Atsushi Kaneko

Fabrication and growth mechanism of YBCO coated conductors by TFA-MOD process

Abstract In the development for the coated conductors, the trifluoroacetates-metal organic deposition (TFA-MOD) process using the multi-coating method was applied to form thicker YBa 2 Cu 3 O y (YBCO) films on the CeO 2 buffered IBAD(Zr 2 Gd 2 O 7 )/Hastelloy tape. As a result, in the 5 times coated film with 1.38 μm in thickness, the J c and I c values achieved to be 1.5 MA/cm 2 and 210 A, respectively. Then, we fabricated a 1 m long YBCO tape by the continuous reel-to-reel system. As a result, we confirmed the tape to be homogeneous by XRD analysis. Additionally, we report the theoretical analysis of YBCO growth during post-annealing in the TFA-MOD process considering both the diffusion in the gas boundary layer and the growth kinetics at the precursor/YBCO interface. This model reveals a basic idea of the growth mechanism to define the steady state growth rate, and could explain the experimental results. In addition, it predicts that the growth rate could be estimated from the partial pressure of the water vapor of the inlet gas.

HTS Current Leads Prepared by the TFA-MOD Processed YBCO Tapes

HTS current leads have been prepared by the TFA-MOD processed YBCO tapes 5 mm in width and around 120 μm in overall thickness. The YBCO superconducting thin layer with 1.5 μm is formed on the oxide buffer layers deposited on Hastelloy substrate tape. The critical current of the YBCO tapes is around 100 A at 77 K in self-field. A current lead unit is composed of a GFRP board and five YBCO tapes soldered to Cu cap at both ends. The transport current of the ten current lead units ranges from 536 A to 408 A at 77 K in liquid nitrogen. The YBCO current lead is assembled from the ten current lead units in parallel. The transport current of 4,000 A at 77 K was stably carried with no voltage generation on the YBCO tapes of the assembled current lead for ten minutes. Furthermore, the trans port current of 5,000 A was successfully applied with voltage of around 200 μV on YBCO tapes. The heat leakage of the assembled YBCO current lead with 150 mm in length between 77 K and 4.2 K is estimated to be 470 mW. Therefore, the heat load of it at transport current of 4000 A corresponds to 0.12 W/kA, which is one order of magnitude smaller than that of conventional Cu current lead. The small heat load results from high current performance and low thermal conductivity in the present YBCO current lead.

Development of TFA-MOD Process for Coated Conductors in Japan

Progresses in the development of the TFA-MOD process for the coated conductors in Japan are reviewed. In the fundamental approaches, some important advantages of Ba-poor starting solution were found such as for obtaining higher Jc, shorter reaction time and wider process temperature window. The high Ic value of 735 A/cm-w@77 K was attained. Additionally, introduction of artificial pinning centers by Sm-substitution and Zr-addition to the Ba-poor films was successfully investigated. The films showed a high minimum Ic value of 42 A/cm-w at 77 K, 1 T. Furthermore, isotropic Ic properties in the angular dependence of applied magnetic fields was realized by the uniform distribution of nanoprticles of Zr-compound. On the other hand, two different process of MOD including batch and reel-to-reel (RTR) types have been developed as an engineering process for low cost and long tape production with high superconducting performances at a high rate. In the batch type, long tape synthesis process could be developed by the optimization of the growth conditions in a large furnace. A 500 m long tape was heat-treated and a high Ic value of 310 A/cm-w in a 250 m tape was confirmed. Concerning the RTR type, a high production rate system was developed after confirmation of the ability of the continuous process in a 90 m long tape. A multi-turn with a vertical gas flow system was developed for this purpose. This system was applied to the fabrication of superconducting films on the lower cost buffer layers including a high deposition rate IBAD-MgO layer. Currently, a 5 m long tape with 250 A/cm-w was obtained at the production rate of 3 m/h using a part of the multi-turn system, which is equivalent to 15 m/h in the entire use of the furnace.