Preparation And Properties Of Sodium Niobate-based Dielectric Ceramics For Energy Storage

Sodium niobate-based ceramics dielectrics have moderate dielectric constants and relatively high electric breakdown strength,making them ideal materials for energy storage capacitors.Under high electric fields,the field-induced ferroelectric phase of sodium niobate causes an increase in the remnant polarization and low energy storage efficiency.Therefore,in this thesis,sodium niobate was chemically modified to disrupt their electric field-induced ferroelectric phase to obtain the new system of sodium niobite-based ceramics suitable for energy storage applications.Low-temperature sintering and spark plasma sintering were used respectively to refine the ceramic grains and further enhance the dielectric energy storage properties of the ceramics.The specific research contents and conclusions are as follows.（1）The relaxor ferroelectric Sr_0.7Bi_0.2Ti O₃（SBT）was selected to dope Na Nb O₃（NN）.The introduction of nanopolar microregions of SBT can disrupt the long-range ordering of the ferroelectric phase in the NN matrix.With the increase of SBT content,the breakdown strength of the ceramic was improved and the antiferroelectric-ferroelectric phase transition electric field was raised above the breakdown strength of the ceramic.The NN-24SBT has the lowest remnant polarization when comparing the SBT-doped samples with different contents at 200 k V/cm.Excellent temperature（25°C-125°C）and frequency（10 Hz-500Hz）stability were also observed.Eventually,NN-24SBT achieved a breakdown strength of350 k V/cm and a recoverable energy storage density of 4.74 J/cm³.（2）The NN-24SBT:Cu O composite ceramics were prepared by using Cu O as the sintering aid to address the problem of high sintering temperature of NN-24SBT and volatility of Na and Bi elements during the sintering process,which led to the sensitivity of the ceramics to the sintering temperature and unstable performance.The reduced sintering temperature and the distribution of Cu O at the grain boundaries both inhibited the growth of the matrix grains and contributed to the improvement of the breakdown strength of the composite ceramics.Ultimately,the composite ceramics obtained a recoverable energy storage density of 6.28 J/cm³ under an electric field of 470 k V/cm,demonstrating the beneficial effect of reducing the grain size on the energy storage performance of sodium niobite-based ceramics.（3）Spark Plasma Sintering（SPS）process was used instead of the low-temperature sintering to avoid the degradation of energy properties due to the diffusion of the second phase.The average grain size of NN-24SBT ceramics prepared by the SPS method was reduced to 1.62μm.The electric field simulation combined with the SEM microstructure of ceramics revealed that more grain boundaries make the electric potential distribution more uniform and the local electric field at the grain boundaries is lower.As a result,the breakdown strength of the fine-grained ceramics was significantly improved.The breakdown field strength of NN-24SBT ceramics prepared by the SPS method was increased to 578 k V/cm,which is 65%higher than that of the conventional solid solution,and the recoverable energy storage density reached 12.21 J/cm³ with an energy storage efficiency of 84.1%.