| The pulse energy storage capacitor is the core of entire pulse power system,and it is required to have the advantages of high energy storage capacity,fast charge-discharge characteristics,wide working temperature range and long service life.The properties of a material depend on its structure.For relaxor ferroelectrics,the charging-discharging process are completed through homogeneous phase transition,thus beneficial to fast charge-discharge characteristics and long service life.However,the relaxor ferroelectrics still have the problems of low energy storage density and poor temperature stability.Therefore,studying the homogeneous phase transition process of relaxor ferroelectrics,analyzing the mechanism of polarization reversal process,and further exploring the influencing factors and solutions of energy storage density and temperature stability are the key to realize high performance pulse energy storage dielectric materials.In order to obtain the relaxor ferroelectric materials with high energy storage properties and pulse charge-discharge characteristics,(Sr0.7Ca0.3)Ti O3ceramics were doped and composited in this paper.The phase structure,phase transition,dielectric properties,energy storage properties and pulse charge-discharge characteristics were systematically researched for the obtained(Sr0.7Ca0.3)1-1.5xBixTi O3 single-phase ceramics,(1-x)Sr0.49Ca0.21Bi0.2Ti O3-x Bi(Mg0.5Zr0.5)O3 complex phase ceramics and Sr0.49Ca0.21Bi0.2(Ti1-xZrx)O3 single-phase ceramics.The conclusions are as follows:(1)To improve the energy storage density,Bi3+ions were introduced to substitute for Sr2+/Ca2+ions in A-site,aiming to change the degree of lattice distortion,thus high relaxation characteristic perovskite dielectric materials were obtained.Through the analysis of structures,it is found that the substitution of Bi3+ions destroys the ordered structure of(Sr0.7Ca0.3)Ti O3.The structure of(Sr0.7Ca0.3)1-1.5xBixTi O3 gradually changes from tetragonal to pseudocubic with the Bi3+ions content increasing.Also,the dielectric constant increases,and the Curie temperature shifts from low temperature to room temperature with the Bi3+ions content increasing.Therefore the temperature stability is improved.The temperature dependence of the dielectric constant shows obvious frequency dispersion,and relaxation characteristics are enhanced.(2)Through the in-depth study,it is found that polar nanoregions exist in the grains of Sr0.49Ca0.21Bi0.2Ti O3 ceramic.It is inferred that the origin of the relaxation is the appearance of polar nanoregions caused by the local inhomogeneity.The Sr0.49Ca0.21Bi0.2Ti O3 ceramic has the best comprehensive performance in(Sr0.7Ca0.3)1-1.5xBixTi O3 system:the energy storage density reaches 2.04 J·cm-3 under the electric field of 260 k V·cm-1,and the energy storage efficiency is 92%,the capacitance temperature variation rate meets the requirements of X7R standard,the power density reaches 23 MV·cm-3 under the electric field of 100 k V·cm-1,the pulse rise time is less than 18 ns,and the power density has no attenuation after 40000 pulse charge-discharge cycles,indicating the Sr0.49Ca0.21Bi0.2Ti O3 ceramic has excellent anti-fatigue characteristics.(3)To further improve the temperature stability of Sr0.49Ca0.21Bi0.2Ti O3-based ceramics,(1-x)Sr0.49Ca0.21Bi0.2Ti O3-x Bi(Mg0.5Zr0.5)O3 composites were obtained.The XRD results show that the combination of Sr0.49Ca0.21Bi0.2Ti O3 and Bi(Mg0.5Zr0.5)O3 will induce the formation of secondary phase Bi4Ti3O12.Because the Bi3+ions are easily precipitated,the liquid phase will appear when the amount of Bi(Mg0.5Zr0.5)O3 increases to some extent.The combination of Sr0.49Ca0.21Bi0.2Ti O3 and Bi(Mg0.5Zr0.5)O3 results in two dielectric constant peaks in the temperature dependence of dielectric constant curves,leading to the improvement of temperature stability at high-temperature range.The capacitance temperature variation rate meets the requirements of X9R standard.However,the interface polarization is enhanced due to the appearance of the secondary phase,so the breakdown strength is gradually reduced,which is harmful to realizing high energy storage density.(4)To avoid the negative impact of interfacial polarization on the breakdown strength,only Zr4+ions with weak electronegativity were used to substitute Ti4+ions in B-site for Sr0.49Ca0.21Bi0.2Ti O3 ceramic.Compared with Sr0.49Ca0.21Bi0.2Ti O3 ceramic,the average grain sizes of Sr0.49Ca0.21Bi0.2(Ti1-xZrx)O3 ceramics decrease prominently,contributing to an increase in grain boundary proportion.Moreover,the substitution of Zr4+ions makes the Raman vibration peak at about 250 cm-1 gradually shift towards the high wavenumber range,indicating the lattice gets more stable.Benefiting from the increase in grain boundary proportion and lattice stability,the breakdown strengths increase from 265 k V·cm-1(for Sr0.49Ca0.21Bi0.2Ti O3 ceramic)to 337 k V·cm-1(for Sr0.49Ca0.21Bi0.2(Ti0.99Zr0.01)O3 ceramic).The results of transmission electron microscopy show that,for Sr0.49Ca0.21Bi0.2(Ti0.99Zr0.01)O3ceramic,the polar nanoregions not only exist in the grain interior but also exist near the grain boundary.This proves that the addition of trace Zr4+ions will deposit near the grain boundary and increase the polar nanocrystals near the grain boundary.Therefore the polarization is enhanced and the grain growth is suppressed.The lattice distortion of Sr0.49Ca0.21Bi0.2(Ti0.99Zr0.01)O3 ceramic is weakened due to the decrease of lattice energy.Benefiting from the enhancement of polarization and breakdown strength,the maximum energy storage density of Sr0.49Ca0.21Bi0.2(Ti0.99Zr0.01)O3 ceramic reaches 3.63 J·cm-3,and it has a high energy storage efficiency of 90%.The power density of Sr0.49Ca0.21Bi0.2(Ti0.99Zr0.01)O3 ceramic reaches 46 MV·cm-3 under the electric field of 100k V·cm-1,and the pulse rise time is less than 17.5 ns,which can ensure the ceramic function well under the high-frequency pulse electric field.The power density has no attenuation after40000 pulse charge-discharge cycles,indicating the outstanding anti-fatigue characteristics of Sr0.49Ca0.21Bi0.2(Ti0.99Zr0.01)O3 ceramic.(5)The activation temperature of polar nanoregions is in accordance with the Curie temperature,so it can be inferred that the thermal activation of polar nanoregions can improve the dielectric constant.Under the electric field,the lattice distortion degree of relaxor ferroelectric is strengthened,and the position of ions is shifted in the crystal cell,resulting in the enhancement of polarization and the increase of polar nanoregions.Phase transition of the relaxor ferroelectrics under the electric field is non-diffusive phase transition,which originates from the ions displacement in cell internal.The ions displacement in crystal cell has a shorter distance,so the formation of polarization needs a very short time.The ion displacement process in the cell is not easily disturbed by the neighboring cells.Therefore,the relaxor ferroelectrics possess a high charge-discharge speed in the pulse charge-discharge process.The relaxor ferroelectrics also possess a high energy storage efficiency because the ions in the crystal cell just need to overcome a small energy barrier to leave the equilibrium position,resulting in a small energy loss.Moreover,the polar nanoregions are small in size,dispersed in the dielectric matrix,and are not easy to be pinned,so relaxor ferroelectrics have strong anti-fatigue characteristics and long service life under the pulse cycle electric field. |
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