Glass Formation Boundary Approach to the Sintering of Alumina

Date

2010-12

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New York State College of Ceramics at Alfred University. Kazuo Inamori School of Engineering.

Abstract

Sintering in alumina has been extensively studied with a wide range of compositions and a correspondingly large variability of reported chemistries and mineralogy in the grain boundaries. Even with recent advances in the understanding kinetics of grain growth and grain boundary structures, only a few studies have attempted to interpret the evolution of the grain boundary phases and chemistries in alumina sintering potentially due to the fact that sintering of ceramics is a decidedly nonequilibrium process. Expanding on a concept used to successfully explain mineralogy of fired porcelain, the glass formation boundary approach is introduced to predict grain boundary evolution in sintered alumina. Microstructural evolution and grain boundary phases were investigated for four CaO:SiO2 ratios in isochronal sintering studies at 1400°C, 1600°C, and 1700°C. The microstructural analysis showed that additive ratios of high silica to calcia ratios yielded amorphous grain boundaries containing anorthite and mullite as secondary phases. A glass formation boundary was applied to explain the formation of mullite without the presence of anorthite. Anorthite found in the samples sintered at 1400°C were proposed to be caused by localized low temperature solid state reactions. A similar argument was used to explain anorthite and gehlenite precipitation at 1400°C at higher CaO ratios. At temperatures above 1600°C amorphous grain boundaries of invert glass compositions and anorthite were found at the grain boundaries and were explainable by an invert glass formation boundary. The glass formation boundary approach adequately provides an explanation for secondary phases reported in the literature and allows for the prediction of grain boundary chemistry in steady-state non-equilibrium conditions experienced when sintering alumina in the presence of a liquid phase (as would be expected in situations with alumina containing silica, alkali, and alkaline earth impurities). The volume of the secondary phases observed correlates with the grain boundary chemistry at the triple points also consistent with the glass formation boundary approach. Furthermore, it is shown that impurities can be viewed as localized concentrations where the volume of secondary phases is directly limited by the concentration of additives. Although this x study has addressed CaO and SiO2 as the impurity chemistry, it is expected that this idea can be more globally applied to explain grain boundary evolution in alumina.

Description

Advisory committee members: Vasantha Amarakoon, Matthew Hall, William Lacourse. Dissertation completed in partial fulfillment of the requirements for the degree of Doctorate of Philosophy in Ceramics at the Kazuo Inamori School of Engineering, New York State College of Ceramics at Alfred University

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