Synthesis of precursor powders for preparation of YBa2 Cu3 O7 coatings by electrophoretic deposition on silver substrates

Abstract

Over the last years, our research group has been investigating the electrophoretic deposition of YBa2Cu3O7 (Y-123) on silver substrates with the aim to improve the magnetic shielding properties. For synthesis purposes, a key characteristic of Y-123 is its peritectic decomposition into solid Y-211 and a Y-035 liquid phase. In this work, we aimed to synthesize the two precursor powders and proceed to electrophoretic deposition on silver substrates. The final Y-123 superconductor should be formed via peritectic recombination. The starting point of this master thesis was to adapt a synthetic procedure of the Y-123 phase to the stoichiometries of Y-211 and Y-035. In this procedure, Y-123 was obtained by self-combustion of sol-gel precursors followed by a final treatment.

In this master thesis, the procedure was successfully adapted to the stoichiometry of Y-211. Stable aqueous solutions containing the metal-citrate complexes could be prepared. Dried residues containing these metal-citrate complexes were obtained via the spray-drying technique with a 95.4% yield. After combustion and complete optimization of the heat treatment, it was found that 880 °C for 6h with a heating rate of 300 °C/h was sufficient to form the desired Y-211 phase. A 250-gram batch of pure Y2BaCuO5 with a median particle size of 1.4 micron could be synthesized via a scaled-up production with a global yield of 86.9 %.

Concerning the synthesis of the liquid phase Y-035, the preparation of the aqueous solutions of metal-citrate complexes was more challenging and did not permit to obtain dried residues via the spray-drying method. Since spray-drying of the EDTA-based solution did not succeed, the spray-drying route was abandoned in favor of the original sol-gel route. Homogeneous gels could be prepared and the stability issue encountered with metal-citrate complexes was solved. Moreover, the amount of organic additives used in the preparation of these gels could be reduced by 23%. Finally, a 125-gram batch of Y-035 could be synthesized in a scaled-up process but high purity could not be reached due to the high content of carbon residues still present after two heat treatments.

Stable butanol-based suspensions could be prepared with each powder. Here, addition of branched poly(ethyleneimine) (PEI) as the dispersing agent permitted to reach optimal conditions of stability in terms of zeta potential. Electrophoretic deposition for 60 s at 50 V allowed to deposit 5.5 g/cm² of Y-211 on Ag substrates. These conditions are optimal to avoid the formation of macro-cracks during the heat treatment. Intermediate heat treatment had to be conducted between each deposition in order to avoid the coating to peel-off. Finally, peritectic recombination was tested on a mixture of Y-211 and Y-035 powders. By XRD, SEM and EDX analysis, we could prove the presence of Y-123 accompanied by Y-211 inclusions, CuO, BaCuO2 and Ag.