Year

1995

Degree Name

Doctor of Philosophy

Abstract

X-ray diffraction was used to characterise the phase composition and to investigate the formation mechanism of the (Bi,Pb)2Sr2Ca2Cu3O10 phase from the precursor with (Bi,Pb)2Sr2CaCu2O8 as the main phase. The reaction is found to be a two-dimensional nucleation (random)-growth type, [-(ln(l-F)]1/2=kt, where F is the conversional fraction of (Bi,Pb)2Sr2CaCu2O8 phase and t is the sintering time. The magnitude of the critical current of the tape is quantitatively related to the conversional fraction of (Bi,Pb)2Sr2CaCu2O8 to the (Bi,Pb)2Sr2Ca2Cu3O10 phase. In the low fraction regime of (Bi,Pb)2Sr2CaCu2Og, the critical current Ic of a tape does not show a simple relationship with the remaining (Bi,Pb)2Sr2CaCu2O8 phase. We argue that other factors such as grain alignment, colony size, contact between colonies, and fine nonsuperconducting particles become important in controlling the critical current density Jc. The predominant weak links seem to be the colony boundaries rather than the (Bi,Pb)2Sr2CaCu2O8 phase in this regime.

The two dimensional behaviour of the critical current in (Bi,Pb)2Sr2Ca2Cu3O10/Ag tapes was observed and analysed by introducing an effective grain misalignment angle, φeff. This angle was found by SEM to be identical to the average crystallographic grain misalignment angle in the superconducting core. Furthermore, after fast neutron irradiation, which is isotropical, the Jc's of the tapes were modified by the introduction of artificial defects, but the φeff's remained the same. This proves that the crystallographic misalignment of the grains determines the critical current anisotropy of the tape in a magnetic field, i.e. Ic (B//tape plane) scales with Ic(B_Ltape plane) with the factor sinφeff Therefore, φeff is an intrinsic property of the tape. φeff is found to be around 10° in different samples. We propose that this typical value is determined by intrinsic mechanical properties of the (Bi,Pb)2Sr2Ca2Cu3O10 compound and is a result of the mechanical deformation of the tape during the tape fabrication. From a comparison of different samples, we propose that the pinning ability is the determining factor of Jc when φeff is small enough.

The transport critical current of (Bi,Pb)2Sr2Ca2Cu3O10/Ag tapes was measured in magnetic fields up to 15 T and at temperatures from of 4.2 to 84 K. At high temperatures, the Jc is strongly anisotropic and the anisotropy increases rapidly with magnetic field, whereas at low temperatures the critical current is less anisotropic and the anisotropy is almost field independent above 1 T. The former case is believed to be in a regime, where pinning limits Jc, at least within some parts of the tape, whereas in the latter case the limitation of Jc by Josephson weak links seems to be the dominant mechanism. In addition, a critical current hysteresis induced by flux trapping in a weak link network is observed, which is more pronounced at low temperatures. From TEM observations of the microstructure we find that the "brick" in the "brick wall" model turns out to be the colony instead of the grain inside the colony. Additionally it is found that colony boundaries parallel to the a,b-plane and intersected boundaries occur much more frequently than boundaries parallel to the c-axis, due to the misalignment of the colonies inside the tape. In a small region near the silver sheath, the colony misalignment is much smaller and boundaries parallel to the c-axis may act as strong links at high temperatures as their interfaces are very clean and well matched.

The transport Jc's in (Bi,Pb)2Sr2Ca2Cu3O10/Ag tapes at 77 K and higher magnetic fields after neutron irradiation are significantly enhanced. This enhancement is attributed to an improvement in the flux pinning capability of this material by the neutron-induced defects. The angular dependence of Jc is still consistent with two-dimensionality, i.e. flux pinning of pancake and/or Josephson vortices is direcdy confirmed by this transport measurement. Fast neutron irradiation also affects the weak links in a way, which agrees with previous results on unaligned ceramics. However, there are some peculiarities which need further investigation.

Short multifilamentary (Bi,Pb)2Sr2Ca2Cu3O10/Ag tapes were fabricated. The sintering parameters were optimised to be 832 °C and 180 h. X-ray diffraction (XRD) results indicate that the multifilamentary tape consists mainly of pure (Bi,Pb)2Sr2Ca2Cu3O10 with a (00l) preferred orientation, like the single filamentary tape. Image analysis microscope (IAM) results show that the filaments in the multifilamentary tape have an inhomogeneous mass distribution over the transverse cross-section. AC susceptibility and transport critical current measurements were carried out to evaluate the tapes. The measurements show that the tapes have a critical transition temperature of 110 K and critical current densities ranging from 13,900 to 17,000 A/cm2 at 77 K and zero magnetic field. The Jc-B characteristics of the tapes are discussed in relation to grain alignment.

The mass densities of the (Bi,Pb)2Sr2Ca2Cu3O10/Ag wire and tape vary during the mechanical deformation process, as one of the steps of the oxide-powder-in-tube technique used to fabricate the composite superconductor. Results show that the rolling has a more significant effect on densifying the tape core, whereas the drawing process can only densify the core to about 75% of the theoretical density. SEM observations of the rolled samples also reveal a very dense morphology, consistent with the mass density calculations. SEM observation also shows that with increasing the deformation extent, the average grain size is reduced. It is proposed that, although the rolling densities the tape core much more significantly, it also destroy the crystallinity of the superconducting phases and results in the formation of an amorphous phase. Since the textured (Bi,Pb)2Sr2Ca2Cu3O10 phase forms by epitaxial growth on the textured (Bi,Pb)2Sr2CaCu2O8 seed crystals, the deformation induced texture is critical. An appropriate deformation extent is necessary, since a too high extent of deformation may change the well aligned grains into the amorphous phase. The formation of the amorphous phase is harmful to the texturing of the (Bi,Pb)2Sr2Ca2Cu3O10 phase, which finally leads to a degradation of critical currents.

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