Microstructure of YBa2Cu3O6 + x sintered in air and its influence on melt-textured microstructure

The microstructure of YBa2Cu3O6 + x (Y123) sintered in air was investigated in the temperature range 900 °C–990 °C which is between the eutectic and peritectic temperatures of the Y-Ba-Cu-O system. It was found that a CuO-rich liquid phase formed above 930 °C owing to the barium loss during fabrication and eutectic melting. As a result of the formation of the liquid phase, the shape of a Y123 grain was changed from the plate-like form to a square form when the sintering temperature increased. The shape change of Y123 grains was explained by the formation of the liquid phases having different yttrium concentrations and the variation in lattice parameters of the unit cell with increasing temperature, which change the Y123-liquid interfacial energy. Furthermore, the formation of free CuO phase leads to the formation of Y2BaCuO5 (Y211) and the liquid phase. A non-uniform microstructure of a mixture of fine Y123 grain structure and the textured Y123 microstructure developed, which seems to be a result of a local peritectic reaction between Y211 and liquid phase. The non-uniform microstructure of the sintered Y123 sample significantly affects the melt-textured microstructure regarding Y211 distribution.