Aerogel–aerogel composites are prepared by embedding highly insulating granular silica aerogel (1–2 mm, 5–58 vol.%) into ambient pressure dried resorcinol-formaldehyde (RF) aerogel. The organic RF aerogel matrix is synthesized via a sol–gel reaction of resorcinol (1,3-dihydroxybenzene) and formaldehyde in deionized water with Na2CO3 as the catalyst. Plates around 90 × 195 mm² with a thickness of 19–25 mm are obtained and can be processed for application by sawing and grinding. A theoretical model for the volume-based surface area was used to show that the matrix aerogel around the silica aerogel grains is affected by their presence. Composites have a density 0.19 ≤ ρ ≤ 0.27 g/cm³ and a thermal conductivity at room temperature between 0.026 and 0.053 W/mK. Composites can be used as thermal insulation material in a normal temperature range < 200 °C due to the decomposition of the organic phase above 200 °C.
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