Evaluation of methods for application of epitaxial buffer and superconductor layers

The recent achievements of critical currents exceeding million amperes per square centimeter at 77K 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. Under the DOE-sponsored program, to carry out an engineering evaluation, first, the process flow schemes were developed for various candidate options identifying the major operating steps, process conditions, and process streams. Next, to evaluate quantifiable parameters such as process severity (e.g. temperature and pressure), coating thickness and deposition rate for HTS material, achieved maximum J{sub c} value (for films >1{micro}m thick) and cost of chemical 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. As a result of this study, MOD, Sol-Gel, MOCVD, E-Beam and PLD options have been identified as the leading candidates for continuous, long-length processing of coated HTS conductors. However, there are several technical concerns related to each of these which need to be addressed first before a commercially viable option can be developed out of any one of them