NANOCOMPOSITES BASED ON FERROELECTRIC POLYMERS FOR ELECTRICAL ENERGY STORAGE

Open Access
Author:
Li, Junjun
Graduate Program:
Materials Science and Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
November 20, 2009
Committee Members:
  • Qing Wang, Thesis Advisor
Keywords:
  • nanocomposite
  • ferroelectric
  • PVDF
  • energy storage
Abstract:
Poly (vinylidene fluoride) (PVDF)-based ferroelectric polymers exhibit much higher electrical energy storage capability than other linear dielectric polymers. These ferroelectric polymers, therefore, find the most promise in manufacturing advanced electronic devices and electric power systems with reduced weight, size, and cost. Polymer-based capacitor technology has the advantages of high energy density, low dielectric loss, easy processing, and good flexibility. In order to further improve and tailor the energy storage capability, carefully selected inorganic nanoparticles are incorporated into polymer matrices. Structural and dielectric properties of the resulting nanocomposites have been examined. Firstly, high dielectric constant BaTiO3 nanoparticles, whose surface were functionalized by ethylene diamine, were used as dopant to prepare nanocomposite with poly (vinylidene fluoride-chlorotrifluoroethylene) [P(VDF-CTFE)] and poly (vinylidene fluoride-trifluoroethylene-chlorotrifluoroethylene) [P(VDF-TrFE-CTFE)]. The morphology, crystalline structure, and dielectric responses were revealed in dependence on the loading of BaTiO3 and selection of polymer matrices. It was concluded that the polymer matrices play a decisive role in determining the energy density of nanocomposites. To obviate the negative influence of dielectric contrast, polymer nanocomposites were then fabricated based on surface-functionalized TiO2 nanoparticles dispersed in a ferroelectric P(VDF-TrFE-CTFE). TiO2 nanoparticles with barium hydroxide (Ba-OH) as surface modifier possess dielectric permittivity of 47, comparable to that of P(VDF-TrFE-CTFE) around 42, were uniformly dispersed in polymer matrix with limited aggregation. The resultant nanocomposites revealed a large enhancement in polarization response at high electric fields and hence the energy density, which might be attributed to the change in polymer microstructure induced by the nano-fillers as well as the interfacial effect. To eliminate the adverse influence of surfactant on the overall dielectric properties, “Graft to” method was also utilized to fabricate polymer nanocomposites. Phosphonic acid group, which is proven to attach to metal oxide surface firmly, was introduced as end functional group of P(VDF-CTFE) by free radical polymerization initiated by a functional benzoyl peroxide (BPO) with phosphonate group followed by transformation of phosphonate to phosphonic acid. The phosphonic acid end-groups were grafted onto the surface of ZrO2 nanoparticles. Comprehensive structural, morphological, thermal, and dielectric studies were conducted on the resultant nanocomposites.