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The study concerns the development of a cost effective multifunctional flexible high temperature superconducting tape (MUST), exhibiting a critical current density in the film Jc>2.10E6 A cm{-2} at 77 K and B=0T, using high power magnetron sputter deposition of 0.5-1 µm thick yttrium(1)barium(2)copper (3)oxygen(7) (YBCO) films on metallic and polymer substrates. To obtain this high current density and cost effectiveness it is necessary to achieve a deposition rate of at least 500 angstroms/min. In turn, this increases the quality requirements of the magnetron targets. To allow a high power throughput, the electrical and thermal conductivity and mechanical strength of the YBCO targets have to be improved by addition of metals that are not detrimental to the superconductivity in the YBCO films. In order to obtain a cost reduction of about 50% in the target production, a low-cost ceramic target fabrication and reconditioning technique based on flame spraying was used. In the flame sprayer, a cooling system for the target is provided, using a device for bringing a cryogenic gas into contact with the target. The cooling device maintains the substrate temperatures between 25 and 70 C. These low temperatures avoid thermal stresses between the coating and the substrate providing a good adhesion, hardness and scratch resistance, thus helping to ensure the long term stability of coatings with a thickness of several mm.
Such a thick layer deposition technique ideally requires good free flowing powders exhibiting a narrow grain size distribution and particle sizes in the range 50 to 80 µm. The optimization of the feeding powder was performed using spray drying. This technique provides homogeneous powder, exhibiting the appropriate rheological properties and correct stoichiometry to generate the required properties in the final coating. By the variation of concentration of the solution and/or the addition of binders and/or the spray drying of slurries instead of solutions, the grain size in the final powder can be controlled.
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