Effect of HNO₃ on crystalline phase evolution in lithium silicate powders prepared by sol–gel processes

摘要

An interesting observation is reported on the dramatic effect of HNO₃ on crystalline phase evolution in the 33.3 mol% Li₂O–SiO₂ glass–ceramic (stoichiometric composition of lithium disilicate Li₂Si₂O₅, LS₂) prepared by sol–gel processes from tetraethylorthosilicate (TEOS) and lithium ethoxide precursors. Nitric acid (65%), in molar ratio HNO₃/TEOS = 0.1, was added either to the precursor sol or to 95 °C dried gel. The product, which is amorphous at temperatures below 450 °C, transforms into crystalline lithium metasilicate (Li₂SiO₃, LS) at around 550 °C (starting temperature ∼450 °C), instead of forming crystalline LS₂. Phase separation in the glassy phase may be responsible for the formation of lithium metasilicate. XRD, 29Si MAS, and 7Li static NMR were used to follow the crystallization evolution and network structures of the materials heat-treated at various temperatures.