PNNL

Abstract

Mesoporous silica thin films about 1 micron thick were prepared by spin casting using several organic templates that provide a range of pore structures from disordered (sponge-like) to more ordered (honeycomb-like) 2D hexagonal arrays. Nanoindentation measurements indicate that the elastic modulus (E), and thus the density, of the pore wall material are substantially lower than for fused silica. The corresponding lower dielectric constant for pore walls was used to calculate film porosities (P) with recent correlations from the literature. Curve fits to the standard modulus vs. porosity correlation, E/Eporewall=(1-P)n, for the films gave lower n=2.2 for the honeycomb-like film with higher E, but higher n=2.5 for the sponge-like film with lower E, in contrast to theoretical expectations (n=2 for sponge-like and n=3 for honeycomb-like). Although the dielectric constant depends primarily on first-order structural information (P), the elastic modulus of these structurally imperfect films required second-order (pore wall thickness/diameter ratio) and third-order (pore connectivity) parameters to resolve the data. The power law exponent n can vary continuously, depending on the details of the mesostructure, and should not be assumed a' priori unless justified by detailed structural information