The Effect Of Doping On The Dielectric Property Of Strontium Titanate Ceramics And Its Mechanism Analysis

With the rise of the requirement of size and weight reduction of impulse power systems, high energy density solid-state dielectric has become a main technical problem to be resolved. If a dielectric has highenergy density, it should possess higher dielectric constant and breakdown strength. Meanwhile, higher dielectric constant of the energy storage dielectric is the precondition of miniaturization of impulse power system. In addition, higher breakdown strength is the basic requirement that the impulse power systems apply to high voltage environment.Hence, to prepare solid-state energy storage dielectric accord with demands of miniaturization of impulse power system and high voltage pulse formation line, Sr Ti O3 based ceramics with higher dielectric constant and breakdown strength was chosen as research object. To gain much higher breakdown strength, equivalent site substitution was applied. Measuring and testing techniques, such as x-ray diffraction(XRD), energy dispersive X-ray spectroscopy(EDS), scanning electron microscope(SEM), alternating current impedance, and so on, were adopted to study the equivalent site substitution for Sr Ti O3 based ceramics by Mg, Ca, Zr and relative preparation, dielectric properties, energy storage properties, high temperature relaxation and conductivity.To gain higher breakdown strength, employ Mg2+ with smaller ionic radius substitute for the A site of Sr Ti O3. The solid-state reaction was taken to prepare the Sr1-x Mgx Ti O3 ceramics. If x?0.02 mol, Mg merely substitutes for A site; else, if x=0.04 mol, Mg partially substitutes for A site and partially substitutes for B site, and forms impurity phases. Mg substitution diminishes the grain size of Sr Ti O3 based ceramics, enhances the interaction between positive and negative ions, raises the breakdown strength, but dielectric constant varies slightly. At room temperature, Mg substituted Sr Ti O3 ceramics possess well dielectric frequency stability and low dielectric loss. Between 20Hz-2MHz, the dielectric loss is always less than 0.006. At the 362 k V/cm field strength, Sr0.99Mg0.01 Ti O3 gains the highest energy storage density 1.86J/cm3, which is 1.29 times to that of the pure Sr Ti O3 ceramics in this experiment(1.44J/cm3), and 2.66 times to that of the pure Sr Ti O3 ceramics in literatures. The energy storage efficiency is higher than 70%. To further improve the breakdown strength and energy storage density, Zr is utilized to substitute for B site of Sr0.99Mg0.01 Ti O3 ceramics. However, Zr substitution is not beneficial for the sintering of Sr0.99Mg0.01 Ti O3 ceramics. Grain size abnormally grows up. It results in the decrease of breakdown strength. Furthermore, lead to the reduction of the energy storage density.Both Ca Ti O3 and Sr Ti O3 are perovskite structure. Ca2+ radius is larger than Mg2+ but less than Sr2+, thus, the energy storage properties of Sr Ti O3 ceramics with A site substituted by Ca might be better than that of Sr1-x Mgx Ti O3 ceramics. Because of Ca substitution, the interaction between positive and negative ions enhances, the breakdown strength rises, and the dielectric constant slightly largen. At room temperature, Sr1-x?Cax?Ti O3 possess well dielectric frequency stability and low dielectric loss too. Between 100 Hz and 2MHz, the dielectric loss is less than 0.06. At 333 k V/cm, Sr0.98Ca0.02 Ti O3 gets the largest energy storage density 1.95 J/cm3, which is 1.35 times to that of the pure Sr Ti O3 in this experiment, and 2.79 times to that of the pure Sr Ti O3 in literatures. The energy storage efficiency is larger than 72%.Zr4+ stabilizes the valence state of the Ti4+ in titanate and diminishes leakage current. Therefore, if Zr substitutes for B site of Sr0.98Ca0.02 Ti O3, it might further improve the breakdown strength and energy storage density. A small quantity of Zr could reduce the sintering temperature of Sr0.98Ca0.02 Ti O3 ceramics. The sintering temperature drops from 1380 ?(Sr0.98Ca0.02 Ti O3) to 1320 ?(Sr0.98Ca0.02Ti0.98Zr0.02O3). XRD, SEM and EDS analysis found that, except Zr partially substitute Sr0.98Ca0.02 Ti O3 and form main crystalline phase perovskite structure, also have oval crystalline phase containing Zr and O. Sr0.98Ca0.02Ti(1-y?)Zry?O3 possess well frequency stability and low dielectric loss. Between 100 Hz and 2MHz, the dielectric loss is less than 0.008. The energy storage efficiency of Sr0.98Ca0.02Ti(1-y?)Zry?O3 is larger than 72%. The energy storage efficiency of Zr substituted Sr0.98Ca0.02 Ti O3 ceramics is higher than that of Sr0.98Ca0.02 Ti O3 ceramics at high field strength. With the raise of the Zr replacement amount, the dielectric constant of Zr substituted Sr0.98Ca0.02 Ti O3 ceramics slightly decreases. At 419 k V/cm, Sr0.98Ca0.02Ti0.98Zr0.02O3 ceramics could get the largest energy storage density 2.77J/cm3. It is 1.42 times to Sr0.98Ca0.02 Ti O3 ceramics without Zr substitution and 1.92 times to that of the pure Sr Ti O3 ceramics in this experiment, and 3.96 times to that of the pure Sr Ti O3 ceramics in literatures. Therefore, the Sr0.98Ca0.02Ti0.98Zr0.02O3 ceramics is the most promising energy storage dielectric used as pulse forming line.Equivalence substitution of A site or B site would result in lattice deformation of Sr Ti O3. However, it is a question need to answer that whether relaxation and conductance are influenced by lattice deformation. For this, the high temperature relaxation and conductance of Sr Ti O3 based ceramics with equivalence substitution of A site or B site are researched. In addition, found that, the high temperature relaxation of Sr1-x Mgx Ti O3 and Sr1-x?Cax?Ti O3 result from short distance migration of O vacancies with thermal activation, and the high temperature conductance arise from long distance migration of O vacancies with thermal activation. In addition, discovered that, the high temperature relaxation of Sr0.99Mg0.01 Ti O3 and Sr0.98Ca0.02 Ti O3 with Zr substitution of B site is caused by short distance migration of O vacancies with thermal activation and induced interfacial polarization. The high temperature conductance Sr0.99Mg0.01 Ti O3 and Sr0.98Ca0.02 Ti O3 with Zr substitution of B site is also lead by long distance migration of O vacancies with thermal activation. The high temperature relaxation and conductance of Sr Ti O3 with equivalence substitution has relation without deformation but with migration of O vacancies with thermal activation. At room temperature, as the O vacancies is unable to be thermally activated, dielectric properties and energy storage properties would be less affected.