Hydrothermal SiO₂ Nanopowders: Obtaining Them and Their Characteristics

The technological mode of obtaining amorphous SiO₂ nanopowders based on hydrothermal solutions is proposed in this study. Polycondensation of orthosilicic acid as well as ultrafiltration membrane separation, and cryochemical vacuum sublimation were used. The characteristics of nanopowders were determined by tunneling electron microscopy, low-temperature nitrogen adsorption, X-ray diffraction, and small-angle X-ray scattering. The scheme allows to adjust density, particle diameters of nanopowders, specific surface area, as well as diameters, area and volume of the pore. Thus, the structure of nanopowders is regulated-the volume fraction of the packing of spherical particles in aggregates and agglomerates, the size of agglomerates, and the number of particles in agglomerates. The pour densities of the nanopowders depend on the SiO₂ content in sols, which were 0.02 to 0.3 g/cm³. Nanoparticles specific surface area was brought to 500 m²/g by low temperature polycondensation. Nanoparticle aggregates specific pore volume (0.2-0.3 g/cm³) weakly depend on powders density. The volume fraction of the packing of SiO₂ nanoparticles in aggregates was 0.6-0.7. Solid samples of compacted nanopowders had a compressive strength of up to 337 MPa. Possible applications of hydrothermal SiO₂ nanopowders are considered.