A new route for preparation of sodium-silicate-based hydrophobic silica aerogels via ambient-pressure drying

Abstract

An in-depth investigation into the synthesis of hydrophobic silica aerogels prepared by the surface derivatization of wet gels followed by subsequent drying at ambient pressure is reported. The following sol-gel parameters were examined for their effect on the physical properties of the derived aerogels: number of gel washings with water, percentage of hexane or methanol in silylating mixture, molar ratio of tartaric acid: Na 2 SiO 3 , gel aging period, weight% of silica, trimethylchlorosilane (TMCS) percentage, and silylation period. These parameters were varied from 1 to 4, 0 to 100%, 0.27 to 1.2, 0 to 4 h, 1.5 to 8 wt.%, 20 to 40% and 6 to 24 h, respectively. The properties of hydrophobic silica aerogels synthesized by this new route were investigated in terms of bulk density, percentage volume shrinkage, percentage porosity, thermal conductivity and contact angle with water, and by Fourier transform infrared spectroscopy (FTIR). The as-prepared hydrophobic silica aerogels exhibited high temperature stability (up to approximately 435• C) as measured by thermogravimetric/differential thermal analysis (TGA-DTA). The optimal sol-gel parameters were found to be a molar ratio of Na 2 SiO 3 : H 2 O : tartaric acid : TMCS of 1 : 146.67 : 0.86 : 9.46, an aging period of 3 h, four washings with water in 24 h and the use of a 50% hexane-or methanol-based silylating mixture. Aerogels prepared with these optimal parameters were found to exhibit 50% optical transparency in the visible range, 84 kg m −3 density, 0.090 W mK −1 thermal conductivity, 95% porosity and a contact angle of 146• with water.

Sol-Gel-Derived Porous Silica: Economic Synthesis and Characterization

“…The efficiency of deionised water washings to free the gel from sodium sulphate was monitored by atomic absorption spectroscopy (AAS) analysis of the silicas obtained after drying. The result indicated that Na + decreased from 36.5% (on the basis of stoichiometry) [25] to 1.6% in the SS-based silica and to 2.1% in the CS based silica.…”

“…The water glass obtained from the basic reaction is similarly hydrolysed to silicic acid which undergoes condensation to disilicic acid and further reaction to a polycondensed hydrogel [24] as shown in Scheme 1. Thereafter, the gel obtained from both precursor compounds were subjected to multiple washings [25] with deionised water to free the gel of sodium sulphate formed during hydrolysis that is trapped in the pores of the gel network. During aging, the 3-D framework of the silicate glass continues to grow and becomes more rigid by contracting and expelling liquid water present inside the pores [26].…”

<strong>Failure analysis of hydrophilic and hydrophobic aerogels under liquid nitrogen thermal shock</strong>

“…after thermal shock. Pore size of hydrophilic aerogel become larger and the number of pores decreased in the same volume [14]. It suggests that part of pore was collapsed, which increase the surrounding space.…”

Synthesis of porous zinc-based/zinc oxide composites via sol–gel and ambient pressure drying routes

“…Therefore, the APD method is a more economical process compared to the SCD method and is able to operate continuously as opposed to producing batches [ 17 ]. Hexane is a well-known low-surface-tension solvent and it has been often used in the APD synthesis of silica aerogels [ 18 – 20 , 23 ]. In this work, we also choose hexane as drying solvent because solvent combination of methanol and hexane has been previously reported in solvent exchange for aerogel preparation [ 23 ], and study different phases of porous Zn-based/ZnO composites by APD method of synthesis.…”