THE ORIGIN OF TEXTURE IN BULK SUPERCONDUCTORS FABRICATED BY THE MELT TEXTURE GROWTH-PROCESS

Melt texture growth YBCO bulk superconductors have been fabricated by making use of the nonzero spatial temperature gradients (VT) at certai n locations and with temporal variation (partial derivative T/partial derivative t) inside a common tube furnace. The texture of the YBa2Cu3 Oy (Y123) single crystal layers and Y2BaCuO5 (Y211) grains of the spec imen are investigated by TEM/EDX, SEM/EDX, XDP, chi-T and R-T measurem ents. The phase distribution results of Y123 and Y211 using EDX coveri ng an area of single crystal layer and Y211 grain indicate that the cr ystal layers are not ''true layers'' but ''bulk crystal with gap lines and planes''. The formation of gap lines and planes depends on how th e Y123 phase grows from the Y211 grain. If the Y211 grain can act as a nucleus of Y123 phase, ''gap plane'' can be formed when two Y123 crys tal blocks grow to touch each other. However, if the Y211 grain cannot take the role of a nucleus, it can only act as a perturbation and gen erate a gap line as the crystal growth front passes through the grain. Moreover, since the orientation of the crystal layers relative to the top and bottom surfaces of the bulk specimen depends on VT and partia l derivative T/partial derivative t, we can control the crystal layer orientation by varying the above two parameters using a simple tube fu rnace in older to produce high quality melt texture YBCO material.