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

In order to study the influence of calcium substitution on the thermal stab ility and phase composition of the YBa2Cu4O8 (Y-124) superconductor, precur sor powders with composition Y:Ca:Ba:Cu = 1 - x : x : 2 : 4 (x = 0.0-0.1) a re prepared using an acetate-tartrate based sol-gel method, The phase compo sition of the products formed after different stages of heat treatment at 8 00 degrees C and under 1 atm oxygen pressure is examined by X-ray powder di ffraction (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 c an react with CuO to form Y-124, but can also decompose into Y-124 and YBa2 Cu3O7 - y (Y-123). The formation of Y-123 is favoured by an increasing amou nt of calcium substitution. We also demonstrate that the TGA technique make s 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 thi s is less obvious using only XRD. (C) 1999 Elsevier Science B.V, All rights reserved.