Electrochemical vapor deposition of yttria stabilized zirconia on porous graphite substrates
Shiau, Lih-Farn
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https://hdl.handle.net/2142/19581
Description
Title
Electrochemical vapor deposition of yttria stabilized zirconia on porous graphite substrates
Author(s)
Shiau, Lih-Farn
Issue Date
1993
Doctoral Committee Chair(s)
Wirtz, Gerald P.
Department of Study
Materials Science and Engineering
Discipline
Materials Science Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, Inorganic
Engineering, Chemical
Engineering, Materials Science
Language
eng
Abstract
The main goal of this study was to deposit stabilized zirconia films on a porous graphite substrate by electrochemical vapor deposition using CO as the oxidizing agent. Five experimental variables, deposition temperature, deposition time, gas flow rate, anode H$\sb2$/Ar ratio, and substrate porosity were varied to investigate their effect on EVD process.
A faceted microstructure of the yttria stabilized zirconia film was observed under the current experimental conditions. Unlike most of the EVD growing films, no conclusive texture has been found in the deposition of yttria stabilized zirconia on porous graphite substrates. The time required for the pore closure decreased with increasing deposition temperature. The formation of ZrC was observed at a higher deposition temperature and a higher H$\sb2$/Ar ratio. It was also found that the addition of H$\sb2$ on the anode side of the local cell resulted in a higher yttria content in the final composition of the film. An increase in the gas flow rate in the feed gas resulted in the agglomeration of small faceted grains and a continuous and dense coating. The film growth rate was affected significantly by the substrate porosity. A higher film growth rate resulted from a higher substrate porosity. This result implies that the reaction mechanism for the deposition of stabilized zirconia on a porous graphite substrate is limited by gas phase transport through the substrate pores. The surface morphology of the EVD film was influenced by the deposition temperature, deposition time, and feed gas composition.
Thermodynamic calculations for the deposition of yttria stabilized zirconia on carbon substrates are presented. The calculations have been described for equilibrium in the feed gas phase and at the EVD surface. The variations in the initial gas composition, such as the amount of H$\sb2$O and H$\sb2$ in the feed gas, were included in the calculations to analyze their effects on the deposition process. A model used to calculate the dissolution energy of Y$\sb2$O$\sb3$ in ZrO$\sb2$ is also proposed. The yield of ZrO$\sb2$ and Y$\sb2$O$\sb3$ was calculated as a maximum conversion fraction. It was concluded from thermodynamic calculations that the highly dehydrated metal chloride sample would result in a higher yttria concentration than the poorly dehydrated sample. (Abstract shortened by UMI.)
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