In this study composite ferroelectric materials have been investigated for their ability to harvest energy from mechanical vibrations via the piezoelectric effect, and store electrical energy as capacitor materials. A combination of modelling and experimental techniques have been used to understand the consequences of using multiphase materials for energy harvesting and storage applications, with particular focus on the significance of interactions between composite structure, electric field distributions and the effective material properties.A detailed investigation into the properties of ferroelectric ceramic-air composites, such as porous barium titanate, is presented. Introducing isotropic, randomly distributed porosity into barium titanate was found to increase the energy harvesting figure of merit from ~1.40 pm^2/N for the dense material to ~2.85 pm^2/N at 60 vol.% porosity. Finite element modelling was used to better understand the poling behaviour of barium titanate with different porous structures (uniform, porous sandwich layer and aligned), enabling the design of materials with improved energy harvesting capabilities. Complex porous structures were found to have enhanced energy harvesting figures of merit, with maximum values achieved of 3.74 pm^2/N and 3.79 pm^2/Nin barium titanate with a 60 vol.% porosity sandwich layer (overall porosity ~34 vol.%) and highly aligned freeze cast barium titanate with 45 vol.% porosity, respectively. Dense and porous barium titanate samples were mechanically excited and the derived electrical energy used to charge a capacitor. The porous barium titanate was found to charge the reference capacitor more effectively than the dense material, demonstrating the benefits of introducing porosity into ferroelectric materials for energy harvesting applications.Ferroelectric composites, in which either a conductive filler was added to a high permittivity ferroelectric matrix or a high permittivity ferroelectric phase was added to a low permittivity polymer matrix, were evaluated for their potential as a new generation of capacitor materials using finite element modelling. The studies suggested that the rise in effective permittivity due to the forming of composites is fundamentally linked to the rapid decline in dielectric breakdown strengths observed in composites, resulting in nearly all cases reported in the literature demonstrating a reduction in the energy storage figure of merit. It is concluded that future efforts into finding the next generation of energy storage materials should focus on single phase, or intrinsic, high permittivity materials rather than composite materials.
|Date of Award||31 Jan 2018|
|Supervisor||Chris Bowen (Supervisor) & John Taylor (Supervisor)|