapes have a high percentage of Bi-2223 phase ( P 95%) together with a small amount of Bi-2212 phase and a trace amount of Sr±Ca±Cu±O phase. The (0 0 l) peaks are very strong and sharp in all the patterns, and non-(0 0 l) peaks are small, indicating good grain alignment in all three tapes. There is no noticeable dierence in the phase composition among the three tapes. This result suggests that the mechanical deformation method did not aect the ®nal
165
Fig. 1. XRD patterns for Bi-2223/Ag superconducting tapes processed by dierent mechanical deformation methods: (a) uniaxial pressing, (b) NR, and (c) SR, but all heat treated under the same thermal conditions (i.e. 840°C for 100 h).
phase purity of the tapes, although it may in¯uence the kinetics of the phase formation [15]. The above XRD results were obtained on tapes deformed at the optimum deformation rate, i.e. the rate at which Jc is the maximum for each deformation method. To study the deformation rate eect, XRD diraction was also performed on tapes deformed to an under-deformation condition (i.e. deformation rate < optimum rate) and tapes in an over-deformation condition (i.e. deformation rate > optimum rate). XRD was also performed on centre and edge portions of tapes to study the uniformity of the phase composition across the width of tapes. In both cases, the XRD patterns did not show any noticeable dierences between among tapes with dierent deformation rates or between ®laments from dierent portion of tapes. Therefore, it was concluded that the deformation method did not aect the phase composition of tapes and that there was no dierence in phase composition between the centre and edge ®laments of the same tape. 3.2. Deformation method and microstructure Shown in Fig. 2a and b are SEM images taken on a transverse cross section of the centre portion
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of a NR tape and a SR tape, respectively. Both tapes were rolled with a thickness reduction rate near their optimal rate (i.e. 20%). It is clear that the SR tape contains denser and better aligned ®laments than the NR tape. Fig. 3a and b shows transverse cross-sectional views of NR and SR tapes rolled at a thickness reduction rate well above their optimum rate. The reduction rate for both tapes was about 33%. Cracks are observed in both tapes. However, compared to the NR tape the SR tape has a much lower incidence of cracks. The size of the cracks in the SR tape is also much smaller. For the rolling method, the main cause for the formation of cracking is the shear force on the tapes. A large
Fig. 3. SEM images of a transverse cross-section of the centre portion of tapes processed by (a) NR and (b) SR. Tapes were deformed at a deformation rate well above the optimum deformation rate for each method.
Fig. 2. SEM images of a transverse cross-section of the centre portion of tapes processed by (a) NR and (b) SR. Tapes were deformed at the optimum deformation rate for each method.
reduction in the number and size of cracks in the SR tape compared to the NR tape means that the SR signi®cantly reduces the shear force compared to the NR method. Fig. 4a and b are SEM images for tapes corresponding to those shown in Fig. 2a and b, but of an edge portion of the tapes. In other words, these are the images of the ®laments located close to the edge of the tapes. It is seen that compared to the central ®laments (Fig. 2a and b), the edge ®laments have a lower density and are less aligned. This is true for both the NR tape and the SR tape. But the dierence between the central and edge ®laments for SR tape is smaller than for NR tape.
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Fig. 4. SEM images of a transverse cross-section of an edge portion of tapes (a) corresponding to Fig. 2a (NR) and (b) corresponding to Fig. 2b (SR). Tapes were deformed at the optimum deformation rate for each method.
The above SEM images suggest that the SR method results in tapes with higher density, better grain alignment, less cracking and more homogeneous ®laments. The dierences in the microstructure between the tapes processed by SR and NR will have an impact on the electrical properties of these two types of tapes, as discussed below. 3.3. Deformation method and Tc Tc was measured on ®laments extracted from centre and edge portions of tapes processed by NR and SR methods. The main focus of this work was to compare these two methods, because the dif-
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ferences between pressing and NR have been well reported in the literature [11,16,17]. The dependence of resistance on temperature obtained for the NR and SR tapes is shown in Fig. 5. Tc values were determined from these curves and the results are summarised in Table 1. The average Tc for SR tape is 107.9 K, which is slightly higher than for NR tape (average Tc 106:7 K). Both are quite normal values for heat treated Bi-2223/Ag tapes. This small dierence could be due to dierence in the grain connectivity between the two types of tapes. As has been seen in the Section 3.2, SR tape has denser and better aligned superconductor ®laments compared to NR tape. The denser and better aligned ®laments result in a better grain connectivity and consequently, a sharper transition from the normal state to the superconducting state in the SR tape compared to the NR tape. Another interesting result to be noticed is that the Tc value is dierent for ®laments extracted from centre and edge portions of the tapes. The Tc for a ®lament from the centre is always higher than for one from the edge of the tapes. This is true for both the NR and SR tapes. More interesting is that the Tc variation from centre to edge is smaller in the SR tape than in the NR rolled tape. For example, while the Tc for the left side and right side ®laments are 1.2 and 1.1 K lower than for the centre ®lament for the NR tape, the corresponding dierence for the SR tape are only 0.6 and 0.5 K, respectively. This again shows that the SR results in a more homogeneous tape than the NR method. 3.4. Deformation method and Jc 3.4.1. Overall Jc Fig. 6 shows the overall Jc as a function of the thickness reduction rate for tapes processed by three deformation methods: pressing, NR and SR. It is evident that all three tapes follow the same trend. Jc initially increases with increasing deformation rate, reaches a maximum and then decreases with further increases in the deformation rate. The deformation rate at which the tape has the maximum Jc value is de®ned as the optimum rate. Surprisingly, the optimum deformation rate is quite similar for all methods, with the rate for
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Fig. 5. Resistance as a function of temperature for ®laments extracted from dierent portion of tapes. Measurements were carried out for both the NR and SR tape.
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Table 1 Results of Tc measured on ®laments extracted from dierent portions of tapes processed by the NR and SR methods Deformation method
Filament number
Filament position
Length (lm)
Width (lm)
Tc0 (K)
Normal rolling
NR1 NR3 NR2
left edge center rightedge
1150 1150 1150
270 350 250
106.3 107.5 106.4
±
1.1
2.27 2.66 2.33
ÔSandwichÕ rolling
SR1 SR3 SR2
left edge center right edge
1250 1000 1000
350 310 300
107.7 108.3 107.8
0.6 ± 0.5
2.68 2.77 2.69
Tc (edge) Tc (centre) (K) 1.2
R300 =R120
The higher Jc value for SR tapes than for NR tapes can be explained by the dierences in their microstructures. As shown in the previous sections, although there is no dierence in phase composition between the two types of tapes, the SR tapes have denser and better aligned microstructure, less cracking and more uniform ®laments than the NR tapes. Also, the Tc for the SR tapes is slightly higher and more homogeneous than for the NR tapes. All these factors contribute to a higher Jc for SR tapes than for the NR tapes.
Fig. 6. Critical current density (Jc ,77 K) vs. deformation rate for Bi-2223/Ag tapes processed by dierent methods: pressing, NR and SR rolling.
pressing (25%) only slightly higher than that for normal pressing and SR (22%). The most noticeable dierence is in the maximum Jc . First, the maximum Jc values for pressed and SR tapes are signi®cantly higher than for NR tape. Second, the maximum Jc for the SR tape is almost identical to that for the pressed tape. As it is well known that pressing usually produces higher Jc than NR [15], the fact that the Jc achieved by SR is similar to that achieved by pressing means that SR has similar mechanical eects to pressing. Since pressing can be used to process only short tapes (a few centimetres to a few tens of centimetres) and SR can be applied to process very long tapes (e.g. 1 km), this result is very signi®cant from the point of view of large scale production of Bi-2223/Ag tapes.
3.4.2. Jc uniformity Fig. 7 compares Jc values measured on ®laments extracted from the left-side edge, centre and right-side edge portions of tapes processed by NR and SR methods. It is noted that for both methods the Jc for a centre ®lament is much higher than for edge ®laments. This is consistent with the microstructural result presented in Sections 3.2 and 3.3, which showed that the centre ®laments have a higher density, better grain alignment and slightly higher Tc than the edge ®laments. It is well known that these factors are all very critical for Jc of the Bi-2223 superconductors [4]. It is also noted that the Jc dierence between a centre ®lament and edge ®laments is smaller for the SR tape than for the NR tape. More speci®cally, while the average ratio of centre ®lament Jc to edge ®lament Jc for the NR tape is 5.5 the ratio for the SR tape is only 2.5. This is again in agreement with the microstructural result. In Section 3.2, we have seen that the microstructural dierences between the centre and the edge ®laments for the SR tape are smaller than for the NR tape. Since Jc is dependent on microstructure, the dierence in ratio
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Fig. 7. Critical current density (Jc ,77 K) measured on ®laments extracted from left-edge, centre and right-edge portions of tapes processed by NR and SR methods.
of centre ®lament Jc to edge ®lament Jc for the NR and SR tapes is understandable. All these results indicate that SR improves the Jc uniformity of Bi2223/Ag tapes compared to NR.
thermore, the Jc distribution across transverse cross the section of SR tapes was much more uniform than for the NR tapes. The Jc value achieved by SR reached the same level as that obtained by pressing.
4. Conclusions A special mechanical deformation technique, SR and two conventional techniques, uniaxial pressing and NR, were used to process Bi-2223/Ag tapes, and their eects on the uniformity of microstructure and electrical properties were investigated and compared. The results obtained showed: 1. The dierent deformation methods did not result in dierences in the phase composition of tapes after ®nal heat treatment. 2. The dierent deformation techniques did result in dierences in tape microstructure. Compared to the conventional rolling method, the SR tape had higher density, better grain alignment, less cracking, and more uniform ®laments across the entire cross-section of the tape. 3. The SR method resulted in slightly higher Tc and more homogeneous Tc among the ®laments than the conventional rolling technique. 4. Compared to the conventional rolling method, SR improved the Jc of tapes signi®cantly. Fur-
Acknowledgements The authors acknowledge the Australian Research Council/Department of Employment, Education, Training and Youth Aairs for ®nancial support. Thanks are also given to Dr. M. Apperley and Mr. G. McCaughey of Metal Manufactures for supplying the green tapes.
References [1] M. Ueyama, T. Hikata, T. Kato, K. Sato, Jpn. J. Appl. Phys. 30 (1991) L1384. [2] Y. Yamada, B. Obst, R. Fl ukiger, Supercond. Sci. Technol. 4 (1991) 165. [3] S.X. Dou, H.K. Liu, Supercond. Sci. Technol. 6 (1993) 297. [4] Y.C. Guo, H.K. Liu, S.X. Dou, Mater. Sci. Eng. B 23 (1994) 58. [5] D.C. Larbalestier, X.Y. Cai, Y. Feng, H. Edelman, A. Umezawa, G.N. Riley Jr., W.L. Carter, Physica C 221 (1994) 299.
Y.C. Guo et al. / Physica C 355 (2001) 163±171 [6] G. Grassp, B. Hensel, A. Jeremie, R. Fl ukiger, Physica C 241 (1995) 45. [7] A.G. Sivakov, A.V. Lukashenko, D. Abraimov, P. Muller, A.V. Ustinov, M. Leghissa, Appl. Phys Lett. 76 (2000) 2597. [8] A.G. Sivakov, A.V. Lukashenko, O.G. Turutanov, I.M. Dmitrenko, D.V. Abraimov, P. Muller, A.V. Ustinov, Physica B 284±288 (2000) 2071. [9] X.Y. Cai, A. Polyanski, Q. Li, G.N. Riley, D.C. Larbalestier, Nature 392 (1998) 906. [10] J.A. Parrell, A. Polyanski, A.E. Pashitski, D.C. Larbalestier, Supercond. Sci. Technol. 9 (1996) 393. [11] M. Polak, J.A. Parrel, A. Polyanski, A. Pashitski, D.C. Larbalestier, Appl. Phys. Lett. 70 (1997) 1034.
171
[12] Z. Han, P. Skov-Hansen, T. Freltoft, Supercond. Sci. Technol. 10 (1997) 371. [13] G. Grasso, A. Jeremie, R. Fl ukiger, Supercond. Sci. Technol. 8 (1995) 827. [14] W.G. Wang, H.K. Liu, Y.C. Guo, P.A. Bain, S.X. Dou, Appl. Supercond. 3 (1996) 599. [15] Y.C. Guo, H.K. Liu, S.X. Dou, T. Kuroda, Y. Tanaka, Supercond. Sci. Technol. 11 (1998) 1053±1056. [16] Q. Li, G.N. Riley Jr., R.D. Parrella, S. Fleshler, M.W. Rupich, W.L. Carter, J.O. Willis, J.Y. Coulter, J.F. Bingert, V.K. Sikka, J.A. Parrell, D.C. Larbalestier, IEEE Trans. Appl. Supercond. 7 (1997) 2026. [17] G. Grasso, A. Perin, B. Hensel, R. Fl ukiger, Physica C 217 (1993) 335.