Review and evaluation of methods for application of epitaxial buffer and superconductor layers1

The recent achievements of critical currents exceeding 1,000,000 amps/cm² at 77 K in YBCO deposited over suitably textured substrate have stimulated interest in the potential applications of coated conductors at high temperatures and in high magnetic fields. Currently, ion-beam assisted deposition (IBAD), and rolling assisted bi-axially textured substrate (RABiTS), represent two available options for obtaining textured substrates. For applying suitable coatings of buffer and high temperature superconductor (HTS) material over textured substrates, several options are available which include sputtering, electron-beam evaporation, laser ablation, electrophoresis, chemical vapor deposition (including metal organics chemical vapor deposition), sol-gel, metal organics decomposition, electrodeposition and aerosol/spray pyrolysis. A commercial continuous long-length wire/tape manufacturing scheme developed out of any suitable combination of the above techniques would consist of operations involving preparation of the substrate and application of buffer, HTS and passivation/insulation materials and special treatment steps such as post-annealing. These operations can be effected by various process parameters that can be classified into chemistry, materials, engineering and environmental related parameters. To carry out an engineering evaluation: (i) the process flow schemes were developed for various candidate options identifying the major operating steps, process conditions, and process streams; (ii) to evaluate quantifiable parameters such as process severity (e.g. temperature and pressure), coating thickness and deposition rate for HTS material, achieved maximum J_c value, and cost of chemicals and material utilization efficiency, the multi-attribute method was used to determine attributes/merits for various parameters and candidate options. To determine similar attribute values for the non-quantifiable parameters, a subjective evaluation was used. Results of the two evaluations were then combined to calculate the overall merit/utility of a given option using weighting factors. To evaluate the effect of arbitrarily determined weighting factors, a sensitivity analysis was carried out by using three different sets of weighting factors. In the end, the results of the overall utility/merit values calculated using different sets of weighting factors were utilized to determine the preliminary ranking among the candidate options. The process flow schemes developed for various options, as well as the results of the engineering evaluation carried out for the candidate options using these schemes, are given in this paper.