Synthesis and characterization of high-T/sub c/ superconducting ceramics

Solid solubility in YBa/sub 2/Cu/sub 3/O/sub 7- delta / was found to be very limited: less than 0.5 at% excess or deficit of Cu, Ba, or Y could be incorporated. Samples deviating from stoichiometry differed sharply from stoichiometric material in their electrical behavior. Tailing of two-step superconductive transitions were found in such samples as well as a decreased T/sub c/, end. The orthorhombic-tetragonal-transformation temperature of YBa/sub 2/Cu/sub 3/O/sub 7- delta / and LnBa/sub 2/Cu/sub 3/O/sub 7- delta / were determined from careful isobaric thermogravimetric measurements. As the ionic radius of the lathanide of Y increased, the transformation temperature tended to increase as well, from 840 K for NdBa/sub 2/Cu/sub 3/O/sub 7- delta / to 980 K for TmBa/sub 2/Cu/sub 3/O/sub 7- delta /. This tendency was interpreted in terms of the repulsion energy between the nearest-neighbor oxygen atoms on the basis of a simple order-disorder transformation model. Reactions were observed at the interfaces between the screen-printed thick YBa/sub 2/Cu/sub 3/O/sub 7- delta / film and the following substrates: quartz, sapphire, MgO, and YSZ. A superconducting thick film with T/sub c/, end=87 K and J/sub c/(77 K)=30 A/cm/sup 2/ was produced on YSZ by using a short firing time (0.1 hours) at 990 degrees C, reducing the interface reactions.<
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