Graduate Thesis Or Dissertation
 

Development of bi-based lead-free piezoelectric materials : thin film piezoelectric materials via PVD and CSD routes

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  • Piezoelectric materials have been widely used in electromechanical actuators, sensors, and ultrasonic transducers. Among these materials, lead zirconate titanate Pb(Zr[subscript 1‐x]Ti[subscript x])O₃ (PZT) has been primarily investigated due to its excellent piezoelectric properties. However, environmental concerns due to the toxicity of PbO have led to investigations into alternative materials systems. Bismuth‐based perovskite piezoelectric materials such as (Bi₀.₅,Na₀.₅)TiO₃ – (Bi₀.₅K₀.₅)TiO₃ (BNT – BKT), (Bi₀.₅,Na₀.₅)TiO₃ – (Bi₀.₅K₀.₅)TiO₃ – BaTiO₃(BNT – BKT ‐ BT), (Bi₀.₅K₀.₅)TiO₃ – Bi(Zn₀.₅,Ti₀.₅)O₃ (BKT – BZT), and (Bi₀.₅,Na₀.₅)TiO₃ – (Bi₀.₅K₀.₅)TiO₃ – Bi(Mg₀.₅,Ti₀.₅)O₃ (BNT – BKT ‐ BMgT) have been explored as potential alternatives to PZT. These materials systems have been extensively studied in bulk ceramic form, however many of the ultimate applications will be in thin film embodiments (i.e., microelectromechanical systems). For this reason, in this thesis these lead‐free piezoelectrics are synthesized in thin film form to understand the structure‐property‐processing relationships and their impact on the ultimate device response. Fabrication of high quality of 0.95BKT – 0.05BZT thin films on platinized silicon substrates was attempted by pulsed laser deposition. Due to cation volatility, deposition parameters such as substrate temperature, deposition pressure, and target‐substrate distance, as well as target overdoping were explored to achieve phase pure materials. This route led to high dielectric loss, indicative of poor ferroelectric behavior. This was likely a result of the poor thin film morphology observed in films deposited via this method. Subsequently, 0.8BNT – 0.2BKT, 85BNT – 10BKT ‐ 5BT, and 72.5BNT – 22.5BKT – 5BMgT (near morphotropic phase boundary composition) were synthesized via chemical solution deposition. To compensate the loss of A‐site cations, overdoped precursor solutions were prepared. Crystallization after each spin cast layer were required to produce phase pure material. Good permittivities and low dielectric loss over the frequency range of 100 Hz to 1 MHz were obtained. Dependent upon annealing conditions, various film morphologies and compositional distributions were observed via electron microscopy and composition measurements. As opposed to previously reported work, good ferroelectric response at low frequency (200 Hz) were found. For BNT – BKT – BMgT, the maximum polarization was over 50 μC/cm² with high d[subscript 33,f] of 75 pm/V were obtained. Additionally, the extrinsic and intrinsic contributions to the dielectric response for solution‐derived BNT – BKT and BNT – BKT – BMgT films were studied via Rayleigh analysis. For sub‐switching fields a good agreement between predicted polarization behavior from Rayleigh analysis and experimentally measured polarization indicated the validity of this approach for BNT‐based thin films. Results of this thesis proved that high quality bismuth‐based piezoelectric thin films with good electrical response can be fabricated with suppression of cation volatility for various processing conditions. Furthermore, these thin films can be considered as alternatives to PZT thin films as potential candidates for piezoelectric‐based microelectromechanical systems (MEMS).
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