Characterization and sintering of gels in the system MgO-Al2O3SiO2

Cordierite aerogels, made by supercritical drying, and xerogels, formed by ambient pressure drying, have been prepared by combining two different reci pes. The chemical composition of the gels varied front stoichiometric cordi erite 2MgO.Al2O3.5SiO(2) to 0.5 MgO.1.4 Al2O3.5SiO(2) due to different proc edures for washing of the gels. The crystallization of nearly stoichiometri c cordierite gels was shown to be relatively complex involving the formatio n of several metastable phases such as mu-cordierite (Mg2Al4 Si5O18), spine l (Al6Si2O13) and sapphirine (Mg4Al8 Si2O20) before the equilibrium phase c omposition was obtained at around 1350 degrees C. On the other hand, during crystallization of gels with stoichiornetry close to 0.5MgO.1.4 Al2O3.5SiO (2) the equilibrium phases mullite, cristobalite and a-cordierite were the major phases formed during heat treatment. A lower densification rate was o bserved for aerogels compared to xerogels due to a larger pore size. A lowe r crystallization temperature in aerogels probably due to heterogeneous nuc leation reduced the densification. For gels with a composition near 0.5 MgO .1.4 Al2O3.5SiO(2) nucleation and densification occur simultaneously and la rge differences in the densification behavior was observed. (C) 1999 Elsevi er Science Limited. All rights reserved.