Effect of heat treatment on hydrophobic silica aerogel

•Secondary particles within silica aerogel gets coalesced when heat treated.•The detailed pyrolysis of hydrophobic silica aerogel can be divided into 3 steps.•Under 350℃ heat treating, the aerogel gets to its optimum for thermal insulation.•C decreases but still exists in the silica aerogel after heat treated. Hydrophobic silica aerogels were heat treated under various conditions. Physical and chemical analyses were conducted to study the effect of the heat treatments on the silica aerogels. The O/Si and C/Si values in the hydrophobic silica aerogels increased and decreased, respectively, with the increase in the heating temperature. C–O, −OH, and CO were detected during pyrolysis. Pyrolysis of the silica aerogels in air could be divided into 3 steps: the hydroxylation of methyl groups, the splitting of the alcoholic hydroxyl, and the oxidisation of CO. When the heat treatment temperature was lower than 350 °C, the properties of the silica aerogels showed little change. With further increase in the heat treatment temperature, the variation in the relevant parameters became more prominent. The secondary particles coalesced with one another, and the mesopores were destroyed. Consequently, the thermal conductivity and bulk density rose greatly. The carbon within the silica aerogels was released after heat treatment. As a result, the heat released in the thermal gravimetry and oxygen bomb analyses dropped remarkably with the increase in the heat treatment temperature.