SiO2-Al2O3 phase diagram and mullite non-stoichiometry of sol-gel prepared monoliths: influence on mechanical properties

The origin of mullite formation has been subject to extensive but controversial investigations [1-10]. Many studies have been concerned with the kaolinite-mullite reaction series and more recently with the crystal- lization of materials prepared by sol-gel route [6-11]. The central controversy has always been focussed on the exothermic reaction around 1000 ° C and its relation with mullite formation. A second controversy occurs about the non-stoichiometry range of mullite [12] and its melting temperature. The different opinions can be summarized as below: (i) mullite is formed directly from amorphous state, (ii) spinel phase is formed before mullite formation and (iii) mullite and spinel phases are simultaneously formed. Furthermore the composition of spinel phase remains discussed (pure alumina or mixed SiO2-A120 + composition). The intricacy of mullite formation is further complicated by the possibility to have nonstoichiometric mullite A12A12+2~Si2_2~OI0_~ with x ranging from 0 to 0.5 according to Cameron [3]. The phase relation sequence is very dependent on authors i.e. from our opinion from their preparation according to resulting inhomo- geneity length and from the methods used to charac- terize the samples. From micronic scale heterogeneous mixing of usual oxide powders to the molecular scale mixing achieved in glass or gel i.e. in method using a liquid precursor, the size of the local inhomo- geneity decreases and thus the temperature needed