This thesis describes investigations performed into the growth and characterisation of Bi-based (Bi<sub>2</sub>Sr<sub>2</sub>Ca<sub>n-1</sub>Cu<sub>n</sub>O<sub>2n + 4 + x</sub>, n=2, 3) ceramic superconducting material in the form of thin films, about 0.5μm thick, grown on single crystal MgO, LaAIO<sub>3</sub> and SrTiO<sub>3</sub> 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 0.9<Bi/Sr<1 for the Bi-2212 (n=2) phase, while for the Bi-2223 phase the Bi-content should be below 1.9 or lower than the Sr-content, for the films not to peel off the substrate during high temperature annealing. T<sub>c- zero</sub> of around 80K is achievable for (Ca + Sr)/Bi ratios between 1.4 and 1.65 while T<sub>c- onset</sub> 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 1.4<Bi<1.8 in as deposited films may provide almost single phase Bi-2223 thin films with Tc values running from 105.5K to 109.5K and Jc>10,sup>4</sup>A/cm<sup>2</sup> 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 (0.9<Pb/Bi<1.5) lowers the formation temperature appreciably and increases the range of sintering temperature (to at least 10K). The Bi-2223 phase starts to form at 835°C from the initial phases (Bi-2212, CuO and Ca<sub>2</sub>PbO<sub>4</sub>) 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 Ca<sub>2</sub>PbO<sub>4</sub> in the film after sintering. On LaAIO<sub>3</sub> and SrTiO<sub>3</sub> perovskite substrates, T<sub>c</sub> is at least 5K lower than in the case of MgO. Nevertheless, LaAIO<sub>3</sub> can provide good microstructure with a critical current density, of 5x10<sup>4</sup>A/cm<sup>2</sup> at 77K. The direction and the range of variation of T<sub>c</sub> 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 T<sub>c</sub> 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 T<sub>c</sub> 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.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:386612 |
| Date | January 1994 |
| Creators | Guldeste, Ayhan |
| Publisher | University of Oxford |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://ora.ox.ac.uk/objects/uuid:681efdc0-8cba-4b3d-83eb-e0021eea5135 |
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