Phase Change Activation and Characterization of Spray-Deposited Poly(vinylidene) Fluoride Piezoelectric Thin Films

Abstract

Structural safety and integrity continues to be an issue of utmost concern in our world today. Existing infrastructures in civil, commercial, and military applications are beginning to see issues associated with age and environmental conditions. In addition, new materials are being put to service that are not yet fully characterized and understood when it comes to long term behavior. In order to assess the structural health of both old and new materials, it is necessary to implement a technique for monitoring wear and tear. Current methods that are being used today typically depend on visual inspection techniques or handheld instruments. These methods are not always ideal for large structures as they become very tedious leading to a substantial amount of both time and money spent. More recently, composite materials have been introduced into applications that can benefit from high strength-to-weight ratio materials. However, the use of more complex materials (such as composites) leads to a high demand of structural health monitoring techniques, since the damage is often internal and not visible to the naked eye. The work performed in this thesis examines the methods that can be used for phase change activation and characterization of sprayable poly(vinylidene) fluoride (PVDF) thin films in order to exploit their piezoelectric characteristics for sensing applications. PVDF is widely accepted to exist in four phases: alpha, beta, gamma, and delta. Alpha phase PVDF is produced directly from the melt and exhibits no piezoelectric properties. The activation or transition from α phase to some combination of beta and/or gamma phase PVDF leads to a polarizable piezoelectric thin film to be used in sensing applications. The work herein presents the methods used to activate phase change in PVDF, such as mechanical stretching, annealing, and chemical composition, to be able to implement PVDF as an impact detection sensor. The results and analysis provided in this thesis will present the possibilities of spray-deposited PVDF thin films in both small-scale and large-scale sensing applications that can be applied to both simple and complex geometries