REBCO (Re=Y, Eu, Gd) coated conductors (CC) based on biaxially textured, thick and homogeneous nanoengineered multilayer structures opened up new application opportunities, such as dissipation-free energy transmission in superconducting grids, highly efficient engines for electrical aviation or compact fusion reactors beyond ITER. However, current carrying capacities of CC could be further improved because they are still far from theoretical limits. As it has been shown, one of the possible robust ways to increase current carrying capacity of CC  is overdoping with oxygen the REBCO structure of CC. 

Treatment of GdBCO_CC under 100 bar of O₂ at 600 °C for 3 h led to an increase in J_c (77K, 0 T) by 6% and a decrease in the c-parameter of Gd123 to 1.17310 nm, which may be associated not only with overdoping with oxygen, but also with silver diffusion into Gd123. No correlation was observed between J_c, T_c, c-parameter of RE123 (RE=Eu, Gd) and carrier density n_H of EuBCO_CC and GdBCO_CC treated at 300-800 ^oC, 1-160 bar O₂ for 3-12 h.

We have started investigating high oxygen pressure treatments of GdBCO and EuBCO-BHO Coated Conductors previously oxygenated with standard treatments. For our experiments we used commercially produced GdBCO and EuBCO-BHO coated conductors provided by Fujikura. The CC samples have been previously oxygenated with their standard process. For the high oxygen pressure post-treatment,  GdBCO and EuBCO-BHO coated conductors with 2 mm Ag layer on the surface of GdBCO CC and without and with Ag layer on the surface of  EuBCO-BHO CC were used. Therefore, before experiments the upper Cu layer (and in some cases the Ag layer) was chemically removed from CCs tapes. In the case of EuBCO-BHO_CCs  the Ag upper layer was removed chemically as well, but for some experiments it was preserved as indicated. The architecture (from  top) of the studied CCs was as follows: (1) FYSC : Ag (2μm)/ GdBCO (1.8μm)/ CeO₂ (700 nm)/ MgO/ / Al₂O₃/Y₂O₃ /Hastelloy (75 μm); (2) FESC : EuBCO (2.5μm)+BHO Nanorods/ CeO₂ (700 nm)/ /MgO/ Al₂O₃/Y₂O₃ / Hastelloy (50 μm); (3) Ag (2μm)/ EuBCO (2.5μm)+BHO Nanorods/ CeO₂ (700 nm)MgO/ / Al₂O₃/Y₂O₃/ Hastelloy (50 μm).

A specially designed tube furnace was used for the oxygenation process. The oxygen pressure in the furnace varied from 1 to 160 bar, the temperature from 300 to 800 °C, and the heating rate was 5 ºC/min. After, a dwell of temperature for a certain time held at the required pressure was attained, and afterwards the heater was turned off.

Results on critical current density, superconducting transition temperature, charge carrier densities, c-lattice parameter and Auger, indicate that high oxygen pressures of 100 and 160 bar can be sustained by these materials when high temperatures are used and that this can be a route to overdope these Coated Conductors. Furthermore, we evidence that these post oxygen treatements should be done without Ag etching so that the REBCO material is not exposed to air during the high pressure treatments. Futher work is on-going to obtain the best conditions to increase the charge carrier concentration and consequently the criticial current density in a robust way.

AcknowledgementS: We acknowledge funds from MUGSUP, UCRAN20088 project from CSIC programme from the scientific cooperation with Ukraine, the Spanish Ministry of Science and Innovation and the European Regional Development Fund, MCIU/AEI/FEDER for SUPERENERTECH (PID2021-127297OB-C21), FUNFUTURE “Severo Ochoa” Program for Centers of Excellence in R&D (CEX2019-000917-S), and HTS-JOINTS (PDC2022-133208-I00),  NAS of Ukraine Project III-7-24 (0788)  Authors also thanks Fujikura for supplying the samples