Influence Of A-site Nonstoichiometry On Structure Of Undoped K_0.5Na_0.5NBO₃Lead-free Piezoelectric Ceramics

K0.5Na0.5NbO3based ceramic possesses excellent piezoelectric properties and high Curie temperature when the Na/K ratio is around50/50because of the presence of a morphotropic phase boundary(MPB). Since Saito el.al. developed a textured KNN-based ceramic whose properties were comparable with PZT system, more and more attentions have been paid to KNN-based piezoceramics. In this paper, the attempts to improve the sinterability and physical properties of K0.5Na0.5NbO3by chemical modification were introduced in detail. And the crystal structure and microstructure of un-doped potassium niobate sodium based lead-free piezoelectric ceramics were studied by changing A-site elements’stoichiometry. The major concerning of (K0.5+xNa0.5+y)NbO3ceramics is the problem of obtaining samples with high density by conventional solid state sintering method. The evaporation of the A-site element K and Na leads to the degration of electric properties and sintering ability, therefore, Investigating the influence of A-site non-stoichiometry on the structure and properties of un-doped KNN based lead free piezoelectric ceramic are of great significance.In this work, we attempt to fabricate KNN samples by traditional solid state method assisted by mechanical activation to prepare nano-scale ceramic powder. From the XRD pattern, When A-site elements(K and Na) stoichiometry have a small deviation, there will be a phase transition from orthorhombic to quasi-tetragonal when the amount of K and Na decrease. The emergence of MPB greatly enhance the sintering properties, relative density and physical properties of the KNN ceramic samples. Combining with the Raman and Fourier infrared spectroscopy can further find that A-site element variation have great impact on niobium octahedral ligand field lead to the alteration of O-Nb-O bond length and energy and the lattice parameter will accordingly change, too. From SEM images, it can be found excess alkaline metal is beneficial to form liquid phase and increase material density, But when the amount of excess K or Na reach to1.5%or1%, respectively, grain abnormal growth appear, and the samples will be easy delinquent.When the deficiency of both K and Na are up to1.5%mol there will be tungsten bronze structure K5.75Nb10.85O30precipitate. However, When the excess amount of K and Na simultaneously reach2%mol, minor NbO2.432which can be detected by XRD pattern will appear as a second phase. The two situation both imply the the variation of A-site stoichiometry can create B-site vacancy, especially in the second situation, the excess K and Na will occupy B-site vacancy, Then oxygen vacancy generates in order to mountain the electrical neutral balance. As it can be seen from the SEM images, the second phase precipitation can reduce the grain size, change the grain morphology, and has a significant impact on the sintering and densification of the KNN sample.