Chemical Method Synthesis, Structure, Piezoelectric Properties And Ferroelectric Properties Of (Na_0.5Bi_0.5) TiO₃ Based Ceramics

Sodium bismuth titanate, (Na0.5Bi0.5)TiO3 (NET), is a kind of perovskite-type ferroelectric with a relatively large remnant polarization (P=38 C/cm2) at room temperature and a relatively high Curie temperature (TC=320 C). For its strong ferroelectricity at room temperature, NBT has been considered to be a promising candidate material for lead-free piezoelectric ceramics. In this dissertation, a citrate method has been employed to prepair NBT-based ceramics, and the synthesis and preparation, structural characteristics, poling process and piezoelectric properties of NBT-based ceramics have been investigated. The influence of the composition and structure on the piezoelectric properties has been studied. The relation between the ferroelectric nature and piezoelectric properties has been established in terms of experimental results.NBT-based fine powders have been synthesized by a citrate method. It was found that the mole ratio of citric acid to total metal cation content (abbreviated as C/Mn+), pH of precursor solution contributed greatly to the formation of homogeneous, transparent sol and gel, while the calcining temperature has a determinant role on crystalline structure and morphology of the calcined powders. The preferred synthesis conditions to prepare fine powders with a pure perovskite-type structure were ascertained and NBT-based fine powders were produced.The structural characteristics, poling process and piezoelectric properties of (l-x)(Na0.5Bi0.5)TiO3-xBaTiO3 (abbreviated as NBT-BT(x)) ceramics were investigated. The piezoelectric properties of the ceramics highly depend on poling field and temperature, while no remarkable effect of poling time on the piezoelectric properties was found. It was confirmed that the morphotropic phase boundary (MPB) of the ceramics lies in the composition near x=0.06 at room temperature and the piezoelectric properties of NBT-BT(x) ceramics made by the citrate method reach maximums values at x=0.06. Compared with the conventional solid-state reaction method, the specimens made by the citrate method exhibit superior piezoelectric properties.The citrate method synthesis, structure and piezoelectric properties of MnO and CoO added NBT-BT6 ceramic were investigated in comparison with those of NBT-BT6 ceramic. It was ascertained that the addition of MnO and CoO did not cause a remarkable change in crystalline structure, with the specimens remaining the coexistence of rhombohedral and tetragonal phases. Compared with NBT-BT6 specimen, the specimen added with proper amount of MnO shows a relative large increase of electromechanical coupling factor (kp) and significantly enhancedmechanical quality factor (Qm) accompanied by a relatively large decrease of piezoelectric constant (d33) and dielectric constant ( 33T/ 0) because of a "soft and hard" doping effect. A considerable degradation of piezoelectric properties was detected for the specimens added with CoO.The ferroelectric properties of NBT-based ceramics were investigated. The results confirm an essential relation between the ferroelectric nature and piezoelectric properties for NBT-based ceramics. Both the remanent polarization (P) and coercive field (Ec) affect the piezoelectric properties of the specimens. This can be qualitatively interpreted as follows. On one hand, a high coercive field is disadvantageous to the sufficient reorientation of ferroelectric domains during electrical poling, generating a negative effect on piezoelectric properties. On the other hand, a high remannet polarization indicates a strong ferroelectricity, which enhances piezoelectric properties. Therefore, the piezoelectric properties of the specimens are dependent on the dual contribution of remanent polarization and coercive field.