Flash vacuum pyrolysis (FVP) of 2,7-dimethyl-2,3:7,8-diepoxy-5-silaspiro[4.4]nonane with cyclotrisiloxanes. Synthesis of spirocyclosiloxanes. Characterization of silica formed by FVP of 2,7-dimethyl-2,3:7,8-diepoxy-5-silaspiro[4.4]nonane