Bismuth based thin film superconductors
This thesis describes investigations performed into the growth and characterisation of Bi-based (Bi2
O2n + 4 + x
, n=2, 3) ceramic superconducting material in the form of thin films, about 0.5μm thick, grown on single crystal MgO, LaAIO3
substrates by r.f. magnetron sputtering. The effect of oxygen content on the Pb doped Bi-2223 (n=3) phase was also studied by changing the cooling process and by annealing in different partial pressures of oxygen at ambient pressure. The films produced have been assessed by considering their initial composition where it is found that Bi/Sr ratios can be between 0.9c- zero of around 80K is achievable for (Ca + Sr)/Bi ratios between 1.4 and 1.65 while Tc- onset remains above 90K for Bi-2212 films. However, the best superconducting properties can be obtained for a (Ca + Sr)/Bi ratio which is quite close the nominal composition. The use of a heavily Pb doped target is an effective way of Pb doping Bi-2223 thin films. A Bi-content of 1.410,sup>4A/cm2 at 77K. The effect of the initial Pb content and annealing conditions on the formation of the Bi-2223 phase was investigated. It was found that high Pb content (0.92PbO4) formed below 835°C and its fraction increases with increasing sintering temperature up to 862°C, while the fraction of initial phases decreases. An annealing duration of 30 min. has provided highly oriented films with c-axis perpendicular to the substrate surface and sharp superconducting transition (<5K). Although Pb/Bi ratio is not critical in the range studied, when it is above 1.3 slow heating and cooling is necessary to prevent retention of excess Ca2PbO4 in the film after sintering. On LaAIO3 and SrTiO3 perovskite substrates, Tc is at least 5K lower than in the case of MgO. Nevertheless, LaAIO3 can provide good microstructure with a critical current density, of 5x104A/cm2 at 77K. The direction and the range of variation of Tc in Bi-2223 films with oxidising process can be related to both the film composition (especially Bi and Pb content) and initial oxygen content. The variation range of Tc with oxidising is controlled by the Pb content. However, the maximum variation is around 4K at ambient pressure. Radiation response measurements were carried out on films patterned into a 150μm wide, and 1 cm long meander-type structure using standard photolithography and wet chemical etching in EDTA. The results showed that the optical response using a continuous wave (cw) He-Ne laser is bolometric, while the microwave response using a 34.5 GHz Gunn diode microwave generator contains a non bolometric component. Such polycrystalline Bi-based high Tc thin films may have interesting applications as sensitive microwave detectors, but they are not particularly good for microwave applications because of their high surface resistance, Rs, at microwave frequencies.