FORMATION AND STABILITY OF HGCAO2, A COMPETING PHASE IN THE SYNTHESISOF HG1-XREXBA2CA2CU3O8+DELTA SUPERCONDUCTOR

Thanks to a novel technique (thermobaric analyser or TEA) for measurin g the in-situ pressure in quartz tubes, we have investigated the forma tion and stability of HgCaO2 and its competition with the Hg,Re-1223 i n the superconductor synthesis. The pressure determination has enabled us to obtain thermodynamic values such as Delta H-f, Delta S-f, from which formation temperatures and stability of the binary oxide were de termined. This fact has made possible to minimise the presence of this impurity applying suitable heating and cooling ramps in the thermal t reatment leading to the formation of Hg,Re-1223. It has been observed that the faster the heating ramp, the less HgCaO2 phase was formed. Ho wever, the cooling ramp only influenced the Hg,Re-1223 stability, bein g more advantageous to perform slow cooling rates. In summary, the in- situ pressure measurement has permitted to ascertain the way to minimi se the HgCaO2 impurity content and hence to improve the quality of the ceramic Hg,Re-1223 superconductor. (C) 1998 Elsevier Science B.V. All rights reserved.