Effect Of Bi-nonstoichiometry On Structure And Electrical Properties Of (Na_0.5Bi_0.5)_0.93Ba_0.07TiO₃ Lead-free Ceramics

Sodium Bismuth titanate((Na_0.5Bi_0.5)TiO₃, abbreviated as NBT) as a kind of perovskite type ferroelectric with complex A-site cations, has a relatively large remnant polarization (P_r=38μC/cm²) and a relatively high Curie temperature (T_c=320 ℃ ). Due to its strong ferroelectricity at room temperature, NBT has been considered to be a promising candidate material for Lead-free piezoelectric ceramics. In this thesis, (Na_0.5Bi_0.50.93Ba_0.07TiO₃ powders with Bi-nonstoichiometry were synthesized by a citrate method and the synthetic process of the citrate method was analyzed. The influence of Bi-nonstoichiometry on the structure and electrical properties of (Na_0.5Bi_0.50.93Ba_0.07TiO₃ lead-free ceramics were investigated. The relation among the composition, structure and electrical properties was also examined. The phase transition behavior upon heating was investigated with respect to the dependence of dielectric constant on measuring temperature and frequency for (Na_0.5Bi_0.50.93Ba_0.07TiO₃ ceramics with Bi-nonstoichiometry. The influence of ferroelectric nature on the piezoelectric properties was also investigated.(Na_0.5Bi_0.5±x0.93Ba_0.07TiO₃ (x=0¹.0%) powders were synthesized by a citrate method. The synthesis process and conditions were modified by means of analyzing the complexing mechanism of involved reactant cations. Fine and homogeneous (Na_0.5Bi_0.5±x0.93Ba_0.07TiO₃ (x=0¹.0%) powders (¹00nm) with a pure perovskite phase were yielded using the citrate method by calcining at 600℃ for lh.The structure and electrical properties of (Na_0.5Bi_0.5+x0.93Ba_0.07TiO₃ ceramic specimens were investigated. It was found that with the increase of excess amount of Bi, a slight change in the crystal structure was detected, while no remarkable evolution in the microstructure was observed. Compared with (Na_0.5Bi_0.50.93Ba_0.07TiO₃ ceramic specimens, Bi excess led to decrease piezoelectric properties. When x≤0.6%, the piezoelectric constant d₃₃ and electromechanical coupling factor k_p tend to slightly decrease with increasing excess amount of Bi. Further increasing the excess amount of Bi above 0.6% caused a rapid degradation of the piezoelectric constant d₃₃and electromechanical coupling factor k_p. Moreover, it was found that the Bi excess resulted in an increased dielectric loss tan5 and a reduced mechanical quality factor Q_m. Saturated P-E hysteresis loop were obtained for the specimens with x<0.4%, and a distortion of measuredloops occurred when x>0.4%. The temperature dependence of dielectric constant ε₃₃^T/ε₀indicates that the depolarization temperature because lower with increasing excess amount of Bi.The structure and electrical properties of (Nao.5Bio.5.x)o.93Bao.o7Ti03 ceramic specimens were investigated. It was found that with the increase of deficient amount of Bi, an abnormal change in the crystal structure and a remarkable evolution in the microstructure was occurred. When x<0.8%, with the increase of deficient amount of Bi, the piezoelectric constant d33 decreased evidently, while the electromechanical coupling factor kp shows a remarkable increase. Moreover, it was found that the Bi deficiency resulted in a decreased dielectric loss tanS and a significantly enhanced mechanical quality factor Qm. With the increase of deficient amount of Bi, the coercive field Ec of the ceramics became obviously larger. Within the temperature range of room temperature to 120°C, saturated P-E hysteresis loop were obtained for most specimens except those with x=0 and x=0.2%.This researched demonstrates that adequately controlling the stoichiometry of Bi plays an crucial role in obtaining desired piezoelectric and ferroelectric properties in (Nao.5Bio.5)Ti03-BaTi03 ceramics with a composition near the morphotrophc phase boundary (MPB).