The Preparation Of High-performance Lead-free Piezoelectric Ceramics Research

Environmental-friendly lead-free piezoelectric ceramics represented by（1-x）Bi_0.5Na_0.5TiO₃-xBaTiO₃（BNBT， BNBTx） and （1-x）Bi_0.5Na_0.5TiO_3-xBi_0.5K_0.5TiO₃（BNKT，BNKTx） has attracted much interest due to the superiorpiezoelectric performance with the x around the morphotropic phase boundary（MPB）. In order to promote their applications to ultrasonic transducers, sensors, andactuators, in present work, ternary solid solutions were designed and fabricatedbased on BNBT and BNKT. The composition, temperature, and frequencydependence of the dielectric, ferroelectric, and piezoelectric properties weresystematically studied. The main works in this dissertation are as follows:1. The phase diagram and electric properties were demonstrated for （0.935-x）Bi_0.5Na_0.5TiO_3-xBi_0.5K_0.5TiO_3-0.065BaTiO3solid solution with0.5%mol Mn doping.A morphotropic phase boundary （MPB） formed between the ferroelectricrhombohedral and tetragonal phases around x of0.04with the MPB tolerance factort of0.984-0.986. The temperature and composition dependence of the dielectric,piezoelectric, ferroelectric properties along with the strain characteristics wereinvestigated in detail and a phase diagram was presented. Around the MPB region,the maximum values of piezoelectric constantd*33of290pC/N, d33of155pC/N,dielectric constant T33/0of1059and low dielectric loss tangent tan of0.017were obtained. In addition, the solid solution with composition x of0.24, exhibitingboth high depolarization temperature Tdof182oC, of156pC/N, d33of130pC/N, will be favorable for high-temperature actuator and sensor applications.2. Ternary solid solutions （0.935-x）Bi0.5Na0.5TiO3-0.065BaTiO3-xSrTiO3and（0.88-x）BNT-0.12BKT-xST （BNKSTx） were designed and fabricated. Temperatureand composition dependence of the ferroelectric, dielectric, and piezoelectricproperties were systematically investigated and a schematic phase diagram wasestablished. The SrTiO3substitution was found to induce a transition fromferroelectric to relaxor pseudocubic phases. For BNBST, around a critical composition x of0.22, large strain response of⁰.2%（under a moderate field of4kV/mm） with normalized strain of490pm/V was obtained. For BNKST, thecorresponding unipolar strain could reach up to⁰.25%with normalized strain of580pm/V. The large unipolar strain response would be of great interest forenvironmental-friendly “on-off” actuators.