Preparation And Dielectric Relaxation Of (Na_1-xK_x)_0.5Bi_0.5TiO₃Lead-free Piezoelectric Thin Films

Relaxor ferroelectrics belong to a special class of disordered materials that, because oftheir extraordinary piezoelectric and dielectric properties, show enormous potential forindustrial applications such as next-generation sensors, actuators and transducers that convertbetween mechanical and electrical forms of energy. However, relaxor ferroelectrics withexcellent properties, such as PLZT, PMN-PZT, PZN-PZT, have been widely used in MEMSdevice, which are detrimental to environment and human health duiring preparation andapplication for they include huge amount of lead oxide （senventy percent of total rawmaterial）. Great interest on lead-free composition with the relaxor behavior for lead-freeferroelectrics has been aroused in recent years because of the environmentally friendlyapplications, such as dielectrics for capacitors, actuators, etc. Because bismuth sodiumtitanate Na0.5Bi0.5TiO₃（NBT） exhibits strong ferroelectricity at room temperature and highCurie temperature, it is regared as one of lead-free piezoelectric materials. Pure NBT suffersfrom its high conductivity and large coercive field. Therefore it is difficult to obtain thedesired piezoelectric properties. In comparison with pure NBT, NBT-based solid solutionsmodified with BaTiO₃and K_0.5Bi_0.5TiO₃（KBT） show the improved piezoelectric anddielectric properties due to the existence of MPB. The material research from bulk to film islargely driven by miniaturization of electronic devices, the new micro-electromechanicalsystems （MEMS） and achievement of breakthrough on the concept of electronic devices.Previous reports focused on NBT-based bulk material, so NBT-KBT-100x relaxor ferroelectricthin films were investigated in details. It is summarized for development, properties andapplications of relaxor materials. In addition, the backgrounds of NBT-based relaxor materialare reviewed, such as bulk, thin film and thick film. Based on the above discussion, the topicand content of this thesis are proposed at last, and the results are listed the following.1. As the MPB of NBT-KBT-100x bulk material locates at the range of0.16-0.20andchemical-solution deposition （CSD） is easy to control chemical composition and precisestoichiometry, NBT-KBT-100x（x=0.15、0.18、0.20、0.25） thin films near MPB were preparedby CSD method.2. We investigated the effects of potassium contents on microstructure, dielectric,ferroelectric and piezoelectric properties of the thin films by X-ray diffraction （XRD）, thefield emission scanning electron microscopy （FE-SEM）, impedance analyzer, ferroelectrictester and SPM. The dielectric temperature spectra of NBT-KBT-18thin film were obtained by combining the impedance analyzer with high temperature sintering furnace. NBT-KBT-18thin film is of the largest effective piezoelectric coefficient, remnant polarization, spontaneouspolarization, dielectric constant, and the lowest dielectric loss among the thin films. Indielectric temperature spectra, NBT-KBT-18thin film is of typical relaxor properties and themechanism is discussed based on dielectric relaxation theory.3. As pyrochlore is high detrimental to microstructural and properties of thin film, theeffects of SrTiO3buffer layer on the microstructural, ferroelectric, leakage current propertiesand phase transition temperatures are investigated based on the above discussion. The resultsshow that SrTiO3buffer layer contributes to good texture and microstructural of the thin film,enhancement of ferroelectricity and increase of phase transition temperatures of NBT-KBT-20thin film, and safe working temperature for potential application in MEMS is broadened bythe buffer layer.