- Piezoelectric materials convert mechanical strain into a dielectric displacement, as well as the converse, allowing these materials to be used as sensors, actuators, and transducers. Currently, lead zirconate titanate (PZT) is the primary material used in these applications. Due to environmental toxicity and safety concerns associated with Pb, development of alternative materials is necessary. Bi-based systems are an attractive area of research in both bulk ceramic and thin film embodiments. Although progress in developing bismuth sodium titanate (BNT)-based solid solutions has been impressive, the combination of cation volatility and high processing temperatures for both bulk ceramic and thin film fabrication can lead to changes in stoichiometry and create defects within the system which can significantly impact material properties.
One of the main sources of defects is often presumed to be related to cation volatility. As such, the diffusion behavior of volatile cations within BNT-bismuth potassium titanate (BKT)-based thin films was studied using transmission electron microscopy, electron energy loss spectroscopy, and energy dispersive x-ray spectroscopy. Both 𝘪𝘯-𝘴𝘪𝘵𝘶 and 𝘦𝘹-𝘴𝘪𝘵𝘶 experiments were performed where the objectives were to: (1) observe crystallization processes in a single layer film, and (2) map the locations of Bi, Na, and K throughout the thin film, bottom electrode, and substrate cross-section. Results indicated that Bi, Na, and K had all diffused into the Pt bottom electrode, and in some cases the underlying buffer layers. In addition to the aforementioned volatilization into atmosphere, this diffusion could also impact film stoichiometry and material properties and should be accounted for.
Multi-layer BNT-BKT-bismuth zinc titanate (BZnT) thin films were fabricated via chemical solution deposition. X-ray diffraction and atomic force microscopy were used to study structure and morphology changes with processing parameters. The dielectric, ferroelectric,and piezoelectric properties were characterized and values of the effective out-of-plane piezoelectric coefficient, d₃₃,f, were extracted from double beam laser interferometry measurements. Dielectric constants and loss ranged from 380-800 and 2-8% respectively as a function of thin film composition. For 0.8 mm diameter top electrodes, the maximum value measured for effective d₃₃,f was approximately 80 pm/V. Lastly, electrical fatigue measurements showed that while the effective d₃₃,f was larger for compositions closer to the BNT-BKT morphotropic phase boundary (MPB), those further away were able to withstand a higher number of cycles, up to three orders of magnitude, at ± 400 kV/cm.
Results from this dissertation were:
(i) An increase in the number of possible end members (e.g. BZnT, BMgT, BaTiO₃ (BT)) for BNT-BKT-based thin films fabricated at OSU
(ii) An expansion in the range of compositions fabricated rather than commonly studied MPB compositions: while the maximum observed values for effective d₃₃ in bulk BNT-BKTBZnT were further towards the BKT-rich side of the ternary phase diagram, the same is not true in thin films
(iii) Motivation for further study of the diffusion of volatile cations in BNT-based thin film systems: cations are not only volatilized into atmosphere during high temperature processing, but can also diffuse out of the thin film into and through the bottom electrode
(iv) If longer device lifetime supercedes the need for the highest obtainable piezoelectric coefficients, compositions away from the BNT-BKT MPB may be of interest