Doctoral Dissertations

Abstract

"Pulsed power applications require large dielectrics (> 10" 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 TiO2 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’’’Ti through the hopping of Nb+5 from one V’’’’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"--Abstract, page iii.

Advisor(s)

Huebner, Wayne

Committee Member(s)

Anderson, H. U. (Harlan U.)
Fahrenholtz, William
Hilmas, Greg
Waddill, George Daniel

Department(s)

Materials Science and Engineering

Degree Name

Ph. D. in Ceramic Engineering

Publisher

University of Missouri--Rolla

Publication Date

Fall 2001

Pagination

xvi, 167 pages

Note about bibliography

Includes bibliographical references (pages 161-166).

Rights

© 2001 Brian Lawrence Gilmore, All rights reserved.

Document Type

Dissertation - Restricted Access

File Type

text

Language

English

Subject Headings

Dielectrics -- ResearchPulsed power systemsEnergy level densities

Thesis Number

T 7978

Print OCLC #

50396779

Electronic OCLC #

905716767

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