Synthesis and characterization of aramid-honeycomb reinforced silica aerogel composite materials

Kurzfassung

In this thesis three different nano-porous silica aerogels filled in the pores of aramid honeycomb structures of different dimensions are investigated. The composites have a prospective use as insulation material. They were prepared by employing a two-step acid–base sol–gel process utilizing the silicon alkoxides methyltrimethoxysilane (MTMS) or tetraethylorthosilicate (TEOS) as precursors and supercritically dried in CO2. In order to drastically improve the mechanical properties of silica aerogel monoliths, different aerogel formulae were synthesized in aramid honeycomb structures of different height and cell size. Each recipe, called super-flexible SA1, semi-flexible SA2 and inflexible SA3, could be synthesized in the honeycomb pore space. For the first two composites, based on super-flexible SA1 and semi-flexible SA2, suitable interface bonding and thermal conductivities of 0.036-0.041 W/mK and 0.0395-0.0455 W/mK could be measured. Honeycomb dominated apparent Young’s moduli of up to 0.137 MPa in-plane / 21.13 MPa out-of-plane for SA1 such as 0.16 MPa in-plane / 20.16 MPA out-of-plane for SA2 could be measured. SA3 on the other hand showed such a bad adhesion to the honeycomb, that further testing was not possible. From both the positive and negative results criteria for successful aramid-honeycomb reinforced aerogel composite fabrication were identified: Primarily reducing shrinkage of the aerogels and honeycomb used such as ensuring good adhesion between the two materials. The results confirm the assumption that the aramid honeycombs have a minor effect on the thermal conductivities of the composites (only ~10-15% increase in comparison to the pure aerogels), though the mechanical properties are increased significantly (peak strength and the range of linear elastic behavior represented by the apparent Young’s modulus, which increased in magnitude by a factor of more than 1000 for SA1-based composites and of more than 100 for SA2-based composites for out-of-plane compression). Hence it is demonstrated that aerogel-honeycomb composite materials have potential to enable new practical applications for silica aerogel materials via diminishing their limiting fragility.