Study On Preparation And Properties Of Na_0.5Bi_0.5TiO₃ Based Textured Ceramics

Na_0.5Bi_0.5TiO₃（NBT）is considered to be one of the most promising lead-free piezoelectric materials to replace the lead-based piezoelectric counterparts.It is well known that controlling the microstructure of ceramics by grain orientation is one of the most effective methods to improve the piezoelectric properties of ferroelectric ceramics.In order to study the effect of matrix powders with different phase structures on the properties of NBT-based textured ceramics,in this thesis,0.88Na_0.5Bi_0.5TiO₃-0.08K_0.5Bi_0.5TiO₃-0.04BaTiO₃（88NBT）with morphotropic phase boundary（MPB）composition,0.89Na_0.5Bi_0.5TiO₃-0.08K_0.5Bi_0.5TiO₃-0.03BaTiO₃（89NBT）with more rhombohedral component near the MPB and 0.90Na_0.5Bi_0.5TiO₃-0.07K_0.5Bi_0.5TiO₃-0.03BaTiO₃（90NBT）with rhombohedral composition textured ceramics were fabricated via the template grain growth method（TGG）using Na_0.5Bi_0.5TiO₃（NBT）and NaNbO₃（NN）as templates.In order to prepare the textured ceramics with higher Lotgering factor,high aspect ratio platelike NBT and NN templates were prepared by the topochemical method.Moreover,research on the textured ceramics show that,both NBT and NN templates can promote texture development in NBT-based ceramics.However,the matrix component will deviate from the original composition if excessive NBT template is used,which is not efficient to improve the piezoelectric performances of NBT-based textured ceramics.In contrast,NN template is more beneficial to improve the piezoelectric properties of NBT-based textured ceramics since there is no reaction occurs between NBT matrix and NN templates.For the 88NBT with MPB composition,compared to the 88NBT randomly oriented ceramics,the piezoelectric properties of 88NBT textured ceramics are improved.The Lotgering factor of 88NBT textured ceramics is 96%with 4 wt%NN as templates.The textured ceramics have a higher d₃₃ of 180 pC/N,which is 20%higher than the randomly oriented ceramics（d33=150 pC/N）.The textured ceramics are observed to possess the brick-like shape with a larger grain size being on the order of10 um.The textured ceramics are found to possess slightly smallerε_max,lower T_d and similar T_m comparing to the randomly oriented counterpart.The d^*₃₃ are 160 and 200pm/V for the randomly oriented and textured 88NBT ceramics at 2 kV/mm,respectively,where the d^*₃₃ of the textured ceramics possess²5%enhancement comparing to randomly oriented one.Of particular significance is that the strain hysteresis of the randomly oriented ceramics is 16%,greatly decreases to 7.6%for the textured 88NBT ceramics.This phenomena can be explained by the texturing characteristics,where all the grains in the textured ceramics are aligned along crystallographic<001>direction,analogue to<001>poled single crystals,being responsible for the enhanced piezoelectric coefficient;meanwhile,the enlarged grain size（thus increased domain size）in the textured ceramics will greatly reduce the hysteresis due to the decreased domain wall density.The Rayleigh analysis shows that the extrinsic contributions of 88NBT randomly oriented ceramics and textured ceramics are 36.7%and 22.2%,respectively.For the 89NBT composition,the d₃₃ and k_p of textured ceramics are 195 pC/N and35%,respectively.Compared with the 89NBT randomly oriented ceramics(d₃₃=120pC/N,k_p=26%),the d₃₃ and k_p are increased by 63%and 36%,respectively.The d^*₃₃ of89NBT randomly oriented ceramics is lower than that of textured ceramics,being on the order of 184 pm/V vs.205 pm/V at 70 kV/cm.The H values are found to be 11.2%and 6.9%at 15 kV/cm,for 89NBT randomly oriented ceramics and textured ceramics,respectively.In addition,the extrinsic contributions of 89NBT randomly oriented ceramics and textured ceramics（with 4 wt%NN）are on the order of 25%and 16.8%,respectively,based on Rayleigh study.In addition,the d₃₃ of the textured 90NBT ceramic is on the order of 175 pC/N,increased by 60%comparing to the randomly oriented ceramics(d₃₃=110 pC/N).Both strain curves of randomly oriented ceramics and textured ceramics show a typical strain feature,thus the d^*₃₃ and hysteresis H increase slightly as the electric filed increases.The Rayleigh analysis shows that the extrinsic contributions of 90NBT randomly oriented ceramics and textured ceramics（with 4 wt%NN）are on the order of 30.1%and 13.9%,respectively.The properties of 88NBT textured ceramics with MPB composition have not been significantly improved when compared to the randomly oriented counterpart,while the piezoelectric properties of 89NBT and 90NBT textured ceramics with more rhombohedral component have been significantly improved.This is due to the fact that enhanced piezoelectric response could be expected in highly textured rhombohedral ceramics poled along<001>direction,as a consequence of facilitated polarization rotation due to the formation of“4R”domain configuration,based on the concept of domain engineering.