New Methods for Preparing Submicrometer Powders of The Tungstate-lon Conductor Sc_2(WO_4)_3 and its Al and In Analogs

摘要

Two new methods for preparing submicrometer powders of M_2(WO_4)_3, M = Sc, In, and Al via combustion synthesis are reported. Stoichiometric combinations of trivalent metal nitrates, ammonium metatungstate, and either urea or carbo-hydrazide as the fuel were reacted at 550℃, producing amorphous or poorly crystallized powders with an average particle size ranging from 164 to 350 nm. Calcining the powders at 800℃ for 1 h produced well-crystallized, phase-pure powders with an average particle size ranging from 210 to 711 nm. Powders sintered at 1000℃ for 14 h resulted in pellets that were 87%-95% of the theoretical density, which is notably higher than typically obtained from powders prepared by solid-state reaction. Whereas there was little difference in the microstructure of Al_2(WO_4)_3 pellets prepared with the two different powders, the carbohydrazide-derived powders resulted in In_2(WO_4)_3 and Sc_2(WO_4)_3 pellets with a larger grain size than those prepared with urea-derived powders. The electrical conductivity of the sintered pellets, while comparable to that reported for polycrystalline M_2(WO_4)_3 prepared by solid-state reaction, was strongly influenced by grain-boundary effects.