Grain boundary misorientation is known to limit the critical current
density of bulk high temperature superconductors. However, the effect of
mechanical deformation on the texture evolution during powder-in-tube (PIT)
processing remains unclear, especially on the microstructural level. In the
present work, the micro- and meso-texture characteristics of PIT-processed
(Bi, Pb)2Sr2Ca2Cu3O10 (Bi2223) superconductor tapes were
investigated. The results for micro-texture show that a/b-axes texture does
exist in PIT-processed tapes. From the meso-texture studies, it was found that
a majority of the grain boundaries were formed by grains with a non-parallel
c-axis. These grain boundaries generally have low mismatch angles of up to
~10°. High-angle misorientation boundaries ranging up to
45° are generally c-axis twist boundaries. Furthermore, about 40% of
the grain boundaries could be coupled strongly.
The current investigation studies the phase transformation process of
Bi-Sr-Ca-Cu-O (BSCCO) tapes processed by cryogenic, 77 K and room temperature
pressing. The work specifically examined the effect of deformation on the
phase content and transformation kinetics of the BSCCO tapes. The results
showed that cryogenic pressing produced tapes with a higher Bi2223 phase content
at lower deformation ratios than room temperature pressed tapes. However, the
phase transformation profiles for both tapes were similar. A minimum phase
content was observed in the profiles that was characteristic to each process.
The minimum occurred between 30-40% deformation for the cryogenically pressed
tapes and between 50-60% for the room temperature pressed tapes. The peculiar
profiles were suggested to be the result of two competing mechanisms that
reduced the free energy of the systems. It was suggested that these two
mechanisms were either Bi2212 re-crystallization or Bi2223 phase formation.
The differences observed between the two processes were attributed to the
amount and way in which the deformation energy was transferred to the tape.
Measurements have been made of the resistivity ρ and of the thermoelectric power Q (vs Pt) of samples of AuCu in various nonequilibrium states. One set of samples was disordered by quenching from 750°C; and annealing curves were obtained at 100°, 149°, 198°, and 258°C. A second set was first ordered and then cold worked to essentially complete disorder, and annealing curves taken at 149°C and 258°C. Of the former set, the Q—ρ curves for 100° and 149°C showed a maximum at ρ≈11.5μ ohm cm while none was observed in the curves for any of the other cases. It follows from the detailed results that to specify the state of the alloy it is necessary to give, in addition to ρ and Q, at least one other quantity such as the coefficient of magnetoresistivity.
It was reported that the critical current density of superconducting tapes can be improved using cryogenic processing. The present paper studies the intrinsic grain alignment characteristics of cryogenically pressed and room-temperature-pressed Bi-Sr-Ca-Cu-O tapes. The grain alignment mechanisms of the two processed tapes were found to be different. The cryogenically pressed tapes showed a step-like increase in grain alignment at approximately 30% deformation, while the increase in grain alignment for the room-temperature-pressed tapes was gradual. The results indicate that the behaviour is reproducible and independent of annealing time and therefore, intrinsic to the deformation steps of each process. The effects of deformation on the alignment are suggested to occur due to two factors:(1) the point at which fracturing of the grains occurs and (2) rotation or movement of these grains into the preferred alignment. The differences observed between the two processes are due to the combined effects of these two factors. The significance of this study is that better grain alignment can be achieved in cryogenically pressed tapes at lower deformation ratios.
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