Lead‐Free Piezoelectric Materials

Quasi-one-dimensional (1D) materials, including nanofibers, nanotubes, nanowires, and nanobelts, have been exploited widely in nanogenerators, sensors, transducers, microelectromechanical systems (MEMS) devices, and other applications, such as microwave varactors and ferroelectric field effect transistors.Currently available nanostructured piezoelectric materials show a low piezoelectric coefficient d 33 of merely 100 pm V -1 , with Pb(Zr, Ti)O 3 (PZT)-based materials at the high end.The health impact of lead poisoning is well known, however, and intensive efforts have begun to discover new lead-free piezoelectric compounds which possess comparable piezoelectric performances to those of the lead-based piezoelectric materials.Recently, a lead-free (1-x)Ba(Ti 0.80 Zr 0.20 )O 3 -x(Ba 0.70 Ca 0.30 )TiO 3 ((1-x)BTZ-xBCT) piezoelectric system with optimal composition of x = 0.5 was reported to show superior room temperature piezoelectricity, with the piezoelectric coefficient d 33 = 620 pC N -1 , the piezoelectric voltage constant g 33 = 15.38 × 10 -3 Vm N -1 , and the electromechanical (converse piezoelectric) response as high as 1140 pm V -1 .These superior piezoelectric properties are comparable to or higher than those of state-ofthe-art PZT or other lead-free piezoelectric compounds, due to the low polarization anisotropy and low energy barrier for lattice distortions in the morphotropic phase boundary (MPB) region.The main work presented in this dissertation is focused on the synthesis and characterization of Ba (Ti 0.80 Zr 0.20 ) O 3 -(Ba 0.70 Ca 0.30 ) TiO 3 (BTZ-BCT) in different forms including ceramics, thin films, and nanofibers.Different structural analysis techniques, including X-ray diffraction (XRD), Raman spectroscopy, and transmission electron microscopy (TEM), have been employed to iii investigate the evolution of the crystal structure and phase content in the samples.The coexistence of two ferroelectric phases, tetragonal and rhombohedral, and crystallization of the ceramics, the fibers, and the thin films in the vicinity of the MPB region have been demonstrated.The lattice constants have been defined using the Rietveld method.The impact of lattice parameter variations on the ferroelectric properties of the (1-x)BTZ-xBCT ceramics has been investigated.Different scanning probe microscopy techniques, including piezoresponse force microscopy (PFM), scanning capacitance microscopy (SCM), and scanning spreading resistance microscopy (SSRM), have been employed to study the piezoelectric, ferroelectric domain switching, and the electrical properties of the nanofibers and thin films.Very large piezoelectricity in low-dimensional BTZ-BCT sintered as thin films (d 33 = 141 pm V -1 ) and nanofibers (d 33 = 180 pm V -1 ) has been achieved.These values are comparable to those of PZT films and nanofibers.Observations of ferroelectric nanodomains with high spatial resolution using SCM and PFM techniques are also presented.The influences of lateral size, geometry, and the clamping effect on the piezoelectric performance are investigated for both the thin films and the nanofibers.The current distribution and resistivity have been studied by SSRM.The results show a uniform distribution of resistance and very high resistance of 10 10 ohms in the BTZ-BCT nanofibers.Combining a high piezoelectric coefficient with environmental benefits, the BTZ-BCT nanostructures provide the superior functions that are in demand for highly efficient piezoelectric devices and electromechanical systems.In the last chapter of the thesis, the synthesis and characterization of biocompatible and piezoelectric (Na,K)NbO 3 (NKN) nanofibers are presented.The X-ray diffraction pattern of the nanofibers reveals a pure single phase with polar structure iv after annealing at 700°C.TEM images and electron diffraction patterns show the growth of NKN single crystals in the form of nanofibers.The ferroelectric domain switching and piezoelectric response of the nanofibers have been investigated using PFM.A higher piezoelectric response is achieved in NKN nanofibers (d 33 = 58 pm V -1 ) than in its thin films (d 33 = 40 pm V -1 ).Owing to the existence of permanently charged regions in the NKN nanofibers known as ferroelectric domains, electrical signals can be generated in them via the piezoelectric effect to provide a new opportunity for construction of a smart biocompatible scaffold that can be used for repair, engineering, and regeneration of damaged tissues.