The kinetics of the above reaction is impracticably slow at room
temperature, and often requires many days to complete. For this
reason, acid or base catalysts are added in increase the reaction
rate. The amount and type of catalyst used in synthesis played
key roles in the microstructural and physical properties of the silica
aerogel product as will be explained later in the paper. Acid catalysts
included hydrofluoric acid and hydrogen peroxide. Basic
catalysts used were ammonium hydroxide and potassium hydroxide.
During the gelation process, metal foam samples were dipped
in the viscous solution and were removed at moderate speed
(about 0.5 cm/s). It was important to perform the process an
appropriate speed to try to obtain a uniform coating. Leaving the
sample in the gel solution for long time would cause the resulting
sample to have solid gel particles in the pores.