Mullite formation from mixtures of alumina and silica sols: mechanism and pH effect

Abstract

Este trabalho relata o efeito do pH no processo de formação de mulita a partir de misturas de sóis de alumina e sílica. O pH das misturas determina as cargas das superfícies das partículas e afeta suas interações e distribuições. A formação de mulita a partir de precursores amorfos na razão molar de 1:3, preparados pela mistura dos sóis, não foi afetada pelo pH. Neste caso, a concentração excessiva de sílica determinou sua distribuição ao redor da alumina, o que levou à formação de mulita tetragonal, conforme o mecanismo de Sundaresan e Aksay. Entretanto, para a formação de mulita a partir de precursores com Al:Si = 3:1, o pH desempenhou um papel muito importante nas interações entre partículas de alumina e de sílica, bem como nas espécies predominantes de alumínio. Em pH 1, íons Al 3+ octaedricamente coordenados predominaram no sol de alumina enquanto que íons Al 3+ tetraedricamente coordenados predominaram no sol a pH ~6. As interações entre as partículas de sílica e de alumina e suas distribuições nesses precursores determinaram a temperatura mínima de formação de mulita ortorrômbica.This work reports the effect of pH on the process of mullite formation from mixtures of alumina and silica sols. The pH of the mixtures determines the charges of particle surfaces and affects their interactions and distributions. Mullite formation from amorphous precursors with an Al:Si molar ratio of 1:3, prepared from the sols mixture, was not affected by pH. In this case, the higher concentration of silica determined its distribution around alumina, which led to tetragonal mullite formation, according to the Sundaresan and Aksay mechanism. However, for mullite formation from precursors with Al:Si = 3:1, the pH played an important role on the interactions between alumina and silica particles, as well as on the predominant aluminum species. At pH 1, octahedrically coordinated Al 3+ ions predominated in the alumina sol while tetrahedrically coordinated Al 3+ ions predominated in the sol at pH ~6. The interactions between silica and alumina particles and their distributions in these precursors determined the minimum temperature required for orthorhombic mullite formation.Keywords: pH effect, mullite formation, alumina sol, silica sol IntroductionMullite (3Al 2 O 3 . 2SiO 2 ) is a very important ceramic material, with high temperature applications, due to its excellent mechanical properties, such as high strength, low thermal expansion coefficient, low thermal conductivity, high thermal shock resistance and creep resistance. [1][2][3][4][5][6] Mullite is also an interesting model for the development of new procedures of ceramic material synthesis, due to the simplicity of its composition.7 This ceramic material is composed solely of silicon, aluminum and oxygen, and is the only crystalline phase of the Al 2 O 3 -SiO 2 system stable at atmospheric pressure. 1,[8][9][10] Studies of mullite formation from alumina and silica sols can be useful to evaluate the interactions of alumina and silica particles in an aqueous medium, as a funct...

Development of hydrophobic clay–alumina based capillary membrane for desalination of brine by membrane distillation

“…Stoichiometrically the excess silica present in clay does not react with the alumina mixed externally [40]. Instead, it transforms into glassy phase and stays in between the voids, which eventually reduces the pore diameter and porosity [34].…”

The kinetic of mullite crystallization: Effect of water content

“…As the sample M(6) crystallized the metastable r-mullite phase with the alumina molar ratio equal to 77 % (x = 0.61) one can conclude that aluminum and silicon chemical species is mixed in a intermediate degree between the monophasic and diphasic gel. Therefore, water presence in the synthesis medium had a negative effect, since the homogeneity levels of the mullite precursors were decreased in the sample in which water was added [20,21,27,28].…”

Preparation and characterization of diphasic sol-gel derived unsupported mullite membranes

“…It is assumed that stable mullites exist in the range of compositions between 3Al 2 O 3 Á2SiO 2 (3:2) and 2Al 2 O 3 ÁSiO 2 (2:1) [3]. Sol-gel processing has been used during the last decade for mullite preparation with chemically homogeneous microstructures [4][5][6][7][8][9]. The observation of mullitization at relatively low temperatures is commonly reported to be due to molecularnano scale mixing of the precursor phases achieved through sol-gel processing.…”