Co-evaporated YBCO/doped-CeO2/Ni-W coated conductors oxygen improved using a supersonic nozzle

A novel process for the coated conductors (CC) deposition, characterized by a single CeO2 buffer layer architecture, and a new oxygenation device for the YBCO layer has been developed. In CC technology, usually the ceria layer thickness must be less than 100 nm to avoid the formation of cracks; in order to ensure an efficient barrier effect, complex and costly multi-buffer layers architectures must be grown. In this work, we describe the way to increase the thickness of crack-free single buffer layer. Single CeO2 buffer layer is grown on biaxially textured Ni-5 at.%W substrates (RABiTS) by e-beam evaporation technique, introducing different doping elements at various percentages in pure Ceria targets. Since the crack formation in cerium oxide layers mainly depends on the stresses introduced by the lattice mismatch with Ni alloy substrate and on the arrangement of the oxygen vacancies in the lattice, it is possible to reduce these stresses by optimizing the doping in the Ceria layer, thus obtaining very thick and crack-free CeO2 buffer layers. The texture quality of our samples is tested by XRD, giving a strong in-plane and out-of-plane orientation (FWHM values of about 5o). Optical and electron microscopy shows dense and crack-free thick layers (up to 450 nm) and confirms excellent insulating properties. A novel technique has been developed to enhance the oxygen pressure on the YBCO film during the deposition as the oxygen content is known to be critical for the growth of YBCO films with high Tc. A supersonic oxygen nozzle (SNEO) incorporated into the vacuum chamber enables a x103 increasing of the local oxygen pressure close to the YBCO layer keeping a 10-4 mbar background pressure. The work demonstrates that the single thick doped-Ceria buffer layer could be a viable way to reduce CC complexity and production costs and that SNEO apparatus can be used to make the oxygenation process of the YBCO films easier and more reliable