Iron/silicon oxy-hydroxide co-precipitate trace-incorporated [-Fe-O]m-[Si-O-]n colloids: A facile strategy for silica xerogel strengthening and toughening
Significant volume shrinkage during the ambient pressure drying is a common defect in the synthesis of silica xerogels, leading to densification, collapse of pore structure, and a decline in mechanical properties. Conventional methods struggle to produce silica xerogels with both low bulk density and high mechanical properties without additional post-treatment. Herein, we introduce a novel approach by ultilizing trace amounts of iron/silicon oxy-hydroxide co-precipitate trace-incorporated [-Fe-O] m -[Si-O-] n colloids as the foundational building blocks for polymethylsilsesquioxane (PMSQ) xerogels. These colloids were synthesized through the condensation of hydrated iron cations with methyltriethoxysilane (MTES) hydrolysates, which also influence the aggregation and sedimentation of MTES sols. The impact of the hydrolysis time of MTES on the chemical compositions, phase structures, microstructures of the iron/silicon oxy-hydroxide co-precipitates, and mechanical properties of PMSQ xerogels were investigated. The optimal hydrolysis time of MTES in the cetyltrimethylammonium bromide/ferric chloride (CTAB/FeCl 3 ) aqueous solution was 5.0 h, endowing MTES sols containing a trace of ∼0.046 mg/mL iron/silicon oxy-hydroxide co-precipitates. This process yielded PMSQ xerogels with a high compression strength of 0.316 MPa and maximum compression strain of 79.3 %. Deviating from this optimal hydrolysis time by 1.0 h might significantly reduce the compression strength of the as-synthesized PMSQ xerogels by 18.0 %-58.5 % and decrease the maximum compression strain by 6.6 %-15.4 %. Notably, the tiny incorporation of the iron phase (∼0.1 mol%) into the Si-O-Si backbone endowed the PMSQ xerogels with a minimal volume shrinkage ratio of 5.5 %, an ultra-low bulk density of 69.8 mg/cm 3 , and the highest specific strength of 4.527 kPa·m 3 /kg recorded among all aerogels and xerogels in the literature. The strategy of forming iron/silicon oxy-hydroxide co-precipitates to strengthen and toughen silica xerogels by modulating the hydrolysis/condensation kinetics of hydrated iron cations and silica sources holds promise for maximizing mechanical properties and minimizing bulk density under industrially relevant conditions.
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