R&D for long tapes with high Ic by advanced TFA-MOD process

Recent progress in the research and development of the TFA-MOD processing for the coated conductors was reviewed. For the higher I-c performance, crack generation in the thick films was a problem, which limits increasing I-c values. It was found that both lower heating rate and low P-H2O in the crystallization step are effective to improve the critical thickness for obtaining crack-free films. However, the high P-H2O is preferable for a high production rate and high J(c) performance. Then, the new heat treatment profile, which could suppress crack formation even in thick films under the high P 1,0, was developed. Consequently, the high I-c value of 470 A at 77 K was realized in the film with 3 mu m in thick. Concerning the long-tape processing, the key factors for high I-c performance by TFA-MOD process were investigated. The highly textured substrate for high crystallinity of the YBCO layer, the thick film with maintaining reasonable high J(c) and full conversion reaction to form the YBCO layer are important to obtain high superconducting performance in long tapes. At present, a 25 m long tape with a reasonable high I-c value of 100 A, and a high I-c value of 250 A in a 4 m long tape were achieved. In order to realize a high production rate, the processes for both calcination and crystallization steps have to be developed. In the calcination step, the multi-turning continuous system was developed to maintain an effective travelling rate even in the multi-coating method. On the other hand, in the crystallization step, it was confirmed that optimization of the processing parameters such as a high water vapour partial pressure, a low total pressure, and a high gas flow rate could make the YBCO growth rate increase. The combination effect of these parameters is experimentally confirmed and the 5 times higher production rate was achieved in the crystallization step. Additionally, the equipment design of the multi-turning system with a new gas flow concept, which is a vertical gas flow system, was investigated by means of both numerically and experimentally, and the concept was confirmed at least. Consequently, a high production rate over 5 m/h could be expected by combining the above mentioned effects. (c) 2006 Elsevier B.V. All rights reserved.