Research On Preparation And Modification Of The Potassium Sodium Niobate (KNN) Lead-free Piezoelectric Thin Films

Lead-free sodium potassium niobate?KNN?piezoelectric thin film has been regarded as one of the best choices to substitute for lead-based materials because of its high Curie temperature and excellent piezoelectric properties together with fine biocompatibility,which can be widely applied to drivers,sensors,actuators and other electronic devices.In this thesis,the KNN thin films were fabricated by sol-gel method successfully.The effects of thermal treatment temperatures,Li⁺/Mn²⁺ doping and introducing buffer layers on structure,and properties of KNN thin film were studied.The main works are as follows:Lower pyrolysis temperature can make the organic incomplete decomposition,which results in a large number of holes.But the serious volatilization of alkaline ions leads to the increase of oxygen vacancy and the decrease of solid solubility at the higher pyrolysis temperature.Lower annealing temperature will make the film crystallize incompletely.However,serious volatilization of the alkaline ions at higher annealing temperature also reduces the quality of the film.When the pyrolysis and annealing temperature are 400? and 600? respectively,the KNN thin films were prepared successfully with the dielectric constant of 712 under 1 k Hz and 2P_r and 2E_c of 6.2 ?C/cm² and 118.6 kV/cm respectively at the applied electric field of 200 kV/cm.Lithium ions mainly take place of the A vacances of KNN thin film,which can refine grain and improve the crystallization so as to make the KNN film denser and more homogeneous.When the amount of Lithium ions is 1.5 mol%,the film has the best crystallinity and optimal electrical properties with ?_r of 449.7 under 1 kHz and 2P_r and 2E_c of 10.3 ?C/cm² and 107.2 kV/cm respectively at the applied electric field 200 kV/cm.Mn doping is effective in reducing the leakage current and improving the electrical properties of thin films,because Mn is a variable valence element,which can absorb the free carrier-holes originated from the volatilization of alkaline ions.Optimal dielectric properties were obtained in KNN films doped with 2 mol% Mn,whose dielectric constant and dielectric loss are 875 and 0.030 at 1 k Hz,respectively.In addition,well-saturated ferroelectric P-E hysteresis loop with large remanent polarization?2P_r?and coercive field?2E_c?of 22.5 ?C/cm² and 130 kV/cm were obtained in 2 mol% Mn-doped KNLN film at the applied electric field of 200 kV/cm.Adding Nb₂O₅ buffer layer to the KNN thin film is an effective method to restrain the mutual diffusion between KNN thin film and Pt substrate,and reduce the lattice mismatch defects caused by interface mismatch.The KNN film with Nb₂O₅ buffer layer of double layers gains the best ferroelectric properties with the dielectric constant of 265 under 1 k Hz and 2P_r and 2E_c of respectively 7.8 ?C/cm² and 75.8 kV/cm at the applied electric field 108.5kV/cm.Introducing SrTiO₃ buffer layer can reduce the crystal lattice mismatch between the film and substrate,and restrain the emergence of non-ferroelectric interface layer.The ferroelectric and dielectric properties of the KNN films are enhanced by adding the seed layer obviously.The dielectric constant of KNN film is up to 446?1 k Hz?,2P_r =10.8 ?C/cm²,2E_c = 121 kV/cm?200 kV/cm?.