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

Deeptanshu Sivaraman,Subramaniam Iswar,Shanyu Zhao,Marco Lattuada,Wim J Malfait

doi:10.1002/adem.202100376

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.

Highlights

Silica aerogels display exceptional properties and great application potential, with skeleton backbone, lend aerogels their a mature market in thermal insulation
We investigate the interplay between mechanical properties of supercritical drying (SCD) aerogels and the spring back effect in ambient pressure drying (APD) materials, with a focus on compression rather than bending experiments,[40] because axial forces are predominant during the drying process.[39,41]
The SCD aerogel densities only depend on silica concentrations and not on aging conditions, and closely track the expected theoretical values based on the solid content in the sol

Summary

Introduction

Silica aerogels display exceptional properties and great application potential, with skeleton backbone, lend aerogels their a mature market in thermal insulation. Both supercritical drying (SCD) and ambient pressure drying (APD) routes are implemented industrially. The exceptional properties: densities ranging from 0.004 to 0.500 g cmÀ3, high specific surface areas (600–1200 m2 gÀ1), and ultralow thermal conductivities (down to 12–15 mW mÀ1 KÀ1).[1] These properties open up a wide array of use-cases in fields such as thermal insulation,[2,3,4] catalysis,[5] and drug delivery.[6] In the current market, silica aerogel is almost exclusively used for thermal insulation (industrial, pipelines, and buildings).[3] Other applications such as in electronic devices,[7] Knudsen pumps,[8] solar panels,[9] waste adsorbents and sensors,[10,11,12] whereas other, nonsilicastrain recovery of the SCD cylinders correlates to the degree of spring back during APD. Malfait of silica aerogel is related to the high cost of raw materials

Results

Discussion

Conclusion