The effect of synthesis conditions and process parameters on aerogel properties

Abstract

Aerogels are remarkable nanoporous materials with unique properties such as low density, high porosity, high specific surface area, and interconnected pore networks. In addition, their ability to be synthesized from various precursors such as inorganics, organics, or hybrid, and the tunability of their properties make them very attractive for many applications such as adsorption, thermal insulation, catalysts, tissue engineering, and drug delivery. The physical and chemical properties and pore structure of aerogels are crucial in determining their application areas. Moreover, it is possible to tailor the aerogel properties to meet the specific requirements of each application. This review presents a comprehensive review of synthesis conditions and process parameters in tailoring aerogel properties. The effective parameters from the dissolution of the precursor step to the supercritical drying step, including the carbonization process for carbon aerogels, are investigated from the studies reported in the literature.

Keywords: aerogel; aging; carbon; polysaccharide; silica; sol-gel; solvent exchange; supercritical.

FIGURE 1

Hydrolysis and polycondensation sol-gel processing.

FIGURE 2

General synthesis of biopolymer aerogels.

FIGURE 3

Representation of phenomena occurring during aging (A) Formed gel and the effect of drying along with the illustration of consequence of wet heat aging (which results in little shrinkage upon drying). Adapted with permission from (Hench and West, 1990) Copyright 1990 American Chemical Society. (B) Particle growth mechanism during aging according to Oswald ripening (left) and neck growth (right). Adapted with permission from (Li et al., 2023a) Copyright 2023 Taylor and Francis Group.

FIGURE 4

Volumetric shrinkage of gels in different solvents after one step solvent exchange. Reprinted from (Subrahmanyam et al., 2015) with permission, Copyright 2015 MDPI.

FIGURE 5

Change in the surface area with respect to solvent concentration achieved before supercritical CO₂ drying for alginate aerogels. Reprinted from (Subrahmanyam et al., 2015) with permission, Copyright 2015 MDPI.

FIGURE 6

The schematic of a typical supercritical extraction apparatus. Reprinted from (Şahin et al., 2017) with permission, Copyright 2017 MDPI.

FIGURE 7

Variation of transparency and occurrence of cracks in monolith structure of silica with respect to the stabilization time (drying time). Reprinted with permission from (Venkateswara Rao et al., 1998) Copyright 1998 Taylor and Francis Group.

FIGURE 8

(A) percent removal of ethanol from the wet-gel for different temperatures and pressure for a constant density of inlet scCO₂, (B) percent removal of ethanol from the wet-gel for different alcogel particle sizes. Reprinted from (Şahin et al. (2019a)), Copyright 2019, with permission from Elsevier.

FIGURE 9

Two-step drying mechanisms of gels by supercritical drying technique combined convective and diffusion mechanisms. Reprinted from (García-González et al., 2011), Copyright 2012, with permission from Elsevier.

FIGURE 10

Distribution of Starbon functional groups as a function of carbonization temperature (color scale indicates relative amounts: black represents the highest amount). Reprinted with permission from (White et al., 2014), Copyright 2014, John Wiley and Sons.