Study On The Structure And Properties Of NaNbO₃ Based Lead-free Energy Storage Ceramics

With the global energy and environmental problems intensifying in recent years,the researches on environmentally friendly dielectric materials with excellent energy storage property has become an important trend in the field of electronic and electrical engineering.NaNbO₃ is a well-known lead-free functional ceramic with antiferroelectricity,which is expected to be applied in pulse power equipment by regulating of its energy storage characteristics.Therefor,NaNbO₃ is chosen as the research object in this paper.NaNbO₃ was modified by Sr Y_0.5Nb_0.5O₃ doping and and Bi³⁺substitution,the doping and preparation process effects on the phase composition,microstructure,dielectric properties,breakdown strength,and energy storage properties of NaNbO₃ ceramics are studied though structural regulation and relaxation enhancement.At first,x Sr Y_0.5Nb_0.5O₃-（1-x）NaNbO₃（x=0.05,0.1,0.15,0.2）ceramics were prepared by traditional solid state method,the structure and properties of Sr Y_0.5Nb_0.5O₃ doped NaNbO₃ceramics were studied.The results showed that the density and grain size of the ceramics increased with the doping of Sr Y_0.5Nb_0.5O₃.Meanwhile,the dielectric constant（e_r）and breakdown strength（BDS,E_b）of the ceramics increased first and then decreased with increasing x.For the x=0.1 sample,the maximum ofe_r and E_b were achieved,which were 1845and 157 k V/cm respectively.At the same time,the recoverable energy storage density(W_rec)reached the peak value of 1.27 J/cm³.In addition,the doping of Sr Y_0.5Nb_0.5O₃ reduced the interfacial polarization and enhanced the relaxation characteristics,which improved the energy storage efficiency（η）of the material.The energy storage efficiency reaches the maximum of68.90%at x=0.2.Considering that the sintering temperature of x Sr Y_0.5Nb_0.5O₃-（1-x）NaNbO₃ ceramics was as high as1370℃and the sintering temperature range was narrow,which may cause serious volatilization of Na and deteriorate the dielectric breakdown strength,the structure and dielectric properties of the Bi³⁺-substitued NaNbO₃ ceramics with a lower sintering temperature were studied.The results showed that the sintering temperature range of Na_（1-3x）Bi_xNbO₃ ceramics was 1175℃～1250℃,which is significantly lower than that of x Sr Y_0.5Nb_0.5O₃-（1-x）NaNbO₃ ceramics.The Bi³⁺addition increased the density and the grain size of the ceramics（the average grain size increased from 4.33μm to 7.98μm with x changed from 0.05 to 0.2）,and madeε_r and E_b increase first and then decrease.When x=0.1,the ceramics processed the low interfacial polarization,the obvious relaxation characteristics,and the optimum energy storage properties:ε_r=1802,E_b=181 k V/cm,W_rec=1.59 J/cm³,η=64.14%.In order to further improve the energy storage characteristics of Na_0.7Bi_0.1NbO₃ ceramics,the effect of different raw materials on the phase structure of the calcined powder and the effect of two-step sintering processes on the structure and properties of the ceramics were investigated.The result showed that the Na_0.7Bi_0.1NbO₃ powder with little impurity could be prepared after calcined at a relatively low temperature 800℃for 6 h by using Na₂CO₃（Sinopharm）,Bi₂O₃（Shanghai Chemical reagents Co.,Ltd.Reagent No.2 Factory）and Nb₂O₅（Guangdong Lingguang New Material Co.,Ltd.）as raw materials.Compared with the traditional sintering process,the two-step sintering significantly increased the density,decrease the grain size,reduce the interface polarization,and improve the breakdown strength of Na_0.7Bi_0.1NbO₃ ceramics.When the window temperature and the holding time of the two-step sintering were 1000℃and 2 h,the average grain size was reduced to 4.26μm,BDS increased to 214 k V/cm,W_rec increased to 2.77 J/cm³,andηincreased to 82.55%.