The effect of cold deformation on the structure, texture and properties of yttrium high temperature superconducting powders and strips

Abstract

Cold deformation of YBa₂Cu₃O_{7 − x} (phase 123, T_c = 88.5–92 K) powders and strips causes partial decomposition of the 123 phase, a reduction in the degree of orthorhombicity of the structure up to almost complete degradation, and a decrease in T_c. When they are deformed, yttrium high temperature superconductors acquire basal (001) [110] texture with high pole density (13–15 arbitrary units), low scattering angle (±6°–7° from the normal direction), and a weak preference for a, b or a + b in the rolling direction; traces of (139) orientations may also be found. This texture is known to be favourable for increasing j_k. The combined effect of cold deformation and a carbon-containing binder leads, however, to a complete loss of superconductivity at 77 K or above. Depending on the regime of subsequent annealing, the following effects may be observed: degradation of the orthorhombic structure with a decrease in T_c; restoration of the orthorhombic structure and T_c of the 123 phase with complete or substantial loss of basal-type texture; change in texture type or retention of the basal texture (p_{α = 0}⁽⁰⁰⁵⁾ = 6–7 arbitrary units) with restoration of the orthorhombic lattice and T_c of the 123 phase. The appearance of a set of orientations from (139) to (001) in the deformation texture is evidence for the process of recrystallization grain growth. This suggests the appearance of plastic deformation of the ceramics (most probably of the shear type). With long annealing and thermocycling, the basal texture changes in accordance with the theory of compromise recrystallization grain growth up to the restoration of basal texture.