Preparation And Properties Of Lead-free 0.94Na_0.5Bi_0.5Tio₃-0.06BaTiO₃ Piezoelectric Thin Films

Piezoelectric materials such as Pb(Zr,Ti)O3 (PZT) are widely used in micro-electro-mechanical systems (MEMS), microactuators, transducers and micromotors due to the superior piezoelectric properties. The lead toxicities of producing, using and recycling are problematic for devices associated with PZT or other lead oxide-based piezoelectrics, therefore lead-free piezoelectric materials are desired inevitably as candidate materials in place of PZT for the sake of biocompatibility and environmental protection. Among various lead-free piezoelectric materials, (Na0.5Bi0.5)TiO3 (NBT) ceramic is recognized as one of key materials because of its strong ferroelectric properties at room temperature. But the application and performance of the material are limited due to the high coercive field and leakage current. In contrast with pure NBT, NBT based solid solutions show the improved dielectric and piezoelectric properties because of the existent rhombohedral-tetragonal morphotropic phase boundary (MPB). For instance, T. Takenaka et al. reported (Na0.5Bi0.5)1-xTiO3-BaxTiO3 (NBT-BTx) have superior piezoelectric properties attributed to the MPB at x=0.06-0.07. Generally, single crystal and ceramic of NBT-BTx are focused on the preparation, ferroelectric, dielectric and piezoelectric properties. However the NBT-BTx thin films are only focused on the preparation, ferroelectric and dielectric properties, and seldom are involved in the piezoelectric performance. In a word, the piezoelectric properties of 0.94Na0.5Bi0.5TiO3-0.06BaTiO3 (NBT-BT6) thin film near MPB are worth studying. The main contents of this thesis are given as follows.1. In chapter 1, the development, properties and applications of piezoelectric materials are summarized firstly. Secondly, the background and current state of lead-free piezoelectric materials, especially NBT-based piezoelectric material, are reviewed. On the basis of the review, the topic and content of this thesis are proposed at last.2. In chapter 2, we introduce the metal-organic decomposition (MOD) method, the corresponding instruments and experimental materials. And then, the flow chart of preparation for NBT-BT6 thin film grown on Pt/Ti/SiO2/Si(100) substrate is described in detail.3. In chapter 3, the NBT-BT6 thin film annealed at 750°C was fabricated by MOD method. By using a scanning probe microscopy system the butterfly-shaped piezoelectric response curve was measured to obtain the electrically induced strain and effective piezoelectric coefficient d₃₃^*. The mechanisms concerning the dependence of the high piezoelectric properties were also discussed. 4. Generally, annealing temperature has important effects on microstructure, ferroelectric, dielectric and piezoelectric properties of thin films. Based on the work of chapter 3, the effects of annealing temperature (650-800°C) on microstructure, ferroelectric, dielectric and piezoelectric properties of NBT-BT6 thin films were studied in chapter 4. We expect that the enhancements on ferroelectric, dielectric and piezoelectric properties of NBT-BT6 thin films can be achieved via adjusting annealing temperature.It is expected that the present research may offer useful guidelines to the design and application of piezoelectric NBT-BTx thin films.