Date of Award

Winter 2007

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Electrical & Computer Engineering

Committee Director

Ravindra P. Joshi

Committee Member

Linda L. Vahala

Committee Member

Karl H. Schoenbach

Committee Member

Gene Hou

Abstract

The development of transmission lines with higher energy storage capabilities is an important goal for compact pulsed power systems. In this context, ceramic dielectrics are promising candidates from the standpoint of high dielectric constants and breakdown strength. Though such materials look promising, their breakdown response characteristics have not been well studied, nor adequately understood. The electrical response of dielectrics also seems to depend on the internal structure and its granularity. For example, the breakdown strength of nano-crystalline insulators such as titania and zinc oxide have been observed to depend on the internal grain size. In general, the hold-off voltage increases monotonically with decreases in grain sizes. For example, nano-crystalline TiO2 exhibits higher breakdown strength as compared to micron size TiO2.

In this dissertation, time-dependent, two-dimensional simulations based on random Voronoi networks have been developed to study the electrical breakdown and thermal failure in ceramic dielectrics in ZnO varistors in response to high-voltage pulsing. Our simulations allow for dynamic predictions of internal failures and to track the progression of hot-spots and thermal stresses in samples. The focus is on internal grain-size variations and relative disorder. Our results predict that parameters such as the device hold-off voltage, the average internal temperature, and average dissipated energy density, and applied pulse-durations would be higher with more uniform grains. Furthermore, scaling down the average grain size offers similar advantages. Finally, it is shown that for the situations studied, the principle failure mechanism arises from internal localized melting, while thermal stresses are well below the thresholds for cracking. In addition, the somewhat surprising observation of lower breakdown fields for TiO2 under pulsed conditions as compared to quasi-DC biasing, was studied. Our simulation results indicated that electrical breakdown of TiO2 under multiple pulsed conditions can occur at lower voltages as compared to quasi-DC biasing. We hypothesize that the lower breakdown voltages observed in TiO2 under pulsed conditions, is a direct rise-time effect, coupled with cumulative detrapping. Finally, the role of granular dielectrics having non-linear, voltage-dependent capacitances on pulse rise-time sharpening was also probed and has been discussed.

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DOI

10.25777/ebz1-zd33

ISBN

9780549329350

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