Synthesis and properties of mullite and cordierite aerogels

A Processable Mullite Precursor Prepared by Reacting Silica and Aluminum Hydroxide with Triethanolamine in Ethylene Glycol: Structural Evolution on Pyrolysis

“…21 The sharp exotherm that is observed at 990°C is typical of crystallization to mullite and/or a ␥−Al 2 O 3 -type spinel phase. 1,4,5,9,10,14,15,17,[23][24][25][26][27][28] No other exotherm is observed at higher temperatures (>1200°C), as expected for single-phase precursors that are reported to exhibit only one exotherm at temperatures of 980°-990°C, because of spinel or mullite (or tetragonal mullite) phase formation. 9,23-25 A more detailed discussion of the meaning of this exotherm is presented below.…”

“…In contrast, in chemical processing, the intimacy of mixing dictates the mullitization temperature and phase purity in the final product. [23][24][25][26][27][28] If the homogeneity is at the atomic level (''single phase''), as in chemical vapor deposition (CVD), spray pyrolysis, and polymer precursor methods, direct mullitization can be observed, as evidenced by an exothermic reaction at ∼980°C. In contrast, if the homogeneity is in the nanometer-to-micrometer range (''colloidal'' or ''diphasic''), mullitization occurs through a transient Al 2 O 3 phase and is often evidenced by a second exotherm at a temperature >1200°C.…”

“…24,28 For example, powders or gels that are formed by the fast hydrolysis of various alkoxides or salts are generally diphasic. 4,10,23,24 Despite the lower processing temperatures (compared to traditional approaches), segregation during microstructural development is often observed.…”

Solgel‐Processed Mullite Coating—A Review

“…ANN and TEOS have been found to be the most popular precursors after Rustum Roy used these two for mullite synthesis first time in 1958. 46,47 Depending on the homogeneity, mullite precursor gel can be classified into two categories, namely monophasic and diphasic gel. [32][33][34]37,38,42 Other sources of starting materials include inorganic salts of alumi-num and silicon or organic species as their alkoxide.…”

“…were also used as solvent. 46,47 Depending on the homogeneity, mullite precursor gel can be classified into two categories, namely monophasic and diphasic gel. Monophasic gel is formed during true precipitation, that is, continuous condensation growth of species with aluminosiloxane (-Al-O-Si-) bond.…”