Aerogel Spring‐Back Correlates with Strain Recovery: Effect of Silica Concentration and Aging

Abstract

Silica aerogels display exceptional properties and great application potential, with a mature market in thermal insulation. Both supercritical drying (SCD) and ambient pressure drying (APD) routes are implemented industrially. Herein, how aging and silica content affect the mechanical properties, and how these in turn determine the shrinkage, spring back, and density during APD are systematically investigated. The APD densities display a U‐shaped dependence of density w.r.t. silica concentration. At low silica concentrations, the gels cannot withstand the capillary forces during APD and dense xerogels are obtained. At intermediate to high concentrations, APD shrinkage is strongly reduced and density increases with silica concentration. A series of cylinders are prepared by SCD and investigated by uniaxial compression and their strain recovery is determined systematically. The mechanical responses are plastic, viscoelastic, and brittle in nature for low, intermediate, and high silica concentrations, respectively. The strain recovery of the SCD cylinders correlates to the degree of spring back during APD. The viscoelastic response of SCD aerogels having 6 wt% corresponds to the silica concentration where a minimum in APD aerogel density is observed. The importance of gel mechanics for silica aerogel spring back during APD, in addition to surface modification and hydrophobization is highlighted.

Springback effect and structural features during the drying of silica aerogels tracked by in-situ synchrotron X-ray scattering

“…Insufficient hydrophobization leads to condensation reactions of silanol groups and irreversible shrinkage of the network 11 . The SBE is believed to occur because of the viscoelastic behavior of hydrophobized gels, which is influenced by the density and aging of the material 12 . The latter increases the stiffness of the network by densification, and when done insufficiently, might lead to a pore collapse 13 .…”

Superinsulating nanocellulose aerogels: Effect of density and nanofiber alignment

“…density, and at higher densities recover a large fraction of their original volume after decompression (Sivaraman et al, 2021), as do some other elastic cellulose composites (Françon et al, 2020;Wong et al, 2015). The diameter of the cylinders did not change significantly after compression and decompression, i.e., the apparent Poisson's ratio is close to zero (Table S1).…”

Research progress and applications of silica-based aerogels – a bibliometric analysis

“…89,90 Mutual repulsion produced by hydrophobic functional groups can maintain the structural integrity during the process of drying, different from the shrinkage of SAs with hydrophilic functional groups. [91][92][93] For example, Wei et al 33 modied silica aerogels with the hydrophobic group in low surface tension solvent. The as-prepared aerogels have a crack-free structure and volume shrinkage of less than 1%.…”