Date of Award

1-1-2011

Document Type

Campus Access Dissertation

Department

Chemical Engineering

First Advisor

Harry J Ploehn

Abstract

The Air Force has increasing demand for high energy density storage systems for pulse power applications. Although electrochemical capacitors offer high energy and power density, their rate capability is limited by mass transfer and faradaic reaction rates - a critical issue for delivering intense pulsed power. Polymer composites, incorporating the high energy storage capability of fillers and graceful failure mechanisms of matrix polymers, offer a solution for the acute need for high voltage, high rate pulse power.

In the first part of this presentation, polystyrene (PS) composites with perovskite fillers such as barium titanate (BT) for dielectric applications will be discussed. The strategy to overcome the poor filler-polymer interactions that form the core of issue in polymer composites is to surface-modify the filler materials. The attempt by our collaborators at Department of Chemistry and Biochemistry at such surface modification fortuitously led to the development of a layered mixed metal phosphonate system (BTPPA). These mixed metal phosphonate fillers have been observed to better disperse in matrix polymer and exhibit higher dielectric permittivity compared to surface modified BT. In addition, an embedded-sphere method was developed to measure dielectric breakdown strength in composites. This technique helped in minimizing low-field breakdown events due to the deleterious effects of the surrounding medium. As a result, breakdown data acquisition has been faster and more reliable. Taken together, these techniques have led to a 3-fold increase in the stored energy density of BTPPA/PS composite system compared to base polymer.

Next, the dielectric integrity of "giant-dielectric" calcium copper titanates (CCTO) fillers in PS and poly (vinylidene fluoride - hexafluropropylene) copolymer (PVDF-HFP) systems will be discussed. An attempt has been made to understand the effect of filler preparation method (sol-gel vs. solid state) and filler loading on dielectric properties of the resulting composites. The composites prepared using sol-gel CCTO fillers, at optimal filler loadings of 20 vol%, show high dielectric permittivities and lower reduction in breakdown strength. The calculated energy density of this system is 12 J/cm3, making it suitable for pulse power applications.

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