Preparation And Electrical Properties Of NaNbO₃ Based Electronic Ceramics

Lead-free dielectric materials can be used to prepare energy storage capacitors,which have the following advantages:1)relatively high dielectric constant,2)relatively low dielectric loss,3)good frequency stability,4)good temperature stability.Among them,Sodium niobate（Na Nb O₃,abbreviated as NN）ceramics have the advantages of low density,low cost,high breakdown field strength,being lead free and non-toxic,and high Curie temperature,so they are considered as one of the most potential green energy storage ceramic materials.In this paper,NN was used as matrix ceramic,doped with ions of different valence states and ferroelectric second element of different structures,to optimize the composition and improve the process,so as to realize the regulation of microstructure,and thus improve the relaxation characteristics.Finally,we developed NN based energy storage ceramics with high energy storage density and high energy conversion efficiency,and studied relevant performances of the prepared ceramics.（1）Proper addition of Bi₂O₃ can inhibit crystal growth and promote the increase of breakdown field E_b;The introduction of Sr²⁺ions can break the long-range ordered structure of ferroelectric domains and induce the generation of polar nanoregions（PNRs）.（1-x）Na Nb O₃-x((Sr_0.7Bi_0.2)Ti O₃)energy storage ceramics（abbreviated as（1-x）NN-x SBT,x=0mol%,40mol%,45mol%,50mol%）,were designed and prepared by doping the second constituent(Sr_0.7Bi_0.2)Ti O₃ into Na Nb O₃.The grain structure,phase production,dielectric ferroelectric properties were measured and analyzed.The results showed that SBT could uniformly enter NN group to form a solid solution.The introduction of SBT made the grain size of the ceramic samples decrease,the corresponding polarization difference increase,and the P-E curves of（1-x）NN-x SBT ceramics changed from"saturated"to"nearly linear".When x=0.45,（1-x）NN-x SBT ceramics showed the best performance:E_b=370 k V/cm,W=2.56 J/cm³,W_rec=1.96 J/cm³,η=76.6%.When x=0.40,（1-x）NN-x SBT ceramics exhibited high energy conversion efficiencyη=85.8%.In addition,0.55NN-0.45SBT ceramics showed significant temperature stability（the variation range of energy storage density was within 9%among 20℃～140℃）and frequency stability（the variation range of energy storage density was within 3.7%among 5Hz～200Hz）at 200k V/cm.They also exhibited high power density（13.16 MW/cm³）,high current density（164.53 A/cm²）and short discharge time（2.10μs）.（2）The introduction of Li⁺ions is conducive to strengthening ionic bonds,and leading to the decrease of P_r.Lead-free energy storage ceramic systems（1-x）Na Nb O₃-x Bi(Li_0.33Ti_0.67)O₃ were constructed by doping Bi(Li_0.33Ti_0.67)O₃ solution into Na Nb O₃ solution.（1-x）NN-x BLT,x=8mol%,10mol%,12mol%,14mol%).In this experiment,the traditional solid phase reaction method was used to prepare the samples,and cold isostatic pressing was used to press the tablet,which avoided the shortcoming of traditional cementing that may lead to the stratification of ceramic pieces.Finally,a series of NN ceramic samples with uniform grains and no obvious pores were prepared.The XRD test of pre-fired powder showed that BLT successfully entered NN ceramics.The test of（1-x）NN-x BLT ceramics at the optimum sintering temperature showed that as the content of BLT increases,the peak dielectric temperature（T_m）of the dielectric temperature spectrum moved to a low temperature gradually,and the dispersion factor was enhanced,which indicated that doping the second component has enhanced the relaxation characteristics of ceramics.0.88NN-0.12BLT ceramics exhibited the best energy storage characteristics:E_b=230 k V/cm,W=2.55 J/cm³,W_rec=1.55 J/cm³,η=61.0%;0.86NN-0.14BLT showed high energy conversion efficiencyη=77.9%at the breakdown intensity E_b=160k V/cm.In addition,0.88NN-0.12BLT ceramics showed good temperature stability under 200k V/cm（the variation range of energy storage density was within 9.5%among 20℃～120℃）and frequency stability（the variation range of energy storage density was within 13%among 5Hz～200Hz）.They also exhibited high power density（14.25 MW/cm³）,high current density（178.08 A/cm²）and short discharge time（1.82μs）.（3）The introduction of Ba²⁺ions can greatly promote the highly excited polarization state of polar clusters.The second component Ba_0.9Sr_0.1Ti O₃（BST for short）was designed.A new first component Na_0.7Bi_0.1Nb O₃（NBN for short）was designed by doping Bi³⁺ions.（1-x）Na_0.7Bi_0.1Nb O₃-x Ba_0.9Sr_0.1Ti O₃（abbreviated as（1-x）NBN-x BST,x=10mol%,20mol%,30mol%,40mol%）ceramics were successfully prepared by the solid state reaction method.The XRD of ceramics pre-fired powder showed a typical perovskite structure.With the gradual increase of BST content,the crystal face spacing increased,and the second components were fully incorporated into the NBN base to form a uniform solid solution.The breakdown field strength of ceramics increases first increased and then decreases,showed the maximum value when x=0.30,E_b=445 k V/cm,0.70NBN-0.30BST ceramics exhibited excellent energy storage characteristics at the optimum sintering temperature:W=4.47 J/cm³,W_rec=3.47 J/cm³,η=77.4%.The energy conversion efficiencies of 0.90NBN-0.10BST and 0.60NBN-0.40BST were greater than 80%.In addition,0.70NBN-0.30BST ceramics showed significant temperature stability under 300k V/cm（the variation range of energy storage density was within 11.2%among 20℃～160℃）and frequency stability（the variation range of energy storage density was within13%among 5Hz～500Hz）.They also showed high power density（9.87 MW/cm³）and short discharge time（1.38μs）.