(Bi,Pb)SrCaCuO superconducting fibres

Magnetic and electric transport properties of Ag/(Bi,Pb) - Sr - Ca - Cu - 0 superconducting fibres* A. Badia, Y.B. Huang, G.F. de la Fuente, M.T. Ruiz, L.A. Angurel, F. Lera, C. Rillo and R. Navarro Instituto de Ciencia de Materiales de Arag6n, CSIC-Universidad de Zaragoza, Centro Polit~cnico Superior de Ingenieros, Maria de Luna 3, Zaragoza E-5001 5, Spain Novel-textured Ag/(Bi,Pb) - S r - C a - Cu - O composite fibres have been grown by the laser floating zone method. Important parameters that have been studied include growing speed, silver content and post-annealing conditions. Polarized optical and scanning electron microscopy, a.c. and d.c. magnetic measurements and d.c. electric transport current have been used to characterize the final products. Silver addition has two main effects: it considerably improves the degree of texture within the fibre, and it lowers the melting point of the material. Moderate critical currents (Jc(77 K) = 100 A cm-2) have been obtained with agreement between transport and inductive values.

Keywords: high Tc superconductors; magnetic properties; transport critical current

Large-scale applications of high-temperature superconductors (HTS) require the development of long conductors with substantial critical current densities (Jc) at 77 K and under large applied fields (B). Present efforts in the accomplishment of new ceramic HTS materials compete against two main limits: weak links and ineffective flux pinning ~. A high degree of texture and the proper morphology to achieve good current transfer between grains in a continuous processing technology is the first goal of all new attempts. Until now, the most promising progress towards practical long conductors has been achieved with BSCCO-Ag tapes 2. At temperatures above 30 K, however, the highly anisotropic nature of Bi-HTS phases seems to produce a dramatic decrease of Jc(B) as the field increases. The so-called irreversibility line H~rr(T), characteristic of HTS materials, would limit the region of effective pinning and of non-resistive electrical transport at fields much lower than the upper critical field Hc2(T). The reasons for such behaviour of BSCCO-Ag tapes are still unknown, and intense worldwide studies are being devoted to this subject. To enhance the mechanical properties, providing thermal stability and avoiding chemical reactivity of the HTS with the atmosphere, commercially useful conductors are likely to be metal- HTS ceramic composites. In addition, such heterogeneous mixtures would also be required to produce low-resistance contacts in

* Paper presented at the conference 'Critical Currents in High Tc Superconductors', 2 2 - 24 April 1992, Vienna, Austria

H T S - H T S connections and in junctions with normal metals. Noble metals, and silver in particular, have provided good results in a variety of textured HTS composites without degradation of their properties including Ag/YBCO melt-textured 3 or Ag fibre-processed YBCO 4"5 and Ag/BSCCO wires and tapes 2"6'7 A new approach to obtaining Ag/BSCCO highly textured composites from liquid-phase processing is provided by the laser floating zone (LFZ) 8 method which has already been used successfully with pure B i - H T S ceramics 9~°. LFZ allows the growth of composite Ag/(Bi,Pb)SrCaCuO superconducting fibres from appropriate molten polycrystalline precursors 't. They melt congruently but solidify separately into an intimate mixture of two different constituents, Ag and BSCCO related phases, whereas the final product remains highly textured. In this paper we present results obtained on a series of Ag/BSCCO composite fibres grown by the LFZ method. Several parameters have been considered of interest for their influence in the superconducting properties of the material: Ag content, fibre growth rate and post-annealing conditions. Macroscopic physical properties studies by means of a.c. and d.c. magnetic techniques and d.c. electrical transport measurements have been combined with microstructural studies performed with polarized optical and scanning electron microscopes in order to characterize the samples. Finally, the importance of Ag addition towards the growth of the 2223 phase and the pinning mechanisms is discussed, comparing the results presented here with those from previous studies in other related systems 6,7.~2.

0011 - 2 2 7 5 / 9 2 / 1 1 0 9 6 9 - 0 6 © 1992 B u t t e r w o r t h - H e i n e m a n n Ltd

Cryogenics 1992 Vol 32, No 11

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Magnetic and electric transport properties: A. Badia et al.

Experimental and analysis Synthesis and fibre growth Powder precursors were obtained by conventional solidstate reaction of component powders based on the stoichiometry Bi2_xPbxSr2CaaCu60:.. Starting materials (% purity, supplier) included Bi203 (99.9, Aldrich), PbO (99.9+, Aldrich), SrCO3 (99.9, Johnson Matthey), CaCO3 (99, Aldrich) and CuO (98+, Aldrich). All these were ball-milled in an agate mortar for one hour. The calcination was made in air in three successive steps at 700°C for 6 h, 780°C for 14 h and 830°C for 24 h with intermediate hand grinding. For pure fibres, the calcined powder was reground and pressed into a rectangular cross-section bar for sintering in air typically at 830°C for 50 h. Precursor phase analysis was performed by powder X-ray diffraction (Cu Ko~ radiation), XRD, giving 2223 and 2212 as the major phases, together with CuO. Composite precursors were obtained by solid-state reaction of the above obtained superconducting powder with high-purity (99.99, Goodfellow) Ag powder. Both components (2246 and Ag) were ball-milled for 30 min, uniaxially pressed into a rectangular cross-section bar, and sintered in air at 830°C for 50 h. XRD analysis gives, in addition to the Ag, the 2212 as the main phase, with small amounts of 2223, CuO and Ca2PbO4. Sintered precursor bars were cut into square crosssection thinner bars used as feed and seed material for the LFZ growth. The LFZ processing was carried out in a recently built apparatus ~3 following a procedure described elsewhere 9. Different weight proportions of Ag up to 40% were used as well as various fibre growth speeds; a summary of the results is given in Table 1. Unless otherwise stated, all the silver content will be given in weight percentages. As has already been stated ~°, as-grown LFZ fibres contain mainly 2212 and 2201 phases and, in order to develop the high Tc phase (2223), require postannealing. In a first attempt we have treated all our samples at 830°C for 20 h, and only some of them display small amounts of 2223 phase. Additional annealing for 50 h at 840°C is needed to develop this phase, except for the 40% Ag fibre.

Electronic and optical microscopy The microstructure was observed with a scanning electron microscope (JEOL, JSM 6400) and with an optical metallographic microscope (Olympus MO2). Samples

were polished by conventional techniques (to 0.3/xm) for optical microscopy observation and for performing elemental analysis (EDS on SEM). A polished longitudinal cut of the 20% Ag fibre (sample D) is shown in Figure 1 as representative of the usual microstructure observed in most LFZ HTS fibres. Large, nearly parallel elongated BSCCO grains are observed well oriented along the fibre growth direction as seen under polarized light. These grains have been systematically observed to exhibit greater parallelism as the centre section of the fibre is approached. Near the edge of the fibre, grains appear misoriented at an angle with respect to the fibre's growth axis. As observed in the micrograph of Figure 1, the silver tends to crystallize with a dendritic habit, sometimes extending along the direction perpendicular to the growth axis (owing to its high crystallization rate). The Ag dendrites form a pattern that also extends, without apparent connectivity, along the fibre's growth axis. This could be due to the predominant oriented solidification of the HTS grains forcing the solidifying silver into the directional spacing between them. Typical grain sizes of the HTS crystallites in pure fibres are 1 - 5 / z m in thickness, 2 0 - 5 0 #m in width and 200-300 #m in length ~4. A perpendicular cross-section of the same sample D is displayed in Figure 2a. A clear-cut distribution of the silver towards the edge of the fibre is observed. In fact this has been observed for all fibres containing less than 30% Ag. As a consequence, all these fibres contain an inner core which is almost silver-free. HTS alignment along the a - b direction, coincident with the fibre's growth axis, is not comparable to that observed in Figure 1 for the orthogonal direction. As can be observed in Figure 2a the HTS grains are aligned only within very small regions or domains which, however, display no long-range orientation along the fibre's crosssection. HTS rectangular-shaped channels with a dissimilar core, that seem to be extended along the fibre, are also observed. As the amount of silver increases above 20% a tendency for it to be more uniformly distributed along the fibre's cross-section is observed. In fact when a 40% Ag fibre (sample C), as shown Figure 2b, is analysed by optical microscopy, Ag grains are much larger in size than in the previous fibres, and they are distributed in an opposite pattern. Moreover a high silver concentration is observed towards the fibre's cross-section centre. The outer part of the fibre has been observed, by both optical and SEM microscopy, to be composed largely of the BSCCO phases.

Table 1 Composition and characteristics of the g r o w n fibre composites (first three rows) and appearance of minor 2223 phase after different annealing processes (fourth and fifth rows). Values of the transport critical current (Jc in A c m - 2 ) on samples with both annealing cycles

Sample

Ag (% wt)

Growth speed (ram h ~)

Diameter (mm)

2 2 2 3 phase (830°C x 20h)

2 2 2 3 phase (840°C x 5Oh)

A B C D E

0 20 40 20 20

15 15 15 10 5

1.8 1.7 1.45 1.2 1.5

no yes no yes yes

yes yes no yes yes

* A t fields of B = 0,1 T Jc = 0 . 4 A cm -2

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Cryogenics 1992 Vol 32, No 11

Jc(77 K) B = OT 8.6 23.8 0 70 18

Jc(70 K) B = 1T 4.92 4.69 O* 3.4 1.94

Magnetic and electric transport properties: A. Badia et al.

Figure 1 Polarized optical micrograph of a section parallel to the fibre growth axis with 20% Ag (sample D). The bar is parallel to the fibre growth axis and indicates a dimension of 50 #m

Magnetic measurements Magnetic measurements were performed in an automated multipurpose cryostat described elsewhere I~. In-phase (X') and out-of-phase (X") components of the a.c. susceptibility were measured at field amplitude h0 = 1 . 1 0 e , frequency v = 114 Hz and at temperatures ranging from 4.2 to 120 K. In order to contrast the results for the different samples directly, similar lengths were taken from the various fibres studied. This, together with the small differences detected in the fibre diameter (see Table 1), accounts for second-order corrections arising from differences in their demagnetizing factors. Values of the non-dimensional quantity 4~rpx vs

T, where p is the average density of the composite and X the in-phase susceptibility component in emu g-~, are displayed for comparison in Figure 3. Figure 3 clearly reveals the onset and values of diamagnetic signals from which we can infer the relative content of BSCCO 2223 ( T c = l l 0 K ) and 2212 (To = 80 K) phases in each sample. The data were taken on samples after annealing for 20 h at 830°C. A noticeable amount of the 2223 phase appears only for samples B, D and E, all containing 20% silver. Pure (sample A) and 40% Ag (sample C) fibres contain only the 2212 superconducting phase. The curves corresponding to samples A, B and E show near 80 K, where the phase coherence takes place in the whole sample at (Tcoh), similar steep changes (ATcoh) which are different from those observed for sample C. It seems that T~oh first increases with Ag content up to 20% and then decreases above this value. More significant is the increase of ATcoh at higher Ag content (40%). Moreover, both components of the a.c. susceptibility of sample D after the two annealing procedures of Table 1 have been measured as a function of the exciting a.c. field amplitude 0 < h0 < 7 . 5 0 e at constant temperature (77 K) and frequency (114 Hz) and at zero d.c. field. Figure 4 shows the X' and 9(" components vs h0. Whereas x'(h0) follows the expected increasing trend with h0, the X"(ho) curve, in addition to an anticipated maximum at h o = 4 - 5 0 e , displays a shoulder at lower fields. This is probably caused by the presence of the 2223 phase. The applied ho fields are small enough to ensure that there is no penetration into the grains and that intergranular currents on the sample's surface partially shield its inner volume. This measurement may thus be used to compute inductive intergranular critical currents Jc, through comparisons with critical-state model (CSM) predictions. Zero-field-cooled (ZFC) and field-cooled (FC) d.c. magnetizations (M) were also measured to study flux pinning and trapping in the sample. Mz~c(T) and MFc(T) curves for the samples (A to E) were recorded from 4.2 to 120 K in an applied d.c. field H = 50 Oe and are represented in Figure 5. In addition, for sample A, we have measured the remanent magnetization 0.2 0

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Figure 3 Scaled in-phase susceptibility for fibres with different Ag content after one annealing for 20 h at 830°C. The nominal density of the composite fibre has been used

Cryogenics 1992 Vol 32, No 11

971

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M,em(T), defined as the magnetization of the fieldcooled sample when the applied field is turned off. In all cases, the magnetic field was applied along the growth axis of the fibre and consequently parallel to the a - b planes 16. By comparison of the d.c. magnetization results with the above a.c. susceptibility it can be simply deduced that the intergranular lower critical field (H~,) at 4.2 K is lower than 50 Oe. This is inferred from the fact that, for instance, in sample A, the value MzFc(4.2 K)/H ( = - 0 . 0 7 emu g-') is higher than the value of X'(4.2 K) ( - 0 . 0 8 emu g-t). Thus the magnetization measurements at the chosen d.c. field would depend on the HTS grains dispersed in the whole sample, and the correspondence would be established in terms of the real superconducting volume Vs~. This has determined the scale of Figure 5, where the experimental M(T) emu g - ' units have been converted to emu cm -3 only for the assumed HTS material. All the MFc(T) data coincide within experimental error, although those of sample C are systematically lower. The same trends, but outside the error bars, are observed in the MzFc(73 data at low temperature. Finally, although the modulus IMr~m(T) l is 25 % lower than IMzFc(T) I, it indicates the existence of an important flux trapping. 3

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(FC). In addition the remanent magnetization Mrem(T) obtained switching o f f the field at 4.2 K is depicted. Only one annealing of 20 h at 8 3 0 ° C w a s p e r f o r m e d on the s a m p l e s after

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Cryogenics 1992 Vol 32, No 11

The d.c. four-probe technique was applied for measurement of the transport critical current (Jc,) in the same multipurpose cryostat used for magnetic property determination. I - V characteristics were recorded at several temperatures (60 K, 70 K and 77 K) and in d.c. fields ranging from 0 to 5 T. Both the current and field were directed along the axis of the fibres. The critical current (It,) was determined through the usual I # V cm -~ criterion. To obtain Jc, from these values we have assumed a homogeneous distribution of Ag and the section occupied by silver was subtracted from the total cross-section of the fibre. Jc,(B,73 was measured for different annealing treatments. After the first cycle, the highest Jc value obtained at 77 K for the best sample was 1 A cm -2. This fact has to be related to the low or non-existent content of 2223 phase in our sample. A second annealing (840°C for 50 h) was performed on the samples in order to improve their properties. Figure 6 displays the results obtained at 77 K. J~ for sample C corresponds to 70 K because at 77 K it was normal even at zero field. Additionally, for this sample Jc values are lower after the second annealing indicating significant degradation of the existing 2223 phase for 40% Ag with this new annealing treatment. The zero field values, also shown in Table 1, are very sensitive to the presence of the 2223 phase; the higher values thus correspond to fibres B, D and E, which contain greater amounts of this phase. Generally, moderate Ag concentration ameliorates the critical current of the composite. Furthermore, similar J~(B) variations correspond to all fibres. Considering th'e 20% silver fibres an optimized route may be established with some uncertainties: lower growth speed and fibres should produce higher alignment in the crystallites and thus yield improved critical currents.

Discussion It should be noted that, at the lowest temperatures, the non-dimensional susceptibility 47rpx for sample A approaches the minimum expected theoretical value for homogeneous samples ( - 1 ) . For the composite fibres, however, lower values (up to 10%) are obtained. As the a.c. susceptibility at the applied exciting fields essentially depends on the surface currents they would reflect the sample's morphology. Accordingly, with the Ag distribution in the micrographs of sample B the superconducting shielding current would flow deeper than in sample A, reducing the shielded volume. (We have not considered the small a.c.-induced currents in the silver.) Since the Ag has a higher density than the BSCCO material, multiplying the experimental X' by the average density results in overestimations of the superconducting volume. The flux pinning and trapping aspects may be analysed using the M~c(T) and MZF~(T) measurements of Figure 5. Furthermore, the M,em(T) data for sample A reveal that a great amount of flux is trapped in the sample when the field is removed. We believe that this phenomenon is related to intergranular flux pinning. The channels parallel to the axis of the fibre which have been identified in the cross-section micrograph (Figure 2a) may

Magnetic and electric transport properties: A. Badia et al. be effective in trapping flux. The rough coincidence of the M(T) data for moderate Ag content points out that the pinning has not changed. In addition, the differences with the results obtained for sample C, which deviate from the others, could be caused by the change in morphology. Sample C has developed an outer shell of superconductors around a matrix of uniform Ag and BSCCO. This is why the pinning centres may be different. In the other cases the presence of similar morphology, although with more favourable crystallization and growing speed, may explain the observed coincidence. Inductive intergranular critical current (Jc,) was also computed for comparison with Jc,. Values for J~, on sample D were obtained using the CSM calculations t7 and the x'(ho) and X"(ho) results at 77 K. The predicted value for the critical current density is Jc, = 216 A cm--' compared to Jc, = 70 A cm -2. We believe that the difference is probably associated with some sort of shunting effect due to the presence of silver when the normal state is reached -~. Thus, if some current is carried by silver, we are underestimating Jc, with the 1 ~tV cm -~ criterion. Furthermore, CSM calculations also permit the evaluation of the fractional volume for superconducting grains in our sample. This is related to the value of the effective permeability/z. . . . . ic'7. Essentially, at 4.2 K the volume fraction of superconducting grains is measured by 1 - #c. . . . ic. In the present cases at 77 K, /~. . . . . ic = 0.35 is obtained. This value compares with the minimum possible value of 0.12 deduced from the Ag volume concentration.

Conclusions In previous studies of Ag/BiPbSrCaCuO composites 6 derived from Ag and HTS powders, two types of morphologies with an inversion in the silver-superconducting matrix at Ag volume of about 30% are observed. Our experiments qualitatively agree with such behaviour. When the Ag concentration increases up to 20%, the silver forms an outer shell from where it branches towards the centre of the sample. For the 40% Ag sample, a different morphology and physical behaviour is observed owing to the formation of a superconductor shell which surrounds a matrix of uniformly distributed silver and BSCCO phases. Jc is very sensitive to the Ag content and increases with increasing content of Ag until 2 0 - 3 0 % . Thus we conclude that the presence of moderate amounts of Ag increases pinning and that the deterioration of Jc observed at higher Ag concentration is due to the morphology changes which would imply different types of junction between the grains. Post-growth optimum annealing conditions have been shown to be related to the silver content of the composite. This can be associated to the change in the melting point of the superconductor with the silver content 7. Pure fibres need a second annealing (840°C × 5 0 h ) for developing the 2223 phase. Samples with 20% Ag had already developed a small amount of this phase at 830°C with 30 h of annealing. Finally, the sample with 40% Ag has already decomposed, owing to a pronounced reduction in its melting point. No relation can be established between the growth speed and the quality of the fibre for the range of

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samples studied. As seen in Figure 6 the best fibre is the one grown at the intermediate rate. We believe that differences arise from the importance of parameters uncontrolled in this study, such as the sample's diameter. In fact, our best sample has the smallest diameter. This agrees with the idea that thin fibres would exhibit optimum properties. All these conclusions can be used to concentrate our future efforts: the growth of small-diameter Ag composite fibres with optimized post-annealing conditions according to their silver content. Transport critical current, magnetic measurements, optical microscopy and SEM provide valuable consistent tools for the characterization of these materials.

Acknowledgements Financial support from Programme MIDAS (REEUNESA-CICYT, Projects 89-3797 and 90-642), ECC (Proj. SCI-0389-C) and Spanish CICYT (MAT 900362) is acknowledged. YBH acknowledges the Spanish Ministerio de Educaci6n y Ciencia for a foreign technologist grant.

References 1 Maley, M.P. J Appl Phys (1991) 70 6189-6193 2 Huang, Y.B., De la Fuente, G.F., Ruiz, M.T., Larrea, A., Badla, A., Lera, F., IRillo, C. and Navarre, R. Cryogenics (to be published) 3 Goyal, A., Funkenbusch, P.D., Kroeger, D.M. and Burns, S.J. Physica C (1991) 182 203-218 4 Tiernan, W.M. and Halleck, R.B. Physica B (1990) 165-166 1383- 1384 5 Tiernan, W.M. and Halleck, R.B. Phys Rev B (1991) 43 10508-10516 6 Savvides, N., Katsaros, A. and Dou, S.X. Physica C (1991) 179 361-368 7 Ullrich, M., Heinemann, K., Schaper, W. and Freyhardt, H.C. Supercond Sci Technol (1992) 5 $228-$231 8 Huang, Y.B., De la Fuente, G.F., Sotelo, A., Badla, A., Lera, F., Navarre, R., Rillo, C., lbdfid, R., Beltrdn, D., SapiNd, F. and Beltr~n, A. Physica C (1991) 185-189 2402 9 De la Fuente, G.F., Beltr~ln, D., Ib~tfiez, R., Martinez, E., Beltdn, A. and Segura, A. J Less-Common Met. (1989) 150, 253 -260 10 De la Fuente, G.F., Navarre, R., Lera, F., Rille, C., Bartelom~,

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Magnetic and electric transport properties: A. Badia et al.

11 12 13

14

J., Badla, A., Beltrlin, D., lbfifiez, R., Beltrdn, A. and Sinn, E., J Mater Res (1991) 6 699-703 De ia Fuente, G.F., Huang, Y.B., Badia, A., Lera, F., Rillo, C., Navarro, R., Ibiiilez, R., Beltriin, D. Adv Mater (submitted) Mukherjee, P.S., Simon, A., Koshy, J., Guruswamy, P. and Damodaran, A.D. Solid State Comman. (1990) 76 659-661 De la Fuente, G.F., Huang, Y., Ruiz, M.T., Sotelo, A., Lera, F., Rillo, C., Badia, A., Navarro, R., Bartolom~, J., Beltriin, D., Iblifiez, R., Sapifia, F. and Beltrfin, A. Bol Sac Esp Cerdlm Vidr (1991) 30 433-437 Snoeck, E., Larrea, A., Roucau, C., De la Fuente, G.F. and

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Huang, Y.B. Physica C (1992) 198 129-136 15 Rillo, C., Lera, F., Badia, A., Angurel, L.A., Bartolom~, J., Palacio, F., Navarro, R. and van Duynevelt, A.J. in: Magnetic Susceptibility of Superconductors and Other Spin Systems (Eds Hein, R.A., Francavilla, T.L. and Licbenberg, D.H.) Plenum Press, New York (1992) 16 Rillo, C., [,era, F., FIor|a, L.M., Navarro, R., Bartolom~, J., Ib~flez, R., Beltr~n, A., Beltrlin, D. and De la Fuente, G.F. Solid State Commun (1989) 72 1003-1008 17 Lera, F., Navarro, R., Rillo, C., Angurel, L.A., Badia, A. and Bartolom~, J. J Magn Magn Mat (1992) 104-107 615-616