(Bi <sub> 0.5 </ Sub> Na <sub> 0.5 </ Sub>) <sub> 0.94 </ Sub> Ba <sub> 0.06 </ Sub> TiO <sub> 3 </ Sub> Piezoelectric Ceramics Preparation And Physical Properties

Sodium bismuth titanate,(Bi_0.5Na_0.5)TiO₃(BNT),is a kind of perovskite-type ferroelectric with a relatively large remanent polarization(P_r=38 pC/cm²) and a relatively high Curie temperature (T_c=320℃).For its strong ferroelectrics at room temperature,BNT has been considered to be a promising candidate material for lead-free piezoelectric ceramics.In this dissertation,a conventional ceramic fabrication method was employed to prepare the BNBT6 ceramic,the synthesis and preparation and poling technics were investigated.The density,phase composition,microstructure, piezoelectric and dielectric properties of BNBT6 samples at different sintering temperature have been investigated.The dielectric relaxation feature for BNBT6 ceramics has been discussed in terms of experimental results.At last properties of CeO₂-doped BNBT6 ceramics(0～1.0 wt%) and(Bi_0.94-(Na_0.94-xLi_x))_0.5 TiO₃ ceramics(x=0.01～0.11) were investigated.(1) The effects of process conditions on the structure of(Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃ ceramics were studied by TG-DTA,XRD,SEM and other analytic methods.The relatively better pre-sintering temperature is in the range of 850～900℃and soaking time is 2h,sintering temperature for ceramic is in the range of 1140～1160℃with 2h soaking time.The piezoelectric properties of BNBT6 ceramics highly depend on poling field and poling temperature,while no remarkable effect of poling time on the piezoelectric properties.Appropriate poling field is 3～4kV/mm,poling temperature is 70～80℃and poling time is 10～20min.(2) BNBT6 piezoelectric ceramics were synthesized at different sintering temperature ranging from 1120 to 1180℃by the conventional ceramics fabrication technique.The density,phase composition,microstructure,piezoelectric and dielectric properties of BNBT6 samples have been investigated.Results show that all the BNBT6 samples are of high compacted density and consisted of both rhombohedral and tetragonal phases.It is found that BNBT6 samples sintered at 1160℃is of the highest density(98%theoretical density),and good physical properties:piezoelectric constant d₃₃=120pC/N,planar electromechanical coupling factor k_p=0.297,dielectric constantε_r=615, dielectric loss tanδ=0.020.The temperature dependence of the dielectric constant and dissipation factor from the room temperature to 500℃was studied,and then the dielectric relaxation feature for BNBT6 ceramics has been discussed.(3)CeO₂-doped BNBT6 piezoelectric ceramics(0.0～1.0 wt%) were studied.X-ray diffraction (XRD) data shows that CeO₂ diffuses into the lattice of BNBT6 ceramics and forms pure perovskite phase.The electrical properties of BNBT6 ceramics have been greately improved by a certain amount of CeO₂ addition.CeO₂-doped BNBT6 ceramics(0.0～1.0wt%) are still relaxation ferroelectric.The temperature dependence of the dielectric constant and dissipation factor shows that the transition temperature from ferroelectric to antiferroelectric phase(T_d) decreases with the addition of CeO₂.At the room temperature,the ceramics doped with 0.4wt%CeO₂ show quite good performance with high piezoelectric constant(d₃₃=128pC/N),high coupling factor(k_p=0.283), high dielectric constant(ε_r=891) and low dissipation factor(tanδ=0.0185) at 1kHz,which indicates that CeO₂-doped BNBT6 sample is of high piezoelectric properties and low dissipation factor.The influence of CeO₂ addition on the dielectric and piezoelectric properties are disscnsed and interpreted.(4)(Bi_0.94(Na_0.94-xLi_x))_0.5Ba_0.06TiO₃ ceramics have been studied for different lithium(x=0.01～0.11) substitutions.X-ray diffraction(XRD) data shows coexistence of rhombohedral and tetragonal phases.With the addition of Li⁺,the piezoelectric properties and dielectric propertiest change,(Bi_0.94 -(Na_0.94-xLi_x))_0.5Ba_0.06TiO₃ ceramics are still relaxation ferroelectric.The temperature dependence of the dielectric constant and dissipation factor shows that the transition temperature from ferroelectric to antiferroelectric phase(T_d) decreases with the addition of Li⁺.When x=0.05,BNBT6 ceramics show quite good performance,piezoelectric constant(d₃₃=128pC/N),coupling factor(k_p=0.30), dielectric constant(ε_r=810).