Study On Preparation,Structure And Properties Of NaNbO₃ Based Energy-Storage Ceramics

Dielectric capacitor can achieve high power density,fast charge and discharge,low cost and long cycle life,which plays an irreplaceable role in energy storage pulse power devices.With the development of electronic components toward miniaturization,light weight,safety and diversification of application fields,the dielectric capacitors with high energy storage characteristics were widely studied.In this paper,broad band gap and low bulk density NaNbO₃ antiferroelectric ceramics were studied to improve the dielectric and energy storage properties of NaNbO₃ by doping with bismuth-based composite perovskite compounds.NaNbO₃ is an antiferroelectric with orthogonal phase symmetry at room temperature,and the small difference in free energy leads to the coexistence of antiferroelectric P-phase and sub-stable ferroelectric Q-phase.The electric field-induced sub-stable ferroelectricity and the volatility of alkali metal Na lead to low breakdown field strength,which greatly limits the application of NaNbO₃ in the field of energy storage.To address the above problems,this thesis employed a strategy of enhancing the antiferroelectricity and constructing a local random electric field to designed and prepared（1-x）NaNbO₃-x Bi(Li_1/3Zr_2/3)O₃、（1-x）NaNbO₃-x Bi(Zn_1/2Zr_1/2)O₃、（1-x）NaNbO₃-x Bi(Mg_2/3Ta_1/3)O₃and（1-x）[0.9NaNbO₃-0.1Bi(Mg_2/3Ta_1/3)O₃]-x(Bi_0.5Na_0.5)_0.7Sr_0.3Ti O₃ series ceramic materials,and the crystal structure,microstructure,dielectric as well as energy storage properties of the above ceramic materials are systematically investigated.The main contents of the full paper are as follows:（1）NaNbO₃ was used as a substrate to introduce(Bi³⁺,Li⁺,Zr⁴⁺)ions to form A-site and B-site ion disorder,which disrupts the ferroelectric long-range ordering,thus converting ferroelectric domains into polar nano-microregions（PNRs）.The fast response of PNRs under the applied electric field is utilized to significantly improve the energy storage density and energy storage efficiency of NaNbO₃ ceramics.It was found that the excellent energy storage properties of（1-x）NaNbO₃-x Bi(Li_1/3Zr_2/3)O₃series ceramic samples were closely related to the enhanced relaxation properties of the system.When x=0.12,the samples achieved high energy storage density(W_rec～3.345 J/cm³)and energy storage efficiency（η～75.8%）.（2）For the high remanent polarization intensity of the antiferroelectric material NaNbO₃ system,in this paper,stabilization of the antiferroelectric phase and construction of a local random electric field were used to reduce the remanent polarization intensity and thus improve the energy storage performance.The introduction of non-equivalent ions at A and B sites can generate local random electric field easily.The introduction of trivalent Bi³⁺ions at A site and Zn²⁺ions with large radius and low polarization rate at B site enhanced the antiferroelectricity and thus reduced the residual polarization intensity.When the doping amount was 12%,the energy storage density of the sample reached 3.14 J/cm³,which is approximately 8times higher than that of pure NaNbO₃,with an efficiency of 84.5%.（3）It was found that the breakdown field strength of NaNbO₃ ceramics was improved by introducing wide band gap Ta₂O₅ and highly insulating Mg O,thus the energy storage properties of the material were optimized.In order to achieve a greater breakthrough,0.9NaNbO₃-0.1Bi(Mg_2/3Ta_1/3)O₃ ceramics were selected as the research subject,and Sr²⁺and strong ferroelectric Bi_0.5Na_0.5Ti O₃ were introduced.The relaxor antiferroelectric ceramics have high driving electric field of antiferroelectric-ferroelectric phase transition and small hysteresis of phase transition.An excellent energy storage density(W_rec～8.0 J/cm³)and ultra-high energy storage efficiency（η～90.4%）have been successfully obtained in NaNbO₃ based relaxor antiferroelectric ceramics.