Abstract
Aerogels are light and porous solids whose properties, largely determined by their nanostructure, are useful in a wide range of applications, e.g., thermal insulation. In this work, as-deposited and thermally treated air-filled silica aerogel thin films synthesized using the sol-gel method were studied for their thermal properties using the 3-omega technique, at ambient conditions. The thermal conductivity and diffusivity were found to increase as the porosity of the aerogel decreased. Thermally treated films show a clear reduction in thermal conductivity compared with that of as-deposited films, likely due to an increase of porosity. The smallest thermal conductivity and diffusivity found for our aerogels were 0.019 W m−1 K−1 and 9.8 × 10-9 m2 s−1. A model was used to identify the components (solid, gaseous and radiative) of the total thermal conductivity of the aerogel.
Highlights
The root mean squared (RMS) roughness of the films was found to decrease as the tetramethyl orthosilicate (TMOS) v/v% increased, with a quasi-plateau for TMOS v/v% values higher than 5% (Fig. 1(c))
Aerogel thin films based on MTMS were synthesized and their functional properties investigated at standard temperature and pressure conditions
The structural optical and thermal properties of the aerogels were modified by inclusion of TMOS and tetraethyl orthosilicate (TEOS) as co-precursors during synthesis
Summary
Silica aerogels exhibit low density (≈100 kg m 3), porosity between 80% and 99.8%, pore sizes in the range of nanometers (≈15 nm), high specific surface area (≈1000 m2 g 1), very low thermal conductivity (≈0.01 W m 1 K 1), optical transparency in the visible spectrum (¿85%), low refractive index (1.05) and superhydrophobicity.[1,2] Their properties have attracted attention due to their wide range of applications, such as in thermal and acoustic insulation,[1] kinetic energy absorption,[3] fiber optics[4] and antireflection coatings.[5]. To reduce costs and synthesis complexity, aerogels have been fabricated in ambient conditions using, for instance, TEOS with a silylation process[7,8,9] or a dedicated precursor such as methyltrimethoxysilane (MTMS).[5,10,11] Both methods produce aerogels covered with methyl groups, providing hydrophobic behavior which prevents the possible collapse of the aerogel that might occur upon drying. Silylation is based on the introduction of silyl groups in the aerogel surface before drying, while MTMS precursors yield aerogels in ambient conditions without any modification, since their molecular structure[11] (Fig. S1 in the supplementary material), including methyl groups, provides hydrophobicity[5,10] and mechanical flexibility[12] to the aerogel. A thermal conductivity vs. density model was used to describe the contribution of the solid particles, the gas inside the pores and the radiative components of the aerogel to the total thermal conductivity
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