Ferroelectric and energy storage characterization of PVDF-based thin films and nanocomposites

Abstract

Functional materials, such as lead zirconate titanate (PZT) and barium titanate (BTO), are widely used in microelectromechanical systems (MEMS) due to their strong piezoelectric properties. However, the inherent stiffness of these ceramics limits their applicability where high mechanical flexibility is required. This limitation has led to the growing interest in piezoelectric polymers, particularly in ferroelectric poly(vinylidene fluoride) (PVDF) and its copolymer with trifluoroethylene (P(VDF-TrFE)). These polymers offer a soft alternative to traditional piezoceramics, making them suitable for flexible MEMS applications. This thesis investigates the electromechanical behaviour of PVDF and P(VDF-TrFE) thin films within the context of polymer science. It specifically explores how processing parameters, surface microstructure, and crystalline phases influence the piezoelectric and ferroelectric properties of these materials. For this purpose, capacitor-type test structures were fabricated by depositing thin films of PVDF and P(VDF-TrFE) via spin-coating. Additionally, capacitors can be applied as energy storage devices, an area where ferroelectric polymers have gained significant interest due to their superior dielectric strength compared to conventional ceramics and higher dielectric permittivity than standard polymeric materials. To leverage these advantageous properties even further, this work focuses on developing P(VDF-TrFE)-based nanocomposites loaded with carboxymethyl cellulose nanofibers mixed with BTO polydopamine-based functionalized high-permittivity nanoparticles (CCNF-BTO@PDA). These nanocomposites were incorporated into spin-cast thin films of 2 μm thickness, which is ten times thinner than those typically reported. Despite challenges such as nanoparticle agglomeration, which became more pronounced in these thinner films and significantly affected the breakdown strength, these findings offer valuable insights for further research in spin-cast fluoropolymer-based nanocomposite thin films, especially when targeting the micrometre thickness range for MEMS applications.