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Ag tapes
Physica C 356 (2001) 53±61
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The eect of intermediate deformation by eccentric rolling on the Jc (B) performance of multicore Bi-2223/Ag tapes P. Kov ac a,*, Y.B. Huang b, I. Husek a, D.M. Spiller b, P. Bukva a a
Institute of Electrical Engineering, Slovak Academy of Sciences, Dubravska cesta 9, 84239 Bratislava, Slovakia b BICC General Superconductors, Oak Road, Wrexham LL13 9PX, UK Received 16 October 2000; received in revised form 2 January 2001; accepted 5 January 2001
Abstract An eccentric rolling (ER) technique has been applied during the intermediate deformation of commercial multicore Bi-2223/Ag tapes. The Jc (B) performance of the tapes deformed by ER and standard 2-high rolling technique (FR) have been compared. It was shown, that at 77 K, the self-®eld Jc can be increased by 12.6% and Jc (1 T) by 21% if an optimum ER deformation is applied. The tapes subjected to ER show smaller Jc drop in low magnetic ®elds (Bex < 0:2 T) as a consequence of reduced number of transverse cracks and improved ®lament density. This is a vital aspect for further Jc improvements by an optimal ER intermediate deformation applied for multicore Bi-2223/Ag tapes. ER tapes also have lower Jc -anisotropy ratio at Bex 0:5 T than FR ones, which may be advantageous to HTS coil manufacturers for various applications. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Bi-2223/Ag tapes; Intermediate deformation; Critical current density; Lorentz force
1. Introduction The powder-in-tube (PIT) processes process requires mechanical deformation (pressing or rolling) combined with subsequent heat treatment cycles so that high current density (Jc ) can be achieved. The aim of the intermediate deformation step is to improve the core density and texture and break up the sintered structure and to facilitate a higher conversion of the precursor powder to 2223 [1]. An optimisation process of parameters for manufacturing mono-®lamentary Bi-2223/Ag tapes
* Corresponding author. Tel.: +421-7-5477-5823-2841; fax: +421-7-5477-5816. E-mail address: [email protected] (P. Kovac).
was done by Grasso et al. [2] showing the importance of deformation-induced texture in colddeformed tape [3]. A non-optimised deformation process resulted in cracks in the oxide core, which were dicult to heal fully during subsequent annealing [1,4,5]. The rates of phase conversion 2212 ) 2223 was measured [6,7] for dierent deformation techniques. The highest content of 2223 phase is usually found at the BSCCO/Ag interface and the lowest at the centre of the core. It is also found that the texture of the 2223 phase, phase assembly, core density distribution, and the distribution of cracks within the core are not uniform [8,9]. This leads to an inhomogeneous distribution of the transport current. It has been shown that the density distribution has a higher impact on Ic than
0921-4534/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 1 ) 0 0 1 2 3 - X
P. Kovac et al. / Physica C 356 (2001) 53±61
54
the density itself [8±10]. Very dense inhomogeneous tapes had low Ic values, and they required longer annealing periods to gain the same Ic as achieved in homogenous ones with lower density [11]. In the case of multi®lamentary Bi-2223/Ag tapes, the shape of ®laments depends also on its positions relative to the tape cross-section. The outer ®laments are usually close to ellipsoidal whereas the ®laments in the centre look like ¯at and wide dumbbells. Dierences in the degree of deformation of ®laments lead to dierences in their microstructure. A ®lament aspect ratio (b=a, widthto-thickness ratio) ranging from 12 at the tape edges up to 55 at the tape centre was presented by Schuster et al. [12]. They showed one order of magnitude dierence in current density at 77 K (from 8 103 A/cm2 , for lowest b=a 12, up to 8 104 A/cm2 , for highest b=a 55). A similar ®lament aspect ratio distribution is observed for most of Bi(2223)/Ag tapes produced by OPIT when drawing and subsequent ¯at rolling are used. In order to reduce the formation of transverse cracks in multicore Bi-2223/Ag tapes has lead to application of continual pressing [13±16] and a new rolling technique, so-called eccentric rolling (ER) [17]. It has been shown that ER results in an increase in BSCCO core density and improved texture of Bi-2212 and 2223 grains caused by a higher shearing force to the powder through the Ag sheath than classical rolling with the comparable roll radius. Consequently, much higher transport current densities (similar to press-sintered tapes) can be reached. The eccentrically rolled samples had a signi®cantly reduced number of transverse cracks (shown by magneto-optical images), which is a vital aspect for increasing the critical current density. Consequently, 1.25± 3.11 times higher transport current densities were reached [18]. Several deforming methods (contin-
uous pressing, rolling with large diameters and ER) have been applied by Skov-Hansen et al. for multicore Bi-2223/Ag tape [19]. The process parameters such as deformation zone size, volumetric strain, reduction ratio and friction parameters were compared. They have shown that there is an optimum in the logarithmic strain ratio region close to ln jew =et j 2 where the largest increase of critical current density in the ®nal heat treatment is located. If this is the case it means that width and thickness strains are very important parameters in the intermediate pressing process. The aim of this contribution is to show the effect of intermediate deformation by ER on the Jc (B) and Jc (e) performance of commercial multicore Bi-2223/Ag tapes. 2. Experimental A Bi-2223/Ag multi®lamentary tape was fabricated by the standard PIT method [20]. The tape was made from commercial Bi-2223 precursor powder and the ®nal ®ll factor (BSCCO/Ag) was estimated at 27%. An Ag sheathed 37-®lament tape was cold rolled to 0:3 3:0 mm2 in crosssection size and heat-treated in air at 835°C for 40 h. After this heat treatment, three kinds of samples (see Table 1) were prepared: (1) intermediate rolling using a standard 2-high rolling mill (FR) with a roll diameter of 150 mm. A 15% reduction was used followed by a second heat treatment in air at 835°C for 140 h (sample YP01); (2) intermediate rolling by the ER [18] using 15% reduction followed by a second heat treatment in air at 835°C for 140 h (YP02); and (3) 2 intermediate rolling by the ER using 15% reduction for each intermediate reduction followed by a further 2 heat treatments in air at 835°C for 20 h and 835°C for 110 h (YP03).
Table 1 Intermediate rolling parameters and transport current densities of compared tapes sintered at 835°=190 h in air Tape no.
Intermediate rolling
Drolls (mm)
Ic (0 T, 77 K) (A)
Jc (0 T, 77 K) (A/cm2 )
Jc (1 T, Bex ==ab) (A/cm2 )
YP01 YP02 YP03
1FR 1ER 2ER
150 140/180 140/180
45.9 55.4 53.1
18 595 20 777 20 945
4290 5190 4517
P. Kovac et al. / Physica C 356 (2001) 53±61
The principle of the ER technique is that deformation occurs between the outer surface of an inner roll with radius R1 and the inner surface of outer hollow cylinder of inner radius R2 [17]. An ER machine with R1 140 mm and R2 180 mm was used in the presented experiment. The working surfaces of both rolls are conical to allow the input and output of the rolled tape. The reduction per pass (RPP) is provided by a variable shift of the rolls axis. The contact length is given by R1 =R2 ratio and by the RPP value. It is apparent that the contact length for an ER is approximately three times longer than that of a two classical rolls of radius R1 . Consequently, the deformation zone is also about three times larger and the angle of textured zone is more shallow. The transport current measurements have been done in self-®eld, in external magnetic ®eld of parallel and perpendicular orientation up to Bex 1 T, and at variable ®eld orientation between Bex ==ab and Bex ==c for Bex 0:1 and 0.5 T, all at 77 K. A standard four-probe set-up and the usual criterion of 1 lV/cm were used to estimate Jc values from measured I±V characteristics. The stress±strain curves and Jc degradation were measured by using a simple uniaxial tensioning system in which the strain value was measured by strain gauge glued directly to the centre of the tape sample (between potential taps). XRD analysis was carried out using a micro XRD technique which allows you to study the phase assemblage and texture of individual ®laments [21]. 3. Results and discussion The values presented in Table 1 show the transport current densities of two eccentrically rolled multi®lamentary Bi(2223)/Ag tapes in comparison to a standard rolling with comparable roller diameter. The measured self-®eld values represent a 20% improvement in Ic for tape YP02 and the highest Jc increase for tape YP03 (by 12.6%) in comparison with a ¯at rolled tape YP01. Fig. 1a shows the ®eld dependence of Jc for all three tapes in a parallel magnetic ®eld (Bex ==ab) up to 1 T and Fig. 1b their normalised values. Sample
55
YP02 (with one intermediate ER rolling) has the best Jc (B) performance in the low (Bex < 0:2 T) as well as in the high ®eld (Bex > 0:2 T) region. This Jc dierence for YP02 in comparison to YP01 can be explained by the depression of transverse cracks (long roll/tape contact length) and better intergrain connections due to higher ®lament density [17] produced by ER. Consequently, a lower decrease of Jc for Bex between 0 and 0.2 T and higher Jc values are observed for ER tapes. Tape YP03 has a higher Jc than YP01, however, the sharper Jc decrease in Bex < 0:2 T (see Fig. 1b) may be attributed to more transverse cracks introduced by double ER intermediate deformation which may not be healed fully during subsequent annealing due to not enough liquid phase being present. At Bex 1 T, the Jc values are by 21% higher for YP02 and by 5.3% for YP03 (see Table 1) in comparison to YP01. The plot of normalised critical current densities for both parallel and perpendicular ®eld orientation are shown by Fig. 2a. Only small Jc (B) differences among these three samples are visible here showing approximately the same 77% Jc decrease for Bex ==ab between 0 and 1 T, but more than two orders decrease for perpendicular ®eld (Bex ==c). The LorentzÕs force was calculated for both ®eld orientation and it is plotted in Fig. 2b. The maxims of FL are localised at the same values of magnetic ®eld: 0.15 T for Bex ==c and 0.8 T for Bex ==ab for FR and ER tapes. Only one maximum of FL for Bex ==ab for YP02 is shifted to 0.9 T which may indicate no apparent change in the pinning after ER deformation and thus to Jc increase explained by improved 2223 grains connection. In order to see more sensitive dierences in the Jc -anisotropy, the angular dependence of Jc was measured. The anisotropy coecients were estimated from measured angular curves as ka Jc 90=Jc 0. Fig. 3 shows Jc (angle) dependence for Bex 0:1 and 0.5 T. Nearly parallel curves are observed for 0.1 T with anisotropy coecient ka ranging from 2.26 for the lowest Jc sample YP01 to 2.44 for the best one YP02 (see Fig. 3a and Table 2). The coecient ka re¯ects the texture of 2223 grains together with the pinning in the ab plane and c-axis direction. At 0.1 T the coupling mechanism and the pinning mechanism
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P. Kovac et al. / Physica C 356 (2001) 53±61
Fig. 1. (a) The measured Jc (B) values for tapes YP01±YP03 in the parallel external magnetic ®eld Bex ==ab. (b) The normalised Jc (B) dependence for tapes YP01±YP03 in Bex ==ab.
act simultaneously, but at 0.5 T only pinning in strongly connected inter-granular current-carrying paths plays a role. Therefore, the way of transport current is dierent and Jc (angular) dependence
does not have the same tendencies for 0.1 and 0.5 T. At 0.5 T (Fig. 3b and Table 2), the highest Jc anisotropy is observed for YP01 and the lowest one for YP03. The decreased ka for YP03 could be
P. Kovac et al. / Physica C 356 (2001) 53±61
57
Fig. 2. (a) The normalised Jc (B) curves of samples YP01±YP03 for parallel and perpendicular external ®eld. (b) Log-lin dependence of the Lorentz force calculated for samples YP01±YP03 measured in Bex ==ab and Bex ==c.
explained by a double ER which leads to reduced texture at the BSCCO/Ag interface. The similar decrease of ka (0.5 T) was observed for 19 ®lament tapes [22] as well as for single cores [23] subjected to several thermo-mechanical steps. The increased
FWHM with the number of mechanical deformations has been presented as each intermediate deformation (rolling or pressing) leads to breaking of large 2223 grains and changes the well de®ned plate-like structure to a more wavy structure [23].
P. Kovac et al. / Physica C 356 (2001) 53±61
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Fig. 3. (a) Angular dependence of the transport critical current density measured at Bex 0:1 T. (b) Angular dependence of the transport critical current density measured at Bex 0:5 T.
Table 2 Texture of 2223 ®laments in the tape's length (L) and width (W) directions related to Jc anisotropy of compared tapes Tape no.
FWHML
FWHMW
ka /FWHML (0.1 T)
ka /FWHML (0.5 T)
ka (0.1 T)
ka (0.5 T)
YP01 YP02 YP03
9.0 8.0 8.5
12.3 14.5 14.0
0.37 833 0.30 375 0.27 294
3.0555 3.3375 2.6470
2.26 2.44 2.33
27.5 26.7 22.6
P. Kovac et al. / Physica C 356 (2001) 53±61
In this case, ER tapes have lower anisotropy ratio then FR one, which could be an advantage for application in coil winding where the radial ®eld component is the limiting factor in coil current. It is interesting that ER appears to reduce the texture in the tape's width direction, perpendicular
59
to transport current, but improves the texture along the tape's length, parallel to the transport current (see Table 2). This eect is probably due to the dierent roll contact length and deforming zone which in¯uence the strain in the tape's length and width directions. For ER (D1 =D2
Fig. 4. (a) The stress±strain curves for samples YP01±YP03 measured at 77K. (b) The dependence of normalised critical current densities versus tension strain.
P. Kovac et al. / Physica C 356 (2001) 53±61
60
140=180 mm), the strain in the tape's length is smaller than for FR (D 150 mm) due to the higher ``eective'' roller diameter. Consequently, the ``waving'' eect of the plate-like structure for ER tapes is lowered in the tapeÕs length, but increased in the tape's width. The double deformation by ER decreases the texture in the tape length, which should be decisive for the transport of current. The ratio of ka /FWHML for the tapeÕs length directions is also given in Table 2. This ratio is quite similar for the ®rst two samples subjected to only one intermediate rolling, but it is decreased more for two times rolled tape YP03. The stress±strain (r±e) curves of compared samples measured at 77 K are shown in Fig. 4a. It is apparent that tapes YP02 and YP03 have better mechanical properties due to the improved density of ®laments caused by ER. The elastic modulus (Ecom ) was estimated from the linear parts of r±e dependence. They range from 19.4 GPa for YP01 to 22.6 GPa for YP02 (see Table 3). Taking into account the not in¯uenced mechanical properties for silver and the same ®ll factor for all samples, the dierences in Ecom may be attributed only to dierent ®laments quality. This indicates the highest mechanical strength for ®laments of YP02 and lowest one for YP01. Fig. 4b shows how the normalised value of transport current is decreasing with increased tension. The most of the points measured for ER tapes lie above those measured for ¯at rolled tape YP01. The estimated slope of Jc ±e (by linear regression, for e < 0:2%) is the highest for YP01 tape and lowest one for YP03. Though the beginning of the sharp Jc degradation starts at eirr 0:21% for YP01 and also for YP02, eirr 0:22% was measured for YP03. More detailed analysis should be required for a more clear explanation of the above dierences, but, it is evident that ER may improve the electro-mechanical properties of multicore Bi2223/Ag tapes. Table 3 The elastic modulus and Jc degradation for YP01±YP03 Tape no.
Ecom (Gpa)
dJcn /de
eirr (%)
YP01 YP02 YP03
19.4 22.6 20.4
14:3 1:36 12:84 1:4 9:5 2:1
0.21 0.21 0.22
4. Conclusion The application of ER for intermediate deformation of commercial multicore Bi-2223/Ag tapes has been studied and compared to a standard 2-high rolling (FR) mill process. The Jc increase measured for ER tape was 12.6% in self-®eld but by 21% for the external ®eld Bex 1 T. The tapes subjected to ER show also better Jc (B) performance especially smaller Jc decrease in low ®elds (Bex < 0:2 T) as a consequence of reduced number of transverse cracks, which is a vital aspect for further Jc improvement by optimalisation of ER for intermediate deformation of multicore Bi-2223/Ag tapes. ER tapes have lower Jc -anisotropy ratio (at the external magnetic ®eld around 0.5 T) then FR one, which could be advantageous for the application in a coil winding where radial ®eld component is limiting the total coil current. Better mechanical properties and slightly increased resistance of the transport current degradation against tension strain have been observed for ER tapes.
Acknowledgements This work was supported by the Grant Agency of the Slovak Academy of Sciences VEGA 2/6056/ 99.
References [1] J.A. Parrell, S.E. Dorris, D.C. Larbalestier, Adv. Cryogenic Engng. 40 (1994) 193. [2] G. Grasso, A. Jeremie, R. Fl ukiger, Supercond. Sci. Technol. 8 (1995) 827. [3] G. Grasso, A. Perin, R. Fl ukiger, Physica C 250 (1995) 43. [4] P. Kovac, I. Husek, W. Pachla, T. Melõsek, V. Kliment, Supercond. Sci. Technol. 8 (1995) 341. [5] J.A. Parrell, A.A. Polyanskii, A.E. Pashitski, D.C. Larbalestier, Supercond. Sci. Technol. 9 (1996) 393. [6] Z. Yi, L. Law, S. Fisher, C. Beduz, Y. Yang, R.G. Scurlock, R. Ridle, Applied Superconduc. Conf., Edinburgh, IOP Publishing, vol. 1, 1995, p. 379. [7] W. Pachla, H. Marciniak, A. Szulc, M. Wroblewski, P. Kovac, I. Husek, T. Melisek, IEEE Trans. Appl. Supercond. 7 (1997) 2090.
P. Kovac et al. / Physica C 356 (2001) 53±61 [8] P. Kov ac, I. Husek, W. Pachla, Proceedings of EUCASÕ95 Conference Applied Superconductivity, IOP Publishing, vol. 1, 1995, p. 367. [9] P. Kov ac, I. Husek, L. Kopera, W. Pachla, Inst. Phys. Conf. Ser. No. 158 presented at Applied Superconductivity, The Netherlands, 30 June±3 July 1977, p. 1343. [10] P. Kov ac, I. Husek, W. Pachla, IEEE Trans. Appl. Supercond. 7 (1997) 2098. [11] P. Kov ac, I. Husek, W. Pachla, H. Marciniak, T. Melõsek, Physica C 261 (1996) 131. [12] Th. Schuster, H. Kuhn, A. Weishardt, H. Konm uller, B. Roas, O. Eibl, M. Leghassa, H.W. Neum uller, Appl. Phys. Lett. 69 (1996) 1954. [13] M.P. James, S.P. Ashworth, B.A. Glowacki, R. Garrre, S. Conti, Inst. Phys. Conf. Ser. No. 148 Applied Superconductivity, IOP Publishing, 1995, p. 343. [14] W. Pachla, Unipress, Warsaw, private communication. [15] M. Tomsic, Superconductor Industry, Spring 1997, pp. 18± 22.
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[16] F. Marti, G. Grasso, Y.B. Huang, R. Fl ukiger, Supercond. Sci. Technol. 8 (1998) 1251. [17] L. Kopera, P. Kovac, I. Husek, Supercond. Sci. Technol. 11 (1998) 433. [18] P. Kovac, I. Husek and L. Kopera, Appl. Superconduc. 1999 (2000) IOP Inst. Phys. Conf. Ser. No. 167, pp. 455± 458. [19] P. Skov-Hansen, Z. Han, R. Fl ukiger, F. Marti, P. Kovac, Appl. Superconductivity 1999, 2000 IOP Inst. Phys. Conf. Ser. No. 167, pp. 623±626. [20] L. Lelay, C.M. Friend, T.P. Beales, Cryogenics 37 (1997) 583±587. [21] Y.B. Huang, J. Juston, D.M. Spiller, Supercond. Sci. Tech. 13 (2000) 1011. [22] P. Kovac, I. Husek, A. Rosova, W. Pachla, Physica C 312 (1999) 179. [23] P. Kovac, C.J. Eastel, W. Pachla, I. Husek, H. Marciniak, C.R.M. Grovenor, M.J. Goringe, Physica C 292 (1997) 322.
www.elsevier.nl/locate/physc
The eect of intermediate deformation by eccentric rolling on the Jc (B) performance of multicore Bi-2223/Ag tapes P. Kov ac a,*, Y.B. Huang b, I. Husek a, D.M. Spiller b, P. Bukva a a
Institute of Electrical Engineering, Slovak Academy of Sciences, Dubravska cesta 9, 84239 Bratislava, Slovakia b BICC General Superconductors, Oak Road, Wrexham LL13 9PX, UK Received 16 October 2000; received in revised form 2 January 2001; accepted 5 January 2001
Abstract An eccentric rolling (ER) technique has been applied during the intermediate deformation of commercial multicore Bi-2223/Ag tapes. The Jc (B) performance of the tapes deformed by ER and standard 2-high rolling technique (FR) have been compared. It was shown, that at 77 K, the self-®eld Jc can be increased by 12.6% and Jc (1 T) by 21% if an optimum ER deformation is applied. The tapes subjected to ER show smaller Jc drop in low magnetic ®elds (Bex < 0:2 T) as a consequence of reduced number of transverse cracks and improved ®lament density. This is a vital aspect for further Jc improvements by an optimal ER intermediate deformation applied for multicore Bi-2223/Ag tapes. ER tapes also have lower Jc -anisotropy ratio at Bex 0:5 T than FR ones, which may be advantageous to HTS coil manufacturers for various applications. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Bi-2223/Ag tapes; Intermediate deformation; Critical current density; Lorentz force
1. Introduction The powder-in-tube (PIT) processes process requires mechanical deformation (pressing or rolling) combined with subsequent heat treatment cycles so that high current density (Jc ) can be achieved. The aim of the intermediate deformation step is to improve the core density and texture and break up the sintered structure and to facilitate a higher conversion of the precursor powder to 2223 [1]. An optimisation process of parameters for manufacturing mono-®lamentary Bi-2223/Ag tapes
* Corresponding author. Tel.: +421-7-5477-5823-2841; fax: +421-7-5477-5816. E-mail address: [email protected] (P. Kovac).
was done by Grasso et al. [2] showing the importance of deformation-induced texture in colddeformed tape [3]. A non-optimised deformation process resulted in cracks in the oxide core, which were dicult to heal fully during subsequent annealing [1,4,5]. The rates of phase conversion 2212 ) 2223 was measured [6,7] for dierent deformation techniques. The highest content of 2223 phase is usually found at the BSCCO/Ag interface and the lowest at the centre of the core. It is also found that the texture of the 2223 phase, phase assembly, core density distribution, and the distribution of cracks within the core are not uniform [8,9]. This leads to an inhomogeneous distribution of the transport current. It has been shown that the density distribution has a higher impact on Ic than
0921-4534/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 1 ) 0 0 1 2 3 - X
P. Kovac et al. / Physica C 356 (2001) 53±61
54
the density itself [8±10]. Very dense inhomogeneous tapes had low Ic values, and they required longer annealing periods to gain the same Ic as achieved in homogenous ones with lower density [11]. In the case of multi®lamentary Bi-2223/Ag tapes, the shape of ®laments depends also on its positions relative to the tape cross-section. The outer ®laments are usually close to ellipsoidal whereas the ®laments in the centre look like ¯at and wide dumbbells. Dierences in the degree of deformation of ®laments lead to dierences in their microstructure. A ®lament aspect ratio (b=a, widthto-thickness ratio) ranging from 12 at the tape edges up to 55 at the tape centre was presented by Schuster et al. [12]. They showed one order of magnitude dierence in current density at 77 K (from 8 103 A/cm2 , for lowest b=a 12, up to 8 104 A/cm2 , for highest b=a 55). A similar ®lament aspect ratio distribution is observed for most of Bi(2223)/Ag tapes produced by OPIT when drawing and subsequent ¯at rolling are used. In order to reduce the formation of transverse cracks in multicore Bi-2223/Ag tapes has lead to application of continual pressing [13±16] and a new rolling technique, so-called eccentric rolling (ER) [17]. It has been shown that ER results in an increase in BSCCO core density and improved texture of Bi-2212 and 2223 grains caused by a higher shearing force to the powder through the Ag sheath than classical rolling with the comparable roll radius. Consequently, much higher transport current densities (similar to press-sintered tapes) can be reached. The eccentrically rolled samples had a signi®cantly reduced number of transverse cracks (shown by magneto-optical images), which is a vital aspect for increasing the critical current density. Consequently, 1.25± 3.11 times higher transport current densities were reached [18]. Several deforming methods (contin-
uous pressing, rolling with large diameters and ER) have been applied by Skov-Hansen et al. for multicore Bi-2223/Ag tape [19]. The process parameters such as deformation zone size, volumetric strain, reduction ratio and friction parameters were compared. They have shown that there is an optimum in the logarithmic strain ratio region close to ln jew =et j 2 where the largest increase of critical current density in the ®nal heat treatment is located. If this is the case it means that width and thickness strains are very important parameters in the intermediate pressing process. The aim of this contribution is to show the effect of intermediate deformation by ER on the Jc (B) and Jc (e) performance of commercial multicore Bi-2223/Ag tapes. 2. Experimental A Bi-2223/Ag multi®lamentary tape was fabricated by the standard PIT method [20]. The tape was made from commercial Bi-2223 precursor powder and the ®nal ®ll factor (BSCCO/Ag) was estimated at 27%. An Ag sheathed 37-®lament tape was cold rolled to 0:3 3:0 mm2 in crosssection size and heat-treated in air at 835°C for 40 h. After this heat treatment, three kinds of samples (see Table 1) were prepared: (1) intermediate rolling using a standard 2-high rolling mill (FR) with a roll diameter of 150 mm. A 15% reduction was used followed by a second heat treatment in air at 835°C for 140 h (sample YP01); (2) intermediate rolling by the ER [18] using 15% reduction followed by a second heat treatment in air at 835°C for 140 h (YP02); and (3) 2 intermediate rolling by the ER using 15% reduction for each intermediate reduction followed by a further 2 heat treatments in air at 835°C for 20 h and 835°C for 110 h (YP03).
Table 1 Intermediate rolling parameters and transport current densities of compared tapes sintered at 835°=190 h in air Tape no.
Intermediate rolling
Drolls (mm)
Ic (0 T, 77 K) (A)
Jc (0 T, 77 K) (A/cm2 )
Jc (1 T, Bex ==ab) (A/cm2 )
YP01 YP02 YP03
1FR 1ER 2ER
150 140/180 140/180
45.9 55.4 53.1
18 595 20 777 20 945
4290 5190 4517
P. Kovac et al. / Physica C 356 (2001) 53±61
The principle of the ER technique is that deformation occurs between the outer surface of an inner roll with radius R1 and the inner surface of outer hollow cylinder of inner radius R2 [17]. An ER machine with R1 140 mm and R2 180 mm was used in the presented experiment. The working surfaces of both rolls are conical to allow the input and output of the rolled tape. The reduction per pass (RPP) is provided by a variable shift of the rolls axis. The contact length is given by R1 =R2 ratio and by the RPP value. It is apparent that the contact length for an ER is approximately three times longer than that of a two classical rolls of radius R1 . Consequently, the deformation zone is also about three times larger and the angle of textured zone is more shallow. The transport current measurements have been done in self-®eld, in external magnetic ®eld of parallel and perpendicular orientation up to Bex 1 T, and at variable ®eld orientation between Bex ==ab and Bex ==c for Bex 0:1 and 0.5 T, all at 77 K. A standard four-probe set-up and the usual criterion of 1 lV/cm were used to estimate Jc values from measured I±V characteristics. The stress±strain curves and Jc degradation were measured by using a simple uniaxial tensioning system in which the strain value was measured by strain gauge glued directly to the centre of the tape sample (between potential taps). XRD analysis was carried out using a micro XRD technique which allows you to study the phase assemblage and texture of individual ®laments [21]. 3. Results and discussion The values presented in Table 1 show the transport current densities of two eccentrically rolled multi®lamentary Bi(2223)/Ag tapes in comparison to a standard rolling with comparable roller diameter. The measured self-®eld values represent a 20% improvement in Ic for tape YP02 and the highest Jc increase for tape YP03 (by 12.6%) in comparison with a ¯at rolled tape YP01. Fig. 1a shows the ®eld dependence of Jc for all three tapes in a parallel magnetic ®eld (Bex ==ab) up to 1 T and Fig. 1b their normalised values. Sample
55
YP02 (with one intermediate ER rolling) has the best Jc (B) performance in the low (Bex < 0:2 T) as well as in the high ®eld (Bex > 0:2 T) region. This Jc dierence for YP02 in comparison to YP01 can be explained by the depression of transverse cracks (long roll/tape contact length) and better intergrain connections due to higher ®lament density [17] produced by ER. Consequently, a lower decrease of Jc for Bex between 0 and 0.2 T and higher Jc values are observed for ER tapes. Tape YP03 has a higher Jc than YP01, however, the sharper Jc decrease in Bex < 0:2 T (see Fig. 1b) may be attributed to more transverse cracks introduced by double ER intermediate deformation which may not be healed fully during subsequent annealing due to not enough liquid phase being present. At Bex 1 T, the Jc values are by 21% higher for YP02 and by 5.3% for YP03 (see Table 1) in comparison to YP01. The plot of normalised critical current densities for both parallel and perpendicular ®eld orientation are shown by Fig. 2a. Only small Jc (B) differences among these three samples are visible here showing approximately the same 77% Jc decrease for Bex ==ab between 0 and 1 T, but more than two orders decrease for perpendicular ®eld (Bex ==c). The LorentzÕs force was calculated for both ®eld orientation and it is plotted in Fig. 2b. The maxims of FL are localised at the same values of magnetic ®eld: 0.15 T for Bex ==c and 0.8 T for Bex ==ab for FR and ER tapes. Only one maximum of FL for Bex ==ab for YP02 is shifted to 0.9 T which may indicate no apparent change in the pinning after ER deformation and thus to Jc increase explained by improved 2223 grains connection. In order to see more sensitive dierences in the Jc -anisotropy, the angular dependence of Jc was measured. The anisotropy coecients were estimated from measured angular curves as ka Jc 90=Jc 0. Fig. 3 shows Jc (angle) dependence for Bex 0:1 and 0.5 T. Nearly parallel curves are observed for 0.1 T with anisotropy coecient ka ranging from 2.26 for the lowest Jc sample YP01 to 2.44 for the best one YP02 (see Fig. 3a and Table 2). The coecient ka re¯ects the texture of 2223 grains together with the pinning in the ab plane and c-axis direction. At 0.1 T the coupling mechanism and the pinning mechanism
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P. Kovac et al. / Physica C 356 (2001) 53±61
Fig. 1. (a) The measured Jc (B) values for tapes YP01±YP03 in the parallel external magnetic ®eld Bex ==ab. (b) The normalised Jc (B) dependence for tapes YP01±YP03 in Bex ==ab.
act simultaneously, but at 0.5 T only pinning in strongly connected inter-granular current-carrying paths plays a role. Therefore, the way of transport current is dierent and Jc (angular) dependence
does not have the same tendencies for 0.1 and 0.5 T. At 0.5 T (Fig. 3b and Table 2), the highest Jc anisotropy is observed for YP01 and the lowest one for YP03. The decreased ka for YP03 could be
P. Kovac et al. / Physica C 356 (2001) 53±61
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Fig. 2. (a) The normalised Jc (B) curves of samples YP01±YP03 for parallel and perpendicular external ®eld. (b) Log-lin dependence of the Lorentz force calculated for samples YP01±YP03 measured in Bex ==ab and Bex ==c.
explained by a double ER which leads to reduced texture at the BSCCO/Ag interface. The similar decrease of ka (0.5 T) was observed for 19 ®lament tapes [22] as well as for single cores [23] subjected to several thermo-mechanical steps. The increased
FWHM with the number of mechanical deformations has been presented as each intermediate deformation (rolling or pressing) leads to breaking of large 2223 grains and changes the well de®ned plate-like structure to a more wavy structure [23].
P. Kovac et al. / Physica C 356 (2001) 53±61
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Fig. 3. (a) Angular dependence of the transport critical current density measured at Bex 0:1 T. (b) Angular dependence of the transport critical current density measured at Bex 0:5 T.
Table 2 Texture of 2223 ®laments in the tape's length (L) and width (W) directions related to Jc anisotropy of compared tapes Tape no.
FWHML
FWHMW
ka /FWHML (0.1 T)
ka /FWHML (0.5 T)
ka (0.1 T)
ka (0.5 T)
YP01 YP02 YP03
9.0 8.0 8.5
12.3 14.5 14.0
0.37 833 0.30 375 0.27 294
3.0555 3.3375 2.6470
2.26 2.44 2.33
27.5 26.7 22.6
P. Kovac et al. / Physica C 356 (2001) 53±61
In this case, ER tapes have lower anisotropy ratio then FR one, which could be an advantage for application in coil winding where the radial ®eld component is the limiting factor in coil current. It is interesting that ER appears to reduce the texture in the tape's width direction, perpendicular
59
to transport current, but improves the texture along the tape's length, parallel to the transport current (see Table 2). This eect is probably due to the dierent roll contact length and deforming zone which in¯uence the strain in the tape's length and width directions. For ER (D1 =D2
Fig. 4. (a) The stress±strain curves for samples YP01±YP03 measured at 77K. (b) The dependence of normalised critical current densities versus tension strain.
P. Kovac et al. / Physica C 356 (2001) 53±61
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140=180 mm), the strain in the tape's length is smaller than for FR (D 150 mm) due to the higher ``eective'' roller diameter. Consequently, the ``waving'' eect of the plate-like structure for ER tapes is lowered in the tapeÕs length, but increased in the tape's width. The double deformation by ER decreases the texture in the tape length, which should be decisive for the transport of current. The ratio of ka /FWHML for the tapeÕs length directions is also given in Table 2. This ratio is quite similar for the ®rst two samples subjected to only one intermediate rolling, but it is decreased more for two times rolled tape YP03. The stress±strain (r±e) curves of compared samples measured at 77 K are shown in Fig. 4a. It is apparent that tapes YP02 and YP03 have better mechanical properties due to the improved density of ®laments caused by ER. The elastic modulus (Ecom ) was estimated from the linear parts of r±e dependence. They range from 19.4 GPa for YP01 to 22.6 GPa for YP02 (see Table 3). Taking into account the not in¯uenced mechanical properties for silver and the same ®ll factor for all samples, the dierences in Ecom may be attributed only to dierent ®laments quality. This indicates the highest mechanical strength for ®laments of YP02 and lowest one for YP01. Fig. 4b shows how the normalised value of transport current is decreasing with increased tension. The most of the points measured for ER tapes lie above those measured for ¯at rolled tape YP01. The estimated slope of Jc ±e (by linear regression, for e < 0:2%) is the highest for YP01 tape and lowest one for YP03. Though the beginning of the sharp Jc degradation starts at eirr 0:21% for YP01 and also for YP02, eirr 0:22% was measured for YP03. More detailed analysis should be required for a more clear explanation of the above dierences, but, it is evident that ER may improve the electro-mechanical properties of multicore Bi2223/Ag tapes. Table 3 The elastic modulus and Jc degradation for YP01±YP03 Tape no.
Ecom (Gpa)
dJcn /de
eirr (%)
YP01 YP02 YP03
19.4 22.6 20.4
14:3 1:36 12:84 1:4 9:5 2:1
0.21 0.21 0.22
4. Conclusion The application of ER for intermediate deformation of commercial multicore Bi-2223/Ag tapes has been studied and compared to a standard 2-high rolling (FR) mill process. The Jc increase measured for ER tape was 12.6% in self-®eld but by 21% for the external ®eld Bex 1 T. The tapes subjected to ER show also better Jc (B) performance especially smaller Jc decrease in low ®elds (Bex < 0:2 T) as a consequence of reduced number of transverse cracks, which is a vital aspect for further Jc improvement by optimalisation of ER for intermediate deformation of multicore Bi-2223/Ag tapes. ER tapes have lower Jc -anisotropy ratio (at the external magnetic ®eld around 0.5 T) then FR one, which could be advantageous for the application in a coil winding where radial ®eld component is limiting the total coil current. Better mechanical properties and slightly increased resistance of the transport current degradation against tension strain have been observed for ER tapes.
Acknowledgements This work was supported by the Grant Agency of the Slovak Academy of Sciences VEGA 2/6056/ 99.
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