|Ph.D Student||Moshe Ruth|
|Subject||Influence of Dopants and Particles on the Kinetics of|
Microstructural Evolution of Alumina
|Department||Department of Materials Science and Engineering||Supervisor||Professor Wayne D. Kaplan|
|Full Thesis text|
The solute drag theory predicts that the presence of solutes in a material will hinder grain boundary movement. However, there are several systems that when doping with specific solutes, the grain boundary mobility increases. In this study, the influence of CaO on the evolving microstructure of alumina and Ni-alumina composites has been studied in a range of concentrations below the solubility limit. The amount of Ca in the alumina was determined by conducting fully standardized wavelength dispersive spectroscopy, and the change in grain boundary mobility as a function of the amount of dopant was characterized using scanning electron microscopy. Unlike segregating dopants which reduce grain boundary mobility by solute-drag, CaO increases the rate of grain growth, and a trend of increased mobility with increasing dopant level was shown. The increased mobility with Ca segregation is believed to be due to an increase in vacancy concentration in the vicinity of the grain boundaries, thus facilitating faster grain boundary motion.
In addition, the solubility limits of Ca and Mg in co-doped alumina and of Si in alumina were measured using wavelength dispersive spectroscopy on a scanning electron microscope. For the determination of the solubility limit of Si in alumina at 1600°C, samples were doped with Si such that the equilibrated material would contain two phases: 3Al2O3•2SiO2 (mullite) and alumina saturated with Si. Thus, the amount of Si measured in the alumina grains represents the solubility limit. Measurements were conducted on water-quenched and furnace-cooled samples. For the quenched samples the Si solubility limit in Al2O3 was found to be 188±7 ppm at 1600°C.
In a similar manner the solubility limits of Ca and Mg co-doped in alumina at 1600°C were determined by equilibrating alumina saturated with Ca and Mg. This resulted in the formation of MgAl2O4 (Mg spinel), CaO•6Al2O3 (CA6), Ca2Mg2Al28O46 (CAM-II) and alumina grains saturated with Mg and Ca. Under these conditions, the amount of Ca and Mg in the alumina grains represents the solubility limits. In the co-doped state, the solubility limit of Ca in alumina was 32±13 ppm, and the solubility limit of Mg in alumina was 210±43 ppm. The presence of Ca results in an increase of the solubility limit of Mg in alumina from 132 ppm to 210 ppm, suggesting that the increased Mg in solution results in more Mg excess at the alumina grain boundaries, thus contributing to a decreased grain boundary mobility by solute-drag.