Study On The Energy Storage Performance And Temperature Stability Of Na_0.5Bi_0.5TiO₃-based Composite Ceramics With Core-shell Structure

Ceramic dielectric capacitors with high energy storage properties have attracted much attention in power electronic systems.However,the mutual constraint of recoverable energy density(W_rec)and temperature stability interval limit their practical application.Na_0.5Bi_0.5Ti O₃（NBT）-based ceramics are energy storage materials with wide application prospects,and their high ferroelectric polarization response is beneficial to obtain high W_rec,by improving their inherent high remanent polarization and low breakdown strength（BDS）and other deficiencies,it is expected to further improve their energy storage characteristics.To this end,in this paper,the interface at mesoscopic scale,grain at microscopic scale,domain structure at nanoscale and defect scales at atomic scale are controlled in NBT-based composite energy storage materials with core-shell structure,so as to achieve the controllable synthesis of NBT-based energy storage composites,and further comprehensively improve their energy storage properties and temperature stability,and provide a certain experimental basis for the structure-activity relationship between the structure,dielectric and energy storage properties of NBT-based materials.1.The wide temperature energy storage properties of composite ceramics with core-shell structure are improved by the interface design.La₂O₃was introduced at the interface of multi-component ceramic phases of NBT and Sr Sn_0.2Ti_0.8O₃（SST）and glass phase Bi₂O₃-B₂O₃-Si O₂（BBS）in the manner of constructing a core-shell structure to achieve interface regulation,thereby improving the wide-temperature energy storage properties of（NBT@La₂O₃）-（SST@La₂O₃）-BBS composite ceramics.The results show that the La₂O₃coating layer can induce a fine-grained and uniform microstructure,and reduce the local electric field distortion at the ceramic/glass interface.Part of the La₂O₃diffused and doped into the ceramic phase lattice can induce strong relaxation and reduce the defect concentration of oxygen vacancies.When the coating amount of La₂O₃is 1.0 mol%and the mass ratio of NBT@La₂O₃and SST@La₂O₃is 58/42,good dielectric temperature stability ofΔε/ε_150°C≤±15%in a wide temperature range of 20-500℃and thermal stability of the W_recchange rate of±10%within 20～150°C can be achieved.And moderate W_recof 2.25 J/cm³andηof 82.1%at 275 k V/cm could also be achieved.It can be seen that the interface regulation of the core-shell structured materials can achieve the purpose of optimizing the energy storage properties,especially the temperature stability,of the composite ceramic materials.2.The wide temperature energy storage properties are improved by the design about grain morphology of composite ceramics with a core-shell structure.The NBT@SST-BBS composite ceramics were obtained by compounding NBT@SST ceramic powders of different grain morphologies including spherical,rod-like and spherical-and-rod mixed with BBS glass.The effects of grain morphology and BBS addition on the energy storage properties of the materials were mainly studied.The experimental results show that the moderate-ε_rSST shell with highly lattice-matched to the core material is beneficial to reduce the interfacial polarization between the ceramic and glass phases and thus improving the BDS,while the nanorod shape of ceramic phase can increase the charge storage sites to induce higherΔP values of 22.1μC/cm².Finally,a moderate W_rec～2.19 J/cm³andη～77.8%at 229 k V/cm were achieved in composite ceramics with nanorod morphology and the BBS addition of 0.05,with a wide temperature range in 30～395°C of the dielectric temperature stability.However,their energy storage properties are slightly lower than that of the spherical morphology samples with a W_recvalue of 2.24 J/cm³at 254 k V/cm³,while the W_recof the spherical-and-rod mixed composites reaches the lowest value of only 1.5 J/cm³at 201 k V/cm³.It can be seen that the design about grain morphology can effectively improve the polarization response,but the optimization of the energy storage properties is not obvious.3.The wide temperature energy storage properties of composite ceramics with core-shell structure are improved by the domain structure design.The ferroelectric phase NBT-K_0.5Bi_0.5Ti O₃（NKBT）,paraelectric phase Ca Zr O₃（CZ）and antiferroelectric phase Na Nb O₃（NN）or（Er,Mn）Na Nb O₃（EMNN）are mainly compounded to prepare NKBTCZ-NN and NKBT@CZ-EMNN composite ceramics.The effects of NN and Er₂O₃additions on the energy storage properties of the materials were mainly studied.It was demonstrated that the introduction of appropriate amounts of NN or EMNN can induce the coexistence of heterogeneous nanodomains,which is beneficial to improveΔP and?.Finally,the W_recvalue of the NKBT-CZ-0.10NN composite ceramic reaches 2.63 J/cm³with moderate?of 83.5%.In addition,it also exhibits excellent stability of dielectric and energy storage properties over a wide temperature range at 72-421°C.When the addition amount of Er₂O₃is2 mol%,the W_recvalue of NKBT@CZ-EMNN composite ceramics reaches 2.79 J/cm³with high?of 89.4%and high BDS of 294 k V/cm.AndΔε/ε_150°C≤15%within 70～500℃,and W_rec>1.32 J/cm³andη>72%can be achieved simultaneously within 20～150℃.It can be seen that the optimization of energy storage properties,especially W_rec,can be achieved through the modulation of the domain structure,but the multiphase recombination will weaken the polarization of ferroelectric materials,which is not conducive to the further improvement of energy storage properties.4.Based on the above studies,the multi-scale regulations of interfaces,grains,domains and defects of NBT-based composite ceramics with a core-shell structure are expected to breakthrough the bottleneck problem that the energy storage properties and temperature stability of composite ceramic materials are difficult to improve synergistically.The composite relaxation phases Sr Zr O₃-Bi Mg_0.5Sn_0.5O₃（SZ-BMS）are mainly coated on the surface of NKBT in the form of core-shell structure,and they are combined with glass phase Si O₂to prepare NKBT@（SZ-BMS）-Si O₂ceramic composites.The effects of BMS addition amount and sintering atmosphere on the energy storage properties of materials were mainly studied.When the BMS addition amount is 15 mol%,the composite ceramics can achieve high BDS of 301 k V/cm and theΔP value of 29.1μC/cm²,and then simultaneously achieve high W_recof 3.94 J/cm³and highηof 87.1%,and have a wide temperature range at 30～467℃of dielectric temperature stability.The samples sintered in pure oxygen atmosphere can achieve both high W_recof 3.66 J/cm³and highηof 91.7%at 349 k V/cm.Therefore,through the multi-scale structural design of NBT-based composite ceramics,a systematic relationship between the microstructure,polarization response and macroscopic electrical properties can be established,so as to achieve the purpose of comprehensively improving the energy storage properties and temperature stability of materials.