Diffusion grown superconducting thick films of Bi-(Pb)-Sr-Ca-Cu-O with zero resistance at 103 K

Physica C 158 (1989) 93-96 North-Holland, Amsterdam

D I F F U S I O N G R O W N S U P E R C O N D U C T I N G THICK F I L M S OF B i - ( P b ) - S r - C a - C u - O W I T H ZERO RESISTANCE AT 103 K S.X. DOU ~, H.K. LIU ~, A.J. BOURDILLON, N.X. TAN and C.C. SORRELL School of Materials Science and Engineering, University of New South Wales, P.O. Box I, Kensington, N S W 2033, Australia

Received 31 January 1989

Thick films of lead doped Bi-Sr-Ca-Cu-O have been grownwith zero resistance above 103 K. The films were grown on nonsuperconductingprecursorsubstrates. These substrates, whichhad the compositionsSrCaCu406and SrCaCu204, were coated by either sedimentation or silk printing with appropriate mixtures of Bi203and PbO. A diffusion treatment at 980°C or 1000°C followed. The films consist of partially oriented platelets 100 ~tmin length.

Although superconducting phases in B i - ( P b ) - S r C a - C u - O systems have been repeatedly observed, these materials demand extremely careful processing. In this paper, we report an easy and flexible diffusion growth technique for producing thick films of bulk superconducting B i - ( P b ) - S r - C a - C u - O with large amounts of high Tc phase. A zero resistance temperature (resistance < 1% of transition sigmoid) was achieved with novel heat treatments in air to overcome frustrating problems in the conventional processing of this material. The improvement is apparently due to the combined effect of (i) physical properties of the substrate, especially its density, (ii) to its variable chemical composition, Sr2_xCaxCu4+y and (iii) to the formation of a liquid phase of Bi203 or Bi203 + PbO at relatively low temperatures. It has been found that the B i - ( P b ) - S r - C a - C u - O materials, as normally processed, consist of superconducting phases, BiESr2CaCu208+y (2212) and BiESr2CaECU30lo+y (2223 ), together with some nonsuperconducting phases such as SrCaCu204 + z (0112 ) and SrCaCu406+z (0114) [ 1-3]. All of these phases allow large deviations from stoichiometry, especially on oxygen, due to the great flexibility of the perovskite structure [4]. 2212 and 2223 phases, when coexistent with the 0114 phase, exhibit high Ca and Cu contents, close to the stoichiometry; while the 2212 Visiting Professors from Northeast University of Technology Shenyang, P.R. of China

phase without co-existent 0112 or 0114 phases is often deficient in Ca and Cu. Thus 0112 and 0114 phases may be used as substrates for growing surface thick films by diffusion. In this paper we report the results of diffusion grown films with zero resistance at 103 K in B i - ( P b ) - S r - C a - C u - O systems. Substrates of 0112 and 0114 were prepared from appropriate mixtures of SrCO3, CaCO3 and CuO by normal powder metallurgy procedures: mixing; calcining at 930°C for 40 h with intermediate grinding; mechanically pressing into pellets, and isostatically pressing at 200 MPa; and finally sintering in air for 12 h at 980°C for 0112 substrates or at 1000°C for 0114 substrates. The sintered materials were examined in a JEOL JSM840 scanning electron microsocope (SEM) and a JEOL 2000FX transmission electron microscope, both equipped with Link Systems energy dispersive spectrometers, and in a Philips type PW 1140/00 powder diffractometer with CuK radiation. The concentration of Cu 3+ ions in the materials was determined by the volumetric measurement technique developed by us [ 5 ] where the evolution of 02 gas by (0112) or (0114) in dilute hydrochloric acid is used to measure the amount of Cu 3+ and hence the oxygen content (i.e. by departure from normal stoichiometry) in the materials. SEM backscattered electron images showed that the (0112) product is a single phase with a typical sintered microstructure corresponding to a density

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S.X. Dou et al. / Diffusion grown superconductingfilms of Bi-(Pb)-Sr-Ca-Cu-O

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of about 90% of the theoretical value. The X-ray powder diffraction pattern is shown in fig. 1. The pattern can be indexed to a triclinic structure with lattice parameters a=0.344 nm, b=0.285 nm, c = 0.831, a = 9 7 . 8 °, fl= 101.5 ° and 7=90 °. The peak at 38.7 ° can be simulated by a change in p. The Cu 3+ concentration was determined to be 1.1% for (0112) and 3.7% of total Cu for (0114) respectively. Thus the exact formula is SrCaCu204+O.Ol] and SrCaCu406+o.075.

Superconducting films can be grown by diffusion on these substrates as follows. A mixture of PbO and

BiO~5 in a molecular ratio of 1:3 was ground and calcined at 700°C to form a melt. The solidified product was ground and made into a slurry with polyethanol as solvent. The film was formed either by silk printing or by sedimentation of the PbO-BiOL5 slurry (or alternatively of a BiOl.5 slurry) on the polished surface of0112 or 0114 substrates. After drying in the oven at 180°C for 6 h the samples were sintered at 800-850°C for between 10 and 40 h in either air or in air enriched in oxygen. The electrical resistivity was measured by the standard four-probe DC technique. Fig. 2 shows a typical temperature dependent DC voltage drop (as a measure of the resistance) for the diffused films on 0112 substrates. It was observed that the diffused films of Bi-Sr-Ca-Cu-O systems exhibit two transitions with the larger DC voltage drops at the higher transition temperature, 110 K while bulk materials made under the same conditions hardly showed the 110 K transition. This contrast indicates that substrates of 0112 or 0114 promote the high Tc phase. Furthermore B i - ( P b ) - S r - C a - C u - O systems have only one transition at 110 K with a zero resistance temperature at 103 K. Fig. 3 shows the morphology of the superconducting thick film on a 0112 substrate. The superconducting phase consists of large plate-like crystals. SEM analysis on unpolished specimens suggests a phase assemblage consisting of {2223} as a major component with smaller portions of {2212}. This is consistent with Tc measurements.

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S.X. Dou et al. / Diffusion grown superconductingfilms of Bi-(Pb)-Sr-Ca-Cu-O

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Fig. 3. The Morphologyof Bi2.sSrL4CaL~Cu3.20~o÷~film on 0112 substrate showing (a) backscatteredelectron image of the top surface, and (b) backscatteredelectron image of the cross section of 0112 substrate (grey) and superconducting phase (white). The processing of the superconducting diffused films contrasts with that for bulk material: in the latter case, careful processing is required including a long period of heat treatment (several hundred hours), low oxygen partial pressure and a narrow range of sintering temperatures (within a few degrees) due to the slow formation of the 2223 phase. In the diffused growth technique an easy and quick route produces the high Tc ( 1 l0 K) superconducting phase. The mechanism for the enhancement o f the high Tc phase is the combined effect of the following features in the processing: (i) 0112 and 0114 have melting points approximately at 1000 and 1020°C respectively, and can be prepared by sintering at

temperatures close to respective melting points which are much higher than the melting point of the superconducting phase, between 900 and 910°C depending on composition. Thus the dense substrates obtained are expected to improve the density of the superconducting phase which is usually very porous for Bi-system materials; (ii) Bi203 and PbO printed onto the substrate have melt at 819°C and 880°C [6 ]. The heat treatment temperature was set above the melting points. Thus a thin liquid phase was formed, and reaction between the liquid phase and the solid substrate took place through dissolution and interdiffusion resulting in a dense superconducting phase. Equilibrium was attained much more quickly

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s.x. Dou et al. / Diffusion grown superconductingfilms of Bi-(Pb)-Sr-Ca-Cu-O

than in solid state sintering. M o r e o v e r the adhesion between substrate a n d superconducting phase is excellent due to the interdiffusion (fig. 3). (iii) Since the 0112 a n d 0114 substrates have large ranges o f n o n s t o i c h i o m e t r y the c o m p o s i t i o n o f the superconducting phase can be adjusted by varying the composition o f the substrates. F o r example the Ca content, which is often deficient in 2212 or 2223 phases can be enriched by increasing the Ca content in the substrates. Finally the diffusion growth technique for making a Bi-(Pb)-Sr-Ca-Cu-O superconductor has imp o r t a n t technical implications. It can be used for processing thick films on bulk materials with high density a n d adjustable composition.

Acknowledgements We are grateful to Metal Manufactures Ltd. for

support (S.X.D.) a n d also to the C o m m o n w e a l t h D e p a r t m e n t o f Industry, Technology a n d C o m m e r c e (H.K.L).

References [ 1] H.K. Liu, S.X. Dou, A.J. Bourdillon, N. Sawides and C.C. Sorrell, Supercond. Sci. and Tech. 1 (1988) 78. [2] S.I. Shah, G.A. Jones and M.A. Subromanian, Appl. Phys. Lett. 33 (1988) 429. [3] A.F. Marshall, B. Oh, S. Spielman, M. Lee, C.B. Iom, R.W. Barton, R.H. Hammond, A. Kepitulnik, M.R. Bessley and T.H. Geballe, Appl. Phys. Lett. 53 (1988) 416. [4] B. Raveau and C. Michel, Proc. of 1st Intern. Symp. on Superconductivity Aug. 28-31, 1988 (in press). [ 5 ] S.X. Dou, H.K. Liu, A.J. Bourdillon, J.P. Zhou, N.. Sawides and C.C. Sorrell. Solid State Comm. 68 (1988) 221. [6] G.V. Samsonov, The Oxide Handbook (IFI/Plenum, New York, 1973) p.105.