STRUCTURAL CHARACTERIZATION OF CAO-B2O3-AL2O3-SIO2 XEROGELS AND GLASSES

Calcium boroaluminosilicate xerogels and glasses with an E-glass composition were synthesized using metal alkoxides and metal salt as precursors. Structural evolution during the gel-to-glass transition was monitored by FTIR and NMR spectroscopies. Kramers-Kronig analysis was performed on the FTIR specular reflectance data to yield the epsilon-2 and Im(-1/epsilon) spectra for the xerogels and the glass. A xerogel treated at 600-degrees-C exhibits distinct structural characteristics relative to the glass. A significantly higher concentration of B(IV) is present in the xerogel due to the large amount of surface boranol groups. Another unique feature of this xerogel is revealed by a red-shift in the high-frequency LO mode of the Si-O stretching vibration relative to the dense glass. This shift is attributed to a surface mode in the vibrational spectrum which is a consequence of the high surface area and the particulate structure of the xerogel. A xerogel treated at 710-degrees-C has FTIR and NMR spectra that are similar to the dense glass. Thus, the bulk network structure of the glass is present in the 710-degrees-C xerogel. A higher surface area (approximately 40 m2/g) and concentration of isolated surface Si-OH and B-OH groups distinguish it from the dense glass. An important conclusion of this study is that the only fundamental difference between the 710-degrees-C xerogel and the glass is the surface of the xerogel. In this sense, monolithic xerogels can provide convenient substrates for studying the surface chemistry of multicomponent glasses.