Effect Of Oxide Additives On Structure And Electrical Properties Of (Na_0.5Bi_0.5)_1-xBa_xTiO₃ Ceramic With A Composition Near MPB

Recently, there is a growing interest of investigating lead-free piezoelectric materials as a possible alternative to widely used lead zirconate titanate (PZT) based ceramics. Sodium bismuth titanate, (Na_0.5Bi_0.5)TiO₃ (NBT) is considered to be one of promising candidates for lead-free piezoelectric ceramics due to its large remanent polarization (Pr=38μC/cm²) at room temperature and high Curie temperature(T_c=320℃). In this dissertation, (Na_0.5Bi_0.5)_1-xBa_xTiO₃ ceramics were prepared by a citrate method, and the effects of various oxide additives on the structure and electrical properties of the ceramics with a composition near the rhombohedral-tetragonal morphotropic phase boundary (MPB) have been investigated. The dielectric relaxation and phase transition behavior of the ceramics with different compositions have been examined with respect to temperature and measuring frequency. An essential relation between the ferroelectric nature and piezoelectric properties has been suggested for the ceramics based on experimental data.(Na_0.5Bi_0.5)_1-xBa_xTiO₃ ceramics were synthesized and prepared by the citrate method, and the structure and electrical properties of the ceramics were systematically investigated. The citrate method was confirmed to be an advantageous alternative to the conventional solid state method in producing NBT based ceramics, which allows producing homogeneous superfine powders together with substantially improving the ferroelectric and piezoelectric properties of the resulting ceramics. The results of crystal structure analysis revealed a MPB composition range of x=0.06-0.10 for (Na_0.5Bi_0.5)_1-xBa_xTiO₃ system at room temperature. It was found that the piezoelectric and ferroelectric properties of (Na_0.5Bi_0.5)_1-xBa_xTiO₃ compositions near the MPB are rather sensitive to phase composition and reach preferred values at x=0.07, where the relative content of the tetragonal phase is slightly higher than that of the rhombohedral phases. (Na_0.5Bi_0.5)_1-xBa_xTiO₃ ceramics presents a gradual decrease in phase diffuse factor (δ) with increasing Ba_xTiO₃ content, implying a degradation of relaxor feature and a transition from relaxor ferroelectrics to normal ferroelectrics.The influence of adding La₂O₃ on the structure and electrical properties of (Na_0.5Bi_0.5)_0.93Ba_0.07TiO₃ composition was investigated. The ceramic specimens added with various amounts of La203 remain the coexistence of the rhombohedral and tetragonal phases, with the relative content of the tetragonal phase enhancing with increasing the amount of La₂O₃. Adding a small amount of La₂O₃ (≤0.2wt%) produced an improved piezoelectric constant (d₃₃) and electromechanical coupling factor (kp) along with an increased dielectric loss (tgδ) and reduced mechanical quality factor (Qm), corresponding to a soft doping effect on electrical properties. Adding higher amounts of La₂O₃ (＞0.3wt%) led to a rapid degradation of piezoelectric properties and a remarkable deformation of measured P-E hysteresis loops, which can be explained in terms of declined stability of ferroelectric domains and resulting formation of antiferroelectric micro-regions in the specimens at room temperature due to increased A site cation vacancy concentration originating from the incorporation of La₃₊ into the lattice.The structure and electrical properties of (Na_0.5Bi_0.5)_0.93Ba_0.07TiO₃+0.2wt%Ln₂O₃ (Ln=La,Pr,Nd,Sm,Eu,Gd) ceramics were examined. No consideration influence of lanthanide cation size on the crystal structure of the specimens was observed. A decline in piezoelectric and ferroelectric properties with smaller lanthanide sizes was found for the specimens. It was suggested that the variation of piezoelectric and ferroelectric properties of the specimens with lanthanide cation size relies on deformation mode and degree of crystal structure.Effect of CoO addition on the structure and electrical properties of (Na_0.5Bi_0.5)_0.93Ba_0.07TiO₃ composition was investigated. The specimens added with various amounts of CoO remain the coexistence of the rhombohedral and tetragonal phases. The addition of CoO resulted in a decreased piezoelectric constant (d₃₃) and dielectric loss (tgδ) as well as an enhanced mechanical quality factor (Q_m), which refers to a hard doping effect on electrical properties. Moreover, an increase of depolarization temperature was detected with the increasing of CoO addition amount, inferring an enhancement in the stability of ferroelectric domains. Effect of CeO₂ addition on the structure and electrical properties of (Na_0.5Bi_0.5)_0.93Ba_0.07TiO₃ composition was investigated. The specimens added with various amount of CeO₂ show the coexistence of the rhombohedral and tetragonal phases, with the relative content of the tetragonal phases gradually enhancing with the increase of CeO₂ addition amount of. Compared with (Na_0.5Bi_0.5)_0.93Ba_0.07TiO₃, the specimen added with a small amount of CeO₂ (≤0.2wt%) display an slightly improved electromechanical coupling factor (k_p) and piezoelectric constant (d₃₃) in conjunction with a reduced dielectric loss (tgδ) and an enhanced mechanical quality factor (Qm). Further increasing the amount of CeO₂ above 0.3wt% led to a rapid decline in piezoelectric properties and a deformation of measured P-E hysteresis loops, which is very similar to the case of La₂O₃ addition.