Effects of pre-aging and concentration of surface modifying agent on the microstructure and dielectric properties of SiO₂ xerogel film

Surface modification is a key process for the preparation of porous SiO₂ xerogel film by ambient pressure drying. The microstructure and surface terminal bonds which determine the properties of the film are closely related with the surface modification. Two kinds of pre-aged film were prepared to investigate the effect of wet-gel microstructure on the surface modification of film. The concentration of surface modifying agent (tetramethylchlorosilane; TMCS) in solvent (n-hexane) was also varied. As concentration of TMCS increased, surface morphology was obtained from a two-dimensional structure to a three-dimensional network structure with pre-aged film for 1 h at 70 °C. This is due to the increase in methyl groups as surface terminal bonds and the suppression of condensation reaction. Variation in film thickness, porosity, dielectric constants, and I-E characteristic according to the TMCS concentration in n-hexane modifying solution was explained in the view-point of the nature of wet-gel structure. However, because of the additional increase in the strength of network and the effective exchange of surface terminal -OH groups with -OSi(CH₃)₃ groups even with low concentration of TMCS, there were no serious changes in the properties of SiO₂ xerogel film pre-aged for 12 h at 70 °C.