Study On Preparation And Property Improvement Of BNT-based Lead-free Piezoelectric Ceramics

Sodium bismuth titanate(Bi_0.5Na_0.5TiO₃, abbreviated as BNT), is a kind of A-sitesubstituted perovskite-type ferroelectric. In this paper, BNT was choosed as the research basematerial. In order to improve the properties of piezoelectric and ferroelectric and achieve thepractical purpose of BNT ceramics, a series of solid solution modification research had beentaken on BNT ceramics according to the research ideas of lead based piezoelectric ceramics.The dense lead-free piezoelectric ceramics （0.944-x）Bi_0.5Na_0.5TiO₃-0.056BaTiO₃-xSrTiO₃（abbreviated as0.944BNT-0.056BT-100xST） and （1-x）(0.93Na_0.5Bi_0.5TiO₃-0.07BaTiO₃)-xKNbO₃（abbreviated as0.93BNT-0.07BT-100xKN） were fabricated via conventional ceramictechnique. In the study of their composition range, the X-ray diffraction （XRD） analysisrevealed that the two sets of ceramic systems form a single perovskite structure, and themorphotropic phase boundary（MPB） between the rhombohedral and tetragonal phases of0.944BNT-0.056BT-100xST was identified in the composition range of0.03≤x≤0.06mol%,the grain size decrease with the increase of ST content, the diffuse phase transitioncharacteristics is significant, The dielectric and ferroelectric properties are further improvedwith the introduce of ST component. When x=0.04mol%, the ceramics exhibit excellentproperties: relative dielectric constant εr=4632, dielectric loss tanδ=0.01576, remnantpolarization Pr=35uC/cm2, and coercive field Ec=21kV/cm, Piezoelectric constant d33=134pC/N.For0.93BNT-0.07BT-100xKN ceramics, series of ceramic materials become easiersintering with the introduce of KNbO₃, the relative dielectric constant decrease, ceramics havehigh coercive field, the maximum piezoelectric constant is118pC/N at x=0.05mol%.BaTiO₃and KNbO₃were added to Na_0.5Bi_0.5TiO₃-SrTiO₃system, the composition pointof BNT-ST system was choosed in the MPB. The dense （0.7-x）Na_0.5Bi_0.5TiO₃-0.3SrTiO₃ -xBaTiO₃（abbreviated as0.7BNT-0.3ST-xBT） and （1-x）(0.72Na_0.5Bi_0.5TiO₃-0.28SrTiO₃)-xKNbO₃（abbreviated as0.72BNT-0.28ST-xKN） ceramics were respectively prepared by thesolid state reaction method. XRD spectrums showed that the crystal structure of0.7BNT-0.3ST-xBT samples are tetragonal structure in their composition ranges. The d33reach themaximum131pC/N at x=0.055mol%; By study modified0.72BNT-0.28ST-xKN solid solutionceramics we can found the introduce of KN does not change the rhombohedral-cubic crystalstructure of the basic material systems. The dielectric loss relaxation phenomenon of0.72BNT-0.28ST-xKN ceramics was quantitatively analysised by three kinds of relaxationmodel. The new glass model coincide well, the phenomenon of dielectric relaxation mainlyoriginate from the polarity of the micro-region to the ferroelectric micro-domain. Thedispersion degree of series materials gradually increase with the introduction of KN in hightemperature dielectric spectrum. The boundaries between Tdand Tmpeaks reduce graduallyand tend to merge.For （1-x）（0.93BNT-0.0665BaTiO₃-0.0035BaZrO₃）-xCaTiO₃（abbreviated as0.93BNT-0.07BZT-100xCT） ceramics, CT does not change the MPB phase of basic material system.The grain size decrease with the increase of CT content. The Curie temperature andferroelectricity raise with introduced appropriate amount of CaTiO₃, the region ofantiferroelectric phase increase for series ceramics. The relaxation degree gradually increasewith the introduce of CT.For （1-x）（0.92BNT-0.06BT-0.02ST）-xLiSbO₃（abbreviated as0.92BNT-0.06BT-0.02ST-100xSL） ceramics, XRD analysis showed that small amount of SL does not change thecrystal structure of0.92BNT-0.06BT-0.02ST, the second phase appear at x=0.06mol%, SEMmicrographs revealed that SL promote the growth of0.92BNT-0.06BT-0.02ST grains, thedielectric temperature characteristic curves showed ceramics gradually transform from twodielectric anomalous peak to one broad dielectric peaks, with the introduction of SL, thehysteresis loops transform from "double hysteresis loops" which are similar to theantiferroelectrics to the elongated single electric hysteresis loops.