Surface Area Reduction and Two Step Sintering of Alumina
Date
2023-02
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New York State College of Ceramics at Alfred University. Inamori School of Engineering.
Abstract
Surface area reduction trajectories were examined for calcined alumina powder over a range of thermal profiles. Variations in trajectory are explained through experimental demonstrations of blended coarse and fine particle alumina systems. This work demonstrates that (1) nanoparticles do not appear to assist conventional sintering, as the nanoparticle surface area drops precipitously with minimal increase in density, and (2) the forming method does not contribute, nor control, surface area reduction during alumina sintering.
Surface area reduction trajectories of >99.9% purity α-alumina powders with various sodium additives levels are compared to a commercial Bayer processed alumina powder to assess the role of sodium on surface area reduction with heat treatment. The results demonstrate that the surface area reduction trajectories of α-alumina are dictated solely by particle size and are independent of the presence of impurities over the range of chemistries studied, indicating that Na impurities do not appear to enhance surface area reduction.
Two step sintering of a calcined alumina powder was investigated. A range of green densities were generated and evaluated, to address the role of processing in two step sintering. Both as received and milled powders achieved relative densities surpassing 99% through two step sintering with the lowest ratio of measured grain size with respect to initial particle size(D50) was observed to be 1.5. Discontinuous grain growth features were observed within a fine matrix for milled powder.
The sintering of graded particle size distributions of alumina were evaluated for dispersed and flocculated systems. Two step sintering (TSS) of a matrix phase composed of fine particles was attempted with respect to the percolation threshold of coarse and medium particles. Relative density and microstructural evolution were evaluated to assess the role of processing and forming relationships with respect to constrained sintering conditions linked to the percolation threshold of large particles.
Description
Thesis 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
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Keywords
Ceramic engineering, Aluminum oxide, Sintering