Role of Alumina in the Preparation of Platelike NaNbO3 Powder by Molten Salt Synthesis and Proposal of New Preparation Method

The preparation of platelike NaNbO3 grains via single-step molten salt synthesis using Bi2O3, Na2CO3, Nb2O5, and NaCl as reactants was examined. When a new alumina crucible was used, platelike NaNbO3 grains were obtained, but a repeatedly used alumina crucible resulted in irregularly shaped NaNbO3 grains. When a platinum crucible was used, even NaNbO3 could not be obtained. Addition of alumina substrates and alumina granules to the reaction mixture in the platinum crucible resulted in the formation of platelike NaNbO3 grains and second-phase grains. The second-phase grains, which were composed of Al2O3, Bi2O3, Na2O, and Nb2O5 and had a pyrochlore structure, could be removed by sieving. The second phase acted as a scavenger for Bi2O3 and hence, the possibility of using another scavenger was attempted. The new scavenger was a mixture of Na2CO3 and Nb2O5, and using them, platelike NaNbO3 grains were successfully obtained with NaNb5Bi2O16 as a byproduct, which could then be removed by sieving.