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Stable and Metastable Equilibria in the System Si02-A1203 I. A. AKSAF* and J. A. PASK*

Summary: Stable and Metastable Equilibria in the System Si02-A1203
I. A. AKSAF* and J. A. PASK*
Inorganic Materials Research Division, Lawrence Berkeley Laboratory and Department of Materials Science and Engineering,
College of Engineering, University of California, Berkeley, California 94720
Concentrationprofilesof AI2O3in diffusioncouplesmade from
sapphire and fused silica were used to determine the stable
mediatecompoundmullite,3A203.2SiO,, meltsincongruently
at1828'5 10C;itsstablesolid-solutionregionrangesfrom70.5
to 74.0wt%A1,03below 1753'C and from 71.6to 74.0wt% at
1813C. The microstructures of diffusion zones and heat-
treated specimens also indicate the incongruency of mullite.
Additionalinformation is given for 3 metastable systems: Si0,-
A120, in the absence of mullite,SO,-"ordered"-mullite in the
absence of alumina, and Si0,-"disordered"-mullite in the
absence of alumina. Under metastable conditions, ordered
mullite melts congruently at =1880C and its solid-solution
rangeextendsupto =77wt%A1203.Thesolid-solutionrangeof
disorderedmulliteextendsto =83wt%A120, with an estimated
congruent melting temperature of == 1900C.The existenceof


Source: Aksay, Ilhan A. - Department of Chemical Engineering, Princeton University


Collections: Materials Science