Large Electromechanical Response in the Piezoelectric BNKT-BST based Ceramics and Ferroelectric ZnSnO3/PVDF-TrFE Nanocomposite
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Piezoceramics are attractive and important materials that are used for many electromechanical applications such as sensors, actuators, and transducers due to their excellent electromechanical properties. However, with respect to environmental concerns and legislation regulating hazardous materials, lead, which is considered toxic, is banned from use in many commercial applications. The enforcement of these regulations and restrictions on using hazardous substances in electronic devices has resulted in efforts to discover lead-free replacements for commercial Pb-based piezoceramics. In this dissertation a suite of new system (1-x)Bi0.5(Na0.40K0.10)TiO3-x(Ba0.70Sr0.30)TiO3 was synthesized. The system was further highly improved with excellent electromechanical properties by doping A-site (La) and B-site (Nb) and the effect of dopants was analyzed and explained in detail. The effects of A- and B-site doping on the phase transitions were confirmed with X-ray diffraction (XRD). Further, an electric-field-dependent XRD was conducted to identify the main source of the large strain in the BNKT-BST ceramics. The ferroelectric, dielectric and impedance studies were carried out and explained. The X-ray crystal structure of the BNKT-BST and with La and Nb addition transformed from a tetragonal to a pseudocubic. It is found that the dielectric constant and Td was decreased with La and Nb contents. The observed change in the behavior of dielectric constant at Td could be considered as an evidence of the transition from ferroelectric (FE) to nonpolar (NP) state at RT as a function of La and Nb concentration and temperature. The significant decrease in remnant polarization (Pr), coercive field (Ec) and along with the small decrease in spontaneous polarization (Ps) suggests that the high ferroelectric order in BNKT-BST is disrupted with the addition of La and Nb ions with a noncubic distortion or a 'nonpolar' phase. The normalized strain (d*33 = Smax/Emax = 650 pm/V) obtained for BNKT-BST ceramic at electric field of 60 kV/cm for the 2 mol% La-content. It was observed that the normalized strain of La-content of 2 mol%, is very temperature- insensitive up to 125oC, even at 125oC the d*33 is as high as ~431pm/V. The d*33 = 634 pm/V was obtained for the 2 mol% Nb-content at 60 kV/cm with relatively slim shape hysteresis of 40%. Our results strongly suggest that the BNKTN-BST ceramics may have potential for future applications in environmentally friendly piezoelectric devices, such as sensors and actuators. Pure polymers are limited by their low intrinsic dielectric constants (? of 2 ??3), although their dielectric breakdown strength is rather high (200 ? 300 kV/mm). The low Tc (~ RT), high dielectric loss (~ 0.4) and temperature coefficient limits the practical applications of the pure polymer. The common approach to enhance the dielectric constants of polymer composites is to introduce ceramic nanoparticles with high dielectric constants such into the polymer matrix. Because of favorable features of ceramics such as high-dielectric-constants, low dielectric loss makes ceramics promising candidates for embedded capacitors used in embedded passive technology. ZnSnO3 is primarily characterized by a large displacement of Zn based on a strong covalent bond between three oxygen and zinc atoms, resulting in the ZnSnO3 having a strong piezoelectric response. Therefore, ZnSnO3 has also attracted significant attention recently. Furthermore, ZnSnO3 nanoparticles can be prepared by a solution method at low temperature. These features of ZnSnO3 allowed us to create a system comprised of ZnSnO3 and polymer nanocomposite. To produce a high dielectric constant, low dielectric loss, temperature and frequency insensitivity and normal ferroelectric behavior, we introduced ZnSnO3 nanoparticles in poly(vinylidene fluoride-trifluoroethylene) P(VDF-TrFE) copolymer. Furthermore, ZnSnO3 nanoparticals prepared by chemical solution process were used as dielectric fillers in P(VDF-TrFE)-based nanocomposites. Enhanced dielectric constant was obtained at a low volume fraction of ZnSnO3 nanoparticles. The enhanced ferroelectric properties in the nanocomposites with Pr (6.0 ?C/cm2 along with very low Ec of 48 MV/m was achieved in the nanocomposites for 5.0 vol% ZnSnO3 nanoparticles. The films exhibited high dielectric constants of up to 39 at 1 kHz and significant low-frequency dielectric properties were discussed. These advantages of a ZnSnO3/P(VDF-TrFE) nanocomposite system make it useful for practical applications with flexible FET, nanogenerators and FeRAM-like devices.