Theses and Dissertations at Montana State University (MSU)
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Item Sintering in ceramics and solid oxide fuel cells(Montana State University - Bozeman, College of Engineering, 2017) Hunt, Clay Dale; Chairperson, Graduate Committee: Stephen W. Sofie; David Driscoll, Adam Weisenstein and Stephen W. Sofie were co-authors of the article, 'Nickel nitrate and molybdenum oxide as a yttria-stabilized zirconia sintering aid' in the journal 'Processing, properties, and design of advanced ceramics and composites' which is contained within this thesis.; Marley Zachariasen, David Driscoll and Stephen W. Sofie were co-authors of the article, 'Current degradation rate quantification of solid oxide fuel cells with and without aluminum titanate' which is contained within this thesis.; David Driscoll and Stephen W. Sofie were co-authors of the article, 'Constant rate of heating definition of the undefined function of density of the Wang and Raj equation for 8YSZ' which is contained within this thesis.; David Driscoll and Stephen W. Sofie were co-authors of the article, 'Constant rate of heating definition of undefined density function for 8YSZ with a sintering aid' which is contained within this thesis.; David Driscoll and Stephen W. Sofie were co-authors of the article, 'Constant temperature definition of the undefined density function for 8YSZ' which is contained within this thesis.; David Driscoll and Stephen W. Sofie were co-authors of the article, 'Constant temperature definition of the undefined density function of 8YSZ with a sintering aid' which is contained within this thesis.Nature's propensity to minimize energy, and the change in energy with respect to position, drives diffusion. Diffusion is a means by which mass transport resulting in the bonding of the particles of a powder compact can be achieved without melting. This phenomenon occurs in powdered materials near their melting temperature, and is referred to as 'sintering'. Because of the extreme melting temperature of some materials, sintering might be the only practical means of processing. The complexity and subtlety of sintering ceramics motivated the evaluation of empirical data and existing sintering models. This project examined polycrystalline cubic-zirconia sintering with and without transition-metal oxide additions that change sintering behavior. This study was undertaken to determine how sintering aids affect the driving force, and activation energy, the energy barrier that must be overcome in order for an atom or ion to diffuse, of the densification occurring during sintering. Examination of commercially-available cubic-zirconia powder sintering behavior was undertaken with dilatometry, which allows monitoring of the length change a material undergoes as it sinters, and with scanning electron microscopy, which facilitates the study of sintered-sample microstructure. MATLAB algorithms quantifying sintering results were developed. Results from this work include proposed definitions of a 26-year-old undefined function of density factor in a well-accepted mathematical model of sintering. These findings suggest activation energy is not changing with density, as is suggested by recent published results. The first numerical integration of the studied sintering model has been performed. With these tools, a measure of the activation energy of densification of cubic-zirconia with and without the addition of cobalt-oxide as a sintering aid has been performed. The resulting MATLAB algorithms can be used in future sintering studies. It is concluded that sintering enhancement achieved with cobalt-oxide addition comes from reduction in activation energy of densification of cubic-zirconia. Further, it is suggested that the activation energy of densification does not change with material density. This conclusion is supported by the sensitivity of the numerical integration of the aforementioned sintering model to changes in activation energy.Item Enhanced sintering of YSZ ceramics with low level nickel oxide dopants(Montana State University - Bozeman, College of Engineering, 2009) Townsend, Zane Douglas; Chairperson, Graduate Committee: Stephen W. SofieTape 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.