| Bismuth sodium titanate (Bi0.5Na0.5TiO3, BNT in abbreviation) based piezoelectric ceramics, as an A site ions substituted perovskite ferroelectrics, is currently being widely studied, and it is considered one of the most promising lead-free material system for its better ferroelectric and piezoelectric properties. In this paper, BNT-based piezoelectric ceramics samples were prepared by conventional solid-state reaction method, and then the influences of the preparation process, substitution and doping modification, forming ternary piezoelectric ceramics and other factors on the phase structure, microstructure, piezoelectric and dielectric properties of the BNT-based ceramics were also studied systematically with the help of modern testing and analysis technology.Firstly, (1-x) Bi0.5Na0.5TiO3-xBaTiO3(BNT-BT) binary piezoelectric ceramics were pepared by solid sintering method at atmospheric pressure. The influences of the calcined temperature, sintered temperature and different Barium contents on the crystal structure, microstructure, piezoelectric properties, dielectric properties and temperature characteristics of the BNT-BT ceramics were studied. According to the X-ray diffraction (XRD) patterns, scanning electron microscope (SEM), density and electrical properties, the suitable calcined temperature, sintered temperature and ceramic compositions in the range of morphotropic phase boundary (MPB) with good performance were determined. The obtained results are as following, (1) the secondary phase is easy to be found in the ceramic samples calcined at lower temperatures, which affacts the synthesis of the main phase. The ceramics samples calcined at excessively high temperatures are too hard to mill, resulting in the decrease of sintering activity, which is not conducive to the formation of ceramics. Suitable calcining temperature can help to reduce pores and defects, mainwhile grains with clear grain boundaries and uniform size were obtained. Sintering temperature increased properly is benefit to improve the density and electrical properties of the ceramics. The samples calcined at 850℃and sintered at 1160℃(holding two hours) show good performance. (2) X-ray diffraction analysis shows that Barium ions(Ba2+) having larger ionic radius substitute Sodium ions(Na+) and Bismuth ions(Bi3+) into the lattices to occupy A bit. With the increasing of the Barium contents, all the ceramics are pure perovskite phase solid solution, at the same time, the peaks of the XRD patterns move toward the lower angles, indicating the expansion of the unit cell volume. The SEM photographs show that the substitution of right amount of Ba2+ can refine the grains and improve the sintering properties and density of the ceramics. The dielectric temperature spectra show that the Barium substitution can decrease the Curie temperature while can increase the dielectric constant and make the ceramics. For all the ceramics, a strong frequency dependence and the broadening of the peak of relative dielectric constant (ε33T/ε0) at Tm suggest that the BNT-BT ceramics are relaxor ferroelectrics. With the increasing of the Barium contents, the piezoelectric constant (d33), planar electromechanical coupling coefficient (KP) and relative dielectric constantε33T/ε0 are inceasing firstly and then decreasing, while the tanδis decreasing firstly and then inceasing. Considering the phase structure, microstructure and electrical properties synthetically, BT content changing form 0.06 to 0.10(mol) is the morphotropic phase boundary (MPB) range of the BNT-BT binary ceramics, in which rhombohedral phase coexists with tetragonal phase, and all the ceramics in MPB are typical relaxor ferroelectrics. When the Batium content is equal to 0.06 mol, the ceramics with compact and homogeneous structure achieved better properties, and the values of bulk density (ρv), d33, KP,ε33T/ε0, tanδand mechanical quality factor (Qm) are equal to 5.664 g/cm3,120 pC/N,0.202,600, 2.1%, and 205, respectively. (3) As a kind of relaxor ferroelectric ceramic materials, BNBT6 has good applied foreground in the ultrasonic filed for the large anisotropy of electromechanical coupling coefficient. However, with the introduction of BT, the depolarization temperature (Td) was decreased, resulting sintered temperature becoming narrow and the thermal stability becoming worse, which will affect the application of the BNT-BT ceramics.In order to improve the properties of BNT-BT ceramics, bismuth lithium titanate (Bi0.5Li0.5TiO3, BLT in abbreviation) and bismuth potassium titanate (Bi0.5K0.5TiO3, BKT in abbreviation) were introduced respectively on the basis of BNBT6, which formulations were (0.94-x)BNT-0.06BT+xBLT (BNT-BT-BLT, x=0,0.01,0.03,0.05,0.07,0.09,0.11) and (0.94-y)BNT-0.06BT+yBKT (BNT-BT-BKT, y=0,0.01,0.03,0.05,0.07,0.09,0.11) respectively. The influences of BLT or BKT contents on the crystalline structure, microstructure, piezoelectric properties and dielectric properties were studied one by one. The results show as follows, (1) X-ray diffraction analysis shows that all the ceramics are pure perovskite phase with the increasing of Lithium ions (Li+) and Potassium ions (K+) respectively. BLT or BKT appears to have diffused into lattice to form a solid solution. For smaller ionic radius of Li+ than Na+, the peaks of the XRD patterns move toward the higher angles, indicating the contraction of the unit cell volume. While because of larger ionic radius of K+ than Na+, the peaks of the XRD patterns move toward the lower angles, indicating the expansion of the unit cell volume. (2) The SEM photographs show that, in the studied composition range, all samples have homogenous microstructure, grain combination is compact, grain boundary is clear and porosity is low, indicating that the substitution of Li+ or K+ can refine the grains and improve the sintering properties of the ceramics. The grain size first decreased and then increased with the increasing Li- or K+ content. When the Lithium content is large excessively, the grain size of some grains is becoming large abnormally, which may be related to the characteristic that the introduction of Li+ can decrease the sintering temperature significantly. (3) The substitution of Li+ for Na+ doesn't change the MPB of the BNBT6. With the increasing of Li content, relative dielectric constantε33T/ε0 and loss tangent tanδof the samples are increasing gradually, while the piezoelectric constant d33 and the planar electromechanical coupling coefficient KP of the samples increase initially and then decrease. When the BLT content is up to 0.05mol, the samples sintered at 1100℃showes optimum properties, values of the d33, KP,ε33T/ε0 and tan 8 are equal to 128 pC/N,0.22,800 and 2.5%, respectively. (4) When the K+ contents are less than 0.09mol, the substitution of K+ for Na+ doesn't change the rhombohedral-tetragonal morphotropic phase boundary in the BNBT6. However, with the incorporation of 0.09mol of BKT, the crystal structure turns into tetragonal symmetry from rhombohedral symmetry. Values of the d33, KP,ε33T/ε0 and tanδof the samples change significantly at the point of phase transition. (5) The moderate introduction of BLT or BKT into BNT-BT is conducive to improve the properties of the ceramics. However, ternary ceramics are multicomponent systems, especially the morphotropic phase boundary of BNT-BT-BKT is uncertain, and further reseach on the relaxor ferroelectric behavior of the two ternary ceramics both BNT-BT-BLT and BNT-BT-BKT is needed, for the purpose of preparing excellent BNT-based lead piezoelectric ceramics. |
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