Spray Forming of Superconducting Oxides High Tc Ceramics

Researchers have reported on the deposition and synthesis of high critical temperature (Tc) ceramic superconductors using dc and RF induction coupled plasma jets (Ref 34, 35). Dense, thick deposits have been produced using prealloyed 1-2-3 YBaCuxOy powders, although the high Tc phase was only formed after a postdeposition heat treatment in oxygen. Researchers have not yet been able to directly plasma spray form the superconducting phase by dc plasma spraying prealloyed powders because the rapid cooling of the "1-2-3" materials from the liquid state suppresses direct formation of the high Tc phase. The microstructures of the deposits were also found to contain many small grains and pores as well as fine cracks that limited the maximum critical currents to very low values. Researchers at the University of Minnesota have developed a method of overcoming this limitation by using a vapor phase, plasma-assisted, reactive plasma spray process (Ref 34). This system uses an RF, induction-coupled, plasma heater in combination with liquid feeding of oxalate precursors of the copper, yttrium, and barium components. The liquids were ultrasonically atomized to generate an aerosol, subsequently carried by an oxygen carrier gas into the RF plasma jet. The longer heating, dissociation, and reaction times available in RF plasma jets have been shown to be able to produce the high-temperature 1-2-3 Tc phase directly. Thin, epitaxially deposited, films of the "1-2-3" high-Tc superconducting oxides were formed without the need for any postdeposition heat treatments or annealing in oxygen. The films were single crystals that exhibited none of the critical current limitations characteristic of the directly plasma sprayed materials.

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