We improved the critical current properties of fluorine-free metal organic decomposition processed YBCO thin films prepared on buffered metallic substrates by increasing the thickness of the films. YBCO films prepared using a conventional one-time growth exhibit a maximum Ic (77 K, ∼0 T) value of ∼100 A per 1 cm width for a film with a thickness of 0.8 μm, and Ic decreases with further increases in the film thickness due to the generation of impurity particles and micro-cracks. In this study, a multiple growth process was adopted for the preparation of thick films to avoid these problems. The Ic of films prepared using this method monotonically increased with the film thickness. YBCO films with a thickness of ∼3.0 μm recorded a quite high Ic (77 K, ∼0 T) value for ∼210 A cm−1 width. Additionally, the in-field Jcs at 20 K and 40 K were found to increase almost proportionally to the film thickness.
Since REBCO melt-textured bulks are produced in an extremely carrier underdoped state after melt-growth, oxygen-annealing is required for a few hundred hours to control the oxygen composition throughout the bulks due to their slow oxygen-diffusivity. In this study, we discovered that the oxygen-annealing time can be largely reduced with the introduction of water vapor to the oxygen-annealing process for Ag-added YBCO melt-textured bulks without any degradation of the superconducting properties. It was revealed that highly dense stacking faults were induced to the matrix during this wet oxygen-annealing, which are considered to act as fast diffusion paths of oxygen. The estimated chemical diffusion coefficients of oxygen at 400 °C for the bulk annealed under wet O 2 was ∼4 .3 ×10 −11 m 2 s −1 , which is almost twice as high as that annealed under standard dry O 2 atmosphere. High J c without a decrease in T c was confirmed for the sample annealed under wet O 2 , when the wet O 2 annealing was performed for a moderate time. In addition, a combined oxygen-annealing process, where wet O 2 was supplied for the first 24 h followed by a dry O 2 process for the remaining period, was found to be more effective to achieve both enhanced oxygen-diffusion and maintain high field trapping properties. These improvements are probably because moderate amounts of defects were introduced to the bulk as a function of the duration of the wet O 2 annealing. The introduction of water vapor to the oxygen-annealing process will become a versatile technique to reduce the total processing time of REBCO melt-textured bulks.
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