Study On Preparation And Dielectric Properties Of Core-shell Sructure BaTiO₃-based Dielectric Ceramics

As the basic device of electronic information technology,multilayer ceramic capacitors?MLCCs?are widely used in aerospace,petrochemical,automotive and other fields.BaTiO₃ possesses the advantages of high dielectric constant,low cost,environmental protection and so on.It was widely used in commercial production of MLCCs,but its Curie temperature is around 130 ^oC limiting its application in ceramic capacitor with wide dielectric temperature stablity.The"core-shell"structure plays an important role in the preparation of BaTiO₃-modified MLCCs materials with wide working temperature range and high dielectric temperature stability.The contribution of different regions of core/shell to dielectric constant is studied,which has a guiding significance on design of core/shell ratio.In this paper,0.25Bi(Zn_0.5Ti_0.5)O₃-0.75BaTiO₃?0.25BZT-0.75BT?powder was prepared by sol-gel method.And commercial 300nm BaTiO₃ powder was used.The impedance spectra of BaTiO₃ and 0.25BZT-0.75 ceramics prepared by conventional sintering and microwave sintering were tested and analyzed.The results show that the impedance spectra of BT ceramics can be fitted to a"3RC"equivalent circuit.The activation energies of grain,grain boundary and electrode interface of conventional sintered BaTiO₃ are 0.86,1.02 and 1.39 eV,respectively.The activation energy of microwave sintered BT decreased and they are 0.82,0.92 and 1.24 eV,respectively.On the other hand,the impedance spectra of 0.25BZT-0.75BT ceramics can be fitted with a"RC"equivalent circuit.The activation energy of the conventional sintered0.25BZT-0.75BT ceramic is 0.94 eV,while the activation energy of the microwave sintered sample corresponds to 0.69 eV.Secondly,0.25Bi(Zn_0.5Ti_0.5)O₃-0.75BaTiO₃[abbreviated as BT@x?0.25BZT-0.75BT?]?x=0.2¹.0?was coated on the surface of BaTiO₃ by sol-gel method.Effects of coated shell ratio on sintering properties,miromorphologies and dielectric properties and impedance spectra of BaTiO₃ ceramics were studied.The dielectric properties of the system show that the 0.25BZT-0.75BT coated shell can significantly improve the dielectric temperature stability on the high temperature side.Especially,BT@0.25BZT-0.75BT ceramic sample with the optimal properties at 1kHz frequency?=2587,tan?=0.06,?C/C?±15%?0¹65 ^oC??,meeting the Z8R specification.AC impedance spectroscopy testing was conducted on the samples.It was found that components of x=0.2⁰.4 could be fitted by a"3RC"equivalent circuit,corresponding to the three elements of grain core,grain shell&grain boundary and ceramic/electrode interface,respectively.While components of x=0.5¹.0 components can be fitted well with a"4RC"equivalent circuit,corresponding to grain core,grain shell,grain boundary and ceramic/electrode interface,respectively.The activation energy of the grain core?x=0.5¹.0?is similar to that of the conventional sintered BaTiO₃ grain's?0.86 eV?,and the activation energy of the shell increases from 0.28 eV to 0.53 eV with the increasing of shell thickness.In this paper,combined with the data obtained by the fitting of impedance spectroscopy and the permittivity-frequency curves of BT and 0.25BZT-0.75BT,the modified two-layer dielectric model was used to calculate the average thickness ratio of the grain core/shell in BT@0.8?0.25BZT-0.75BT?ceramics(d_gc/d_gs=7.26).Finally,the dielectric constant of BT@0.8?0.25BZT-0.75BT?ceramics at room temperature was simulated by Lichtenecker's equation.The results of the simulation are in good agreement with the actual measured values.Besides,considering the limitations of ceramics with"core-shell"structures prepared by conventional sintering,sol-gel method and microwave sintering are combined to prepare BT@x?0.25BZT-0.75BT??x=0.5¹.0?ceramics.The AC impedance spectra of the system prepared by this method are similar to those of conventional sintering,and can also be fitted by the"4RC"equivalent circuit.The activation energy of grain core is similar to that of microwave sintered BaTiO₃ grain?0.82 eV?,and the activation energy of grain shell is obviously increased compared with the conventional sintering,increasing from 0.53 eV to 0.80 eV with the increasing of shell thickness.The dielectric constant of BT@0.8?0.25BZT-0.75BT?ceramics was simulated by Lichtenecker's equation.The simulation results were in good agreement with the actual measurements.