Study On Structures And Properties Of Lead-free Ferroelectric K_0.5Na_0.5NbO₃ Films Derived From The Chemical Solution Depostion Method

Lead-based ferroelectric/piezoelectric materials have been widely used in the practical application for many years because of their superior piezoelectric properties and ajustable composition. However,due to the toxicity of lead oxide that is largely used as the start chemical, the lead-based derivatives have harmful effects on human beings and the environment during their production, use and dispose. In addition, the lead and other hazardous substances are banned by many legislations recently. So it is significant to study the lead-free, environmentall friendly piezoelectric matrials. Among the lead-free materials, the potassium sodium niobate, K_0.5Na_0.5NbO₃ (KNN)is considered as one of the promising candidates due to its high Curie temperature, high piezoelectric and electro-mechanical coupling coefficients. With the development of micro-electro-mechanical systems, integrated sensors and transducers, the films have also attracted the interest of many researchers. Therefore, the dissertation focuses on the preparation of lead-free ferroelectric KNN films derived from the chemical solution depostion (CSD) method.The KNN films prepared by a conventional CSD method can not show good electrical properties. However, polyvinylpyrrolidone (PVP)-modified method show greatly improved performances for the KNN films as the thickness is over 2μm. The dielectric constant can be up to 800-900 with a lower dielectric loss of 5-7 %,and the typically saturated ferroelectric hysteresis loops with the highest remnant polarization of 16.4μC/cm² can be easily obtained. In addition, the high piezoelectric coefficient of 61 pm/V is also obtained by laser scanning vibrometer.In the study of the effect of thicknesses on the structures and electrical properties of KNN films, it was found that there exists a critical thickness between 1.3 and 2.5μm. When the thickness of KNN films is lower than the critical thickness, their electrical properties are very poor. Once the thickness is higher than the critical thickness, the electric properties are significantly improved. In addition, the variation of electric properties of KNN films with 2 the thickness tends to be stable. The study on the residual stresses shows the tensile stresses exist in the KNN films with small thickness, and the compressive ones exist in those with larger thickness, which can be released with the thickness.Since introduction of PVP greatly improved the electrical properties of KNN films, the mechanism of PVP on KNN was studied with analyses of thermogravimetric analysis-differential scanning calorimetry (TGA-DSC), thermogravimetric analysis-mass spectrometry (TGA-MS) and x-ray photoelectron spectrocopy (XPS). It was found that the introduction of PVP with the appropriate molecular weight (360,000) can obviously decrease the crystallization temperature, reduce the losses of alkaline ions, hence promote the crystallization of KNN perovskite phase and improve the electrical properties. The introduction of PVP with a very large molecular weight of 1,300,000 results in porous morphologies of KNN films so that the electrical properties were degraded. The PVP with a small molecular weight can not reduce the volatility of alkaline ions but results in more loss of alkaline ions, and lead to the form of secondary phases. With the TGA-MS analyses, it was found that the alkaline ions for all KNN films were mainly volatilized at a lower temperature range of 300-500℃before crystallization and were lost mainly as the oxides of KO and NaO. In addition, it can be seen that the introduction of PVP effectively supressed the volatility of alkaline ions after the formation of KNN perovskite phase. It is found that loss of Na⁺ ions is higher than that of K⁺ ions, so the secondary phase always appeared as the potassium niobate.The improved electrical properties were obtained in the KNN films with thickness higher than 2μm with the introduction of PVP. To make the KNN films with small thickness also show well saturated and typical hysteresis loops, the Mn²⁺ and Co²⁺ dopants were introduced into the KNN precursor solutions. It was found that the KNN films with the thickness of 1.3-1.6μm show the improved electrical properties, especially the ferroelectric properties which the typically saturated hysteresis loops were obtained. The 2 mol% Mn-doped KNN film shows the high dielectric constant of 521 and the remnant polarization, Pr of up to 7.2μC/cm². While the 2 mol% Co doped one shows the higher dielectric constant of 629 and the maximum Pr of 7.5μC/cm². The introduction of Mn²⁺ and Co²⁺ ions can promote the formation of KNN perovskite phase and significantly decrease the leakage current density. The improved electrical properties can be obtained for the doped KNN films. The x-ray photoelectron spectrocopy analyses indicates that the initial Mn²⁺ and Co²⁺ ions were oxidized into Mn³⁺ and Co³⁺ ions. Mn³⁺ and Co³⁺ ions substitute the alkaline ions in A-site and Nb5+ ions in B-site of KNN perovskite structure, respectively.