Kurt Richard Mikeska

Grain boundary chemistry of alumina by high-resolution imaging SIMS

Abstract The unique capabilities of the high-resolution scanning ion microprobe developed at the University of Chicago (UC-SIM) are described and its utility is demonstrated in a study of grain boundary chemistry of alumina ceramics. When polycrystalline alumina is doped singly with either MgO or SiO 2 , strong segregation of the individual ions to grain boundaries is observed: (1) for Mg segregation C gb / C grain ∼400; (2) for Si segregation C gb / C grain ∼300. However, on codoping with both MgO and SiO 2 , grain boundary segregation is significantly diminished by a factor of five or more over single doping as both cations are redistributed into the bulk alumina lattice. A defect compensation mechanism is proposed to explain this mutual solid solubility of Mg and Si in alumina. One important consequence of this chemical redistribution is a change in abnormal grain growth morphology from facetted grains in SiO 2 singly doped alumina, to non-facetted grains with curved boundaries in MgO and SiO 2 codoped alumina. As the Mg/Si dopant ratio exceeds the equimolar concentration, abnormal grain growth development ceases. These findings provide a physical mechanism to explain the role of MgO as a sintering aid to control microstructure evolution in alumina. Another significant consequence of SiO 2 redistribution on MgO doping is an observed improvement in the corrosion resistance of alumina to aqueous HF. Siliceous grain boundary films readily corrode and compromise the intrinsically good corrosion resistance of bulk alumina. MgO doping in amounts greater than the SiO 2 concentration prevents the formation of these corrodable silica-based phases leading to the development of aluminas for use in aqueous HF-containing ambients.

Luminescent properties of nanostructured Dy3+- and Tm3+-doped lanthanum chloride prepared by reactive atmosphere processing of sol-gel derived lanthanum hydroxide

Dy3+- and Tm3+-doped lanthanum chloride was prepared via reactive atmosphere processing (RAP) of sol-gel derived hydroxide powders. The low-phonon energy chloride host facilitated the 1.3-μm emission from Dy3+ and the 1.2- and 1.4-μm emissions from Tm3+. The emission intensities of the Dy3+ and Tm3+ ions increased logarithmically with increasing RAP temperature. The dependence of emission intensities and lifetimes on the rare-earth (RE) concentration was also investigated. The emission intensity was found to increase up to 1 mole % RE for both ions. Luminescence quenching was observed for concentrations exceeding 1 mole % for Tm3+ and 0.1 mole % for Dy3+.

Role of magnesia and silica in alumina microstructure evolution

The effects of MgO and SiO2 additive distributions on alumina grain morphology have been characterized using high-resolution imaging secondary ion mass spectrometry (HRI-SIMS). In alumina samples singly-doped with MgO, the concentration of Mg segregated to grain boundaries is independent of grain boundary length for a majority of grain boundaries studied. Mg segregant therefore redistributes from grain boundaries to microstructural sinks, such as pores and/or second phases, during grain coarsening. In samples singly-doped with SiO2, abnormal grain growth develops and the concentration of Si at grain boundaries is also independent of grain boundary length. Redistribution of segregants is again necessary in this case to maintain constant grain boundary composition. Codoping with Mg/Si > 1 suppresses abnormal grain growth as a result of increased mutual solid solubility of both ions and an associated decrease in grain boundary segregation. Grain growth kinetics for doped aluminas are reconsidered in light of these observations.