Study On Structure And Electric Properties Of (Na_0.5Bi_0.5)TiO₃-Based Lead-free Piezoelectric Ceramics

Sodium bismuth titanate,(Na_0.5Bi_0.5)TiO₃（NBT）, is a kind of perovskite-typeferroelectric with a relatively large remanent polarization （Pr=38μC/cm²ï¼‰ at roomtemperature and a relatively high Cuire temperature （Tc=320℃）. Because of its strongferroelectrics at room temperature, NBT has been considered to be a promising candidatematerial for lead-free piezoelectric ceramics. In this dissertation, a conventionalsolid-state reaction method had been employed to prepare the NBT-based ceramic. Therelation between electric properties and the composition and structure was investigated.The phase transition feature for NBT-base ceramics have been studied through thetemperature dependences of dielectric and ferroelectric properties during heating. Themain research content is comprised of three parts as follow:（1）（1-x）(Na_0.9K_0.1)_0.5Bi_0.5TiO₃-xBa_0.7Ca_0.3TiO₃[（1-x）NKBT-xBCT] lead-freepiezoelectric ceramics. The effects of different BCT content on the structure andelectrical properties of ceramic samples were investigated. The results show（1-x）NKBT-xBCT materials possess the pure perovskite structure. The morphotropicphase boundary（MPB） of the system locates in the composition range of0.04≤x≤0.06.Both of the piezoelectric constant d33and planar coupling factor kpof ceramics firstincrease and then decrease, and the depolarization temperature Tddecrease with theincreasing of BCT content. The relaxor characteristic of the system ceramics enhancefirstly then weaken with the increase of x value. The series ceramics exhibit goodelectrical properties: the best kp=0.29at x=0.05, and when x=0.06, the ceramic exhibitsan optimum performance: d33=168pC/N, kp=26%, relative dielectric constantr=1280,dielectric loss tan=3.7%, remnant polarization Pr=37μC/cm², coercive field Ec=18.8kV/cm. The temperature dependences of ferroelectric and dielectric properties suggestthat the ceramics may contain both the polar and non-polar regions at temperatures aboveTd.（2） Bi-compensated （0.84-x）Na_0.5Bi_0.5TiO₃-_0.16K_0.5Bi_0.5TiO₃-xSrTiO₃（abbreviatedas NBT-KBT-xST） lead-free piezoelectric ceramics. The effects of different ST dosageson the structure and electrical properties of ceramic samples were investigated. Theresults indicate that full solid solution with a pure perovskite structure is formed after theST is added. The phase structure of the ceramics at room temperature changed fromrhombohedral to tetragonal with increased ST content, and the morphotropic phaseboundary（MPB） of the system locates in the composition range of0.03≤x≤0.04, whichexhibit good ferroelectric and piezoelectric properties. When x=0.04, the ceramics exhibitthe best performance: the piezoelectric constant d33=156pC/N, planar coupling factor kp =0.29, relative dielectric constantr=1116, dielectric loss tan=4.1%, remnantpolarization Pr=3_0.5μC/cm², coercive field Ec=23.9kV/cm. All the ceramic sampleshows frequency dispersion phenomena at Tdand a broad transition peak at Tm,suggesting that the material presented a typical characteristic of relaxor ferroelectrics.（3） MnCO₃doped (Bi_0.5Na_0.5)_0.852(Bi_0.5K_0.5)_0.11Ba_0.038TiO₃（abbreviated asNBKBT-Mn）lead-free piezoelectric ceramics. The influence of different MnCO₃additiveon the structure and electrical properties of the NBKBT-Mn ceramics was investigated.All the ceramics samples are pure perovskite phase and maintain a coexistence ofrhombohedral and tetragonal phase. It can be observed that the grain size and the relativedensity can be improved by adding a small amount of MnCO₃（x=0-0.4）, thus increasingthe ferroelectrical and piezoelectrical properties in some degree. When x=0.4, theceramics exhibit the best performance: the piezoelectric constant d33=185pC/N, planarcoupling factor kp=0.25, relative dielectric constantr=1335, dielectric loss tan=2.3%.The temperature dependences of dielectric properties suggest the relaxor characteristicsbecome more obvious with the increasing of MnCO₃doping amount.