Fabrication Of BSCCO And MgB₂ Novel Superconducting Tapes Via Chemical Methods

In this dissertation, BSCCO superconducting tapes were prepared by sol-gel method. The experimental technique was studied. In addition, the phase composition and surface morphology were investigated in detail. Furthermore, MgB2 superconducting tapes were prepared on different substrates (cathodes) via molten salts electrochemical technique, and the influence of temperature on MgB2 was investigated. The main content of the dissertation are as follows:1. The influence of different sol-gel techniques on the formation of Bi-2212 phase. In the process to prepare Bi-2212 powders, Metal nitrates were used as starting materials and ethylenediaminetetracetic acid (EDTA) was used as chelating agent. The influence of concentration of precursor solution and the heating devices on Bi-2212 phase were studied, and thereby the phase purity of Bi-2212 was improved. Furthermore, a novel technique named the secondary gel technique was devised and settled, and Bi-2212 powders were obtained through conventional gel technique and secondary gel technique for different sintering time. Results reveal that the optimized sintering times for the conventional and secondary gel technique were 10h and 5h, respectively. Furthermore, the property of sample derived from the secondary gel technique is better than the one derived from the conventional gel technique. That is to say, the secondary gel technique can not only decrease the sintering time to form Bi-2212 phase, but also improve the Bi-2212 phase purity. The reason lies in the improvement of ingredient homogeneity of the secondary gel during the preparation of Bi-2212 powders.2. The mechanism of different gel techniques. According to the experimental results and the recognition of gel models, the models of gels in the experiment were devised. In the meanwhile, structures of the conventional and secondary gel were optimized and energies of them were calculated by the Dmo13 module of Material Studio software. According to the current results, energy of the secondary gel is lower than that of the conventional gel, and thereby the bridging ability of the secondary gel is stronger. The results derived are in accordance with the experimental results.3. The fabrication of BSCCO tapes. First of all, before the fabrication of BSCCO tapes, the treatment of substrates and the heat treatment conditions were investigated. Then, two routes were adopted during the experiment:(1) the precursor sol was spin-coated on NiO/Ni substrates, and BSCCO tapes with texture were prepared through the heat treatment; (2) Bi-2212 powders were used as solute, and sol, acetone and alcohol were used as solvent to prepare suspension via stirring and ultrasonic vibration. Then the suspension was spin-coated on NiO/Ni substrates to prepare precursor tapes, and the BSCCO tapes were derived through the process of heat treatment. The results of XRD, SEM and optical microscopy indicate that for route (1), the treatment of substrates can affect the surface wettability and thickness of NiO buffer layer. In addition, the sintering conditions are crucial to the preparation of tapes with good properties, and BSCCO tapes with Bi-2201 single phase and Bi-2212/Bi-2201 mix phases with preferential orientation have been obtained. For route (2), the suspension consisting of acetone and Bi-2212 powders are favorable to the preparation of BSCCO tapes on NiO/Ni substrates.4. Fabrication of MgB2 superconducting tapes. In the investigation, the system of the molten salts and the electrolytic conditions of the experiment were investigated in detail, and the ranges of electrolysis parameters were fixed. During the experiment, the electrolysis cell was simplified, and the difficulty to control the distance between the two electrodes was decreased. Then, MgB2 films were successfully prepared on graphite, stainless steel and copper cathodes. The influence of electrolytic temperature on the purity and crystallinity of MgB2 was investigated, and the optimized electrolytic temperatures for different cathodes were obtained. In addition, pulse power source was introduced on the stainless steel cathode, and the purity of MgB2 phase was improved.