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T. L. Aselage, E. L. Venturini, J. A. Voigt, D. L. Lamppa, and S. B. Van Deusen Department 1153/MS 0345, Sandia National Laboratories, Albuquerque, New Mexico 87185-0345 (Received 7 March 1994; accepted 27 May 1994)

Stable conditions have been identified for two-zone processing of the superconducting thallium cuprate Tlo.5Pbo.5(Sro.8Bao.2)2Ca2Cu3Or With P(O 2 ) of 0.8 atm, P(T12O) of 4.4 X 1(T 3 atm, and a sample temperature of 920 °C, single-phase Tlo.5Pbo.5(Sr08Bao.2)2Ca2Cu30J, is produced with a Tc of 115 K, complete diamagnetic shielding, and Meissner fraction greater than 70%. Although a small amount of melting occurs under these conditions, a comparison of the low-field diamagnetic shielding for these samples with samples of Pb- and Sr-free TlBa2Ca2Cu3O>, and Tl2Ba2Ca2Cu3Oj, suggests that such melting is not necessary to produce the triple-CuO2 -layer superconductors.

The substitution of Pb for half of the Tl atoms within the Tl—O layers of the thallium superconductors was originally found to stabilize the (Tl05Pb0.5)Sr2CaCu2Oj: and (Tl05Pbo.5)Sr2Ca2Cu30>, phases.1 More recent magnetization studies2'3 have reported enhanced flux pinning at 77 K in samples of composition Tlo.5Pbo.5 (Sr0.8Ba0.2)2Ca2Cu3Oy that were prepared by partial melting and subsequent anneals. The superconducting transition temperatures of these samples were within the range 114-118 K.2^ These results have spurred substantial interest in Pb- and Sr-substituted Tl superconductors for bulk applications. Ag-clad tapes of Pb- and Sr-substituted samples have been prepared by powder-in-tube processes with lengths up to 24 m.4 The application of such tapes is limited, however, by a rapid decrease in transport critical currents with small increases in magnetic field.3'5 This phenomenon is believed to be caused by weak intergranular links resulting from a lack of alignment of superconducting grains. Grain alignment in thallium superconductors has been achieved6 through partial melting and free surface resolidification of thick films of TlBa 2 Ca 2 Cu 3 0 >) . The transport critical currents of the resulting films exhibited a relatively weak dependence on the applied magnetic field.7 These thick films were synthesized using a two-zone furnace. Such a system allows for the independent control of oxygen partial pressure, P(O2), and thallous oxide partial pressure, P(T12O), by heating the sample in one temperature zone of the furnace with an independent source of T12O vapor, usually T12O3, in a separate temperature zone. In quasi-closed systems, P(T12O) is nearly constant over the T12O3 source and the sample. A similar two-zone approach has not as yet been reported for the Pb- and Sr-substituted 1223 phase. In this communication, we describe conditions of sample temperature and T12O partial pressure under which Tl0.5Pbo.5(Sro.8Bao.2)2Ca2Cu30>, is stable for extended two-zone anneals in 0.8 atm of O 2 . The loss of 2470

http://journals.cambridge.org

J. Mater. Res., Vol. 9, No. 10, Oct 1994

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Pb by evaporation from the samp