Processing and Characterisation of Single-Phase Superconducting Thin Films in the Bi-Sr-Ca-Cu-O System from Chemical Pre
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PROCESSING AND CHARACTERISATION OF SINGLE-PHASE SUPERCONDUCTING THIN FILMS IN THE Bi-Sr-Ca-Cu-O SYSTEM FROM CHEMICAL PRECURSORS. S. J. GOLDEN*, T. E. BLOOMER*, F. F. LANGE*, A. M. SEGADAES*, K. J. VAIDYA* AND W. L. OLSON** * Materials Department, University of California, Santa Barbara, CA 93106 "**Superconductor Technologies, Inc., 460 Ward Drive, Suite F, Santa Barbara, CA 93111. ABSTRACT Thin films in the Bi-Sr-Ca-Cu-O system have been synthesized from liquid ethyl hexanoate precursors by spin pyrolysis. An extensive solid solution range was determined for the two Cu-layer phase through the study of c-axis oriented, singlephase thin films fabricated on single-crystal MgO (100). Extensive cation nonstoichiometry was observed in all cases. The variation of important thin film properties with composition within the solid solution range have been described. Utilising the results from the thin film study together with data relating to liquid formation in mixtures of bulk material a working model describing the formation of the two-layer phase has been formulated. The two-layer phase is formed as the result of precipitation from a fugitive liquid at temperatures exceeding 730 "C. INTRODUCTION One of the oxide systems discovered recently showing high temperature superconductivity is the Bi-Sr-Ca-Cu-O system [ 1, 2 ] . Although some aspects of the cation non-stoichiometry and cation deficiency in the Cu layer phases have been elucidated [ 3, 4, 5, 6 la systematic study of the extent of the solid solution range and the implications with regard to material properties has not been carried out. Other important issues that require investigation include a basic understanding of phase relations and formation relating to the superconducting phases. Studies concerning phase relations and phase equilibria in parts of the BiO 1 .5 - SrO - CaO - CuO system have been carried out [ 7 - 11 ];however certain key
issues remain unresolved such as low temperature liquid formation and its role in the formation of the superconducting phases. In the present work intimate mixtures of the starting components through the use of soluble ethyl hexanoate precursors of the various cations are utilised in the fabrication of two Cu-layer thin films of varying composition. A bulk study utilising mixtures of separate oxides and hydroxides was also carried out and useful comparisons were made both in terms of formation of the superconductive two-layer phase and low temperature liquid formation. EXPERIMENTAL Ethyl hexanoate precursors of Bi, Sr, Ca and Cu were mixed to form solutions containing various cation ratios. Thin films were fabricated on single crystal MgO( 100) using a spin-pyrolysis technique. A more detailed discussion of the fabrication process and compositional characterisation using Xray microanalysis (EDS) is described elsewhere [121. Other techniques used were Xray diffraction and four-point resistivity-temperature measurements. Bulk studies Mat. Res. Soc. Symp. Proc. Vol. 169. 91990 Materials Research Society
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