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On the Half Unit Cell Intergrowth of Bi2Sr2Ca3Cu4Ox with Other Superconducting Phases in Two-step Annealed LFZ Fibers

Laser floating zone melting (LFZ) is a non-equilibrium texturing technique. The c-axis of Bi-based superconducting phases (Bi2Sr2Can-1CunOx, n=1, 2, 3) in fibres produced by this technique is predominantly perpendicular to the fibre growth direction [1]. The fibres without Pb and Ag additions are most suitable for the phase evolution research. It was found in our recent work that the fibres with nominal composition of BiSr2Ca3Cu5Oz after annealing at 870°C for 10 min. and then at 830°C for 72 h, had the superconductivity much improved. The onset Tc is about 105K. Below a second transition temperature of about 95K, the diamagnetic signal dropped steeply [2]. In this paper, the two-step annealed Bi-1235 LFZ fibres are investigated by transmission electron microscope (HITACHI-9000NA) and X-ray energy dispersive spectrometry (RONTEC). The samples were prepared by conventional ion milling under liquid nitrogen cooling. HREM images of the superconducting grains reveal important features of the microstructures. Half unit cell intergrowth of 2234 phase with 2223 phase, or 2212 phase is observed. The results provide not only explanations to the properties but also to the mechanism of the phase evolution.