Solid state 29Si and 11B NMR studies of sol-gel derived borosilicates

Abstract

Gels in the system SiO 2B 2O 3 were prepared from metal alkoxides. The structural evolution of these gels as a function of thermal treatment was studied by high field 29Si and 11B NMR spectroscopy, using magic angle spinning. The results corroborate earlier work on this system using infrared spectroscopy to follow the formation of borosiloxane bonds during heat treatment. While incorporation of boron into the silica backbone is negligible in gels dried at room temperature only, thermal treatment drives the borosiloxane bond formation with removal of excess water. Initial boron incorporation involves the formation of terminal BOSi groups with tetrahedral boron environments. Further heating causes the condensation of boron into symmetric trigonal BOSi sites. Boron is fully incorporated by 450°C. In addition to the symmetric trigonal boron environment of borosiloxane bonds, a small signal is also observed from 11B nuclei in asymmetric trigonal environments in the gels heated to ⩾ 450°C, presumably due to the formation of borate chains (BOB) which were previously observed with infrared spectroscopy.