Study on Microwave-assisted Hydrothermal Synthesis and the Properties of KNbO_3 Powders

KNbO3 powders with different phases and morphologies were successfully synthesized through the reaction between KOH and Nb2O5 by microwave-assisted hydrothermal method at 200℃.Pure KNbO3 crystals were obtained when the concentration of KOH was maintained from 10mol/L to 14mol/L.According to the XRD,FESEM and TEM characterizations,it could be observed that as increasing the concentration of KOH from 10mol/L to 15mol/L,the KNbO3 phases changed from rhombohedral to orthorhombic then to tetragonal,and the corresponding morphology of the KNbO3 powders changed from pyramid-like to rod-like then to cubic-like.Some Nb2O5 phases were found when the concentration of KOH increased to 15mol/L.KNbO3 ceramics were sintered from the KNbO3 powders by conventional processing.The piezoeletric properties such as the piezoeletric constant d33,the relative permittivity e33/e0,the dielectric loss tanδ,the electromechanical coupling factors kp and the mechanical quality factor Qm of the sintered KNbO3 ceramics were 80 pC/N,302,0.023,0.17,70,respectively.The tetragonal to orthorhombic and cubic to tetragonal phase transitions temperatures were 223℃ and 420℃,respectively.