Oxide Films for the Protection of Cuprate Superconductors
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OXIDE FILMS FOR THE PROTECTION OF CUPRATE SUPERCONDUCTORS Ronald H. Baney*, Debora F. Bergstrom, Leslie E. Carpenter%, Donald R. Petersen, Dennis F. Elwell#, Andrew A. Shapiro#, and Paul S. Fleishner# *Dow Corning Corporation, Midland MI #Hughes Aircraft Company, Newport Beach, CA ABSTRACT The cuprate superconductors rapidly degrade in moisture and in the presence of organic compounds. A protection method has been developed with a coating process using sol/gel routes to the low temperature formation of oxide films. In contrast to traditional protective films, amorphous silica films formed from a solution precursors gave excellent protection and caused little degradation of the superconductors. The protective abilities of the films were assessed by monitoring the degradation of Y1Ba 2Cu 3O7 -d in the presence of 85% relative humidity at 85 C. Volume magnetic susceptibility, four-point probe resistivity, current density measurements and x-ray diffraction were performed. Without protection, degradation was almost complete within 30 minutes of exposure. With protection, superconducting properties were maintained even after 48 hours of exposure. INTRODUCTION The YBa2Cu307.d superconductor is a reactive oxide which can be decomposed at various temperatures in the presence of moisture and carbon dioxide,[1-4] organic solvents, vapors or matrices,[5-8] and ceramic substrates or matrices.[9-12] Before the high temperature cuprate superconductors can be used in practical thin film applications, methods for protection of the materials from the environment must be found. The systems which have been studied for the protection of the superconductors include noble metals,[13-161 fluorinated polymers,[17] organic polymers,[1 8] and vapor deposited oxides and halides.[19] This work describes a method for protection of the superconductors using nonreactive silica deposited from solution precursors at low temperatures and the performance of such protective coatings. EXPERIMENTAL Commercially obtained YBa 2 Cu307d powder was restored to the superconducting state using a one hour pyrolysis in flowing 02 at 930950 C followed by a 12-15 hour pyrolysis at 450 C. The powder was pressed into 13mm diameter pellets. The pellets were cut into small pieces using a diamond saw cooled with a fluorocarbon solvent. The samples were coated with protective coating solution precursors by alternately evacuating and pressurizing the samples in the coating solutions. The protection films were formed by firing the samples coated with precursors in flowing 02 at 400 C for one hour except for the following: (a) Si3 N4 precursor [20] which was heated at 400 C for one hour in NH3 (b) polystyrene samples which were dried under vacuum, and (c) commercially obtained polycarbosilane SiC (Nicalonq precursor which was heated at 400 C in Ar for one hour. Samples were then placed in an environmental stress chamber at 85% relative humidity and 85 C for one hour. The volume magnetic susceptibilities of the coated, stressed samples, with coated, unstressed and uncoated,
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