Design,Preparation And Polarization Energy Storage Properties Of Sodium Niobate-based Antiferroelectric Ceramics

In recent years,the development of pulsed power technology and the gradual promotion of various renewable energy related technologies such as wind power generation and hybrid vehicles,have spawned a large number of requirements for capacitors with high energy storage density,high operating temperature,high operating voltage,good temperature stability and high reliability.Currently,the widely used ferroelectric Ba Ti O₃-based Class II capacitors are gradually difficult to meet the needs in this regard,since the ferroelectric dielectrics have low energy storage density at high electric fields,low bias field dielectric constant and relatively poor temperature stability.In contrast,antiferroelectric and relaxor type dielectrics are more likely to avoid these drawbacks.In particular,antiferroelectric dielectrics have unique characteristics of double hysteresis loops,which not only have a large potential energy storage density and power density,but also have the characteristics of reaching a maximum dielectric constant at a certain bias field.At present,lead-based antiferroelectric MLCCs have been successfully applied in pulsed power equipment and power semiconductor systems.However,lead-containing materials are harmful to the environment in the process of raw material mining,production and post-processing.With the increase of people’s environmental protection awareness and the increasingly stringent environmental regulations,the restrictions on the application of lead-containing materials continue to increase.Therefore,it is necessary to explore and develop lead-free antiferroelectric material systems.Among them,the NaNbO₃ is one of the few known antiferroelectrics perovskite,which has a large saturated polarization（～40μC/cm²）and a high Curie temperature（>360°C）.However,due to the metastable characteristics of the ferroelectric phase induced by the electric field,its P-E curve usually presents the characteristics of ferroelectric hysteresis loop,and the potential antiferroelectric and the corresponding energy storage performance are seldom studied.The researches in this dissertation focus on sodium niobate-based materials and aim to improve the antiferroelectric properties of NaNbO₃-based materials and deepen the understanding of the corresponding antiferroelectric properties by forming solid solution and doping,as well as exploring new types of NaNbO₃-based relaxor energy storage system.The main research contents are as follows:Firstly,by applying the strategy of adjusting tolerance factor and ion polarizability,（1-x）NaNbO₃-x Ca Sn O₃（NNCS100x）ceramic system was designed.It is proved that with the increase of CS content,the antiferroelectricity of the NNCS system is enhanced by XRD,TEM and Raman spectroscopy,etc.The results of electrical characterization show that the highest P_max/P_r value（6.6）and a high backward phase transition field（45k V/cm）in the NN system at room temperature are simultaneously achieved for the x=0.04 composition.Secondly,CuO and BiMnO₃ were used to modify the NNCS04 and pure NN ceramics respectively.The results show that the incorporation of CuO reduces the ceramic sintering temperature and promotes densification,and at the same time increases the ceramic resistivity,thereby improving the cyclic fatigue performance of NNCS ceramics and reducing the leakage current and remanent polarization after electric field loading.The highest backward phase transition field for the NaNbO₃ceramic system（～60 k V/cm）is realized in the sample with large grain size.The introduction of BiMnO₃ in NaNbO₃ ceramic leads to donor and acceptor impurities simultaneously,making the ceramics easy to be completely polarized under a lower electric field（60 k V/cm）at temperatures 150°C,and also significantly reducing the coercive field of the metastable ferroelectric phase of NaNbO₃ ceramics.The hysteresis loops under hydrostatic pressure shows that the remanent polarization gradually decreases to zero as the pressure rises to 200 MPa,indicating a great potential in ferroelectric-type pulsed power application.Thirdly,rare-earth La doping is carried out in NNCS04 composition to improve the antiferroelectric properties of ceramics and study its potential energy storage properties.It is found that La doping can effectively reduce the hysteresis and remanent polarization,and achieve an energy storage density of 2.1 J/cm³ and efficiency of 62%.Regarding the phenomenon that the rare-earth doping cannot increase backward phase transition field,it is proposed that this may be caused by the large mass difference between the substitutive and the origin ions in the NaNbO₃ system via an explanatory defected diatomic chain model.Finally,taking advantage of the high polarization and high Curie temperature of NaNbO₃,with the aim of achieving relaxor energy storage in NaNbO₃-based ceramics,（1-x）Na NO₃-x Bi(Mg_2/3Nb_1/3)O₃（NN-x BMN）binary relaxor ceramic system was designed and its energy storage and charge-discharge performances were systematically studied.The results show that a high energy storage density（2.4 J/cm³）and efficiency（90%）are obtained simultaneously in x=0.15 composition at 300 k V/cm were obtained simultaneously.The pulse charge-discharge test results show that this composition have a high discharge capability close to linear dielectrics.On this basis,the potential energy storage properties of the ternary systems（1-3x）NN-x BMN-2x Ca Ti O₃（NNBMNCT）and 0.85NN-0.05Bi Al O₃-0.10Ca Ti O₃/Sr Ti O₃（NNBACT/ST）were explored,and the studies show that NNBACT has a high breakdown strength,an energy storage density of 3.5 J/cm³ and efficiency of 85%are obtained at 390 k V/cm.In addition,using NN as a solid solution component,with the aim to achieve the fineness and uniformity of the ceramic grain size hence to improve the breakdown strength,the high-entropy ceramic system（Na,Ba,K,Bi,Ca）_0.2Nb_0.2Ti_0.8-xZr_xO₃（SZr100x）was designed and successfully prepared.It is found that ceramics have both high polarization and relatively high breakdown strength.The x=0.06 composition achieves an energy storage density of 4.6 J/cm³ efficiency of 86%at 320 k V/cm.