Study On Preparation, Structure And Electric Properties Of Na_0.5Bi_0.5TiO₃-based Lead-free Piezoceramics

Sodium bismuth titanate, (Na0.5Bi0.5)TiO3 (NBT), is a kind of perovskite-type ferroelectric with a relatively large remanent polarization (Pr =38μC/cm2) at room temperature and a relatively high Curie temperature (Tc=320℃). For its strong ferroelectrics at room temperature, NBT 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 NBT-based ceramic, and the synthesis and preparation, structure characteristics and electric properties were investigated. The relation between piezoelectric properties and the composition and structure was also studied. The dielectric relaxation feature and phase transition for NBT-based ceramics and the influence of ferroelectric feature on the piezoelectric properties have been established in terms of experimental results.(1) 1Synthesized temperature of powder for (1-x)Na0.5Bi0.5TiO3-xK0.5Bi0.5TiO3 ((1-x)NBT-xKBT) ceramic system decreases as the increasing of substituting amount of KBT. Appropriate synthesized temperature is in the range of 850-900℃ and sintering temperature for ceramic is in the range of 1160-1175℃ with 2-6 h soaking time. By the contrary, the synthesized temperature of powder for (1-y)Na0.5Bi0.5TiO3-yBaTiO3 ((1-y)NBT-yBT) ceramic system increases as the increasing of BT. Appropriate synthesized temperature is 850-950℃. (2)The piezoelectric properties of the NBT-based ceramics highly depend on poling field and temperature, while no remarkable effect of poling time on the piezoelectric properties. Appropriate poling field is 3~3.5kV/mm, poling temperature is 70 - 80℃ and poling time is 10-15 min.(2) For (l-x)NBT-xKBT lead-free ceramic system: (1)It was found that the morphotropic phase boundary (MPB) with rhombohedral and tetragonal co-existed crystalline structure for the ceramics lies in the range of x=0.16-0.20. When x=0.20, the piezoelectric constant d33 is 122pC/N and electromechanical coupling factor kp is 28.5% which are the best piezoelectric properties. (2)The relaxor characteristic of the system ceramics increases after substituted by KBT, and the relaxation feature increases with the increasing of KBT composition. (3)Dielectric properties as a function of temperature and XRD results at different temperatures reveal that when the crystal structure is rhombohedral or rhombohedral and tetragonal co-existed at room temperature, the ceramics will occur phase transition from rhombohedral ferroelectric to tetragonal anti-ferroelectric and tetragonal anti-ferroelectric to tetragonal paraelectric phase transitions in the heating process. While it is tetragonal symmetry at room temperature, it will occur to anti-ferroelectric macro-domain to anti-ferroelectric micro-domain and anti-ferroelectric to paraelectric phase transitions. (4)The relation between ferroelectric properties and compositions indicates that when it is tetragonal symmetry at room temperature, it is anti-ferroelectrics validated by raman spectra and has no piezoelectric properties after poling. However, the composition near MPB with tetragonal structure can be taken place from anti-ferroelectric to ferroelectric phase transition induced by outer strong electric field. It also has piezoelectric properties after poling. When the crystal structure of ceramicis rhombohedral or rhombohedral and tetragonal co-existed at room temperature, it shows ferroelectric characteristic and has piezoelectric properties after poling.(3) For (1-y)NBT-yBT lead-free piezoelectric ceramic system: (1)It was found that the MPB for the ceramics lies in the range of x=0.06-0.10. The best properties of the ceramics are piezoelectric constant d33 = 137pC/N at y=0.10 and electromechanical coupling factor kp is 28.6% at y=0.06, respectively. (2)This ceramic system has distinctly different relaxation characteristics from the (1-x)NBT-xKBT system. The ceramic is relaxor at y<0.10. But the relaxation feature transforms from relaxor ferroelectric to normal-relaxor ferroele...