Robust superconducting tapes based on Bi2Sr2Ca2Cu3O10−δQiang Li, J. E. Ostenson, M. M. Fang, and D. K. Finnemore Citation: Applied Physics Letters 59, 2895 (1991); doi: 10.1063/1.105844 View online: http://dx.doi.org/10.1063/1.105844 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/59/22?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Decomposition products of superconducting (Bi,Pb)2Sr2Ca2Cu3O10+δ J. Appl. Phys. 76, 1111 (1994); 10.1063/1.357831 Limiting factors for critical current densities in Bi2Sr2Ca2Cu3O10Ag composite superconducting tapes atelevated temperatures Appl. Phys. Lett. 63, 3515 (1993); 10.1063/1.110112 Anisotropic superconducting properties for caxis aligned powders of the (2223) compound(Bi,Pb)2Ca2Sr2Cu3O10+δ AIP Conf. Proc. 251, 467 (1992); 10.1063/1.42102 ac losses in powderintube Bi2Ca2Sr2Cu3O10 tapes at power frequencies Appl. Phys. Lett. 60, 252 (1992); 10.1063/1.106953 Critical currents in silversheathed (Bi,Pb)2Sr2Ca2Cu3O10− y superconducting tapes Appl. Phys. Lett. 59, 3171 (1991); 10.1063/1.105774

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Robust superconducting tapes based on Bi2Sr2Ca2Cu3010-~ Qiang Li, J. E. Ostenson, M. M. Fang, and D. K. Finnemore Ames Laboratory, USDOE and Department of Physics, Iowa State University, Ames, Iowa 50011

(Received 9 September 1991; accepted for publication 27 September 1991)

A rugged conductor based on the Bi,SrzCa,Cu,O,, _ 6 has been developed by dispersing Ag throughout the matrix on a micrometer scale. The Ag seems to effectively blunt crack tips and provides regions for plastic flow. The performance degrades relatively little with the strains of handling, soldering and quenching to 77 K. Tapes having a critical current density of 4,500 A/cm2 at 77 K and zero field will show over 20 000 A/cm2 at 4.2 K and zero field. At 20 T for the field perpendicular to the tape surface, these conductors show 2 800 A/cm2 at 4.2 K and 2 200 A/cm2 at 20 K. The critical current density falls roughly linearly with strain and decreases from about 22 000 A/cm2 at zero strain to about 10 600-A/cm” at 1.0% bending strain.

Monolithic conductors of Bi2Sr,Ca,Cu30,c _ fi Bi(2223), and Bi,Sr,Ca,Cu30s-s Bi(2212), both have rather limited strain tolerance. Ekin et al, ’ and Togano et aZ.2 find that strain beyond 0.2% exceeds the elastic limit and that there is very serious degradation of the critical current density J, for strain beyond this point. Some im- provement in this strain tolerance is desirable if these ma- terials are to be used to wind magnets.

Two methods have been used to improve the situation. Sato et al.” fabricated a multifilamentary conductor by stacking thousands of filaments about IO-pm thick in an Ag matrix and found far superior bending strain tolerance. In a rather different approach, Miller et al4 and Li et al5 dispersed Ag in the superconducting Bi(2212) matrix and found that a rather gradual decrease of J, all the way out to 2% bending strain. If these conductors are to be used widely, it is important to have easy methods of fabrication.

The dispersion of Ag in the superconducting matrix serves at least two purposes. First, it blunts crack tips and prevents catastrophic cracking; second, it also provides re- gions of plastic flow to relieve some the stress. To obtain a maximum effect, it is important to be able to control the location and distribution of the Ag. In this regard, Bi(2212) and Bi(2223) are quite different because the fab- rication process is not the same in the two cases. For Bi( 22 12), the melt processing at temperatures in the range from 860 to 880 “C! produces considerable liquid;415 for Bi(2223), the superconductor forms at 830 to 840 “C and rather little liquid forms. The purpose of the work reported here is to show that a fine dispersal of Ag in Bi(2223) yields a robust and strain tolerant conductor that is supe- rior in some ways to improved tapes based on Bi(2212).

Two types of conductor were prepared for the strain tolerance and high field performance studies. For the Bi( 2212) tapes, a precalcined powder of Bi( 2212) is mixed and ground together with 20% by volume of Ag. The Ag powder has a grain size of about 1 pm and the Bi(2212) powder has a-grain size less than 15 pm. The mixture is packed into an Ag tube having an outer diameter of 6.25 mm and an inner diameter of 4.35 mm. This composite is swaged and drawn to 1.1 mm outer diameter wire and then rolled to a tape with a thickness of 0.32 mm. Partial-melt-

processing was carried out in a flowing mixture of Nz and O2 in the ratio of 10 to 1 by ramping to 880 “C at 100 “C/h, ramping from 880 to 860 “C at 4 “C/h, holding at 860 “C for 48 h and then cooling to room temperature at 100 “C/h. For the Bi(2223) tapes, calcined powder of kd’bo.,,Srl.~, Ca,03Cu3,,0, and 20% by volume Ag are mixed and worked to tape in the same fashion as for Bi (22 12). The calcined powder had a very clean supercon- ducting transition at 107 K. The tapes were heat treated at 830 “C! for 80 h in a Nz to O2 mixture with a ratio of 10 to 1. The tapes were uniaxially pressed at 1 GPa and heat treated another 80 h at 830 “C in the same O2 partial pres- sure. Another uniaxial pressing at 1 GPa and a heat treat- ment of 830 “C in the same 0, partial pressure completed the thermomechanical treatment. The final thickness of the tape is 0.28 mm; the core thickness containing Bi(2223)/ Ag mixture is 0.16 mm with a cross section of 0.2 mm’. An optical microscope study of a polished transverse section through the tape after reaction shows that the Bi(2223) tape has a fine dispersion of Ag particles dispersed through the Bi(2223) material. The Ag particles are roughly equi- axed and 5 ,um across. A scanning electron microscope study shows that the Bi(2223) is in the usual plate-like structure but the texturing is not nearly as high as that reported by Sato et aZ.3 and Togano and co-workers.6 Transport J, measurements were made using samples about 25 mm long in a standard four probe configuration. The voltage leads are about 10 mm apart and a 1 yV/cm criterion was based to define JC These values are lower than the linear intercept method.” The core area including the 20% by volume Ag was used to calculate J,. Strain measurements were made by bending the sample around a mandrel and measuring the sample in the deformed con- dition. A 1% strain was achieved by bending the tape around an 8 mm radius mandrel. All measurements were made with the sample immersed in liquid He or liquid H,.

These tapes are not as highly textured as those re- ported by Sato et aZ.3 but J, values parallel to the tape face are consistently higher than those perpendicular to the tape face. As shown in Fig. 1, the tapes typically start at 22,000 A/cm2 at zero field and fall to 2,800 A/cm” at 20 T parallel to the tape surface (solid circles). For this orientation, an

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1021 0 4 8 12 16 20

FIG. 1. Comparison of J, values for magnetic fields both parallel and perpendicular to the tape surface at 4.2 K (solid symbols) and at 20 K (open symbols).

increase of the temperature to 20 K at 20 T decreases J, to only about 2,200 A/cm2 (open circles). For H 1 to the tape face, J, values at 4.2 K fall to 2,000 A/cm2 at about 12 T (solid squares); at 20 K, J, falls to 2000 A/cm2 at about 8 T (open squares).

With the application of bending strain, J, falls from about 20,000 A/cm2 at zero strain to about 10 000 A/cm2 at 1.0% strain or about a factor of 12 as shown by the solid. circles on Fig. 2. At 20 T, the corresponding data are a drop from 2,500 A/cm2 at zero strain to 1,000 A/cm2 at 1.0% strain. From the gradual way the J, falls with strain, there probably is a great deal of microcracking of the struc- ture but the Ag prevents catastrophic cracks and a sub- stantial supercurrent finds its way through the sample. It is important to remember that the sample has been unsol- dered and resoldered for every strain run and for every shift from 4.2 to 20 K. This involves considerable handling and thermal shock as the sample is dipped into liquid N2 to precool it and then dipped either into liquid He or liquid Hz for the measurement. Data taking at the National Mag- net Laboratory, as we do it, is not a gentle enterprise. By the time a sample is being measured at 1.0% strain, it has be soldered, unsoldered, and quenched about a dozen

102 0 0.2 0.4 0.6 0.8 I

Bending strain (%)

FIG. 3. J, surface for Bi(2223) measured at 4.2 K for (a) 41 tape face, 3 and (b) HI tape face. -

times. A summary of the data for one of the samples mea- sured at 4.2 K is shown on the three dimensional’ plot of Fig. 3 (a) where J, is plotted as a function of both magnetic field and strain for fields parallel to the tape surface. The lowest point in the surface at the front corner is 1,000 _ A/cm2 which corresponds to 20 T and 1.0% strain. A similar plot, for a different sample, and for the field per- _~ pendicular to the tapes surface is shown in Fig. 3(b). The field and strain dependence of J, for a sample of Bi(2212) i with 20 ~01% Ag is shown for comparison in Fig. 4. This surface corresponding to H parallel to the tape surface is _

FIG. 2. Strain dependence of J, for fields parallel and perpendicular the FIG. 4. J, surface for Bi(2212) at 4.2 K for H/I to the tape surface. The tape surface at 4.2 K. solid circles are data for H perpendicular to the tape surface.

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much flatter than for Bi(2223). The values of J, in zero strain fall only from 10,000 A/cm2 to 3,500 A/cm2 at 20 T. At 20 T, then J, falls only from 3500 to 2200 A/cm2 at 1.0% strain. After the measurements were completed at 1.0% strain for H parallel to the tape surface, the sample was rotated to have H perpendicular to the tape surface. These data are plotted as the solid circle in Fig. 4. For this configuration, J, falls from 5,200 A/cm” at zero field to 1,500 A/cm2 at 20 T. This performance is substantially better than the earlier studies4j5 that were made on samples with 30 ~01% Ag.

In summary, a new family of Bi(2223) composites ca- pable of withstanding considerable punishment has been developed. The basic idea of using a dispersion of Ag to blunt crack tips and provide regions of plastic flow seems to work as well for Bi(2223) as it does for Bi(2212).314 Tapes can be soldered and resoldered dozens of times with very little degradation. Tapes having a core that is 160qm thick can be bent around an 8 mm radius mandrel and still

2897 Appl. Phys. Lett., Vol. 59, No. 22, 25 November 1991

carry over- 1,000 A/cm2 at 20 T. These are very robust conductors.

Ames Laboratory is operated for the U.S. Department of Energy by Iowa State University under Contract No. W-7405ENG-82. This work was supported by the Super- conductor Technology Program and the Office of Basic Energy Sciences of the Department of Energy.

’ J. Ekin (private communication). ‘K. Togano, H. Kumakura, J. Kase, (2. Li, J. E. Ostenson, and D. K. Finnemore, J. Appl. Phys. 70, 6966 (1991).

‘K. Sato, T. Hikata, H. Mukai, M. Ueyama, N. Shibuta, T. Kate, T. Masuda, M. Nagata, K. Iwata, and T. Mitsui, IEEE Trans. Magn. MAG-27, 1231 (1991).

“T. A. Miller, J. E. Ostenson, Q. Li, A. Schwartzkopf, D. K. Finnemore, J. Righi, R. A. Gleixner, and D. Zeigler, Appl. Phys. Lett. 58, 2159 (1991).

5Q. Li, J. E. Ostenson, and D. K. Finnemore, J. Appl. Phys. 70, 4392 (1991).

“J. Kase, K. Tagano, H. Kumakura, D. R. Dietderich, N. Irisawa, T. Morimoto, and H. Maeda, Jpn. J. Appl. Phys. 29, L1096 (1990).,

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