Lead-free piezoelectric nanofibers and nanotubes for energy harvesting applications

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Embargoed until 2019-12-01
Copyright: Ghasemian, Mohammad Bagher
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Abstract
One dimensional (1D) piezoelectric materials such as nanofibers, nanowires and nanotubes have been applied largely in sensors, nanogenerators, microelectromechanical systems (MEMS), transducers etc. due to their remarkable transport of charge carriers, large piezoelectricity and high sensitivity to small mechanical motions through the efficient conversion of ubiquitous mechanical energy to electricity owing to their lightweight, small-size, high elastic compliance, and high strain tolerance. In this project, different 1D lead-free piezoelectric materials including (Bi0.5Na0.5)TiO3 and (K, Na)NbO3 were synthesized through a hydrothermal method under optimized conditions. The structural, chemical and electrical properties of the products were investigated by a suite of advanced materials characterization techniques such as XRD, SEM, EDS, TEM, Raman, NMR and PFM. The first chapter presents an introduction to piezoelectricity, different types of piezoelectric nanostructures, their typical synthesis methods and characterization techniques. The motivation, novelty and significance of this project were also given. The second chapter - Literature Review - introduces a background about lead-based and lead-free piezoelectric materials, especially the advantage and progress in 1D lead-free piezoelectric materials. In chapter three, lead-free piezoelectric bismuth sodium titanate (BNT) nanostructures were synthesized using a low-temperature hydrothermal technique. It was found the phase and morphology of the products were strongly dependent on the composition and concentration of the precursors, as well as the processing conditions. By optimising the synthesis parameters, well-crystallized BNT nanofibers with 150 - 200 nm in diameter and ~5 µm in length were obtained. The BNT fibers showed a pure perovskite phase and an orthorhombic structure with (011) orientation along the length direction. A piezoelectric constant of d33 = ~15 pm/V in the diameter direction was observed in these BNT nanofibers. In chapter four, the facile low-temperature hydrothermal method accompanied with a moderate annealing step was employed to create highly crystalized lead-free piezoelectric 0.94(Bi0.5Na0.5)TiO3-0.06BaTiO3 (BNBT) nanofibers where sodium ions in the (Bi0.5Na0.5)TiO3 (BNT) lattice are substituted with larger barium ions. The BNBT nanofibers were typically 150 – 200 nm in diameter composed of a confirmed pure perovskite phase with the orientation of (101) along the fiber length direction. A rhombohedral lattice structure was confirmed by TEM measurements after introducing barium to BNT structure. Solid-state 23Na NMR evidenced a reduced degree of disorder in the A site of the BNBT as compared to the BNT, along with the formation of additional Na environments, which were ascribed to an inhomogeneous distribution of Ba in the system. A significant piezoelectric constant of d33 = ~26 pm/V in the diameter direction was found in these BNBT nanofibers. In chapter five, a large piezoelectric coefficient of 76 pm/V along the diameter direction, approaching that of lead-based piezoelectrics is obtained in hydrothermally-synthesized Bi0.5(Na0.8K0.2)0.5TiO3 nanotubes. The 30-50 nm diameter nanotubes are formed through a scrolling and wrapping mechanism without the need for a surfactant or template. A molar ratio of KOH/NaOH=0.5 for the mineralizers yields the Na:K ratio of ~0.8:0.2 corresponding to an orthorhombic-tetragonal phase boundary composition. XRD patterns along with TEM analysis ascertain the coexistence of orthorhombic and tetragonal phases with (110) and (001) orientations along the nanotube length direction, respectively. 23Na NMR spectroscopy confirms the higher degree of disorder in BNKT nanotubes with O/T phase coexistence. These findings present a significant advance towards the application of 1D lead-free piezoelectric materials. In chapter six, potassium sodium niobate (K1-xNaxNbO3) piezoelectric nanostructures with different morphologies and compositions were synthesized through hydrothermal method. It was found that elemental compositions of the products were different from the K/Na ratio in the starting materials. The effect of hydrothermal processing parameters on the structure and morphology of K1-xNaxNbO3 materials was investigated. One-dimensional K0.5Na0.5NbO3 nanostructure was obtained under the conditions of KOH/NaOH = 5 and hydrothermal temperature of 160 ˚C through an oriented attachment mechanism, while higher temperatures led to a cubic morphology. K0.5Na0.5NbO3 nanofibers with ~50 nm diameter presented a morphotropic phase boundary (MPB) between orthorhombic and monoclinic phases with a growth direction of [001] and [110], respectively. The coexistence of O and M phases in KNN nanofibers was further confirmed by nuclear magnetic resonance (NMR) measurements, i.e. the nanofibers exhibited higher structural disorder than the nanocube counterparts possessing a single O phase. A high piezoelectric coefficient of 60 pm/V was confirmed in K0.5Na0.5NbO3 nanofibers through PFM measurements.
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Author(s)
Ghasemian, Mohammad Bagher
Supervisor(s)
Wang, Danyang
Li, Sean
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Publication Year
2018
Resource Type
Thesis
Degree Type
PhD Doctorate
UNSW Faculty
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