Papers by Author: Ji Wei Zhai

Abstract: BaxSr1-xTiO3 (x=0.4, 0.5, 0.6) ceramics were fabricated by the conventional solid-state reactions method. The temperature dependences of the dielectric constant and tunability were investigated under high DC electric field. It was found that the change of dielectric constant and tunability under the applied electric field were closely related to ferroelectric phase, phase transition region and paraelectric phase states. The Curie temperature (Tc) was gradually shifted to higher temperature and were broadened and depressed with increasing of DC electric field. The tunability dependence of temperature exhibits different trends in a wide temperature range and reaches a maximum value near the ferroelectric-paraelectric phase transition. These results may be helpful in understanding the mechanism of dielectric response under higher electric field.

Abstract: Lead-free piezoelectric ceramics, (Ba0.85-xSrxCa0.15)(Zr0.1Ti0.9)O3 (BSCZT, x=0.01-0.07), were prepared via a solid-state reaction route. The dielectric properties, ferroelectric properties, piezoelectric and strain properties of BSCZT ceramics were studied. The phase structure and microstructure were investigated by X-ray diffraction and scanning electron microscope, respectively. Results showed that dense ceramics with pure perovskite phase were obtained. At room temperature, the samples with x=0.03 exhibited excellent properties with large piezoelectric coefficient d33=534pC/N, planar mode electromechanical coupling coefficient kp=47.7%, thickness mode electromechanical coupling coefficient kt= 42% and high strain levels of 0.34%. In addition, the study of electrical properties suggested that the Curie temperature decreased linearly from 92oc to 73oc with the increasing doping content of strontium in BCZT ceramics. The remnant polarizations, piezoelectric coefficient and strain levels were all increased as the Sr content increased and then decreased with further increased Sr doping level, giving the maximum values at the Sr content of 3mol%. These results indicated that the BSCZT system is a promising lead-free material for applications in the future.

Abstract: For energy storage devices such as capacitances, pulsed power applications and other any electronic systems, high permittivity and high breakdown strength (BDS) are necessary for dielectric materials used in high energy storage capacitors. Ba_0.4Sr_0.6TiO₃ nano-powders were directly synthesized from solution and then wrapped by a thin surrounding shell with composition of zinc oxides dopants in aluminum oxides. This core-shell structure was characterized by a conjunction of XRD, and TEM analysis. The core-shell nanoparticles were pressed into pellets with 10 mm diameter and then sintered at different temperature in air. The specimen coated with 2.0wt% compounds of aluminum oxides doped with zinc oxides showed the maximum breakdown strength of 475kV/cm and the energy storage density of 4.7J/cm3 while the tanδ was only 0.002, whereas the uncoated Ba_0.4Sr_0.6TiO₃ was only 86.9kV/cm, 0.23J/cm3 and 0.005, respectively. And the energy storage density and the BDS decreased when the shell content was more than 2.0wt%.

Abstract: Calcium bismuth niobate (CaBi₂Nb₂O₉, CBN) is a high Curie temperature (Tc) piezoelectric material, however, their piezoelectric properties are very poor. In order to improve the piezoelectric performance of CBN, the Eu modification and texture method (templated grain growth, TGG) were applied in this material system. The piezoelectric properties of CBN ceramics were significantly enhanced by Eu modification. The 5mol% Eu dopants in CBN ceramics have a higher piezoelectric constant and higher Tc (d₃₃=12.8pC/N, Tc=945°C) than that of undoped CBN ceramics (d₃₃=6.2pC/N, Tc=930°C). Highly textured 5mol% Eu doped CBN ceramics with 92% Lotgering factor were obtained by the TGG technique. The piezoelectric properties of textured and modified CBN ceramics are anisotropic on different directions of perpendicular (d₃₃=0.1pC/N) and parallel (d₃₃=20.5pC/N) to tape stacking direction. Furthermore, they had excellent thermostable piezoelectric properties from room temperature to near Curie temperature. The textured Eu modified CBN ceramics with high Tc and high piezoelectric properties might be a promising candidate for high-temperature piezoelectric applications.