Dielectric capacitors traditionally have very high power and short response time but low energy storage capability compared to batteries and electrochemical capacitors. For efficient and reliable energy storage of intermittent sources such as wind and solar, energy storage devices would ideally have both high power and energy densities. The studies of high energy density capacitor dielectrics presented in this thesis are part of the effort to move toward this paradigm.

Ceramic antiferroelectric compositions such as

Pb_{0.99Nb0.02[(Zr0.57Sn0.43)1-yTiy]0.98O3 (PNZST 43/100y/2) show promise as dielectrics in high energy density capacitors due to a sharp and highly tunable phase transition from antiferroelectric (AFE) to ferroelectric (FE). This transition results in a significant increase in polarization at a critical electric field, storing a large amount of electrical energy that can be released during unloading if the material undergoes the reverse transition at a relatively high electric field.}

These compositions also display thermally induced phase transitions, which must be understood in order to more fully understand how antiferroelectric properties develop. Several thermal characterization methods (dielectric constant and loss tangent, storage modulus and mechanical loss tangent, thermal expansion, and calorimetry) have been utilized to uncover the nature of complex phase transitions in lead-free pseudo-antiferroelectric composition (Bi_{1/2Na1/2)0.93Ba0.07TiO3 (BNT-7BT) and the lead-containing PNZST 43/8/2 composition. These experiments reveal the first order nature of the ferroelectric to antiferroelectric and the antiferroelectric to multi-cell cubic transitions, and the second order nature for the multi-cell cubic to single-cell cubic transition in PNZST43/8/2. In the BNT-7BT, the dielectric anomalies are not accompanied by any structural transitions in the unpoled state. However, after electrical poling to a ferroelectric phase with large domains, the thermal depolarization process corresponds to a first order structural transition.}

In general, the antiferroelectric to ferroelectric transition is accompanied with a volume expansion. Therefore, the critical field may be altered in specimens with varying electrode size, where the outer unpoled material exerts radial pressure on the expanding electroded material. The impact of electrode coverage on antiferroelectric PNZST43/100y/2 capacitors has been investigated at a series of temperatures in a series of compositions. Self-exerted mechanical confinement was found to shift the critical electric fields of the transitions to higher values and moderately increased the energy storage density. Phase field modeling reveals that, in addition to the self-confinement, material defects also contribute to these enhancements.