Microstructural properties and heat transfer characteristics of in-situ modified silica aerogels prepared with different organosilanes

Abstract

• In-situ modified silica aerogels were synthesized in ambient pressure by a facile sol-gel method. • Various organosilanes MTMS, MTES, VTMS, MEMO and GLYMO were utilized as silica precursors. • Heat transfer mechanisms were investigated both theoretically and experimentally. • SA-MTMS had the lowest density (92 kg⁄m 3 ) and high thermal stability (up to 700 °C). • All samples exhibited similar insulation performances (λ in between 42 and 47 mW/mK). • The density was the most decisive parameter for different heat transfer modes in aerogels. • Coupling thermal conductivity term is the key contributor to the total thermal conductivity. Conducting an in-situ modification during the sol-gel reactions, without any post gelation-modification, offers a cost-effective and time-saving strategy in the production of silica aerogels. Therefore, in this study, in-situ modified silica aerogels were synthesized as potential thermal insulation materials via a two-step sol-gel process under ambient pressure. Different organosilanes having methyl (SA-MTMS), ethyl (SA-MTES), vinyl (SA-VTMS), epoxide (SA-GLYMO), and methacrylate functional (SA-MEMO) groups were used as silica sources. According to the selected precursor, the change in the heat transfer properties were investigated both by thermal conductivity measurements and by theoretical models. SA-MTMS possessed the lowest density (92 kg / m 3 ) and high thermal stability (up to 700 °C). Despite the existence of bulk organic groups in its structure, the SA-GLYMO had the highest specific surface area (531 m 2 / g ) . Regardless of their morphology, all samples have exhibited similar insulation performances (λ=42–47 mW/mK). However, the prominent heat transfer mechanisms were directly affected by the bulk density and the underlying microstructure. For low-density samples, gas-contributed thermal conductivities were the key contributor to the total thermal conductivity whereas solid thermal conduction was the dominant mechanism in high-density samples. The coupling thermal conductivity term was also crucial as the total conductivity would have been significantly underestimated without it.