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MATERIALS RESEARCH

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Al2 O3 additions for isothermal melt processing of Bi2 Sr2 CaCu2 Oy T. G. Holesinger Superconductivity Technology Center, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (Received 19 January 1996; accepted 20 April 1996)

It is shown that additions of Al2 O3 (1.0 wt. %) can significantly aid in the isothermal melt processing of Bi2 Sr2 CaCu2 Oy (Bi-2212). Al2 O3 additions provide a means for grain refinement of phases present in the partial melt. By limiting grain growth in the partial melt, solidification to form Bi-2212 proceeds more efficiently, resulting in fewer secondary phases and improved transport properties. Aluminum does not substitute into the superconducting phase to any appreciable extent and is present in fully processed material as small, secondary grains of approximate composition Sr22x Cax AlOy or Bi2 Sr42x Cax Al3 Oy . Al2 O3 additions were applied to the isothermal melt processing of Bi-2212 thick films and current leads.

I. INTRODUCTION

It has been found that melt-processing is beneficial, if not necessary, for producing practical conductors of Bi2 Sr2 CaCu2 Oy (Bi-2212).1–4 However, controlling the phase assemblage during and after partial melting can be difficult due to the incongruent melting of Bi-2212. During conventional melt processing of Bi-2212 in air, several alkaline-earth cuprate phases develop in the partial melt.5,6 Depending on the temperature, time spent in the melt, and oxygen partial pressure, these phases can grow to sufficient sizes that they become metastable and are not completely consumed on cooling through the peritectic to form Bi-2212.7 A similar problem arises during isothermal melt processing where the partial melt is formed by melting in an inert atmosphere.8–11 Depending on the temperature and time spent in the partial melt, large grains of CaO and Bi2 Sr32x Cax Oy (2 : 3 Cu-free) can form, which can lead to the development of other secondary phases in fully processed material. In a recent study, for example, large alkaline-earth cuprates, alkaline-earth bismuthates, and the low-temperature superconductor, Bi2 Sr22x Cax CuOy (Bi-2201), were found in thick films isothermally melt processed above 800 ±C.12 In either approach to the melt processing of Bi2212, the underlying problem is the uncontrolled growth of solid phases present in the partial melt. A large grain size affords a degree of metastability. This phase may then be only partially consumed during the latter stages of processing where the Bi-2212 phase is formed. Since the phases present in the partial melt, either during conventional or isothermal melt processing, are typically devoid of one or more elements [i.e., Sr12x Cax Cux O2 (1 : 1 AEC), Sr22x Cax CuO3 (2 : 1 AEC), 2 : 3 Cu-free, and/or CaO], remnants of phases from the partial melt result in a preferential removal of material J. Mater. Res., Vol. 11, No. 9, Sep 1996

from the system. From a mass balance perspective, the preferential loss of material inhibits Bi-2212 development and leads t