Deposition of CeO_2 Buffer Layers and YBa_2Cu_3O_(7-δ) Superconducting Layers Using an Aqueous sol-gel Method

摘要

Recently, the sol-gel method has been developed as a potential alternative to the conventional vacuum techniques (e.g. pulsed laser deposition and magnetron sputtering) towards the synthesis of a coated conductor. The advantages of the sol-gel method include a more simple and faster way to deposit films, deposition at a lower cost and the possibility to incorporate it in a continuous system. Possible applications of these coated conductors are long lengths of superconducting cables, strong magnets, fault current limiters,... In our work, the coated conductor consists of a CeO_2 buffer layers and a YBa_2Cu_3O_(7-δ) superconducting film, coated on a textured substrate. Two kinds of substrates have been used in this work: ceramic R-cut (1102) sapphire substrate and metallic (001) Ni-alloy tape. For both substrates, a cleaning procedure was developed to improve the wettability. An almost complete wettability is necessary as an aqueous sol-gel method is used in this work. Dip-coating the cleaned substrates in the aqueous metal-citrate solution leaves a wet film on the substrate which can be converted to the ceramic oxide by thermal treatment. The dip-coating technique and thermal treatment were first optimized onto the sapphire substrate, as in the case of Ni-alloy, special attention is needed to avoid oxidation of the substrate. Due to the larger lattice mismatch, no biaxially textured CeO_2 could be obtained on R-cut sapphire substrate. However, on Ni-alloy tape, perfectly (001)-oriented CeO_2 is formed when the thermal treatment is performed in an Ar-H_2 atmosphere to avoid oxidation of the substrate. The results of microscopic and structural study of both CeO_2 and YBa_2Cu_3O_(7-δ) will be presented. Special attention will be given to the XRD and pole figure results, as these results give a good indication of the texture of the layers. Texture is of huge importance in the case of coated conductors, as in this way high critical current densities might be obtained.