| Bismuth sodium titanate, (Bi0.5Na0.5)Ti03 (BNT) is a perovskite-structured ferroelectric having Bi3+ and Na+ complex on the A site of ABO3-type compounds with a rhombohedral symmetry at room temperature. Because of its strong ferroelectricity and a high Curie temperature, the BNT ceramic has been considered as one of the promising candidates for lead-free piezoelectric ceramics. In this dissertation, a conventional ceramic fabrication method was employed to prepare the BNT-based ceramic, their properties of sintering behavior, phase structures, microstructures, dielectric, ferroelectric, piezoelectric and strain properties were investigated. The relation among electrical properties and the composition and structure was also studied.Lead free piezoelectric ceramics exhibiting superior electromechanical responses have been formulated near the morphotropic phase boundary (MPB), since compositions near MPB possess instability of the polarization state so that the polarization direction can be easily rotated by external stress or electric field, thereby resulting in a high piezoelectricity and permittivity. (1-x)Bi0.5Na0.5TiO3-x(Ba0.7Ca0.3)TiO3 (BNT-xBCT,0≤x<≤0.15) solid solutions have been synthesized by a conventional solid state sintering method for obtaining a morphotropic phase boundary (MPB) with good piezoelectric properties. XRPD patterns show that BCT has completely diffused into the BNT lattice forming a complete solid solution and the MPB of BNT-xBCT system lies in a compositional range of 0.09<≤x≤0.12 at room temperature. The specimen with x=0.09 has the good piezoelectric properties:d33=125 pC/N and kp=0.33. SEM photographs show that the average grain size of BNT-xBCT ceramics decreases slightly with an increase BCT content x. A dielectric relaxation behavior was observed during phase transitions from antiferroelectric to paraelectric in specimens with x exceeding 0.06. The ceramics with x=0.09 show optimized ferroelectric properties (Pr=35.00μC/cm2 and Ec=3.20 kV/mm). Thus, the proper addition of (Ba0.7Ca0.3)TiO3 greatly lowers Ec without degrading remnant polarization Pr of pure BNT. The good ferroelectric and piezoelectric properties lie in MPB composition range, which is attributed to an increase in the number of possible spontaneous polarization directions for compositions near the phase boundary due to the coexistence of the rhombohedral and tetragonal phases. On the other hand, the good piezoelectric properties can also be assigned to the suppression of clamping effect caused by oxygen vacancies which restrains the reversal of spontaneous polarization of ferroelectric domains under an applied electrical field. In lead-free piezoelectric ceramics,0.94Bi0.5Na0.5TiO3-0.06BaTiO3 (BNBT) ceramics have received a great deal of attention due to their excellent ferroelectric and piezoelectric properties and their near rhombohedral-tetragonal (MPB) compositions, and theoretical calculations predict that BiAlO3(BA) has a very large spontaneous polarization of about 76μC/cm2, we introduced BA into BNBT to form solid solutions. (1-x)(0.94Bi0.5Na0.5TiO3-0.06BaTiO3)-xBiAlO3 (0≤x≤0.07) (BNBT-xBA) ceramics was fabricated by an ordinary sintering technique. XRPD patterns show that the ceramics exhibit a pure perovskite structure at x≤0.04, suggesting that BA has diffused into the BNBT-xBA lattice to form a homogeneous solid solution, and a secondary phase appeared at x≥0.05. With addition of BCT, the distortion of the crystal structure decreased, resulting in the crystal structure transformation of BNBT-xBA ceramics from both rhombohedral and tetragonal symmetry to a pseudocubic phase. SEM photographs show that the grain size of BNBT-xBA ceramics decreases obviously with an increase BA content. A dielectric relaxation behavior was observed during phase transitions from antiferroelectric to paraelectric in all the specimens. The polarization hysteresis loop of pure BNBT exhibited typical ferroelectric behavior, having a large remnant polarization and coercive field, when x≥0.03, there is a drastic decreace in remnant polarization and coercive field, which is probably due to the phase transition temperature from ferroelectric to antiferroelectric shifting to room temperature. The electric field-induced strain curves measured at room temperature show that pure BNBT-xBA ceramics without the addition of BA exhibit a butterfly shaped curve typical of ferroelectric material, as BA content increases, the curve changes shape and the negative strain decreased. As x= 0.03, the positive strain reached a maximum value of 0.31% and the value of Smax/Emax reached 523 pm/V. A largest electrostriction coefficient (Q=0.022m4c-2) was obtained for the specimen with x=0.07. |
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