The microstructure study of Co-doped YBCO system

Abstract A series of YBa 2 Cu 3− x Co x O y ( 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. YBa 2 Cu 3− x Co x O y undergoes a structure phase transition from orthorhombic to tetragonal between x =0.12 and 0.15. Twin structure predominates when x is low ( x 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 τ 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 τ 1 and the concentrations of oxygen vacancy and twin (tweed) boundary. The experimental results can be satisfactorily explained by this model.