Wayne D. Kaplan: The mechanisms behind solute-drag and solute-acceleration during microstructural evolution of alumina
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IISS

Ruth Moshe, Rachel Marder, Leon Rudnik, Wayne D. Kaplanty

 

Department of Materials Science and Engineering, Technion – Israel Institute of Technology, Haifa, Israel

 

Alumina is one of the most used ceramic materials, and as such understanding its sintering and densification processes is important. It is known that the microstructural evolution is strongly affected by dopants, such as MgO which promotes sintering and limits grain growth. Key impurities, such as CaO and SiO2, are known to cause exaggerated grain growth. Over the years various models have been proposed to explain the influence of defects, but experimental limitations, such as knowledge of high temperature solubility limits, has prevented corroborated evidence of impurity adsorption affecting grain boundary mobility, compared to liquid-phase enhanced grain boundary mobility.

This presentation will focus on the influence of CaO and/or MgO on the evolving microstructure of alumina for a range of concentrations below the solubility limit. The amount of dopant in the alumina was fully quantified by conducting wavelength dispersive spectroscopy, and the change in grain boundary mobility as a function of measured dopant concentration was characterized using scanning electron microscopy via grain size measurements. Annealing experiments were conducted in a graphite furnace under flowing He, and the mobilities were compared to samples annealed in air.

Unlike segregating dopants which reduce grain boundary mobility by solute-drag (such as MgO), CaO increases the rate of grain growth, and a trend of increased mobility with increasing dopant level will be demonstrated. The increased mobility due to Ca segregation is believed to be due to an increase in vacancy concentration in the immediate vicinity of the grain boundaries. Co-doping with Mg and Ca leads to a higher Mg solubility limit, and thus more Mg at the grain boundaries in balance with the Mg in solution, and a reduced grain boundary mobility. Presumably grain boundary motion in alumina is via the motion of disconnections, which has been experimentally demonstrated for SrTiO3. How dopants, including carbon, interact with disconnections will be discussed.