Development of high energy density dielectrics for pulse power applications

Pulsed power applications require large dielectrics (>10&inches; diameter and ≈0.125 thick) with high breakdown strength (BDS). The presence of porosity and increasing grain size lowers the BDS. Testing conditions such as sample thickness, electrode area, electrode design, and form of applied voltage (DC or pulsed) can also influence the BDS. The effect of electrode design is particularly important in that the design can induce local field enhancements and reduce the BDS during testing. The purpose of this study was to characterize the effects of such microstructural features and testing conditions on the BDS.; TiO₂ was chosen as the candidate material for BDS characterization studies and was fabricated using slip casting techniques. The BDS of TiO ₂ decreased by a factor of 2–3 with increases in the thickness and electrode area due to the “weakest link” theory. The BDS also decreased with increasing amounts of porosity, but defects with sizes larger than pores seem to be also influencing the BDS at densities >95%. A breakdown mechanism was proposed consisting of gas discharge within a microstructural defect followed by electron avalanche.; Field modeling showed field enhancements were induced at the triple points where, the surrounding medium, electrode, and dielectric all came into contact. Field enhancements factors (FEFs) were 1.74, 1.03, and 1.23 for the planar, dimpled, and edge radius electrode designs, respectively. Field modeling showed resistive gradients induced by donor doping TiO₂ with Nb +5 would mitigate these field enhancements. Therefore, diffusion of Nb+5 was studied as a function of annealing time, temperature, and pO₂ in both single crystal and polycrystalline TiO₂. Diffusion was found to be about an order of magnitude faster in the (001) versus the (100) direction due to the open c-channels that exist in the rutile structure. Plots of log D versus log pO₂ yielded slopes of 0.20–0.30, indicating that the diffusion of Nb+5 was associated with $V\prime \prime \prime \prime Ti$ through the hopping of Nb+5 from one $V\prime \prime \prime \prime Ti$ to another. Activation energies of 6.97 eV and 6.92 eV were found for diffusion in the (100) and (001), respectively. Grain boundary diffusion in polycrystalline TiO₂ was three to four orders of magnitude higher than bulk diffusion with activation energies of 5.6 eV and 5.0 eV for grain sizes of 32 μm and 8 μm, respectively.