N substitution O-site in K0.5Na0.5Nb0.7Ta0.3O3 piezoceramics: Preparation, characterization and piezoelectric properties

Abstract

In the study, the N-doped K0.5Na0.5Nb0.7Ta0.3O3-xNx (x = 0.035, 0.055, 0.073) lead-free piezoceramics were successfully synthetized by the reaction of KNNT powders with urea. We systematically researched the influences of N3− partially substituted O2− on ceramics phase structure, microstructure, domain structures, piezoelectric properties, and oxygen vacancy defects. The results indicated N3− partially substituted O2− in KNNT ceramics without changing its phase structure, and remained a pure phase perovskite structure. N-doped KNNT ceramics had caused lattice expanded along the c-axis, leading to the increase of the asymmetry of KNNT lattice. In addition, with the increase of N content in KNNT ceramics, TC and TO-T of ceramics gradually decrease. Besides, the doped substitution of N to the O-site in KNNT ceramics would make the ceramics domains more easily reversed. The increase of lattice asymmetry and the easier reversal of electric domains caused by N doping would have positive effects on the d33 of KNNTN ceramics. Nevertheless, as the amount of N in the KNNT ceramics increased, the forbidden band width of the ceramics became smaller and the defects increased; and the oxygen vacancy defect concentrations were 18.9%, 20.2%, 20.9% and 22.5%, which corresponded to KNNTN (x = 0), KNNTN (x = 0.035), KNNTN (x = 0.055) and KNNTN (x = 0.073), respectively. The increase of the oxygen vacancy defect concentrations of KNNT ceramics would have negative effects on its piezoelectric properties. Hence, there was a competitive relationship between the positive and negative effects of N3− doping substituting the O-site in KNNT ceramics. When the doping amount of N was x = 0.035, its positive effects was greater than negative effects, and its relative density also reached the maximum value of 91.12%, making its d33 reached the maximum value of 207 pC/N.