Abstract:
It is believed that the textured microstructure of the superconductor strongly affects the
critical current density of the (Bi, Pb)2Sr2Ca2Cu3O10+χ (Bi2223) tapes: a higher degree of
texture generally results in a higher critical current density. Mechanical deformation
and thermal treatment are the essential processes in the fabrication of industrial scale
BSCCO superconductors with high critical current densities. In the present work, the
effects of mechanical deformation and thermal treatment processes on microstructures
of BSCCO superconductors has been investigated in detail. It includes: (1) the effects
of mechanical deformation and annealing on the grain alignment of (Bi,
Pb)2Sr2CaCu2O8+x (Bi2212) and Bi2223 superconductors, (2) the effects of the silver
sheath on the annealing texture of Bi2223 superconductors, (3) the distribution of
texture microstructure produced by mechanical deformation in Bi2212 and Bi2223
superconductors, (4) the effects of mechanical deformation on phase transformation of
BSCCO superconducting materials, (5) the texturing and grain growth behaviours of the
deformed Bi2223 superconducting oxide etc. The crystal fracture and lattice distortion
behaviours of BSCCO oxides, and the effects of energy storage on the recrystallisation
processes have also been studied to understand the mechanisms of the microstructural
changes.
The results indicated that there are different grain alignment behaviours in Bi2212 and
Bi2223 oxides which are introduced by mechanical deformation and thermal treatment.
Heavy mechanical deformation produces a high degree of grain alignment for Bi2212 in
the early stage of the Bi2223 fabrication but not for the Bi2223 phase in the later processing stages. Mechanical deformation also produces a deeper texture distribution
in Bi2223 than in Bi2212. The texturing mechanism ofBi2223 oxide under mechanical
deformation was also revealed by the investigation of the fracture behaviour of Bi222j
oxide. However, The subsequent annealing clearly improves the grain alignment for
Bi2223, Bi2212 and Bi22223 + Bi2212, but the increase in the texture degree during the
phase transformation process is much greater than in the recrystallization process. On
the other hand, the silver sheath serves as a wall that restricts the outward growth of the
Bi compound, and also promotes the recrystallization and growth of the a-b plane
oriented Bi2223 grains. However, the results of mechanical deformation effects on
lattice distortion and crystal size denotes that only a small part of the energy from
mechanical deformation can be transferred to strain energy stored in Bi2223 crystals as
the result of lattice distortion, and the lattice distortion does not seem to have a direct
effect on the grain size after recrystallization as in the case of metals. The
characteristics of Bi2223 oxide under mechanical deformation show that the texturing
processes consist of two stages - reconnection of the broken crystals and
recrystallization/growth of the crystals. Two growth mechanisms, ledge-growth and
“screw dislocation-growth”, of Bi2223 crystals during recrystallization annealing were
also indicated in the present work. Finally, the effects of mechanical deformation on the
phase transformation of BSCCO superconductors was studied. The results show that
the mechanical deformation has an important influence on the Bi2223 phase
transformation. The Bi2223 phase abundance decreased with increasing mechanical
deformation, and then increased as the deformation ratios increased beyond a thickness
reduction of 60%. The 60% mechanical deformation provided the lowest abundance of
Bi2223 phase during annealing.
The present dissertation is aimed at experimental and theoretical investigations of
microstructure control for further industrial applications. The goals of this thesis are to
understand the effects of mechanical deformation thermal treatment on the
microstructure of BSCCO superconductors, to develop methods in optimising the
textured microstructure and reducing grain boundary weakness in an effort to improve their electrical transport properties.