ULTRASONIC SPRAY-PYROLYSIS OF A CHELATED PRECURSOR INTO SPHERICAL YBA2CU3O7-X HIGH-TEMPERATURE SUPERCONDUCTOR POWDERS

YBa2Cu3O7-x powders have been prepared directly by ultrasonic spray py rolysis using nitrate salts as precursors and citric acid and ethylene glycol as chelating agents. This method consists of ultrasonically at omizing a precursor solution into droplets, thermally chelating, dryin g, decomposing and solid state reacting these droplets in a carrier ga s flowing through a tube furnace, forming a well characterized powder. The chelated precursor adjusted to pH 8 forms bidentate bonding betwe en the cations and the chelating agents. Thermal analysis and infrared spectroscopy identify the decomposition steps of the precursor. The d ry gel of the chelated precursor is nearly amorphous indicating intima te mixing on the atomic level. X-ray diffraction suggests the mechanis m of forming the 1:2:3 crystalline phase. Spherical powders are produc ed with diameters ranging from 0.2 to 0.8 mu m depending on the ultras onic frequency and the solution concentration. The spherical particles are hollow or solid depending on the precursor type and the furnace t emperature. The primary crystallite size is about 10-50 nm. X-ray diff raction data and infrared spectra show that the spray pyrolysed powder from the chelating precursor forms the YBa2Cu3O7-x phase at 800 degre es C, which is 100 degrees C lower than that formed from unchelated pr ecursors.