Influence of calcium substitution on the formation and thermal stability of the YBa2Cr4O8 superconductor

Abstract

In order to study the influence of calcium substitution on the thermal stability and phase composition of the YBa2Cu4O8 (Y-124) superconductor, precursor powders with composition Y:Ca:Ba:Cu = 1 - x : x : 2 : 4 (x = 0.0-0.1) are prepared using an acetate-tartrate based sol-gel method, The phase composition of the products formed after different stages of heat treatment at 800 degrees C and under 1 atm oxygen pressure is examined by X-ray powder diffraction (XRD) and thermogravimetric analysis (TGA). We found that in the first stage of the heat treatment, among stable mixed oxides and carbonate, mainly Y2Ba4Cu7Oy (Y-247) and CuO are formed. Upon further heating Y-247 can react with CuO to form Y-124, but can also decompose into Y-124 and YBa2Cu3O7 - y (Y-123). The formation of Y-123 is favoured by an increasing amount of calcium substitution. We also demonstrate that the TGA technique makes it possible to detect and distinguish between the presence of the Y-123, Y-124 and Y-247 when they are simultaneously present in a sample, while this is less obvious using only XRD. (C) 1999 Elsevier Science B.V, All rights reserved.