Lead-free Relaxor Ferroelectric Thin Films: Design, Processing, and Characterization for Advanced Energy Storage Capacitor Applications

The demand for capacitors with high energy and power density has been steadily increasing in advanced electronic technologies. To meet this demand while addressing environmental regulations, the development of lead-free dielectric thin film capacitors with high performance and reliability is essential.
This thesis investigates the composition-processing-property relationships in Bi0.5Na0.5TiO3-based perovskite thin film capacitors substituted with BaTiO3, NaNbO3, and BiMg2/3Nb1/3O3. Particular attention is given to compositional modification as a strategy to tailor structural and functional properties, and to processing optimization to maximize energy storage performance. A reproducible route for preparing metal-organic precursor solutions was established, enabling the fabrication of multilayer thin film capacitors on platinized silicon substrates. In addition, the influence of processing parameters – specifically heating rate and annealing temperature – was systematically optimized to control grain growth, texture, and film nucleation.
The thin films were structurally and microstructurally characterized using grazing-incidence X-ray diffraction, Raman spectroscopy, atomic force microscopy, and scanning electron microscopy. Electrical characterization was carried out through polarization-electric field (P-E) hysteresis loops, dielectric spectroscopy, dielectric breakdown measurements, and reliability testing under cyclic and temperature-dependent conditions.
BaTiO3-substituted Bi0.5Na0.5TiO3 thin films exhibited slim hysteresis loops and improved energy storage performance near the morphotropic phase boundary (6 mol% BaTiO3). Further substitution with Zr enhanced relaxor-like behavior, yielding slimmer hysteresis loops, higher breakdown strength, and recoverable energy densities up to 29 J cm-3. NaNbO3 substitution improved fatigue and thermal reliability, maintaining good energy storage performance. BiMg2/3Nb1/3O3 substitution further strengthened the relaxor character by suppressing long-range ferroelectric ordering, resulting in ultrahigh energy storage properties. After optimizing the processing route, the thin films achieved recoverable energy densities up to 61 J cm-3 with efficiencies exceeding 70% at a breakdown field of more than 2.7 MV cm-1, demonstrating electrical stability up to 180 °C and endurance over 105 cycles.
Overall, the results demonstrate that the systematic tuning of composition and the precise control of processing parameters, when coordinated effectively, enable the realization of ultrahigh-performance lead-free dielectric thin film capacitors for advanced energy storage applications.