THERMOANALYTICAL INVESTIGATION OF THE FORMATION OF YBA2CU3O6.5

The sol-gel process is a versatile technique generally used to obtain fine, homogeneous powders. The quality of the final product and the ki netics and mechanism by which compound formation takes place through t he sol-gel process is dependent on the process parameters, such as pH, water-to-salt molar ratio and temperature of hydrolysis. In the prese nt study, acetates of yttrium, barium and copper were employed to synt hesize the high-temperature superconductor YBa2Cu3O7 (123) by the sol- gel process. Differential thermal analysis (DTA), thermogravimetry (TG ) and X-ray diffraction (XRD) techniques were employed to study the ef fect of pH on the kinetics and mechanism of formation of 123. The form ation of 123 was found to take place either in a single step or in two steps, depending on the pH of the solution of the starting materials. The formation of 123 occurred in two steps in the samples prepared at pHs 6, 7 and 8, whereas only a single step of formation was observed in the samples prepared at pHs 6.5 and 7.5. To identify intermediate p roducts during the formation of 123, samples were quenched at respecti ve differential thermogravimetry (DTG) peak temperatures and character ized by XRD. Based on the kinetic analysis of the TG data, the mechani sm for formation of 123 was identified as nucleation and growth follow ing Avrami's kinetics. Samples prepared at pH 6 resulted in the format ion of pure 123 compound, whereas samples prepared at other pHs result ed in impurity phases such as Y2BaCuO5 (211) along with 123. The activ ation energy for the formation of 123 was evaluated as 223 kJ mol-1.