Effect Of Rare Earth Content On Microstructure And Superconducting Properties Of (Gd,Y)BCO Coated Conductors Fabricated By Multi-pass MOCVD

The irreversible magnetic field for REBCO film is the highest for superconductors discovered so far in almost the entire temperature range of 4-77 K.Therefore,the second generation high temperature superconducting tapes based on REBCO are of potential of the best in-field performance and the highest critical current density.It is the irreplaceable practical feature for the second generation high temperature superconducting tapes that determines their unlimited possibility of market prospects.In this paper,REBCO(RE=Gd+Y,with Gd=Y)superconducting tapes with RE contents of 1,1.1,1.2,1.3,1.4,1.5 were prepared by multi-pass metal organic chemical vapor deposition(MOCVD)on Hastelloy substrate with an IBAD MgO based buffer architecture.Critical currents for sample tapes with different film thicknesses and rare earth contents were measured at various temperatures and applied magnetic fields using standard four probe DC transport method.The microstructures for the tapes were analyzed using ICP-AES,Step Profiler,X-ray diffraction(XRD),Scanning Electron Microscope(SEM)and Transmission Electron Microscope(TEM).The rare earth content of 1.2 shows the highest critical current in the first pass REBCO film samples,while the rare earth contents of 1.4 and 1.5 provide higher critical current in the second pass and third pass.The results are related to the surface morphology and the crystal structure.The structural analyses indicate that higher RE-content favors in-plane-alignment,minimizes Ba-Cu-O phases and reduces surface roughness.Furthermore,the excess amount of RE elements was found to result in precipitates of RE2O3 in the films,which are effective pinning centers to increase the critical current.Therefore,RE contents of 1.2 to 1.5 would be good for multi-pass MOCVD.The micro structural analysis on REBCO tapes with high RE contents fabricated by three pass MOCVD reveals clearly extended RE2O3 nanoprecipitates along the a-b plane,which are effective pinning centers to increase the critical current.Angular dependences of the tapes in an applied magnetic field show significant anisotropy.For REBCO film samples with higher rare earth contents,a-b plane aligned RE2O3 formed by excessive rare earth elements enhances the magnetic flux pinning alone the ab plane.