| Bismuth sodium titanate (Bi0.5Na0.5TiO3, abbreviated as BNT) with a rhombohedral symmetry (R3c) at room temperature is a kind of A-site complexly occupied perovskite type ferroelectric ceramics, which has a high Curie temperature (Tc～320℃) and a relatively large remanent polarization (Pr～38 μC/cm2). Therefore, it was considered as one of the most promising lead-free piezoelectric and ferroelectric ceramics. In this dissertation, a conventional solid-state reaction method was adopted to fabricate ceramics and the phase composition dependent properties were also investigated in detail. As the same time, the phase structure transformation, dielectric relaxor properties, piezoelectric and electrostrain properties in BNT-based ceramics were also analyzed. Based on this, the fabrication process and electrical properties were preliminarily investigated in the BNT-based lead-free ceramics/epoxy 1-3 piezocomposites and the relations between the electrical properties and the ceramic volume fraction were also discussed.This dissertation can be divided into three parts:1. The pure BNT ceramic has its intrinsic bottlenecks such as high coercive electric field and conductivity, which restricted its extensive applications. By incorporating PT into BNT matrix, the overall electrical properties can be improved to some degree. As known to all, the phase structure of 0.84BNT-0.16PT is tetragonal at room temperature, when the material with rhombohedral structure was added into this matrix to form MPB, electrical properties could be enhanced further. Lead magnesium niobate (PMN) has pseudocubic structure at room temperature with a relatively high dielectric permittivity and strong dielectric dispersion. The tetragonality of this matrix decreased gradually by adding PMN into 0.84BNT-0.16PT and when the mole fraction of PMN was 37.5%, piezoelectric properties reached optimal performance due to the coexistence of tetragonal and pseudocubic phase. When the content of PMN reached to 42.5%, a relatively large strain can be induced under the external electric field of 7 kV/mm, which was due to the fact that this composition was in the vicinity of the boundary of nonergodic and ergodic relaxor phase. Especially notable is that the current density loops appeared abnormal phenomenon in a particular composition after modulating different compositions and temperature. Hence we adopted in-situ high-resolution synchrotron X-ray diffraction measurement and other characteristic methods to explore the cause of this phenomenon. Based on the analysis of previous chapter, we fixed the PMN content and synthesize the ternary solid solution (0.7-x)(Bi1/2Na1/2)TiO3-xPbTiO3-0.3Pb(Mg1/3Nb2/3)O3. As the same time, the phase structure transformation, composition-dependent dielectric relaxor properties, piezoelectric and ferroelectric properties were discussed as a function of the PT content. Moreover, in this chapter, we tempted to summarize the evolution of ferroelectric hysteresis loops, bipolar-unipolar strain curves and current density loops under different electric fields. What’s more, according to various zones, the different morphology of domains can be draw under different electric fields.2. Barium nickel niobate Ba(Ni1/2Nb1/2)O3 (BNN) is a perovskite-structured ferroelectric ceramic, which displays a cubic structure and possesses a diffuse phase transition at room temperature. Moreover, BNN has a relatively large tolerance factor of t=1.064 (RBa2+=1.61 A, RNi3+=0.56 A and RNb5+=0.64 A). Due to the fact that BNT has a rhombohedral symmetry (R3c) at room temperature, when BNN was incorporated into the BNT matrix to form a solid solution, not only a morphotropic phase boundary (MPB Ⅱ) was formed, but also a relatively large electrostrain appeared at the side of pseudocubic relaxor phase. In the BNT-xBNN system, because of the coexistence of ferroelectric rhombohedral phase and pseudocubic phase, the x=0.045 sample exhibited optimal piezoelectric and electromechanical coupling properties of d33-121 pC/N and kp～0.27. When the BNN content reached to x=0.05, an electrostrain of 0.3%(d33*～425 pm/V) under the external electric field of 7kV/mm appeared together with the degradation of piezoelectric and electromechanical properties.3. Fine-scale 0.96Bi0.5(Na0.84K0.16)0.5TiO3-0.04SrTiO3 ceramic rod/epoxy (BNKT-ST/epoxy) 1-3 piezocomposites were fabricated via a modified dice-fill method. Through optimizing the ceramic volume fraction (v) and when v=0.276, the 1-3 composite exhibits promising electrical properties:a relatively high piezoelectric strain constant d33～104 pC/N, a large piezoelectric voltage coefficient g33～91.5×10-3 m2/C, a high thickness coupling coefficient kt-0.547, moderate dielectric permittivity εr～128.4 and low acoustic impedance Z～9 Mrayls. Moreover, a single thickness mode can be obtained with a relatively high resonance frequency with more than 2 MHz. These promising results indicate that the BNKT-ST ceramic/epoxy 1-3 composite has great potential to be used in biomedical ultrasonic transducers as well as nondestructive evaluations. |
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