Investigating the Microstructure Evolution of Industrial Al2O3 with Glass Phase Chemistry in the CaO-Al2O3-SiO2 System

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New York State College of Ceramics at Alfred University. Inamori School of Engineering.
While extensive research has been performed on sintering of high purity alumina (>99.9%), the role of liquid phase properties on microstructural evolution of industrial Al2O3 (88-98% Al2O3) has been studied to a limited extent. This study investigates the microstructure evolution of industrial Al2O3 (99.8%) sintered with glass phase chemistries in the CaO-Al2O3-SiO2 (CAS) system. Two ancillary results demonstrate: (1) industrial Al2O3 contains significant agglomeration after processing which can be removed via either wet milling or with a sedimentation technique, and (2) optimized etching conditions for Al2O3 with significant glass phase necessitate a two-part chemical-thermal etch for imaging of Al2O3 grains. Al2O3 samples were created with glass compositions within the CAS system of varying SiO2:CaO ratios and compared to samples of as-received Al2O3, with no additional glass forming additives. A statistical Design of Experiments (DOE) was used to investigate the validity of statistical analysis to empirically model densification with varying glass phase compositions. The relative significances of sintering time, temperature, and Al2O3 levels were determined for each composition system. It can be concluded that full factorials more completely represent densification in liquid-phase sintered systems than partial factorials due to the asymptotic nature of densification curves. It was observed that the SiO2:CaO ratio strongly influenced both densification and grain size under identical sintering conditions, and these behaviors can be grouped by the relative ratios of SiO2:CaO in the system (according to SiO2:CaO >1, ≈1, and <1). The compositions of the glass phases and secondary phase formation can be predicted based on the established Glass Formation Boundary approach to sintering. Al2O3 grains were observed to exhibit normal grain growth in all samples; however, significant secondary crystallization occurred with increasing CaO content, which limited the growth of Al2O3 grains. The average grain size of Al2O3 grains increased with increasing CaO content until the SiO2:CaO ratio fell below 1:1.5, where excessive secondary phase formation occurred and reduced the average Al2O3 grain size.
Dissertation completed in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Ceramic Engineering at the Inamori School of Engineering, New York State College of Ceramics at Alfred University
Aluminum oxide, Glass