The microstructure study of Co-doped YBCO system

A series of YBa2CU3-xCoxOx (x = 0-0.50) samples have been studied by means of SEM, TEM, XRD and positron annihilation technology. Oxygen contents of the samples have been measured using a new volumetric method The microstructure evolution of the Co-doped YBCO is very similar to that in the Fe-doped YBCO. YBa2Cu3-xCoxOx undergoes a structure phase transition from orthorhombic to tetragonal between x = 0.12 and 0.15. Twin structure predominates when x is low (x < 0.05). With the increasing of x, tweed structure appears and its proportion increases till it becomes dominant. When x > 0 12, the tweed structure disappears completely. The grain boundary density increases with x and shows a sudden drop when x > 0.12. Similarly, the positron short lifetime component tau(1) shows a sudden drop between x = 0.12 and 0.15 Therefore, positron can be used as a sensitive probe for the O-T phase transition in this system. A possible model is proposed to describe the relationship between positron short lifetime component tau(1), and the concentrations of oxygen vacancy and twin (tweed) boundary. The experimental results can be satisfactorily explained by this model. (C) 2002 Elsevier Science B.V. All rights reserved.