Effect Of Grain Boundary Regulation On Properties Of CaCu₃Ti₄O₁₂ Based Dielectric Materials

With the miniaturization,intelligence,integration and multi-function of electronic components,the dielectric properties of the materials need improving urgently.Therefore,the giant dielectric constant materials are attached more importance in the field of microelectronics,which is employed for the key material of electronic components including dynamic memories,multilayer ceramic capacitors and filters.CaCu3Ti4O12(CCTO)has a perovskite-like structure with good temperature stability and frequency stability.The dielectric constant is as high as 105 at low frequencies,and remains almost constant in the range of 300 K to 600 K.In addition,CCTO can exhibit giant dielectric properties without doping rare earth.Therefore,CCTO has great prospects in the electronic component industry,which has attracted great attention.However,CCTO ceramics still have a large dielectric loss and are still difficult to put into practice.As an electronic material with semiconducting grains and insulating grain boundaries,the relationship between microstructure and properties is studied in depth,and the conductive mechanism is described,which the performance of such materials can be further optimized and its practical application can be promotedIn this paper,the nano-scale CaCu3Ti4O12 powders were synthesized by sol-gel method.The phase structure,composition distribution and element valence state of the powders were characterized by X-ray diffraction(XRD),energy dispersive spectroscopy(EDS),X-ray photoelectron spectroscopy(XPS),which shows that the obtained CaCu3Ti4O12 powders have perovskite structure with high purity.CaCu3Ti4O12 ceramics were prepared by ceramic process using nano-CaCu3Ti4O12 powders.The microstructures of ceramics were characterized by XRD,SEM,EDS and XPS.Simultaneously,the effects of different sintering temperatures and time on the electrical properties of CaCu3T14O12 ceramics were systematically investigated.It was found that the sintering conditions can significantly affect the electrical properties of the ceramics.When the sintering temperature is around 1100 0 C and the sintering time is 8?12 hours,the dielectric constant at the room temperature of pure phase CaCu3Ti4O12 ceramics is larger than 3×104(1 kHz).The relative studies on microstructure and performance show that the change of Cu ion content in the lattices can obviously affect the dielectric properties of CCTO ceramics.The impedance analysis is used to study the change of the conductivity of ceramic grains and grains boundaries.The results reveal that the ceramic has an internal barrier structure composed of insulating grain boundaries and semiconducting grains.In order to improve the dielectric properties,it is necessary to optimize the microstructure of CCTO ceramics.Firstly,the grains should have good conductivity for producing electric charges.Secondly,the thin and high-resistance grain boundaries can benefit the accumulation of charges.Based on the theory of defect chemistry,the conduction mechanism of CaCu3Ti4O12 is reasonably explained.The effect of N2 sintering on the microstructure and properties of CaCu3Ti4O12 ceramics was studied.It was found that there was a large amount of CuO phases in the ceramic after sintering.The possible reason is that the low oxygen pressure promoted the decomposition of CaCu3Ti4O12 phase in N2 environment.According to the analysis,the excess Cu2+ vacancies in CCTO will combine with the donors to form the complex defects,which balances the charge of the donor defects and weakens the conductivity of the grains.Therefore,above complex defects deteriorate the dielectric properties of the ceramic.To further reducing the dielectric loss,the hydrothermal method was used to coat SiO2,ZrO2 and Fe2O3 on the surface of nano-sized CaCu3Ti4O12 powder particles to form a core-shell structure,which is helpful to control the performance of CaCu3Ti4O12.It's proved by various characterization methods that the core-shell structure formed by coating silicon compounds is composed of Ca2SiTiO6 phase.the core-shell structure formed by coating ZrOCl2,the shell layer is ZrO2 phase.the core-shell formed by coating Fe2O3,the shell layer is FeTiO3 phase.It indicates that ion exchange occurs between the core and the shell,and the oxides shell layer will cause the change of Cu and Ti ions concentration in the core of CCTO,which significant affects the dielectric properties.Among them,Fe3+ ion coating does not improve the dielectric performance,while Si4+ and Zr4+ ions coating can significantly reduce the dielectric loss.The dielectric constants at the room temperature of CCTO@SiO2 and CCTO@ZrO2 are all larger than 104,and the dielectric loss is less than 0.04 and 0.035(1 kHz),these performances are close to satisfy the application requirement of capacitors.