TRANSMISSION ELECTRON-MICROSCOPY STUDY OF THE ENVIRONMENTAL DEGRADATION IN BA2YCU3O7-Y

A Ba2YCu3O7-y high Tc superconductor degrades with time in air. In this paper we discuss the early stage of the degradation from a microscopic standpoint by using transmission electron microscopy (TEM). It has been shown by a high resolution TEM that the degradation has two modes. One is the formation of amorphous and polycrystalline phases, the latter of which are perhaps BaCO3. It has been clarified that the origin of the formation of the amorphous and polycrystalline phases is the reaction of Ba2YCu3O7-y phase with the water vapor, and the polycrystalline phase is formed sequentially after the formation of the amorphous phase. The other is the formation of planar defects which are introduced between Ba-O and Ba-O layers even in dry air. It is thought that the planar defects are formed to relax the strain energy of the dislocation caused by the migration of ions.