Abstract
Silica aerogels are highly porous, translucent materials with the lowest thermal conductivities among solids. They are produced via sol-gel process and subsequent drying to replace the pore liquid by air without significant alterations of the silica skeleton. During ambient-pressure drying (APD), evaporation generates capillary forces that put the liquid under tension, which is balanced by the compression of the silica skeleton. This results in substantial drying shrinkage that is made partially reversible by surface modification of silica gels. Past a critical shrinkage, the gels re-expand through the spring-back effect (SBE), demonstrating a remarkable elasticity for a silica-based material. However, the description of the evaporation and deformation mechanisms remains mostly theoretical and few experiments were performed to investigate the APD process in-operando. This thesis aims to provide an empirical description of the evolution of silica gels properties during drying, thermal treatment and mechanical compression. The main motivation is to unveil the underlying mechanisms of the drying shrinkage and spring-back by addressing the kinetics of the gels' phase composition during APD. Another incentive is to quantify the capability of silica aerogels to recover large deformations related to drying shrinkage and uniaxial compression and evaluate the corresponding changes in the aerogels' nanostructure. Silica gels were prepared by a sol-gel process from tetraethyl orthosilicate and were casted as 16 mm tall cylinders, followed by modification with trimethylchlorosilane. Specimen were dried at ambient-pressure to produce monolithic aerogels. The average phase composition of the gels during APD was successfully computed by developing a novel quantitative imaging workflow based on in-operando X-ray micro-computed tomography (µCT). The emergence of the SBE was correlated to an equal volume fraction of silica skeleton, hexane and gas in the gels. To this regard, the re-expansion was arguably caused by a local relaxation of the drying stress, indicating a depletion of solvent in some pores. Simulations on unmodified gels supported the incidence of condensation reactions during drying. Further analysis of µCT data allowed to map the distribution of liquid and vapor in the gels during drying, which notably uncovered evidence of evaporation of the pore liquid by cavitation. This was supported by estimations using classical nucleation theory, and by separate in-operando wide-angle X-ray scattering experiments showing a significant volume of gas in the gels prior to the SBE. The onset of evaporation by meniscus recession was manifested by a drying front travelling across the specimen and was correlated to a heterogeneous SBE. Initially limited after drying, the spring-back was completed by thermal annealing resulting in a "two-step" SBE. The related nanostructural changes were evaluated by small-angle X-ray scattering, giving insights on the causes of partial re-expansion after drying. Annealed silica aerogels showed significant compressibility and a plastic behavior under uniaxial compression, and it was found that residual deformations were completely recovered by a second thermal annealing at the macro- and nano-scale. The deformation mechanisms upon compression were addressed by analyzing changes in fractal dimension and mean cluster size, which supported a phenomenology of compaction by sequential shrinkage of large pores. To this day, the size of monolithic silica aerogels produced by APD remains limited due to the susceptibility of silica gels to mechanical failure. This thesis contributes to a better understanding of the evaporation mechanisms, which is relevant to evaluate and predict the stress state in gels during drying. Moreover, the occurrence of cavitation brings new aspects on the APD process and highlights the potential of reducing the drying stress and thus the risk of cracks by tuning the gels' preparation and drying conditions to promote cavitation. Re-expansion of silica aerogels can be controlled by thermal activation thus qualifying aerogels as programmable materials, which broadens their potential utilization in several fields such as thermal insulation.
