Ultrasonic spray pyrolysis of a chelated precursor into spherical YBa₂Cu₃O_7−xhigh temperature superconductor powders

Abstract

YBa₂Cu₃O_7−x powders have been prepared directly by ultrasonic spray pyrolysis using nitrate salts as precursors and citric acid and ethylene glycol as chelating agents. This method consists of ultrasonically atomizing a precursor solution into droplets, thermally chelating, drying, decomposing and solid state reacting these droplets in a carrier gas flowing through a tube furnace, forming a well characterized powder. The chelated precursor adjusted to pH 8 forms bidentate bonding between the cations and the chelating agents. Thermal analysis and infrared spectroscopy identify the decomposition steps of the precursor. The dry gel of the chelated precursor is nearly amorphous indicating intimate mixing on the atomic level. X-ray diffraction suggests the mechanism of forming the 1∶2∶3 crystalline phase. Spherical powders are produced with diameters ranging from 0.2 to 0.8 μm depending on the ultrasonic frequency and the solution concentration. The spherical particles are hollow or solid depending on the precursor type and the furnace temperature. The primary crystallite size is about 10–50 nm. X-ray diffraction data and infrared spectra show that the spray pyrolysed powder from the chelating precursor forms the YBa₂Cu₃O_7−x phase at 800 °C, which is 100 °C lower than that formed from unchelated precursors.