Effect Of Double Doping On Dielectric Properties And Energy-storage Density Of BNT-based Ceramics

There is an urgent need to develop materials with high energy/power densities to meet the growing demands of portable electronics,electric vehicles,and large-scale energy-storage devices.Ultrafast charging/discharging rates and ultra-high power density make dielectric capacitors an indispensable component in modern electrical and electronic devices,especially pulsed power systems.Among them,ceramic dielectric capacitors have the advantages of high temperature resistance and high dielectric constant compared to polymer dielectric capacitors.Lead-based ceramics have excellent energy-storage properties,but their toxicity has raised concerns about their use in consumer applications.Therefore,it is necessary to develop the lead-free dielectric energy-storage ceramics with excellent comprehensive properties.Bi_0.5Na_0.5Ti O₃-based（BNT-bsaed）materials are considered to be an important family of lead-free perovskites.They have the advantages of high polarization,high dielectric constant,easy synthesis and relaxor properties,so they are expected to become lead-based compounds in possible alternatives in various applications.Therefore,a series of high-performance lead-free relaxor ferroelectric ceramics were prepared by traditional solid-state method with high polarization Bi_0.47Na_0.47Ba_0.06Ti O₃ceramic as matrix and Ca Hf O₃,Sr Hf O₃ and Bi_0.2Sr_0.7Ti O₃ as solid solution components.By studying the changes of micro morphology and phase structure of the ceramics and their effects on electrical properties,the modification mechanism of doped components is analyzed and the laws are summarized,so as to provide reasonable and effective design ideas for the development of lead-free ceramic system with high performance.（1）A relaxor ferroelectric ceramic with chemical composition of（1-x）Bi_0.47Na_0.47Ba_0.06Ti O₃-x Ca Hf O₃ was designed and prepared.Hf⁴⁺with large ion radius,high relative atomic mass and wide band gap is doped into the B-site of the matrix ceramic,which reduces the grain size,increases the band gap and improves the breakdown strength to 280 k V/cm.Doping small-size and low-quality Ca²⁺at the A-site is beneficial to induce relaxor properties and reduce residual polarization.Therefore,the BNBT-12CH ceramic exhibits a recoverable energy-storage density of 4.2 J/cm³ and an energy-storage efficiency of 68%.At a moderate electric field of 180 k V/cm and a temperature range of 20～140°C,the energy-storage density and efficiency of BNBT-12CH ceramic gradually increased with increasing temperature,showing good thermal stability.（2）In view of the problem that the energy-storage efficiency is not ideal in the previous work,Sr Hf O₃ is used to replace Ca Hf O₃ as the doping component,and the doping amount is increased.With the increase of Sr Hf O₃content,the residual polarization intensity is greatly reduced,the energy-storage efficiency continues to increase to more than 80%,and the breakdown electric field gradually increases to above 400 k V/cm.The BNBT-20SH ceramic shows a recoverable energy-storage density of up to 5.83J/cm³ and an energy-storage efficiency of 83.67%at an applied electric field of400 k V/cm.In the charge-discharge test,the BNBT-20SH ceramic also exhibits an ultra-high discharge energy density of 5.33 J/cm³ under 380k V/cm.（3）In order to overcome the compromise between polarization strength and breakdown electric field,BNBT-6CH ceramic with moderate breakdown strength（～190 k V/cm）and ultra-high saturation polarization（～53.4μC/cm²）was selected as the matrix,and Bi_0.2Sr_0.7Ti O₃ with typical induced relaxor was doped.The incorporation of BST increases the local random field,resulting in a large number of randomly distributed PNRs,which improved the polarization behavior under high applied electric fields.With the increase of BST content,the content of R3c phase decreases,and the P4bm phase gradually occupies the principal part.An Excellent comprehensive energy-storage performance was obtained in the ceramic with x=0.4 at 410k V/cm,with a W_rec of 6.19 J/cm and aηof 93.5%.And in the charge-discharge performance test,the ceramic with x=0.4 also shows a W_d very close to the W_rec under the same electric field.