Fabrication And Properties Of Multilayer Coated BaTiO₃-based Dielectric Ceramics For Capacitors Application In Wide Temperature Range

As with high dielectric constant, low cost and environment friendly features,BaTiO3 ceramic are first applied to the production of MLCC, however, the Curie temperature(Tc) of BaTiO3 is about 130 oC, which limits the application in X9 R ceramics, especially in electronic industry. Based on the theory of ‘core-shell'structure, the material selection of core and shell, the design of multilayer ‘core-shell'structure were systematically investigated in this paper to improve the dielectric-temperature stability.To improve the dielectric temperature stability at high temperature section,xBi(Zn0.5Ti0.5)O3-(1-x)BaTiO3(Abbreviated as xBZT-(1-x)BT) was chosen to study in this paper. The xBZT-(1-x)BT(x=0.1~0.5) ceramics were synthesized by polymer net-work method. The phase structure, microstructure and dielectric properties with the various doping contents were investigated. The results show that, with the contents of BZT increase, the Tm increased then decreased and the peak of dielectric constant was suppressed and broadened, showing obvious relaxation behavior.Dielectric properties of three kinds of xBZT-(1-x)BT coated by Nb2O5 were investigated. It turned out that the core material with low Tm may result dielectric peaks combination of core and shell. Only when the temperature difference between dielectric peaks of core and shell was big enough, the dielectric temperature stability could be improved by ‘core-shell' structure.It was easy to obtain a wider dielectric temperature stability when dielectric temperature stability of the core material was good at high temperature section.The mono dispersed Ba TiO3 powders was synthesised by alkoxide-hydroxide sol-precipitation method to improve the agglomeration during the coating process.The powders were characterized by XRD and SEM, and the powders were coated with 0.3BZT-0.7BT and Nb/Co oxides respectively. The dielectric properties of coated samples showed that the thickness and composition of coating material had influence on the core BaTiO3 crystal transformation. According to this, the inner stress model of ‘core-shell' structure was introduced to explain the phenomenon of abnormal. The BaTiO3 successfully turned to tetragonal phase from cubic phase byadjusting the thickness and composition of coating material.In order to improve the low temperature end and the high temperature end dielectric properties of BaTiO3 ceramics at the same time, a multilayer ‘core-shell'structure was designed and synthesised by polymer net-work method that tetragonal phase Ba TiO3 as core material, 0.3BZT-0.7BT as the first coating layer, and Nb2O5 as second layer, which was confirmed by TEM-EDS test. The results showed that the dielectric properties of Ba TiO3-based samples with multilayer core-shell structure were found to meet the X9 R specification when x=3, y=0.15. The dielectric constant was 1190 and the dielectric loss was 1.3% at room temperature.