Low temperature hydrothermal sintering process for the quasi-complete densification of nanometric α-quartz

We report the first successful densification of polycrystalline alpha-quartz. The innovative hydrothermal sintering process was used as a low temperature route on sol-gel silica nanoparticles (350 MPa, 300 degrees C, 90 min) in presence of water or aqueous sodium hydroxide solution (from 0.2 M up to 5 M). For the lowest NaOH concentrations, amorphous silica densification is observed, reaching a compactness of 86%. The basic physical-chemical mechanism proceeds through dissolution-precipitation. When NaOH concentration increases, the kinetics of dissolution are faster, nanoparticles totally dissolve and alpha-quartz is stabilized during precipitation. 98% dense alpha-quartz is obtained, the crystallite size increases with NaOH concentration. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.