REBa2Cu3Oy coated conductors have been widely manufactured by various methods. Among them, fluorine-free MOD (FF-MOD) method is one of the most cost-effective processes to prepare REBCO tapes because of simple chemical reaction under ambient pressure with very high processing rate. Our recent studies revealed that addition of moderate amount of hydrochloric acid to the starting solution resulted in generation of bi-axially aligned oxychloride Ba2Cu3O4Cl2 (Ba2342) fine crystals which assist biaxial growth of YBCO layers. Therefore, highly textured YBCO films can be successfully prepared with high reproducibility in Cl-added FF-MOD method. In this study, further improvement of critical current(Ic) characteristics has been attempted for FF-MOD processed YBCO thin films on clad substrates covered by biaxially oriented oxide layers by increasing the thickness of YBCO layer.
Propionate-based solution with a nominal composition of Y: Ba: Cu: Cl = 1: 2.1: 3.15: 0.1 was coated on clad substrates by spin-coating. Coating and calcination at 500°C under the flowing oxygen atmosphere were repeated 1–9 times to control the film thickness. The calcined films were sintered in the tube furnace under flowing O2/Ar gas with PO2 = 10 Pa, followed by oxygen annealing at 450°C for 12 h. Ic values (A/cm) of the prepared films were evaluated by an inductive method in liquid nitrogen without an external field.
Optimizations of heat-treatment processes, such as calcination, sintering and oxygen annealing, for YBCO films (~0.5 mmt) resulted in a decrease in sum of all the heat-treatment time to half of the typical one with maintaining relatively high Ic ~90 A/cm. Ic of YBCO films with various thickness (0.17–1.5 mmt) sintered at 800°C for 1 h was highest (~100 A/cm) for a film with ~0.8 mmt and thicker films more than 1.0 mmt showed relatively low Ic due to an increase of impurity particles and/or generation of cracks. On the other hand, intermediate sintering was found to be effective for suppressing degradation of Ic for thick YBCO films. A YBCO film (~1.0 mmt) sintered twice exhibited higher Ic ~126 A/cm.
 T. Motoki et al., Supercond. Sci. Technol. 27 095017 (2014).
 T. Motoki et al., Supercond. Sci. Technol. 29 015006 (2016).