CRYSTALLIZATION IN THE SIO2-AL2O3 SYSTEM FROM AMORPHOUS POWDERS

The crystallization of amorphous chemically homogeneous powders in the SiO2.Al2O3 system has been studied by differential scanning calorimet ry and X-ray diffraction. Up to 1300 degrees C only one exotherm has b een observed. Only mullite crystallizes for compositions less than or equal to 69 mol% Al2O3 and spinel for those greater than or equal to 8 0%. The crystalizations into mullite and spinel are sharp and exotherm ic, with an enthalpy of 250-300 J/g. The chemical composition of the c rystallized mullite regularly increases from 68 to 76 mol% Al2O3 with increasing bulk composition from 60 to 75 mol% Al2O3.