Enhanced sintering of YSZ ceramics with low level nickel oxide dopants
Townsend, Zane Douglas.
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Tape casting is an effective method for obtaining thin YSZ electrolytes used in solid oxide fuel cells. While the costs of YSZ have decreased from demand and improved manufacturing, large amounts of energy are required to properly densify the ceramic material inflating of the overall cost of this material. YSZ must be completely dense to ensure that the fuels used for power generation do not combine resulting in failure of the cell. The purpose of this research is to determine the effectiveness of low level nickel oxide dopants as a sintering aid to reduce the high sintering temperature process while also evaluating the mechanical and ionic conduction behavior. Baseline samples, of both 8YSZ and 3YSZ, were tested and compared to samples doped with concentrations of nickel oxide up to 1 mole %. Transition metal additives may have the potential to increase the electrical conductivity and/or decrease the ionic conductivity of the YSZ, therefore only a maximum of 1 mole % nickel was added to the ceramic powder. It was determined that one mole percent of nickel oxide added to the traditional tape casting slurry achieved the greatest improvement in theoretical density, sintering rate, and onset of densification at 1300°C using standard micron sized powders. Utilizing impedance spectroscopy, it was shown that overall the nickel dopant yielded a negligible change in total conductivity, however, it is noted that a slight increase in the grain boundary contribution was observed for the doped specimens. Mechanical strength testing was conducted with a concentric ring on ring compressive flexural strength standard according to ASTM guidelines indicating an average 40% increase in strength over the undoped specimens for all sintering temperatures. Evaluation of grain size and fracture patterns for doped and undoped specimens indicates significant grain growth and a transition to transgranular fracture with the doped specimens.