Controlled partial melting during isobaric and isothermal processing of dipcoated Bi-2212/Ag tapes
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Controlled partial melting during isobaric and isothermal processing of dipcoated Bi-2212yyAg tapes A. L. Crossley and J. L. MacManus-Driscoll Centre for High Temperature Superconductivity, Imperial College of Science, Technology and Medicine, London SW7 2BZ, United Kingdom (Received 21 July 1997; accepted 5 February 1998)
A detailed study has been made of the control and optimization of partial melting of dipcoated Bi2 Sr2 Ca1 Cu2 O81d Ag0.1 (Bi-2212) tapes using reduced oxygen partial pressures. A coulometric titration technique has been employed to vary the oxygen partial pressure in a region of the phase diagram corresponding to binary melting, and the amount of partial melting has been quantified. Using this information, tapes have been processed using both isothermal and isobaric techniques. An optimum processing route was determined which combined isothermal and isobaric processes. Highly aligned material at the point of optimum melting was obtained.
I. INTRODUCTION
It has been found that melt processing is necessary for producing practical conductors of Bi2 Sr2 Ca1 Cu2 O81d on silver (Bi-2212). During melt processing the material is heated above the decomposition temperature of Bi-2212 in order to produce sufficient liquid such that upon solidification the Bi-2212 phase is dense. The phase assemblages during and after partial melting are complex since Bi-2212 melts incongruently. Conventional melt processing in air often results in large alkaline earth cuprate (AEC) phases and copper-free phases which can cause grain misalignment, obstruct current paths, and lower the critical current density (Jc ).1 One approach to improving phase purity and grain alignment has involved processing in different oxygen partial pressures. By choosing a region of the stability diagram that will produce secondary phases that do not grow as fast as the AEC phase in air, it may be possible to both minimize residual liquid and reduce grain misalignment. Oxygen partial pressure ( pO2 ) and its effects on the phase stability of Bi-2212yAg has previously been documented.2 The information has been used to melt process Bi-2212 at low pO2 , in a region of the phase diagram corresponding to binary melting as described by Eq. (1), where L is the liquid phase.3 Bi-2212 1 Ag ! Ls1Agd 1 sCa12x Srx d2 CuO3 1 O2 . (1) The following study will also be restricted to this region where the pO2 , 6 3 1023 atm and the temperature , 830 ±C. This region is preferable because only one precipitated crystalline phase, (Ca12x Srx )2 CuO3 , has been observed, and the phase grows more slowly than the secondary phases which form in air. Another possible advantage of processing in low pO2 is that the effect of “bubbling” in powder in tube tape (PIT) is reduced.4 3580
http://journals.cambridge.org
J. Mater. Res., Vol. 13, No. 12, Dec 1998
Downloaded: 14 Mar 2015
The universal feature of the Bi-2212 melt process is that the reaction kinetics below the peritectic of the precipitated phases are slow giving remna
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