Synthesis And Physical Properties Of (1-x)KNN-xLN Lead-free Ferroelectric Thin Films

Potassium sodium niobate (KNN)-based piezoelectric ferroelectric ceramics is a kind of lead-free environmentally friendly materials, and has high Curie temperature, relatively large piezoelectric constant and electromechanical coupling coefficient. Thus, KNN-based ceramics is considered as one of the most promising alternatives for the traditional lead-based piezoelectric ferroelectric ceramic materials such as lead zirconate titanate (PZT). At the same time, the requirements for miniaturization and integration of electronic components are greatly promoting the preparation and research of KNN-based thin film materials.The deviation of chemical composition caused by the volatilization of alkali metal elements in preparation process will reduce the properties of KNN thin film, which is one of the bottlenecks of the applications of KNN ferroelectric thin film. So the main purposes of this thesis are improving the properties of KNN thin film by doping and improving the process conditions.In this thesis,(1-x)K0.5Na0.5NbO3-xLiNbO3(referred to as (1-x)KNN-xLN) lead-free ferroelectric thin films with thickness between1-2μm were prepared by chemical solution deposition (CSD) method on Pt(111)/Ti/SiO2/Si(100) substrates. And the influence of annealing temperature, thermal treatment atmosphere and Li doped content on crystalline phase, microstructure, chemical composition, dielectric properties, leakage current, ferroelectric properties and piezoelectric properties of (1-x)KNN-xLN thin films were studied systematically. The conclusions are as follows:1. By exploring and optimizing the process conditions of the chemical solution deposition method, we successfully got the (1-x)KNN-xLN thin films with perovskite structure ((100),(110) diffraction peaks are the strongest), smooth surface and no crack.2. Piezoelectric response of the KNN thin films increases with the increasing of Li doped content, for example, the piezoelectric response of0.90KNN-0.10LN thin film is nearly four times that of pure KNN thin film, which indicates that Li doping could significantly improve the piezoelectric properties of KNN thin film. But at the same time, Li doping causes the KNN thin film leakage current increasing, crystallization becoming worse and the RMS roughness increasing slightly.3. The relationship between annealing atmosphere and properties of KNN-LN thin films. Unlike the air atmosphere annealing, the oxygen annealing atmosphere can not only maintain the role of Li to improve micro structure and enhance electrical properties of KNN thin film, and can effectively eliminate or reduce the adverse effects which caused by Li doping. The advantages of oxygen atmosphere annealing for Li-doped KNN in our study are as follows:1) Effectively inhibited the cracks on film surface which caused by Li doping;2) Excellently inhibited the volatilization of Na and K, made the chemical composition ratio of Na/K and (K+Na)/Nb close to the ideal ratio of1:1. The effect above probably due to the oxygen annealing atmosphere can accelerate the crystallization of the film, so Na and K could quickly integrate into the stable perovskite structure, and thus less volatilization compared to the simple physical adsorption when prior to crystallization;3) Reduced the leakage current of the KNN-LN thin films. For example, under the external electric field of120kV/cm, the leakage current density of the0.94KNN-0.06LN thin film which annealed in oxygen atmosphere at650℃is2.9×10-4A/cm2, this value is more than one order of magnitude smaller than that of the0.94KNN-0.06LN thin film which annealed in air atmosphere at650℃;4) Among all samples, the0.94KNN-0.06LN thin film which annealed in oxygen atmosphere at650℃has the largest dielectric constant and a very small dielectric loss, at1kHz,εr=471.1, tanδ=0.0302, this dielectric constant is larger than that of the0.94KNN-0.06LN thin film which annealed in air atmosphere as much as107.4%;5) Among all samples, the0.94KNN-0.06LN thin film which annealed in oxygen atmosphere at650℃has the largest remanent polarization Pr=7.2μC/cm2, the coercive field Ec=88 kV/cm, and observed that its ferroelectric domain achieved a180°polarization reversal under external electric field;6) The0.90KNN-0.10LN thin film which annealed in oxygen atmosphere achieved the largest piezoelectric response among all samples.4. The influence of oxygen atmosphere annealing on the non-doped lithium KNN thin film is opposite from that on the KNN-LN thin films which mentioned above, such as exacerbating the volatilization of Na, K in the pure KNN thin film and reducing its dielectric properties. From the results above we can know that thermal treatment atmosphere and Li doped content interact with each other to affect the properties of KNN thin films.