Effects of the starting material and hydrothermal treatment conditions on the crystallization of ultrafine silica

摘要

The effects of the starting material and hydrothermal crystallization conditions on the phase com- position and particle size of the resulting nanocrystalline silica powders were studied. Hydrothermal treatment of silica gels, xerogels, and Aerosils in water and aqueoous NaF and NaOH solutions at temperatures in the range 250-500 deg. C and pressures of up to 100 MPa causes gradual dehydration and densification of the materials, accompanied by the formation of crystalline SiO_2 varieties, such as more or less ordered opaline silicas, cristo- balite, keatite, and quartz.