Phase Separation in Alumina-Chromia

摘要

The alumina-chromia system shows complete mutual solubility and is represented by an isomorphous phase diagram. However, the alumina-chromia system exhibits an asymmetric miscibility gap under 1300℃. Using existing data from the literature, the alumina-chromia system was assessed using thermodynamic modeling by Kim and Sanders. Regular and subregular solution models for the liquid and solid phases were used to define the phase boundaries for the miscibility gap in this system. Using this thermodynamic representation of the miscibility gap to select temperatures of interest, 75 mole percent Al_2O_3 samples were synthesized via combustion of powders, followed by pressing into pellets and heat-treated for various times and temperatures. Both X-ray and TEM analysis showed evidence of spinodal decomposition after heat-treatment. X-ray analysis showed that decreasing the heat-treatment temperature increases the compositional difference between the phases present. The experimentally observed microstructures exhibit lamella-like structures that vary in spacing from 8nm to 3nm as the heat-treatment temperature varies from 400℃ to 800℃.