Thermally activated sintering-coarsening-coalescence-polymerization of amorphous silica nanoparticles

An onset sintering-coarsening-coalescence-polymerization (SCCP) event of amorphous SiO2 nanoparticles (ca. 40-100 nm in size) by isothermal firing in the 1150-1300 °C range in air was characterized by an N2 adsorption-desorption hysteresis isotherm coupled with X-ray diffraction and vibrational spectroscopy. The apparent activation energy of such a rapid SCCP process was estimated as 177±32 kJ/mol, based on 30% reduction of a specific surface area with an accompanied change of medium range orders, i.e. forming Si2O5 while retaining the Si-2ndO yet losing the Si-2ndSi without appreciable crystallization. The minimum temperature of the SCCP process, as of concern to industrial silica applications and sedimentary/metamorphosed sandstone formation, is 1120 °C based on the extrapolation of steady specific surface area reduction rates to null.