Solid-state 1H and29Si NMR studies of silicate and borosilicate gel to glass conversion

Abstract

Silicate and borosilicate gels were prepared by the sol-gel process and thermally treated in the 150-850°C temperature range. Solid-state 1H MAS and 29Si CP/MAS NMR spectroscopy were used to investigate the effects of heat treatments on the silicate gel to glass conversion process. The 1H NMR Isotropic chemical shifts and the relative intensities of hydrogen bonded and isolated silanol groups have been used to access the information concerning the dehydration process on the silicate gel surface. The 29Si NMR isotropic chemical shifts affected by the local silicon environment have been used to determine the degree of crosslinking, i.e. the number of siloxane bonds. These NMR results suggest that the silicate gel to glass conversion process is occurred by two stages which are dependent on the temperature; (1) the formation of particles up to 450°C and (2) the formation of large particles by aggregation of each separated single particle above 450°C. In addition, the effects of B atom on the formation of borosiloxane bonds in borosilicates have been discussed.