Genki Honda

Dramatic effects of chlorine doping on Jc and microstructure of fluorine-free MOD Y123 thin films

Chlorine doped Y123 thin films have been successfully prepared by the fluorine-free metal organic decomposition (FF-MOD) method on SrTiO3(100) single crystal substrates. The rectangular Ba2Cu3O4Cl2 (Ba2342) columns were epitaxially grown in the film without degrading crystallinity and transition temperature (Tc) of Y123. Cl-doped Y123 films maintained high Tc of 90.5–91.5 K and they showed higher critical current density (Jc) than the Cl-free films particularly in magnetic fields. The high Jc feature was observed in thick films up to ~0.93 μm in thickness. In addition, Cl and Hf co-doped Y123 films maintained high crystallinity and showed further improved Jc of 1.1 MA cm−2 at 40 K under 5 T. These suggested that Ba2342 precipitates have two effects on FF-MOD processed Y123 films. One is an increase in pinning force and the other is improvement of crystallinity of Y123 because of the good lattice matching between Y123 and Ba2342.

Microstructures and improved J c–H characteristics of Cl-containing YBCO thin films prepared by the fluorine-free MOD method

Undoped, Cl-doped, (Cl, Hf) co-doped and (Cl, Sn) co-doped YBa2Cu3O y (YBCO) thin films have been prepared by the fluorine-free metal–organic decomposition (FF-MOD) method on SrTiO3(100) single-crystalline substrates. Cross-sectional microstructures of these films were investigated in detail using scanning transmission electron microscopy (STEM). Rectangular-shaped oxychloride precipitates (Ba2Cu3O4Cl2) and fine particles (BaSnO3) were clearly observed in the (Cl, Sn) co-doped films. The magnetic angular dependence of the critical current density (J c–H–θ) of these films was evaluated. The existence of c-axis-correlated type pinning centers was suggested in Cl-containing YBCO films, whereas this type of pinning is not common in MOD-processed films. J c values were enhanced by Cl doping and further by (Cl, Sn) co-doping in all magnetic field directions at 77 K. This improved J c–H–θ property with c-axis-correlated pinning sites is the first report in FF-MOD-processed YBCO films.

Improvement of Recovery Characteristics of REBCO Tape With Several Surface Conditions for Resistive Fault Current Limiter

Superconducting fault current limiters (SFCLs) are required to return to the superconducting state within the proper time (recovery time) after limiting a fault current according to a relay operation scheme in a power system. In order to apply SFCLs to power systems, the recovery time is important property and deeply depends on the heat transfer property of the heated surface of the superconducting tape (normal conducting state) to liquid nitrogen. It has been confirmed that REBCO tapes with various shaped fins or PTFE coating improved cooling property. In this paper, to clear the mechanism of cooling property improvement of fins or PTFE coating, boiling phenomena on REBCO tapes with some types of fins or PTFE coating was observed by use of a high-speed video camera. The influence on the cooling improvement property of cooling fins and PTFE coating was discussed according to the photographical view of the boiling phenomenon.

Dramatic effects of chlorine addition on expanding synthesis conditions for fluorine-free metal–organic decomposition YBa2Cu3O y films

The synthesis conditions of fluorine-free metal–organic decomposition (FF-MOD)-processed YBa2Cu3O y (YBCO) films on buffered metallic substrates have been systematically investigated. Chlorine addition to the starting solution was found to be quite effective for expanding the synthesis conditions of highly c-axis-oriented YBCO films. YBCO films showing a high critical current, ~100 A/cm (77 K, ~0 T), were successfully obtained by sintering at 740 °C, which is ~50 °C lower than the typical sintering temperature for FF-MOD-processed YBCO films. This strongly indicated that chlorine addition is promising for the development of long and homogeneous YBCO tapes even by sintering at a low temperature of ~740 °C.