The Effect Of Ba_0.67Sr_0.33TiO₃-based Ceramics Composition Change On The Structure And Properties

Multilayer ceramic capacitors (MLCC) are important electric components, the micromation, the integration, the low cost and multifunctionality of MLCC depends on preparing ceramics with high properties, including the preparation of porcelain raw material, the properties control of ceramics and so on. The Ba1-xSrxTiO3is one of the main systems. In the thesis, Bao.67Sro.33Ti03-based ceramic was selected as candidate material, the preparation methods of powder, the preparation technology of ceramics, the effect of Bao.67Sro.33Ti03-based ceramic composition change on permittivity as well as temperature stability and so on were investigated systematically, and we explore the intrinsic or extrinsic factors impacting properties for offering proof of obtaining ceramics with high quality.The main works and conclusions are summarized as follows:For the preparation of BST ceramics with optimum properties, it is necessary to obtain highly active powder first. We study the effect of preparation processes on the structure and properties of BST powder as well as ceramics using the solid method. And we futher investigate the influence of grain size on the properties of BST ceramics. The experiments show that when calcined at950℃, the BST powder present high purity, the better single dispersivity, small particle diameter (about110nm) and homogeneous size distribution. High relative density(98.6%), fine grains (about250nm) of BST ceramic sintered at1300℃show better dielectric properties and ferroelectric properties. The high room dielectric constant and low dielectric loss: εr=7676, tanδ=0.01; the large remanent polarization and low coercive field are Pr=9.2μC/cm2, Ec=l.58kV/cm, respectively.Considering the low melting point of CBT(CaBi4Ti4O15) as well as containing Ca, Bi ions with diffusion, in order to decrease sintering temperature and improve the dielectric temperature stability of BST ceramics effectively, the structures and properties of BST ceramics doped with various CBT content were investigated. The results show that CBT doping decrease the sintering temperature of BST ceramics to 1100℃, moreover restrain grain growing significantly. In addition, CBT improve the temperature characteristics of BST ceramics obviously, and the range of dielectric temperature stability(ΔC/C≤±15%) cann't be broaden obviously when the CBT content x>6%. The BST ceramics exhibit obvious ferroelectric relaxation behaviors by doping CBT, and the BST ceramics show an increased diffuseness of the dielectric peak and an increased degree of relaxor characteristics with increasing CBT content.It is a common and effective method to enhance the dielectric properties of BST ceramics by adding rare-earth elements. The structure and dielectric properties of La-doped BST ceramics were investigated as well as the relation between the properties of BST ceramics and the different defect compensation mechanisms resulting from La doping. The results show that proper rare earth La dopant (0.2≤x≤0.7) may greatly increase the dielectric constant of BST ceramics, also flat the dielectric peak and improve the temperature stability evidently. According to the current-voltage I-V(J-V) characteristics, the proper La doped BST ceramics may reach the better semiconductivity at the optimal densification condition, when the less and more La doping, the ceramics are insulators. In addition, the current-voltage basically follows the ohm's law for insulating samples; while for the semiconductive samples, the conduction mechanism is different at different voltage range, and the current is mainly controlled by the Schottky emission at comparatively high voltage range.By the Schottky barrier model and electric microstructure model to analyze the cause of the high dielectric constant behavior and the dielectric relaxation of La doped BST ceramics. The results show that the surface or grain boundary potential barrier height and the width of depletion layer do play an important role in impacting dielectric properties; and the surface potential barrier have a great contribution to the high permittivity of BST ceramics. CuO has been used to modify the electrical properties of BST, and it can be observed that the permittivity was further increased and the dielectric loss was decreased obviously as well as the broken voltage is improved evidently. Three various dielectric relaxations of BST ceramics are observed in the measured frequency range. On the complex impedance spectroscopy analysis, the surface barriers and grain boundary barriers have been further confirmed in the semiconductive BST ceramic. The formation of such electrical potential barriers leads to the electrical nonlinearity in the I-V characteristics and different interface polarizations resulting in very large apparent dielectric constant.