MSE Seminar: The Mechanisms Behind Solute-drag and Solute-acceleration During Microstructural Evolution of Alumina

Abstract: Alumina is one of the most used ceramic materials and a paradigm for sintering and microstructural evolution studies. 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, have 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.  

Bio: Wayne D. Kaplan is a professor in the Department of Materials Science and Engineering at the Technion, where he holds the Karl Stoll Chair in Advanced Materials. He completed his bachelor's degree in mechanical engineering and his master's degree and doctorate in materials at the Technion. He then spent a year as a Humboldt Fellow at the Max-Planck Institute in Stuttgart, Germany before joining the Technion faculty in 1995. Since 1995, Kaplan's research activities at the Technion have focused on the structure, chemistry and energy of interfaces between metals and ceramics, where his main focus has been on merging atomistic structural data (gathered using advanced electron microscopy) with thermodynamic data mostly gathered from solid-solid and solid-liquid wetting experiments. In addition to his fundamental research in materials engineering, Kaplan works on the development of electron microscopy techniques for characterization at the nano-scale level. Kaplan is the author of more than 145 scientific articles, including four publications in Science, as well as two textbooks: Joining Processes and Microstructural Characterization of Materials, both published by Wiley. In 2006, he received the Henry Taub Prize for Academic Excellence. He is a fellow of the American Ceramic Society, a member and past chair of the Israel Microscopy Society, and was an editor of the Journal of Materials Science (Springer). In October 2020, he will receive the Sosman Award from the American Ceramic Society. For the last five years, Kaplan has served as executive vice president for research of the Technion (his term ended Sept. 30, 2019). In this role, Kaplan coordinated all research funding and collaborative research efforts between the Technion and external bodies, including industry. This includes operations of the Technion Research and Development Foundation Ltd., a for-profit company owned by the Technion, where technology transfer from the Technion is implemented. For five years previous to this, Kaplan served as dean of the faculty of materials science and engineering.  

Host: Xiaoqing Pan