Energy Storage Behavior And Mechanism Of Linear Dielectric Tailored Ferroelectric Materials

Under the main background of the global energy crisis and environmental protection,high-performance energy storage equipment is crucial to realize clean and intelligent modern electrical power systems.Among the current mainstream energy storage devices,dielectric capacitors play a leading role in the application of advanced pulsed power systems,owing to their high power density,extremely quick discharge time and extended service life.However,the relatively low energy storage density has been a major obstacle to the integration of advanced devices.Therefore,researchers have devoted themselves to researching and developing dielectric materials with high energy storage density.Combined with the theme of environmental protection,the development of high-performance lead-free ceramic capacitors has aroused extensive research interest.Among the typical dielectric materials,linear dielectric（LD）and relaxor ferroelectric（RFE）have attracted much attention due to their high-efficiency energy storage behavior.As a typical linear dielectric,calcium titanate（Ca Ti O₃,abbreviated to CT）has a high theoretical breakdown field strength and a wide band gap,which renders it promising potential to be explored in both modified components and main energy storage dielectric material.Hence,this thesis selects CT as the central research point to explore the energy storage behavior of CT tailored(Na_0.5Bi_0.5)Ti O₃（BNT）-based and(K_0.5Bi_0.5)Ti O₃（BKT）-based relaxor ferroelectric ceramics.On the other hand,the quasi-linear CT-based relaxor ferroelectric with giant comprehensive capacitive energy storage is also achieved.The breakthrough in energy storage performance（ESP）was achieved through the multiscale optimization strategy and construction of nanoscale polarization heterogeneous regions,which promoted the development of lead-free ceramic capacitors.Energy storage behavior of CT-modified BNT-based binary and ternary relaxor ferroelectric ceramics:The introduction of CT into BNT-based ceramics has a significant impact on its energy storage behavior.The binary ceramic BNT-0.3CT achieves a high W_recof 5.65 J/cm³and a highηof 88.1%at 420 k V/cm.Via multiscale optimization design,ternary relaxor ferroelectric ceramics（BNT-BAT）-0.4CT successfully achieved a breakthrough in the performance,which achieved a super-high value ofη≈7.8%and a high value of W_rec≈5.81 J/cm³,and excellent charge-discharge performance(P_D≈6.2 MW/cm³,t_0.9≈37.6 ns).The correlation between small grain size and enhanced E_bwas revealed by a phase-field breakdown model.TEM and PFM tests confirmed the formation of high dynamic PNRs under manipulating domain structure.Variable-temperature Raman and PFM provide favorable support for（BNT-BAT）-0.4CT ceramics to obtain excellent temperature,frequency,and cycle stability on the basis of physical evolution.Energy storage behavior of BNT-based material modified linear dielectric CT:In the study of CT,the introduction of BNT-based material effectively improved the disadvantage of its low polarization.The binary linear ceramic CT-0.3BNT obtained the best performance W_rec≈5.87 J/cm³andη≈7.7%.The effective construction of nanoscale polarization heterogeneous regions makes the novel relaxor ferroelectric ceramics CT-0.2（BNT-BAT）with extraordinary ESP,W_recup to 6.39J/cm³,and a high value ofη≈4.4%.It has achieved excellent energy storage stability under a high electric field of 500 k V/cm,and also exhibited excellent charge-discharge performance(t_0.9≈28.4 ns,P_D≈161.3 MW/cm³),showing its superiority as an excellent candidate material for dielectric capacitors.TEM and PFM reveal the nanoscale polarization heterogeneous regions of extremely small PNRs with high dynamic,multiphase coexistence and local structural heterogeneity,which contribute to excellent ESP.Variable-temperature Raman,PFM,and XRD have been explored to disclose the physical origin of the superior stability of materials.Energy storage behavior of CT-modified BKT-based binary and ternary relaxor ferroelectric ceramics:Linear dielectric CT also showed a very effective regulatory effect on BKT-based ceramics,which are similar to the observations on both bismuth-based ceramics and BNT-based ceramics.The binary relaxation ferroelectric ceramic BKT-0.5CT has achieved satisfactory ESP under a high electric field of 500 k V/cm,including high W_rec≈6.03 J/cm³,η≈1.2%,good temperature/frequency/cycle stability,and superior charge-discharge performance(P_D≈119.7MW/cm³,t_0.9≈81.2 ns),showing a good application prospect.At the same time,by the introduction of CT,the ternary ceramic（BKT-BT）-0.4CT also behaves enhanced relaxation behavior,thus producing a good ESP of W_rec≈4.95 J/cm³,η≈85.6%.