"The use of titania (TiO2) dielectrics for high energy density applications such as pulse-forming networks requires the breakdown strength (BDS) of TiO2 to be on the order of 700kV/cm. In essence a higher BDS translates into smaller systems. Since the intrinsic BDS of ceramic dielectrics depends primarily on the micro structure, this study focused on maximizing the density and decreasing the grain size through the use of nanometer-size TiO2. In addition, the influence of electrode design, thermal annealing and surface roughness were characterized since each of these extrinsic factors can impact the electric field enhancement, and in turn the BDS.
Uniaxial pressing of nanometer-sized TiO2 was chosen as the processing technique to fabricate components. Initial dispersion studies using phosphate ester and fish oil with a PVB binder system did not yield uniform microstructures. Surprisingly, nearly fully dense samples with uniform microstructures could be pressed without a dispersant, most likely due to the powder’s broad particle size distribution and effective “Van der Waals” binder. Titania dielectrics with a grain size ranging from 77nm to 11µm were achieved using sintering temperatures from 700 to 1400⁰C for 2h in air. Full density samples with a minimum grain size were obtained by sintering at 850⁰C in air. Sintering in oxygen promoted densification at lower temperatures, with full density and a grain size of 0.27 µm obtained at 800⁰C. Hence a range of grain sizes from 0.27 to 11 µm were prepared to study the influence of grain size on the BDS.
Field modeling showed that planar electrodes create a non-uniform, enhanced field gradient at the triple points, with field enhancement factors (FEFs) of 2.27 and 1.54 for anode and cathode triple points, respectively. This lead to the use of a dimpled electrode design that yields a more uniform field with FEFs for triple points at the anode and cathode being 1.03 and 1.76, respectively. This dimpled electrode design lead to breakdown occurring systematically at the bottom of dimple where the highest field occurred, yielding reproducible data.
The observed BDS of the TiO2 dielectrics depended upon surface finish and grain size. Thermal annealing at 900 C was too low for surface flaw recovery and no BDS improvement was observed. However, dielectrics with an 8pm grain size, polished with 0.1 µm diamond paste showed a higher BDS than those polished only by 1 µm diamond.
The BDS was studied as a function of grain size from 0.27 to 11 µm. The data fit the relation, E oc G-2, which could be explained by a grain boundary model. The BDS of pressed TiO2 sintered in O2 with grain size of 0.27µm was the highest reported in the literature (~1754kV/cm). Even porous TiO2 pellets sintered at 750 and 800⁰C in air, with 6.7 and 1.2% porosity, respectively, exhibited a high BDS of ≈1000kV/cm. The breakdown mechanism in these studies was ascribed to a combination of discharging and thermal breakdown"--Abstract, pages iii-iv.
Materials Science and Engineering
M.S. in Ceramic Engineering
University of Missouri--Rolla
xiii, 61 pages
© 2003 Yang Ye, All rights reserved.
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Ye, Yang, "Grain size dependence of the dielectric breakdown strength of titanium dioxide dielectrics" (2003). Masters Theses. 2443.
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