Kinetics and formation of (Pb-0.5,Cd-0.5)Sr-2(Y-0.6,Ca-0.4)Cu2O7-delta superconductor by directed reaction process

We report the formation of superconducting (Pb-0.5,Cd-0.5)Sr-2(Y-0.6,Ca-0.4 )Cu2O7-delta (1212) monophasic compound via ternary precursors. Monophasic superconducting 1212 compound has been synthesized by a directed reaction p rocess (DRP) at an ambient pressure. Starting materials used for the synthe sis of 1212 were monophasic SrCuO2, Sr(Pb-0.5,Cd-0.5)O-3 and (Y-0.6,Ca-0.4) (2)Cu2O5 prepared by standard solid state reaction method. The formation te mperature of these compounds was studied by employing a simultaneous therma l analyser namely simultaneous measurement of differential thermal analysis /thermogravimetric analysis. The activation energy (E-a) for the formation of SrCuO2 and Sr(Pb-0.5,Cd-0.5)O-3 phases were estimated to be 343 and 376 kcal/mol. The simultaneous thermal analysis (STA) thermogram for the monoph asic (Pb-0.5,Cd-0.5)Sr-2(Y-0.6,Ca-0.4)Cu2O7-delta compound shows less amoun t of weight loss (1.75%) compared to conventional method (14%) in which oxi des/carbonates were used as precursors. This clearly shows that the stabili ty of 1212 phase prepared from DRP is higher than that of the conventionall y prepared 1212 compound. From STA, an endothermic event was observed at 10 20 degreesC and it represents the formation of monophasic 1212 compound and it was further confirmed from X-ray diffraction studies. From Kissinger me thod, the activation energy for the formation of 1212 phase from directed r eaction process has been estimated to be 325 kcal/mol, whereas for conventi onal method it is estimated to be 410 kcal/mol. The highest T-c.onset of 50 K was observed for the material obtained from DRP and the T-c enhancement was 5 K above the conventionally prepared superconductor. (C) 2001 Elsevier Science B.V. All rights reserved.