New Methods for Preparing Submicrometer Powders of The Tungstate‐Ion Conductor Sc 2 ( WO 4 ) 3 and its Al and In Analogs

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 carbohydrazide as the fuel were reacted at 550°C, producing amorphous or poorly crystallized powders with an average particle size ranging from 164 to 350 nm. Calcining the powders at 800°C for 1 h produced well‐crystallized, phase‐pure powders with an average particle size ranging from 210 to 711 nm. Powders sintered at 1000°C 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.