New Lead-free Ferroelectric Thin Films Preparation Using Chemical Solution Deposition Method And Engineered Domain Configurations In Ferroelectric Materials

As one of the most important active materials, ferroelectric materials hold a large ratio in the field of electronic materials. The sales volume of ferroelectric ceramics in the world increases at about 15 percent in recent years. With the rapid development of information industry, ferroelectric devices, such as ferroelectric memory, transducer, actuator, filter, resonator, frequency discriminator, have been extensively applied in the field of audio, video, communication, and computer devices.However, environmental issues are world-widely concerned. Ferroelectric devices which is mainly made from lead materials are facing great challenges both for the voice from ever-rising environmental protection and the development trend of electronic devices along the digital, high-frequent, multifunctional, thin, light, small and portable direction. This dissertation takes this as the cutting point and makes a deep study in ferroelectric materials and devices from mesoscale to macroscale. We prepare ferroelectric thin films by chemical solution deposition method. We analyze a rectangular ceramic plate in electrically forced thickness-twist vibration as a piezoelectric transformer. Meanwhile, we explore the relationship between the macro behavior of ferrolectric materials and their inner microstructure in mesoscale.The main work of this dissertation is as follows:1. We prepared the lead-free BNT thin films by chemical solution deposition. Phase identification, crystalline orientation and degree of crystallinity of the films were studied by a D/max-rA X-ray diffractometer. Scanning electron microscopy(SEM) was used to determine the thickness and the surface morphology of the thin films. The ferroelectric and dielectric measurements were performed using a RT66A ferroelectric tester and a HP4194A impedance analyzer equipped with a micrometer probe station. P-E hysteresis loop showed the chemical solution deposition derived BNT thin films were a potentially important candidate for non-volatile random access memory devices.2. A rectangular ceramic plate with appropriate electrical load and operating mode is analyzed for piezoelectric transformer application. An exact solution from the three-dimensional equations of linear piezoelectricity is obtained. The solution simulates the real operating situation of a transformer as a vibrating piezoelectric body connected to a circuit. Transforming ratio, input admittance, and efficiency of the transformer are obtained.3. We propose a mechanism for enhanced piezoelectricity that takes advantage of both intrinsic crystalline anisotropy of ferroelectric crystal and extrinsic 90°domain switching of ferroelectric variants. The intrinsic and extrinsic piezoelectric responses of barium titanate single crystals under different crystallographic orientations are calculated using an energy minimization theory, where it is observed that the piezoelectric coefficient is significantly enhanced by the 90°domain switching, especially under the small field measurement where the domain wall movement is reversible. The optimal crystallographic orientation is also identified.